ASNT NDT Handbook Volume 9 visual Testing

at 50× can be resolved but difficulties are Wear can be determined in a similar
encountered at higher magnifications. manner by replicating and comparing a
With a low viscosity compound, slightly worn surface to an unworn surface
better resolution is obtained but curing (Fig. 8). Fracture surfaces with rough
times are long and not suited to field contours can be easily replicated with
applications. The lower viscosity medium silicone (taking an acetate replica of such
is also known to creep with time and is surfaces is difficult). However, the
not recommended for applications where resolution characteristics of a silicone
very accurate dimensional studies are replica are not as good as acetate replicas
needed. and this limits the amount of
interpretation that can be performed.
Silicone Replicating Materials Macroscopic details such as chevron
markings can be easily located with the
Silicone replicating materials are supplied silicone technique to determine crack
in two parts: a base material and an propagation direction or to trace a
accelerator. Although it is best to follow fracture path visually to its origin.
the recommended mixing ratios, these
can be altered slightly to change the Conclusion
working time of the material. The two
parts are mixed thoroughly and spread Cellulose acetate tape and silicone
over the subject area. Additional material impression materials are commonly used
can be added to thicken the replica. for nondestructive visual tests of surface
Molding clay can also be used to build a phenomena such as corrosion, wear,
dam around a replicated area. The dam cracking and microstructures.8 Both types
supports the replica as its sets and allows of replicating material have advantages
thicker replicas to be made. and limitations but when used in the
correct application, can provide valuable
Measurements of pit depth and surface information.
finish can be obtained easily because of
the silicone’s ability to flow into crevices In terms of resolution, the silicone
on the test object. To evaluate pit depth replica typically does not have the
and surface finish, the replica is cut and capability to copy fine detail above 50×.
the cross section is examined with a
microscope or a macroscopic measuring FIGURE 8. Silicon replicas used to determine wear variance
device (a micrometer or an optical on failed pinion gear.
comparator).

FIGURE 7. Creep damage curve showing
typical relationship of strain to time for
material under stress in high temperature
atmosphere. Development of creep related
voids in this alloy occurs early in service life
— their eventual linkage is shown
schematically on curve.10

Fracture

Creep strain D

C
B
A

Exposure time

Legend
A. Isolated cavities.
B. Oriented cavities.
C. Microcracks.
D. Macrocracks.

Techniques Allied to Visual Testing 295

The acetate replica can reveal detail up to The silicone material is not as restrictive
50 000× on a transmission electron in terms of the surface features it can
microscope. The acetate replica is limited, copy. The need for fine, resolvable detail
however, by the roughness of the versus macroscopic features normally
topography it can copy. On rough fracture indicates whether acetate or silicone
surfaces, difficulty is encountered in both replicas are best for the application.
applying and removing an acetate replica.

296 Visual Testing

PART 3. Etching12

The information contained in this text is and location of visual tests. Specific areas
simplified for general instruction. Local may contain discontinuities from forming
authorities for health and environment operations such as casting, rolling, forging
(including the Occupational Safety and or extruding. Weld tests may be full
Health Administration and Environmental length or random spots and typically
Protection Agency) should be consulted cover the weld metal, fusion line and
about the proper use and disposal of heat-affected zone. The service of a
chemical agents. For reasons of safety, all component may also indicate problem
chemicals must be handled with care, areas requiring inspection.
particularly the concentrated chemicals
that aid visual testing. Location of the test site directly affects
surface preparation. The test site may be
In visual nondestructive testing, prepared and nondestructively inspected
chemical techniques are used to clean and in situ. Removal of a sample for laboratory
enhance test object surfaces. Cleaning examination is a destructive alternative
processes remove dirt, grease, oil, rust and test method that typically requires a
mill scale. Contrast is enhanced by repair weld.
chemical etching before visual testing.
Etching is described by standards13-15 and Surface Preparation
is well documented in the literature.16,17
Preparation of the test object before
Macroetching is the use of chemical etching may require only cleaning or a
solutions to attack material surfaces to process including cleaning, grinding and
improve the visibility of discontinuities fine polishing (improper grinding is
for visual inspection at normal and low shown in Fig. 10). The extent of these
power magnifications. Caution is required operations depends on the etchant, the
in the use of these chemicals — the use of material and the type of discontinuity
protective clothing and safety devices is being sought.
imperative. Test object preparation and
the choice of etchant must be appropriate Solvent Cleaning
for the inspection objectives. Once the
desired etch is achieved, the metal surface Solvent cleaning can be useful at two
must be flushed with water to avoid stages in test object preparation. An initial
excessive etching. cleaning with a suitable solvent removes
dirt, grease and oil and may make rust
Test Object Selection and mill scale easier to remove.

Figure 9 shows typical test objects One of the most effective cleaning
removed from their service environment. solvents is a solution of detergent and
Governing codes, standards or water. However, if water is detrimental to
specifications may determine the number the test object, organic solvents such as
ethyl alcohol, acetone or naphthas have
FIGURE 9. Components removed from service for visual been used. These materials generally have
testing. low flash points and their use may be

FIGURE 10. Improper surface preparation. Grind marks mask
indications, and even severe etchant does not give good test
results.

Techniques Allied to Visual Testing 297

prohibited by safety regulations. Safety Etching
solvents such as the chlorinated
hydrocarbons and high flash point Choice of Etchant
naphthas may be required to meet safety
standards. The etchant, its strength, the material and
the discontinuity all combine to
Removing Rust and Scale determine surface finish requirements
(Table 2). Properly selected etchants
Rust and mill scale are normally removed chemically attack the test material and
by mechanical methods such as wire reveal welds (Fig. 11), pitting (Fig. 12),
brushing or grinding. If appropriate for a grain boundaries, segregation, laps, seams,
particular test, the use of a severe etchant cracks and heat-affected zones. The
requires only the removal of loose rust indications are highlighted or contrasted
and mill scale. Rust may also be removed with the surrounding base material.
chemically. Commercially available rust
removers are generally inhibited mineral Safety Precautions
acid solutions and are not often used for
test object preparation. Etchants are solutions of acids, bases or
salts in water or alcohol. Etchants for
Most surface tests require complete macroetching are water based. Etching
removal of rust and mill scale but a solutions need to be fresh and the
coarsely ground surface is often adequate primary concerns during mixing are
preparation before etching. purity, concentration and safety.

Grinding may be done manually or by TABLE 2. Chemicals for etchants.
belt, disk or surface grinding tools. Surface
grinders are usually found only in Formula Name Concentration
machine shops. Hand grinding requires a
hard flat surface to support the abrasive Acid hydrochloric (muriatic) 37 percent
sheet. Coolant is needed during grinding hydrofluoric 48 percent
and water is the preferred coolant but HCl nitric 70 percent
kerosene may be used if the test material phosphoric
is not compatible with water. HF sulfuric 30 percent

Grinding and Polishing HNO3 ethanol
H3PO4 methanol
Fine grinding and polishing are needed H2SO4
for visual tests of small structural details, Alcohol hydrogen peroxide
welds and the effects of heat treatment.
Finer grinding usually is done with 80 to C2H5OH ammonium hydroxide
150 abrasive grit followed by 150 to CH3OH sodium hydroxide
180 grit and finally 400 grit (in a gaging Aqueous
of grit size in the United States, 400 being iodine
the finest). At each stage, marks from H2O2
previous grinding must be completely Base ammonium persulfate
removed. Changing the grinding direction ammonium sulfate
between successive stages of the process NH4OH copper ammonium chloride
aids the visibility of previous coarser NaOH cupric chloride
grinding marks. Coolant is required for ferric chloride
grinding and typical abrasives include Element potassium iodide
emery, silicon carbide, aluminum oxide
and diamond. I2
Salt
If the required finish cannot be
achieved by fine grinding with 400 grit (NH4)2S2O8
abrasive, the test surface must be (NH4)2SO4
polished. Polishing is generally done with 2NH4Cl·CuCl2·2H2O
a cloth-covered disk and abrasive particles CuCl2
suspended in paste or water. Common FeCl3
polishing media include aluminum oxide, KI
magnesium oxide, chromium oxide, iron
oxide and diamond with particle sizes FIGURE 11. Example of contrast revealing weld in stainless
ranging from 0.5 to 15 mm (0.02 to steel.
0.6 in.).

During polishing, it is critical that all
marks from the previous step be
completely removed. If coarser marks do
not clear, it may be necessary to repeat a
previous step using lighter pressure before
continuing. Failure to do so can yield false
indications.

298 Visual Testing

Safety precautions are necessary during Other materials that are especially
the mixing and use of chemical harmful in contact with skin are
etchants.18 Chemical fumes are concentrated nitric acid, sulfuric acid,
potentially toxic and corrosive. Mixing, chromic acid, 30 and 50 percent hydrogen
handling or using etchants should be peroxide, sodium hydroxide, potassium
done only in well ventilated areas, hydroxide, bromine and anhydrous
preferably in an exhaust or fume hood. aluminum chloride. These materials also
Use of an exhaust hood is mandatory produce vapors that cause respiratory
when mixing large quantities of etchants. irritation and damage.
Etching large areas requires the use of
ventilation fans in an open area or use of Containers
an exhaust hood.
Containers used with etchants must be
Contact of etchants with skin, eyes or rated for mixing, storing and handling of
clothes should be avoided. When pouring, chemicals. Glass is resistant to most
mixing or handling such chemicals, chemicals and is most often used for
protective equipment and clothing should containment and stirring rods.
be used, including but not limited to Hydrofluoric acid, other fluorine based
glasses, face shields, gloves, apron or materials, strong alkali and strong
laboratory jacket. A face-and-eye wash phosphoric acids can attack glass,
fountain is recommended where requiring the use of inert plastics.
chemicals and etchants are sorted and
handled. A safety shower is recommended Generation of Heat
when large quantities of chemicals or
etchants are in use. Heat may be generated when chemicals
are mixed together or added to water.
Should contact occur, certain safety Mixing chemicals must be done using
steps must be followed, depending on the accepted laboratory procedures and
kind of contact and the chemicals caution. Strong acids, alkalis or their
involved. Skin should be washed with concentrated solutions incorrectly added
soap and water. Chemical burns should to water, alcohols or other solutions,
have immediate medical attention. Eyes cause violent chemical reactions. To be
should be flushed at once with large safe, never add water to concentrated
amounts of water and immediate medical acids or alkalis.
attention is mandatory. Hydrofluoric and
fluorosilic acids cause painful burns and In general, the addition of acidic
serious ulcers that are slow to heal. materials to alkaline materials will
Immediately after exposure, the affected generate heat. Sulfuric acid, sodium
area must be flooded with water and hydroxide or potassium hydroxide in any
emergency medical attention sought. concentration generate large amounts of
heat when mixed or diluted and an ice
FIGURE 12. Effect of etching: (a) unetched component with bath may be necessary to provide cooling.
shiny appearance at rolled area; (b) pits visible in dulled area Three precautions in mixing can reduce or
after etching with ammonium persulfate. prevent a violent reaction.

(a) 1. Add the acid or alkali to the water or
to a weaker solution.
(b)
2. Slowly introduce acids, alkali or salts
to the water or the solution.

3. Stir the solution continuously to
prevent layering and a delayed, violent
reaction.

Concentration

The strength of an acid, base or salt in
solution is expressed by its concentration
or composition. Etchants are typically
mixtures of liquids or solids in liquids.
The concentration of liquid mixtures is
expressed as parts or percent by volume.

For solids in liquids, units of
concentration are parts or percent by
weight. Generally, etchants are mixed in
small quantities. Table 2 lists a variety of
etchants.

Techniques Allied to Visual Testing 299

Chemical Purity Swabbing

Chemicals are available in various grades Etching may also be done by swabbing
of purity ranging from technical to very the test surface with a cotton ball, cotton
pure reagent grades. For etchants, the tipped wooden swab, bristled acid brush,
technical grade is used unless a purer medicine dropper or a glass rod. The
grade is specified. For macroetchants, the cotton ball and the cotton tipped wooden
technical grade is generally adequate. swab generally are saturated with etchant
and then rubbed over the test surface.
Water is the solvent used for most
macroetching solutions and water purity Tongs and gloves should be used for
can affect the etchant. Potable tap water protection and the etchant applicator
may contain some impurities that could must be inert to the etchant. For example,
affect the etchant. Distilled water has a strong nitric acid and alkali solutions
significantly higher purity than tap water. attack cotton and these etchants must be
For macroetchants using technical grade applied using a fine bristle acid brush. A
chemicals, potable tap water is usually glass or plastic medicine dropper may be
acceptable. For etchants in which high used to place etchants on the test object
purity is required, distilled water is surface and a suitable stirring rod can be
recommended. used to rub the surface. The test object
may be immersed in etchant and swabbed
Disposal while in the solution.

Before disposing of chemical solutions, Etching Time
check environmental regulations (federal,
state and local) and safety department Etching time is determined by (1) the
procedures. The steps listed here are used concentration of the etchant, (2) the
only if there are no other regulations for surface condition and temperature of the
disposal. Spent etchants are discarded and test object and (3) the type of test
must be discarded separately — mixing of material. Details about the chemicals and
etchant materials can produce violent their concentrations, handling and
chemical reactions. applications are available in the
literature.12-17 During etching, the
Using a chemical resistant drain under material surface loses its bright
an exhaust hood, slowly pour the spent appearance and the degree of dullness is
etchant while running a heavy flow of tap used to determine when to stop etching.
water down the drain. The drain is Approximate dwell times are given in the
flushed with a large volume of water. procedures but experience is important as
well.
Using Etchants
Test Object Preservation
After proper surface preparation and safe
mixing of etchants, the application of Cleaning and coating may be required for
etchants to the test object may be done preservation of the test object. Rinsing
with immersion or swabbing. The removes the etchant by flushing the
technique is determined by the surface thoroughly under running water.
characteristics of the etchant being used. Cold water rinsing usually produces better
surface appearance than hot water rinsing.
Immersion Hot water rinsing does aid in drying.

During immersion, a test object is If adherents are a problem, the test
completely covered by an etchant object can be scrubbed with a stiff bristled
contained in a safe and suitable material brush or dipped in a suitable desmutting
— glass can be used for most etchants solution. The test object should be dried
except hydrofluoric acid, fluorine with warm dry air. Shop air may be used
materials, strong alkali and strong if it is filtered and dried. After visual
phosphoric acid. inspection, the test surface may be coated
with a clear acrylic or lacquer but such
A glass heat resistant dish on a hot coatings must be removed before
plate may be used for heated solutions. subsequent tests. If the component is
The solution should be brought to returned to service, a photographic record
temperature before the test object is of the macroetched area should be made.
immersed. Tongs or other handling tools
are used and the test object is positioned Closing
so that the test surface is face up or
vertical to allow gas to escape. The Visual testing is performed in accordance
solution is gently agitated to keep fresh with applicable codes, standards,
etchant in contact with the test object. specifications and procedures. Chemical
aids enhance the contrast of

300 Visual Testing

discontinuities making them easier to results on a properly cleaned and prepared
interpret and evaluate. This enhancement
is attained by macroetching — a surface. Chemicals for etching must be
controlled chemical processing of the
surface. Macroetching gives the optimum mixed, stored, handled and applied in
strict accordance with safety regulations.18

Techniques Allied to Visual Testing 301

References

1. Generazio, E.R. and D.J. Roth. 10. Neubauer, B. and U. Wedel. “Restlife
Section 1, “Other Applications of Estimation of Creeping Components
Visual and Optical Tests”: Part 1, by Means of Replicas.” Advances in Life
“Interface of Visual Testing with Other Prediction. D.A. Woodford and J.R.
Nondestructive Testing Methods.” Whitehead, eds. New York, NY:
Nondestructive Testing Handbook, American Society of Mechanical
second edition: Vol. 8, Visual and Engineers (1983): p 307-314.
Optical Testing. Columbus, OH:
American Society for Nondestructive 11. ASME FFS/API 579, Fitness-for-Service.
Testing (1993): p 304-312. New York, NY: ASME International
(2008). Dallas, TX: American
2. Generazio, E.R. “The Role of the Petroleum Institute (2000).
Reflection Coefficient in Precision
Measurement of Ultrasonic 12. Parrish, D. Section 4, “Basic Aids and
Attenuation.” Materials Evaluation. Accessories for Visual Testing”: Part 7,
Vol. 43, No. 8. Columbus, OH: “Chemical Aids.” Nondestructive Testing
American Society for Nondestructive Handbook, second edition: Vol. 8,
Testing (July 1985): p 995-1004. Visual and Optical Testing. Columbus,
OH: American Society for
3. Truell, R., C. Elbaum and B.B. Chick. Nondestructive Testing (1993):
Ultrasonic Methods in Solid State Physics. p 118-124.
New York, NY: Academic Press (1969).
13. ASTM E 381, Standard Method of
4. Generazio, E.R. “Ultrasonic Macroetch Testing Steel Bars, Billets,
Attenuation Measurements Determine Blooms, and Forgings. West
Onset, Degree and Completion of Conshohocken, PA:
Recrystallization.” Materials Evaluation. ASTM International (2006).
Vol. 46, No. 9. Columbus, OH:
American Society for Nondestructive 14. ASTM E 407, Standard Practice for
Testing (August 1988): p 1198-1203. Microetching Metals and Alloys.
West Conshohocken, PA:
5. Generazio, E.R. “Scaling Attenuation ASTM International (2007).
Data Characterizes Changes in
Material Microstructure.” Materials 15. QW 470, “Etching — Processes and
Evaluation. Vol. 44, No. 2. Columbus, Reagents.” ASME Boiler and Pressure
OH: American Society for Vessel Code: Section IX, Welding and
Nondestructive Testing Brazing Qualifications. New York, NY:
(February 1986): p 198-202. American Society of Mechanical
Engineers (2008).
6. Meiley, S. Section 4, “Basic Aids and
Accessories for Visual Testing”: Part 5, 16. CRC Handbook of Metal Etchants.
“Replication.” Nondestructive Testing Baton Rouge, LA: CRC Press (1990).
Handbook, second edition: Vol. 8,
Visual and Optical Testing. Columbus, 17. Microengineering of Metals and Ceramics:
OH: American Society for Part 2: Special Replication Techniques,
Nondestructive Testing (1993): Automation, and Properties. Baton
p 108-113. Rouge, LA: Wiley-VHC (2005).

7. ASTM E 1351, Standard Practice for 18. Lewis, R.J. Sax’s Dangerous Properties of
Production and Evaluation of Field Industrial Materials, tenth edition.
Metallographic Replicas. Philadelphia, Three volumes. New York, NY:
PA: ASTM International (2001). Wiley-Interscience (2000).

8. ASM Handbook: Vol. 9, Metallography
and Microstructures. Materials Park, OH:
ASM International (1985, 2004).

9. Marder, A.R. “Replication Microscopy
Techniques for NDE.” Metals
Handbook, ninth edition: Vol. 17,
Nondestructive Evaluation and Quality
Control. Materials Park, OH:
ASM International (1985): p 52-56.

302 Visual Testing

13

CHAPTER

Visual Testing Glossary

Introduction

Purpose

Standards writing bodies take great pains to ensure that
their standards are definitive in wording and technical
accuracy. People working to written contracts or
procedures should consult definitions referenced in
standards when appropriate. For example, persons who
work in accordance with standards published by ASTM
International are encouraged to refer to definitions in the
ASTM standards.1

The definitions in this Nondestructive Testing Handbook
volume should not be referenced for tests performed
according to standards or specifications or in fulfillment
of contracts. This glossary is provided for instructional
purposes. No other use is intended.

On References

Many definitions in this glossary are adapted from other
volumes of the Nondestructive Testing Handbook series,
especially from the second edition’s Visual and Optical
Testing (1993).2

Some terms apply generally to nondestructive testing
and are not specific to visual testing — terms on subjects
such as metallurgy, quality control and personnel
qualification. Many of these definitions come from the
second edition volume Nondestructive Testing Overview
(1996)3; some are rephrased in the third edition’s most
recent volume, Magnetic Testing (2008).4

Entries from other volumes in the Nondestructive
Testing Handbook series are reprinted but generally not
referenced below.

Definitions

A ASNT Recommended Practice
No. SNT-TC-1A: See Recommended
acceptable quality level (AQL): Practice No. SNT-TC-1A.
Maximum percent defective (or the
maximum percentage of units with ASNT: American Society for
rejectable discontinuities) that, for the Nondestructive Testing.
purposes of sampling tests, can be
considered satisfactory as a process automated system: Acting mechanism
average. that performs required tasks at a
determined time and in a fixed
acceptance criterion: Benchmark against sequence in response to certain
which test results are to be compared conditions or commands.
for purposes of establishing the
functional acceptability of a part or axial: Of or pertaining to a direction
system being examined.4 along the length of an oblong object
and perpendicular to its radius — for
acceptance level: Measured value or example, down the length of a
values above or below which test cylinder. Compare radial.
objects are acceptable, in contrast to
rejection level.4 B

acceptance standard: (1) Specimen, background cylinder and difference
similar to the product to be tested, cylinder: Two devices used to
containing natural or artificial calculate illuminance by using the
discontinuities that are well defined equivalent sphere illumination
and similar in size or extent to the technique.2,5
maximum acceptable in the product.4
(2) Document defining acceptable binary system: In metallurgy, a
discontinuity size limits. See also two-element alloy system. See also
standard. isomorphous binary system.

accommodation: Of the eye, adjustment birefringence: Splitting of a light beam
of the lens’ focusing power by into two parts through a translucent
changing the thickness and curvature material.
of the lens through its movement by
tiny muscles. black body: Theoretical object that
radiates more total power and more
ACGIH: American Conference of power at any wavelength than any
Governmental Industrial Hygienists. other source operating at the same
temperature.2,5
acuity: See neural acuity, vision acuity.
adaptive thresholding: Threshold value blackbody: See black body.
black light: Term sometimes used for
varying with inconstant background
gray level. ultraviolet radiation, particularly in
adhesive wear: See wear, adhesive. the near ultraviolet range of about
alpha ferrite: Form of pure iron that has 320 to 400 nm.
a body centered cubic structure stable blacklight: See black light.
below 910 °C (1670 °F). Also called blind spot: Portion of the retina where
alpha iron. the optic nerve enters, without rods
alpha iron: See alpha ferrite. and cones and hence insensitive to
ambient light: Light in the environment light.2,6
as opposed to illumination provided blister: Discontinuity in metal, on or near
by a visual testing system. the surface, resulting from the
ampere (A): SI unit of electric current.4 expansion of gas in a subsurface zone.
angle: See field angle. Very small blisters are called pinheads
angstrom (Å): Disused unit of length. 1 Å or pepper blisters.4
= 0.1 nm. blotch: (1) An irregularly spaced area of
anomaly: (1) In nondestructive testing, a color change on a surface. (2) The
nonrelevant indication. (2) In nonuniform condition of a surface
nondestructive testing, an characterized by such blotches.
unintentional or undesired material blowhole: Hole in a casting or a weld
condition that may qualify as a defect. caused by gas entrapped during
Compare defect; discontinuity. Some solidification.4
anomalies, such as inadequate case blue light hazard: Danger posed to the
hardening or rough surface finish, eye by long term exposure to high
may be defects but, because there is no frequency visible light at intensities
interruption in the material structure, and durations that may damage the
are not discontinuities. retina.

304 Visual Testing

borescope: Industrial endoscope; a borescope, video: Borescope transmitting
periscope or telescope using mirrors, image electronically.
prisms, lenses, optic fibers or
television wiring to transmit images borescope, waterproof/vaporproof:
from inaccessible interiors for visual Borescope completely sealed and
testing. Borescopes are so called impervious to water or other types of
because they were originally used in fluid, used for internal tests of liquid,
machined apertures and holes such as gas or vapor environments.
gun bores. There are both flexible and
rigid borescopes. borescope, wide field: Borescope with
rotating objective prism to provide
borescope, angulated: Borescope bent for fields of view up to 120 degrees.
viewing at forward oblique, right angle
or retrospective angles for visual borescopy: Viewing or inspection with a
testing of surfaces not accessible with borescope.
conventional borescopes.
brinelling: Repeated stripe indentations
borescope, calibrated: Borescope with made by a spherical object. False
gage on external tube to indicate the brinelling refers to a type of surface
depth of insertion during a test. wear.
Borescopes with calibrated reticles are
used to determine angles or sizes of burr: Raised or turned over edge occurring
objects in the field when held at a on a machined part and resulting from
predetermined working distance. cutting, punching or grinding.2,5

borescope, cave: Multiangulated, burst: In metal, external or internal
periscopic borescope used for remote rupture caused by entrapped gas.
observation of otherwise inaccessible
areas. butt weld: Weld that joins the edges of
two work pieces in the same plane.
borescope, fiber optic: Industrial
endoscope, or fiber optic borescope, C
that uses glass or quartz fibers to
transmit light and the optical path to calibration: (1) Ratio of the output from a
and from the test object. device to a reference input. Knowledge
of this ratio helps to infer a device’s
borescope, indexing: Borescope that can input from its output. (2) Act of
be bent 90 degrees by rotation of a returning an instrument to the
knob after the instrument has been parameters and settings of the original
inserted through an aperture. A knob equipment manufacturer.
at the eyepiece can rotate the objective (3) Statement of the scale of a device.
head through 360 degrees for
scanning a circumferential weld seam. candela (cd): Base SI unit of luminous
intensity, in a given direction, of a
borescope, micro-: Borescope with an monochromatic radiation source that
outside diameter generally from 1 to has a frequency of 5.4 × 1014 Hz and
5 mm (0.04 to 0.2 in.), typically using that has a radiant intensity in that
quartz filaments. Compare miniature direction of 1.464 mW·sr–1.4
borescope.
candle: Former name for candela.
borescope, miniature: Borescope with an cavitation fatigue: Form of pitting,
outside diameter generally less than
13 mm (0.5 in.). Sometimes called caused by erosion from vibration and
miniborescope. See also microborescope. movement in liquid environments.
CCD: See charge coupled device.
borescope, panoramic: Borescope with a cementite: Iron carbide (Fe3C), a hard
scanning mirror mounted in front of and brittle substance present in steels.
the objective lens system. Rotation of certification: With respect to
the mirror is adjusted at the ocular nondestructive test personnel, the
end of the instrument to scan in process of providing written testimony
forward oblique, right angle and that an individual has met the
retrospective directions. qualification requirements of a specific
practice or standard. See also certified
borescope, retrospective: Borescope that and qualified.
looks backward more than 90 degrees certified: With respect to nondestructive
from the distal line of interrogation test personnel, having written
normal to the plane of a conventional testimony of qualification. See also
objective lens. certification and qualification.
CGS system: Obsolete system of
borescope, rigid: Borescope that does not measurement units based on the
bend, typically in order to keep the centimeter, gram and second.
geometrical optics in alignment Compare SI.
through a light train system. chafing: See wear, fretting.
channels: In biology, mechanisms
borescope, ultraviolet: Borescope functioning as band pass filters in the
equipped with ultraviolet lamps, filters visual cortex of mammals, causing
and special transformers to transmit sensitivity to visual stimuli in
radiation of ultraviolet wavelengths. particular frequencies and ranges.

Visual Testing Glossary 305

charge coupled device (CCD): Solid state corrosion, crevice: Corrosion found in
image sensor. Charge coupled devices tight crevices or pores (pits) and
are widely used in inspection systems accelerated by galvanic activity from
because of their accuracy, high speed high ion concentrations.
scanning and long service life.
corrosion-erosion: Simultaneous
check cracking: Surface crack caused by occurrence of erosion and corrosion.
overheating and having cross hatched
pattern. See grinding crack. corrosion, fretting: Corrosion facilitated
by fretting, particularly where a
closing: In image processing, dilation protective surface has been chafed in a
followed by erosion. A single pixel by corrosive environment.
closing connects a broken feature
separated by one pixel. See also corrosion, poultice: Corrosion occurring
opening. under a layer of foreign material (for
example, under mud in automobile
closure: Process by which a person rocker panels).
cognitively completes patterns or
shapes that are incompletely cosine law: Physical law stating that the
perceived. illumination of a surface varies as the
cosine of the incidence angle.
cocoa: Debris (usually oxides of the Maximum illumination is obtained
contacting metals) of fretting wear, where the cosine equals one and when
retained at or near the site of its the source is perpendicular to the
formation — a condition easily surface.
identified during visual tests. With
ferrous metals, the debris is brown, red crack: (1) Stress induced break, fissure or
or black, depending on the type of rupture, sometimes V shaped in cross
iron oxide formed. For this reason, section and relatively narrow. By
ferrous debris is called cocoa or, when convention, a crack is called linear if it
mixed with oil or grease, red mud. is at least three times longer than it is
wide. (2) Propagating discontinuity
code: Standard enacted or enforced as a caused by fatigue, corrosion or stresses
law. such as heat treating or grinding. May
be difficult to detect unaided because
coefficients of the filter: Values in a of fineness of line and pattern (may
mask that serves as a filter in image have a radial or latticed appearance).
processing.
creep: Gradual and permanent change of
cold light: Disused word for fluorescence. shape in a metal under constant load,
color: Visual sensation by means of which usually at elevated temperature.
Occurs in three stages: primary creep,
humans distinguish light of differing secondary creep and tertiary creep. See
hue (predominant wavelengths), also deformation.
saturation (degree to which those
radiations predominate over others) crevice corrosion: See corrosion, crevice.
and lightness.
color blindness: Deficiency in ability to D
perceive or distinguish hues.
color discrimination: Perception of dark adaptation: Process by which the
differences between two or more hues. retina becomes adapted to a
color temperature: Rating of a light luminance less than about
source for color vision. 0.034 cd·m–2.2,6 In dark adaptation,
compound microscope: See microscope, the pupils dilate and the two types of
compound. photoreceptors in the retina change
cone: Part of the eye; color sensitive chemical balance. After a finite
photoreceptor at the inner region of amount of time, possibly 10 min,
the retina. Cones assist with mesopic vision will change from photopic
vision and are responsible for vision to mesopic or scotopic low
photopic vision. Compare rod. illumination vision.4
confidence level: Level of assurance for
detecting a specified discontinuity size dark adapted vision: See accommodation,
with a specified probability. See also visual; scotopic vision.
probability of detection.
constitution diagram: See phase diagram. defect: Discontinuity whose size, shape,
contrast: (1) Difference in color or orientation or location (1) makes it
brightness between a test indication detrimental to the useful service of its
and background. (2) Difference host object or (2) exceeds an
between the amount of light reflected accept/reject criterion of an applicable
or transmitted by an object and by the specification. Some discontinuities do
background in the field of view. not exceed an accept/reject criterion
control: See in control, process control and and are therefore not defects.
quality control. Compare crack; discontinuity; indication.
corrosion: Loss or degradation of metal
because of chemical reaction. deformation: Change of shape under
load. See also creep and elastic
deformation.

306 Visual Testing

delta ferrite: Solid solution with body discontinuity, artificial: Reference
centered cubic structure and iron as discontinuity such as hole,
solvent. Also called delta iron. indentation, crack, groove or notch
introduced into a reference standard
delta iron: See delta ferrite. to provide accurately reproducible
depth of field: In photography, the range indications for determining test
sensitivity levels.4
of distance over which an imaging
system gives satisfactory definition discontinuity, inherent: Material
when its lens is in the best focus for a anomaly originating from
specific distance. solidification of cast metal. Pipe and
dewetting: In soldering, the flow and nonmetallic inclusions are the most
retraction of solder, caused by common inherent discontinuity and
contaminated surfaces, dissolved can lead to other types of
surface coatings or overheating before discontinuities in fabrication.2,5
soldering.
difference cylinder: See background discontinuity, primary processing:
cylinder. Discontinuity produced from the hot
dilation: In image processing, the or cold working of an ingot into
condition of a binary image where the forgings, rods, bars and other
pixel in the output image is a 1 if any shapes.2,5
of its eight closest neighbors is a 1 in
the input image. See also closing, discontinuity, secondary processing:
erosion and opening. Discontinuity produced during
diopter: Unit used to express the machining, grinding, heat treating,
resolving power of lenses, equal to the plating or other finishing
inverse of the length (in meters) of the operations.2,5
optical axis.
directional lighting: Lighting provided discontinuity, service induced:
on the work plane or object Discontinuity caused by the intended
predominantly from a preferred use of the part.
direction.2,6
direct photometry: Simultaneous distal: In a manipulative or interrogating
comparison of a standard lamp and an system, of or pertaining to the end
unknown light source.2,6 opposite from the eyepiece and
direct substitution alloy: Alloy in which farthest from the person using the
the atoms of the alloying element can system. Objective; tip.
occupy the crystal lattice spaces
normally occupied by the atoms of the E
parent metal.
direct viewing: (1) Viewing of a test elastic deformation: Temporary change
object in the viewer’s immediate in shape under a load. The material
presence. The term direct viewing is returns to its original size and shape
used in the fields of robotics and after the load is removed. Elastic
surveillance to distinguish deformation is the state in which most
conventional from remote viewing. metal components are used in service.
(2) Viewing of a test object during
which the light image is not mediated elasticity: Ability of a material to resume
through a system of two or more its former shape after deformation.
lenses (as in a borescope) or
transduced through an electronic electric arc welding: Joining of metals by
signal (as with a charge coupled heating with electric arc. Also called
camera). The term direct viewing is used arc welding.
in some specifications to mean
viewing possibly with a mirror or endoscope: Device for viewing the
magnifier but not with a borescope. interior of objects. From the Greek
Compare indirect viewing; remote words for inside view, the term
viewing. endoscope is used mainly for medical
direct vision instrument: Device offering instruments. Nearly every medical
a view directly forward. A typical endoscope has an integral light source;
scene is about 20 mm (0.75 in.) wide many incorporate surgical tweezers or
at 25 mm (1 in.) from the objective other devices. See borescope.
lens.
discontinuity: Interruption in the equilibrium diagram: Phase diagram
physical structure or configuration of a showing the phases present at
test object. After nondestructive equilibrium in a material system.
testing, a discontinuity indication can
be interpreted to be a defect.4,7 equivalent 20/20 near vision acuity:
Compare defect; indication. Vision acuity with remote viewing or
other indirect viewing that
approximates 20/20 direct viewing
closely enough to be considered the
same for visual testing purposes.

equivalent sphere illumination: Level of
perfectly diffuse (spherical)
illuminance that makes the visual task
as photometrically visible within a
comparison test sphere as it is in the
real lighting environment.

Visual Testing Glossary 307

erosion: (1) Loss of material or fiber optics: Technology of efficient
degradation of surface quality through transmission of light through
friction or abrasion from moving transparent fibers such as glass, quartz
fluids, made worse by solid particles in and plastic by means of total internal
those fluids or by cavitation in the reflection.
moving fluid. See wear. (2) In image
processing, condition of a binary fiberscope: Jargon for fiber optic
image where the pixel in the output borescope.
image becomes a 1 if each of its eight
neighbors is a 1 in the input image. field: In video technology, one of two
See also closing, dilation and opening. video picture components that
together make a frame. Each picture is
erosion-corrosion: Simultaneous divided into two parts called fields
occurrence of erosion and corrosion. because a frame at the rate of thirty
frames per second in a standard video
etch crack: Shallow crack in hardened output would otherwise produce a
steel containing high residual surface flicker discernible to the eye. Each
stresses, produced in an embrittling field contains one half of the total
acid environment.2,5 picture elements. Two fields, then, are
required to produce one complete
eutectic liquid: Liquid metal having a picture or frame so the field frequency
proportion of metals such that two or is sixty fields per second and the frame
more solid phases form at the same frequency is thirty frames per second.
temperature during cooling.
field angle: Included angle between those
eutectic point: Temperature and points on opposite sides of a beam
proportion of metals at which two or axis at which the luminous intensity is
more phases of a eutectic liquid form. 10 percent of the maximum value.
Compare eutectoid. This angle may be determined from an
illuminance curve or may be
eutectoid: Similar to eutectic but in a approximated by use of an incident
solid system during cooling. light meter.2,6

evaluation: Process of deciding the field of view: Range or area where things
severity of a condition after an can be seen through an imaging
indication has been interpreted, to system, lens or aperture. Compare
determine whether it meets depth of field.
acceptance criteria.
field of vision: Range or area where
eye sensitivity curve: Graphic expression things can be perceived by eyesight at
of vision sensitivity characteristics of a point in time, assuming the eye to
the human eye. In the case of a be immobile.
physical photometer, the curve should
be equivalent to the standard observer. fillet weld: Weld of approximately
The required match is typically triangular cross section joining two
achieved by adding filters between the surfaces approximately at a right angle
sensitive elements of the meter and to each other.
the light source. See photopic vision.
filter: Processing component or function
F that excludes a selected kind of signal
or part of a signal.
false brinelling: Fretting wear
indentations. Compare brinelling. filtering: See low pass filtering.
fit up: To secure one or more joint
false indication: Test indication that
could be interpreted as originating members with special external
from a discontinuity but that actually fixturing in order to prevent
originates where no discontinuity movement during welding.2,5
exists in the test object. Compare flakes: Short discontinuous internal
indication, nonrelevant; defect.4 fissures in ferrous metals attributed to
stresses produced by localized
farsightedness: Vision acuity functionally transformation and/or decreased
adequate for viewing objects at a solubility of hydrogen during cooling
distance, generally farther than arm’s usually after hot working. On a
length. Also called hyperopia. Compare fractured surface, flakes appear as
nearsightedness. bright silvery areas; on an etched
surface, flakes appear as short,
far vision: Vision of objects at a distance, discontinuous cracks.2,5
generally beyond arm’s length. flaw: Rejectable anomaly.4 See also defect.
Compare near vision. fluorescence: Phenomenon of absorption
of electromagnetic radiation and its
feature extraction: From an enhanced reemission at a lower energy (longer
image, derivation of some feature wavelength). In visual testing,
values, usually parameters for fluorescence is typically a response to
distinguishing objects in the image. ultraviolet radiation.

ferrite: Solid solution of one or more
other elements in alpha iron.

fiber optic borescope: See borescope, fiber
optic.

308 Visual Testing

focus: Position of a viewed object and a galvanic series: List of metals, alloys and
lens system relative to one another to graphite (a nonmetal) in sequence
offer a distinct image of the object as with the most anodic (easily corroded)
seen through the lens system. See in liquids at one end of the list and
accommodation and depth of field. the most cathodic (least easily
corroded) at the other end. For
focus, principal plane of: Single plane in practical reasons, this sequence is
focus in a photographic scene. compiled using seawater as the
electrolyte — 3 to 5 percent sodium
focusing, automatic: (1) Feature of a chloride and other salts dissolved in
camera whereby the lens system water.
adjusts to focus on an object in the
field of view. (2) Metaphorical gamma iron: Iron with face centered
attribute of a borescopic instrument’s cubic structure formed by slow cooling
depth of field (the range of distance in of delta ferrite. This characteristic
focus). The depth of field is so great in lattice structure is stable between
the case of video borescopes that 906 °C (1663 °F) and 1391 °C
focusing is unnecessary for most (2535 °F). Also called austenite.
applications. Despite the name, no
mechanism is actively adjusted. The gas metal arc welding (GMAW): Inert
large depth of field is due both to the gas shielded metal joining process that
small diameter of the lens aperture uses a continuous and consumable
and to the proximity of the lens to the wire electrode. Also called MIG (metal
charge coupled device. inert gas) welding. Compare gas
tungsten arc welding and shielded metal
focusing, primary: Focusing by the lens arc welding.
of the image onto a fiber optic bundle
at the tip of a probe. gas tungsten arc welding (GTAW): Inert
gas shielded metal joining process that
focusing, secondary: Focusing at the uses a nonconsumable tungsten
eyepiece of a borescope or other electrode. Filler material, when
optical instrument, specifically the needed, is manually fed into the
manual refocusing needed when the molten weld puddle. Also called
viewing distance changes. tungsten inert gas (TIG) welding.
Compare gas metal arc welding; shielded
footcandle (ftc): Disfavored unit of metal arc welding.
illuminance, where
1 ftc = 1 lm·ft–2 = 10.76 lx. gasket seal: Resilient ring, usually virgin
polytetrafluoroethylene (PTFE), in a
footlambert (ftl): Disfavored unit of piping or tubing connection. Compare
luminance, where 1 ftl = 3.426 cd·m–2. interference sealing thread.

forging crack: Stress induced general examination: In personnel
discontinuity formed during qualification, a test or examination of
mechanical shaping of metal; see a person’s knowledge, typically (in the
crack. case of nondestructive testing
personnel qualification) a written test
fovea centralis: Region of sharpest vision on the basic principles of a
in the retina, where the layer of blood nondestructive test method and
vessels, nerve fibers and cells above general knowledge of basic equipment
the rods and cones is far thinner than used in the method. (According to
in peripheral regions. ASNT’s guidelines, the general
examination should not address
foveal vision: See photopic vision. knowledge of specific equipment,
fracture mechanics: Field of solid codes, standards and procedures
pertaining to a particular application.)
mechanics that deals with behavior of Compare practical examination and
cracked bodies subjected to stress and specific examination.
strain.
frame: Complete raster scan projected on geometrical optics: Mathematical study
a video screen. There are thirty frames of how light rays are reflected and
per second in a standard video output. refracted and practical techniques
A frame may be comprised of two based on such understanding,
fields, each displaying part of the total including the transmission of images
frame. See also field. by lenses and mirrors. Also called lens
frequency: Number of times per second optics.
that a cyclical waveform repeats. The
unit of frequency is hertz (Hz). glare: Excessive brightness (or brightness
fretting corrosion: See corrosion, fretting. varying by more than 10:1 within the
fretting wear: See wear, fretting. field of view) that interferes with
friction oxidation: See wear, fretting. observation or interpretation of a test
response. Glare may be caused by
G reflection, whether specular (smooth
surface) or diffuse (rough surface), of
galling: Type of surface damage caused by light or radiation sources.
friction.

Visual Testing Glossary 309

glare, blinding: Glare so intense that for heat affected zone: Portion of base metal
an appreciable length of time after it not melted during brazing, cutting or
has been removed, no object can be welding but with mechanical
seen.2,6 properties altered by the heat.4

glare, direct: Glare resulting from high heat checking: Surface cracking caused
luminances or insufficiently shielded when metal rapidly heated (or cooled
light sources in the field of view. and heated repeatedly) is prevented
Direct glare is usually associated with from expanding freely by colder metal
bright areas, such as luminaires, below the surface. Friction may
ceilings and windows which are produce the heat. Heat checking is
outside the visual task or region being sometimes called thermal fatigue.
viewed.2,6
heat wave: Thermally produced variation
glare, reflected: Glare resulting from in flue gas density that distorts images
specular reflections of high of objects in a firebox.
luminances in polished or glossy
surfaces in the field of view. It usually hot tear: Fracture formed in a cast metal
is associated with reflections from during solidification and due to
within a visual task or nearby areas.2,6 extensive tensile stress associated with
volumetric shrinkage. Hot tears often
gloss meter: Reflectometer used to occur where areas of different
measure specular reflectance.2,6 thicknesses adjoin.

gnomon: Artifact intended to cast a hue: Characteristic of light at a particular
shadow. The shadow may be used to bandwidth; commonly associated with
measure time or distance. the color’s name.

gouge: Surface indentation caused by human factors: Factors in the overall test
forceful abrasion or impact or flame sensitivity based upon mental and
cutting. Also called nick. Compare tool physical condition of the inspector,
mark. training, experience level and the
physical conditions under which the
grain: Solid particle or crystal of metal. As inspector must work.
molten metal solidifies grains grow
and lattices intersect, forming irregular hyperopia: See farsightedness.
grain boundaries. hyperthermia: Heating so excessive that

grain boundary: Interface that forms it can damage or kill plant or animal
between grains of solidifying metal as cells.
the random oriented crystal lattices
meet. See grain. I

gray body: Radiator whose spectral illuminance: Intensity of radiant energy
emissivity is uniform for all (density of luminous flux) on a
wavelengths. surface, in the visible light spectrum.
Illuminance is measured in lux.
gray level: Integer number representing
the brightness or darkness of a pixel illuminate: Cast light on (something).
or, as a composite value, of an image illumination: Act of illuminating or state
comprised of pixels.
of being illuminated. See also
green rot: Form of attack due to illuminate. Compare illuminance.2,6
simultaneous carburization and image: Visual representation of an object
oxidation of stainless heating elements or scene.
common to nickel chromium and image enhancement: Any of a variety of
nickel chromium iron alloys, image processing steps, used singly or
especially in furnace environments. in combination to improve the
detectability of objects in an image.
grinding crack: Shallow crack formed in image guide: Fiber bundle that carries the
the surface of relatively hard materials picture formed by the objective lens at
because of excessive grinding heat or the distal end of a fiber optic
the brittleness of the material. borescope back to the eyepiece.
Grinding cracks typically are oriented image orthicon: Television tube that uses
90 degrees to the direction of the photoemission method. Compare
grinding.4 vidicon tube.
image processing: Actions applied singly
H or in combination to an image, in
particular the measurement and
Hadfield’s steel: Austenitic manganese alteration of image features by
specialty steel, easily work hardened. computer. Also called picture processing.
image segmentation: Process in which
halitation: Rings of light visible around a the image is partitioned into regions,
spot on a video screen where an each homogeneous.
electron scanning beam is held. in control: Within prescribed limits of
process control.
heading: Upsetting wire, rod or bar stock incandescence: Emission of visible
in dies to form parts having some of radiation as a result of heating.
the cross sectional area larger than the
original. Examples are bolts, rivets and
screws.1

310 Visual Testing

indication: Nondestructive test response inverse square law: Physical law for a
that requires interpretation to point source of energy. The quantity
determine its relevance.4 See also or strength is inversely proportional to
defect; discontinuity; false indication; the square of the distance from the
nonrelevant indication. origin.

indication, nonrelevant: Indication that iris: Ring of variable area around the
has no relation to a discontinuity that pupil and in front of the lens of the
might constitute a defect.1 Test eye. The surface area of the iris adjusts
response caused by geometry or by a spontaneously to change the amount
physical condition that is not a of light entering the eye.
discontinuity.
irradiance: Radiant power falling upon a
indication, relevant: Indication from a known surface area at a given angle.
discontinuity (as opposed to a false Compare radiance. See also radiometer.
indication) requiring evaluation by a
qualified inspector, typically with IshiharaTM plates: Trade name for a kind
reference to an acceptance standard, of pseudoisochromatic plates, used for
by virtue of the discontinuity’s size or color differentiation vision testing.
location.1
isomorphous binary system: Two
indirect viewing: Viewing of a test object element alloy system in which both
during which the light image is elements are completely soluble in
mediated through a system of two or each other in the liquid and the solid
more lenses (as in a borescope) or states, in all proportions at all
transduced through an electronic temperatures.
signal (as with a charge coupled
camera). Compare direct viewing; remote J
viewing.
jaeger eye chart: Eye chart used for near
infrared radiation: Electromagnetic vision acuity examination.
radiant energy of wavelengths longer
than 770 nm.2,6 K

interference fitted thread: See interference kinetic vision acuity: Vision acuity with
sealing thread. a moving target. Studies indicate that
10 to 20 percent of visual efficiency
interference objective: In a microscope, a can be lost by target movement.
small, metallized glass mounted in
contact with the test object and L
adjustable for tilt to control fringe
spacing. laboratory microscope: Conventional
compound microscope. See microscope.
interference sealing thread: Piping seal
using a tapered connection made up lambert cosine law: See cosine law.
under great pressure, forcing the lambertian: Having a surface that reflects
mating surfaces together more tightly
than is possible with a spiral thread. light diffusely and uniformly rather
Compare gasket seal. than specularly. See matte. Most
objects have a lambertian surface.
interlaced scanning: Process whereby the Compare specular.
picture appearing on a video screen is lap: Forging discontinuity caused by a
divided into two parts. Interlaced folding over of metal. Laps are found
scanning reduces flicker by increasing in rolled bar stock and at or near
the electron beam’s downward rate of diameter changes.2,5
travel so that every other line is sent. laser: Acronym (light amplification by
When the bottom is reached, the stimulated emission of radiation). A
beam is returned to the top and the device, the laser that produces a
alternate lines are sent. The odd and highly monochromatic and coherent
even line scans are each transmitted at (spatial and temporal) beam of
1/60 s, totaling 1/30 s per frame and radiation.2
retaining the standard rate of leaked visible light: Unwanted
30 frames per second. The eye’s electromagnetic radiation that has a
persistence of vision allows the odd wavelength between 400 and 800 nm
and even lines to appear as a single that is generated by a UV-A source but
image without flicker. not filtered out of the emission
spectrum. Leaked visible light is
interpretation: Determination of the generally perceived as purple or dark
cause, significance and relevance of blue light and not accurately measured
test indications. using a photometric sensor.4
lens: Transparent object that refracts light
interstitial alloy: Alloy in which the passing through it in order to focus
atoms of the alloying element fit into the light.
the spaces between the atoms of the lens optics: See geometrical optics.
parent metal. light adapted vision: See photopic vision.

Visual Testing Glossary 311

light guide bundle: Bundle of filaments, luminance: Photometric brightness of a
usually glass, that carries noncoherent light source defined by the density of
light from a high intensity source its luminous intensity, measured as
through a fiber optic borescope to luminous flux per unit solid angle per
illuminate an object. unit area in a given direction.
Reported in candela per square meter
light: Radiant energy that can excite the (cd·m–2).
retina and produce a visual sensation.
The visible portion of the luminosity: Luminous efficiency of
electromagnetic spectrum extends radiant energy.
from about 400 to 800 nm.2,6
luminous efficacy: Ratio of the total
lighting, back: Placement of light source luminous flux of a light source to the
and image sensor on opposite sides of total radiant flux or to the power
the test object, used when the input. Sometimes called luminous
silhouette of a feature is important. efficiency.

lighting, flash: See lighting, strobe. luminous efficiency: See luminous
lighting, front: Placement of light source efficacy.

and image sensor on the same side of luminous flux: Radiant energy’s time rate
the test object. of flow. Measured in lumens.
lighting, strobe: Lighting that flashes
intermittently at a rate that may be luminous intensity: Measure of a light
adjusted and is often perceived as a source’s power output per unit solid
flicker, used to image moving objects angle emitted or reflected from a
or still objects with potential point, when weighted by the photopic
movement. spectral luminous efficiency response
lighting, structured: Combining a light curve. Luminous intensity is measured
source with optical elements to form a in candela. Compare luminance.4
line or sheet of light.
light meter: See photometer. Compare lux (lx): SI unit of illuminance, equal to
radiometer. one lumen per square meter
limited certification: Of a person, (1 lx = 1 lm·m–2).
certified only for specific operations;
usually called limited Level (I or II) or M
designated as having limited
certification because they are not machine vision: Automated system
qualified to perform the full range of function of acquiring, processing and
activities expected of personnel at that analyzing images to evaluate a test
level of qualification, for a given object or to provide information or
method. interpretation for human
line pair: Pair of adjacent, parallel lines interpretation. A typical machine
used to evaluate the resolution of a vision system consists of a light
specific imaging system. See also source, a video camera, a video
minimum line pair. digitizer, a computer and an image
lot tolerance percent defective: In display.
quality control, the percent defective
at which there is a 10 percent macula lutea: Irregular, diffuse ring of
probability of acceptance in a yellow pigment which partly overlaps
production run. the fovea and surrounds it out to
low pass filtering: In image processing, around 10 degrees and which absorbs
linear combination of pixel values to blue light, thus changing the color of
smoothen abrupt transitions in a the light reaching receptors beneath.
digital image. Also called smoothing.
lumen (lm): SI photometric unit of martensite: (1) Acicular (needlelike)
luminous flux, weighted according to microstructure produced by fast
the photopic vision response. One cooling or quenching of metals and
lumen equals the light emitted by one alloys such as steel. (2) The hard steel
candela (cd) point source into one with such microstructure produced by
steradian (sr) solid angle (1 lm = fast cooling of austenite. Martensite is
1 cd·sr–1). a constituent commonly found in
lumen method: Lighting design quenched steel.
procedure used for predetermining the
relation between the number and martensite finish temperature:
types of lamps or luminaires, the room Temperature at which martensite
characteristics and the average formation is completed as steel cools.
illuminance on the work plane. It
takes into account both direct and martensite start temperature:
reflected flux. Also called flux Temperature at which martensite starts
method.2,6 to form as steel cools.

mask: (1) A spatial filter in the sensing
unit of a surface inspection system.
(2) An n × n square matrix with
different values that serves as a filter
in image processing.

312 Visual Testing

match bend effect: Optical illusion microscope, polarizing: Microscope with
whereby an area of uniform brightness polarizing elements to restrict light
appears to be nonuniform because of vibration to a single plane for studying
contrast with the brightness of an material with directional optical
adjacent area. properties. As fibers, crystals, sheet
plastic and materials under strain are
mathematical morphology: Image rotated between crossed polarizers on
processing technique of expanding the microscope stage, they change
and shrinking. The basic operators in color and intensity in a way that is
mathematical morphology are dilation related to their directional properties.
(expanding), erosion (shrinking),
opening and closing. MIG welding: See gas metal arc welding.
miniature borescope: See borescope,
matte: Having a surface that reflects light
diffusely rather than at an angle equal miniature.
to the angle of incidence; not shiny. miniborescope: Jargon for miniature
Also called lambertian. The term matte
is generally applied to smooth surfaces borescope.
or coatings. Compare specular. minimum line pair: Closest distance that

mesopic vision: Vision adapted to a level a specific imaging system can resolve
of light between photopic at between a pair of adjacent, parallel
3.4 × 10–2 cd·m–2 (3.2 × 10–3 cd·ft–2) lines (line pair) used to evaluate
and scotopic at 3 × 10–5 cd·m–2 system resolution.
(2.7 × 10–6 cd·ft–2). modulus of elasticity: Ratio between
stress and strain in a material
metallograph: Short term for deformed within its linear elastic
metallographic microscope. range.
monochromatic: Light from a very small
metallographic microscope: See portion of the spectrum.
microscope, metallographic. monochromator: Device that uses prisms
or gratings to separate or disperse the
metallography: Science and practice of wavelengths of the spectrum into one
microscopic testing, inspection and band.
analysis of a metal’s structure, morphology: See mathematical
typically at magnifications from 50 × morphology.
to 2500 ×. mottle: Apparently random positioning of
metallic flakes that creates an
metallurgical microscope: See microscope, accidental pattern.
metallurgical. multipass weld: Weld made by many
passes, one pass at a time.
microborescope: See borescope, micro-. multiphase alloy: Alloy in which several
microscope: Instrument that provides phases are present.

enlarged images of small objects. N
microscope, compound: Conventional
NDE: (1) Nondestructive evaluation.
microscope, using geometrical optics (2) Nondestructive examination. See
for magnification. Also called nondestructive testing.
laboratory microscope.
microscope, interference: Magnifier NDI: Nondestructive inspection. See
using the wavelength of light as a unit nondestructive testing.
of measure for surface contour and
other characteristics. NDT: See nondestructive testing.
microscope, metallographic: near ultraviolet radiation: Ultraviolet
Metallurgical microscope
incorporating a camera. Also called a radiation with wavelengths ranging
metallograph. Most metallographic from about 320 to about 400 nm.
microscopes share these features: Formerly called black light.
(a) stand with concealed shock near vision: Vision of objects nearby,
absorbers, (b) intense light source, generally within arm’s length.
(c) inverted stand so that the test Compare far vision.
object is face down, (d) viewing nearsightedness: Vision acuity
screens for prolonged tasks such as dirt functionally adequate for viewing
count or grain size measurements, objects nearby, generally within arm’s
(e) bright, dark and polarized length. Also called myopia. Compare
illumination options. farsightedness.
microscope, metallurgical: Microscope necking down: Localized reduction in
designed with features suited for area of a specimen or structural
metallography. member during welding under
microscope, phase contrast: Laboratory overload.2,5
microscope with two additional negative sliding: Rolling and sliding of
optical elements to transmit both meshing gears or rollers when the
diffracted and undiffracted light, rolling and sliding are in opposite
revealing refractive index directions.
discontinuities in a completely
transparent test object.

Visual Testing Glossary 313

neural acuity: Ability of the eye and O
brain together to discriminate patterns
from background. Discrimination is objective: In discussion of a lens system
influenced by knowledge of the target (camera, borescope, microscope,
pattern, by the scanning technique telescope), of or pertaining to the end
and by the figure-to-ground or lens closest to the object of
relationship of a discontinuity. The examination — at the end opposite
figure/ground relationship can be from the eyepiece. Distal; tip.
referred to as having a level of visual
background noise. OCTG: Oil country tubular goods.
oil country tubular goods: Hollow
nick: Surface indentation caused by
forceful abrasion or impact. Also called cylindrical components, such as pipes,
gouge. Compare tool mark. used in petroleum wells to case the
hole and to convey petroleum and
nit: Obsolete unit for measuring related products.4
luminance, equivalent to one candela opening: Image processing operation of
per square meter. Abbreviated nt. erosion followed by dilation. A single
opening eliminates isolated single
noble metals: Cathodic metals (such as pixels. See also closing.
gold, platinum and silver), which opsin: See visual purple.
strongly resist corrosion. optic disk: Area in the retina through
which the fibers from the various
nondestructive evaluation (NDE): receptors cross the inner (vitreous
Another term for nondestructive humor) side of the retina and pass
testing. In research and academic through it together in the optic nerve
communities, the word evaluation is bundle. This transitional area is
often preferred because it emphasizes completely blind.
interpretation by knowledgeable optics: Physical science of the
personnel. transmission of radiation, especially of
light. See geometrical optics.
nondestructive examination (NDE): organoleptic: Relying on or using sense
Another term for nondestructive organs, such as the human eye.
testing. In the utilities and nuclear orthicon: See image orthicon.
industry, examination is sometimes
preferred because testing can imply P
performance trials of pressure
containment or power generation parafoveal vision: See scotopic vision.
systems. parallax: Apparent difference in position

nondestructive inspection (NDI): of an imaged point according to two
Another term for nondestructive differently positioned sensors.
testing. In some industries (utilities, pass: In welding, a single bead of weld
aviation), the word inspection often metal along the entire joint or the
implies maintenance for a component process of laying down that bead.
that has been in service. pearlite: Platelet mixture of cementite
and ferrite in steels or in alpha and
nondestructive testing (NDT): beta phases in nonferrous alloys.
Determination of the physical peripheral vision: Seeing of objects
condition of an object without displaced from the primary line of
affecting that object’s ability to fulfill sight and outside the central visual
its intended function. Nondestructive field.2,6
test methods typically use an phase: In metallurgy, a physically
appropriate form of energy to homogeneous portion of a material
determine material properties or to system, specifically the portion of an
indicate the presence of material alloy characterized by its
discontinuities (surface, internal or microstructure at a particular
concealed). Sometimes called temperature during melting or
nondestructive evaluation, nondestructive solidification.
examination or nondestructive inspection. phase contrast microscope: See
microscope, phase contrast.
nonferromagnetic material: Material not phase diagram: Graph showing the
magnetizable and essentially not temperature, pressure and composition
affected by magnetic fields.4 limits of phase fields in a material
system. Also called a constitution
nonrelevant indication: See indication, diagram. Compare equilibrium diagram.
nonrelevant. photochromic lens: Eyeglass material
that automatically darkens to reduce
numerical analysis: Technique to light transmission when exposed to
generate numbers as the solution to a ultraviolet radiation.
mathematical model of a physical
system; used in place of a closed form
analytic expression; usually requires
digital computation.4

314 Visual Testing

photoconduction: Method by which a pixel: Addressable point in a digital
vidicon television camera tube image. The image from a conventional
produces an electrical image, in which computer is an array of pixels, and
the conductivity of the photosensitive each has a numerical value. The
surface changes in relation to the higher the number for a pixel, the
intensity of the light reflected from brighter it is. Formerly called picture
the scene focused onto the surface. element.
Compare photoemission.
plane of focus: See focus, principal plane
photoelasticity: Effect of a material’s of.
elastic properties on the way that it
refracts or reflects light. platelet: Flat crystallites in certain phases
of steel.
photoelectric effect: Emission of
electrons from a surface bombarded by polarizing microscope: See microscope,
sufficiently energetic photons. Such polarizing.
emissions may be used in an
illuminance meter and can be porosity: Discontinuity in metal resulting
calibrated in lux.2,6 from the creation or coalescence of
gas. Very small pores open to the
photoemission: Method by which an surface are called pinholes.2,5
image orthicon television camera tube
produces an electrical image, in which positive sliding: Rolling and sliding of
a photosensitive surface emits meshing gears or rollers when the
electrons when light reflected from a directions of rolling and sliding are
viewed object is focused on the the same.
surface. Compare photoconduction.
poultice corrosion: See corrosion, poultice.
photometer: Device used to measure practical examination: In certification of
illuminance. The sensor is filtered
such that its response closely matches nondestructive testing personnel, a
the spectral responsivity curve of the hands-on examination using test
human eye. In nondestructive testing, equipment and sample test objects.
photometers measure lux. Compare Compare general examination; specific
radiometer. examination.
primary creep: First stage of creep,
photometric brightness: Luminance of a marked by elastic strain plus plastic
light source. strain.
principal plane of focus: See focus,
photometry: Study and measurement of principal plane of.
electromagnetic radiation with probability of detection (PoD): Statistical
approximate wavelengths between 400 statement from a specific test
and 800 nm, within the human eye’s procedure indicating how likely a
spectral responsivity. See also given discontinuity length may be
photometer; photopic vision; radiometry; reliably found.
relative photometry. process: Repeatable sequence of actions to
bring about a desired result.
photon: Particle of light, hypothesized to process control: Application of quality
explain those behaviors of light in control principles to the management
which its behavior is corpuscular of a repeated process.
rather than wavelike. pseudocolor: Image enhancement
technique wherein colors are assigned
photopic vision: Average spectral to an image at several gray scale
responsivity curve of the human eye intervals.
when adapted to well lit conditions pseudoisochromatic plate: Image used
(greater than 0.034 cd·m–2). The for color vision examinations. Each
photopic spectral luminous efficiency plate bears an image which may be
response curve is governed by an difficult for the examinee to see if his
averaged retinal cone response with or her color vision is impaired. See
sensitivity peaks centered at about also Ishihara™ plates.
555 nm. Also known as foveal vision psychophysics: Interaction between
and light adapted vision. Compare vision performance and physical or
mesopic vision and scotopic vision.2,6 psychological factors. One example is
the so-called vigilance decrement, the
photoreceptor: Light sensor. degradation of reliability based on
picture element: See pixel. performing visual activities over a
picture processing: See image processing. period of time.
pipe: Longitudinal centerline pupil: Black aperture in the center of the
eye’s lens, through which light enters
discontinuity inherent in ingots, the lens to impinge on the retina.
imparted to some rolled metal and purple: See visual purple.
consisting of a concavity or voids.
pitting: Discontinuity consisting of
surface cavities. See also cavitation
fatigue.

Visual Testing Glossary 315

Q raster: Repetitive pattern whereby a
directed element (a robotic arm or a
qualification: Process of demonstrating flying dot on a video screen) follows
that an individual has the required the path of a series of adjacent parallel
amount and the required type of lines, taking them successively in turn,
training, experience, knowledge and always in the same direction (from top
abilities.4-6 See also certification and to bottom or from left to right),
qualified. stopping at the end of one line and
beginning again at the start of the
qualified: Having demonstrated the next line. Following a raster pattern
required amount and the required makes it possible for electron beams to
type of training, experience, form video pictures or frames and for
knowledge and abilities. See also a sensor-bearing armature to cover a
certified and qualification. predetermined part of the surface of a
test object.
quality: Ability of a process or product to
meet specifications or to meet the rat’s tooth principle: (1) The tendency
expectations of its users in terms of for hard material on a tooth’s front
efficiency, appearance, reliability and surface to wear more slowly than soft
ergonomics.4-6 material on the back surface, keeping
the edge sharp. (2) Mechanism of wear
quality assurance: Administrative actions whereby adjacent hard and soft
that specify, enforce and verify surfaces wear at different rates,
quality.4-6 producing a self-sharpening edge.

quality control: Physical and Recommended Practice No. SNT-TC-1A:
administrative actions required to Set of guidelines published by the
ensure compliance with a quality American Society for Nondestructive
assurance program. Quality control Testing, for employers to establish and
may include nondestructive testing in conduct a qualification and
the manufacturing cycle.4-6 certification program for
nondestructive testing personnel.4-6
quality of lighting: Level of luminance in
a visual task or environment. recommended practice: Set of guidelines
or recommendations.4-6
R
recovery: Reduced stress level and
radial: Of or pertaining to direction from increased ductility of metal after work
center of a circle (or a sphere or cross hardening. See creep.
section of a cylindrical object) to its
surface, and perpendicular to its axis. recrystallization: Changes in
Compare axial. microstructure and properties upon
heating of cold worked metal.
radiance: Radiant flux per unit solid
angle and per unit projected area of red mud: Debris (usually oxides of the
the source. Measured in watts per contacting metals) of fretting wear,
square meter steradian. Compare mixed with oil or grease and retained
irradiance. at or near the site of its formation. See
also cocoa.
radiant energy: Energy transmitted
through a medium by electromagnetic reference standard: Object containing
waves. Also known as radiation. known discontinuities at known
distances and representing accept or
radiant flux: Radiant energy’s rate of reject criteria.
flow, measured in watts.
reflectance: Ratio of reflected wave
radiant intensity: Electromagnetic energy energy to incident wave energy. Also
emitted per unit time per unit solid known as reflectivity.
angle. Measured in watts per steradian.
reflection: General term for the process
radiant power: Total radiant energy by which the incident energy leaves a
emitted per unit time. surface or medium from the incident
side, without change in frequency.
radiometer: Device used to measure Reflection is usually a combination of
irradiance. In nondestructive testing, specular and diffuse reflection.2,6
radiometers are used to measure UV-A
output, or leaked visible light, in reflectometer: Photometer used to
microwatt per square centimeter measure diffuse, specular and total
(µW·cm–2). See also irradiance. reflectance.
Compare photometer.
reflector: Device used to redirect the
radiometric photometer: Radiometer for luminous flux from a source by the
measuring radiant power over a process of reflection.2,6
variety of wavelengths.
refraction: Reorientation of radiation’s
radiometry: Study and measurement of path by the medium through which it
electromagnetic radiation emitted by a passes.
source or falling upon a surface.

316 Visual Testing

relative photometry: (1) Evaluation of a resolving power: Ability of vision or
desired photometric characteristic other detection system to separate two
based on an assumed lumen output of points. Resolving power depends on
a test lamp. (2) Measurement of an the angle of vision and the distance of
uncalibrated light source relative to the sensor from the test surface.
another uncalibrated light source. Resolving power is often measured
using parallel lines. Compare
remote viewing: (1) Indirect viewing of a resolution.
test object far from the viewer’s
immediate presence — for example, retina: In the eye, the tissue on back,
viewing with telemetry or crawlers. inside surface of the eyeball, opposite
The term remote viewing is used in the the pupil, where light sensitive rods
fields of robotics and surveillance to and cones sense light.
distinguish conventional from distant
viewing tasks. (2) Viewing of a test retinene: See visual purple.
object during which the light image is rhodopsin: See visual purple.
mediated through a system of two or robotic system: Automated system
more lenses (as in a borescope) or
transduced through an electronic programmed to perform purposeful
signal (as with a charge coupled actions in variable sequences.
camera). This use of the term remote rod: Retinal receptor that responds at low
viewing in some specifications is a levels of luminance even down below
misnomer, intended merely to the threshold for cones. At these levels
distinguish borescopy from direct there is no basis for perceiving
viewing. Compare borescope; direct differences in hue and saturation. No
viewing; indirect viewing. rods are found in the fovea centralis.2,6
Concentrated toward the outer region
replica: Piece of malleable material, such of the retina, rods assist with mesopic
as polyvinyl or polystyrene plastic vision and are responsible for scotopic
film, molded to a test surface for the night vision. Compare cone.
recording or analysis of the surface root mean square (rms): Statistical
microstructure. measure of the magnitude of a varying
quantity, such as current. Square root
replica, metallographic: Replica suitable of the mean square of a set of
for microscopic examination. See measures, usually a time series.
metallography.
S
replication: Method for copying the
topography of a surface by making its sampling, partial: Testing of less than
impression in a plastic or malleable 100 percent of a production lot.
material.
sampling, random partial: Partial
reserve vision acuity: Ability of an sampling that is fully random.
individual to maintain vision acuity
under poor viewing conditions. A sampling, specified partial: Partial
visual system with 20/20 near vision sampling in which a particular
acuity under degraded viewing frequency or a sequence of sample
conditions has considerable reserve selection is prescribed. An example of
vision acuity compared to that of an specified partial sampling is the testing
individual with 20/70 near vision of every fifth unit.
acuity.
saturation: Relative or comparative color
resolution: Aspect of image quality characteristic resulting from a hue’s
pertaining to a system’s ability to dilution with white light.
depict objects, often measured by
distinguishing (resolving) a pair of scalar: Quantity completely specified by a
adjacent objects or parallel lines. single number and unit.4

resolution test: Procedure wherein a line scaling: (1) Forming a layer of oxidation
is detected to verify a system’s product on metals, usually at high
sensitivity. temperature. (2) Deposition of
insoluble constituents on a metal
resolution threshold: Minimum distance surface, as in cooling tubes and water
between a pair of points or parallel boilers.2,5
lines when they can be distinguished
as two, not one, expressed in minutes scoring: (1) Marring or scratching of any
of arc. Vision acuity, in such a case, is formed part by metal pickup on a
the reciprocal of one-half of the period punch, die or guide. (2) Reducing
expressed in minutes.2,6 the thickness of a part along a line
to weaken it purposely at a specific
location.2,5

Visual Testing Glossary 317

scotopic vision: Dark adapted vision, SI (International System of Units):
using only the rods in the retina, Measurement system in which the
where differences in brightness can be following seven units are basic: meter,
detected but differences in hue mole, kilogram, second, ampere,
cannot. Vision is wholly scotopic kelvin and candela.
when the luminance of the test
surface is below 3 × 10–5 cd·m–2 signal electrode: Transparent conducting
(2.7 × 10–6 cd·ft–2). Also known as film on the inner surface of a vidicon’s
parafoveal vision. Compare mesopic faceplate and a thin photoconductive
vision and photopic vision. layer deposited on the film.

scuffing: Type of adhesive wear. signal processing: Acquisition, storage,
seam: Linear discontinuity formed by a analysis, alteration and output of
digital or analog data.
lack of metal from folds produced by
an underfilled pass during metal signal-to-noise ratio: Ratio of signal
rolling. Squeezed tight on subsequent values (responses that contain relevant
passes, the underfill runs parallel to information) to baseline noise values
the longitudinal axis of the bar. (responses that contain nonrelevant
second stage replica: Positive replica information).4
made from the first cast to produce a
duplicate of the original surface. signal: Physical quantity, such as voltage,
secondary creep: Second stage of creep, that contains relevant information.4
in which deformation proceeds at a
constant rate and less rapidly than in simple magnifier: Microscope having a
primary creep. Essentially an single converging lens.
equilibrium condition between the
mechanisms of work hardening and smoothing: In image processing, linear
recovery. See also defect and combination of pixel values to smooth
discontinuity.4 abrupt transitions in a digital image.
sensitivity: Ability of a sensor or system Also called low pass filtering.
to distinguish a signal or indication
from background noise. See also SNT-TC-1A: See Recommended Practice No.
probability of detection. SNT-TC-1A.
sensitization: Precipitation of chromium
carbides in the grain boundaries of a spalling fatigue: See subcase fatigue.
corrosion resistant alloy, resulting in specific examination: In certification of
intergranular corrosion that would
otherwise be resisted. nondestructive testing personnel, a
sensor: Device that detects a material written examination that addresses the
property or mechanical behavior (such specifications and products pertinent
as radiation or displacement) and to the application. Compare general
converts it to an electrical signal. examination and practical examination.
shadow casting: Nondestructive specific gravity: Unitless ratio of the
technique of vapor depositing a thin density of a material divided by the
metal film onto a replica at an oblique density of water. Water has a density
angle in order to obtain a micrograph of about 1 g·cm–3, or 1000 kg·m–3.
of a test surface of an opaque test specification: Set of instructions or
object. standards invoked to govern the
shear break: Open break in metal at the properties, results or performance of a
periphery of a bolt, nut, rod or specific set of tasks or products.4-6
member at approximately a 45 degree spectral irradiance: Measure of energy
angle to the applied stress. Shear emitted by a radiation source as
breaks occur most often with flanged function of wavelength. Units of
products. Also called shear crack.2,5 spectral irradiance are watts per square
shear crack: See shear break. meter and are often plotted versus
shielded arc welding: Joining of metals wavelength.
by heating them with an electric arc spectral power distribution: Radiant
between electrode(s) and the work power per unit wavelength as a
piece, using an inert gas to shield the function of wavelength. Also known
electrode(s). See also gas tungsten arc as spectral energy distribution, spectral
welding. density and spectral distribution.
shoulder: Cylindrical metal component spectral reflectance: Radiant flux
(pipe) surface, machined to receive reflected from a material divided by
threading indentations but in fact not the incident radiant flux.
threaded, where the thread stops on spectral responsivity: Measure of a
the outside surface of the pipe. photometric or radiometric sensor’s
sensitivity over a wavelength range of
interest, often presented as percent
versus wavelength. Photometric
sensors should exhibit a bell shaped
spectral responsivity curve over the
visible light range, whereas
radiometric sensors may exhibit a flat
or other response curve.

318 Visual Testing

spectral transmittance: Radiant flux stick welding: See shielded metal arc
passing through a medium divided by welding.
the incident radiant flux.
strain: Deflection or alteration of the
spectrophotometer: Instrument used for shape of a material by external forces.
spectrophotometry.
stress: (1) In physics, the action in a
spectrophotometry: Measurement of material that resists external forces
electromagnetic radiant energy as a such as tension and compression.
function of wavelength, particularly in (2) Load per unit of area.
the ultraviolet, visible and infrared
wavelengths. stress concentration: Region where force
per unit area is elevated, often because
spectroradiometer: Instrument used for of geometric factors or cracks. Also
spectroradiometry. known as a stress raiser.

spectroradiometry: Measurement of stress raiser: Contour or property change
electromagnetic radiant power and that locally increases stress magnitude.
spectral emittance, used particularly to
examine colors and to measure the stress riser: See stress raiser.
spectral emittance of light sources. stringer: In wrought materials, an

spectroscope: Instrument used for elongated configuration of
spectroscopy. microconstituents or foreign material
aligned in the direction of working.
spectroscopy: Spectrophotometry or Commonly, the term is associated
spectroradiometry in which the with elongated oxide or sulfide
spectrum, rather than being analyzed inclusions in steel.
only by a processing unit, is presented subcase fatigue: Fatigue originating below
in a visible form to the operator for the case depth.
organoleptic examination. subcase origin fatigue: See subcase
fatigue.
spectrum: Representation of radiant subsurface fatigue: Fatigue cracking that
energy in adjacent bands of hues in originates below the surface. Usually
sequence according to the energy’s associated with hard surfaced or shot
wavelengths or frequencies. A rainbow peened parts but may occur any time
is a well known example of a visible subsurface stresses exceed surface
spectrum. stresses.

specular: Pertaining to a mirrorlike T
reflective finish, as of a metal.
Compare lambertian. tarasov etching technique: Way of
visually inspecting for the presence of
specular reflection: When reflected deleterious effects in hardened steels
waves and incident waves form equal by using specific etching solutions and
angles at the reflecting surface. methods of inspection.

speed of light: Speed of all radiant temperature diagram: See time
energy, including light, is temperature transformation (TTT)
299 792 458 × 108 m·s–1 in vacuum. In diagram.
all materials the speed is less and
varies with the material’s index of tempering: Process of heating a material,
refraction, which itself varies with particularly hardened steel to below
wavelength.2,6 the austenite transformation
temperature, to improve ductility.
speed of vision: Reciprocal of the
duration of the exposure time required tertiary creep: Third stage of creep,
for something to be seen.2,6 marked by steady increase in strain to
the point of fracture under constant
standard: Object, document or concept load.
established by authority, custom or
agreement to serve as a model or rule test object: Physical part or specimen
in the measurement of quantity or the subject to nondestructive testing.
establishment of a practice or
procedure.4,8 See also reference standard threshold: (1) A value in a phenomenon
and acceptance standard. where a large change of output occurs.
(2) Setting of an instrument that
standardization, instrument: Adjustment causes it to register only those changes
of instrument readout before use to a in response greater or less than a
specified reference value.4 specified magnitude.4 See adaptive
thresholding, resolution threshold.
standard observer response curve: See
eye sensitivity curve. thresholding: Digital data processing
technique that reduces a gray level
steel: Iron alloy, usually with less than image into a binary image.
two percent carbon.
throat, actual: Shortest distance from the
stereo photography: Close range root of a fillet weld to its face, as
photogrammetric technique involving opposed to theoretical throat or weld
the capture and viewing of two images size.
of the same object in order to
reconstruct a three dimensional image
of the object.

Visual Testing Glossary 319

throat, effective: In fillet welds, the weld UV-A: Electromagnetic radiation with
throat including the amount of weld wavelengths between 315 and
penetration but ignoring excess metal 400 nm. Fluorescent nondestructive
between the theoretical face and the testing has historically used ultraviolet
actual face. energy centered at 365 nm. See also
irradiance and radiometer.
throat, theoretical: Distance from the
beginning of the root of a fillet weld V
perpendicular to the hypotenuse of
the largest right triangle that can be video: Pertaining to the transmission and
inscribed within the cross section of display of images in an electronic
the fillet weld. Compare weld size. format that can be displayed on a
monitor or screen.
throat, weld: Distance from the root of a
fillet weld to its face. Compare weld videoscope: Jargon for video borescope. See
size and throat, actual. borescope, video.

TIG welding: Tungsten inert gas welding. vidicon tube: Analog television tube that
See gas tungsten arc welding. uses the photoconduction method.
Compare image orthicon.
time temperature transformation (TTT)
diagram: Graph showing time vigilance decrement: Degradation of
required at any temperature to reliability during performance of
transform austenite to pearlite, bainite visual activities over a period of time.
or martensite. See also psychophysics.

tip: Distal or objective end of a borescope. visibility: Quality or state of being
tool mark: Shallow indentation or groove perceivable by the eye. In many
outdoor applications, visibility is
made by the movement of defined in terms of the distance at
manufacturing tools over a surface. which an object can be just perceived
Compare gouge or nick. by the eye. In indoor applications it
trace: Line formed by electron beam usually is defined in terms of the
scanning from left to right on a video contrast or size of a standard test
screen to generate a picture. object, observed under standardized
tracer: In leak testing, a gas that is sensed view conditions, having the same
as it escapes from confinement. threshold as the given object.2,6
transformation diagram: See time
temperature transformation (TTT) visible light: Radiant energy with a
diagram. wavelength between 400 and 800 nm
troland: Unit of retinal illuminance equal as measured in photometric units of
to that produced by a surface whose lux.
luminance is 1 cd·m–2 when the pupil
measures 1 mm2. vision: Perception by eyesight. See far
tubing string: Pipe with which oil or gas vision, machine vision, mesopic vision,
has contact as it is brought to the near vision, peripheral vision, photopic
Earth’s surface.4 vision, scotopic vision, and speed of
tungsten inert gas (TIG) welding: See gas vision.
tungsten arc welding.
vision acuity: Ability to distinguish fine
U details visually at a given distance.
Quantitatively, it is the reciprocal of
ultraviolet borescope: See borescope, the minimum angular separation in
ultraviolet. minutes of two lines of width
subtending one minute of arc when
ultraviolet radiation: Electromagnetic the lines are just resolvable as
radiation with wavelengths between separate.2,6
40 and 400 nm. See also irradiance and
UV-A. visual acuity: See vision acuity.
visual angle: Angle formed by lines
ultraviolet radiometer: Meter, usually
calibrated at 365 nm, used in drawn from center of eye subtended
fluorescent liquid penetrant and by an object or detail at the point of
magnetic particle testing to measure observation. It usually is measured in
output of ultraviolet lamp. minutes of arc.2,6
visual background noise: Formations on
undercut: Undesirable groove left unfilled or signals from a test object that
by weld metal, created during welding constitutes the background to a
and located in base plate at the toe of discontinuity. The higher the level of
a weld. visual background noise, the more
difficult it is to distinguish a
Unified Numbering System: discontinuity.
Alphanumeric system for identifying
alloys according to a registry
maintained by ASTM International
and SAE International.4

320 Visual Testing

visual efficiency: Reliability of a visual wear, adhesive: Degradation of a surface
system. The term visual efficiency uses by microwelding and consequent
20/20 near vision acuity as a baseline fracture due to the sliding of one
in the United States for 100 percent surface against another. Types include
visual efficiency. fretting, galling and scuffing.

visual field: Locus of objects or points in wear, fretting: Surface degradation by
space that can be perceived when head microwelding and microfractures on
and eyes are fixed. The field may be surfaces rubbing each other. Also
monocular or binocular.2,6 called chafing, friction oxidation and
wear oxidation. See also cocoa and false
visual perception: Interpretation of brinelling.
impressions transmitted from the
retina to the brain in terms of weld size: Thickness of weld metal — in a
information about a physical world fillet weld the distance from the root
displayed before the eye. Visual to the toe of the largest isosceles right
perception involves any one or more triangle that can be inscribed in a
of the following: recognition of the cross section of the weld.
presence of something (object,
aperture or medium); identifying it; weld throat: See throat, weld.
locating it in space; noting its relation welder’s flash: Clinical condition,
to other things; identifying its
movement, color, brightness or specifically keratoconjunctivitis,
form.2,6 commonly caused by overexposure to
ultraviolet radiation of welding arc.
visual performance: Quantitative white light: Light combining all
assessment of the performance of a frequencies in the visible spectrum
visual task, taking into consideration (wavelengths from 380 to 780 nm)
speed and accuracy.2,6 and in equal proportions.
work hardening: Increase in hardness
visual purple: Chromoprotein called accompanying plastic deformation of
rhodopsin, the photosensitive pigment a metal. Usually caused in a metal by
of rod vision. The mechanism of repeated impacting, bending or
converting light energy into nerve flexing. Compare creep and recovery.
impulses is a photochemical process in working standard: Work piece or energy
the retina. Chromoprotein is source calibrated and used in place of
transformed by the action of radiant expensive reference standards. In
energy into a succession of products, calibrating of photometers, the
finally yielding the protein called opsin standard would be a light source.
plus the carotenoid known as retinene.

visual task: Appearance and immediate
background of those details and
objects that must be seen for the
performance of a given activity. The
term visual task is a misnomer because
it refers to the visual display itself and
not the task of extracting information
from it. See visual field.

visual testing: Method of nondestructive
testing using electromagnetic radiation
at visible frequencies.

voids: Hollow spots, depressions or
cavities. See also discontinuity.

volt (V): Measurement unit of electric
potential.4

VT: Visual testing.

W

wavelength: Distance between repeating
values of a wave. For example, the
distance from one peak to the next
peak on a sine wave.

wear: See erosion; rat’s tooth principle; wear,
adhesive; and wear, fretting.

wear oxidation: See wear, fretting.

Visual Testing Glossary 321

References

1. ASTM E 1316, Standard Terminology for
Nondestructive Examinations. West
Conshohocken, PA: ASTM
International (2007).

2. Nondestructive Testing Handbook,
second edition: Vol. 8, Visual and
Optical Testing. Columbus, OH:
American Society for Nondestructive
Testing (1993).

3. Nondestructive Testing Handbook,
second edition: Vol. 10, Nondestructive
Testing Overview. Columbus, OH:
American Society for Nondestructive
Testing (1996).

4. Nondestructive Testing Handbook, third
edition: Vol. 8, Magnetic Testing.
Columbus, OH: American Society for
Nondestructive Testing (2008).

5. EPRI Learning Modules. Charlotte, NC:
Electric Power Research Institute.

6. IES Lighting Handbook: Reference
Volume. New York, NY: Illuminating
Engineering Society of North America
(1984).

7. Lovejoy, D. Magnetic Particle Inspection:
A Practical Guide. New York, NY:
Chapman & Hall (1993).

8. TO33B-1-1 (NAVAIR 01-1A-16)
TM43-0103, Nondestructive Testing
Methods. Washington, DC: United
States Department of Defense, United
States Air Force (June 1984).

322 Visual Testing

Index

A arc burns focus adjustment, 136, 137, 141, 144
on jet engine turbine blades, 274 heat damage to, 276
abrasive wear, 203 on pipe threads, 228 historical background, 28, 29-32
acceptance criteria, 15 in liquid penetrant testing, 286, 289
ACCP (ASNT Central Certification Program), 18 arc strikes, near welds, 196, 248 in magnetic particle testing, 286, 289
acoustic emission testing, 10 arc welding, 236, 237-239 optical systems, 144-145
adhesive wear, 205 area lighting, 117 rigid, 89, 138, 141-142
aircraft area measurement, 149, 150, 151, 152 rubber eyeshields, 136
ASME Boiler and Pressure Vessel Code, 17 sectioned, 146
borescope applications, 31, 32, 139, 266-267, systems based on, 145
276-277 brazed joints in power plants, 248 types of, 138, 145-147
historical background, 27 boundary detection, visual, 62-63
composite materials, 280-281 reactor pressure vessels, 250, 252 Bozzini, Philipp, 28
diffracted light technique, 267, 281 vision acuity for testing welds, 241, 242 brazed joints, 248
engine inspection, 85, 139, 274-277 weld joint porosity, 246 brazing, 235-236
fatigue cracking of fuselage, 3 weld joint undercut, 247 brightness
landing gear pitting corrosion, 271 ASME International, 17 color value and, 96
lap joints, enhanced visual ANSI/ASME B31.1, 246 of digital image, 98
ASME B 31.8, 218 as luminance, 35
inspection, 173-176 ASME FFS, 294 of photographic image, 87, 91-92
magnifying lenses for inspection, 266 Association Connecting Electronics Industries, 17 bursts in metals, 186-187, 262
rudder flange cracks, 270-271 astigmatism, 65 in bolting materials, 261
slat drive bell crank, 268 ASTM International, 16, 17 butt joint, 234
spoiler lubrication hole cracks, 268-269 automated inspection. See also machine vision discontinuities in, 243, 244, 247
spoiler torsion bar corrosion pitting, 268 early patents, 28-29 butterfly valves, 259
structural cracking and corrosion, 139, lighting for, 118-119
automotive applications C
266-273 borescopes in, 139
surface crack detection algorithm, 174-175 corrosion of auto body, 201 calibration, of photometers, 54-56, 59
vision acuity for crack detection, 77-78 fatigue cracking, 206, 208 calipers, 130-131
wing cracks under panel, 269-270 ferritic stainless steels in, 209 cambridge gage, 132, 133, 242
wing spar cap cracks, 271-272 camera based measurement, 148-154
wing spar exfoliation corrosion, 85 B camera borescope, 138, 148
Alhazen, 25-26 camera interface, for machine vision, 163
aluminum alloys ball valves, 259 camera obscura, 25-26
fretting wear producing black powder, 206 bifocals, 73 CameraLink®, 163
stress corrosion cracking, 201, 202 binarization, 166-167 cameras. See also photography
threshold for elevated temperature binocular magnifiers, 121
bit map, 95-96 charge coupled device in, 102-103, 161, 162
behavior, 207 black light, 36 colorimetric functions of, 129
aluminum panels, exfoliation corrosion, 281 blackbody, 48, 49, 54 complementary metal oxide semiconductor
American Concrete Institute, 17 blowholes
American National Standards Institute (CMOS), 161
in cast ingots, 183 for general digital photography, 86, 87-88
(ANSI), 17, 18 in welds, 192 for machine vision, 158, 161-163
ANSI/ASME B31.1, 246 blue hazard, 22-23 microscopes with, 125
ANSI/ASNT CP-105, 18 boiler tubes, borescope applications, 138 in other nondestructive testing methods, 286
ANSI/ASNT CP-189, 16, 241 boilers. See also ASME Boiler and Pressure Vessel pan, tilt and zoom, 153, 154
Recommended Practice No. SNT-TC-1A, push cameras, 153-154
Code; pressure vessels for reactor vessel testing, 253-255
16, 18-19, 32-33 early history of inspection, 27 on robot or crawler, 138, 154
American Petroleum Institute (API), 17 failure caused by discontinuities, 3 underwater, 91, 253-254
power boilers, weld acceptance criteria, 216 candela, 35-36, 52, 53-54
API RP 8B, 222 bolts, 259-261 carbides, precipitated, 293, 294
API RP 7G, 221 borescopes, 8, 136-137, 141-147. See also video carburization, 209
API SPEC 5B, 224, 225 castings
pipe and tube specifications, 17, 220, 221, borescopes discontinuities in, 182-184, 262
in aircraft engine inspection, 139, 276-277 surface roughness, 128
224, 225 in aircraft structural inspection, 31, 32, 139, thermal cracking of molds, 209
storage tank standards, 17, 218 cathode ray tubes (CRTs), 106
American Society for Nondestructive Testing 266-267 cellulose acetate replication, 291-294, 295, 296
angles of view, 141-142, 144, 145-146 center line hot cracking, in welds, 193
(ASNT), 16, 18-19, 32-33, 241 calibrated, 147 Central Certification Program, 18
ACCP (ASNT Central Certification clamps or tweezers for manipulation, 137 ceramics
components of, 137 injection molded turbine blades, 274-275
Program), 18 in composite materials inspection, 280, 282 ultrasonic attenuation in, 290
ANSI/ASNT CP-105, 18 eyeglasses and, 136 certification of personnel, 16, 18-19, 32-33
ANSI/ASNT CP-189, 16, 241 factors determining setup for, 136-137 for visual testing of welds, 241
American Welding Society (AWS), 17, 32, 33 field of view, 89, 136, 137, 142, 143,
AWS D1.1M, 246
Structural Welding Code, 17, 191, 241, 245 145, 146
angle of illumination, discontinuity detectability fixed focus, 141, 145
flexible, 28, 30, 138, 142-144, 266-267
and, 78
angle of viewing, in direct visual testing, 112-114
anisotropic surfaces, 128
Annual Book of ASTM Standards, 16
anomalous trichromatism, 69-70
Arab optics, medieval, 24, 25-26

Index 323

chaplets, unfused, on surface of casting, 262 correlated color temperature, 50, 54 diopter, 36, 64
charge coupled devices, 102-103, 161 corrosion, 200-201. See also exfoliation corrosion; direct visual testing, 112-133

color information with, 161, 162 stress corrosion cracking defined, vi, 13, 112
of video borescope, 148 aircraft lap joints, enhanced visual inspection, dimensional measurement, 130-133
charge injection devices, 161-162 illumination for, 116-120
chemical industry, 139, 212-214 173-176 magnification for, 121-126
chills, unfused, on surface of casting, 262 aircraft spoiler torsion bar, 268 mirrors in, 114-115
chromatic aberration, 46, 47, 122 crevice corrosion, 201, 261 observer’s attitude, 112
chromatic contrast, 116 fluid flow at high temperature and, 208 surface characteristics, 127-129
chromatic correction, 121 landing gear pitting corrosion, 271 as technique, 112
chromaticity. See hue replication for analysis of, 291 viewing angle, 112-114
circle gage, 149, 150, 151, 152 cosine cubed law, 43-44 discontinuities, 4-5, 6
cleaning cosine law, 43-44 defined, 180, 215
of fracture surface, for replication, 291 costs detected by simple visual test, 8
precleaning of discontinuities, 13 of manufacturing, 3, 4, 7 revealed by etching, 298
to prepare for etching, 297-298 of mechanical failure, 4 visual contrast and detection of, 116
cleanliness, visual testing of, 127 crack detection discontinuities in composite
before joining or painting, 213 computer algorithm for, 174-175
CMOS (complementary metal oxide vision acuity and, 77-78 materials, 279, 280-281
cracks, 4, 5, 8. See also fracture surface discontinuities in metals. See also steels
semiconductor) camera, 161 in ceramic turbine blades, 275-276
coaxial cable, 104 in composite materials, 278, 279, 280, 282 in aircraft engine components, 274
cobalt base alloys, elevated temperature photography of, 84, 85 in aircraft structures, 266
cracks in metals. See also fatigue cracks; fatigue in bolts, studs, washers, and nuts, 260-261
behavior, 207 in cast ingots, 182-184, 262
cocoa (ferrous debris), 206 cracks in aircraft; hydrogen cracking; stress in drawn products, 263
coddington magnifier, 122, 123 corrosion cracking; tears in metals elevated temperature discontinuities, 206-209
codecs, 103 of aircraft structures, 77-78, 266, 268-273 in extruded products, 263
coherent light, 119 cooling cracks, 185-186 factors affecting detectability, 180
coil breaks, in formed steel, 188, 190 creep cracking, 207 in forgings, 185, 186-187, 262
coil weld, in steel, 188 in forgings, 262 manufacturing stages affecting, 180-181
cold cracking, in welds, 193 revealed by etching, 298 primary processing discontinuities,
cold lap. See overlap of welds in rolled products, 262, 263
cold shuts, in castings, 182, 262 from secondary processing, 197-199 185-190, 260
cold start, of weld, 194 in welds, 192-194, 195, 247 in rolled metal, 185-190, 195, 198, 262-263
collimated light source, 118 Crampton, George, 30-32 secondary processing
color, 49-50 crater cracking, in welds, 194
crawlers, 138, 154, 214 discontinuities, 197-199, 260
with charge coupled device, 161, 162 creep, 206-207 service induced discontinuities, 200-209, 260
in digital images, 96-97 high temperature, 293, 295 from straightening after heat treating, 197
direct visual testing of, 128-129 microscopy of replicated microstructure, 293 of welds, 191-196, 215-216, 243-248
color balance, 97 creep cracking, 207 discriminant analysis, in image
color blindness, 69 creep growth of high alloy castings,
color discrimination, 62, 63, 69 photogrammetry of, 94 processing, 166-167
abnormalities of, 69-70 crevice corrosion, 201 distance measurement, 148, 149, 150-152
clinical testing of, 74-77 crevice corrosion cracks, on bolts, 261 distribution temperature, 54
in leak testing, 286-287 critical angle, 47 double-pass retroreflection surface inspection
lighting for, 116-117 crypt, borescope inspection, 139, 140
in liquid penetrant testing, 287 cutting tools, self-sharpening, 205-206 technique, 173
in magnetic particle testing, 288-289 doublet magnifier, 122, 123
color intensity, 128 D dyes
color saturation, 96, 98, 116, 128
color temperature, 49-50, 53, 54, 59 dark adapted vision, 51 in leak testing, 287
color value, 96, 128 data storage, 104-105 in visual testing of composite materials, 280
colorimeter, 129 data transfer, 104-105 dynamic response, 5, 6
comparison measurements, 150, 152. See also defect, defined, 180, 215 dynamic stress, 3
delaminations, in composite
reference standards E
complementary metal oxide semiconductor materials, 279, 280, 281
delayed cracking, of welds, 193 eddy current testing, 9
(CMOS) camera, 161 depth measurement, 149, 150, 152-153 edge breaks, in formed steel, 189
composite materials, 278-282 depth of field edge detection
Compressed Gas Association, 17, 230
computer display, 95-96, 97, 105, 106 borescope, 137, 141, 145 image processing algorithms, 164-165, 174
computerized design, inspection principles camera, 88-89, 90, 92 as visual ability, 62-63
machine vision system, 159 edge joint, 234
and, 133 magnifier, 121, 122 edge-of-light surface inspection technique,
condenser reflector, 118 diaphragm valves, 259
cones, retinal, 51, 62, 64-65, 69 dichromatism, 69, 70 173, 267
diffracted light technique, 267 Edison, Thomas, 28
abnormalities of, 69-70 for composite panel examination, 280-281 electric power industry. See also nuclear reactor
consultants, 14 diffuse front illumination, 119
contact reticle, 123-124 diffuse light source, 118 applications
contrast diffuse reflection, 45 borescope applications, 138-139
digital cameras, 87-88 push cameras, 153
in digital image, 98 digital image processing. See image processing electrical discharge damage, on jet engine turbine
etching for enhancement of, 297, 298 digital images, 95-99
photographic, 86, 91, 92 digital video, 100-101, 103-104, 105-106, 107 blades, 274-275
in visual inspection, 116, 117 dimensional anomalies, of welds, 215-216, electromagnetic spectrum, 5, 6, 44
contrast sensitivity, 68-69 electromagnetic test methods, 5, 6
luminous, 62 243-246 electromagnetic theory of light, 42
convolution operator, 164-165 dimensional measurement, 130-133 electron microscope analysis, of fracture surface
cooling cracks in metal, 185-186
copper alloys, stress corrosion cracking, 201, 202 in petroleum industry, 213, 221-222 replicas, 291-292
corner joint, 234 dimensional testing, 6 elevated temperature discontinuities, in metals,
corrective lenses, 68 dimensions, 5, 6
borescope use and, 136 206-209
crack detection and, 77, 78 embedded vision computer, 158
presbyopia and, 73 emissivity, 47-48
engines

aircraft, 139, 274-277
automotive, 139
thermal fatigue, 208

324 Visual Testing

equivalent sphere illumination photometer, 57 fluorescence, 63 H
erosive wear, 203-204 digital image and, 97
in liquid penetrant testing, 288 heat checking, 208
on bolts, 261 in magnetic particle testing, 288-289 of die casting molds, 209
fluid flow at high temperature and, 208 photography and, 87
etchants, 298-300 spectral response of eye and, 51 heat treating
etching, 297-301 creep properties related to, 207
for detection of adhesive wear in hardened fluorescent lighting sources, 119, 161 discontinuities caused by, 197, 198
fluting, in formed steel, 187
steels, 205 focal length helicopter blades, borescopy, 31-32
for microstructural replication, 293 herring bone porosity, in welds, 192
etching cracks, 198 camera, 90 history of visual testing, 24-33
ethernet, 104 lens, 46 hot cracking, in welds, 193-194
Euclid, 24, 25 magnifier, 121, 122 hot tears
European Committee for Standardization, 17 focal number, 92
exfoliation corrosion focal point, 88 in castings, 182-183, 262
aircraft wing spar, 85 focus in welds, 193-194
wrought aluminum plate, around fastener borescope, 136, 137, 141, 144 hough transforms, 171-173
camera, 88-89 hue, 96, 116, 128
holes, 281 folds, in bolting materials, 261 Hunayn ibn Ishaq, 26
eye forgings Huygens, Christian, 42
discontinuities in, 185, 186-187, 262 hydrogen blistering, 202, 203
components, 63-65 surface roughness, 128 hydrogen cracking, 202-203
focusing power, 64, 65 fovea centralis, 64, 65 pickling and, 198, 199, 202
spectral response, 51, 52 fracture. See cracks plating and, 198, 199, 202
eye protection filters, 23 fracture surface replication of microstructure, 293
eye relief, 121, 122 cellulose acetate replicas of, 291-292, 296 of welds, 193, 203
eyeglasses. See corrective lenses cleaning of, 291 hydrogen embrittlement, 198, 202
debris analysis, 291 hydrogen flakes, 185, 187
F silicone replicas of, 295 hyperopic eye, 65
fracture toughness, 206
f number, 92 frame grabber, 158, 163 I
farnsworth D 15 test, 76 frame rate, 100, 106, 107
farnsworth-munsell 100 hue test, 75-76 frequency, relationship to wavelength and Ibn al-Haytham, 25-26
farsightedness, 65, 68 Ibn Sahl, 24, 25, 45
fatigue cracks, 206 velocity, 44 IEEE 1394, 105, 163
fretting wear, 205-206 illuminance, 35, 53, 54
in petroleum industry, 213, 222 friction digs, in formed steel, 189
of shafts, fretting wear prior to, 205-206 furnaces cosine laws, 43-44
thermal, 207-208, 209 inverse square law, 43
in wrought aluminum panels, diffracted light petrochemical, photogrammetry of, 93, 94 for liquid penetrant testing, 288
power plant, periscope and camera for magnetic particle testing, 289
technique, 281 recommended levels for visual tasks, 116, 117
fatigue cracks in aircraft inspection, 139 retinal, 36
steel carburization in, 209 illuminance meters, 55
in B-47 bomber fleet, 31 future usefulness, and nondestructive testing, 2 illumination, 35-36. See also light sources
fuselage, 3 angle of, discontinuity detectability and, 78
slat drive bell crank, 268 G borescopy, 141, 144
spoiler lubrication hole cracks, 268-269 direct visual testing, 116-120
wing cracks under panel, 269-270 gages, 131-132, 242 indirect visual testing, 136, 137, 138
wing spar cap, 271-272 profile gage for pipe threads, 221, 222, 228 machine vision, 118-119, 159-161
Federal Aviation Administration, 17, 73, 74 photography, 86, 87, 91-92
ferromagnetic materials galvanic corrosion, 200-201 image processing
defined, 212 galvanizing tank material, resistance to molten composite materials and, 282
discontinuities in castings, 182 with diffracted light technique, 267
discontinuities in forgings, 185 zinc, 209 digital image characteristics in, 95-99
magnetic particle testing of, 8 gas metal arc welding, 239 for digital photography, 87
secondary processing discontinuities, 197 of digitized radiographic images, 288
service induced discontinuities, 200 lack of fusion in, 194 with edge-of-light surface inspection
tubes in chemical and petroleum industry, 212 gas, natural. See petroleum industry
fiber optic borescopes, 138, 142-144, 266-267 gas tungsten arc welding, 238-239 technique, 173, 267
fiber optic light guide, 144 gasket seal, oil field, 223-224 for indirect measurements, 148, 150
field of view gate valves, 257-258 integrity of image and, 98-99
borescope, 89, 136, 137, 142, 143, 145, 146 geometric transformations, in image processing, for machine vision, 158, 164-170
camera, 89 for machine vision in aircraft
machine vision system, 159 168, 169-170
magnifier, 121, 122 ghost lines, in formed steel, 188 inspection, 173-176
observer, 113 GigE Vision®, 163 for nondestructive testing in general, 286
file formats, digital, 95-96 girth welds, acceptance criteria, 217-218 image segmentation algorithms, 166-167
metadata and, 99 glare, 116, 117, 119 incandescent light, 119
video, 103-104 globe valves, 258 inclusions
fillet weld gage, 131-132, 242 gloss, 129 in bolts, 260
fillet welds gloss meters, 56 in cast ingots, 182, 183-184, 262
acceptance criteria, 216, 217, 218 gnomon, 151 in forgings, 262
components and terminology for, 240 gold alloys, stress corrosion cracking, 202 in rolled products, 262, 263
discontinuities in, 243-245, 247 goniophotometer, 57 in welds, 191, 192, 195
finishing operations, surface roughness produced gouging wear, 204 index of refraction, 44, 45
indexing borescope, 147
by, 128 on bolts, 261 indirect measurement, 148-153
Firestone, Floyd, 28 grain boundaries, revealed by etching, 298
flash drives, 104 graphite epoxy panels, diffracted light
flash line tears, in forgings, 186
flashlights, 117 inspection, 281
floppers, in formed steel, 189 gray scale image, 96-97
fluid flow at high temperature graybody, 48-49
Greek optics, 24-25
corrosion of components, 208 green rot, of stainless steel heating elements, 209
erosive wear of components, 208 grinding, to prepare for visual testing, 297, 298
grinding cracks in metals, 197-198
grinding wear, 204

Index 325

indirect visual testing, 13, 136-154. See also light sources. See also luminance McMaster, Robert, vi
borescopes for automated inspection, 118-119 measurement
for liquid penetrant testing, 287-288
applications, 138-140 for machine vision, 161 with calipers, 130-131
defined, vi, 136 measurable characteristics of, 53-54 of color, 128-129
designs for maneuvering cameras, 153-154 for photography, 86, 87, 91-92 dimensional, 130-133
illumination, 136, 137, 138 safety with, 20-23 with gages, 131-132
measurement, 148-153 standard for color vision testing, 75 of gloss, 56, 129
in petroleum industry, 214 test material characteristics and, 118 indirect, 148-153
test object factors in, 136-137 for visual testing, 116, 117, 118, 119 by photometry, 51-57
types of instruments for, 137-138 of ultraviolet radiation, 36
infrared radiation hazards, 20-21, 22 lighting. See illumination units for, 34-36, 53-54
infrared testing, 11-12 liquation cracking, in welds, 193-194 measuring magnifier, 123-124
infrastructure applications, 139 liquid metal contact, 209 mechanical gages, 131
push cameras in, 153-154 liquid penetrant testing, 8 medical endoscopy, 28
ingots. See castings mercury, stress corrosion cracking from contact
in-house programs, 14 visual aspects of, 287-288
integrating sphere, 54-55 logMAR vision acuity chart, 71 with, 209
integrating sphere photometer, 56-57 loupe, 122, 123 mesopic vision, 51
interference microscope, 125-126 lumens, 35, 42, 53, 54 metal joints. See brazed joints; soldered joints;
interference sealing threads, 223 luminance, 35, 36, 53, 54. See also light sources
International Commission on Illumination (CIE), welds
of pixel, 98 metallographic microscope, 125
color system, 129 luminance contrast, 116 metallurgical instabilities, 208
International Electrotechnical Commission, 17 luminescent light, 119 metallurgical microscope, 125
International Organization for Standardization luminous bodies, 43 metals. See also discontinuities in metals
luminous contrast sensitivity, 62
(ISO), 17, 19 luminous efficacy, 51, 53 cooling methods for components, 209
inverse square law, 43 luminous efficiency function, V (λ), 52, 54, 55 general oxidation of, at elevated temperature,
irradiance, ultraviolet, 36 luminous exitance, 53, 54
Ishihara® color plates, 75 luminous exposure, 53, 54 208-209
luminous flux, 35, 42, 53, 54 liquid, cracking of metals caused by, 209
J replication testing of surfaces, 291-296
integrating sphere photometer and, 56-57 scaling of, at elevated temperature, 208, 209
jaeger near point chart, 73-74 luminous flux calibration, 54-55 thresholds for elevated temperature
Japanese Institute of Standards, 17 luminous intensity, 53-54, 55
jet engines, 274-277. See also engines lux, 35, 53, 54. See also illuminance behavior, 207
JPG files, 95, 96 metal-to-metal seal, oil field, 223, 224
M metric units, 34
L michelson contrast, 68
machine shops, borescope applications, 138 microborescope, 266
laboratory microscope, 125 machine vision, 158-176 microdensitometry, in radiographic testing, 288
lamellar tearing, of welds, 195 micrometers, 131
laminations, in rolled metal, 185, 262-263 basic system architecture, 158 microscopes, 124-126
Lang, John, 32 camera calibration, 162
lanthony desaturated D 15 test, 76-77 camera interface, 158, 163 for fracture surface analysis of replication,
lap joint, 234 cameras, 158, 161-162 291-292
laplacian operator, discrete, 165 defined, 158
laps in metals, 185, 186, 262 illumination for, 118-119, 159-161 photography with, of composite
image processing for, 158, 164-170 materials, 278
in bolting materials, 261 for nondestructive testing, visible
revealed by etching, 298 microstructure, 6
laser light, 119 light, 173-176 metallurgical instabilities and, 208
hazards of, 20, 23 optics, 159 replication techniques, 291, 292-293,
LCDs (liquid crystal displays), 105 pattern recognition for, 170-173 295-296
lead, stress corrosion cracking, 202 machining tears in metals, 198
leak testing, 11 macro lens, 87, 90 microwelding, 205
visual aspects of, 286-287 macro photography, 90 mill scale, removal of, before etching, 297, 298
LEDs (light emitting diodes), 105, 161 macroetching, 297, 298, 300, 301 miniature borescope, 142
length. See distance measurement macula, 64 mirrors
lens law, 46 magnesium alloys, stress corrosion cracking, 202
lens of eye, 64 magnetic particle testing, 8 in direct visual testing, 114-115
aging, 70 visual aspects of, 97, 222, 288-289 in visual testing of welds, 242
lenses. See also corrective lenses; magnifiers magnification miter joint welds, acceptance criteria, 217-218
borescope, 145 borescope, 145 MPEG (Moving Picture Experts Group), 103-104
optical principles, 46-47 for direct visual testing, 121-126 multiple-segment length measurement, 149, 150,
photographic, 87-91 for indirect visual testing, 136, 137
lift check valves, 259 photographic, 91 151, 152
light single lens magnifier, 122 munsell color system, 129
generation of, 43 thin lens, 46 myopic eye, 65
health hazards, 19-20, 21-23 magnifiers
measurable characteristics of, 53 for aircraft applications of composite N
overview, 42
speed, 44, 45, 48 materials, 278, 280 Natural Color System, 128
theories of, 42-43 in aircraft maintenance, 266 Navy, US, visual weld testing, 32, 33
units for measurement of, 35-36 descriptive factors, 121 near vision acuity testing, 72-74
wavelength range, 42, 43, 44 illuminated, 124 nearsightedness, 65, 68
wavelength relationship to frequency and in magnetic particle or liquid penetrant necking, in drilling pipe, 222
necking down, of bolts, 261
velocity, 44 testing, 286 neural network, in crack classification
light adapted vision, 51 types, 121-124
light intensity. See illuminance Manhattan Project, 31, 32 algorithm, 174-175
light meters. See photometers Manufacturers Standardization Society, 17 Newton, Isaac, 42
Mars® letter contrast sensitivity chart, 69 nickel, stress corrosion cracking, 202
materials nickel base alloys, elevated temperature
demand for higher quality, 4
nondestructive characterization, 2, 6 behavior, 207
mathematical morphology, 167-169 nickel chromium steel, stress corrosion
Maxwell, James Clerk, 42
cracking, 202
nickel copper alloys, stress corrosion cracking, 202
nitrides, precipitated, 293, 294
Nitze, Max, 28
noise in digital images

exposure sensitivity and, 86
noise reduction algorithms, 165-166

326 Visual Testing

nondestructive testing, 2-12 magnification and, 91 plating cracks, 198-199, 202
defined, 2 as permanent record of testing, 84-85 plug valves, 259
methods classification, 4-6 supersession of technologies, 84 point-to-line measurement, 149, 150, 151, 152
methods overview, 7-12 videography and, 84, 94 polarized light, 119
methods versus techniques, 2-3, 112 photometers, 51-52, 120 polarizing microscope, 125
objectives and attributes, 4-6 NIST standard, 55, 57 polishing
purposes, 3-4 precautions for, 55
scope, 2 reference plane of, 55 for microstructural replication, 292-293
value, 7 related instruments, 56-57 to prepare for visual testing, 297, 298
photometric brightness, 54. See also luminance porosity
nuclear reactor applications photometric quantities, 53-54 in cast ingots, 182, 183, 262
borescopes in, 139 photometry, 51-57, 120 in ceramic turbine blades, 275, 276
leak testing, visual aspects of, 286 calibration, 54-55, 56, 57 in ceramics, ultrasonic attenuation and, 290
need for visual inspectors, 32 cosine laws in, 43-44 in forgings, 262
pan, tilt and zoom cameras in, 153 early methods, 52 in welds, 191-192, 246
pressure vessels, 250-256 instruments, 51-52 poultice corrosion. See crevice corrosion
underwater camera in, 91, 253-254 inverse square law in, 43 preattentive processing, 62, 63
radiometry and, 52-53 presbyopia, 64, 73
Nuclear Regulatory Commission, 250 spectral response of eye and, 51, 52 pressure vessels. See also ASME Boiler and Pressure
photopic vision, 51
O photosensitizers, 23 Vessel Code; boilers
phototubes, 101 bolts of, 259
object discrimination, visual, 62, 66-68. See also physical photometry, 52 for chemical and petroleum industry, 212-213
resolution pickling cracks, 198, 199, 202 leak testing with bromocresol purple, 287
pillowing nuclear reactor, 250-256
observer’s attitude, in direct visual testing, 112 aircraft lap joints, 174, 175 weld acceptance criteria, 216
oil. See petroleum diffracted light technique and, 267 primary colors, 50
oil field tubing and casing, 223. See also pipes, pinchers, in formed steel, 189 prisms, 47
pinhole camera, 26 profile gage, for pipe threads, 221, 222, 228
petroleum industry calibration for machine vision, 162-163 profilometry, of surface roughness, 127
types of seal, 223-224 pipe program management, visual testing, 13-14
opaque materials, 118, 119, 125 in bolts, 260 pseudocolor, 288
optics, 45-47. See also lenses in castings, 181, 182, 183 pseudoisochromatic plates, 75
historical development, 24-26 in forgings, 262 Ptolemy, 24, 25
orange peel discontinuity, in castings, 183, 184, in rolled products, 262, 263 pumps, 256-257
pipe and socket welds, acceptance criteria, 217 push cameras, 153-154
187 pipe threads, 223-228
overlap of welds, 196, 215-216, 246 arc burns on, 228 Q
oxyacetylene gas welding, 236-237 black crested, 224-225, 228
burrs on, 225, 226, 227 qualification of personnel, 16, 18-19, 32-33
P buttress thread areas, 225 for visual testing of welds, 241
buttress thread criteria, 227
P tile technique, 166 chamfer criteria, 225 quantum theory, 42
panoramic borescope, 146 of completely tightened makeup, 227-228 quenching of metals, cracks related to, 197, 198
parallax, 93-94, 151 coupling threads, 226-227
parallax bar, 94 galling on, 221, 225, 226 R
pattern recognition, 170-173 makeup triangle on, 227
Pelli-Robson letter sensitivity chart, 68-69 pin threads, 224-225 radiant energy measurements, 53
peripheral vision, 113 profile gage for, 221, 222, 228 radiant energy theories, 42-43
periscopes, 139, 146-147 round thread areas, 224 radiant flux, 42
personnel qualification and certification, 16, round thread criteria, 226
shoulders and, 228 measurement of, 101
18-19, 32-33 on used drill pipe, 221-222 radiation resistant optics, 31, 32
for visual testing of welds, 241 pipe welds radiographic testing, 9-10
perspective, 24 full length visual testing, 220-221
petrochemical furnaces, photogrammetry, 93, 94 linear porosity in weld root, 192 visual aspects of, 288
petrochemical plants, 139, 212-214 transmission pipe acceptance criteria, 218 radiometers, 57
petroleum industry applications pipelines, petroleum industry, 213 radiometry, 52-53
borescopes in, 29, 139, 220, 221, 222 linear porosity in weld root, 192 raster images, 95-96
casing in, 223 weld acceptance criteria, 218-219 RAW file format, 95, 96, 97
oil field hoisting equipment, 222 pipes, petroleum industry, 220-222. See also pipe reading borescope, 147
pipe and tube testing, 212, 220-222 reading glasses, 73
pipe threads, 221-222, 223-228 threads Recommended Practice No. SNT-TC-1A,
sealing types, on tubes and casings, 223-224 couplings, 221
role of visual testing in, 212-214 full length visual testing, 220-221 16, 18-19, 32-33
welds, acceptance criteria, 215-219 grades and minimum yield strengths, 220 red mud (ferrous debris), 206
phosphorescent materials, 51 straightness, 221 reference standards
photoelectric sensors, 101-102 used drill pipe, 221-222
photoemissive devices, 101-102 piping, weld porosity in form of, 192 digital image processing and, 99
photogrammetry, 92-94 pitting for indirect measurement, 150, 152
of surface roughness, 127 diffracted light technique and, 267 for welding, 241
photography, 84-92. See also cameras; image revealed by etching, 298 reflectance, 129
pitting corrosion, aircraft structures reflection, 24, 25, 45, 47
processing; video landing gear, 271 reflective characteristics of test object, 118, 137
advantages of digital methods, 85, 99 spoiler torsion bar, 268 reflectometers, 56
aperture, 92 pixel counting, 149, 152 refraction, 24, 25, 45-46, 88
brightness, 87, 91-92 pixels, 95-96, 97-98 colors and, 50
capturing maximum detail, 89-91 of charge coupled device, 102-103 by eye, 64, 68
of composite materials, 278 Planck, Max, 42 remote visual testing. See indirect visual testing
contrast, 86, 91, 92 Planck’s radiation law, 49 replication, 291-296
documentation of, 85, 99 plasma displays, 105 resistance welding, 236, 237
equipment, 86, 87-88 plastic deformation. See creep resolution
exposure time, 86-87, 92 charge coupled device, 103
flash, 86, 87, 92 digital image, 97-98
focus, 88-89 digital video display, 105
high temperature limitations, 94 for indirect visual testing, 136
illumination for, 86, 87, 91-92 machine vision system, 159
of indications, 85-87 video system, 101

Index 327

reticle. See also gnomon space shuttle, 77 structure, 5, 6. See also microstructure
borescope with, 147 spatial discrimination, 62, 66-68 sunlight, 119
contact, 123-124 special process, 16 supersession of technologies, 84
specifications, for visual testing, 15-16, 27 surface characteristics, 127-129
retina, 64-65 spectral distribution of lighting, 116-117
retinal damage, 20, 21-23 spectral emissivity, 48-50 indirect visual testing and, 136-137
retinal illuminance, 36 spectral luminous efficiency function, for photopic surface comparator, 123
retrospective borescope, 146 surface methods of nondestructive testing, 5
right angle borescope, 145-146 vision, 52 surface tears, in bolting materials, 261
rigid borescopes, 89, 138, 141-142. See also spectral power distribution, 53, 54, 57 surface texture, 127-128
swing check valves, 259
borescopes color assessment and, 129
robotic inspections, 138 spectrophotometers, 57 T
rods, retinal, 51, 62, 64-65 spectroradiometers, 57
rolled metal spectrum tape measures, 130
task lighting, 117
discontinuities evident after welding, 195 colors of, 50 tears in metals. See also cracks in metals; hot tears
discontinuities in, 185-190, 262-263 electromagnetic, vi, 5, 6, 44
scale rolled into surface, 198 of radiant energy, 48 in bolting materials, 261
room lighting, 117 specular reflectance measurement, 56, 129 flash line tears in forgings, 186
roughness, surface, 127-128 specular reflection, 45 lamellar tearing of welds, 195
rulers, 130 specular surface, 24 machining tears, 198
rust, removal of, before etching, 297, 298 spherical porosity, in welds, 191-192 in rolled products, 263
spot projector, 118 techniques of nondestructive testing, 2-3
S stainless steels, 209 direct viewing as, 112
standards for visual testing, 16, 17 tee joint, 234
safety frequent changes in, 84 telemetric instrument, 138
in etching, 298-299, 300 petroleum industry, 230 telephoto lens, 90, 91
public demands for, 4 standard illuminant C, 75 template matching, with digital image, 170-171
with ultraviolet sources, 21, 63 Statue of Liberty, 140, 201 texture, surface, 127-128
in visual testing, 19-23 statues thermal fatigue, 207-208
crevice corrosion, 201 thermal testing, 11-12
safety factor, modern reductions in, 3-4 indirect visual testing, 139, 140 threads. See pipe threads
sampling, 2 steam power plants, 138-139 three-dimensional viewing. See stereo
scanning electron microscopy, of replicated steam turbines, borescope applications, 30, 31 thumb flash drives, 104
steels TIF files, 95, 96
microstructure, 291-292, 293 adhesive wear, 205 titanium, stress corrosion cracking, 202
Schindler, Rudolph, 28 carburization, 209 titanium alloys, elevated temperature
scotopic vision, 51 limitations of inspection, 180
seams nonmetallic inclusions, 184 behavior, 207
plating cracks, 198-199 tolerance standards, 132-133
in bolting materials, 261 rolling of sheet, discontinuities evident after, tool and die shops, borescope applications, 138
revealed by etching, 298 tool marks, in bolts, 261
in rolled metal, 185, 262, 263 187-190 translucent test objects, 118-119
secondary colors, 50 segregation of alloying elements, 184 transmission factor, 35
segregation stainless, 209 transmission pipe welds, acceptance criteria, 218
in cast ingots, 182, 184 stress corrosion cracking, 201, 202, 209 transmittance, 35
revealed by etching, 298 surface colors, 128 transparent materials, 118
self-sharpening tools, 205-206 surface inspection for discontinuities, 180 triangulation, 149
separations, 6 threshold for elevated temperature triplet magnifier, 122, 123
service companies, 14 tripod, 86, 87
shadow measurement technique, 150, 151-152 behavior, 207 troland (Td), 36
shadows weld discontinuities related to properties of, tubes. See also pipes, petroleum industry
discontinuity detectability and, 78
lighting and, 117, 119 193, 194 boiler tubes, borescope applications, 138
shear breaks, in bolts, 261 Stefan-Boltzmann law, 49 in chemical and petroleum industry, 212, 223
shielded metal arc welding, 237-238 stereo measurement, 150, 151 Tuboscope®, 29
shop microscope, 124-125 stereo microscopes, 124 turbine blades, jet engines, 274-276
SI (International System of Units), 34-36 stereo photogrammetry, 93-94
signature analysis, 5, 6 stereoscopic machine vision system, 173-176 U
silicone rubber replicas, 294-296 sticker breaks, in formed steel, 190
size reduction, stress increase due to, 3, 4 stop check valves, 258 ultrasonic testing, 10-11
skew measurement, 148, 150, 152 storage tanks, petroleum industry, 213 borescope image for probe placement, 280
slag inclusions, in welds, 192, 195 diffracted light technique compared to, 281
slag removal weld acceptance criteria, 216, 218 visual aspects of, 289-290
in weld inspection, 242 strain, in formed steel, 187
in welding, 240, 243 strain replication, 293-294 ultraviolet borescope, 31, 147
slip marks, in drilling pipe, 213, 222 stress ultraviolet photographs, 87
slivers, in formed steel, 188 ultraviolet radiation
smart camera, 158 creep and, 207
snellen chart, 67 fatigue cracking caused by, 206, 207-208 eye’s response to, 51
snellen fraction, 66 modern demands on machines and, 3-4 fluorescence and, 63
snellen letters, 66-68 testing of, 5, 6 hazards of, 21, 63
snellen near point chart, 74 stress corrosion cracking, 201-202 units of measurement, 36
Snell’s law, 25, 45 aircraft spoiler torsion bar, 268 underbead cracking, of welds, 193
SNT-TC-1A. See Recommended Practice landing gear, 271 undercut, of welds, 195-196, 241, 247
liquid metal contact leading to, 209 uniform corrosion, 201
No. SNT-TC-1A as metallurgical instability, 208 units, 34-36
soldered joints, 248 replication analysis of microstructure, US Navy, visual weld testing, 32, 33
soldering, 235 USB (universal serial bus), 104, 163
solid state image amplifier, 101, 102 293, 294
solidification cracking, in welds, 193 stretcher strains, in formed steel, 187
South African Bureau of Standards, 17 string shot, of drilling pipe, 222
stringer bead along weld, 239
stringers, in rolled metal, 185, 262, 263
Structural Welding Code, 17, 191, 241, 245

328 Visual Testing

V VistechTM chart, 69 weber contrast, 68
visual abilities, 62-63 Weber’s law, 63
valves, 257-259 visual angle, 74 weight reduction, stress increase due to, 3, 4
vanadium bearing fuel oils, ash attack on visual photometry, 52 weld gages, 131-132, 242
visual processing, 62 welds
metals, 208 visual search, 63
vernier caliper, 131 basic processes for forming, 235-239
video prolonged, 77 complete penetration, 234-235
visual testing, 7-8. See also direct visual testing; dimensional anomalies in, 215-216, 243-246
analog connectors, 105 discontinuities, 191-196, 215-216, 243-248
analog transmission and recording, 106-107 indirect visual testing discontinuity classification, 215
analog versus digital, 100-101 adapting to new technologies for, 84 joint configurations with, 234-235
camera interfaces, 163 five basic elements of, 136 lack of fusion, 194, 196
codecs for, 103 limitations, 13 lack of penetration, 194-195, 247
data transfer and storage, 104-105 other nondestructive testing methods and, multipass, 240
digital displays, 105 partial penetration, 235
digital file formats, 103-104 286-290 revealed by etching, 298
photoelectric sensors for, 101-103 personnel qualification and certification, 16, terminology for, 239-241
in ultrasonic testing, for data display, 289-290 welds, visual testing
video borescopes, 137, 138, 148 18-19, 32-33 acceptance criteria, 216-219, 248
with image processing, 282 procedures, 14-15 borescope inspection, 139, 140, 147
in jet engine maintenance, 276 program management, 13-14 equipment for, 242
microelectronic, 267 reliability of results, 15-16 historical background, 32, 33
video cameras safety, 19-23 personnel qualification, 241
laboratory microscope with, 125 selection of, 13 in petroleum industry, 213
in nondestructive testing, 286 specifications, 15-16, 27 procedure, 241-243
for reactor pressure vessel testing, 253-256 standards, 16, 17 recording and reporting, 249
video probe, 138, 148 voids. See also pipe; porosity reference standard for, 241
video scope. See video probe in composite materials, 279, 280 surface preparation for, 242
video tape, 106, 107 creep voids in microstructure, 294, 295 white balance control, on digital camera, 87, 97
videography, 84, 94 volumetric methods, 5 Wien’s displacement law, 49
of composite materials, 278 Wolf, Georg, 28
viewing angle, in direct visual testing, 112-114 W Wolf, Richard, 28
vigilance decrement, 77 Wolf, Robert, 28
vision acuity, 62, 66-70 wagon tracks, in weld, 192 wormhole, in weld, 192
in borescope use, 136, 141 Wappler, Reinhold, 28
causes of reduction in, 68 warping Z
corrective lenses and, 68, 73, 77, 78
occupational requirements, 77-78 simple visual test for, 8 zoom, digital, 90
in radiographic interpretation, 288 straightening of metal, with cracking, 197 zoom, optical, 89-90
vision acuity charts, 71-72 washouts, of drilling pipe, 222 zoom cameras, 153, 154
vision acuity testing, 71-78 water treatment applications, push cameras zoom lens, 87-88, 90-91
color vision, 74-77
frequency of, 71 in, 153
general methodology, 72-74 wavelength, 44
near versus far, 72-74, 78
occupational requirements, 77-78 color and, 50
standardization, 71-72 wavelet methods, in image processing for crack

detection, 174, 175
waviness, surface, 127, 128
wear, 203-206

erosive, 203-204, 208, 261
silicone replicas of, 295

Index 329

Figure Sources

Chapter 1. Introduction to Visual Chapter 6. Indirect Visual Testing
Testing
Figure 8a — Lenox Instrument, Trevose, PA (1987).
Figure 13a, 16, 18, 19a — Public domain. Not Figure 8b — Olympus Corporation, Lake Success, NY
copyrighted.
(1986).
Figure 15 — Carnegie Mellon University, Pittsburgh, PA. Figure 9 — Olympus Industrial Systems Europa, Essex,
Figure 17 — ASME International, New York, NY (ASME
United Kingdom (Christopher I. Collins).
library, 1985). Figures 20-31 — General Electric, Skaneateles Falls, NY
Figure 19b — AMF Tuboscope, Houston, TX
(Trevor Liddell).
(Alfred E. Crouch, 1985).
Chapter 8. Visual Testing of
Chapter 2. Light Metals

Figures 2 to 9, 13b, 14 — Dover Books, New York, NY. Figures 4, 5, 10-21, 27 — American Iron and Steel
Not copyrighted. Institute, Warrendale, PA.

Figures 1b, 1c, 10, 11, 16, 18-19 — Illuminating Figures 28, 31, 32 — ASM International, Materials
Engineering Society of North America, Park, OH
New York, NY.
Chapter 9. Chemical and
Figures 15, 17, 20-21 — National Institute of Standards Petroleum Applications of Visual
and Technology, Gaithersburg, MD. Not Testing
copyrighted.
Figures 4-5. American Welding Society, Miami, FL.
Chapter 3. Vision Acuity for Figures 6-9. ASME International, New York, NY.
Nondestructive Testing Figure 10. American Petroleum Institute, Dallas, TX.
Figure 11. Exxon Production Research, Exxon Mobil,
Figure 5 — Not copyrighted (Joel Schneider, 2002).
Figure 6 — Mars Perceptrix, Chappaqua, NY. Irving, TX.
Figure 7 — National Vision Research Institute of Australia,
Chapter 10. Electric Power
Carlton, Victoria, Australia. Applications of Visual Testing
Figure 8 — Western Ophthalmics Corporation,
Figures 1, 7, 8, 10, 30, 32, 33, 36 — Magnaflux Division
Lynnwood, WA. of ITL, Glenview, IL.

Chapter 4. Imaging of Visual Tests Figures 2, 16, 21, 27 — ASM International, Materials
Park, OH.
Figures 4-5, 7, 9-11, 14, 19, 20 — General Electric,
Skaneateles Falls, NY (Trevor Liddell). Figures 17-20 — The Welding Institute (formerly British
Welding Institute), Abingdon, United Kingdom.
Chapter 5. Direct Visual Testing
Figure 23 — American Welding Society, Miami, FL.
Figure 4 — KD Marketing, Danaher Tool Group,
Sparks, MD. Chapter 11. Aerospace
Applications of Visual Testing
Figure 5 — After original illustration by Illuminating
Engineering Society of North America, Figures 11, 13 — Federal Aviation Administration. Not
New York, NY. copyrighted.

Figures 7, 8 — Edmund Scientific Corporation,
Barrington, NJ (1992).

Figure 10 — Bausch and Lomb, Rochester, NY.
Figure 12 — After original illustration by Hommel

America. ASNT is unable to contact Hommel
America as of 2009.

330 Visual Testing

1

CHAPTER

Introduction to
Visual Testing

Mohamed El-Gomati, University of York, Heslington,
North Yorkshire, United Kingdom (Part 3)
William J. Lang, Lenox Instrument Company, Trevose,
Pennsylvania (Part 3)
Marvin W. Trimm, Savannah River National Laboratory,
Aiken, South Carolina (Part 2)

PART 1. Nondestructive Testing

Scope of Nondestructive pressure testing is a form of proof testing
Testing that sometimes destroys the test object.

Nondestructive testing is a materials A gray area in the definition of
science concerned with many aspects of nondestructive testing is the phrase future
quality and serviceability of materials and usefulness. Some material investigations
structures. The science of nondestructive involve taking a sample of the test object
testing incorporates all the technology for for a test that is inherently destructive. A
process monitoring and for detection and noncritical part of a pressure vessel may
measurement of significant properties, be scraped or shaved to get a sample for
including discontinuities, in items electron microscopy, for example.
ranging from research test objects to Although future usefulness of the vessel is
finished hardware and products in service. not impaired by the loss of material, the
Nondestructive testing examines materials procedure is inherently destructive and
and structures without impairment of the shaving itself — in one sense the true
serviceability and reveals hidden test object — has been removed from
properties and discontinuities. service permanently.

Nondestructive testing is becoming The idea of future usefulness is relevant
increasingly vital in the effective conduct to the quality control practice of
of research, development, design and sampling. Sampling (that is, less than
manufacturing programs. Only with 100 percent testing to draw inferences
appropriate nondestructive testing can the about the unsampled lots) is
benefits of advanced materials science be nondestructive testing if the tested sample
fully realized. The information required is returned to service. If steel bolts are
for appreciating the broad scope of tested to verify their alloy and are then
nondestructive testing is available in returned to service, then the test is
many publications and reports. nondestructive. In contrast, even if
spectroscopy in the chemical testing of
Definition many fluids is inherently nondestructive,
the testing is destructive if the samples are
Nondestructive testing (NDT) has been poured down the drain after testing.
defined as those methods used to test a
part or material or system without Nondestructive testing is not confined
impairing its future usefulness.1 The term to crack detection. Other anomalies
is generally applied to nonmedical include porosity, wall thinning from
investigations of material integrity. corrosion and many sorts of disbonds.
Nondestructive material characterization
Nondestructive testing is used to is a field concerned with properties
investigate specifically the material including material identification and
integrity or properties of a test object. A microstructural characteristics — such as
number of other technologies — for resin curing, case hardening and stress —
instance, radio astronomy, voltage and that directly influence the service life of
current measurement and rheometry the test object.
(flow measurement) — are nondestructive
but are not used specifically to evaluate Methods and Techniques
material properties. Radar and sonar are
classified as nondestructive testing when Nondestructive testing has also been
used to inspect dams, for instance, but defined by listing or classifying the
not when used to chart a river bottom. various techniques.1-3 This approach to
nondestructive testing is practical in that
Nondestructive testing asks “Is there it typically highlights methods in use by
something wrong with this material?” In industry.
contrast, performance and proof tests ask
“Does this component work?” It is not In the Nondestructive Testing Handbook,
considered nondestructive testing when the word method is used for a group of test
an inspector checks a circuit by running techniques that share a form of probing
electric current through it. Hydrostatic energy. The ultrasonic test method, for
example, uses acoustic waves at a
frequency higher than audible sound.
Infrared and thermal testing and

2 Visual Testing

radiographic testing are two test methods satisfaction and maintain the
that use electromagnetic radiation, each manufacturer’s reputation, (5) to aid in
in a defined wavelength range. The word better product design, (6) to control
technique, in contrast, denotes a way of manufacturing processes, (7) to lower
adapting the method to the application. manufacturing costs, (8) to maintain
Through-transmission immersion testing uniform quality levels and (9) to ensure
is a technique of the ultrasonic method, operational readiness.
for example.
These reasons for widespread and
Purposes of profitable nondestructive testing are
Nondestructive Testing sufficient in themselves but parallel
developments have contributed to the
Since the 1920s, the art of testing without technology’s growth and acceptance.
destroying the test object has developed
from a laboratory curiosity to an Increased Demand on Machines
indispensable tool of fabrication,
construction, manufacturing and In the interest of greater performance and
maintenance processes. No longer is reduced cost for materials, the design
visual testing of materials, parts and engineer is often under pressure to reduce
complete products the principal weight. Weight can be saved sometimes
nondestructive test for quality. by substituting aluminum alloys,
Nondestructive tests in great variety are in magnesium alloys or composite materials
worldwide use to detect variations in for steel or iron but such light parts may
structure, minute changes in surface not be the same size or design as those
finish, the presence of cracks or other they replace. The tendency is also to
physical discontinuities, to measure the reduce the size. These pressures on the
thickness of materials and coatings and to designer have subjected parts of all sorts
determine other characteristics of to increased stress levels. Even such
industrial products. Scientists and commonplace objects as sewing
engineers of many countries have machines, sauce pans and luggage are also
contributed greatly to nondestructive test lighter and more heavily loaded than ever
development and applications. before. The stress to be supported is
known as dynamic stress or dynamic
How is nondestructive testing useful? loading, as opposed to static stress. It
Why do thousands of industrial concerns often fluctuates and reverses at low or
buy the test equipment, pay the high frequencies. Frequency of stress
subsequent operating costs of the testing reversals increases with the speeds of
and even reshape manufacturing modern machines, so components tend to
processes to fit the needs and findings of fatigue and fail more rapidly.
nondestructive testing? Modern Another cause of increased stress on
nondestructive tests are used by modern products is a reduction in the
manufacturers (1) to ensure product safety factor. An engineer designs with
integrity and in turn reliability, (2) to certain known loads in mind. On the
avoid failures, prevent accidents and save supposition that materials and
human life (Figs. 1 and 2), (3) to make a workmanship are never perfect, a safety
profit for the user, (4) to ensure customer
FIGURE 2. Boilers operate with high internal steam pressure.
FIGURE 1. Fatigue cracks caused damage to aircraft fuselage, Material discontinuities can lead to sudden, violent failure
causing death of flight attendant and injury to passengers with possible injury to people and damage to property.
(April 1988).

Introduction to Visual Testing 3

factor of 2, 3, 5 or 10 is applied. However, accident. This demand for personal safety
a lower factor is often used that depends has been another strong force in the
on considerations such as cost or weight. development of nondestructive tests.

New demands on machinery have also Rising Costs of Failure
stimulated the development and use of
new materials whose operating Aside from awards to the injured or to
characteristics and performances are not estates of the deceased and aside from
completely known. These new materials costs to the public (because of evacuations
could create greater and potentially occasioned by chemical leaks, for
dangerous problems. For example, an example), there are other factors in the
aircraft part was built from an alloy whose rising costs of mechanical failure.
work hardening, notch resistance and
fatigue life were not well known. After These costs are increasing for many
relatively short periods of service, some of reasons. Some important ones are
the aircraft using these parts suffered (1) greater costs of materials and labor,
disastrous failures. Sufficient and proper (2) greater costs of complex parts,
nondestructive tests could have saved (3) greater costs because of the complexity
many lives. of assemblies, (4) a greater probability that
failure of one part will cause failure of
As technology improves and as service others because of overloads, (5) the
requirements increase, machines are probability that the failure of one part
subjected to greater variations and will damage other parts of high value and
extremes of all kinds of stress, creating an (6) part failure in an integrated automatic
increasing demand for stronger or more production machine, shutting down an
damage tolerant materials. entire high speed production line. In the
past, when production was carried out on
Engineering Demands for Sounder many separate machines, the broken one
Materials could be bypassed until repaired. Today,
one machine is often tied into the
Another justification for nondestructive production cycles of several others. Loss
tests is the designer’s demand for sounder of such production is one of the greatest
materials. As size and weight decrease and losses resulting from part failure.
the factor of safety is lowered, more
emphasis is placed on better raw material Classification of Methods
control and higher quality of materials,
manufacturing processes and The National Materials Advisory Board
workmanship. (NMAB) Ad Hoc Committee on
Nondestructive Evaluation classified
An interesting fact is that a producer of techniques into six major method
raw material or of a finished product categories: visual, penetrating radiation,
sometimes does not improve quality or magnetic-electrical, mechanical vibration,
performance until that improvement is thermal and chemical/electrochemical.3
demanded by the customer. The pressure A modified version of their system is
of the customer is transferred to presented in Table 1.1
implementation of improved design or
manufacturing. Nondestructive testing is Each method can be completely
frequently called on to confirm delivery characterized in terms of five principal
of this new quality level. factors: (1) energy source or medium used
to probe the object (such as X-rays,
Public Demands for Greater Safety ultrasonic waves or thermal radiation),
(2) nature of the signals, image or
The demands and expectations of the signature resulting from interaction with
public for greater safety are widespread. the object (attenuation of X-rays or
Review the record of the courts in reflection of ultrasound, for example),
granting high awards to injured persons. (3) means of detecting or sensing
Consider the outcry for greater resultant signals (photoemulsion,
automobile safety as evidenced by the piezoelectric crystal or inductance coil),
required automotive safety belts and the (4) means of indicating or recording
demand for air bags, blowout proof tires signals (meter deflection, oscilloscope
and antilock braking systems. The trace or radiograph) and (5) basis for
publicly supported activities of the interpreting the results (direct or indirect
National Safety Council, Underwriters indication, qualitative or quantitative and
Laboratories, the Occupational Safety and pertinent dependencies).
Health Administration, the Federal
Aviation Administration and other The objective of each method is to
agencies around the world are only a few provide information about one or more of
of the ways in which this demand for the following material parameters:
safety is expressed. It has been expressed (1) discontinuities and separations (such
directly by passengers who cancel as cracks, voids, inclusions and
reservations following a serious aircraft

4 Visual Testing

delaminations), (2) structure or Classification by Test Object
malstructure (such as crystalline structure,
grain size, segregation and misalignment), Nondestructive test techniques may be
(3) dimensions and metrology (such as classified according to how they detect
thickness, diameter, gap size and indications relative to the surface of a test
discontinuity size), (4) physical and object. Surface methods include liquid
mechanical properties (such as reflectivity, penetrant testing, visual testing and moiré
conductivity, elastic modulus and sonic testing. Surface/near-surface methods
velocity), (5) composition and chemical include tap, holographic, shearographic,
analysis (such as alloy identification, magnetic particle and electromagnetic
impurities and elemental distributions), testing. When surface or near-surface
(6) stress and dynamic response (such as methods are applied during intermediate
residual stress, crack growth, wear and manufacturing, they provide preliminary
vibration), (7) signature analysis (such as assurance that volumetric methods
image content, frequency spectrum and performed on the completed object or
field configuration) and (8) heat sources. component will reveal few rejectable
discontinuities. Volumetric methods
Material characteristics in Table 1 are include radiography, ultrasonic testing
further defined in Table 2 with respect to and acoustic emission testing.
specific objectives and specific attributes Through-boundary techniques include
to be measured, detected and defined. leak testing, some infrared thermographic
techniques, airborne ultrasonic testing
Methods that use electromagnetic and certain techniques of acoustic
radiation (Table 3) can be divided emission testing. Other less easily
according to the segment of the spectrum classified methods are material
each uses as interrogating energy: radar, identification, vibration analysis and
thermography, visual testing and strain gaging.
X-radiography (Fig. 3). Methods using
vibration and ultrasound are in a different No one nondestructive test method is
spectrum: the acoustic. all revealing. In some cases, one method
or technique may be adequate for testing
The limitations of a method include a specific object or component. However,
conditions (such as access, physical in most cases, it takes a series of test
contact and surface preparation) and methods to do a complete nondestructive
requirements to adapt the probe to the test of an object or component. For
test object. Other factors limit the example, if surface cracks must be
detection or characterization of detected and eliminated and if the object
discontinuities or attributes and limit or component is made of ferromagnetic
interpretation of signals or images.

TABLE 1. Nondestructive test method categories. Test Objectives
Categories

Basic Categories

Mechanical and optical color; cracks; dimensions; film thickness; gaging; reflectivity; strain distribution and magnitude; surface
Penetrating radiation finish; surface flaws; through-cracks
Electromagnetic and electronic
cracks; density and chemistry variations; elemental distribution; foreign objects; inclusions; microporosity;
Sonic and ultrasonic misalignment; missing parts; segregation; service degradation; shrinkage; thickness; voids

Infrared and thermal alloy content; anisotropy; cavities; cold work; local strain, hardness; composition; contamination;
corrosion; cracks; crack depth; crystal structure; electrical conductivities; flakes; heat treatment;
Chemical and analytical hot tears; inclusions; ion concentrations; laps; lattice strain; layer thickness; moisture content;
Auxiliary Categories polarization; seams; segregation; shrinkage; state of cure; tensile strength; thickness; disbonds; voids

Image generation crack initiation and propagation; cracks, voids; damping factor; degree of cure; degree of impregnation;
Signal image analysis degree of sintering; delaminations; density; dimensions; elastic moduli; grain size; inclusions;
mechanical degradation; misalignment; porosity; radiation degradation; structure of composites;
surface stress; tensile, shear and compressive strength; disbonds; wear

anisotropy; bonding; composition; emissivity; heat contours; plating thickness; porosity; reflectivity;
stress; thermal conductivity; thickness; voids; cracks; delaminations; heat treatment; state of cure;
moisture; corrosion

alloy identification; composition; cracks; elemental analysis and distribution; grain size; inclusions;
macrostructure; porosity; segregation; surface anomalies

dimensional variations; dynamic performance; anomaly characterization and definition; anomaly
distribution; anomaly propagation; magnetic field configurations

data selection, processing and display; anomaly mapping, correlation and identification; image
enhancement; separation of multiple variables; signature analysis

Introduction to Visual Testing 5

material, then magnetic particle testing then ultrasonic testing or radiography
would be the appropriate choice. If the would be chosen. The exact technique in
material is aluminum or titanium, then each case depends on the thickness and
the choice would be liquid penetrant or nature of the material and the types of
electromagnetic testing. However, if discontinuities that must be detected.
internal discontinuities are to be detected,

TABLE 2. Objectives of nondestructive test methods.

Objectives Attributes Measured or Detected

Discontinuities and Separations

Surface anomalies roughness, scratches, gouges, crazing, pitting, imbedded foreign material
Surface connected anomalies
Internal anomalies cracks, porosity, pinholes, laps, seams, folds, inclusions

cracks, separations, hot tears, cold shuts, shrinkage, voids, lack of fusion, pores, cavities, delaminations,
disbonds, poor bonds, inclusions, segregations

Structure molecular structure; crystalline structure and/or strain; lattice structure; strain; dislocation; vacancy;
Microstructure deformation

Matrix structure grain structure, size, orientation and phase; sinter and porosity; impregnation; filler and/or reinforcement
distribution; anisotropy; heterogeneity; segregation
Small structural anomalies
Gross structural anomalies leaks (lack of seal or through-holes), poor fit, poor contact, loose parts, loose particles, foreign objects

assembly errors; misalignment; poor spacing or ordering; deformation; malformation; missing parts

Dimensions and Measures linear measurement; separation; gap size; discontinuity size, depth, location and orientation
unevenness; nonuniformity; eccentricity; shape and contour; size and mass variations
Displacement, position film, coating, layer, plating, wall and sheet thickness; density or thickness variations
Dimensional variations
Thickness, density

Physical and Mechanical Properties

Electrical properties resistivity; conductivity; dielectric constant and dissipation factor
Magnetic properties polarization; permeability; ferromagnetism; cohesive force, susceptibility
Thermal properties conductivity; thermal time constant and thermoelectric potential; diffusivity; effusivity; specific heat
Mechanical properties compressive, shear and tensile strength (and moduli); Poisson’s ratio; sonic speed; hardness; temper

Surface properties and embrittlement
color, reflectivity, refraction index, emissivity

Chemical Composition and Analysis

Elemental analysis detection, identification, distribution and/or profile
Impurity concentrations contamination, depletion, doping and diffusants
Metallurgical content variation; alloy identification, verification and sorting
Physiochemical state moisture content; degree of cure; ion concentrations and corrosion; reaction products

Stress and Dynamic Response heat treatment, annealing and cold work effects; stress and strain; fatigue damage and residual life
wear, spalling, erosion, friction effects
Stress, strain, fatigue corrosion, stress corrosion, phase transformation
Mechanical damage radiation damage and high frequency voltage breakdown
Chemical damage crack initiation, crack propagation, plastic deformation, creep, excessive motion, vibration, damping,
Other damage
Dynamic performance timing of events, any anomalous behavior

Signature Analysis potential; intensity; field distribution and pattern
Electromagnetic field
Thermal field isotherms, heat contours, temperatures, heat flow, temperature distribution, heat leaks, hot spots, contrast
Acoustic signature
noise, vibration characteristics, frequency amplitude, harmonic spectrum, harmonic analysis, sonic
Radioactive signature emissions, ultrasonic emissions
Signal or image analysis
distribution and diffusion of isotopes and tracers

image enhancement and quantization; pattern recognition; densitometry; signal classification, separation
and correlation; discontinuity identification, definition (size and shape) and distribution analysis;
discontinuity mapping and display

6 Visual Testing

Nondestructive Testing’s methods. The following section briefly
Value describes major methods and the
applications associated with them.
In manufacturing, nondestructive testing
may be accepted reluctantly because its Visual Testing
contribution to profits may not be
obvious to management. Nondestructive Visual testing is the subject of the present
testing is sometimes thought of only as a volume and of a volume in the previous
cost item and can be curtailed by industry edition.4
downsizing. When a company cuts costs,
two vulnerable areas are quality and Principles. Visual testing (Fig. 4) is the
safety. When bidding contract work, observation of a test object, either directly
companies add profit margin to all cost with the eyes or indirectly using optical
items, including nondestructive testing, so instruments, by an inspector to evaluate
a profit should be made on the the presence of surface anomalies and the
nondestructive testing. The attitude object’s conformance to specification.
toward nondestructive testing is positive Visual testing should be the first
when management understands its value. nondestructive test method applied to an
item. The test procedure is to clear
Nondestructive testing should be used obstructions from the surface, provide
as a control mechanism to ensure that adequate illumination and observe. A
manufacturing processes are within design prerequisite necessary for competent
performance requirements. When used visual testing of an object is knowledge of
properly, nondestructive testing saves the manufacturing processes by which it
money for the manufacturer. Rather than was made, of its service history and of its
costing the manufacturer money, potential failure modes, as well as related
nondestructive testing should add profits industry experience.
to the manufacturing process.
Applications. Visual testing is widely used
Nondestructive Test on a variety of objects to detect surface
Methods discontinuities associated with various
structural failure mechanisms. Even when
To optimize nondestructive testing, it is other nondestructive tests are performed,
necessary first to understand the visual tests often provide a useful
principles and applications of all the supplement. When the eddy current
testing of process tubing is performed, for
example, visual testing is often performed
to verify and more closely examine the

TABLE 3. Nondestructive test methods and corresponding parts of electromagnetic spectrum.

Interrogating Energy Test Method Approximate Approximate
Wavelengths (m) Frequencies (Hz)

X-rays or gamma rays radiography (RT) 10–16 to 10–8 1024 to 1017
Ultraviolet radiation various minor methodsa 10–8 to 10–7 1017 to 1015
Light (visible radiation) visual testing (VT) 4 × 10–7 to 7 × 10–7 1015
Heat or thermal radiation infrared and thermal testing (IR) 10–6 to 10–3 1015 to 1011
Radio waves radar and microwave methods 10–3 to 101 1011 to 107

a. Ultraviolet radiation is used in various methods: (1) viewing of fluorescent indications in liquid penetrant testing and
magnetic particle testing; (2) lasers and optical sensors operating at ultraviolet wavelengths.

FIGURE 3. Electromagnetic spectrum.

Radiation wavelength (nm)

106 105 104 103 102 10 1 10–1 10–2 10–3 10–4 10–5 10–6

Visible X–rays
light
Radio Infrared Ultraviolet Cosmic rays

Gamma rays

10–9 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10–1 1 10 102 103

Photon energy (MeV)

Introduction to Visual Testing 7

surface condition. The following surface as dry particles or as wet particles
discontinuities may be detected by a in a liquid carrier such as water or oil.
simple visual test: surface discontinuities,
cracks, misalignment, warping, corrosion, Applications. The principal industrial uses
wear and physical damage. of magnetic particle testing include final,
receiving and in-process testing; testing
Magnetic Particle Testing for quality control; testing for
maintenance and overhaul in the
Principles. Magnetic particle testing transportation industries; testing for plant
(Fig. 5) is a method of locating surface and machinery maintenance; and testing
and near-surface discontinuities in of large components. Some discontinuities
ferromagnetic materials. It depends on the typically detected are surface
fact that when the test object is discontinuities, seams, cracks and laps.
magnetized, discontinuities that lie in a
direction generally transverse to the Liquid Penetrant Testing
direction of the magnetic field will cause a
magnetic flux leakage field to be formed Principles. Liquid penetrant testing (Fig. 6)
at and above the surface of the test object. reveals discontinuities open to the
The presence of this leakage field and surfaces of solid and nonporous materials.
therefore the presence of the Indications of a wide variety of
discontinuity is detected with fine discontinuity sizes can be found regardless
ferromagnetic particles applied over the of the configuration of the test object and
surface, with some of the particles being regardless of discontinuity orientations.
gathered and held to form an outline of Liquid penetrants seep into various types
the discontinuity. This generally indicates of minute surface openings by capillary
its location, size, shape and extent. action. The cavities of interest can be very
Magnetic particles are applied over a small, often invisible to the unaided eye.
The ability of a given liquid to flow over a
FIGURE 4. Visual test using borescope to surface and enter surface cavities depends
view interior of cylinder. on the following: cleanliness of the
surface, surface tension of the liquid,
configuration of the cavity, contact angle
of the liquid, ability of the liquid to wet
the surface, cleanliness of the cavity and
size of the surface opening of the cavity.

Applications. The principal industrial uses
of liquid penetrant testing include
postfabrication testing, receiving testing,
in-process testing and quality control,
testing for maintenance and overhaul in
the transportation industries, in-plant and
machinery maintenance testing and
testing of large components. The
following are some of the typically
detected discontinuities: surface
discontinuities, seams, cracks, laps,
porosity and leak paths.

FIGURE 6. Liquid penetrant indication of
cracking.

FIGURE 5. Test object demonstrating
magnetic particle method.

8 Visual Testing

Eddy Current Testing wavelength or particulate radiation
(X-rays, gamma rays and neutrons).
Principles. Based on electromagnetic Different portions of an object absorb
induction, eddy current testing is perhaps different amounts of penetrating radiation
the best known of the techniques in the because of differences in density and
electromagnetic test method. Eddy variations in thickness of the test object
current testing is used to identify or or differences in absorption characteristics
differentiate among a wide variety of caused by variation in composition. These
physical, structural and metallurgical variations in the attenuation of the
conditions in electrically conductive penetrating radiation can be monitored
ferromagnetic and nonferromagnetic by detecting the unattenuated radiation
metals and metal test objects. The method that passes through the object.
is based on indirect measurement and on
correlation between the instrument This monitoring may be in different
reading and the structural characteristics forms. The traditional form is through
and serviceability of the test objects. radiation sensitive film. Radioscopic
sensors provide digital images. X-ray
With a basic system, the test object is computed tomography is a
placed within or next to an electric coil in three-dimensional, volumetric
which high frequency alternating current radiographic technique.
is flowing. This excitation current
establishes an electromagnetic field Applications. The principal industrial uses
around the coil. This primary field causes of radiographic testing involve testing of
eddy currents to flow in the test object castings and weldments, particularly
because of electromagnetic induction
(Fig. 7). Inversely, the eddy currents FIGURE 7. Electromagnetic testing:
affected by characteristics (conductivity, (a) representative setup for eddy current
permeability, thickness, discontinuities test; (b) inservice detection of
and geometry) of the test object create a discontinuities.
secondary magnetic field that opposes the
primary field. This interaction affects the (a) Primary Direction of
coil impedance and can be displayed in primary alternating
various ways. electromagnetic current
field
Eddy currents flow in closed loops in
the test object. Their two most important Coil in
characteristics, amplitude and phase, are eddy current
influenced by the arrangement and
characteristics of the instrumentation and probe
test object. For example, during the test of
a tube, the eddy currents flow Induced field
symmetrically in the tube when
discontinuities are not present. However, Induced field
when a crack is present, then the eddy
current flow is impeded and changed in Direction of Conducting
direction, causing significant changes in eddy current test object
the associated electromagnetic field.
Eddy current intensity
Applications. An important industrial use decreases with
of eddy current testing is on heat
exchanger tubing. For example, eddy increasing depth
current testing is often specified for thin
wall tubing in pressurized water reactors, (b)
steam generators, turbine condensers and
air conditioning heat exchangers. Eddy
current testing is also used in aircraft
maintenance. The following are some of
the typical material characteristics that
may affect conductivity and be evaluated
by eddy current testing: cracks, inclusions,
dents and holes; grain size; heat
treatment; coating and material thickness;
composition, conductivity or
permeability; and alloy composition.

Radiographic Testing

Principles. Radiographic testing (Fig. 8) is
based on the test object’s attenuation of
penetrating radiation — either
electromagnetic radiation of very short

Introduction to Visual Testing 9

where there is a critical need to ensure service and acoustic emission testing is
freedom from internal discontinuities. used because it gives valuable additional
Radiographic testing is often specified for information about the expected
thick wall castings and for weldments in performance of the structure under load.
steam power equipment (boiler and Other times, acoustic emission testing is
turbine components and assemblies). The selected for reasons of economy or safety
method can also be used on forgings and and loading is applied specifically for the
mechanical assemblies, although with acoustic emission test.
mechanical assemblies radiographic
testing is usually limited to testing for Applications. Acoustic emission is a
conditions and proper placement of natural phenomenon occurring in the
components. Radiographic testing is used widest range of materials, structures and
to detect inclusions, lack of fusion, cracks, processes. The largest scale events
corrosion, porosity, leak paths, missing or observed with acoustic emission testing
incomplete components and debris. are seismic; the smallest are microscopic
dislocations in stressed metals.
Acoustic Emission Testing
The equipment used is highly sensitive
Principles. Acoustic emissions are stress to any kind of movement in its operating
waves produced by sudden movement in frequency (typically 20 to 1200 kHz). The
stressed materials. The classic sources of equipment can detect not only crack
acoustic emission are crack growth and growth and material deformation but also
plastic deformation. Sudden movement at such processes as solidification, friction,
the source produces a stress wave that impact, flow and phase transformations.
radiates out into the test object and Therefore, acoustic emission testing is also
excites a sensitive piezoelectric sensor. As used for in-process weld monitoring, for
the stress in the material is raised, detecting tool touch and tool wear during
emissions are generated. The signals from automatic machining, for detecting wear
one or more sensors are amplified and and loss of lubrication in rotating
measured to produce data for display and equipment, for detecting loose parts and
interpretation. loose particles, for preservice proof testing
and for detecting and monitoring leaks,
The source of acoustic emission energy cavitation and flow.
is the elastic stress field in the material.
Without stress, there is no emission. Ultrasonic Testing
Therefore, an acoustic emission test
(Fig. 9) is usually carried out during a Principles. In ultrasonic testing (Fig. 10),
controlled loading of the test object. This beams of acoustic waves at a frequency
can be a proof load before service; a too high to hear are introduced into a
controlled variation of load while the material for the detection of surface and
structure is in service; a fatigue, pressure subsurface discontinuities. These acoustic
or creep test; or a complex loading waves travel through the material with
program. Often, a structure is going to be some energy loss (attenuation) and are
loaded hydrostatically anyway during reflected and refracted at interfaces. The
echoes are then analyzed to define and
FIGURE 8. Representative setup for locate discontinuities.
radiographic testing.
FIGURE 9. Acoustic emission monitoring of floor beam on
Radiation suspension bridge.
source

Test object

Void Sensor

Image plane Discontinuity
images

10 Visual Testing

Applications. Ultrasonic testing is widely pressurized components or into evacuated
used in metals, principally for thickness components. The principles of leak testing
measurement and discontinuity detection. involve the physics of liquids or gases
This method can be used to detect flowing through a barrier where a pressure
internal discontinuities in most differential or capillary action exists.
engineering metals and alloys. Bonds
produced by welding, brazing, soldering Leak testing encompasses procedures
and adhesives can also be ultrasonically that fall into these basic functions: leak
tested. In-line techniques have been location, leakage measurement and
developed for monitoring and classifying leakage monitoring. There are several
materials as acceptable, salvageable or subsidiary methods of leak testing,
scrap and for process control. Also tested entailing tracer gas detection (Fig. 11),
are piping and pressure vessels, nuclear pressure change measurement,
systems, motor vehicles, machinery, observation of bubble formation, acoustic
railroad stock and bridges. emission leak testing and other principles.

Leak Testing Applications. Like other forms of
nondestructive testing, leak testing affects
Principles. Leak testing is concerned with the safety and performance of a product.
the flow of liquids or gases from Reliable leak testing decreases costs by
reducing the number of reworked
FIGURE 10. Classic setups for ultrasonic products, warranty repairs and liability
testing: (a) longitudinal wave technique; claims. The most common reasons for
(b) transverse wave technique. performing a leak test are to prevent the
loss of costly materials or energy, to
(a) prevent contamination of the
environment, to ensure component or
Crack Back system reliability and to prevent an
surface explosion or fire.
Time
Bolt Infrared and Thermal Testing

Principles. Conduction, convection and
radiation are the primary mechanisms of
heat transfer in an object or system.
Electromagnetic radiation is emitted from
all bodies to a degree that depends on
their energy state.

Thermal testing involves the
measurement or mapping of surface
temperatures when heat flows from, to or
through a test object. Temperature

Transducer Crack FIGURE 11. Leakage measurement dynamic leak testing using
Crack vacuum pumping: (a) pressurized system mode for leak
(b) testing of smaller components; (b) pressurized envelope
mode for leak testing of larger volume systems.

(a)

Envelope

Leak detector

System
under test

Source of tracer gas

(b)

Envelope

Entry surface System
Crack under test

Leak detector

Source of tracer gas

Introduction to Visual Testing 11