ASNT NDT Handbook Volume 10 OVERVIEW

THERMOGRAPHY AND OTHER SPECIAL METHODS I 483

more colors or levels of gray. Typical operating ranges begin instantaneous modulation of electron backscatter, whose
intensity depends on the thermal information at the particu-
at -50 °C to O °C (-58 to 32 °F) and may go as high as lar point being scanned. The backscatter intensity is
detected by a high sensitivity electron multiplier, mounted
+2,000 °C (3,600 °F). coaxially around the electron gun. The amplified current
signal is then fed through processing circuits to a synchro-
Scanning radiometry. The test surface can be optically nized television monitor. The result is a standard television
scanned at high speed by mechanical deflection of mirrors image of the thermogram.
and prisms in the radiometer. For example, the horizontal
trace on the raster is provided by a rotating prism and the Detector arrays. The greatest advantage of using an
vertical trace by a nodding mirror. Moving back and forth array detector rather than a scanned detector is that each
and up and down on the surface, the scan covers the surface element of a detector array can monitor the emissions from
the object's surface for the full duration of the video frame
completely in a small fraction of a second, producing the exposure whereas the scanned detector has to collect infor-
thermographic image. A variety of optimizing adjustments mation "on the fly." Where objects are near room tempera-
are provided to select spot size, scan area, scan rate, focus, ture, as is often the case for nondestructive testing, the
sensitivity (contrast), etc. Figure 10 shows a functional intensity for infrared emissions can be very low and the
sketch of an optical scanning radiometer. Note that the increased dwell time of the detector in the full field array
indium antimonide detector requires cooling, a normal situ- dramatically improves the level of photon noise. Linear
ation for high sensitivity infrared detector materials. detector arrays are suited for production environments,
where they provide data from a cross section of product as it
Some scanning radiometers will yield an image several passes by on line; full field arrays, however, offer the famil-
inches wide and several inches high or surface area sizes iar scenic view as required for maintenance inspections.
determined by the optics. A field of view of 5 to 30 degrees is Single chip arrays of 128 x 128, 256 x 256 and 512 x 512 ele-
common. Detection distances are usually from about 1 m (a ments apiece are used for full field imaging sytems. The
few feet) to infinity. Standard frame speeds of 30 frames per most common array detectors are made of platinum silicide
second will produce flicker-free images, which are especially or indium antimonide, mercury cadmium telluride, com-
advantageous for visuallyobserving static or moving objects. mon for single element detectors, has also been introduced
for multiple arrays.
Real time radiometry. Radiometers of one type, gener-
ally called imagers, feature real time image readout by using Emissivity Variables
an infrared sensitive vidicon tube. Mechanical manipulation
of the optics is replaced with a scanning electron beam. As Infrared thermography is most successful when used
shown in Fig. 11, the thermal image from the surface of the with surfaces with high emissivity. High emissivity provides
test material is focused on a photoconductive retina plane several important effects. First, as seen in the Stefan-Boltz-
next to the receiving face of the tube, converting the ther- mann Law, surfaces with a high emissivity E emit a higher
mal image into a conductivity pattern by the photoconduc- intensity radiation at a given temperature, thereby providing
tive action of the retina. An electron beam focused from the a larger signal for the infrared detector. Second, high emis-
other end of the tube is raster scanned across the entire sivity surfaces are, by definition, poor reflectors. Low emis-
inside surface of the retina, usually at a rate of 30 frames per sivity surfaces tend to reflect radiation from other sources.
second. During electron scanning, an electrical charge pat- The infrared detector thus senses energy unrelated to the
tern corresponding to the conductivity pattern is developed temperature of the object being examined. This contributes
on the inside surface of the retina. This action causes an to the noise of the test and reduces the sensitivity to details
of interest. Third, high emissivity surfaces also absorb more
FIGURE 9. Block diagramof typicalinfrared radiant energy. Radiant sources can therefore be effectively
inspectionsystem used to induce a thermal gradient in the test sample, a con-
dition required for many infrared thermal tests. Table 2 pro-
VCR - INFRARED TEST vides a list of typical surface emissivities for a variety of
CAMERA materials in a temperature range of about Oto 40 °C.3,6
DOPANEL
FRAME HEAT Emissivity changes can cause severe errors in radiomet-
GRABBER SOURCE ric detection methods, unless something is done to keep
emissivity constant. This has been successfully accom-
COMPUTER 1-------1 IMAGE plished primarily by coating the test surface with materials
IMAGE PROCESSING MONITOR that possess uniform, high emissivityvalues (typically E = 0. 7
to 0.9). Uniform emissivity is necessary for temperature
SYSTEM

484 I NONDESTRUCTIVE TESTING OVERVIEW

measurement accuracy while high emissivity is desirable to scanning one ahead of the other. The result is an emissivity
provide a larger radiant intensity (per the Stefan-Boltzmann independent readout (even on very rough surfaces) for
Law). It was once thought that coatings needed to be dark in regions of constant temperature. Another method uses a
color, such as lampblack suspended in a polymeric binder. single radiometer that samples the reflected and emitted
However, transparent polymeric coatings also have been infrared radiation. Although considerably more simple, this
successful. Metal surfaces especially must be coated and design is suited only for very smooth surfaces.
nonmetal surfaces to a lesser degree; however, high emissiv-
Both passive and active infrared nondestructive testing
ity coating will help. After testing, the coatings are usually find a variety of applications throughout diverse industries.
removed from the material surface. The development of satisfactory (sensitive) thermograms
depends on several important factors. It must be remem-
The emissivity problem can also be reduced by special bered that, because of transient heat flow, a thermogram of
design of the radiometer system. One ingenious method a given area changes with respect to observation time (heat-
involves two radiometers, where the infrared signal from ing time). When heat is applied to an inspection surface, as
one radiometer is delayed, a constant signal added to it and shown in the common setup in Fig. 12, a thermogram will
the sum divided by the other radiometer's output. Sensing develop that is a function of the material, the nature of the
delay time between the two radiometers is accomplished by

FIGURE10. Functionalsketchof radiometershowingone opticalscanningtechniquefor producing
infraredimages

VERTICAL SYNCHRONIZATION SIGNAL

HORIZONTAL SYNCHRONIZATION SIGNAL

MOTOR I PHOTOCELL PHOTOCELL MOTOR 2
PICKUP PICKUP

LIQUID VIDEO
N2 SIGNAL
TO
DISPLAY
UNIT

16 CYCLES
PER SECOND

GERMANIUM LENS

MIRROR
GERMANIUM LENS

OSCILLATING
PLANE MIRROR

THERMOGRAPHY AND OTHER SPECIAL METHODS I 485

TABLE 2. Total radiation emissivities at all 3. The observation time required for a temperature dif-
wavelengths ference to reach its maximum value over a discontinu-
ity increases as the thermal diffusivity decreases, as
Material Emissivity the depth of the discontinuity increases and the size
(range of O to 1 .0) increases. At a given depth, small discontinuities reach
their peak intensities on the thermogram before
Metallic 0.02 to 0.04 larger discontinuities reach their peak intensities.
Highly polished aluminum,
silver. gold, brass, tin 0.03 to 0.08 4. The duration of the transient temperature difference
Polished brass, copper. steel, nickel, over a discontinuity decreases as the discontinuity size
chromium, platinum, clean mercury 0.08 to 0.20 decreases and as the discontinuity depth increases. At
Dull, smooth, clean aluminum and a given depth, infrared testing is less sensitive to
alloys, copper. brass, nickel, stainless 0.15 to 0.25 smaller discontinuities than to larger ones.
steel, iron, lead, zinc
Rough ground or smooth machined 0.20 to 0.4 5. Discontinuities parallel to the test surface are usually
castings; steel mill products, sprayed easier to detect than ones perpendicular to the surface.
metal, molten metal 0.30 to 0.55
Smooth, slightly oxidized aluminum, FIGURE 11 . Functional sketch of infrared
copper. brass, lead, zinc 0.60 to 0.85 vidicon tube
Bright aluminum, gilt or bronze paints
Heavily oxidized and rough iron, steel, 0.80 to 095 RETINA
copper. aluminum
0.85 to 0.95 INFRARED
Nonmetallic LENS
White or light colored paint plaster. 0.92
brick, tile, porcelain, plastics, asbestos 0.93 ELECTRONMULTIPLIER IMAGE
Red, brown, green, buff, and other 0.96
colors of paint tile, inks, clays, stone 0.85 COOLING FINGER
Concrete 0.93 FROM GENERALELECTRICCOMPANY. REPRINTEDWITH PERMISSION.
Brick, Common red 0.90
Water 0.98 FIGURE 12. Common radiometer setup for
Snow at -1 0 °C f+ 14 °F) 0.90 active testing
White bond paper 0.85 to 0.95
Sand RADIOMETER
Human skin 0.90 to 0.97
Wood: planed oak
Gla_ss and translucent plastics, glass
fiber composites, oil, varnish, ice
crystals
Carbon black, tar. asphalt matte black
paints, carbon fiber composites

discontinuity,the observation time and the heat intensity. A INFRAREDLAMP INFRARED LAMP
complicated relationship exists among these variables. How-
ever, a practical summary follows. TEST MATERIAL

1. Transient temperature differences over discontinu- ______.I THERMOGRAM
ities in materials having higher thermal diffusivities
(metals) are shorter in duration and lower in magni-
tude than they are in materials having lower thermal
diffusivities (nonmetals).

2. The magnitude of the surface temperature difference
over a discontinuity decreases with the depth of the
discontinuity. Near surface anomalies are much eas-
ier to detect than deep ones.

486 I NONDESTRUCTIVE TESTING OVERVIEW

PART 3

THERMOGRAPHIC APPLICATIONS

Typical thermographic applications involve the introduc- than the thermal conductivity of the matrix material. This
tion of a controlled thermal load on the object of interest. leads to dramatically higher in-plane conductivity than
Variations in thermodynamic properties of the object then through-thickness conductivity, a condition that is usually
produce surface temperature patterns that can be detected exactly the opposite of what should be desired for ideal ther-
with an infrared imaging system. Applications of infrared mography. Computer simulations and analyses of the thermal
thermography cover a wide range of industries, temperature diffusion process in a composite material are frequently use-
ranges, material types and test configurations. The flexibility ful in understanding the results of the test and drawing quan-
of the techniques, blended with the ingenuity of practitioners titative conclusions from the data.8-11.13-18
has led to an extremely diverse array of test techniques. The
great diversity of these techniques defies a simple classifica- Heat Conduction Analysis
tion into a small number of well defined groups.
Infrared thermographic techniques have frequently
The following discussion is organized by the general cate- been applied to composite materials for the characterization
gory of application rather than by test technique. There is of impact damage.7-10,13 Use of thermography for this appli-
some overlap in the discussion of thermographic techniques. cation is attractive for a variety of reasons: (a) infrared ther-
Discussion for each application is limited to practical tech- mography provides a rapid, noncontact approach suitable
niques. It must be understood that in nearly all cases alterna- for evaluating large areas of structure quickly; (b) impact
tive techniques can be developed that make effective use of damage can be difficult to spot with the unaided eye, yet
test equipment with different performance characteristics. typically produces a pattern of delaminations lying parallel
to the surface, a condition readily detected by thermogra-
Composite Materials and Structures phy; (c) the required equipment can be relatively light
weight and portable; (d) impact damage represents one of
Structures and components fabricated using advanced the principle damage mechanisms occurring in service and
fiber reinforced composite materials are, in many ways, can substantially reduce the load carrying capability of the
excellent candidates for infrared thermographic examina- structure (particularly in compressive loading where column
tion. These materials tend to have relatively high emissivi- buckling may be a concern).
ties and are therefore suited for examination with or without
surface treatments. The nature of the materials is such that By using pulsed thermal techniques and time sensitive
many of the damage mechanisms produce discontinuities analysis of the results, discontinuity depth characterization
oriented roughly parallel to the surface, thereby providing a can be effectively made.8-10•13-16 In most cases, these tech-
suitable blockage to flow of heat. Further, the diffusion rate niques use a very short duration thermal pulse to provide a
in these materials tends to be relatively slow, permitting a temporal resolution of the thermal transient. Thermal exci-
reasonable period of time to visualize, capture and evaluate tation events are often generated by the discharge of one or
the results of a test. Numerous applications of thermogra- more xenon strobe units. Combined discharge energies usu-
phy for composite materials and structures have been ally range from 4 to about 50 kJ (3.8 to 47 BTU), deposited
reported, particularly for detection of impact damage.7-14 over only a few milliseconds. Relatively short bursts from
quartz lamps have also initiated the thermal pulse.
At the same time, these materials frequently offer some
interesting challenges to the interpretation of the infrared When such a pulse is used to generate an event, the
thermal results. Just as the elastic properties of these materi- thermal diffusion front propagates in a manner somewhat
als tend to be nonuniform, the thermal properties of the fiber analogous to a dispersive wave19 with a time dependent
reinforcements are frequently dramatically different from velocity, such that the effective average velocity:
those of the matrix materials leading to substantial anisotropy
in the heat flow patterns. This fact can significantly compli- z a (Eq. 8)
cate the quantitative interpretation of the infrared thermal z
results. The problem is particularly severe with carbon fiber Where:
composites, which represent some of the most commonly t = time;
used high performance materials. The thermal conductivity z = depth to a feature; and
of carbon fibers is commonly two orders of magnitude higher a = thermal diffusivity.

THERMOGRAPHY AND OTHER SPECIAL METHODS I 487

Where the thermal pattern for the heated surface is mon- in which the time to peak contrast is monitored for each
portion of the image (frequently for each pixel). The image
itored, time to develop a response to a discontinuity can be can then be analyzed and presented on the basis of the time
to peak contrast, thereby stepping down through the thick-
approximated as the square of its depth divided by the ther- ness of the material. One factor that must be considered in
these tests is lateral diffusion around small discontinuities.
mal diffusivity. Following the introduction of pulsed heating,
Lateral diffusion can prematurely suppress the contrast in
the surface layer of the composite ( or any material) initially the surface indication associated with small discontinuities,
particularly in materials with high in-plane conductivity.8
heats up rapidly during surface heating and then cools as the
Time dependent processing of infrared thermal images
heat both diffuses into the material and is transferred into can also be achieved using step function heating.13 In this
technique the imaged surface is continuously heated by a
surrounding matter (mostly air) by conduction, convection heat source uniform throughout the duration of the test. In
this case, the surface temperature continues to rise as
and radiation. The heating and cooling curves follow a pat- energy is continuously deposited. The resulting tempera-
ture profile for discontinuities at varying depths are shown
tern that depends on the depth to a feature of high thermal in Fig. 15. The principle advantage of this technique is that
greater contrast can be generated for deep lying discontinu-
resistance. A family of such curves showing the effect of dis- ities. Two precautions are that the heat source must be very
uniform to prevent generation of thermal contrast patterns
continuity depth on the shape of the cooling pattern is shown unrelated to discontinuities in the test object and that the
in Fig. 13. If one subtracts the heating and cooling curve for heat source must be chosen so that reflected energy does
a very thick material from each of the curves, the result is a not enter the infrared imager and contaminate the image.
family of curves as shown in Fig. 14. Three distinct features Diffuse laser heating is frequently used in these tests to
can be seen in these curves. First, the initial portion of each address these precautions.
curve is flat at zero, indicating that there is initially no differ-
ence between the pattern over a discontinuity and the pat-
tern for the thick material. Second, the time at which each
curve leaves the baseline and subsequently peaks, increases
with discontinuity depth. Finally, the peak contrast gener-
ated decreases with discontinuity depth.

Correlation between the time to peak contrast and the
depth to a discontinuity has led to development of a ther-
mographic tool, frequently called thermal tomography, io.u

FIGURE 13. Coolingcurvesfor pulse thermal FIGURE14. Thermal contrastcurvesgenerated
event over delaminationsat varyingdepths by subtractingfull thicknesscurvefrom each of
the other curves
250
60

200 50

.....J
z<(
o J 50 I- 40

vi ~
z1- 30
0
0u 20
LU

~O:::'. JOO
L~L

50 JO

0 8 0 8
0 0
TIME TIME
...LEGEND (microseconds) LEGEND (seconds)

-+- REFERENCE ­­1:r­ 0.50 mm f0.02 in.) DEEP

~ 0.50 mm (0.02 in.) DEEP "'*'" 0.75 mm f0.03 in.) DEEP

0.75 mm (0.03 in.) DEEP __._ 1.00 mm (0.04 in.) DEEP
__. 1.25 mm (0.05 in.) DEEP
--- 1.00 mm (0.04 in.) DEEP
1.25 mm (0.05 in ) DEEP

488 I NONDESTRUCTIVE TESTING OVERVIEW

Thermal images of impact· damage detected using the sample was heated by a xenon strobe pulse of approximately

one-sided pulsed heat technique are shown in Figs. 16 to 2.4 kJ (574 cal). Figure 18 is a thermal tomogram, or slice
iamppagroe,xiomfaateslyam7plJe that had been subjected to an impact of
18. Figure 16 shows the thermal tpoattaern3.4geJne(r2a.t5ed in a (5.2 ft-lb.) and heated with a thermal
graphite-epoxy sample subjected ft-lbr) pulse from an array of six xenon strobe units.
impact. The thermal loading was produced using a short
Thermography of composite structures can also be con-
pulse (about 5 s) from an array of four 600 W quartz lamps.
ducted using "through-transmitted" heat. This technique pro-
Figure 17 shows the thermogram for a 2g7rapJ h(i2t0e-efpt-olbxy.). lami-
nate subjected to an impact of about This vides somewhat more uniform sensitivity to discontinuities at

FIGURE 15. Discontinuity depth dependent FIGURE 1 7. Thermogram of graphite-epoxy
heating curves for step function heating sample with 27 J (20 ft-lbr) impact and strobe
heat pulse
0.5
FIGURE 1 8. Thermal tomogram of
w 04 graphite-epoxy sample with 7 J (5 ft-lbr) impact

C::: T
::)
25 mm
~w 0.3
Q.. 1(i in.)
2 I.Omm

>Ill-.J 0.2 REFERENCE CURVE
THERMALLY THICK
w 2024-B
ALUMINUM
~w 0.1

C:::

0 0.1 0.2 0.3 04 0.5
0

SQUARE ROOT TIME
(root-seconds)

FIGURE 16. Thermogram of graphite-epoxy
sample with 3.4 J (2.5 ft-lbr) impact and quartz

lamp heat pulse

THERMOGRAPHYAND OTHER SPECIALMETHODS I 489

varied depths. Figure 19 compares the one-sided versus through-transmission model while only the near surface dis-
through-transmitted thermal contrast for near side, mid- continuity shows significant contrast in the one-sided test.
plane and far side discontinuities in a 25 mm (1.0 in.) thick
fiberglass laminate subjected to pulsed thermal loading. The improved uniformity of sensitivity for discontinuities
Note that all three show significant thermal contrast in the of varying depth make the through-transmitted test configu-
ration particularly attractive for test cases involving thicker
FIGURE19. Thermal contrastcurvesfor laminate sections. The through-transmitted test configura-
10 mm (0.4 in.) thick fiberglasslaminate tion has been used effectively to characterize the extent of
computedfor thermal pulseheating in relative damage in marine composite structures.7 Figure 20 shows a
scalewhere O is value for materialwithout thermogram for a large fiber glass test panel used to evaluate
discontinuity:(a) with one-sidedimaging; candidate material systems for a United States Navy applica-
tion. The panel measures about 3.4 by 4.0 m (11 by 13 ft) by
(bl with through-transmittedimaging 90 to 100 mm (3.5 to 3.9 in.) thick. The panel had been sub-
jected to shock loading simulating an underwater explosion
(a) 16 near the surface of the panel. The panel was positioned with
the sun shining on one surface with the infrared camera posi-
14 tioned to image the other surface. The sun had produced a
temperature difference of about 15 °C (27 °F) across the
·~c 12 thickness of the panel. Subsequent destructive tests con-
10 firmed the indicated areas of delamination. Effective results
:::i have also been obtained using a through-transmitted config-
->(~lJ 8
4 °uration for fiberglass and cored laminate construction in sail-
~ 2
ing yachts.7•2 For these tests, effective results have been
0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 obtained by heating the interior spaces of the vessel and
imaging the external surfaces of the hull and deck.
TIME
(seconds) FIGURE20. Thermogramof 90 to 100 mm
(3.5 to 3.9 in.) thick fiberglass laminateusing
LEGEND solar heating and imagingon shaded surface

= IO PERCENT DEEP
= 20 PERCENT DEEP
= 30 PERCENT DEEP
= 40 PERCENT DEEP
= 50 PERCENT DEEP

(b) 0 /,:..:-: .:-: :::::.::: .
-0.1

vi' -0.2

~ -0.3 \-</. /.
:::i -0.4
. . ......
~ -0.5
-~ -0.6 5 IO 15 20 25 30 35 40 45 50 55 60

~ -0.7 TIME
- -0.8 (seconds)

-0.9

0

LEGEND

= FAR SIDE DISCONTINUITY
= MID-PLANE DISCONTINUITY
= NEAR SIDE DISCONTINUITY

490 I NONDESTRUCTIVE TESTING OVERVIEW

Buildings effects significantly influence the thermal pattern seen on a
structure when a temperature differential exists across the
Verification of Construction Details structure. Thermographic methods provide effective detec-
tion of moisture in walls22•23 and roofing materials.24·2·5 Eval-
Infrared thennography is used rather broadly in the eval- uations of roofing materials can be conducted from the
uation of the structure and condition of building envelopes. rooftop or from an airborne platform, either helicopter or
Therrnographic techniques can be effectively used to evalu- fixed wing aircraft. Areas with moisture intrusion can be
ate new construction for compliance with design specifica- readily detected. Figure 21 shows the visual and thermal
tions. 21 For these tests the best examination time is after the pattern produced by moisture in the insulation material of a
building envelope is completed and the heating, ventilating school roof.!? The ability to isolate the location(s) of mois-
and air conditioning (HVAC) system is operational. More ture intrusion in a roof structure and the extent of moisture
uniform results can be obtained if the examination can be in the insulation permits the accurate planning and estima-
performed before installation of interior treatments which tion of needed repairs. This frequently makes the difference
could influence the thermal properties near the interior sur- between a relatively inexpensive repair of a leaking roof and
face. Successful evaluation of the building envelope for a considerably more expensive roof replacement.
masomy structure typically requires a temperature differen-
tial across the thickness of the wall of 10 °C for at least 3 h. FIGURE 21. Section of roof: (a) photograph;
This thermal loading should be successful for most masonry (bJ thermogram showing wet insulation as
structures but variations in the materials used, construction warmer (lighter colored)
design and viewing conditions will impact the suitability of fa)
these conditions. Ideally the HVAC system should he used
to introduce the thermal gradient in a controlled fashion fbJ
and the influence of solar loading should be minimized,
because the coverage and loading produced by the sun may
result in variable thermal loading. This uncontrolled ther-
mal loading can make interpretation of the results difficult.

With controlled and uniform thermal loading, details of
the construction can be readily seen in the thermograrn. For
example, insulation voids or gaps can be seen as areas of
increased thermal conduction, voids in precast concrete
panels can be seen as areas of decreased conduction and
solid core reinforcements can be seen as areas of greater
thermal mass. The thermal polarity of the reinforcements
will depend on the thermal loading conditions used. For
example, if solar loading has been applied to the exterior
surface for an extended period of time and then removed,
the reinforcements will retain heat for a longer period of
time than the surrounding material.

Where use of the building HVAC system is not possible,
it may be possible to use an ASTM E 799 blower door and
temporary heating or cooling systems to induce the required
thermal loading.

Moisture Detection

Infrared thermography has proven to be a particularly
effective tool for the detection and characterization of mois-
ture in building structures. The infusion of moisture into
building and insulation materials typically has two noted
effects on the thermal properties of the structure. First, the
moisture increases the heat capacity of the material, fre-
quently by a significant amount. Second, the moisture
increases the conductivity of the material. Both of these

THERMOGRAPHY AND OTHER SPECIAL METHODS I 491

Other Building Applications A modest increase in resistance can dramatically increase
the heat generation, thus raising the temperature of compo-
Infrared thermography has been effectively used to nents in the immediate vicinity of the problem area. A loose
characterize a variety of other building related features. or corroded connection, broken strands in a conductor, dirty
One novel application uses plastic film screens to visualize contact surfaces and other problems can be sources of ele-
air flow patterns in building HVAC systems.26 The small vated electrical resistance. Several other sources can also be
mass and low emissivity of gases makes infrared thermo- responsible for elevated temperature in high voltage equip-
graphic detection of gaseous material temperature very dif- ment.F For example, leakage to ground through cracked
ficult unless gas temperature varies greatly from the insulators can generate substantial temperatures that are
ambient. For this application a screen of plastic film is readily detected by infrared thermography. Induced voltage
placed in the air flow near a register and situated parallel to in conducting structures placed close to current carrying
direction of air flow. As the warm or cool air flows across the components can also result in energy lost to ground thereby
screen, the plastic film material is heated or cooled. Emis- generating heat.
sivity of the plastic screen material can be quite high and is
readily imaged by a scanning infrared radiometer. High voltage systems also represent a significant safety
hazard. Systems must frequently be taken off line before
In an application analogous to the roof inspection, the they can be approached with traditional test equipment
insulation on steam lines and/or HVACducts can be checked requiring contact with the current carrying components.
for moisture or gaps in the insulation. In this case, the ther- Because the infrared thermographic images can be gener-
mal load is provided by the energy in the steam or the air in ated from a safe distance, electric distribution systems can
the HVAC duct. Infrared thermography has also been used usually be tested on line.
successfully to examine buildings for air leaks around doors
and windows, electrical fixtures and other through-wall Diagnosis is more complex than simply determining how
openings in exterior walls. As with the HVAC air flow test, it hot things are. Many components have low emissivity, and
is not the leaking air that is imaged in the thermogram, but the effects of load and wind can further complicate image
the effect of the air on the temperature of the surrounding interpretation.
structure. For example, with a slight negative pressure inside
the building, leakages of warmer outside air (during the cool- Usually, direct visual access to high voltage equipment
ing season) can be seen as warm regions in the interior wall can represent a significant challenge. Critical components
surface near fixtures, windows and doors. may be protected behind cabinet doors equipped with
safety interlocks.28 Without direct visual access, infrared
Electric Power Distribution and thermography cannot be effectively performed. It may be
Transmission Systems possible to detect the presence of a severe problem by
detecting an elevated temperature for the enclosure; how-
High voltage electric power distribution systems repre- ever, the extent of the problem cannot be usefully deter-
sent a natural application for infrared thermography. Many mined in this manner. The amount of ventilation, separation
of the potential problems in these systems result from ele- between the source of the heat and the enclosure surface
vated electrical resistance. The energy losses associated with and the nature of the enclosure can all dramatically influ-
these problems manifest themselves as heat. The rate of ence the ability to detect such conditions. Furthermore, if
heat generation in a component or circuit is proportional to the problem can be detected, diagnosis of the problem will
the product of the current flowing through the circuit and be very difficult without specific knowledge of the source of
the voltage drop across the component or circuit. This can heat. It can be dangerous to apply techniques without fur-
also be expressed as the product of the resistance in the ther testing their suitability for the specific application.
component and the current squared:
Pavement, Bridge Decks and
H Elt (Eq. 9) Subterranean Surveys
4.18
Maintenance and repair of paved surfaces requires the
Where: timely detection of delaminations and voids or erosion pock-
ets beneath the paved surface. Resurfacing over a delami-
H heat generated (joules); nated section will only lead to premature pot holes and
E potential (volts) across the component or circuit; subsurface erosion pockets can jeopardize the integrity of
I current (amperes) flowing through the circuit; both highway and parking lot surfaces. In many cases, these
R resistance (ohms); and situations are addressed only after they become serious
t time (seconds). problems. In other cases, sounding techniques have been

492 I NONDESTRUCTIVETESTING OVERVIEW

used in an effort to identify pavement and bridge deck convection losses. Testing must not be conducted at tem-
delaminations before resurfacing. Sounding techniques typ- peratures below O °C (32 °F) because ice in a delamination
ically involve dragging chains behind a vehicle and listening could invalidate the test. Bridge decks are tested by driving
for a change in the sound over a delaminated surface. the vehicle at speeds not exceeding 16 km-h-' (10 mi-h+) to
collect data for one traffic lane. A separate pass is made for
Infrared thermography is effectively used to evaluate each traffic lane. If the shoulders are wider than 1 m (3.3 ft),
paved surfaces in each of these situations. In fact, an ASTM a separate pass is made for the shoulder.
Standard Test Method28·29 exists to address use of infrared
Typically, successful testing can be performed during the
thermography for detecting delaminations in bridge decks. winter months on Portland cement concrete decks with 4 h
Solar loading can produce a substantial thermal gradient of direct sunshine and winds less than 6.6 m-s! (15 mi-h'").
across the depth of a paved surface. This gradient produces a These conditions should be sufficient to produce at least an
heat flow from the upper surfaces of the pavement into the 11 °C (20 °F) temperature rise in the pavement. Asphalt
lower layers and base for the paved surface. Voidsand delam- decks require about 6 h of direct sunshine under similar
inations in the pavement or base can produce increased resis- conditions to achieve the 11 °C (20 °F) temperature rise in
tance to the flow of heat through the pavement. the pavement. While the ASTM standard specifically
addresses bridge decks, similar methods can be used to
Thus, during the day while the solar loading is causing evaluate the condition of highway pavements and parking
the heat flow down through the pavement, delaminations lot surfaces.
will show as hotter regions because the delaminations
reduce the ability of the lower layers to conduct heat away Infrared thermography can also help find underground
from the surface. Through the course of the day, the solar storage tanks, voids, buried industrial waste etc. Such sur-
loading increases the temperature of the pavement and the veys can be very, effective in evaluating possible construc-
ground beneath. At night, the deposition of solar energy tion sites 'to identify potential problem areas. As with the
ceases and the air temperature drops so that the direction of pavement examinations, solar loading can be used to pro-
heat flow reverses. The energy stored in the pavement and vide a suitable thermal gradient. The infrared scanner can
base begins to flow back to cooler ambient surroundings. be ground vehicle mounted or operated from a helicopter or
Once again, delaminations reduce the ability of this heat to fixed wing aircraft. Correlation of infrared thermal results
flow. Thus, the heat in the upper layer of pavement above with alternative techniques, such as ground penetrating
the delamination quickly dissipates without drawing addi- radar, can provide effective interpretation of the results.P
tional heat energy from the layers below. The pavement sur-
face over a delamination therefore cools more quickly than Aerial thermographic surveys of city streets have been
the surrounding area and can be imaged as a cooler region. effectively used to locate and identify steam line breaks.
Data from a global positioning system may be fed to the
Thermographic examination of paved surfaces can be video recorder along with the infrared image data, to pro-
conducted with aerial surveys, ground vehicle surveys, or, vide a position reference for the thermographic data.
particularly in the case of a parking lot surface, from the roof
of an adjacent building. ASTM Standard Test Method D Automotive Applications
4788 describes the use of vehicle mounted thermographic
examination to evaluate bridge deck pavement. The stan- Little has been published regarding automotive applica-
dard calls for the infrared scanner to be mounted on the tions of infrared thermography, primarily because much of
front of a vehicle at a height to permit an image width of at the work is proprietary. However, major advances have
least 4.5 m (14 ft). The scanner must have a minimum tem- taken place in thermography in that industry.
perature resolution of 0.2 °C (0.36 °F) at ambient tempera-
tures. A standard color video camera is also mounted on the One novel application is the verification of operation of
front of the vehicle to produce a field of view comparable to the automobile's windshield defrosting system. For this test,
that of the infrared scanner. Both the infrared scanner and the car's engine is run until the engine warms up and the
the color video camera are connected to separate video defroster system is turned on. An infrared scanner can then
recorders to document the test data. A distance measuring be used to image the temperature pattern on the wind-
device, accurate to within 76 mm (3.0 in.) over the span of shield. The pattern can then be analyzed for functionality,
the bridge is also attached to the vehicle. The output of the uniformity and symmetry. Problems in air flow patterns can
distance measuring device is fed to both video recording quickly be identified.
decks to aid in the location of indications. The pavement
deck must have been dry for at least 24 h before conducting Infrared thermographic techniques can also be used to
the thermal test and the deck must have received a mini- characterize the performance of internal combustion
mum of 3 h of direct sunshine to produce a minimum of engines, both for automotive and industrial applications.31
0.5 °C (0.9 °F) temperature differential over voids. Wind Operating engines can be examined for uniformity of cylin-
conditions must be under 6.6 m-s! (15 mi-h ") to minimize der heating and for exhaust leaks.

THERMOGRAPHY AND OTHER SPECIAL METHODS I 493

Thermography has been used in the design stage for Second, the conductivity and therefore the rate of diffusion
thermal mapping of floor pans and under-hood analysis. for aluminum structures is quite high. Time to peak thermal
One of the most useful applications has been to validate the contrast is generally not measured in seconds or minutes, as
placement of thermocouples as well as to reduce the quan- with the composite structure, but in milliseconds. Special
tity needed. Tracer gases and infrared sensing have been care must therefore be given to the selection of an infrared
investigated for air flow visualization. Suspension alignment recording system with a suitable temporal resolution.
and tire wear problems correlate to thermal patterns. Many
functions in automobiles are controlled by printed circuit Several approaches have been used to modify the surface
boards, and thermography has become an important tool in emissivity of bare metal surfaces to improve the perfor-
the design and placement of these heat sensitive devices. mance of thermal infrared examination. (Note that these
techniques are not required for some painted surfaces.)
During production, infrared thermography has been One approach is to coat the surface with a water washable,
used to inspect die casting, injection molding, and blow high emissivity material that can be easily removed follow-
molding equipment and processes.32·33 Thermal studies of ing the test.·38 Another approach is to coat the test area with
ovens have dramatically reduced paint problems, one of the a strippable plastic film coating.37 If good, bubble-free
most costly items on a vehicle. Welding, induction heating, adhesion is achieved with the plastic film, this approach has
and hardening processes are all monitored with infrared been shown to produce satisfactory results.
thermography. As more and more composite materials have
been incorporated into vehicles, bond integrity applications Another novel approach that has been used to image
have been added into production line inspection processes. more stable thermal events in metallic surfaces is to use a
Heat produced in cutting and boring operations provides high emissivity material that is placed in contact with the
direct feedback as to the efficiency of those processes. Even surface for a period of time and thus becomes "imprinted"
in the cutting and sewing of fabric and trim, thermography with the thermal pattern of the metallic surface.39 When the
has dramatically reduced scrap rates and improved quality. high emissivity material is removed from the surface, the
thermal pattern can be imaged by the infrared scanner. A
Infrared thermography has been used in hundreds of rolling motion is typically used to continuously bring the
automotive manufacturing facilities, both for solving manu- high emissivity material into contact with the test piece sur-
facturing problems as well as for basic machine and facilities face. Figures 22 to 24 illustrate possible test configurations.
maintenance. The inspection of robotic welding machines, In each case, provision is made to "erase" or normalize the
precision spindles and hydraulic presses has been particu- temperature pattern on the high emissivity material before
larly valuable to improving production uptime and reliabil- being again brought into contact with the test part surface.
ity.34 Major automobile manufacturers have made a The relatively low specific heat of the transfer material
concerted effort to educate parts suppliers about the value helps ensure that the temperature pattern detected is dom-
of using thermography in design and production. Infrared inated by the pattern in the metallic component and the
thermography continues to grow in importance in the auto-
motive industry as smaller, higher resolution focal plane FIGURE 22. Thermal transfer imaging system
arrays come into wider use. using solid roller of compliant material; thermal
gradient may be input as through-transmission
Bonded Materials and Structures (heater AJ or one-sided (heater BJ39

Recent increased emphasis on the need to provide viable ERASER ROLLER
nondestructive inspection procedures for aging aircraft
structures has led to numerous evaluations of infrared ther- COMPLIANT ROLLER
mography as a tool for characterizing adhesive bond
integrity in bonded lap joints.35-37 Thermographic examina- a>-;>' INFRARED
tion of these details typically presents two substantial obsta- CAMERA
cles. First, while surface emissivity of a composite surface is
frequently in excess of 0.9, a bare aluminum surface has an HEATER A SAMPLE MOTION
emissivity that is generally less than 0.1. This means that the
radiant energy relating to the surface temperature is very
low. Worse yet, it means that the surface of the object acts as
an infrared mirror. The infrared imager will tend to see not
so much the temperature of the test object as the tempera-
ture of those objects reflected in the test object surface.

494 I NONDESTRUCTIVE TESTING OVERVIEW

lower conductivity of the transfer material helps stabilize Welding and Welded Structures
the image for infrared detection.
Welding entails considerable heating of the work piece.
The second major area of concern when dealing with The way heat is generated is frequently critical to the success
high conductivity materials is the short time periods avail- of the welding process. As a result, infrared thermography
able to capture transient events. Several approaches are has been evaluated by several researchers as a tool for on-
used to deal with the short time periods. One approach is to line process control of gas metal arc welding and gas tung-
modify the detection system to work with infrared detection sten arc welding Because the cooling rate of the weld puddle
systems that can provide more rapid frame rates.t? This pro- affects the microstructure of the completed weld, infrared
vides raw data at shorter time intervals to aid in capturing techniques have been evaluated as a tool to monitor the cool-
short duration events. The faster frame rates can be ing rate.41 The measured cooling rate could be used in a
achieved in at least a couple of ways; one is to work with feedback control to produce the desired weld microstruc-
higher speed focal plane array detectors and another is to ture. A thermal line scan across the weld bead immediately
work with a line scan, rather than a full field imaging system. behind the weld torch can also be used to characterize sev-
A second approach to aiding the analysis of rapid transient eral features of the weld being produced. First, the height of
events is to use time derivative image processing. This the line scan (representing the peak temperature excursion
approach has been shown to produce effective results for from ambient) has been found to correlate linearly with weld
characterizing bonded lap joints.36,37 penetration, while the width of the line scan correlates lin-
early with the width of the resulting weld bead.42 In fact the
FIGURE 23. Thermal transfer imaging system inflection points in the line scan, as shown in Fig. 25, have
using closed loop membrane; air jets provide been shown to correspond to the liquid-to-metal interface.P
image erasing mechanism39 The change in slope can be explained by the difference in
emissivity for the liquid and solid metal.
/ CLOSED LOOP MEMBRANE
The symmetry of the heat distribution pattern has also
/ / INFRARED CAMERA been used to determine the alignment of the weld torch with
the weld joint centerline.44 If the torch begins to wander off
!<,{~ the weld centerline, more heat is deposited in one plate than
the other and the heat distribution becomes asymmetric.
This effect is illustrated in the two thermograms shown in
Fig. 26. The thermogram in Fig. 26a shows an asymmetric
pattern in which the torch is offset from the centerline

COMPLIANT ROLLER FIGURE 25. Line scan of temperature pattern
across weld pool, in relative grayscale units;
SAMPLE MOTION inflection points correlate to liquid-to-metal
interface43

FIGURE 24. Thermal transfer imaging using 255
closed loop membrane with thermal excitation
and detection from within membrane 240 \ -,r>,

zi:'.: 220 I: .\
200 I:
vi
~l..1.J IBO .

STRETCHING CLOSED LOOP MEMBRANE I- 160
ROLLERS
II.,.-.,--------~ 140 ,/-'

INFRARED 1=::,J"' zI- 120 /\
CAMERA ::t;; 100
0 80
WATER v .HEATER 60 /\
SPRAY' ..,1\, .':' ~ 40

20

0

DISCONTINUITY~ 4 24 44 64 84 I 04 124 144 164 184 204 224 244

SAMPLE MOTION DISTANCE
(pixels)

THERMOGRAPHY AND OTHER SPECIAL METHODS I 495

whereas the thermogram in Fig. 26b shows a symmetric pat- Diverse Applications

tern produced with the torch centered over the centerline. Recent investigations have evaluated infrared thermog-
raphy as a tool to detect and measure the extent of corrosion
Infrared thermography has also been used to character- in metallic components. Some success has been achieved in
both steel46 and aluminum=" samples. In both cases, step
ize the integrity of spot welds in a post-manufacturing exam-
ination. 45 In this application, the spot welds were used in the FIGURE27. Schematicof spotwelded panels45
fabrication of a corrugated stainless steel structure as shown
in Fig. 27 to make lightweight ship structure. Thermo-
graphic examination was performed in much the same man-
ner as many of the composites applications. The panels were
examined in a painted condition, so surface emissivity was
not a problem. The thermal gradient is typically input with a
swept quartz lamp. Incomplete or fractured spot welds can
readily be detected in the resulting thermogram as shown in
Fig. 28.

FIGURE26. Thermogramsof weld pool •••••• 0 •••••• o
showingasymmetricand symmetricheat 0 e
patterns44 0
0 0
faJ 0 0
0 0
0

FIGURE28. Thermogramof spotwelded
corrugatedstainlesssteel panel

fbJ

496 I NONDESTRUCTIVE TESTING OVERVIEW

function heating was used to induce a thermal gradient. The applications. Many novel applications are possible and many
time at which the rate of temperature increase changes is have been evaluated. Discussion contained in this section is
then related to sample thickness. This technique is certainly intended to provide the reader with a taste of some of the
analogous to thermal tomography techniques discussed in diverse applications that are possible. Many additional test
the composites applications section, however, problems applications are possible and have been developed in the
associated with surface emissivity and very short response 1990s. As innovative concepts of thermographers around
times make signal analysis problems more difficult. the world meet with advances in infrared detection systems,
image processing and data analysis techniques, the applica-
Infrared thermography is a very flexible nondestructive tions for infrared thermography will continue to grow.
evaluation method and is applicable to a broad range of

THERMOGRAPHY AND OTHER SPECIAL METHODS I 497

PART4

OPTICAL METHODS

Grid and Moire Nondestructive Under the right circumstances, when two sets of periodi-
Testing47 cally repeated elements are superimposed, moire fringes
become visible. Moire fringes are the result of the modula-
Methods for the measurement of deformations of struc- tion of light intensity when light is passed in succession
tures are of interest because local and global deformations through these repetitive patterns. The amount of light that
can reveal the severity of loads on the structure. Even
before the age of instrumented strain measurement, engi- FIGURE 29. Opticalsetups for four basic grid
neers would apply markings on the surface of structures to methods:(a) fixed grid methodfor in-plane
see deformation by tracking the displacements of the mark-
ings when the structure is loaded. It is well known that most deformations;(b) fixed grid methodfor
structural materials are too stiff to exhibit local elastic defor-
mations directly to the eyes. Additional instrumentation and out-of-planedeformation;(c) projectedgrid
magnifying optics with high resolving powers are needed. methodfor out-of-planedeformations;

From the point of view of applications to technical prob- (dJ reflectedgrid methodfor out-of-plane
lems, there are four varieties of grid based phenomena. deformations
D O 11(a)
1. In the intrinsic moire orfixed grid technique, a grid is / DOMINANT IN-PLANE DEFORMATIONS
printed on a flat surface to show displacement of lines
that result from deformation. I TELECENTRIC FILM.GLASS PLATE.
I LENS CHARGE COUPLED
2. The intrinsic moire technique may also be used on out- I
of-plane (curved) surfaces to measure out-of-plane DEVICE OR CHARGE
deformations and displacements of surface points. I GRID FIXED

3. Inprojected, or shadow, moire technique, the shadow ON SURFACE INJECTED DEVICE
of a grating is projected on a matte, or light diffusing,
surface. The grating and its projection on the surface TEST SPECIMEN BASIC RESULTS STRAINS: Epo,EPR,YoPR
produce a moire pattern for measuring changes in the
contour of a surface. D O 1r(bJ
I I DOMINANT OUT-OF-PLANE DEFORMATIONS
4. In reflectedmoire, a grating and its reflection on a non-
diffusing, or mirrorlike, surface produce a moire pat- I TELECENTRIC FILM.GLASS PLATE.
tern that gives the loci of equal projected slopes. The I LENS CHARGE COUPLED
reflected technique is useful for small, out-of-plane I GRID ON ~~~EEg~~~~GE
forms and for changes in curvature distributions.
SURFACE
The phenomena are analyzed by using a common frame-
work. Figure 29 shows the schemes of the optical setup of O(dJ ANY DEFORMA:Ni;-::::1-~r~~~~D"---':
each of these four basic grid techniques. The optical setup is \t.:.­­3.- .: - -fsMALL ,...,1---,1-...,._._.BRIGHTGRID
the heart of a grid technique. Moire techniques differ from RROR LIKE -;,~ PRODUCER
the pure grid techniques in that the optical system produces
interference fringes to highlight displacements. SURFACE /'---'...----~

Moire is a French word for a silk fabric that shows dark GRID MILKY BASIC RESULTS
bands; the word can be traced to Chinese. Moire is histori-
cally the first member of a family of optical methods GLASS LOCAL SURFACE
(including holography and speckle interferometry) that can CURVATURES
be used to measure displacements. All these methods have
some common elements and the moire method provides the
basis to comprehend the basic aspects of the whole family.

498 I NONDESTRUCTIVE TESTING OVERVIEW

gets through depends on the relative positions of the two surface. Other metrological methods, such as some electro-
patterns; if the two patterns move one relative to the other magnetic and ultrasonic techniques, are noncontacting but
or if the observer moves, the fringes also move. Conse- require that the detection equipment be close to the test
quently, the observation system plays an important role in specimen, that is, within 25 mm (1.0 in.). In contrast, holo-
the formation of the fringes. graphic methods require only visual access to the specimen's
surface and so can be used for remote sensing. Further-
Moire Interferometry more, as a noninvasive process, holography can detect the
information of interest in its purest form, without altering
Moire interferometry measures in-plane displacements the information by mechanical effects as some acoustic
with very high sensitivity. It is effective for a broad range of
engineering problems, including analyses of deformation of techniques might.
very stiff materials and investigations of micromechanics. Another advantage of holography is that it provides full
Being a strictly geometric effect, it responds equally well to
elastic, inelastic, isotropic, anisotropic and composite bod- field detection in a single acquisition. Other nondestructive
ies. Displacement fields are determined throughout an testing methods (such as ultrasonic C-scanning) can provide
extended field of view by contour lines of equal displace- a full field map of data but may require a time consuming
ment. Because of the high sensitivity and abundance of dis- rastering of the specimen surface. Scanning can be pro-
placement data, reliable strain fields can be extracted from hibitive in the study of dynamic or inline processes.
the data. Thermal deformations can be measured, too, as Although full field detection does limit spatial resolution of
well as mechanically induced deformations.48 holography, the typical imaging systems for holographic
readout (employing a closed circuit device camera and com-
Holography49 puter digitization) provide minimum resolutions of 0.002 x
full field dimension, which is sufficient for most applica-
Laser based methods of nondestructive testing include
holographic interferometry, speckle techniques (including tions.
shearography) and point triangulation profilometry. They all Another advantage of holographic techniques is their
use light polarized into a coherent beam by lasers so that
strain and deformation can be inferred from how waves are flexibility in meeting a broad scale of requirements. A single
reflected from the test surface. The laser based methods are holographic system can be used to simply and rapidly iden-
a noncontacting means of determining material conditions tify gross discrepancies in a test specimen's behavior by
that contribute to failure in critical parts ranging from lami- direct inspection of holographic fringe patterns or to quanti-
nated composites to pressurized containers. tatively analyze the exact character of that behavior with
high accuracy and resolution by automated fringe pattern
Holography has received much attention in the nonsci- interpretation.
entific community as providing a "three-dimensional pic-
ture" but images like those in the motion picture Star Wars Holographic techniques have limitations, particularly
give a distorted impression of the technology. Holography is with regard to experimental stability. Because a hologram is
more accurately understood as a method for constructing actually formed by the test specimen's complex interference
not irrwges but wavefronts. pattern known as its speckle pattern, any morphological
changes in that speckle pattern during holographic exposure
The term holography ( or holographic interferometry) (or between exposures, for holographic interferometry) will
has been applied to three similar methods in nondestructive decorrelate the hologram, rendering it useless. For exam-
testing. One is acoustic holography, which is treated in a ple, the rumble of a truck passing nearby may cause the test-
section on acoustic methods. The other two holographies ing room to vibrate, invalidating precise holographic
differ in the way the wavefront is extracted from the holo­ measurements. Details of the relationships between object
gram. The first of these techniques is known as electronic condition and speckle pattern morphology are covered in
holography, television holography or video holography; it is the discussion of speckle interferometry, but for now suffice
a form of speckle interferometry and is discussed later. The it to say that excessive motion of the specimen or deforma-
third method is conventional holography (the present sub- tion of its surface will cause decorrelation. Therefore, the
ject), in which a full optical wavefront is captured and stored
on a recording medium, usually film. applicability of holographic nondestructive testing for a
given test condition may be dependent on the extent to
The biggest advantage of holography in nondestructive which the local environment can be controlled. In addition,
testing is that it, like other optical techniques, is inherently holographic analyses are differential in nature; that is, they
noninvasive and does not require contact with the specimen measure a change in the test specimen's condition, so an
object must either be undergoing change on its own or be
subjected to an externally applied, albeit reversible, change
(i.e., deformation, translation, reillumination etc.).

Figure 30 shows an example of an experimental arrange-

ment for an off-axis hologram.

THERMOGRAPHY AND OTHER SPECIAL METHODS I 499

Background Although there are some applications of simple hologra-
phy to nondestructive testing (as is discussed below), most
Holography involves the recording (and subsequent nondestructive testing requires a much higher precision of
reconstruction) of the entire optical wavefront coming from measurement than can be achieved by direct inspection of a
an object. This wavefront has both amplitude and phase as holographic image. In these situations, it is necessary to
does any wave propagation and both must be recorded. apply holographic interferometry. Whereas conventional
Because conventional photographic materials can record laser interferometry involves the interference of two laser
only intensity (amplitude squared) information, the phase beams, holographic interferometry requires the interference
information must be encoded in some way in that intensity of two complex optical wavefronts. Consider the arrange-
map. This is accomplished by interfering multiple wave- ment of Fig. 30b. If the object is deformed slightly after
fronts at the film plane. A brief description follows; a com- holographic exposure and then reilluminated along with the
plete theory of holography is available elsewhere.50,5l
FIGURE 31 . Basicsof two-beaminterference:
Figure 31 shows the basic concept of two beam interfer- (a) configurationof incidentbeams A and B;
ence. In Fig. 3la two plane waves (A and B) from a single (b) resultinginterferencepattern (enlargedto
coherent source are incident on a film plate. When the film show detail)
is developed, the film's resulting intensity distribution forms
a sinusoidal grating (Fig. 3lb). The periodicity of this grat- (a) INTERFERENCE
ing is such that if the developed film is reilluminated by one
of the original plane waves, say wave A, a portion of the SCREEN
transmitted light will be diffracted into a direction that cor-
responds exactly to the propagation of wave B, as if both
waves were present.

FIGURE 30. Experimentalconfigurationfor 0
off-axis holography:(a) holographic 0
construction;(b) holographicreconstruction
LEGEND
(a) = INCIDENT BEAMS

MIRROR - = INTERFERENCE PATTERN

FILM PLATE (b)

OBJECT

OBJECT BEAM

(b)

MIRROR

VIRTUAL
OBJECT

~-

500 I NONDESTRUCTIVE TESTING OVERVIEW

developed hologram, two wavefronts will be seen emanating tested and yields full field surface strain information without
from the object's position - one real, the other holographi- limitations associated with other methods.
cally reconstructed. Each of these wavefronts will represent
the object in a different state of deformation and their inter- One major difference between shearography and other
ference will be manifest as a contour map of that differ- methods of nondestructive testing is the way discontinuities
ence. 52 The contour map can then be interpreted to provide are revealed. Methods such as penetrant and magnetic parti-
quantitative information on the object's deformation. cle testing reveal surface or subsurface discontinuities by
enhanced visual means and techniques such as ultrasonic and
Although most nondestructive testing uses of holography radiography detect internal discontinuities by detecting het-
are interferometric, there are some situations in which erogeneities in materials. Many discontinuities are unimpor-
straightforward holography can be helpful to the nonde- tant; others have a serious influence on the performance of
structive testing engineer. Often a measurement is not even the material/structure. Most nondestructive testing methods
required for nondestructive testing and visual testing is the help decide whether a discontinuity requires additional study
primary means of inspection. to determine its criticality. In contrast, shearography is basi-
cally a strain measuring tool, revealing discontinuities that
Shearography49 alter the strain distribution and hence weaken the structure.

Shearography is a nondestructive method for evaluation of Shearography reveals discontinuities by looking for dis-
quality and for discontinuity detection in manufactured continuity induced strain anomalies and thus provides more
goods. Shearography is tolerant to the hostile environment direct information about critical discontinuities. Further-
found in many manufacturing plants and, because of its speed more, shearography is a full field optical method that does
of application, can be developed into an online inspection not require scanning or contact. Thus, the inspection rate of
tool. A full field method, shearography can easily distinguish shearography is, inherently higher than other techniques.
between meaningful and nonmeaningful discontinuities. One major limitation of shearography is the need to impose
stresses (or additional stresses) on test objects.
Shearography is an interferometric method that permits
full field observation of surface strains in a test object. Point Triangulation Profilometry49
Because a discontinuity in an object usually induces strain
concentrations, shearography reveals discontinuities by The principle of using optical triangulation to determine
identifying the discontinuity induced strain concentrations range _was first documented by early Greeks. Euclidean
which are translated into anomalies in the fringe pattern. geometry is used to determine one side of a triangle if two
Although shearography measures surface strains, both sur- other angles and a side are given. In practical use, a range
face and internal discontinuities can be detected. This is finder consists of a long stick with two viewing sights serving
because the internal discontinuities unless very remote from as the base of the triangle. One sight is oriented until a dis-
the surface also affect the surface deformation. tant target is aligned within both sights. The distance to the
target can then be solved using trigonometry because the
Shearography has been used routinely in the aerospace two viewing angles and base leg are known. This same prin-
and rubber tire industry. The Federal Aviation Administra- ciple is used in an electrooptical manifestation of the range
tion has endorsed shearography as a standard method for finder, termed a point triangulation sensor. The sensor con-
inspecting aircraft tires. With the current trend of increased sists of an optical source and beam forming optics to project
use of polymers and composites and associated problems of light from a known direction, an imaging system, a spatially
adhesive joining, it becomes increasingly challenging to sensitive detector and signal processing electronics. This
detect discontinuities such as disbonds, delamination and chapter discusses the theory of optical triangulation, design
other material/structural deficiencies in manufactured principles and applications. The discussion is broken down
goods. As shearography seems to be ideally suited in these into light sources, detectors and electronics and processing.
applications, the following discussion will emphasize the Point triangulation represents an emerging nondestructive
inspection of composites. evaluation measurement technique. It produces an absolute
range signal, is noncontacting, can operate at high temporal
Shearography is a laser based optical method originally and spatial frequencies and can achieve resolutions in the
developed for strain measurement. Other methods of exper- submicron range.
imental stress analysis require laborious mounting of a large
number of strain gages, or application of a contoured pho- A diagram of a general point triangulation optical sensor
toelastic coating or application of a brittle coating upon the is shown in Fig. 32. As the distance between the sensor and
surface of the material structure. In most cases, these other the surface changes, the incident light, scattered by the sur-
methods either reinforce or alter the surface. Shearography face and imaged in parallax by a lens, creates a focused spot
is noncontacting, does not reinforce the structure being

THERMOGRAPHY AND OTHER SPECIAL METHODS I 501

of light that moves across a spatially sensitive photodetector. always assumed to be O degrees. A special case is when the
Processing electronics convert the detector output into an viewing angle a = y, the so-called retroreflection condition.
This arrangement occurs when there is very little light
absolute range signal. The optical source can be an incan- returned from a dark or diffuse scattering surface. This con-
dition places stringent demands on the viewing lens and
descent bulb, a light emitting diode (LED) or laser diode. electronics design. When a + y = 90 degrees the geometry
has the highest geometric gain, producing the maximum
The viewing optics can be as simple as a pinhole but are signal change for an incremental change in surface height. A
usually a single lens or multielement optical lens system. common arrangement is the case when a = y = 45 degrees.
This type of sensor uses retroreflection and also yields the
The detector can be a bicell, a lateral effect photodiode or a largest movement of the imaged spot on the detector for a
change in height at the target surface.
charge coupled device ( CCD) array.
Usually the incident angle y = 0 degrees. Otherwise Light can be produced from almost any source, although
most types of point range sensors use light emitting diodes,
changing the distance between the sensor and surface laser diodes or gas lasers. The advantages of a laser include
would cause the incident beam to traverse over different high brightness, single wavelength and phase coherence.
regions of the surface. Unless otherwise indicated, y is The single wavelength attribute is useful for blocking ambi-
ent light to a sensor using a wavelength specific filter. The
FIGURE 32. Elements of a point triangulation most popular gas laser is the helium-neon. For small sen-
sensor sors, however, gas lasers cannot be easily used due to their
large size. The source is operated continuously, or it is mod-
INCIDENT BEAM SPATIAL ulated. Most high accuracy systems employ modulated
DETECTOR sources to remove offsets due to the electronics or ambient
lighting conditions. The typical electronic technique used to
I recover information uses an alternating current (AC) cou-
pling technique to the front end amplifiers followed by
DEPTH some form of synchronous detection.
OF
As described above, laser based profilometry uses a
FOCUS reflected signal to map a surface. In the case of internal sur-
face cracking, both surface perturbations and momentary
2ZR lack of surface reflection can be used to locate and charac-
terize areas affected by corrosion fatigue cracking. As shown
in Fig. 33, the laser beam, in this case about 0.05 mm

FIGURE 33. Helical scan of crack

SURFACE LIGHT
CRACK BEAM

LEGEND

APERTURE DIAMETER
Do INPUT BEAM DIAMETER AT

LENS LI
do BEAM DIAMETER AT WAIST
t, VIEWING LENS
L; INCIDENT LENS
m MAGNIFICATION; q/p

p = OBJECT DISTANCE

Po INCIDENT POWER
q IMAGE DISTANCE
ZR RAYLEIGH RANGE

~ex VIEWING ANGLE
TILT ANGLE
y INCIDENT ANGLE
/\, WAVELENGTH
0 INCLUDED ANGLE

502 I NONDESTRUCTIVE TESTINGOVERVIEW

(0.002 in.) in diameter, proceeds on its helical scan of the advantages of this technology are speed, accuracy, quantita-
surface until it "drops" into a crack In addition to the radial tive results and its adaptability to small packages.
displacement information that the laser sensor generates,
the momentary loss of signal can be correlated to locate the Taken together, the laser based methods of nondestruc-
presence of internal cracks and define their shape. tive testing offer a noncontacting means of measuring strain
and deformation and hence material conditions that con-
This technology will continue to mature as a stand alone tribute to or cause failure in critical parts ranging from lam-
method for nondestructive testing and quality control. inated composites to pressurized containers. Wide
Laser based profilometry systems are useful in any applica- availability of coherent sensors, combined with the increas-
tion where detailed internal dimension information must ing power of microprocessors, promise an increased variety
be obtained by means of a noncontact sensor. The key of applications for these methods in the twenty-first century.

THERMOGRAPHY AND OTHER SPECIAL METHODS I 503

PART 5

OTHER SPECIAL METHODS

Alloy Identification Electromagnetic Special Methods

Normally, nondestructive testing is viewed in terms of Potential Drop Method
evaluating the integrity of materials, including locating and
sizing of discontinuities, determining the effects of corro- Cracks and other discontinuities in metals have been
sion, sampling for proper shape and thickness etc. One test studied for many years by measuring the way the disconti-
that is often overlooked is that of determining or verifying nuity alters the flow of a test current in the metal.54-56 Usu-
identity and keeping traceability of a material from original ally, the voltageproduced at the surface of the sample by the
melting to final application. metal's resistivity is measured and the voltage for the sus-
pected part is compared with that for a part known to be
In 1992, Committee E-7 on NondestructiveTestingof the good. Because voids, cracks and slag inclusions tend to be
American Society of Testing and Materials (ASTM) estab- electrically nonconductive, the sensitivityof the technique
lished section E 07.10.05under the jurisdiction of Subcom- can be very high.
mittee E 07.10 on Emerging NDT Methods. The section is
concerned with material identification methods and pub- Both direct current and alternating current techniques
lished its first document in 1992, E 1476,Standard Guidefor have been used. The very high frequency alternating cur-
Metals Identification, Grade Verificationand Sorting.53 rent techniques57 are more sensitive and easier to design
instrumentation for because the skin effect concentrates the
Metal identification and verification is a major concern current in a thin layer near the surface of the sample and
of producers, warehousers and users. A common practice is because it is easier to measure small alternating current
voltages. However, surface roughness and the effects of
the use of standard quality assurance measures and proce- magnetismcan make the results difficult to interpret. Direct
dures throughout the various stages of manufacturing and current measurements require large currents and are per-
processing - warehousing, receiving, fabrication and final turbed by thermoelectric and contact potential effects.54
They are much more difficult to make than alternating cur-
installation of the product. These practices typically involve rent measurements. However, it is easier to interpret the
standard chemical analysisand physical tests to meet prod- observed potentials in terms of crack depth and analytic
uct acceptance standards. Sample pieces from a production results have been obtained for many common crack geome-
run are usually destroyed or rendered unusable through tries. Thus, laboratory measurements of fatigue or stress
mechanical and chemical testing and the results are used to corrosion cracks have tended to use direct current tech-
assessthe entire lot using statisticaltechniques. These tech- niques, whereas field measurements of boilers, welds etc.
niques are effective; however, mixed grades, impure chem- have tended to use the alternating current techniques.
istry and nonstandard physical properties remain primary
causes for claims in the metals industry. Additionally,in- For both the alternating current and direct current mea-
house material mixescan be commonplace. surements, two techniques have been used to examine the
sample. Some measurements have been made with current
Nondestructive means are availableto supplement con- leads attached to parts of the test piece remote from the
ventional verification techniques and to monitor chemical area being tested so that the test current flowsthrough large
portions of the sample. Measurements have also been made
and physicalproperties at selected production stages to help in which the voltage and current contacts have been built
maintain the identities of metals and their consistency in into a single four-point probe. Depending on the details of
mechanical properties. These methods have potential for sample and crack geometry, either the separated current
monitoring properties such as hardness and case depth and lead (which we will refer to as two-point) or the four-point
for verifying the effectivenessof heat treatment, cold work- probe arrangement may be easier to use and interpret. The
ing and the presence or absence of coatings. Nondestructive four-point arrangement has the advantage that, because the
methods are often used in the field for solving problems current is concentrated in the region near the voltage
involvingmixed materials. probes, much larger voltages are developed than with the

The nondestructive methods in Table 3 provide both
direct and indirect responses to the sample being evaluated.
Direct methods are classified as quantitative and indirect
methods classifiedas qualitative.

504 I NONDESTRUCTIVETESTINGOVERVIEW

TABLE 3. Capabilities of alloy identification methods

X-Ray Optical ThermoelectricChemical

Metal Fluorescence Emission ElectromagneticPotential {Seebeck) Spot TriboelectricSpark
Properties
SpectrometrySpectrometry Method Drop Method Testing Method Testing

Chemical composition E, N E,A G, 8 F, 8 G, 8 G,N F, gc G, 8
Identification of E,A E,A G, Bd F G, ge
E FG
specific alloy E, A
Response to E, A F-G, B F, 8 E, 8 E NN
N
surface chemistry N N F-G, gr.g N9 G, gg.h N NN
Physical properties N N9 G, gg.h
Surface hardness N N F, gf.g N NN
Tensile Yield
Monitoring process variables1 N F-G, gr N G, gh N NN
F-G, gr.g F, 89 F-G, gg.h
N N NN

A DIRECTREADING, QUANTITATIVE.
B INDIRECT READING, QUALITATIVE.
E EXCELLENT.

F FAIR.
G GOOD.
N NOT APPLICABLE.
p POOR.

a. NOT SUITABLEFOR LOWATOMIC NUMBER ALLOYSSUCH AS CARBON, SILICONAND PHOSPHORUS..

b. SINGLE ELEMENT PER SPOTTEST.
c. NOT SUITABLEFOR STEELS.
d. RESPONDSWELL TO MANGANESE REVERSIONSIN STEELS.

e. RESPONDSWELL TO ELECTRICALLYOR THERMALLYACTIVE ELEMENTS,OR BOTH.
f. REQUIRESCONTROLLED PROCESSING,COMPOSITION, ETC.

g. RESPONSESARE ALL RELATIVEAND BASEDON KNOWN SURFACERESPONSESTO MEASUREDVARIABLES.
h. THERMOELECTRICPROPERTIESINFLUENCED BY METALLURGICALEXCHANGE IN FERROUSMATERIALS.

I. PROCESSVARIABLESINCLUDE COLD WORK, WARM WORK, ANNEALING, CASE/THROUGH HARDENING, TEMPERING.

two-point probe, where the current is spread over the entire quantitative nature of magnetic resonance imaging to pro-
sample cross section or surface area. vide contrast within the image, it is possible.to spatially map
concentration variations within the sample, leading to infor-
Alternating current measurements inherently require mation on cracks, voids, inclusions, heterogeneities etc.
that the crack be open and that the test contacts be applied Additionally, by using other mechanisms to provide image
on the cracked surface. Direct current measurements can, contrast, it is possible to map a variety of other parameters
in principle, be used to detect cracks or voids that are not such as cross link density and stress in polymers or diffusion
connected to the surface being tested but the interpretation and flow in oil cores. Magnetic resonance imaging has
of the measurements can be very complex. already proven to have an extremely broad potential for
materials characterization. Spurred by continuing technique
Magnetic Resonance lmaging58 and hardware improvements, the development of materials
characterization applications is accelerating. In a time when
Magnetic resonance imaging (MRI), or nuclear mag- advanced materials and their methods of production are
netic resonance imaging (NMRI), is a noninvasive, noncon- increasing in complexity, applications of magnetic resonance
tact, nondestructive evaluation technology. The technology imaging will fill a unique and important role for the nonde-
is based on measurement of the net magnetization vector structive testing community.
caused by the alignment and precession of "sensitive" nuclei
in the presence of a strong magnetic field, as discussed Barkhausen Noise Analysis59
below. By using magnetic resonance imaging it is possible to
nondestructively obtain multidimensional (one-, two- and The magnetization curve of ferromagnetic materials has
three-dimensional) images of the internal structure of the a fine structure consisting of discrete jumps in the magnetic
specimen. Images produced through magnetic resonance flux density as the magnetic field intensity changes. Abrupt
imaging are very sensitive to the chemical, physical and irreversible movements of the magnetic domain walls as
morphological properties of the specimen. This sensitivity they break away from pinning sites produce random jumps
provides a versatility not commonly found in nondestructive in electric impulse. The random electric impulses are called
testing techniques. For example, by taking advantage of the

THERMOGRAPHY AND OTHER SPECIAL METHODS I SOS

Barkhausen noise, after Heinrich Barkhausen, who first Acoustic Methods

reported the effect in 1919. The Barkhausen power spec- Tap Testing62

trum starts at the magnetization frequency and extends to Tap testing is a common and inexpensive form of inspec-
about 250 kHz. The intensity of the Barkhausen noise tion. The inspector taps the surface of the test structure and
evaluates the sound that is generated. The inspector either
depends on the structural composition of the material and listens directly to the sound or uses a specially designed
receiver to analyze the sound and compare the response
its stress state. The Barkhausen noise can be detected with with one from a discontinuity-free part.
an induction coil to measure the stress in a material.
Probably the simplest inspection method to ensure a
The main commercial application of the magnetic Bark- bond exists between the honeycomb and facing sheet is coin
tapping. It is also useful for detecting near surface delamina-
hausen effect (MBE) method, also known as Barkhausen tions in composite laminates. A disbond is readily apparent
noise analysis (BNA), is the detection and measurement of by a change in tone or frequency of sound when tapped with
a coin or rod as compared to the sound for a bonded area.
residual stresses in steel. The presence and distribution of Disbonds generate a damped, low pitch sound characteristic
elastic stresses will influence the way domain walls behave, that can be easily recognized for large discontinuities. Tap
producing a magnetoelastic interaction. In materials having a testing is limited to detecting disbonds larger in diameter
positive magnetostriction coefficient (iron, cobalt and most than 13 mm (0.5 in.). Discontinuities can be detected under
steels), compressive stresses in the direction of the applied a 1 mm (0.040 in.) thick skin of fiberglass laminate.
field decrease the intensity of Barkhausen noise while tensile
stresses increase it. If the magnetic field is applied normal to Coins such as silver dollars or half-dollars are widely
the stress the reverse is true. Materials with negative magne- used for tap testing. Although a coin may be sufficient to
tostriction exhibit the opposite behavior. Thus, in principle, serve as a tapping tool, standard tap hammers have been
Barkhausen noise analysis can be used to measure the resid- developed to help inspectors perform the test. Automated
ual stresses and to determine their principal directions. tapping devices that provide uniform impacts have also
been introduced.
The principal application of Barkhausen noise is the
evaluation of residual stresses when microstructural vari- Tap testing with a coin or small aluminum rod or ham-
ables are relatively constant. It has generally been reported mer is useful for locating voids or disbonds of about 38 mm
that the useful range of measurement extends to about 50 (1.5 in.) or larger in diameter in metal-to-metal, composite-
percent of the yield strength with a resolution of 30 MPa to-metal or thin facing-sheet honeycomb assemblies. Tap
(4,350 lbr · in.-2). Examples of components where Bark- testing is limited to detection of disbonds or voids between
hausen noise analysis has been applied to determine resid- upper facing sheet and adhesive. It will not detect voids or
ual stresses include turbine diffuser cases, compressor disbonds at second, third or deeper adherends. The test
blades, compressor disks, weld seams, pipes, camshafts, method is subjective and may yield variation in test results.
gear teeth and ball bearings.
There are several drawbacks to manual tap testing. On
Grinding bums can produce both residual tensile thin facing sheets, coin tapping produces undesirable small
stresses and hardness changes that are detectable as varia- indentations. For large bonded structures, such as the Sat-
tions in Barkhausen noise. Barkhausen noise analysis has urn booster, the manual tap test is time consuming. Also,
also been used to measure variations in heat treatment. without an electronic instrument, no record is produced and
Microstructural measurements include grain size in low car- • the reliability of the discontinuity identification may be
hon ferritic and ferritic-pearlitic steels, aging in carbon questionable because it depends on the operator's subjec-
steels and pearlite morphology in steel wires. In addition, it tive discrimination of sounds.
has been shown that the Barkhausen noise is sensitive to
plastic stress and its potential for fatigue damage detection Vibration Analysis
has been investigated.
Vibration analysis is a relatively new and frequently mis-
Microwave Testing understood method of nondestructive testing. There are
two very different branches of engineering concerned with
Often considered a special method, microwave nonde- vibration; one is destructive and one is nondestructive.
structive testing is treated in the electromagnetic volume of Destructive vibration testing is one of a number of environ-
the Nondestructive Testing Handbook, second edition.f" mental stress screening methods, sometimes referred to col-
Radio waves and radar are microwaves.The term radar in the lectively as shake and bake. The test piece is placed in a
context of nondestructive testing, however, implies far field
applications such as underground structure location rather
than near field applications such as discontinuity detection.
Microwaves are especiallywell suited for material characteri-
zation, thickness measurement, and discontinuity detection
in dielectric materials such as ceramics and plastics.61

506 I NONDESTRUCTIVE TESTING OVERVIEW

special device to see if it can survive abrupt and repeated populations of subcritical discontinuities. These are factors
changes in direction at high speeds. Laboratories that per- that influence both singly and collectively acoustoultrasonic
form this sort of vibration testing also may subject test measurements and so correlate with mechanical property
pieces to impact and compression tests and extreme tem- variations. 70
peratures.
The combination of an acoustic emission sensor with an
Nondestructive vibration analysis62-67 is a very different active pulser forms the basis for an acoustoultrasonic sys-
technology used to monitor the behavior of rotating machin- tem. Calibration of acoustic emission sensors is convention-
ery, such as turbines, whose vibrations betray problems ally done with lead pencil or glass capillary breaks, electric
before failure. Every rotating machine exhibits a character- sparking and piezotransducers. Conceptually, this comprises
istic vibration signature uniquely its own. The signal is the the first step toward an acoustoultrasonic configuration.
sum total of the design, manufacture, application and wear
of each of its parts. If the maintenance mechanic or engi- The general objective of testing is to rate the relative
neer responsible for a piece of machinery takes the time to efficiency of stress wave propagation in a material. The basic
become acquainted with the nature of the vibration of his attribute measured is stress wave energy transfer and, there-
machinery, it will not be long before he can effect substan- fore, better transmission and distribution of dynamic strain
tial cost savings for his company. energy.

Nondestructive vibration analysis is one of a number of The working hypothesis is that more efficient strain
technologies useful in predictive maintenance and condition energy transfer and strain redistribution during loading cor-
monitoring. It is often part of a program that includes ther- responds to increased strength and resistance in composites.
mal monitoring and oil analysis to confirm that generators, This hypothesis is based on the stress wave interaction con-
for example, are functioning as designed. cept, which holds that spontaneous stress waves at the onset
of fracture will promote rapid macrocracking unless their
Acoustoultrasonic Testing68 energy is dissipated by other mechanisms - for example,
plastic deformation of microcrack deflections." In lieu of
Acoustoultrasonic(AU) testing belongs to a class of tech- these mechanisms, prompt and efficient dissipation of stress
niques that includes (coin) tap testing, dynamic resonance wave energy away from crack nucleation sites is needed to
and structural damping measurements. These are in addi- ensure that the energy is not focused or localized in a way
tion to more directly related techniques like acoustic emis- that causes catastrophic fracture. One very important impli-
sion, pulse echo and guided Lamb wave methods. Sonic cation of the hypothesis is that the wave attenuation proper-
(coin) tap testing can be considered a primitive version of ties of a material are pivotal. For composites, low attenuation
acoustoultrasonic testing, except that acoustoultrasonic tap­ will usually indicate high strength and impact resistance.
ping is usually done with a piezotransducer and listening is
done with a second piezotransducer and appropriate elec- Resistance Strain Gaging
tronic instrumentation.
The experimental verification and measurement of
The acoustoultrasonic method was devised to assess dif- stresses and strains is a vital part of modern technology in all
fuse discontinuity populations of the mechanical properties fields of engineering. 12-75
of composites and composite-like materials. The acousto-
ultrasonic (also known as the stress wave factor technique) Strain measurement is of prime importance in the experi-
has been used to evaluate fiber reinforced composites, mental analysis of materials, structures and machines. The
adhesive bonds, lumber, paper and wood products, cable very definition of strain as a unit or percentage change in
and rope and also human bone. The acoustoultrasonic length creates the impression that its measurement is no
method has been demonstrated to be sensitive to interlami- more complex than measurement of changes in length.
nar and adhesive bond strength variations and has been Indeed, for many years, calipers and rulers were standard
shown to be useful in assessing microporosity and microc- strain measuring instruments. These were succeeded by
racking produced by fatigue cycling. 69 extensometers: mechanical lever systems, gear trains and opti-
cal, acoustical and pneumatic systems. Typical examples are
Acoustoultrasonic testing combines aspects of acoustic the Berry, Huggenberger, Whittemore and Tuckerman units.
emission testing and ultrasonic materials characterization The most accurate strain measuring devices, frequently used
methodology. An acoustic emission system in its elementary as standards, are optical in nature and include both interfero-
form constitutes half of an acoustoultrasonic testing system: metric means and optical lever arm techniques.
passive listening but no active interrogation. Unlike acoustic
emission testing, acoustoultrasonic testing is not concerned Subsequent developments occurred in the electrical
with source location and characterization. Instead, it deals fields, where capacitance, inductance, reluctance and resis-
primarilywith the assessment of the integrated effects of dif- tance were made the bases of numerous extensometer
fuse discontinuity states, thermomechanical degradation and

THERMOGRAPHY AND OTHER SPECIAL METHODS I 507

instruments. A basic and popular strain measuring device is cranes, earth moving equipment and automobiles and wher-
ever a stress problem has required their use.
the electric resistance strain gage.
Several other methods of stress analysis have been Load cells operating on the strain gage principle are used
in process and control industries for chemical batching,
proven extremely helpful in conjunction with the resistance automatic mixing, continuous conveyor belt weighing and
gage technique of measuring strains at individual points totalizing. They have been applied to servo and control sys-
over the surface of a structure. The brittle coating method tems of all types and have proven to be reliable components
lets the experimenter obtain an excellent qualitative presen- of such systems. Two major fields of use of resistance strain
tation of the stress distribution, both in magnitude and gages are 'differentiated mainly by the widely different pre-
direction, over the entire surface of two- or three-dimen- cisions achievable in each case: (1) stress analysis by strain
sional specimens. The entire test part is sprayed with a brit- measurement and (2) load measuring transducers.
tle coating designed to crack at known values of mechanical
strain in the material beneath the coating. The cracks indi- FIGURE 34. Grid configurationsfor foil
cate both magnitude and direction of strain. This permits resistance straingages: (a) axial; (bJ shear;
intelligent selection of the locations and directions of indi- (cJ rosette
vidual strain gages for more accurate analysis.76 Photoelastic
stress analysis uses coatings cemented or sprayed onto the (a)
surface of two- or three-dimensional parts to determine
strain distributions. 77 Other stress analysis tools fall into the
large category of analogies such as electrical, electrical field,
fluid flow, membrane and arithmetical methods.

The idea of bonding a length of fine wire directly to the
part to be tested, with the cement acting as an electrical insu-
lator, made possible the wide applications of resistance strain
gages. The bonded gage is used to measure acceleration,
force or pressure as well as strain. Strain gages and strain
gage based transducers have become indispensable tools in
stress analysis, instrumentation, load and process control.

Resistance Strain Gage Applications and Limitations (b)
(c)
Resistance strain gages have been used to measure
strains as low as fractions of a millionth of a meter per
meter, as well as strains up to 20 percent on rubber and plas-
tics. They have withstood environments from liquid helium
to 1,100 °C (2,000 °F) combustion exhaust gases. They have
operated submerged in sea water on ships' hulls for periods
in excess of a year and a half and have been used in hydro-
static environments at pressures above 345 MPa (5 x
104 lbr·in.-2). Resistance strain gages can faithfully follow
strains from zero frequency (direct current) to over 50 kHz
and upper frequency limits have not yet been determined.
They have been mounted to withstand 6,000 km·s-2
(600,000 G) loading on rotating machinery, and signals have
been obtained from them at rotational speeds of close to
100,000 revolutions per minute. When used to measure
quantities other than strains, such as load, pressure and
torque, they have exhibited accuracies better than 0.1 per-
cent for periods of 20 years.

Resistance strain gages have been used on almost all
metals, on concrete, cement, brick, bones (live and dead),
wood, rubber and plastics. They have even been woven into
fabrics. They have been used in and on operating gas tur-
bines, reciprocating engines, airplanes, submarine hulls,

508 I NONDESTRUCTIVE TESTING OVERVIEW

For stress analysis, the strain gage is used either singly, transducers exhibit accuracies from 1 percent to better
in pairs or in rosette configurations. The gage is mounted at than 0.05 percent, depending on the application. Linearity
the spot where strain is to be measured and the gage's resis- and hysteresis figures are usually well within these values
tance change is correlated with the stress produced in the and can be obtained from most transducer manufacturers.
structure. It is not usual, and actually is extremely difficult,
to calibrate an individual gage and then to use the same Gage Materials and Construction
gage to measure strains. Instead, strain gages are calibrated
by the manufacturer on a statistical basis. The gagefactors For many years, strain gages were constructed with wire
are usually given to plus or minus 1 percent tolerance on sensing grids but foil gages have replaced wire gages in most
each package of gages - strain gages are not expected to applications. Both wire and special foil gages may be used at
exhibit accuracies better than one or two percent of error very high temperatures. .
for stress analysis. It is optimistic to expect better than two
to three percent overall accuracies on stress analysis tests Wire from 25 to 150 mm (1 to 6 in.) long is required to
with strain gages. produce a wire gage. Wires with diameters from 13 to
25 um (5 x 10-4 to 1.0 x 10-3 in.) are used most commonly.
Strain gages are frequently attached to specimens Foils from 3 to 25 µm (1.2 x 10-4to 1.0 x 10-3 in.) thick are
designed to act as load transducers. For example, a column used for etched foil gages. Lead wires are attached to the
may be used to transduce a force applied to the column fine, strain sensitive filament by soldering or welding. The
into a strain in the column material. A strain gage attached resulting miniature postage stamp size of a sandwich is a
to the column then acts as a transducer, changing this strain gage ready to be cemented to any location where
strain into an equivalent resistance variation in the gage. strain is to be measured (see Fig. 34). Occasionally, and
The overall instrument is then a load measuring instru- especially for high temperature applications, either foil or
ment and no longer a strain measuring device. These com- wire gages are made without backing or with a backing that
posite structures are frequently called strain gage strips off. Foil gages are produced by chemical etching of
transducers. They are widely used in the measurement of the foil rather than by the winding procedure used for wire.
force, torque, pressure, acceleration, surface finish, speed,
displacement, straightness, dimensions, blood pressure Other special grid configurations are embodied in the
and many other variables. Because such transducers can T gage and rosette gage configurations, which may contain
be calibrated individually before use, a magnitude of accu- two, three or four separate strain sensitive grids so that
racy not attainable with individual commercial strain gages strains may be measured in different directions at essen-
can be achieved. Commercial instruments using such tially a single point. This procedure permits the determina-
tion of the complete state of stress in two dimensions.

THERMOGRAPHY AND OTHER SPECIAL METHODS I 509

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menti, eds. New York, NY: Plenum Press (1992):
p 447-451.

51 0 I NONDESTRUCTIVE TESTING OVERVIEW

18. Cowell, S.D. and D.D. Burleigh. "Numerical Model- 26. Sundberg, J. "Use of Thermography to Register Air
ing of Thermographic Nondestructive Testing for Temperatures in Cross Sections of Rooms and to
Graphite Epoxy Laminates." Thermosense XII: An Visualize the Air Flow from Air Supply Diffusers."
International Conference on Thermal Sensing and Thermosense XV· An International Conference on
Imaging Diagnostic Applications. Vol. 1,313. S.A. Thermal Sensing and Imaging Diagnostic Applica­
Semanovich, ed. Bellingham, WA: International Soci- tions. Vol. 1,933. L.R. Allen, ed. Bellingham, WA:
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(1993): p 61-66.
19. Cielo, P., X. Maldague, A.A. Deom and R. Lewak.
"Thermographic Nondestructive Evaluation of 27. Hurley, TL. "Infrared Qualitative and Quantitative
Industrial Materials and Structures." Materials Eval­ Inspections for Electric Utilities." Thermosense XII:
uation. Vol. 45, No. 4. Columbus, OH: American An International Conference on Thermal Sensing and
Society for Nondestructive Testing (1987): p 452-460. Imaging Diagnostic Applications. Vol. 1,313. S.A.
Semanovich, ed. Bellingham, WA: International Soci-
20. Pfund, B. "Repairing Major Damage." Professional ety for Optical Engineering (Society of Photo-Optical
Boatbuilder. No. 25. Brooklin, ME: Wooden Boat Instrumentation Engineers) (1990): p 6-24.
Publishers (October-November 1993): p 18-27.
28. D 4788, Test Method for Detecting Delaminations in
21. McMullen, P.C. "Masonry Building Envelope Analy- Bridge Decks Using Infrared Thermography. Philadel-
sis." Thermosense XV: An International Conference phia, PA: American Society for Testing and Materials.
on Thermal Sensing and Imaging Diagnostic Applica­
tions. Vol. 1,933. L.R. Allen, ed. Bellingham, WA: 29. Weil, G.J. "Review of the ASTM Standard on IR Test-
International Society for Optical Engineering (Soci- ing of Concrete Bridge Decks." Thermosense XI: An
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(1993): p 43-54. ing for Diagnostics and Control. Vol. 1,094. G.B.
Mcintosh, ed. Bellingham, WA: International Society
22. Lyberg, M.D., M. Mattsson and J. Sundberg. "Detec- for Optical Engineering (Society of Photo-Optical
tion of Moisture Damage in Buildings Using Ther- Instrumentation Engineers) (1989): p 32-37.
mography." Thermosense XII: An International
Conference on Thermal Sensing and Imaging Diag­ 30. Weil, G.J. and R.J. Graf. "Nondestructive Remote
nostic Applications. Vol. 1,313. S.A. Semanovich, ed. Sensing of Hazardous Waste Sites." Thermosense XV
Bellingham, WA: International Society for Optical An International Conference on Thermal Sensing and
Engineering (Society of Photo-Optical Instrumenta- Imaging Diagnostic Applications. Vol. 1,933. L.R.
tion Engineers) (1990): p 100-105. Allen, ed. Bellingham, WA: International Society for
Optical Engineering ( Society of Photo-Optical In-
23. Bison, P.G., C. Bressan, E. Grinzato, S. Marinetti and strumentation Engineers) (1993): p 252-261.
V Vavilov. "Active Thermal Testing of Moisture in
Bricks." Thermosense XV: An International Confer­ 31. Wurzbach, R.N. and J.E. Hart. "Increasing Main-
ence on Thermal Sensing and Imaging Diagnostic tainability of Emergency Diesel Generators with
Applications. Vol. 1,933. Lee R. Allen, ed. Belling- Thermographic Inspections." Thermosense XV· An
ham, WA: International Society for Optical Engineer- International Conference on Thermal Sensing and
ing (Society of Photo-Optical Instrumentation Imaging Diagnostic Applications. Vol. 1,933. L.R.
Engineers) (1993): p 207-214. Allen, ed. Bellingham, WA: International Society for
Optical Engineering (Society of Photo-Optical In-
24. Knehans, A. and J. Ledford. "The Impact of Aerial strumentation Engineers) (1993): p 14-25.
Infrared Roof Moisture Scans on the U.S. Army's
ROOFER Program." Thermosense XV An Interna­ 32. P:rystay, M. et al. "Application of Thermographic
tional Conference on Thermal Sensing and Imaging Temperature Measurements in Injection Molding
Diagnostic Applications. Vol. 1,933. L.R. Allen, ed. and Blow Molding of Plastics." Thermosense XII: An
Bellingham, WA: International Society for Optical International Conference on Thermal Sensing and
Engineering (Society of Photo-Optical Instrumenta- Imaging Diagnostic Applications. Bellingham, WA:
tion Engineers) (1993): p 67-73. International Society for Optical Engineering (Soci-
ety of Photo-Optical Instrumentation Engineers)
25. Feit, E. "Infrared Thermography Saves Shelburne (1996): p 5-14.
Middle School Time and Money." Materials Evalua­
tion. Vol. 45, No. 4. Columbus, OH: American Soci-
ety for Nondestructive Testing (April 1987):
p 400-401.

THERMOGRAPHY AND OTHER SPECIAL METHODS I 511

33. Prystay, M. et al. "Optimization of Cooling Channel 40 Spicer, J.WM., WD. Kerns, L.C. Aamodt, R. Osian-
Design and Spray Patterns in Aluminum Die Casting der and J.C. Murphy. "Time-Resolved Infrared
Using Infrared Thermography." Therrnosense XII: An Radiometry (TRIR) Using a Focalplane Array for
International Conference on Thermal Sensing and Characterization of Hidden Corrosion." Therrnosense
Imaging Diagnostic Applications. Bellingham, WA: XV· An International Conference on Thermal Sensing
International Society for Optical Engineering (Soci- and Imaging Diagnostic Applications. Vol. 1933. L.R.
ety of Photo-Optical Instrumentation Engineers) Allen, ed. Bellingham, WA: International Society for
Optical Engineering (Society of Photo-Optical
(1996): p 15-24. Instrumentation Engineers) (1993): p 148-159.

34. Green, T. and J. Snell. "Thermographic Inspection of 41. Lukens, W.E. and R.A. Morris. "Infrared Temperature
Hydraulic Systems."ProceedingsofTherrrwsenseXVIII: Sensing of Cooling Rates for Arc Welding Control."
An International Conference on Thermal Sensing and Welding Journal. Miami, FL: American Welding Soci-
Imaging Diagnostic Applications. Bellingham, WA: ety (January 1982):p 27-33.
International Society for Optical Engineering (Society
of Photo-Optical Instrumentation Engineers) (1996): 42. Wang, Y. and B.A. Chin. "On-Line Sensing of Weld
p 25-30. Penetration Using Infrared Thermography." Optical
Techniques for Industrial Inspection. Vol. 665.
35. Howell, P.A. and WP. Winfree. "Numerical Solutions Bellingham, WA: International Society for Optical
for Heat Flow in Adhesive Lap Joints." Review of Engineering (Society of Photo-Optical Instrumenta-
Progress in Quantitative Nondestructive Evaluation. tion Engineers) (1986): p 314-320.
Vol. llA. D.O. Thompson and D.E. Chimenti, eds.
New York, NY: Plenum Press (1992): p 457-464. 43. Banerjee, P. and B.A. Chin. "Infrared Sensor-Based
On-Line Weld Penetration Control." Thermosense
36. Winfree, WP., B.S. Crews and P.A. Howell. "Compar- XV· An International Conference on Thermal Sensing
ison of Heating Protocols for Detection of Disbonds and Imaging Diagnostic Applications. Vol. 1,933.
in Lap Joints." Review of Progress in Quantitative L.R. Allen, ed. Bellingham, WA: International Society
Nondestructive Evaluation. Vol. llA. D.O. Thomp- for Optical Engineering (Society of Photo-Optical
son and D.E. Chimenti, eds., New York, NY: Plenum Instrumentation Engineers) (1993): p 83-92.
Press (1992): p 471-478.
44. Nagarajan, S. and B.A. Chin. "Infrared Image Analy-
37. Winfree, WP. "Thermal QNDE Detection of Air- sis for On-Line Monitoring of Arc Misalignment in
frame Disbonds." Proceedings of the 1991 Interna­ Gas Tungsten Arc Welding Processes." Materials
tional Conference on Aging Aircraft and Structural Evaluation. Vol. 48, No. 12. Columbus, OH: Ameri-
Airworthiness. NASA Publication 3160. Washington, can Society for Nondestructive Testing (1990):
DC: National Aeronautics and Space Administration p 1,469-1,472.
(1991): p 249-260.
45. Jones, T.S. and R. Cervera. "Infrared Inspection of
38. Theilen, D.A., R.J. Christofersen, B.G. Dods, D.C. Spot Welded Stainless Steel Structures." ASNT 1990
Emahiser and B.H. Robles. "Infrared Thermographic Fall Conference Paper Summaries. Columbus, OH:
Inspection of Superplastically Formed/Diffusion American Society for Nondestructive Testing (Octo-
Bonded Titanium Structures." Therrnosense XV· An ber 1990): p 100-102.
International Conference on Thermal Sensing and
Imaging Diagnostic Applications. Vol. 1,933. L.R. 46. Linkous, A. and B. McKnight. "Using Thermography
Allen, ed. Bellingham, WA: International Society for to Detect and Measure Wall Thinning." Therrnosense
Optical Engineering (Society of Photo-Optical XV· An International Conference on Thermal Sensing
Instrumentation Engineers) (1993): p 174-177. and Imaging Diagnostic Applications. Vol. 1,933.
L.R. Allen, ed. Bellingham, WA: International Society
39. Maldague, X., J.-C. Krapez and P. Cielo. "Subsurface for Optical Engineering ( Society of Photo-Optical
Instrumentation Engineers) (1993): p 26-30.
Flaw Detection in Reflective Materials by Thermal
Transfer Imaging." Optical Engineering. Vol. 30, No. 1. 47. "Grid and Moire Nondestructive Testing." Nonde­
Bellingham, WA: International Society for Optical structive Testing Handbook, second edition: Vol. 9,
Engineering (Society of Photo-Optical Instrumenta- Special Nondestructive Testing Methods. Columbus,
tion Engineers) (January 1991):p ll 7-125. OH: American Society for Nondestructive Testing
(1995): p 19-102.

512 I NONDESTRUCTIVE TESTING OVERVIEW

48. Post, D., B.T. Han and P. Ifju. High Sensitivity Moire: 59. "Barkhausen Noise." Nondestructive Testing Hand­
Experimental Analysis for Mechanics and Materials. book, second edition: Vol. 9, Special Nondestructive
New York, NY: Springer-Verlag (1994). Testing Methods. Columbus, OH: American Society
for Nondestructive Testing (1995): p 421-422.
49. "Laser Based Nondestructive Testing Methods."
Nondestructive Testing Handbook, second edition: 60. Nondestructive Testing Handbook, second edition:
Vol. 9, Special Nondestructive Testing Methods. Vol. 4, Electromagnetic Testing: Eddy Current, Flux
Columbus, OH: American Society for Nondestructive Leakage and Microwave Testing. Columbus, OH:
Testing (1995): p 103-157. American Society for Nondestructive Testing (1986):
p 461-560.
50. Collier, R.J., C.B. Burckhardt and L.H. Lin. Optical
Holography. New York, NY: Academic Press (1971). 61. Zoughi, R. "Microwave and Millimeter Wave Nonde-
structive Testing: A Succinct Introduction." Materials
51. Caulfield, H.J. Handbook of Optical Holography. Evaluation. Vol. 53, No. 4. Columbus, OH: American
New York, NY: Academic Press (1979). Society for Nondestructive Testing (April 1985):
p 461-462.
52. Powell, R.L. and K.A. Stetson. "Interferometric
Vibration Analysis by Wavefront Reconstruction." 62. "Tap Testing." Nondestructive Testing Handbook,
Journal of the Optical Society of America. Vol. 55, second edition: Vol. 9, Special Nondestructive Testing
No. 12. New York, NY: Optical Society of America Methods. Columbus, OH: American Society for Non-
(1965): p 1,593-1,598. destructive Testing (1995): p 220-227.

53. E 1476, Standard Guide for Metals Identification, 63. Goldman, S. Vibration Spectrum Analysis: A Practi­
Grade Verification and Sorting. Philadelphia, PA: cal Approach. New York, NY: Industrial Press Incor-
American Society for Testing and Materials [revised porated (1991).
annually].
64. Lyon, R.H. Machinery Noise and Diagnostics.
54. Halliday, M.J. and C.J. Beevers. "The D.C. Electrical Boston, MA; London, United Kingdom: Butter-
Potential Method for Crack Length Measurement." worths (1987).
The Measurement of Crack Length and Shape during
Fracture and Fatigue. C.J. Beevers, ed. Warley, 65. Smith, J.D. Vibration Measurement and Analysis.
United Kingdom: Engineering Materials Advisory Boston, MA; London, United Kingdom: Butter-
Services, Limited (1980): p 85-112. worths ( 1989).

55. Knott, J.F. "The Use of Analogue and Mapping Tech- 66. White, G. Introduction to Machine Vibration. Bain-
niques with Particular Reference to Detection of bridge Island, WA: DLI Corporation (1993).
Short Cracks." The Measurement of Crack Length
and Shape during Fracture and Fatigue. C.J. Beev- 67. Crawford, A.R. and S. Crawford. The Simplified
ers, ed. Warley, United Kingdom: Engineering Handbook of Vibration Analysis: Volume 1, Introduc­
Materials Advisory Services, Limited (1980): tion to Vibration Analysis Fundamentals. Knoxville,
p 113-135. TN: Computational Systems, Incorporated (1992).

56. Dover, W.D., F.D.W. Charlesworth, K.A. Taylor, 68. "Acoustoultrasonic Nondestructive Testing." Nonde­
R. Collins and D.H. Michael. "The Use of A-C Field structive Testing Handbook, second edition: Vol. 9,
Measurements to Determine the Shape and Size of a Special Nondestructive Testing Methods. Columbus,
Crack in a Metal." Eddy­Current Characterization of OH: American Society for Nondestructive Testing
Materials and Structures. G. Birnbaum and G. Free, (1995): p 247-277.
eds. Philadelphia, PA: American Society for Testing
and Materials (1981): p 401-427. 69. Duke, J.C., Jr., ed. Acousto­Ultrasonics ­ Theory
and Applications. New York, NY: Plenum Press ( 1988).
57. Webborn, T.J.C. "Crack Depth Measurements Using
AC Potential Drop." Materials Evaluation. Vol. 41, 70. Vary, A., ed. Second International Conference on
No. 2. Columbus, OH: American Society for Nonde- Acousto­ Ultrasonics: Topical Conference Proceedings
structive Testing (February 1983): p 156-159. Book. Columbus, OH: American Society for Nonde-
structive Testing (1993).
58. "Application of Magnetic Resonance Imaging to the
Nondestructive Testing of Materials." Nondestructive 71. Vary, A. Solid Mechanics Research for Quantitative
Testing Handbook, second edition: Vol. 9, Special Nondestructive Evaluation. J.D. Achenbach, ed.
Nondestructive Testing Methods. Columbus, OH: Dordrecht. Netherlands: Martinus Nijhoff Publishers
American Society for Nondestructive Testing (1995): (1987): p 134-152.
p 398-420.
72. "Special Nondestructive Testing Methods for Strain
Measurement." Nondestructive Testing Handbook,
second edition: Vol. 9, Special Nondestructive Testing
Methods. Columbus, OH: American Society for Non-
destructive Testing (1995): p 431-498.

THERMOGRAPHYAND OTHER SPECIALMETHODS I 513

73. Murray, WM. and P.K. Stein. Strain Gage Tech­ 76. "Brittle Coating Tests." Nondestructive Testing Hand­
niques. Bethel, CT: Society for Experimental Me- book, first edition. R.C. McMaster, ed. Sec. 52.
chanics ( 1959). Columbus, OH: American Society for Nondestructive
Testing (1959).
74. Hannah, R.L. and S.E. Reed. Strain Gage Users'
77. "Photoelastic-Coating Tests." Nondestructive Testing
Handbook. Bethel, CT: Society for Experimental Handbook, first edition. R.C. McMaster, ed. Sec. 53.
Mechanics [New York, NY: Elsevier Applied Science] Columbus, OH: American Society for Nondestructive
(1992). Testing (1959)
75. Reese, R.T. .and W. Kawahara. Handbook on Struc­
tural Testing. Lilburn, GA: Fairmont Press (1993).



14SECTION

NONDESTRUCTIVE TESTING.
GLOSSARY

516 I NONDESTRUCTIVE TESTING OVERVIEW

Introduction acceptance criteria: The standard against which test
results are to be compared for purposes of establishing
Most of the definitions in this glossary are adapted from the functional acceptability of a part or system being
the text of Volumes 1 through 9 of the second edition Non­ examined.
destructive Testing Handbook.r" The definitions in this
glossary have been modified to satisfy peer review and edi- acceptance level: A test level · above or below which test
torial style. For this reason, references given_ in this glossary objects are acceptable in contrast to rejection level. 4.13
should be considered not attributions but rather acknowl-
edgments and suggestions for further reading. acceptance standard: A specimen similar to the test
object containing natural or artificial discontinuities
The definitions in this Nondestructive Testing Handbook that are well defined and similar in size or extent to the
volume should not be referenced for inspections performed maximum acceptable in the product. See reference
according to standards or specifications or in fulfillment of standard and standard. 4.5.7
contracts. Standards writing bodies take great pains to
ensure that their documents are definitive in wording and accommodation:Of the eye, adjustment of the lens' focus-
technical accuracy. People working to written contracts or ing power by changing the thickness and curvature of
procedures should consult definitions referenced in real the lens by the action of tiny muscles attached to the
standards when appropriate. lens.8

This glossary is provided for instructional purposes. No accumulationtest technique: Detecting the total amount
other use is intended. of leakage by enclosing the component under test
within a hood, bag, box, shroud or container. For pres-
A sure testing, any gas leaking from the component accu-
mulates in the space (volume) between the component
A-scan display: A display in which the received signal
amplitude is shown as a vertical excursion from the hor- and the enclosure. For vacuum testing, any gas leaking
izontal sweep time trace. The horizontal distance into the component accumulates in the leak detector
between any two signals represents the material dis- sampling the evacuated component. Accumulation of
tance between the two conditions causing the signals.10 tracer gas in a measured time period provides a mea-
In a linear system, the vertical excursion is proportional sure of the leakage rate.1
to the amplitude of the signal.7 accuracy:The degree of conformity of a measurement to a
standard or true value.1
absolute coil: A coil of electrically conductive wire that acoustic emission: The transient elastic waves resulting
responds to the electromagnetic properties of that region from local internal micro displacements in a material.
of the test object that is within the magnetic field of the The term also describes the technical discipline and
coil, without comparison to the response of a second coil measurement technique relating to this phenomenon. 5
at a different location on the same or similar material.4 acoustic emission activity: The number of bursts (or
events, if the appropriate conditions are fulfilled)
absolute measurement: Measurement made with an detected during an acoustic emission test.5
absolute coil.4 acoustic emission count: The number of times the signal
amplitude exceeds the preset reference threshold. 5
absolute pressure: Pressure above absolute zero value, or acoustic emission event: A microstructural displacement
pressure above that of empty space. Equal to sum of that produces elastic waves in a material under load or
local atmospheric pressure and gage atmosphere.1 stress.5
acoustic emission rate: The number of times the ampli-
absolute temperature:Temperature measured from abso- tude has exceeded the threshold in a specified unit of
lute zero temperature, expressed in Kelvin (K) in SI.1 time.5
acoustic emission testing (AE): Nondestructive test
absorbed dose: The amount of energy imparted to matter method that uses acoustic emission.·
by an ionizing particle per unit mass of irradiated mate- acoustic impedance: A material property defined as the
rial at the place of interest. Absorbed dose is expressed product of sound velocity and density of the mate-
in Gray (Cy) or rads." rial.1,12

absorption coefficient, linear: The fractional decrease in acousticmicroscopy:High resolution, high frequency ultra-
transmitted intensity per unit of absorber thickness. It sonic techniques used to produce images of features
is usually designated by the symbolµ and expressed in beneath the surfaceof a test object.7
units of crrr-'. 1,12
acuity:See neural acuity, vision acuity.
acceptable quality level (AQL):The maximum percent adaptive thresholding: Threshold value varying with in-
defective (or the maximum number of units with
rejectable anomalies per hundred units) that, for the constant background gray level. 8
purposes of sampling tests, can be considered satisfac-
tory as a process average.8 adhesive wear: See wear, adhesive.

NONDESTRUCTIVE TESTING GLOSSARY I 51 7

AE: Acoustic emission testing. amplitude response: That property of a test system
age hardening: A process of aging that increases hardness whereby the amplitude of the detected signal is mea-
sured without regard to phase. See also harmonic ana­
and strength, but that ordinarily decreases ductility. lyzer and phase analysis.4,13
Also known as precipitation hardening. 3
agency: The organization selected by an authority to per- amplitude, echo: The vertical height of a received signal on
ftioornmonropnudrecshtrausectiovredetre. s2 ting, as required by a specifica- an A-scan, measured from base to peak for video or peak
air dried: Drying of any item such as a core or mold with- to peak for radio frequency presentation. 7
out application of heat.3
air injection machine: A die casting machine in which air analog-to-digital converter: A circuit whose input is
pressure acts directly on the surface of molten metal in a information in analog form and whose output is the
closed pot (gooseneck) and forces the metal into a die.3 same information in digital form.4.14
air flow:In leak testing, the flow of air from the probe inlet
to the sensitive element of the halogen leak detector that angle: See field angle.
carries the tracer gas from the leak to the sensing diode.1 angle beam: An ultrasound beam traveling at an acute
algorithm: A prescribed set of well defined rules or pro-
cesses for the solution of a mathematical problem in a angle into a medium. The angle of incidence or refrac-
finite number of steps.4.14 tion after entry is measured from the normal to the
alkali ion diode: A sensor for halogen gases. In this device, entry surfaceJ,12
positive ions (cations) of an alkali metal are produced on
the heated surfaces (usually platinum) of the diode. One angle of incidence: The included angle between the beam
electrode is at a negative potential and attracts cations axis of the incident wave and the normal to the surface
that are released when a halogen gas passes between the at the point of incidence.U''
sensor electrodes. Provides an output current to operate
the indicator on the halogen leak detector.1 angle of reflection: The included angle between the beam
alpha ferrite: The form of pure iron that has a body cen- axis of the reflected wave and the normal to the reflect-
tered cubic structure stable below 910 °C (1,670 °F). ing surface at the point of reflection. 7,1o
Also called alpha iron. 8
alpha particle: A positively charged particle emitted by angle of refraction: The angle between the beam axis of a
certain radioactive materials. It is made up of two neu- refracted wave and the normal to the refracting inter-
trons and two protons; hence it is identical with the face. 7.10
nucleus of a helium atom.'!
angle testing: A method of ultrasonic testing in which
alternating current: An electrical current that reverses its transmission of ultrasound is at an acute angle to the
direction of flow at regular intervals.6 entry surface. Usually called angle beam testing. 7,10

alternating current field: The varying magnetic field pro- angle transducer: A transducer that transmits or receives
duced around a conductor by an alternating current ultrasonic energy at an acute angle to the surface. This
flowing in the conductor.6 may be done to achieve special effects such as setting
up shear or surface waves by mode conversion at an
alternating current magnetization: Magnetization by a interface. 7.1o
magnetic field that is generated when alternating cur-
rent is flowing.6,15 angstrom: A unit of distance abbreviated A and used to

ambient light: Light in the environment as opposed to illu- express wavelengths of electromagnetic radiation. 1he SI
mination provided by a visual testing system. 8 unit nanometer (nm) is now preferred; 1 nm= 10 A.2,8
anisotropy: The characteristic of exhibiting different values
ambient or atmospheric temperature: Temperature of of a property (velocity, for example) in different direc-
surrounding atmosphere. Also called dry bulb tempera­ tions in the material. 2
ture.
annealing: Process of heating and cooling a material, usu-
ampere: A unit of electric current. Abbreviated A or amp.6 ally to reduce residual stresses or to make it softer. 2,3,8
ampere per meter: The SI base unit for magnetic field
annular coil clearance: The mean radial distance between
strength in air at the center of a single-tum circular coil the inner diameter of an encircling coil assembly and
having a diameter of 1 m, through which a current of test object surface in electromagnetic examination. See
1 A is flowing. Abbreviated A,m-1 or Alm. 1 A-m-1 = 1.3 fill factor.4.13
x 10-2 Oe (see oersted). 5.15
anode: (1) In radiography, the positive electrode of an elec-
ampere turns: The product of the number of turns of a coil tron tube. (2) Negatively charged terminal, which may
and the current in amperes flowing through the coil.6,16 corrode electrochemically during production of an
electric current. Compare cathode. 8
amplitude distortion: See harmonic distortion.
anomaly: A variation from normal material or product

quality.4
antinode: A point in a standing wave where certain charac-

teristics of the wave field have maximum amplitude.l!''

AOQ: Average outgoing quality.

AOQL: Average outgoing quality limit.

518 I NONDESTRUCTIVE TESTING OVERVIEW

AQL: See acceptable quality level. attenuation: ( 1) Decrease in energy or signal magnitude in
arbor: A bar or mandrel on which a core is built. 3
arc: A luminous high temperature discharge produced transmission from one point to another. Can be

when an electric current flows across a gaseous gap.6·15 expressed in decibels or as a scalar ratio of the input
arc strikes: Localized bum damage to an object from the arc magnitude to the output magnitude.4.14 (2) The loss in

caused by breaking an energized electric circuit. Also acoustic energy that occurs between any two points of
called arc bums.6.16
travel. This loss may be caused by absorption, reflec-
arc welding: See electric arc welding. tion, scattering or other material characteristics.!"
arcing: Current flow through a gap, often accompanied by
(3) The change in signal strength 7c(a4u)seInd by an elec-
intense heat and light.6.17 tronic device such as an attenuator. radiography,

Argand diagram: A graphical representation of a vector the decrease in exposure rate of radiation caused by
used in complex notation.4•14 passage through material. 11

array: (1) A group of transducers used for source location.5 attenuation coefficient: A factor which is determined by
(2) An arrangement of sensors used for image building.
(3) A group of transducers arranged for beam shaping the degree of diminution in sound wave energy per unit
or beam control. distance traveled. Composed of two parts, one (absorp-

array transducer: A transducer made up of several piezo- tion) proportional to frequency, the other (scattering)
electric elements individually connected so that the sig-
nals they transmit or receive may be treated separately dependent o7·n18 the ratio of grain size or particle size to
or combined as desired." wavelength.

articulated pole pieces: On a magnetizing yoke, indepen- attenuator: A device for causing or measuring attenuation.
dently adjustable magnetic elements enabling the mag- Usually calibrated in decibels.l-'?
netization of irregular test object profiles.6
austenite: A solid solution with iron as the solvent in a face
artifact: In nondestructive testing, an indication that may
be interpreted erroneously as a discontinuity.2 centered cubic structure formed by slow cooling of
delta ferrite. Characteristic lattice structure is stable
artificial discontinuity standard: See acceptance standard. between 906 °C (1,663 °F) and 1,390 °C (2,535 °F).
artificial discontinuity: Reference point, such as a hole,
Also called gamma iron.8
groove or notch, that are introduced into a reference
standard to provide accurately reproducible sensitivity automated system: Acting mechanism that performs
levels for nondestructive test equipment.t+' A manu- required tasks at a determined time and in a fixed
afancdturreefdermenacteesrtiaanl daanrdo.m6 aly. See acceptance standard
artificial flaw standard: See acceptance standard. sequence in response to certain conditions.8
artificial source: In acoustic emission, a point where elastic
waves are created to simulate an acoustic emission event. B
The term also defines devices used to create the waves.5
ASNT: American Society for Nondestructive Testing. B-scan: A data presentation method typically applied to
ASNT Recommended Practice No. SNT­TC­lA: A set of pulse echo techniques. It produces a two-dimensional
guidelines for employers to establish and conduct a
nondestructive testing personnel qualification and cer- view of a cross sectional plane through the test object.
tification program. SNT­TC­lA was first issued in 1968
by the Society for Nondestructive Testing (SNT, now The horizontal sweep is proportional to the distance
ASNT) and has been revised every few years since.8 along the test object and the vertical sweep is propor-
atmosphere: See standard atmospheric conditions.
tional to depth, showing 7t.h12e front and back surfaces and
abnospheric pressure: Ambient pressure caused by the discontinuities between.
weight of the earth's atmosphere. Because the weight of
the earth's overlying atmosphere decreases with increase back draft: A reverse taper on the pattern that prevents its
in altitude, barometric pressure decreases at higher ele- removal from the mold. 3
vations above sea level. Also called barometric pressure.
At sea level, standard barometric pressure is taken as back reflection: The signal received from the far boundary
101.325 kPa, equivalent to an absolute pressure of or back surface of a test object. 7·10
14.696 lbrin.-2. It is also equal to the pressure exerted by back scatter: See backscatter.
a mercury column 760 mm (29.92 in.) high - that is,
equal to 760 mm Hg (29.92 in. Hg) or 760 torr.1 background: The surface of the test object on which the

indication is viewed in surface methods such as liquid

penetrant and magnetic particle testing. It may be the

natural surface of the object or the developer coating
on the object surface. This background may contain

irrelevant information that can interfere with the visi-
bility of the indication.2·6.16

background contamination: Tracer gases that accumu-

late in the test area, making it difficult to keep a leak

detector zeroed. They may also be a health hazard. 1

background cylinder and difference cylinder: Two

devices used to calculate illuminance by using the
equivalent sphere illumination technique. s,19

NONDESTRUCTIVE TESTING GLOSSARY I 519

background fluorescence: Fluorescent residues observed bead: A half-round cavity in a mold or a half-round projec-
over the general surface of the test object during fluo- tion or molding on a casting.3
rescent penetrant testing and fluorescent magnetic par-
ticle testing.2 beam exit point: See probe index.

backgroundnoise: The signals that originate from the test beam spread:The divergence of the sound beam as it travels
object, the test instrument and their surroundings and through a medium.t? Specifically, the solid angle which
that interfere with test signals of interest. It has electri- contains the main lobe of the beam in the far field.7
cal and mechanical origins. Sometimes called grass or
bearding: See furring.
hash. 5·7•10
background signal: A steady or fluctuating output signal bed-in: A method of ramming the drag mold without
rolling over it.3
of a test instrument caused by the presence of acoustic,
chemical, electrical or radiation conditions to which the bedding a core: Placing an irregularly shaped core on a
sensing element responds.1 bed of sand for drying.3
backing board: A second bottom board where molds are
opened.3 bentonite: A plastic, adhesive clay that swells when wet. It
backscatter: ( 1) In radiography, radiation scattered from is derived from decomposed volcanic ash and is used
the floor, walls, equipment and other items in the area for bonding molding sand. 3
of a radiation source.!' (2) In ultrasonic testing, scat-
tered signals that are directed back to the Berthold penetrameter: A magnetic flux indicator con-
transmitter/receiver. 7 taining an artificial discontinuity in the shape of a cross,
mounted below an adjustable cover plate.6.15
baked core: A core that has been heated or baked until it is
thoroughly dry.3 beta particle: An electron or positron emitted from a
nucleus during decay.'!
baked permeability: The property of a molded mass of
sand heated at a temperature above llO °C (230 °F) beta ray: A stream of high speed electrons of nuclear ori-
until dry and cooled to room temperature to permit gin. This radiation is more penetrating than alpha radi-
passage of gasses. 3 ation but ionizes less strongly.'!

band pass filter: An electromagnetic frequency filter that betatron:A circular electron accelerator that is a source of
has a single transmission band between two cutoff fre- either high energy electrons or X-rays. The electrons
quencies, neither of the cutoff frequencies being zero are injected by periodic bursts into a region of an alter-
or infinity.4·14 nating magnetic field. After acceleration, the electrons
are brought out directly or directed against a target to
bandwidth:The difference between the lower and upper produce X-rays.11
cutoff frequencies.vI"
billet: A solid semifinished round or square product that
barium clay: A molding clay containing barium, used to has been hot worked for forging, rolling or extrusion.2
eliminate or reduce the amount of scattered or sec-
ondary radiation reaching the film.3 binary system: In metallurgy, a two element alloy system.8

barometer: Absolute pressure gage used to measure the binder: A material used to hold the grains of sand together
atmospheric pressure at a specific location.1 in molds or cores. It may be cereal, oil, clay or natural
or synthetic resins.3
barometric pressure: Ambient pressure caused by the
weight of the Earth's atmosphere.1 See atmospheric birefringence: The splitting of a light beam into two parts
pressure. through a translucent material. s

baseline: The horizontal trace across the A-scan cathode black body: See blackbody.

ray tube display. It represents time and is generally black light: Disfavored term for electromagnetic radiation
related to material distance or thickness.7 or light energy in the near ultraviolet range with wave-
basin:A cavity on top of the cope into which metal is poured lengths from 320 to 400 nm, just below the wavelengths
before it enters the sprue. Alsocalled pouring basin.3 of visible light. Also a term for the ultraviolet light
basis calibration: Standardizing an ultrasonic testing source used in fluorescent nondestructive testing.
instrument using calibration reflectors described in an Black light sources often have a predominant wave-
application document. 7 length of 365 nm. See the preferred term, ultraviolet
radiation. 2·6·8·16
bath: The water or oil used as a vehicle for wet method
black light filter:A filter that transmits ultraviolet radiation
magnetic particles.6 The liquid penetrant testing mate- between 320 and 400 nm wavelengths while absorbing or
rials (penetrant, emulsifier, developer) into which test suppressing the transmission of the visible radiation and
objects are immersed during the testing process and hard ultraviolet radiation with wavelengths less than
penetrant materials retained in bulk in immersion tanks 320 nm.6.16
intended for reuse.2 See suspension.
black light intensity: See intensity.

520 I NONDESTRUCTIVE TESTING OVERVIEW

blackbody: Hypothetical radiation source that yields the borescope, angulated: Borescope bent for viewing at for-
maximum radiation energy theoretically possible at a ward oblique, right angle or retrospective angles for
given temperature. A blackbody will absorb all incident visual testing of surfaces not accessible with conven-
radiation falling upon it and has an emissivity of 1.0. See tional borescopes. 8
also emissivity. 9
borescope, fiber optic: Borescope that uses fiber optic
blacklight:See black light. materials (such as glass or quartz) in the optical path
and for transmission of light to and from the test sur-
bleed: Refers to molten metal oozing out of a casting. face."
Stripped or removed from the mold before complete
solidification.' borescope, micro-: Borescope with an outside diameter
generally from 1 to 5 mm (0.04 to 0.2 in.), typically
bleedback: The ability of a penetrant to bleed out of a dis- using quartz filaments. Compare borescope, miniature.8
continuity subsequent to removal of the indication
without reapplying the penetrant. 2 borescope, miniature: Borescope with an outside diame-
ter generally less than 13 mm (0.5 in.). Sometimes
bleedout: The action by which a penetrant exudes from called miniborescope. See also borescope, micro­.8
discontinuities onto the surface of a material. Action of
the entrapped penetrant in spreading out from surface borescope, rigid: Borescope that does not bend, typically
discontinuities to form an indication.s in order to keep the geometrical optics in alignment
through a light train system.8
blended sand: A mixture of sands of different grain sizes
and clay content that is needed to produce a sand pos- borescope, ultraviolet: Borescope equipped with ultravi-
sessing more suitable characteristics for foundry use.3 olet lamps, filters and special transformers to transmit
radiation of ultraviolet wavelengths.8
blind riser: An internal riser that does not reach to the
exterior of the mold.3 bottom board: The board or plate on which the mold
rests.3
blind spot: Portion of the retina where the optic nerve
enters, without rods and cones and hence insensitive to bottom echo: See back reflection.
light.8 bottom pour mold: A mold grated at the bottom.3
boundaryecho: A reflection of an ultrasonic wave from an
blister: A discontinuity in metal, on or near the surface,
resulting from the expansion of gas in a subsurface interface.7·12
zone. Very small blisters are called pinheads or pepper branch gates: Gates leading into a casting cavity from a
blisters.2
single runner and sprue.f
blotch: ( 1) An irregularly spaced area of color change on a brazing: Joining of metals and alloys by fusion of nonfer-
surface. (2) Nonuniform condition of a surface charac-
terized by such blotches.8 rous alloys that have melting points above 430 °C
(806 °F), but below melting points of materials being
blotting:The action of the developer in soaking up the pen-
etrant from the surface of a discontinuity so as to cause joined.2
maximum bleedout of the liquid penetrant for bridging: Premature solidification of metal across a mold
increased contrast and sensitivity.2
section before the metal below or beyond solidifies.3
blowhole: A hole in a casting or a weld caused by gas brinell hardness: A measure of metal hardness. Deter-
entrapped during solidification. 2·3
mined by pressing a hard steel ball into the smooth sur-
blue hazard: Exposure to high frequency visible light at face under standard conditions.
intensities and durations that may damage the retina, brinelling: Stripe indentations made by a spherical object.
particularly in conjunction with overheating.8 False brinelling refers to a type of surface wear. 8
brittle crackpropagation:A very sudden propagation of a
bobbin coil: See ID coil. crack with the absorption of no energy except that
stored elastically in the body. Microscopic examination
bond:A cohesive material used to bind sand.3 may reveal some deformation even though it is not visi-

bond clay: Any clay suitable for use as a bonding material ble to the unaided eye.2
in molding sand.3 brittleness: The quality of a material that leads to crack

bond strength:The degree of cohesiveness that the bond- propagation without appreciable plastic deformation.2
ing agent exhibits in holding sand grains together. 3 broad .handed: Having a relatively wide frequency band-

book mold:A split mold hinged like a book.3 width. Describes pulses that display a wide frequency
spectrum and receivers capable of amplifying them.
borescope: An industrial endoscope; a periscope or tele- Opposite to narrow banded or tuned.7
scope using mirrors, prisms, lenses, optic fibers or tele- bubbler: See water column.
vision wiring to transmit images from inaccessible bucking coils: See differentialcoils.
interiors for visual testing. Originally used in machined buckle: Indentation in a casting, resulting from expansion
apertures such as gun bores. There are both flexible of the sand.3
and rigid, fiber optic and geometric light borescopes."

NONDESTRUCTIVE TESTING GLOSSARY I 521

bumper: A machine used for packing molding sand in a capillary action: The tendency of liquids to penetrate or
migrate into small openings, such as cracks, pits or fis-
flask by repeated jarring or jolting.3
sures. The positive force that causes movement of cer-
bumping: Ramming sand in a flask by repeated jarring and
tain liquids along narrow or tight passages.2
jolting.3 carrier fluid: (1) A fluid that acts as a carrier for the active
materials. (2) The fluid in which fluorescent and visible
burning: Extreme overheating. Makes grains excessively
dyes are dissolved or suspended, in liquid penetrants or
large and causes the more fusible constituents of steel leak tracers.2 (3) The liquid vehicle in which fluores-

to melt and run into the grain boundaries or it may cent or nonfluorescent magnetic particles are sus-
pended for ease of application. See vehicle. 6,16
leave voids between the grains. Steel may be oxidized to case crushing: A mechanism producing fracture of the

the extent that it is no longer good and cannot be cor- case, like subcase fatigue but attributable to static over-
rected by heat treating but it can be remelted. 2
loading rather than to fatigue alone. In many instances
burnt-in sand: A discontinuity consisting of a mixture of
sand and metal cohering to the surface of a casting. 3 the movement of the subcase causes the case to crack

burr: A raised or turned over edge occurring on a machined or spall.8
casing: The many strings of pipe that are used to line the
part and resulting from cutting, punching or grinding.s,19
burst: ( 1) A signal whose oscillations have a rapid increase hole during and after drilling of a gas or oil well.8
casing string: Tubular structure on the outer perimeter of
in amplitude from an initial reference level (generally
that of the background noise), followed by a decrease a gas or oil well hole. The casing string is a permanent
(generally more gradual) to a value close to the initial
value.5 (2) In metal, external or internal rupture caused part of the well and many are cemented into the forma-
by improper forming.8
burst counting: A measurement of the number of bursts tion."
detected relative to specified equipment settings such cassette: A lightproof container that is used for holding the
as threshold level or dead time.f
burst duration: The interval between the first and last time radiographic films in position during the radiographic
the threshold was exceeded by the burst.5
burst emission:A qualitative term applied to acoustic emis- exposure and that may or may not contain intensifying
sion when bursts are observed.5 and/or filter screens.U
burst rate: The number of bursts detected in a specified
time:5 cast structure: The internal physical structure of a casting
burst rise time: The time interval between the first thresh-
old crossing and the maximum amplitude of the burst.5 evidenced by shape, orientation of grains and segrega-
butt weld or butt joint: Weld joining two metal pieces in tion of impurities.e-'
the same plane.8
cast weld assembly: 3An assembly formed by welding one
c casting to another.

C-scan: A data presentation technique applied to pulse casting: Object of shape obtained by solidification of a sub-
echo and transmission techniques. It yields a two- stance in a mold.
dimensional plan view of the object but no depth indi-
cations unless special gating procedures are used.7,10,12 casting shrinkage: Total shrinkage includes the sum of
three types: (1) liquid shrinkage (the reduction in vol-
calibration reflector: A reflector with a known dimen-
sioned surface established to provide an accurately ume of liquid metal as it cools to the liquidus), (2) solid-
reproducible reference level.7
ification shrinkage (the change in volume of metal from
candela: Base unit of measure in SI for measuring lumi-
nous intensity. The luminous intensity in a given direc- the beginning to ending of solidification); and (3) solid
tion of a source that emits monochromatic radiation of shrinkage (the reduction in volume of metal from the
frequency 540 x 1012 Hz and that has a radiant intensity
in that direction of 1.4641 mwsr :'. Symbolized ed. solidus to room temperature).2·3
Formerly known as candle. 8
casting strains: Strains in a casting caused by casting
candle: Former name for candela. 8 stresses that develop as the casting cools.3
capacitor discharge technique: A single-shot magnetiza-
casting stresses: Stresses set up in a casting because of
tion technique using dischargefrom a bank of capacitors.
A means by which electrical charge is built up and stored geometry and casting shrinkage.3
until a sufficient level is achieved to provide a predeter-
mined magnetic field in a test object, usually saturation.6 cathode: ( 1) In radiography, the negative electrode of an

electron tube. (2) Positively charged terminal in an

arrangement that produces current by chemical reac-
tions. Compare anode. 8

cathode ray: A stream of electrons emitted by a heated fil-

ament and projected in a more or less confined beam
under the influence of a magnetic or electric field.7·12
cathode ray tube (CRT): A vacuum tube containing a

screen on which nondestructive testing or other signals

may be displayed. Used for ultrasonic A-scans or
B-scans.i.l2

522 I NONDESTRUCTIVE TESTING OVERVIEW

cavitation fatigue: A form of pitting caused by erosion chuck: A small bar set between crossbars to hold sand in
from vibration and movement in liquid environments.8 the cope.3

cavity:The die impression that gives a casting its external circular magnetic field: The magnetic field surrounding
shape.3 panasseeledctlroincgailtucdoinndaullcytothr r(oteusgthotbhjeeccto) nwduhcetnora. 6·c16urrent is

CCD: See charge coupled device. circular magnetization: The magnetization in an object
cementite: Iron carbide (Fe3C), present in steels." resulting from current passed longitudinally through
central conductor: An electric conductor passed through the object itself or through an inserted central conduc-
tor. 6,16
the opening in a part with an aperture, or through a
hole in a test object, for the purpose of creating a circu- circumferential coil: See encircling coil.
lar magnetic field in the object.6.10 circumferential magnetization: See circular magnetiza­
centrifugal casting:A casting made in a mold (sand, plas-
ter or permanent) that rotates while the metal solidifies tion.
under the pressure developed by centrifugal force.3 cire perdue process: The lost wax process.3
certification: The process of providing written testimony clean: Free from interfering solid or liquid contamination
that an individual is qualified. See also certified. a
certified: Having written testimony of qualification. See on the test surface and within voids or discontinuities.2
also certification. a cleaner: Volatile solvent employed to clean a surface before

chafing:See wear, fretting. penetrant application. The cleaner is sometimes re-
chalk test: A past method of locating cracks by applying ferred to as the solvent remover.2
cleanup or cleanup time: The time required for a leak
penetrating liquid to an object and then removing the testing system to reduce its signal output to 37 percent
excess from the surface. The surface is coated with of the signal indicated when the tracer gas ceases to
whiting or chalk. After a short period of time the pene- enter the leak testing system.1
trant seeps out of the cracks into the whiting or chalk,
causing an appreciable difference in whiteness.2 cleavage: The fracture of a crystal on a crystallographic
channels: In biology, mechanisms functioning as bandpass plane of low index.2
filters in the visual cortex of mammals, causing sensitiv-
ity to visual stimuli in particular frequencies and cleavage fracture: A fracture, usually of a polycrystalline
ranges." metal, in which most of the grains have failed by cleav-
chaplet: A metal support used to hold a core in place on a age, resulting in bright reflecting facets. It is one type of
mold.3 crystalline fracture. Contrast with shear fracture. 2
charge coupled device (CCD): Solid state image sensor
widely used in inspection systems because of their closing: In image processing, dilation followed by erosion.
accuracy, high speed scanning and long service life." A single pixel closing connects a broken feature sepa-
Charpy test: A destructive mechanical test in which a rated by one pixel. See also opening. a
notched 10 x 10 x 55 mm rectangular bar, supported at
both ends as a simple beam, is broken by the impact of a closure: Process by which a person cognitively completes
fallingpendulum. Energy absorbed in breaking the bar is patterns or shapes that are incompletely perceived."
a measure of the impact strength of the bar material and
indicates the material's resistance to brittle fracture.2 cocoa: Debris (usually oxides of the contacting metals) of
chatter: (1) In machining or grinding, a vibration of the fretting wear, retained at or near the site of its forma-
tion - a condition especially helpful during visual tests.
tool, wheel or workpiece producing a wavy surface on With ferrous metals, the debris is brown, red or black,
the work. (2) The finish produced by such vibration.2 depending on the type of iron oxide formed. For this
checks:Numerous, very small cracks in metal or other mate- reason, ferrous debris is called cocoa or, when mixed
rial caused in processing. Minute cracks as in a die with oil or grease, red mud. a
impression, usually at a corner, caused by forging strains.
Also called grinding checks and check marks.2 code: A standard enacted or enforced as a law.8
chill: (1) A metal insert embedded in the surface of a sand
mold or core or placed in a mold cavity to increase the coefficient of thermal expansion: The linear expansion
cooling rate at that point. (2) White iron occurring on a or contraction per unit length per degree of tempera-
gray iron casting, such as the chill in the wedge test. 3 ture change between specified lower and upper tem-
chipping: (1) Removing seams and other surface disconti- perature limits.2
nuities in metals manually with chisel or gouge or by
continuous machining, before further processing. coefficients of the filter: Values in a mask that serves as a
(2) Removing excessive metal.2·3 filter in image processing.a

coercive force: The reverse magnetizing force needed to
remove remanent or residual magnetism and thereby
demagnetize the object.6

coil: One or more loops of a conducting material. In eddy
current testing, a single coil may be an exciter and
induce currents in the material or it may be a detector
or both simultaneously.4

NONDESTRUCTIVE TESTING GLOSSARY I 523

coil clearance: See liftoff color contrast penetrant: A penetrant incorporating a

coil shot: A technique of producing longitudinal magneti- dye, usually nonfluorescent, sufficiently intensive to

zation by passing electric current through a coil encir- give good visibility to discontinuity indications under
cling the test object.6,10
visible light.2
coil spacing: In electromagnetic testing, the axial distance
between two encircling coils of a differential system.v':' color discrimination: The perception of differences
between two or more hues.f
coil technique: A method of magnetization in which all or
columnarstructure:A coarse structure of parallel columns

a portion of the object is encircled by a current-carrying of grains, having the long axis perpendicular to the cast-
coil.6,16 ing surface.2·3

cold cathode ionization gage: Discharge current results combination die: A die having two or more different cavi-
from the application of a high voltage between anode ties for different castings.3

and cathode. The discharge current magnitude is a comparativemeasurement: In electromagnetic testing, a

function of the gaseous pressure within the gage cham- measurement based on the unbalance in a system using
ber. The external permanent magnet facilitates ioniza-
comparator coils. In contrast to differential and abso-
tion by forcing the electrons into a spiral path between lute measurements. See also comparator coils. 4,13

the two electrodes. The discharge current is displayed comparative test block: A penetrant comparator in the
on a meter over the absolute pressure range of less than
10 mPa ( 10--4 torr) .1 Also known as Philips discharge form of a block See comparator, penetrant.

comparator coil: In electromagnetic testing, two or more

gage or Penning gage. coils electrically connected in series opposition and
cold chamber machine:A die casting machine where the arranged so that there is no mutual induction (cou-

metal chamber or plunger are not heated.3 pling) between them. Any electromagnetic condition

cold cracks:Discontinuities appearing as straight lines usu- that is not common to the test object and the standard

ally continuous throughout their length and generally will produce an unbalance in the system and thereby
existing singly. Cold cracks start at the surface and result yield an indication. See differential coils. 4•13

from cold working or stressing of metallic materials.2 comparator,penetrant: A test block or reference panel
cold light: Obsolete word forfluorescence.8
with artificial cracks or special surface conditions, typi-
cold shut: (1) Casting discontinuity caused by two streams
cally having two separate but adjacent areas for applica-
of semimolten metal coming together inside a mold but
failing to fuse. Cold shuts are sometimes called misruns tion of different penetrants or processing materials or
operation, so that a direct visual comparison can be made
but the latter term correctly describes incomplete fill-
between different penetrant processes or materials.2
ing of the mold.3 (2) A discontinuity that appears on the
surface of test metal as a result of two streams of liquid compensator:An electrical matching network to compen-
sate for electrical impedance differences.7.l2
meeting and failing to unite. A cracklike discontinuity
complete testing: Testing of an entire production lot in a
caused by forging, where two surfaces of metal fold
prescribed manner. Sometimes complete testing entails
against each other to produce a discontinuity at the the inspection of only the critical regions of a part. One
point of folding. This is usually at some angle to the sur-
hundred percent testing requires the inspection of the
face. It may also be a separate piece of metal forged
into the main component. See lap. (3) A portion of the entire part by prescribed methods. Compare sampling,
partial:"

surface of a forging that is separated in part from the complex plane: A plane defined by two perpendicular ref-
main body of metal by oxide.2,3
erence axes, used for plotting a complex variable (such
cold work: Permanent deformation produced by an exter- as impedance) or functions of this variable (such as a
transfer function). See impedance analysis. 4,14
nal force in a metal at temperature below its recrystal-

lization temperature.2 compound microscope: See microscope, compound.

collimator:A device for limiting effects of beam spread. 7 compressional wave: A wave in which particle motion in

color: Visual sensation by means of which humans distin- the material is parallel to the wave propagation direc-

guish light of differing hue (predominant wavelengths), tion. Also called longitudinal wave. 7
saturation (degree to which those radiations predomi-
conditioned water: Water with an additive or additives
nate over others) and lightness.
that impart specific properties such as proper wetting,
color blindness: Deficiency in the ability to perceive or particle dispersion or corrosion resistance. 6

distinguish hues.8 conditioning agent: An additive to water suspensions that

color contrast dye: A dye that can be used in a penetrant imparts specific properties such as proper wetting, par-
ticle dispersion or corrosion resistanoe.c-lf
to impart sufficient color intensity to give good color

contrast indications augnadinesrt vtihsieblebalcigkhgtr.o2und on a test conductance: 1Property of a gas flow system that permits
surface when viewed gas to flow.

524 I NONDESTRUCTIVE TESTING OVERVIEW

conduction: Heat transfer occurring when warmer atomic control echo: A reference signal from a constant reflector,
particles collide with - and thus impart some of their
heat energy to - adjacent cooler (slower moving) par- such as the back reflection from a smooth, regular sur-
ticles. This action is passed on from one atom (or free face. 7.12
electron) to the next in the direction of cooler regions.
Thus, heat always flows from a warmer to a cooler cooling stresses: Residual stresses resulting from nonuni-
region.9
form distribution of temperature during cooling.2·3
cone: In biology, a retinal receptor that dominates the reti-
nal response when the luminance level is high and pro- cope: The upper or topmost section of a flask, mold or pat-
vides the basis for the perception of color. Compare tern."
rod.s,20
core: (1) A specially formed material inserted in a mold to
confidence level: The probability that the true leakage
rate will not exceed the upper confidence limit.1 shape the interior of another part of a casting that can-

constitution diagram:See phase diagram. not be shaped as easily by the pattern. (2) In a ferrous
contact head: Electrode assembly used to clamp and sup- alloy, the inner portion that is softer than the outer por-

port an object to facilitate passage of electric current tion or case.3
through the object for circular magnetization.v-"
contact method: (1) The ultrasonic testing method in core blower: A machine for making foundry cores, using
which the transducer makes direct contact with the test
object through a thin film of couplant.U'' (2) The cur­ compressed air to blow and pack the sand into the core
rentflow technique in magnetic particle testing.6 box.3
contact pad: Replaceable metal pad, usually made of lead
or copper braid, placed on electrodes to give good elec- core pin: A core, usually a circular section, having some
trical contact, thereby preventing damage such as arc taper or draft.3
strikes to the test object.6·16 core plate: A plate on which a green core is baked. 3
contact transducer: The transducer used in the contact
method.1 core wash: A liquid with which cores are painted to pro-
continuous annealing furnace: A furnace in which cast- duce smoother surfaces on the casting.3
ings are heat treated, by being passed through different
heat zones kept at constant temperatures.3 corner effect: The strong reflection obtained when an
continuous casting:A casting technique in which an ingot,
billet, tube or other shape is continuously solidified ultrasonic beam is directed toward the intersection of
while being poured so that its length is not determined two or three mutually perpendicular surfaces.l+'
by mold dimensions.3
continuous emission: A qualitative term applied to acous- corrosion: The deterioration of a metal by chemical or
tic emission when the bursts or pulses are not dis-
cernible. 5 electrochemical reaction with its environment.
Removal of material by chemical attack, such as the
continuous technique: Applying of magnetic particles to
form satisfactory discontinuity indications while the rusting of automobile components.2
magnetizing field is simultaneouslyapplied.6
corrosion, crevice: Type of galvanic corrosion caused by
continuous wave: A single frequency wave that continues differences in metal ion concentrations in neighboring
without interruption.7 portions of the corrodent. 8

contracted sweep: A misnomer that refers to extending corrosion emhrittlement: The severe loss of ductility of a
the duration of the sweep to permit viewing discontinu-
ities or back reflections from deeper in the test object. metal, resulting from corrosive attack, usually inter-
The sweep appears to be compressed. 7
granular and often not visually apparent.2
contrast:( 1) In radiography, the measure of differences in
the film blackening resulting from various radiation corrosion fatigue: Fatigue cracking caused by repeated
intensities transmitted by the object and recorded as load applications on metal in a corrosive environment.2
density differences in the image. Thus, difference in
film blackening from one area to another. 11 (2) The dif- corrosion, fretting: Corrosion facilitated by fretting, par-
ference in visibility (brightness or coloration) between ticularly where a protective surface has been chafed in
an indication and the surrounding surface.2
a corrosive environment.8
control: See in control, process control and quality control.
corrosion, poultice: Corrosion occurring under a layer of
foreign material (e.g., under mud in automobile rocker

panels).8

corrosion-erosion: Simultaneous occurrence of erosion

and corrosion.8

count rate: See acoustic emission rate.

couplant: A substance (usually liquid) used between an

ultrasonic transducer and the test surface to permit or

improve transmission of ultrasonic energy into the test
object. 7·12
'

coupled: Two electric circuits that have an impedance in
common so that a current in one causes a voltage in the
other.4.13

coupling: The percentage of magnetic flux from a primary
circuit that links a secondary circuit. The effectiveness

of a coil in inducing eddy currents in the test object."

NONDESTRUCTIVE TESTING GLOSSARY I 525

coupling coefficient: (1) The fraction of magnetic flux crack, quenching: Ruptures produced during quenching
of hot metal due to more rapid cooling and contraction
from one test coil that threads a second circuit ( test of one portion of a test object than occurs in adjacent
portions.2
object). (2) The ratio of impedance of the coupling to
crack, transverse: Cracks at right angles to the length of
the square root of the product of the total impedances the test object.2

of similar elements in the two meshes.v'" crack, weld: Cracks in weld fusion zones or adjacent base
metal. Usually a result of thermal expansion or contrac-
coupon: A piece of metal from which a test object is pre- tion stresses related to temperature changes during
welding.2
pared, often an extra piece, as on a casting or forging.3
crater: (1) In machining, a depression in the cutting tool
cover half: The stationary half of a die casting die.3 face eroded by chip contact. (2) In arc or gas fusion
welding, a cavity in the weld bead surface, typically
crack: (1) A break, fissure or rupture, usually V shaped and occurring when the heat source is removed and insuffi-
relatively narrow and deep. A discontinuity that has a cient filler metal is available to fill the cavity.2
relatively large cross section in one direction and a
small or negligible cross section when viewed in a direc- creep: Gradual and permanent change of shape in a metal
tion perpendicular to the first.2 (2) Propagating discon- under constant load, usually at elevated temperature.
tinuities caused by stresses such as heat treating or Occurs in three stages: primary creep, secondary creep
grinding. Difficult to detect unaided because of fine- and tertiary creep. See also deformation. 8
ness of line and pattern (may have a radial or latticed
appearance). 6 creep strength: The constant nominal stress that will cause
a specified creep rate at constant temperature. 2
crack contaminant: Material that fills a crack and that may
prevent penetrants from entering or from forming indi- crevice corrosion: See corrosion, crevice.
cations.f critical angle: The incident angle of an ultrasound beam

crack, base metal: Cracks existing in base metal before a above which a specific mode of refracted energy no
manufacturing or welding operation or occurring in longer exists.7·10
base metal during the operation. 2 cross line grating: In moire and grid nondestructive test-
ing, a grating with bars, furrows or lines parallel to
crack, cold: Cracks that occur in a casting after solidifica- orthogonal xy axes.9
tion, due to excessive stress generally resulting from cross talk: The unwanted signal leakage (acoustical or elec-
nonuniform cooling.2 trical) across an intended barrier, such as leakage
between the transmitting and receiving elements of a
crack, cooling: Cracks in bars of alloy or tool steels result- dual transducer. Also called cross noise and cross cou­
ing from uneven cooling after heating or hot rolling. pling.1,12
They are usually deep and lie in a longitudinal direc-
tion, but are usually not straight.2 CRT: See cathode ray tube.
crush: A casting discontinuity caused by a partial destruc-
crack, crater: A multisegment crack in a weld crater. Seg-
ments radiate from a common point, often called star tion of the mold before the metal was poured.3
cracks. crushing: The pushing out of shape of a sand core or sand

crack, fatigue: Progressive cracks that develop in the sur- mold when two parts of the mold do not fit properly
face and are caused by the repeated loading and where they meet.3
unloading of the object.2 crystal: See transducer.
crystal mosaic: Multiple crystals mounted in the same
crack, forging: Cracks developed in the forging operation plane on one holder and connected so as to cause all to
due to forging at too low a temperature, resulting in vibrate as one unit.7.12
rupturing of the steel.2
crystal, X-cut: A cut such that the cut face is perpendicu-
crack, grinding: Thermal cracks caused by local overheat- lar to the X-direction of the piezoelectric crystal. In a
ing of the surface being ground.2 quartz slice so cut, a thickness mode of vibration
occurs when the slice is electrically stimulated in the
crack, hot: Cracks that develop before the casting has com- X-direction.7.12
pletely cooled, as contrasted with cold cracks, that
develop after solidification.2 crystal, Y-cut: In Y-cut, the cut face of the piezoelectric
crystal is perpendicular to the Y-direction. In quartz, a
crack, longitudinal: Cracks parallel to the length of the shear mode of vibration is obtained when the slice is
test object.2 electrically stimulated in the Y-direction.7•12

crack, machining: Cracks caused by too heavy a cut, a dull cumulative bursts: The number of bursts detected from
tool or chatter. Typically called machining tears. 2 the beginning of the test.5

crack, pickling: Cracks caused by immersing objects with
high internal stresses in an acid solution.2

crack, plating: Cracks similar to pickling cracks, but occur-
ring during plating when the object is immersed in a
strong electrolyte.s

526 I NONDESTRUCTIVE TESTING OVERVIEW

cumulative characteristic distribution: A display of the dark adapted vision: See scotopic vision.

number of times an acoustic emission signal exceeds a daubing: The act of filling cracks in cores.3
preselected characteristic as a function of the charac-
teristic. 5 dead zone: In ultrasonic contact testing, the interval fol-
lowing the initial pulse at the surface of a test object to
cumulative count: The number of times the amplitude of the nearest inspectable depth.l'' Any interval following
a reflected signal where additional signals cannot be
an acoustic emission signal has exceeded the threshold detected.7
since the start of a test.f
deburring: Removing burrs, sharp edges or fins from metal
cumulative events: The number of events detected from objects by filing, grinding or rolling the work in a barrel
with abrasives suspended in a suitable liquid medium.
the beginning of a test. Use of this term is restricted in Sometimes called burring. 2,3
the same way as event counting.5
decarburization: The loss of carbon from the surface of a
cup fracture: Fracture, frequently seen in tensile test ferrous alloy as a result of heating in a medium that
places of a ductile material, in which the surface of fail- reacts with the carbon at the surface.2

ure on one portion shows a central flat area of failure in decibel: A unit for expressing power relationships in sonic
and acoustic measurements. Equal to ten times the
tension, with an ceuxpt­earinodr­ceoxnteenfrdaecdturreim. 2 of failure in base ten logarithm of the ratio of two powers. The unit
shear. Also called for voltages is twenty times the base ten logarithm of
the ratio of two voltages, provided the voltages are mea-
Curie point: The temperature at which ferromagnetic sured across equal impedances. 7

materials lose residual magnetism and can no longer be deep drawing: The forming of deeply recessed parts by
means of plastic flow of the material.2
magnetized by outside forces (between 650 and 870 °C
[1,200 and 1,600 °F] for most metals).6.16 deep etching: Severe etching of a metallic surface for
examination at a magnification of ten diameters or less
current flow technique: Magnetizing by passing current to reveal gross features such as segregation, cracks,
porosity or grain flow.2
through an object using prods or contact heads. The
current may be alternating current or rectified alternat- defect: A discontinuity whose size, shape, orientation or
location make it detrimental to the useful service of its
ing current.6·16 host object or which exceeds the accept/reject criteria
current induction technique: Magnetization in which a of an applicable specification.v!" Note that some dis-
circulating current is induced in a ring component by continuities may not affect serviceability and are there-
the influence of a fluctuating magnetic field.6.16 fore not defects.2 All defects are discontinuities.2
Compare discontinuity and indication. 8,19
cutoff frequency: Upper or lower spectral response of a
deformation: Change of shape under load. See also creep
filter or amplifier, at a specified amount less (usually 3 and elastic deformation. 8
or 6 dB) than the maximum response. 7
degasifier: A substance that can be added to molten metal to
cycle:A single period of a waveform or other variable. See remove soluble gases that might otherwise be occluded
or entrapped in the metal during solidification.3
period.
degassing: Removing gases from liquids or solids.3
D
degreasing fluid: Solvents or cleaners employed to
damping: (1) Limiting the duration or decreasing the remove oil and grease from test surfaces before the liq-
amplitude of vibrations, as when damping a transducer uid penetrant is applied.2
element.12 (2) A deliberate introduction of energy
absorbers to reduce vibrations.7 delamination: A laminar discontinuity, generally an area of
unhanded materials.7
damping capacity: A measure of ability of a material to
dissipate mechanical energy.7.18 delay line: A material (liquid or solid) placed in front of a
transducer to cause a time delay between the initial
damping material: A highly absorbent material used to pulse and the front surface reflection.7•12
cause rapid decay of vibration.7
delayed sweep: An A-scan or B-scan sweep, the start of
damping, transducer: A material bonded to the back of which has been delayed, thereby eliminating the
the piezoelectric element of a transducer to limit the appearance of early response data on the screen.7,21
duration of vibrations.v'"
delayed time base: See delayed sweep.
damping, ultrasonic: Decrease or decay of ultrasonic
wave amplitude with respect to time or distance.U?

dark adaptation: (1) Adjustment of the eye over time to
reduced illumination, including increased retinal sensi-
tivity, dilation of the pupil and other reflex physical
changes.2·6·16 (2) Process by which the retina becomes
adapted to luminance less than about 0.034 cd-m-2.8,20

NONDESTRUCTIVE TESTING GLOSSARY I 527

delta effect: Reradiation of energy from a discontinuity.12 detector probe: An adjustable or fixed device through
which air and/or tracer gas is drawn into the leak test
The reradiated energy may include waves of both inci-
dent mode and converted modes (longitudinal and instrument and over the sensing element or d1 etector.
Also called a sampling probe or a sniffer probe.
shear)." detector probe test: A pressure leak test in which the

delta ferrite: Solid solution with body centered cubic leakage of a component, pressurized with a tracer rich
structure and iron as solvent. Also called delta iron. 8
mixture, is detected by scanning the test object bound-
delta iron: See delta ferrite.
ary surface with a sniffer probe connected to an elec-
delta t (M): The time interval between the detected arrival tronic leak detector. Leakage tracer gas is pulled from

of an acoustic emission wave at two sensors.5 the leak through the probe inlet to the sensing element

demagnetization:The reduction of residual magnetism to to cause a visible or audible signal on the indicator of

an acceptable level.6·16 the leak test instrument.1 Also called sniffer test.

demagnetizing coil: A coil of conductive material carrying detergent remover:A penetrant remover that is a solution
alternating current used for demagnctizatton.f-P
of a detergent in water.2
demodulation: A modulation process wherein a wave developer: (1) In penetrant testing, a material that is

resulting from previous modulation is employed to applied to the test piece surface after the excess pene-
derive a wave having substantially the characteristics of trant has been removed and that is designed to enhance

the original modulating wave.t'" the penetrant bleedout to form indications. May be a

dendrite: A crystal that has a treelike branching pattern, fine powder, a solution that dries to form a dry powder
tbheeinsgolmidoifsitceavtiiodnenrat ningec.a2s,3t metals slowly cooled through or a suspension (in solvent or water) that dries leaving

deoxidizing: (1) The removal of oxygen from molten met- an absorptive film on the test surface.2 (2) In radiogra-

als by use of suitable deoxidizers. (2) Sometimes refers phy, a chemical solution that reduces exposed silver
to the removal of undesirable elements other than oxy- halide crystals to metallic silver.!'

developer, dry:A dry, fine powder applied to the test piece
gen by the introduction of elements or compounds that after the excess penetrant is removed and the surface
readily react with them. (3) In metal finishing, the
dried in order 2to increase the bleedout by means of
removal of oxide films from metal surfaces by chemical capillary action.

or electrochemical reaction.3 developer, nonaqueous: See developer, solvent.

depth compensation:See distance amplitude correction. developer,soluble: Fine particles completely soluble in its
carrier (fnoortmaansuasdpesonsripotniveocfoaptoinwgd. 2er in a liquid) that
depth of field: In photography, the range of distance over dries to
which an imaging system gives satisfactory definition
when its lens is in the best focus for a specific distance.f developer, solvent: Fine particles suspended in a volatile

depth of fusion:The depth to which the base metal melted solvent. The volatile solvent helps to dissolve the pene-

during welding.2 trant out of the discontinuity and brin~s it to the sur-
face. It then dries, fixing the indication.
depth of penetration: In electromagnetic testing, the
developer,wet: A penetrant developer usually supplied as
depth at which the magnetic field strength or intensity dry particles that is mixed with water to form a suspen-
of induced eddy currents has decreased to 37 percent
sion of particles.2
of its surface value. The square of the depth of penetra-
developing time: Elapsed time necessary for the applied
tion is inversely proportional to the frequency of the developer to absorb and show indications from pene-
signal, the conductivity of the material and the perme-
trant entrapments.2
ability of the material. Synonymous terms are standard dewaxing:Removing the expendable wax pattern from an

depth of penetration and skin depth. 2S,4,e13e joint penetra­ investment mold by heat or solvent.3
tion, root penetration and skin effect.
dewetting: The flow and retraction of liquid on a surface,
descaling: Removing the thick layer of oxides formed on
caused by contaminated surfaces or dissolved surface
some metals at elevated temperatures.2 coatmgs.f

deseaming: Analogous to chipping, the discontinuities diamagnetic material: A material whose relative perme-

being removed by gas cutting.2 ability is less than unity. The intrinsic induction Bi is

detail: In radiography, the degree of sharpness of outline of oppositely directed to applied magnetizing force H.4J3
the image. If a radiograph does not show a clear defini-
tion of the object or a discontinuity in the object, it is of A material with magnetic permeability less than 1.6
die casting: (1) A casting made in a die. (2) A casting pro-
little value although it may have sufficient contrast and
density.!' cess where molten metal is forced under high pressure
detector coil: See sensing coil. 4
into the cavity of a metal mold.3

difference cylinder: See background cylinder.

528 I NONDESTRUCTIVE TESTING OVERVIEW

differential amplifier:An amplifier whose output signal is directional properties: Properties whose magnitudes de-

proportional to the algebraic difference between two pend on the relation of the test axis to the specific
direction in the metal, resulting from pSreefeearrneidsotorroiepnyt.a2-
input signals.4·14 tion or from fibering in the structure.
differential coils: Two or more physically adjacent but
directional solidification: The solidification of molten
mutually uncoupled coils connected in series opposition
such that an unbalance between them, causing a signal, metal in a casting in such manner that feed metal is

will be produced only when the electromagnetic condi- always available for that portion that is just solidifying.3

tions are different in the regions beneath two of the discernible image: Image capable of being recognized by

coils. In contrast, comparator coils are not adjacent." sight without the aid of magnification.2
discontinuity: An intentional or unintentional interrup-
differential measurement: In electromagnetic testing,
tion in the physical structure or configuration of a
the imbalance in the system is measured using differen- part.6,8,16,22 After nondestructive testing, unintentional
tial coils - in contrast to absolute measurement and
comparative measurement. 4•13 discontinuities cianllteedrpflraewtesdoarsdedfeetcrtism.6enCtaolmpinartehdeefhoecstt,
differentiated signal: In electromagnetic testing, an out- object may be
dislocation and indication.
put signal proportional to the input signal's rate of discontinuity,artificial:Reference discontinuities such as
change with respect to time.4.13
holes, indentations, cracks, grooves or notches that are
diffraction:In ultrasonic testing, the deflection of a wave-
introduced into a reference standard to provide accu-
front when passing the edge of an ultrasonically opaque
rately reproducible indications for determining sensi-
object.12 tivity levels.2
diffuse indications: Indications that are not clearly
discontinuity, inherent: Material anomaly originating
defined as, for example, indications from surface con-
tamination.f from solidification of cast metal. Pipe and nonmetallic

diffuse reflection: Scattered, incoherent reflections from inclusions are the most common and can lead to other
rough surfaces.I-!"
types of discontinuities in fabrication.8·19

diffusion:The process by which molecules intermingle as a discontinuity, primary processing: Material anomaly
produced from the hot or cold working of an ingot into
result of concentration gradients or thermal motion.2
forgings, rod and bar.8.19
Spreading of a gas through other gases within a volume.
discontinuity, secondary processing: Material anomaly
dilation: In image processing, the condition of a binary
produced during machining, grinding, heat treating,
image where the pixel in the output image is a 1 if any plating or other finishing operations.s-l''
of its eight closest neighbors oipseani1ngi.n8 the input image.
See also closing, erosion and discontinuity,service induced: Material anomaly caused
by the intended use of the part. 8
dip rinse:A means of removing excess surface penetrant in dislocation: Void or discontinuity in the lattice of a metal

which othreretmesot voebrj.e2 cts are dipped into a tank of agitated crystalline structure.8 Two basic linear types are recog-
water
dispbneiirnzseaidtoion(ne:sdTghaeneddviapsrlaoiracttiaiaotlinodnoisfalponhcdaastsiceornevswelaodrceiistlmyowcoasitttihponrfer)evbqauluetencntoc.m2y.-7
direct contact magnetization: See current flow tech­

nique.
direct current: An electric current flowing continually in

one direction through a conductor.6·17 dispersive medium: A medium in which propagation

direct current field: An active magnetic field produced by velocity depends on the wave frequency.7
direct current flowing in a conductor or coil.6.17 displacement resolution: In moire and grid nondestruc-

direct photometry: Simultaneous comparison of a stan- tive testing, measurement precision expressed as the
dard lamp and an unknown light source.8,20
smallest displacement that can be determined with rea-

direct substitution alloy: Alloy in which the atoms of the sonable reliability.9

alloying element can tohcecuaptoymtsheofctrhyestpaal rleanttticmeestapla.c8es dissociation: The breakdown of a substance into two or
normally occupied by
more constituents.2

direct viewing: Viewing of a test object in the viewer's distal:In a manipulative or interrogating system, of or per-

immediate presence. The term direct viewing is used in taining to the end opposite from the eyepiece and far-
thest from the person using the system. Objective; tip.8
the fields of robotics and surveillance to distinguish
conventional from remote viewing.8 distance amplitude correction (DAC):Compensation of

direct vision instrument: Device offering a view directly gain as a function of time for difference in amplitude of

forward. A typical scene is about 19 mm (0.75 in.) wide reflections from equal reflectors at different sound

at 25 mm (1 in.) from the objective lens.8 travel distances. Refers also to compensation by elec-
tronic means such as sswenespittivgiatiyn,timtime ecocnotrrroelc.7t·e12d gain,
directional lighting: Lighting provided on the work plane time variable gain and

or object predominantly from a preferred direction.8•20

NONDESTRUCTIVE TESTING GLOSSARY I 529

divergence: A term used to describe the spreading of ductility: The ability of a material to deform plastically
ultrasonic waves beyond the near field. It is a function without fracturing, being measured by elongation or
of transducer diameter and wavelength in the reduction of area in a tensile test, by height of cupping
medium.7 in an Erichsen test or by other means.2

domain: A saturated macroscopic substructure in ferro- dwell time: The total time that the penetrant or emulsifier
magnetic materials where the elementary particles is in contact with the test surface, including the time
(electron spins) are aligned in one direction by inter- required for application and the drain time.2
atomic forces. A saturated permanent magnet.6,10
dynamic creep: Creep that occurs under conditions of
dose: The amount of ionizing radiation energy absorbed fluctuating load or fluctuating temperature.2
per unit mass of irradiated material at a specific loca-
tion, such as part of the human body. Measured in rems dynamicrange:The ratio of maximum to minimum reflec-
and rads.t? tive areas that can be distinguished on the cathode ray
tube at a constant gain setting.7,23,24
dose rate: The radiation dose delivered per unit time and
measured, for instance, in rems per hour. See also E
dose+'
echo: A signal indicating reflected acoustic energy.7
dosimeter:A device that measures radiation dose, such as a ECT: Eddy current testing.
film badge or ionization chamber.11 eddy current:An electrical current induced in a conductor

double crystal method: A method of ultrasonic testing by a time varying magnetic field.4
that uses two transducers, one transmitting and the eddy current testing: A nondestructive testing method in
other receiving.I-"
which eddy current flow is induced in the test object.
drag:The bottom section of a flask, mold or pattern.3 Changes in the flow caused by variations in the object
dragout: The carryout or loss of penetrant materials as a are reflected into a nearby coil, coils, Hall effect device
or other magnetic flux sensor for subsequent analysis by
result of their adherence to the test pieces.2 suitable instrumentation and techniques.v':'
drain time: That portion of the dwell time during which edge or end effect: In electromagnetic testing, the distur-
bance of the magnetic field and eddy currents due to
the excess penetrant, emulsifier, detergent remover or the proximity of an abrupt change in geometry. The
developer drains off the test piece.2 effect generally results in the masking of discontinuities
drop: A discontinuity in a casting due to a portion of the within the affected region.4.13
sand dropping from the cope or overhanging section of effective depth of penetration: In electromagnetic test-
the mold.> ing, the minimum depth beyond which a test system
drop out: The falling away of green sand from the walls of a can no longer practically detect a further increase in
mold cavity when the mold is closed.3 object thickness. If the minimum thickness for the fre-
dross:The scum that forms on the surface of molten metals quency used is not exceeded or the object thickness is
largely because of oxidation but sometimes because of not rigidly controlled, the test may be influenced by the
the rising of impurities to the surface.3 object thickness.13 Depending on the criteria, this min-
dry bulb temperature: Alternate term for ambient or at­ imum thickness is three to seven times the skin depth.4
mospheric temperature .1 effective penetration: In ultrasonic testing, the maximum
dry powder: Finely divided ferromagnetic particles select- depth in a material at which discontinuities can be
ed and prepared for magnetic particle testing.6,10 detected.U"
effective throat:In welding, the weld throat including the
dry technique: A magnetic particle testing technique in amount of weld penetration but ignoring excess metal
which the ferromagnetic particles are applied in a dry between the theoretical face and the actual face.8
powder form.6.16 elastic constants: Modulus of elasticity, either in tension,
compression or shear and Poisson's ratio.2
drying oven: An oven used for drying rinse water from test elastic deformation:Temporary change in shape under a
load. The material returns to its original size and shape
pieces.2 after the load is removed. Elastic deformation is the
drying time: The time allotted for a rinsed or cleaned test state in which most metal components are used in ser-
vice.8
piece to dry.2 elastic limit:The maximum stress to which a material may
dual response penetrant: A penetrant that produces dis- be subjected without any permanent strain remaining
on complete release of stress.2
continuity indications that can be seen under either
ultraviolet light or visible light.2
dual transducer:A single transducer containing two piezo-
electric elements, one for transmitting and one for
receiving.7,12

ductile crack propagation:Slow crack propagation that is
accompanied by noticeable plastic deformation and
requires energy to be supplied from outside the body.2

530 I NONDESTRUCTIVE TESTING OVERVIEW

elasticity: The ability of a material to resume its former emulsification time: In liquid penetrant testing, the
shape after deformation. 8 period of time that an emulsifier is permitted to com-
bine with penetrant before removal. Also called emulsi­
electric arc welding: Joining of metals by heating with fier dwell time. 2
electric arc. Also called arc welding. 8
emulsifier: A liquid that combines with an oil based pene-
electric field: A vector field of electric field strength or of trant to make it water washable.2

electric flux density.4·14 emulsion: A dispersion of fine droplets of one liquid in
another that can be stabilized by the addition of an
electrical center: The center established by the electro- emulsifier.2

magnetic field distribution within a test coil. A constant . encircling coil: In electromagnetic testing, a coil or coil
assembly that surrounds the test object. Such coils are
intensity signal, irrespective of the circumferential posi- also called annular, circumferential or feed­through
tion of a discontinuity, is indicative of electrical center- coils. 4·13 See coil technique.

ing. The electrical center may be different from the endoscope: Device for viewing the interior of objects.
physical center of the test coil.4.13 From the Greek words for inside view, the term endo­
scope is used mainly for medical instruments. Nearly
electrical noise: Extraneous signals caused by externally every medical endoscope has an integral light source;
many incorporate surgical tweezers or other devices.
radiated signals or electrical interferences within an Compare borescope. 8
ultrasonic lnstrument.l'' A component of background
equilibrium diagram: A phase diagram showing the
noise.7 phases present at equilibrium in a material system.8

electrochemical corrosion: Corrosion that occurs when equivalent 20/20 near vision acuity: Vision acuity with
remote viewing or other nondirect viewing that approx-
current flows between cathodic and anodic areas on imates 20/20 direct viewing closely enough to be con-
metallic surfaces.2 sidered the same for visual testing purposes.8

electrode: A conductor by which a current passes into or equivalent sphere illumination: Level of perfectly dif-
out of a test object. 5,15 fuse (spherical) illuminance that makes the visual task
as photometrically visible within a comparison test
electromagnet: A soft iron core surrounded by a coil of sphere as it is in the real lighting environment. 8

wire that temporarily becomes a magnet when an elec- erosion:( 1) Loss of material or degradation of surface quality
tric current flows through the wire.6.16 through friction or abrasion from moving fluids, made
worse by solid particles in those fluids or by cavitation in
electromagnetic acoustic transducer: An electromag- the moving fluid. See wear. (2) Inimage processing, con-
netic device using Lorentz forces and magnetostriction dition of a binary image where the pixel in the output
image is a 1 if each of its eight neighbors is a 1 in the
in conductive and ferromagnetic materials to generate input image. See also closing, dilation and opening.8

and receive acoustic signals for ultrasonic nondestruc- erosion-corrosion: Simultaneous occurrence of erosion
tive tests.7 and corrosion.8

electromagnetic testing (ET): A nondestructive test ET:Electromagnetic testing.
etch cracks: Shallow cracks in hardened steel containing
method for materials, including magnetic materials,
high residual surface stresses produced by etching in an
that uses electromagnetic energy, both alternating and acid. 2,s,19

direct current, to yield information regarding the qual- etching: A cleaning process for the controlled removal of
ity and characteristics of the tested material.s-P surface material by chemical agents before liquid pene-
trant application.s Subjecting the surface of a metal to
electrostatic spraying: A technique of spraying wherein prerveefearlesntrtiuacltucrhael mdiectaalilso.r2 electrolytic attack in order to
the material being sprayed is given a high electrical
eutectic alloy: The composition in a binary alloy system
charge (potential) while the test piece is grounded.2 that melts at a minimum temperature. More than one
eutectic composition may occur in a given alloy
element: A chemical substance that cannot be divided into
system.2
simpler substances by chemical means. Examples are eutectic liquid:A liquid having a proportion of metals such

hydrogen, lead and uranium.2 that two or more solid phases form at the same temper-

elongation: In tensile testing, the increase in the gage ature during cooling.8

length, measured after fracture of the object within the

gage length, luesnugathll.y2 expressed as a percentage of the
original gage

EMAT:See electromagnetic acoustic transducer.

embrittlement: Reduction in the normal ductility of a

metal due to a physical or chemical change.2

emissivity:Variable ratio of the total energy radiated by a

given surface at a given temperature to the total energy

radiated by a blackbody at the same temperature. Sur-
face phenomenon depending on the surface condition

and composition. Smooth materials have lower emissiv-
ities than rough or corroded materials.9

NONDESTRUCTIVE TESTING GLOSSARY I 531

eutectic point: Temperature and proportion of metals at false brinelling: Fretting wear indentations. Compare
brinelling. 8
which two or more phases of a eutectic liquid form.
Compare eutectoid. 8 false indication:A test indication that could be interpreted

eutectoid: Similar to eutectic but in a solid system during as originating from a discontinuity but which actually

cooling.8 nooringrienlaetveasnwt hinerdeicnaotiodnis. 2coCnotminpuaitrye exists.7 Distinct from
evaluation:Process of determining the magnitude and sig- defect.8

nificance of a discontinuity after the indication has been family: An obsolete term, formerly denoting a complete

interpreted as relevant. Evaluation determines if the series of materials from one manufacturer necessary to

test object should be rejected, repaired or accepted. perform a specific process of penetrant testing.2
See indication and interpretation=s.! far field: The zone beyond the near field in front of the

evanescent wave: A disappearing wave.7 transducer in which signal amplitude decreases mono-

event: A micro adciospulsatcicememenistsigoinvienvgenrits.e5 to transient elastic tonically in proportion to distance from the transducer.
waves. See Also called the Fraunhofer zone. 7

event counting:A measurement of the number of acoustic far vision: Vision of objects at a distance, generally beyond
arm's length. Compare near vision. 8
emission events. Because an event can produce more
than one burst, this term is used in its strictest sense
farsightedness:Visionacuity functionally adequate for view-
only when conditions allow the number of events to be
ing objects at a distance, generally beyond arm's length.
related to the number ofbursts.5 Alsocalled hyperopia. Compare nearsightedness. 8
event rate: The number of events detected in a specified
fatigue fracture:The progressive fracture of a material that
unit of time. Term is restricted in the same way as event
begins at a discontinuity and increases under repeated
cycles of stress. The phenomenon leading to fracture
counting.5
examination:The process of testing materials, interpreting under repeated or fluctuating stresses having a maxi-
and evaluating test indications to determine if the test mum value less than the tensile strength of the material.2
object meets specified acceptance criteria. 6
feature extraction: From an enhanced image, derivation

examination medium: A powder or suspension of mag- of some feature values, usually parameters for distin-

netic particles applied to a magnetized test surface to guishing objects in the image.8
determine the presence or absence of surface or feed-through coil: See encircling coil.
slightly subsurface discontinuities.S'"
feeder: A reservoir of molten metal connected to, but not a
excitation coil: Coil that carries the Seexecidtaettieocntocrucrroeinl.t4. Also
called primary coil or winding. part of, the casting and designed to remain liquid while

exfoliation:Corrosion that progresses approximately paral- the casting is solidifying. It is located so that it will feed
liquid metal to the larger portions of the casting that are
lel to the outer surface of the metal, causing layers of
the last to solidify. Sprues, gates, risers and runners fre-
the metal to be elevated by the formation of corrosion quently function in this manner.3

product.2 felicity effect: The appearance of significant acoustic emis-

expanded sweep: A short duration horizontal sweep posi- sion at a stress level below the previous maximum ap-
tioned to provide close examination of a particular sig- plied. 5

nal or material volume.7 felicity ratio:The measurement of the felicity effect. Ratio

external discontinuities:Discontinuities on the outside or between ( 1) the applied load or pressure at which

exposed surface of a test object.2 acoustic emission reappears during the next application
eye sensitivity curve: Graphic expression of vision sensi- of loading and (2) previous maximum applied load.5

tivity characteristics of the human eye. In the case of a ferrite: (1) Any of several magnetic substances that consist
physical photometer, the curve should be equivalent to
essentially of an iron oxide combined with one or more
the standard observer. The required match is typically
metals (manganese, nickel or zinc) having high magnetic
achieved by adding filters between the sensitive ele- permeability and high electrical resistivity.4,25 (2) Solid
ments of the meter and the light source.8
solution of one or more other elements in alpha iron.8

ferromagnetic material: Material such as iron, nickel or

cobalt whose relative permeability is considerably

F greater than unity and depends on the magnetizing

facing:Any material applied in a wet or dry condition to the mfoarcgen.e4·t1i4smMaarteerciaallsledthfaetrraormeamgnoesttics.t2rongly affected by
face of a mold or core to improve the surface of the
casting.3 fiber optic borescope: See borescope, fiber optic.

fiber optics: The technology of light transmission through

crystalline fibers such as plastic, glass or quartz.8

fiberscope: Jargon forfiber optic borescope. 8

532 I NONDESTRUCTIVE TESTING OVERVIEW

field: In video technology, one of two video picture compo- fine crack: A discontinuity in a solid material with a very
nents that together make a frame. Each picture is fine opening to the surface, but possessing length and
divided into two parts called fields because a frame at depth greater than the width of this opening. Usually
the rate of thirty frames per second in a standard video the depth is many times the width.2
output would otherwise produce a flicker discernible to
the eye. Each field contains one half of the total picture finite element analysis: A numerical technique for the
elements. Two fields are required to produce one com- analysis of a continuous system whereby that system is
plete picture or frame so the field frequency is sixty decomposed into a collection of finite sized elements.4
fields per second and the frame frequency is thirty
frames per second.8 fit up: To secure one or more joint members with special
external fixturing to prevent movement during weld-
field angle: The included angle between those points on ing.8,19
opposite sides of the beam axis at which the luminous
intensity from a theatrical luminaire is 10 percent of the flakes: Short discontinuous internal fissures in ferrous met-
maximum value. This angle may be determined from an als attributed to stresses produced by localized transfor-
illuminance curve or may be approximated by use of an mation ancl/or decreased solubility of hydrogen during
incident light meter. 8,20 cooling usually after hot working. On a fractured sur-
face, flakes appear as bright silvery areas; on an etched
field flow technique: See magnetic flow technique. 6 surface they appear as short, discontinuous cracks.8·19
Also called shatter cracks and snowflakes. 2
field of view: The range or area where things can be seen
through an imaging system, lens or aperture. Compare flash magnetization: Magnetization by a current flow of
depth offield. 8 brief duration. See capacitor discharge technique. 6·16

field of vision:The range or area where things can be per- flash point: The lowest temperature at which vapors above
ceived organoleptically at a point in time, assuming the a volatile, combustible substance ignite in air when
eye to be immobile." exposed to flame.6•16

fill factor: For encircling coil electromagnetic testing, the flask:A metal or wood frame for making and holding a sand
ratio of the cross sectional area of the test object to the mold. The upper part is called the cope and the lower
effective cross sectional core area of the primary encir- part is called the drag.3
cling coil (outside diameter of coil form, not inside
diameter that is adjacent to the object).4·6,13,15 For inter- flat bottom hole: A type of reflector commonly used in
nal probe electromagnetic testing, the ratio of the reference standards. The end (bottom) surface of the
effective cross sectional area of the primary internal hole is the reflector.7
probe coil to the cross sectional area of the tube inte-
rior.4.13 flaw: A rejectable anomaly or unintentional discontinuity.
See also defect and discontinuity. 2
fill factor effect: The effect of fill factor on coupling
between coil and test object. See coupling coefficient. 4 flaw inversion:A method for measuring some dimensions
of a discontinuity by the application of a mathematical
filled crack:A cracklike discontinuity, open to the surface, algorithm to the discontinuity signal.4
but filled with some foreign material, such as oxide,
grease, etc., that tends to prevent penetrants from flaw location scale: A specially graduated ruler that can be
entering.2 attached to an angle beam transducer to relate the posi-
tion of an indication on the cathode ray tube screen to
fillet weld: Weld at the corner of two metal pieces.8 the actual location of a discontinuity within the test
object.7
film badge: A package of photographic film worn as a
badge by radiography personnel (and by workers in the fluidity:The ability of molten metal to flow readily.Typically
nuclear industry) to measure exposure to ionizing radi- measured by the length of a standard spiral casting.3
ation. Absorbed dose can be calculated by degree of
film darkening caused by irradiation.'! fluorescence: The emission of visible light from a material
in response to ultraviolet or X-radiation. Formerly
film holder: A light tight carrier for films and screens.'! called cold light.8

film speed: Relative exposure required to attain a specified fluorescent penetrant: A highly penetrating liquid used
density.!' in the performance of liquid penetrant testing and
characterized by its ability to fluoresce under ultravio-
filter: ( 1) A network that passes · electromagnetic wave let light.2
energy over a described range of frequencies and atten-
uates energy at all other frequencies.s-P (2) A process- fluorescent magnetic particle testing:The process using
ing component or function that excludes a selected kind finely divided ferromagnetic particles that fluoresce
of signal or part of a signal.8 when exposed to ultraviolet light (320 to 400 nm).6·17

filtering: See low pass filtering. fluorescent penetrant testing: Technique of liquid pene-
trant testing that uses fluorescent penetrant.

flux density: See magnetic flux density.