ASNT NDT Handbook Volume 10 OVERVIEW

ACOUSTIC EMISSION TESTING I 333

As seen in Fig. 38, the acoustic emission counts (display- The printer's storage element consists of a parallel recir-
A) and envelope strength counts (display-B) are accumu- culating stack of 128 bytes that can store information from
lated during the envelope generated window. At the end of
the window, the accumulated counts are displayed in light acoustic emission bursts occurring less than 10 us apart; pri-
emitting diode (LED) counters. The counts are also sent to
a printer where the values are stored and then printed at the ority is given to storage rather than printing. The recirculat-
unit's earliest convenience.
ing concept actually expands the memory capability to more
than 128 bytes. Once the information in each memory loca-
tion is printed, it is no longer needed and can be written

FIGURE37. Blockdiagramof crackdetectionsystemappliedto rotationalsolderingoperationthat
joins metal caps to each end of capacitorscasing

SOLDER IRON CERAMIC

PRINTER

SENSOR ACOUSTIC ESTNRVEENLGOTPHE __ ._1
EMISSION COUNT
COUNT

ROTATING CAPACITOR

FROM BELLLABORATORIES.REPRINTEDWITH PERMISSION.

FIGURE38. Functionsof crack detectionsystemfor monitoringceramiccapacitorsoldering
operations

ENVELOPE. ENVELOPE DISPLAY-8
DETECTOR STRENGTH

ACOUSTIC
EMISSION IN

TRIGGER ACOUSTIC SPECIAL DISPLAY A
EMISSION CIRCUITRY
THRESHOLD ALARM
ALARM DRIVER B
DRIVER A

0.Dc __) ALARM DECISION

( _

FROM BELLLABORATORIES.REPRINTEDWITH PERMISSION.

334 I NONDESTRUCTIVE TESTING OVERVIEW

over during subsequent cycles. The electronics feed acous- transfer of the acoustic emission system from the laboratory
tic emission burst data to the printer until all the informa- to the manufacturing environment.
tion that is stored is printed.
Tubulation Pinchweld on the
The printout allows the operator to further analyze rele- Repeater Housing
vant acoustic emission bursts for indications of cracking in
the ceramic. This is done by determining the ratio of the The housing of the entire undersea repeater is designed
envelope strength counts (display-B) to the acoustic emis- to resist more than 75 MPa (11,000 lbrin.-2) of water pres-
sion counts (display-A). sure on the ocean floor. The tubulation pinchweld seals off
the pressurized nitrogen gas in the repeater housing and a
Experimental results show the following: if the count poor quality weld could cause premature failure of the cir-
ratio BIA falls within a specified range experimentallydeter- cuitry by admitting salt water into the unit.
mined to be 1.6 to 2.9, the burst is considered a signal
caused by cracking in the ceramic. In the past, the quality of the pinchweld was determined
with a radioisotope test, trying to force isotopes through
After successful work in the laboratory, the ceramic cracks or voids that might exist in the pinchweld area. Water
capacitor crack detector was installed in the production containing isotopes of cesium was used as the medium. After
line. The system included a calibrator that generated simu- the pinchweld area was subjected to this high pressure water
lated acoustic emission signals to check normal functions of for a specific time period, a Geiger counter was passed over
the transducer and the acoustic emission electronics.
Installation was accomplished by replacing a standard
nylon pivot on the soldering station with a specially modi-
fied transducer (Fig. 36) that also acted as a pivot for the
rotating capacitor. This was the only change required for

FIGURE39. Summaryof relationshipsamong weld currentflow time, timing windows,acoustic
emissionand acousticemissionenvelopes

TIME

WELD

CURRENT I
i - --- -- - ---·-- -....---L--DJ-.-~-~-Y_-l_-=--i_---_, l-I _-1_H·_1-1~21H1,~1,~1t+14-{11~11~1,~

I
II IPLASTIC DEFORMATION
WINDOW A

I =iL~PLAY2 132 ms

I COLD CRACKING
I WINDOW C
125 ms 245 ms

DISPLAY 3

HOT CRACKING

ACOUSTIC EMISSION ENVELOPES

DISPLAY 4 HOT CRACKING ENVELOPES

HOT CRACKING
MECHANICAL NOISE

I I DISPLAYS-]
7 CYCLES
AC WELD CURRENT POST WELD CRACKING
MECHANICAL NOISE
__ JII

FROM BELL LABORATORIES. REPRINTED WITH PERMISSION.

ACOUSTIC EMISSION TESTING I 335

the repeater housing to check for leakage. The time con- If the count exceeds an experimentally predetermined max-
sumed, the complexity of the facilities and the difficulties imum value, the weld is rejected because hot cracking has
associated with this test made its replacement desirable. occurred.

Pinchweld Analyzer An additional capability of the pinchweld analyzer is that
it can distinguish weld cracking from mechanical noise.
The pinchweld analyzer differs from the conventional Mechanical noise releases energy very slowly while the
acoustic emission resistance spot weld analyzer in that it can acoustic emission signals generated by weld cracking are a
perform the following functions: rapid energy release phenomenon. Weld cracking is identi-
fied by taking the ratio of the respective acoustic emission
1. measuring the degree of plastic deformation related cracking count (value in light emitting diode counter two or
to pinchweld thickness;
FIGURE40. Upper and lower controllimits:
2. detecting hot cracking during the welding cycle and (a) for acousticemissioncountsof window A in
cold cracking during the postweld period; and Fig. 39; (b) lower limit correspondsto
insufficientweld melting; (c) upper limit
3. recognizing mechanical noise generated during the representsexcessivemelting and possible
welding operation. materialexpulsion,indicatedby arrow

The pinchweld analyzer has five light emitting diode (aJ LOWER UPPER LIMIT
counter displays; three show acoustic emission counts and
two show envelope counts. The five displays are controlled LIMIT
by three time windows. The time windows are generated
externally by the weld current; that is, a time window is 110 144)
formed whenever the weld current surpasses a threshold,
and persists as long as the current remains above the thresh- 7 1.05 142) OUTSIDE DIAMETER: 3.2 mm (0. 13 in.)
old. Figure 39 summarizes the relationships among the weld 1.00 140) WALL THICKNESS 0.6 mm (0.025 in.)
current flow time, the timing windows, the acoustic emis- 0 0.95 138)
sion and the acoustic emission envelopes. A calibrator gen- 0. 90 (36) • 00 4P SENSOR 1 MHz
erates simulated acoustic emission signals used to check the ~x 0.85 134) :BAND-PASS: 200 kHz to 2.2 MHz
normal functions of the transducer and pinchweld analyzer. L~U 2_, 0.80 (32) ec oo
I~ .Q.....i, o.75 1301 oI
Window-A in Fig. 39, the first window, is opened during I
the time of welding current flow. This detects the number of I- ~
acoustic emission counts associated with material plastic oo I
deformation and material expulsion. Acoustic emission ·E
counts collected in this window are displayed in the first 0 .. •
light emitting diode counter. Upper and lower acoustic
emission count control limits for this window have been 0.70 128)
established experimentally (shown in Fig. 40). The lower
limit corresponds to insufficient weld melting and the upper 1,400 2,200 3,000 3,800 4,600 5,400 6,200
limit represents excessive melting, possible material expul-
sion, and therefore porosity formation. 1,800 2,600 3,400 4,200 5,000 5,800
ACOUSTIC EMISSION COUNT IN WINDOW A
To monitor postweld cold cracking, a window-B opens
after the weld period is completed. This acoustic emission (0 to I 32 milliseconds)
count is displayed in the second light emitting diode
counter. A higher count corresponds to the occurrence of (bJ (cJ
cold cracking and therefore a bad weld. An upper count
limit is established empirically; if the postweld cracking (20xj (200xj
count exceeds this value, the weld is rejected.
FROM BELLLABORATORIES. REPRINTEDWITH PERMISSION.
Hot cracking is monitored by window-C which opens
during the time that each half cycle of weld current is decay-
ing. Acoustic emission signals generated during this time are
related to residual stress buildup occurring during the cool-
ing period as weld current decreases. This acoustic emission
count is displayed in the third light emitting diode counter.

336 I NONDESTRUCTIVE TESTING OVERVIEW

three) to the corresponding envelope count occurring dur- quality analysis of the soldering operation has been achieved
ing the same period (fifth light emitting diode counter for and the reliability of the capacitor has been enhanced.
cold cracking and fourth light emitting diode counter for hot
cracking). If the ratio is less than an experimentally prede- In the case of the pinchweld analyzer, its ability to iden-
termined value, it is attributed to cracking. For a ratio
higher than the predetermined value, it is attributed to tify poor quality welds by detecting amounts of plastic
mechanical noise. deformation, material expulsion, hot and cold cracking and
mechanical noise interference has made it an excellent sys-
Over a six month field trial of the pinchweld analyzer, the tem for the undersea repeater pinchweld application.
acoustic emission results compared favorably with those of
the existing isotope test method, which was subsequently The cost of adding the acoustic emission weld analyzer to
discontinued. The ceramic capacitor crack detector is the pinchweld production line is minimal and is far out-
shown in production line operation in Fig. 41. weighed by eliminating the previous isotope test. The earlier
test facilityoccupied a lead lined room containing six test sites,
The detector provides a means of checking for cracking at which six repeaters simultaneouslyunderwent lengthy tests
during the soldering operation. Previously, only a visual in high pressure water. This space is now available for other
inspection was made after the capacitor was soldered. As a purposes. In addition, radioactive testing materials, requiring
result of using the acoustic emission technique, in-process strict adherence to Federal government regulations and
inspection procedures, have also been eliminated.

FIGURE 41 . Acousticemission crack detectorused during solderingof ceramic capacitors

FROM BELLLABORATORIES.REPRINTEDWITH PERMISSION.

ACOUSTIC EMISSION TESTING I 337

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342 I NONDESTRUCTIVE TESTING OVERVIEW

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ACOUSTIC EMISSION TESTING I 343

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97. Fourth International Symposium on Acoustic Emis­
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(July-September 1990): p 215-222.

10SECTION

INTRODUCTION TO ULTRASONIC
TESTING

Albert S. Birks, AKZO Nobel Chemicals, Axis, Alabama
Matthew J. Golis, Columbus, Ohio
Robert E. Green, Jr., Johns Hopkins University, Baltimore, Maryland

346 I NONDESTRUCTIVE TESTING OVERVIEW

PART 1

BASIC ULTRASONIC TESTING

Historically, nondestructive testing has been used almost Versatility
exclusively for detecting macroscopic discontinuities in
structures after they have been in service for some time. It The ultrasonic method permits testing of a wide range of
has become increasingly evident that it is practical and cost sizes and geometries. The technique detects internal, hidden
effective to expand the role of nondestructive testing to discontinuities that may be deep below the surface. Trans-
include all aspects of materials production and application. ducers and coupling wedges are available to generate waves
of several types, including longitudinal, shear and surface
Research efforts are being directed at developing and waves. Applications range from thickness measurements of
perfecting ultrasonic techniques capable of . monitoring thin steel plate to internal testing of large turbine rotors.
(1) materials production processes; (2) material integrity fol-
lowing transport, storage and fabrication; and (3) the Most nonporous, resilient materials used for structural
amount and rate of degradation during service. In addition, pmposes (steel, aluminum, titanium, magnesium and
efforts are under way to develop techniques capable of ceramics) can be penetrated. Even large cross sections can
quantitative discontinuity sizing, permitting determination be tested successfully for minute discontinuities.
of material response using fracture mechanics analysis, as
well as techniques for quantitative materials characteriza- Sensitivity and Directivity
tion to replace the qualitative techniques used in the past.
The use of a high frequency, well defined beam of sound
Advantages of Ultrasonic Tests permits detection and location of the smallest critical dis-
continuities. Disk shaped discontinuities or cracks of almost
Because only low amplitude, inaudible sonic waves are zero thickness can be detected.
used, the test object is not damaged by the test.
Depth can be indicated within millimeters, even in thick
Ultrasonic waves are mechanical vibrations, so ultrasonic test objects. The ultrasonic beams typically have good direc-
testing is especially suited to detection of elastic anomalies tivity, being confined to cones of low divergence by their
and measurement of physical properties such as porosity, short wavelengths. Angle beam techniques permit direction
structure and elastic constants. Optical, magnetic, chemical of the beam toward almost any area in the object.
and other properties are not ordinarily indicated. In order of
detectability, the anomalies typically determined with ultra- Convenience
sonics include geometric variations, gross discontinuities,
minute discontinuities and minute structure. There is no hazard to the operator or to nearby person-
nel during the use of ultrasonic testing. Access to only one
Ultrasonic test instrumentation is electronic and indica- surface is typical.
tions may be obtained in real time. This characteristic per-
mits rapid scanning with automatic positioning, plotting and Some equipment can be used in shop, laboratory, ware-
alarming. The ultrasonic beam almost instantaneously tra- house or field settings, allowing on-site testing. Moderate
verses the complete volume of material under the trans- power is needed from an alternating current line or a small
ducer extending from the front to the back surface of the generator. Portable units are battery powered.
test object. Each incremental scan requires only a fraction
of a second. With a 25 mm ( 1 in.) transducer and a pulse Training can be simple for many specific applications
repetition rate of 60 Hz, a steel roll 3 m ( 10 ft) long can be where the problem is clear cut and technique is established.
tested at the rate of a quarter ton per second. Because the
instrument response time is negligible, practical testing Ultrasonic testing techniques are widely accepted for
speeds are determined by factors such as the scanning quality control and materials testing in major heavy indus-
mechanism, handling equipment, human reaction time and tries, including electric power generation, production of steel,
pulse repetition rate. aluminum and titanium, in the fabrication of airframes, jet
engine manufacture and ship building. Ultrasonic techniques

INTRODUCTION TO ULTRASONIC TESTING I 347

are used to prevent failure of rolling stock axles, press Potential Applications
columns, earth moving gear, mill rolls, mining equipment and
other machines and components. Investigations indicate the possibility of applying ultra-
sonic techniques to a variety of problems such as:
Established Applications
1. metallurgical analysis and control of case depth and
Ultrasonic methods are commonly used for discontinuity hardness, precipitation of alloy constituents and grain
detection and thickness measurement. Examples of discon- refinement;
tinuity detection include:
2. detection of creep and microcracking in high temper-
1. mill components (rolls, shafts, drives and press ature components;
columns);
3. detection of incipient metal fracture;
2. power equipment (turbine forgings, generator rotors 4. theoretical crystallography and metallurgy; and
and pressure piping); 5. mechanical properties of liquids and plastics.

3. jet engine components (turbine blanks and compres- Limitationsof Ultrasonic Tests
sor rotors);
Conditions may exist that prevent the use of ultrasonics
4. airframe components (forging stock and frame sec- in certain testing problems. Some of these difficulties are
tions); related to one or more of these limiting factors: discontinu-
ity detectability, coupling and scanning problems and eco-
5. machinery materials (die blocks, tool steels and drill nomics of testing.
pipe);
Discontinuity Detectability
6. rolling stock (axles, wheels and crank pins);
7. railroad track maintenance (rail joint areas); Detectability is related to sensitivity, resolution and noise
8. nuclear reactors (clad fuel elements and heat discrimination. Briefly, sensitivity refers to the ability of the
instrument to detect a minute amount of sound energy
exchanger tubing); and reflected from a discontinuity. Resolution measures the
9. welds in pipelines and steel structures. ability to separate indications occurring close together from
two nearby points in an object. Noise discrimination is the
The discontinuities to be detected may be voids, cracks, capacity for indicating desired signals from discontinuities
inclusions, segregations, laminations, bursts, flakes, welding in the presence of simultaneous, unwanted signals of either
anomalies or other types. They may originate in the raw electrical or acoustical origin.
material, result from fabrication and heat treatment or
occur in service from fatigue, corrosion and other causes. Testing may be marginal or unsatisfactory if the test
Testing may be performed manually or automatically, by object is extremely thin, geometrically complicated or has
immersion or by contact, depending on the conditions. exceedingly coarse grain structure.

Thickness measurements are made principally on in- Ordinarily the size or type of discontinuity is not an
service piping, aircraft propellers, wing sections, armor actual limit in practice. For example, in the case of fatigue
plate, steel castings, submarine hulls and other structural cracks in axles near press fitted wheels, the smallest and
components. Numerous special instruments are available shallowest crack that can be detected is limited less by the
for thickness ranges from fractions of millimeters to several instrumentation or the material than by the acoustic noise
meters. generated by the press fits. Similarly, testing of threaded
components for shallow root cracks may not be feasible.
Instrumentation already developed can be applied to
many different problems, provided special techniques are Coupling and Scanning
established. Among such applications are:
Problems of coupling and scanning occur to some extent
1. rate of fatigue crack growth; in every application. Liquid or paste couplant that wets the
2. detection of bore hole eccentricity; surface must typically be used between transducer and test
3. measurement of elastic moduli; object. Certain soft solids can occasionally be used as cou-
4. study of press fits; plants. Total immersion of the object sometimes can be
5. investigation of corrosion rates;
6. control of weld quality;
7. metallurgical research relating to structure; and
8. examination of grinding wheels, ceramics and

concrete.

348 I NONDESTRUCTIVE TESTING OVERVIEW

considered. Extremely rough surfaces may prevent effective factors common to its successful use are clear definition of
sound coupling. Scanning of rough, complicated or small test problems, qualification of operators, adequate refer-
objects may be difficult or impossible for practical ultrasonic ence standards, practical test specifications or procedures,
testing. realistic acceptance standards, detailed test records and fre-
quent testing where required.
Criteria for Successful Testing
These criteria are common to the success of all types of
An appraisal of some specific industrial applications in nondestructive testing. Because ultrasonic testing repre-
which ultrasonic testing has been particularly successful sents considerable investment in equipment and labor and
indicates certain keys to success with the technique. Among can generate delays in production, careful attention to these
factors of success is always warranted. Where safe usage of
the product is contingent on careful testing, rigorous atten-
tion to these factors is essential.

INTRODUCTION TO ULTRASONIC TESTING I 349

PART 2

ULTRASONIC WAVES IN MATERIALS

Ultrasonic techniques play a role in evaluating micro- waves. Anisotropy is even more pronounced in composite
structures, associated mechanical properties, as well as materials than in crystalline metals and ceramics because of
detecting microscopic and macroscopic discontinuities in the filament placement processes.
solid materials. Although the possibility of using ultrasonic
waves for nondestructive testing applications was suggested Definitionof Wave and Wave
by Russian and German scientists as early as 1929 and 1931, Properties
it was Floyd Firestone's development of the Supersonic
Reflectoscopein 1942 that led to the practical instruments A wave is a disturbance that conveys energy through
and techniques used in the United States today. space in a manner that depends on both position and time.

Wave Propagation Rayleigh Waves

An ultrasonic wave propagating through a solid body can It has been observed experimentally in a variety of appli-
be used to measure material properties and property alter- cations that large amplitude waves propagate in solid mate-
ations throughout the volume of an object. Such an internal rials along the bounding surfaces. These waves are
examination offers definite advantages over other tech- constrained to lie near the surface and hence expand in only
niques that rely on surface measurements alone. Some two dimensions. Because of this fact, the effect of these
properties exhibited by the surface layer of a solid are not waves can be felt at greater distances from the wave source
exhibited by the bulk material. If surface characteristics are than the three-dimensional body plane waves. As an exam-
of interest, ultrasonic surface waves may also be used to ple of two-dimensional spreading, it is often observed that
evaluate the zone near the surface. Because the penetration the major damage following an earthquake is caused by
depth of such waves increases with decreasing frequency, waves that propagate at velocities slightly slower than the
tests can be performed at various material depths. More- phase speed of the transverse body wave.
over, a variety of guided waves is finding increased applica-
tion for testing interfaces between different materials. At ultrasonic frequencies in the megahertz region and
above, sound waves attenuate rapidly in air. Ultrasonic
The use of ultrasound in nondestructive tests has as a waves traveling in solid material almost totally reflect at a
prerequisite the careful documentation of the propagational boundary with air. For all practical purposes, the wave is
characteristics of ultrasonic waves. Ultrasonic testing is typi- constrained to remain in the solid and the boundary is con-
cally conducted with low amplitude waves that normally sidered to be in free space. Thus, the assumption of zero
obey linear elasticity theory and thus exhibit consistent and stresses on the boundary is very reasonable.
predictable propagation behavior.
Higher frequency waves are constrained to lie closer to
Material Anisotropy the surface of the solid than lower frequency waves. Thus by
varying the frequency of the surface wave used in an ultra-
Although most practical uses of ultrasonics are applied to sonic test, material properties may be determined at varying
solid materials (polycrystalline aggregates assumed to be depths in the material.
isotropic), with crystalline solids and composites the condi-
tion of ideal isotropy is nearly impossible to attain. The pri- The particle displacement associated with the Rayleigh
mary reason for this is that the solidification or forming and wave has components in both Y and Z directions. Hence
heating associated with materials processing do not permit Rayleigh waves cannot be classified as longitudinal or trans-
random distribution of crystallographic orientations among verse. Rather, Rayleigh waves have a combination of the
the aggregation of grains and often do not produce uniform motions associated with longitudinal and transverse waves.
grain structure. Particle displacement on the surface of the specimen moves
through an elliptical path as one complete cycle of the wave
Therefore, most polycrystalline aggregates possess a tex- passes a point on the surface. This motion is similar to the
ture that strongly influences the mechanical properties of the
anisotropic material, including its propagation of ultrasonic

350 I NONDESTRUCTIVETESTING OVERVIEW

path followed by a buoy floating on the surface of a lake as a (of an appropriate frequency) to interact with a variety of
water surface wave passes. The normalized longitudinal and discontinuities, including cracks, inclusions, precipitates,
transverse displacements for a plane Rayleigh surface wave grain boundaries, interphase boundaries, voids and disloca-
are a function of depth in the material. tions.

Linear Elastic Waves in Anisotropic Materials NonlinearElasticWaves

In general, three different linear elastic waves may prop- Nonlinear effects associated with ultrasonic wave propa-
agate along a given direction in an anisotropic material. gation arise from several causes. First, the amplitude of the
These three waves are usually not pure modes because each elastic wave may be sufficiently large so that nonelastic
wave typically has particle displacement components, both strains arise. Second, a material may behave in a nonlinear
parallel and perpendicular to the wave normal. One of these fashion when infinitesimal ultrasonic waves are propagated,
components is usually much larger than the other. The wave provided that a sufficient amount of external static stress is
with a large parallel component is called quasilongitudinal. superimposed or residual stress is present. This can occur
Waves with a large perpendicular component are called even if the material behaves in a linear fashion in its unde-
quasishear. In the event that the material is isotropic, then formed state. Finally, the material itself may contain various
all modes become pure modes - the particle displacements energy absorbing mechanisms so that it is locally nonlinear.
are either parallel or perpendicular to the wave normal and
the two quasishear modes degenerate into one pure shear Nonlinear elastic waves differ from linear elastic waves
mode. in several important ways. An initially sinusoidal nonlinear
longitudinal elastic wave of a given frequency distorts as it
The energy flux vector is also of practical importance to propagates and energy is transferred from the fundamental
elastic wave propagation in anisotropic materials. The direc- to the harmonics that appear. The degree of distortion and
tion of energy flow per unit time per unit area (the energy harmonic generation is directly dependent on the amplitude
flux vector) does not generally coincide with the wave nor- of the wave. A pure mode nonlinear longitudinal wave may
mal as it does in the isotropic case. That is, the ultrasonic propagate alone but a pure mode nonlinear transverse wave
energy fluxvector can be skewed even for normal incidence. cannot propagate without an accompanying longitudinal
wave. On the other hand, a nonlinear transverse wave does
Ultrasonic Attenuation not distort when it propagates in a solid free of discontinu-
ities.
For most solids, the assumption of pure linear elasticity
is an approximation because ultrasonic waves are attenuated Nonlinear elastic waves can interact with other waves in
by means other than geometrical (diffraction) spreading as the solid. At the intersection of two ultrasonic beams, addi-
they propagate. Then, to the lowest order of approximation, tional ultrasonic frequencies can be generated. Interaction
the general propagation characteristics of such waves in with thermal vibrations causes energy loss from the wave.
solid materials are found to be identical with linear elastic- The degree of interaction in all cases depends directly on
ity. This is true of the wave speeds, particle displacements, the amplitude of each wave.
energy flux vectors and diffraction spread. However, as a
result of various mechanisms, there is energy lost from these Ultrasonic Waves in Inhomogeneous Materials
propagating waves.
Additional problems arise with ultrasonic wave propaga-
Although geometric effects can cause energy loss, such tion in inhomogeneous materials. The presence of a single
losses are not indicative of intrinsic loss mechanisms associ- bounding surface complicates the propagational character-
ated with the-rnicrostructure. Once proper precautions are istics and can lead to erroneous interpretation of velocity
taken to eliminate or control these geometric effects, ultra- and attenuation measurements. The presence of many
sonic attenuation measurements serve as a very sensitive bounding surfaces, such as those in grossly porous or large
indicator of internal loss mechanisms caused by microstruc- grained ceramics and composites, complicates the propaga-
tures and microstructural alterations in the test material. tion characteristics even more. Except for a few special
This sensitivity derives from the ability of ultrasonic waves cases, these problems have not been solved analytically.

INTRODUCTION TO ULTRASONICTESTING I 351

PART 3

IMPLEMENTATION OF ULTRASONIC
TESTING

Transmission and Reflection 1 and 25 MHz. However, applications exist for frequencies
Techniques as low as 25 kHz and as high as hundreds of gigahertz. Vari-
ous ultrasonic testing techniques and instruments have been
Ultrasonic testing is typically performed in two ways. A developed to send ultrasonic energy directly through test
beam of ultrasonic energy is directed into the test object objects.
and (1) the energy transmitted through it is measured or
(2) energy reflected from discontinuities in the object is Low frequency resonance methods (in which the entire
measured. The first technique is called through­transmis­ test object is caused to vibrate at sonic frequency) should
sion ultrasonic testing; techniques of the second type are not be confused with ultrasonic methods whose probing
called pulse echo or reflection techniques. beam is typically restricted to a small fraction of the test
object volume.
Such tests are possible because an ultrasonic beam trav-
els with little loss through homogeneous material. Energy Ultrasonic Stress Ranges
loss occurs when the ultrasonic beam is intercepted and
reflected by grain boundaries or discontinuities in the elas- All mechanical testing methods use similar phenomena
tic continuum. Figure 1 shows these two basic techniques as described by the fundamental laws of mechanics and acous-
they are used for internal discontinuity detection. In Fig. la, tics. The various methods differ primarily in frequency and
the discontinuity is detected by the decrease of transmitted magnitude of stresses developed in the test material. Sonic
energy at the receiver. In Fig. 1b, it is detected by energy and ultrasonic nondestructive tests use low amplitude stresses
reflected to the receiver. that operate well below the yield stress of the material and do

Ultrasonic Test Systems FIGURE 1 . Basic ultrasonic testing methods:
(a) discontinuity detected by decrease of
Parts common to all ultrasonic test setups are shown in energy at receiver; (b) discontinuity detected
Fig. 1. They include (1) a transmitting transducer, (2) cou- by energy reflected to receiver
plant to transfer acoustic energy to the test object, (3) the
test object, (4) couplant to transfer acoustic energy to the /faJ TRANSMITTER TRANSDUCER
receiver and (5) a receiving transducer. - ACOUSTIC COUPLANT

Equipment selection, design and arrangement depend - TEST OBJECT
primarily on the specific characteristics of ultrasonic wave INTERNAL DISCONTINUITY
propagation being used for detection and measurement of
test object properties. The phenomena involved may fbJ ~ RECEIVER TRANSDUCER
include (1) velocity of wave propagation, (2) beam geometry RECEIVER TRANSDUCER
(focusing field pattern or dual transducer systems),
(3) energy transfer (reflection, refraction or mode conver-
sion) or (4) energy losses (scattering, absorption).

Ultrasonic Frequency Ranges INTERNAL
DISCONTINUITY
Ultrasonic testing uses high frequency elastic waves to
nondestructively inspect manufactured materials. Most com-
mercial ultrasonic testing is done at frequencies between

352 I NONDESTRUCTIVE TESTING OVERVIEW

not permanently affect the test object. Destructive mechan- wave trains to obtain reflections from minute discontinu-
ical tests, such as static physical tests and forced vibration ities. 6·7 This development was aided by a contemporary
fatigue testing, typically involve high amplitude stresses. growth of electronic instrumentation and technology. Dur-
These may cause heating, nonlinear effects, permanent ing the 1940s, Firestone's efforts led to the marketing of
deformation and eventual rupture of the component. practical ultrasonic discontinuity detectors in the United
States and abroad. In the same period, ultrasonic test equip-
Applications of Ultrasonic Techniques ment was developed independently in the United King-
dom." As with early industrial X-ray equipment, the first
Because ultrasonic techniques are basically mechanical ultrasonic instruments were primarily considered laboratory
phenomena, they are particularly adaptable for determining tools, typically installed in metallurgical research facilities.
the structural integrity of engineering materials. Their prin-
cipal applications consist of ( 1) discontinuity detection, Early Industrial Applications
(2) thickness measurement, (3) determination of elastic
moduli, (4) study of metallurgical structure and (5) evalua- Production applications were soon found for this tech-
tion of the effect of processing variables on the component. nology and ultrasonic testing was applied to many critical
quality control problems. Among the outstanding early
Advantages and Disadvantages of Ultrasonic Tests applications was the testing of railway axles and the first jet
engine rotor forgings for internal discontinuities.9
The desirable features of ultrasonic tests include ( 1) high
sensitivity, permitting detection of minute discontinuities; In the meantime, fundamental and applied research
(2) good penetrating power, allowing examination of continued and many significant contributions were made.
extremely thick sections; (3) accuracy in the measurement Firestone and his associates at the University of Michigan
of discontinuity position and estimation of discontinuity investigated transducer mechanisms, 10 polarized sound
size; (4) fast response, permitting rapid and automated test- using shear waves, 11 applications of Rayleigh or surface
ing; and (5) need for access to only one surface of the test waves, 12 a device for variable angle testing, 13 the delay col-
object for most applications. umn for near surface testing, 14 a pulsed resonance method
for thickness measurement15 and a variety of Lamb or plate
Test conditions that may limit the application of ultra- wave techniques.16 Other developments included frequency
sonic methods typically relate to one of the following fac- modulated resonance thickness gages, 17.18 improved immer-
tors: (1) unfavorable test object geometry (size, contour, sion testing systems19·20 and several ultrasonic imaging and
surface roughness, complexity and discontinuity orienta- discontinuity plotting techniques.21-26
tion) and (2) undesirable internal structure (grain size,
structure porosity, inclusion content or fine, dispersed pre- Ultrasonic Sources
cipitates).
Mechanical vibrations for nondestructive tests can be
Development of Tests generated by electromechanical transducers - devices with
the ability to transform electrical energy into mechanical
The possibility of using ultrasonic waves for nondestruc- energy and vice versa. For ultrasonic testing at frequencies
tive testing was recognized in the late 1920s and early 1930s above 200 kHz, piezoelectric transducers are generally used
in Germany by 0. Mulhauser,' A. Trost,2 R. Pohlman3 and (they can be used also at lower frequencies).
in Russia by S. Sokoloff,4 all of whom investigated various
continuous wave techniques. Discontinuity detection equip- Such materials generate electric charges when mechani-
ment was eventually developed, based on the principle of cally stressed and conversely become stressed when electri-
ultrasonic energy interception by a gross discontinuity in the cally excited. Piezoelectric elements mounted for ultrasonic
path of the beam. This technique later became known as the testing are commonly identified as transducers, search
through­transmission method. An ingenious transmission units, probes, transmitters (or receivers) or crystals.
system developed by Pohlman produced shadow images of
internal discontinuities.5 Later, several transmission discon- Ultrasonic Transducer Types
tinuity detectors were marketed.
Examples of common transducer materials having char-
During this early period, efforts were also made to use acteristics suitable for ultrasonic transducers are ( 1) natural
reflected waves as well as transmitted ultrasonic waves. quartz crystals, (2) lithium sulfate monohydrate crystals and
These tests were intended to overcome certain limitations of (3) polarized polycrystalline ceramics and lead zirconate
earlier methods, especially the need for access to both test
object surfaces. No practical method was found until Floyd
Firestone invented an apparatus using pulsed ultrasonic

INTRODUCTION TO ULTRASONICTESTING I 353

titanate. Transducer elements that operate as thickness Figure 2a shows a straight beam contact transducer.
expanders are widely used. These produce motion similar to These use thin wear plates to prevent crystal breakage and
that of an oscillating piston and generate compressional to protect the front electrode, which provides internal
waves in the test object. grounding. Facings of ceramic, metal, plastic and rubber
have been used. Applications include tests of rough surfaces
Typical Transducer Characteristics and electrical nonconductors.

Properties of a typical transducer element with frequency Angle beam transducers direct the ultrasonic beam away
for area A can be found using the relationship for thickness: from normal incidence toward selected areas within a test
object using a wedge between the crystal and the test object
(see Fig. 2b). These wedges or shoes are usually made from
plastic materials.

!1... (Eq. l) FIGURE2. Typicalultrasonictesting
transducers:(a) straightbeam contactunit;
f (b) angle beam transducer;(c) immersion
transducer
and for capacitance:
fa) CASE
c c1 x f x A (Eq.2)

Where:

thickness (millimeters); / SGNAf
1 MHz thickness (millimeters); CONNCEOCATXOIARL~'. ... ·.·.:' CONNECTION
frequency (megahertz);
capacitance (picofarads); .. . . ." : :; . . . • MOUNT AND
unit capacitance (capacitance per square
millimeter); and CON~~~~~ ·.·/·•·•••• /\·~ :~::~DES
A area (square millimeters).
\ <, PIEZOELECTRIC

ELEMENT

WEAR PLATE

Temporal Resolution and Sensitivity SIGNAL
CONNECTION
The temporal resolution of a transducer is directly pro-
portional to its bandwidth !if and is primarily a function of MOUNT AND
the damping produced by mechanical loading. Conversely, BACKING
the number of cycles required for crystal vibration to reach
full amplitude when driven by constant alternating current PIEZOELECTRIC
voltage is given by its mechanical Q (the reciprocal of band- ELEMENT
width expressed in percentage).
(c) COAXIAL WEDGE
Because the sensitivity of a given system increases CONNECTOR ---~
directly with the Q of its components (transducer, pulser ELECTRODES CASE
and electrical amplifier), a compromise is needed to achieve SIGNAL
optimum sensitivity and resolution. For a given Q, resolu- CONNECTION
tion increases directly with system frequency.
GROUND CONNECTION
Transducer Design Requirements

Inthe design of practical transducers for various applica-
tions, additional requirements include mechanical (contact
area, wear resistance, waterproofing and connectors), elec-
trical (voltages, wave shapes, capacitance and grounding)
and acoustic (noise level, cross coupling, damping and face
plates). Construction of three types of transducer assem-
blies is shown in Fig. 2.

354 I NONDESTRUCTIVE TESTING OVERVIEW

Angle beam transducers produce shear or surface waves beam area is likely to be much smaller than the test object
by refraction and mode conversion. Most of the commercial cross section and alignment is critical.
angle beam units operate in the 1 to 5 MHz range. Special
types include curved adapter shoes for pipe testing and vari- Discontinuity detectability of such a system depends on
able angle units for axial testing. (1) the ratio of discontinuity area to beam size, (2) the dis-
tance from discontinuity to transducers and (3) discontinu-
Transducers used in immersion testing (Fig. 2c) are sep- ity interference with wave propagation. In addition to these
arated from the test object by a couplant of considerable limitations, other problems may arise from this system con-
thickness. The transducer mounting must be thoroughly figuration, including spurious signals from multipath reflec-
tions, amplitude variations caused by minor geometry
waterproofed and a grounding electrode may be provided changes, undesirable resonances of test object or couplant
on the front face. Such transducers are available for all stan- and direct electrical cross talk between transducers. If con-
dard test frequencies'from 200 kHz to 25 MHz tact coupling is used, pressure effects are large. If immer-
sion is used, standing waves can sometimes occur when
The addition of plastic acoustic lenses to the front face of waves interfere after reflection. To minimize such variables,
immersion transducers makes possible the focusing of ultra- the electric signal applied to the transducer is often pulsed.
sonic beams. Cylindrical curvatures permit focusing the In this way, the impact of standing waves is reduced.
sound energy for entering cylindrical surfaces or along a line
focus. Spherical lens curvatures focus the sound at a point
determined by the radius of curvature.

Special Transducer Configurations Pitch and Catch Contact Testing

For applications requiring uncommon transducers, vari- Angle beam contact testing can also be conducted using
ous special styles, sizes and frequencies have been devel- two transducers in a pitch and catch mode. The transmitting
oped, including ( 1) dual elements with a common holder, transducer pitches a sound beam that skips in the plate and
(2) large paintbrush elements of 25 x 100 mm (1 x 4 in.) is caught by a receiving transducer (see Fig. 5).
(3) larger, multiple-element arrays, (4) small piezoelectric
crystals of 3 mm (0.12 in.) diameter and less, (5) high fre- FIGURE 3. Delay line test using buffer; buffer
quency (up to 100 MHz fundamental) elements, (6) alter- delay time is also called water path for
nate materials (lithium sulfate, fired titanate ceramics, immersion transducers
lithium niobate), (7) sandwich and tandem arrangements and fa)
(8) Y-cut quartz crystals for direct contact shear wave gener-
ation. FIRST BACK SURFACE ECHO

Delay Line Mode ------I-1fb) INTERFACE ECHO
FIRST BACK SURFACE ECHO
In the delay line mode, a solid or liquid buffer or delay JPOSSIBLEMULTIPLES
line is used to separate the transmit signal from the interface
signal of the test object surface. This configuration and the INITIAL
resulting test signals are shown in Fig. 3. The main difference PULSE
between this mode and the contact mode is that the buffer
delay time must be accommodated in signal processing. ·------·
BUFFER DELAY TIME
Through-Transmission Systems

Transmission systems depend on the principle that spe-
cific material characteristics of a test object produce signif-
icant changes in the intensity of an ultrasonic beam passing
through it. The entire thickness of the object can be tested
with this technique but no depth information is obtained.

Structurally, the equipment must consist only of an
ultrasound source, receiver, test object and couplant (see
Fig. 4). Typically, however, a scanning mechanism, gating
and a recording or alarm device are needed because the

INTRODUCTION TO ULTRASONIC TESTING I 355

FIGURE 4. Types of through-transmission testing systems

CONTINUOUS RADIO FREQUENCY RECTIFIER
AMPLIFIER
(al RADIO FREQUENCY

OSCILLATOR

(bl MODULATED RADIO FREQUENCY RECTIFIER
RADIO FREQUENCY AMPLIFIER
I
OSCILLATOR I

MODULATOR II .. -----, I , . . ALTERNATING

I

L- ~ DETECTOR - - ~' '\ CURRENT

J '- .,' VOLT METER

FREQUENCY SWEEP BANDPASS AMPUFIER 1-1 RECTIFIER

(cl RADIO FREQUENCY

OSCILLATOR

I ------- . .,DISPLAY
I-~'lI _ .J... ----,
SWEEP -1 I DETECTOR r-~ I
GENERATOR
lJ

l----------~---------- __ J

H~~
.. (S)(di FREQUENCY
PULSED RADIO

OSCILLATOR PULSE AMPLIFIER DETECTOR PEAK VOLT METER

RATE
GENERATOR

PEAK VOLT METER

PULSED PULSE AMPLIFIER DETECTOR GATE

(el RADIO FREQUENCY

OSCILLATOR

RATE
GENERATOR

356 I NONDESTRUCTIVE TESTING OVERVIEW

The distance between the two transducers can be cali- TABLE 1 . Types of ultrasonic system displays
brated to maximize the received signal amplitude. As shown
in Fig. 5 a planar discontinuity perpendicular to the plate Display Means of Indication
surface in the path of the sound beam deflects the sound Type Presentation
beam and prevents it from reaching the receiving trans-
ducer. Therefore, a loss of signal on the monitor indicates A-scan oscilloscope discontinuity depth
the presence of a discontinuity. and amplitude of signal
8-scan recording paper
If the transmit/receive switch is set in the open position, C-scan and computer discontinuity depth and
then it is possible to operate in the pitch and catch or through- Gated system monitor distribution in cross
transmission modes. In this configuration, separate transduc- sectional view
ers are used to generate and receive ultrasonic signals. recording paper
and computer discontinuity distribution
Amplitude and Transit Time Systems monitor in plan view

The most versatile techniques for ultrasonic testing of electricalsignal determined by technique,
solid materials use two parameters simultaneously: ( 1) the used for marker;
amplitude of signals obtained from internal discontinuities facsimileor chart
and (2) the time or phase shift required for the beam to recorder
travel between specific surfaces and these discontinuities.
This cycle is repeated (at rates varying from 60 Hz to
A-Scan Equipment 10 kHz). A continuous indication is displayed on an oscillo-
scope or digital display panel. The pulse repetition rate on a
The basic components of a pulse echo system are shown pulse echo system is adjusted so that reverberations within
in Fig. 6. Similar techniques are used in some radar and the test object decay completely between pulses.
depth sounding equipment, although the issues relating to
interpretation, resolution and frequency range are very dif- Several types of instrumentation are possible, depending
ferent. A short electric pulse is generated and applied to the principally on the methods of timing and indicating. Many
electrodes of the transducer. This produces a short train of applications can also be used, depending on the number and
elastic waves that are coupled into the test object. Timing style of transducers, the frequency range covered and the
circuits then measure the intervals between the transmittal means for coupling or scanning. In addition, numerous vari-
of the initial pulse and the reception of signals from within ations and combinations of circuit components can be incor-
the test object. porated into the equipment. A breakdown of basic
equipment types relates primarily to the method of present-
FIGURE 5. Pitch and catch angle beam testing ing discontinuity data as shown in Table 1.

(aJ TRANSMITTER RECEIVER A-Scan Presentation

Most ultrasonic test systems use a basic A-scan presenta-
tion (see Fig. 7). The horizontal baseline on the display indi-
cates elapsed time (from left to right) and the vertical

FIGURE 6. Pulse echo testing system

AMPLIFIER

(bJ TRANSMITTER RECEIVER

BACK SURFACE TIMER DISPLAY

INTRODUCTION TO ULTRASONIC TESTING I 357

TABLE 2. Values for total reflection time in typical FIGURE 8. Basic ultrasonictestingmethods:
materials (a) through-transmission(b; ) doubletransducer
reflection;(c) singletransducerreflection;
Material LongitudinalWaves Shear Waves (d) angle beam contact;(e) immersion
f microseconds f microseconds
faJ
per millimeter) per millimeter)

Liquid 1.35 1.43
Acrylic resin 0.74 0.62
Steel 0.34

deflection shows signal amplitudes. For a given ultrasonic fbJ u-- 2
velocity in the test object, the sweep can be calibrated fcJ
directly in terms of distance or depth. Conversely, when the fdJ I --- 2
through-wall dimensions of the test object are known, the feJ
sweep time can be used to determine ultrasonic velocities
from which elastic moduli can be calculated. The signal
amplitudes represent the intensities of transmitted or
reflected beams. These may be related to discontinuity size,
test object attenuation, beam spread and other factors.

As shown in Fig. 8, several test techniques can be used,
including through-transmission or reflection, single or double
transducer systems and contact or immersion coupling. Other
possibilities include angle beam and surface wave techniques
using single or dual transducers and various combinations of
methods. Single transducer operation with single or dual ele-
ments is used for most reflection testing applications.

The time intervals for sweep length can also be com-
puted from the known velocities in the materials of interest.
The values in Table 2 are approximated for the total reflec-
tion time in typical materials.

FIGURE 7. Diagram of pulse echo A-scan system

JAMPLIFIER PULSER DRANSDUCER
TEST
OBJECT

SWEEP ·-----. GRAETNEERATOR
GENERATOR

LEGEND
T = TRANSDUCER TRANSMITTING SIGNAL

R = TRANSDUCER RECEIVING SIGNAL

I = SCANNING POSITION ONE
2 = SCANNING POSITION TWO

358 I NONDESTRUCTIVETESTINGOVERVIEW

A-Scan Sensitivity TABLE 3. Alternative circuit components for
ultrasonic test systems
In industrial testing, the sensitivity of commercial A-scan
instruments is typically adequate for detecting the smallest Component Options
discontinuities of concern (including cracks, inclusions and
porosity), although other limitations such as resolution may Synchronizer
exist. Pulse repetition rate fixed, adjustable with fixed range,

Reasonable amplifier linearity is desirable for calibration stepped with sweep range, variable
and discontinuity comparison. Reading accuracies of one
part in twenty are sufficient, because discrepancies due to (50 Hz to 1,000 Hz)
other variables such as coupling and alignment predomi-
nate. Precision markers of high sweep linearity are ordinar- Locking signal line voltage (or harmonic), internal
ily not required for discontinuity detection, although one or
the other may be necessary for accurate thickness or veloc- Pulser impulse, spike, gated sine wave,
ity measurements. In some cases precision markers are pro- Wave shape damped wave train
vided as special system accessories.
Type tunable, impedance matched, variable
The temporal resolution of A-scan systems (the ability to
detect a small signal immediately after a large one) is limited amplitude
to about 5 cycles, depending on the relative signal ampli-
tudes involved. Circuit thyratron, pulsed oscillator

Alternative System Features and Construction Amplifier

Among the alternative configurations for ultrasonic cir- Type tuned radio frequency.wide band
cuit components are those listed in Table 3. Response linear; sharp cut off, logarithmic

Special instrumentation needed for some testing appli- Sensitivity time variable gain or constant
cations may include the following: (1) provision for dual Controls gain, input attenuation, reject, variable
transducer operation, (2) interference elimination circuitry,
(3) compensation for long transducer cables, (4) stabiliza- bandwidth
tion for extreme line voltage changes, (5) exponential cali-
brator for attenuation measurements, (6) exceptional Signal display
portability for field use, (7) remote indicators, (8) extended
frequency range (above 100 MHz), (9) high resolution and Type radio frequency wave train, video
(10) computer interface and on-board digital memory.
Source radio frequency output rectified
8-Scan Presentation
(envelope),differentiated video
When the shape of large discontinuities or their distribu-
tion within a test object cross section is of interest, the Sweep logarithmic. high linearity, conventional
B-scan display is the most useful. A B-scan shows a cross Type adjustable, automatic, none
sectional view of the part by displaying depth as a function Delay
of transducer lateral translation. In addition to the basic
components of the A-scan unit, provision must be made for Expansion fixed, adjustable, related to sweep
these additional B-scan functions: (1) intensity modulation
or brightening of the oscilloscope spot (pixel) in proportion Marker
to the amplitude of the discontinuity signal, (2) deflection of
the display panel trace in synchronism with the motion of Type fixed scale on cathode ray tube,
the transducer along the test object and (3) retention of the precisionelectronic, adjustable square
image by a long persistence display screen.
wave, movable step mark
Often a B-scan display is used in conjunction with A-scan
testing or as an attachment to standard A-scan equipment. Source crystaloscillator,adjustable
Therefore, the system design criteria depend on A-scan
multivibrator; precision integrator

Display superimposedon signals, alternate

sweep, separatetrace, intensity

modulated

Signal gate amplitude proportional
Type direct current level, modulated,
Output

rectangular wave, pulse stretched

System

Frequencyrange single, selectable, continuous tuning, low
(50 to 200 kHz), intermediate (200 kHz

to 5 MHz), high (5 to 25 MHz)

INTRODUCTION TO ULTRASONIC TESTING I 359

equipment and the testing application. Where high speed Gated Systems
scanning is required, the longer persistence of the B-scan
display may be an advantage to the operator. In general, gating is needed for all automatic systems that
alarm, mark, record, chart or otherwise replace visual inter-
The effectiveness of the B-scan in showing discontinuity pretation. Such gating circuits may be built into the disconti-
detail depends on the relationship of discontinuity size, nuity detector or supplied as separate attachments.
beam area and wavelength. Optimum results are obtained
with larger discontinuities, smaller transducers and higher Commercial recording attachments typically provide at
frequencies. For other conditions, beam sharpening tech- least two gates, one to indicate the presence of discontinu-
niques such as focusing and electronic contrast enhance- ities in the fest object and the second to show a decrease in
ment may be needed. back reflection. Some units provide additional discontinuity
gates so that two or more alarm levels can be set or so that
C-Scan Presentation different depth increments can be tested.

By synchronizing the X,Y position of the display spot If the cross section of the test object varies during the
(pixel) with the transducer scanning motion along two coordi- scanning cycle in an automatic test, the gating periods must
nates and modulating the intensity proportional to the discon- be simultaneously adjusted. In addition, other functions
tinuity indication, a C-scan, or plan view, of the test object can such as sensitivity, transducer angle and recorder speed may
be developed. have to be controlled.

In addition to the circuitry required for a B-scan, provi- System Calibration
sion must be made for eliminating unwanted signals such as
the initial pulse, interface echo or back reflection, which Interpretation of test indications is discussed in detail in
obscure internal discontinuity signals. An electronic gate is several subsequent sections. To permit such an evaluation,
used to render the display circuits sensitive only for the appropriate calibration techniques and reference standards
short intervals of sweep time when signals from the desired are necessary. This is particularly true in application of pulse
depth range occur (see Fig. 10). echo instruments because of the large number of variables.

In certain cases, hybrid systems present some data about There is no equivalent of the radiographic penetrameter
discontinuity size and location with a sacrifice in discontinu- in ultrasonic testing. In most ultrasonic tests, only a consis-
ity shape and position detail. tent calibration procedure is practical. This typically involves

FIGURE 9. Diagram of typical ultrasonic 8-scan FIGURE 10. Diagram of simple ultrasonic
presentation C-scan presentation

RATE
GENERATOR

DISPLAY

INTENSITY

-X-AXIS TRANSDUCER

~SITION,........-.:;;;...____, ~L,~- --··_~· I
---~TESTING
DEPTH

360 I NONDESTRUCTIVE TESTING OVERVIEW
FIGURE 11 . Parameters affecting signals in ultrasonic pulse echo tests

--- ------- -INSTRUMENT CONTROLS RECEIVER ----------
ELECTRONICS
AMPLIFIER
PULSER

TRANSDUCER BACKING
TRANSDUCER

LEGEND p2 MATERIAL DENSITY
V2 MATERIAL VELOCITY
v; PULSER OUTPUT
Cl2 MATERIAL ATTENUATION
VR DISCONTINUITY SIGNAL f1 TEST OBJECT SURFACE
Z1 INPUT IMPEDANCE f2 DISCONTINUITY SURFACE
<!>1 = ANGLE OF INCIDENCE
Z2 COUPLING IMPEDANCE
D1 TRANSDUCER DIAMETER <!>2 = ANGLE OF REFRACTION
D2 REFERENCE HOLE DIAMETER
p I COUPLANT DENSITY <1>3 = DISCONTINUITY ANGLE
V1 COUPLANT VELOCITY
a1 COUPLANT ATTENUATION

INTRODUCTION TO ULTRASONICTESTING I 361

comparison of test condition indications with indications FIGURE 1 2. Typical reference blocks used for
from reference blocks containing drilled test holes, notches studies of ultrasonicsystem calibration
or from reference surfaces within the test object. (aJ

Neither method provides a direct calibration of system d
response to discontinuities that may be located anywhere
within the test object cross section. Except for certain types (bJ T~l-·-j~
of curved test objects, signals from the front face (interface ,--- -,
reflection) or back surface (first back reflection) typically d
have amplitude too great for use as references during tests
at high sensitivity for small discontinuities. However, the use (cJ
of precision decibel attenuators makes it possible to deter-
mine the ratio between back reflection and discontinuity d
reflection amplitudes. The use of small reflecting surfaces
placed at some point in the beam path but external to the (dJ
test object in the manner of radiographic penetrameters has
only limited advantages and is usually impractical, except for
certain conditions in immersion testing. For the most part,
calibration of ultrasonic instruments is done with sets of

special reference blocks intended for specific applications.
The required accuracy of calibration can vary greatly

with the application. In some cases, only a crude check on
instrument performance is necessary. In other tests, only
reference settings are needed, so that results can be dupli-
cated later. When acceptance of material is based on rigid
ultrasonic test specifications, considerable attention must be
given to calibration techniques. Fabrication of duplicate sets
of reference blocks required for such applications can prove
to be a serious problem.

Major System Parameters d

The components of an ultrasonic testing system and (eJ
their principal variables are shown in Fig. 11. Some of the
characteristics of such a system can be anticipated from fun- (fJ ~~MLLSUPPORT
damental principles of ultrasonic testing. Others may be
related primarily to the specific components used in the ~~
instrument or to the transducers, wave shape, amplifier
recovery time and transducer field patterns. IMMERSION TRANSDUCER STAINLESS STEEL BALL
LEGEND
Operator Controlled Parameters
T = TEST SURFACE
The major parameters controlled by the operator relate
directly to choice and operation of the test system. Instru- D = DIAMETER OF TEST BLOCK
ment type and transducer type and size are an initial deci-
sion, followed by the choices of ( 1) technique (coupling d = DIAMETER OF TARGET
method, scanning sequence and peaking procedure) and a = ANGLE OF TEST SURFACE
(2) the control settings (frequency, pulse length, electronic
distance amplitude correction [DAC]). L = LENGTH (TEST SURFACE TO TARGET)

Parameters Controlled by Testing Application

Some parameters are determined by the specific test-
ing problem, including: (1) test object properties (velocity,

362 I NONDESTRUCTIVE TESTING OVERVIEW

impedance, geometry, surface, attenuation and noise level) United States Air Force yielded considerable data for many
and (2) discontinuity conditions (depth, size, shape, testing problems. Figure 12 shows the several types of test
impedance and orientation). blocks used for the studies and Fig. 13 presents normalized
amplitude distance data (semilog plot) in aluminum for sev-
Evaluation of System Parameters eral standard contact transducers and test frequencies (from
the block in Fig. 12a). Figure 14 represents the beam profile
Many studies have been made to isolate and evaluate the for one of these curves (the relative amplitudes obtained as
many factors involved in calibration. Early projects by the the transducer traverses a discontinuity at different depths).

FIGURE 13. Amplitude distance data for seven FIGURE 14. Narrow band beam profile
standard contact transducers variation with increasing distance from 25 mm
(1 in.) to 300 mm (12 in.) at S MHz for 32 mm
DEPTH (1.1 in.) diameter quartz contact transducer
millimeters (inches)
__ a
~N ::a­ 000 0 _a 0 0 0 Oc:i 0 AMPLITUDE
~ ~==-Q. 9_ Q. Q.:::.0 0 0z, 0 ~00 ==- (measured from base line)
s; Oc:i §_ :Q ~0
:Q
0
:::'~ ~ ~'.:'. '0.:'. 00 0 0 0 00 I !~3",5, ~, ~, ~, '".
01.1'\ 01.1'\ 0
NN NN =-0 0 00 0
1.1'\ 01.1'\ 0
N·r,J Lri

·,\\ \'r._) 1,2.0fl ~
\
j~
\./; i~
I\ ~ \. :::. EE

\\ 1-1 ~ 0
'~:~~ l/ ~ ' N 0
I/ ~ "3 --- {Y)
,~- __ 1 DEPTH
2 ~...~' ~ ~4 ­r­!:'-:-'...
vi -/ DEPTH (BEAM DISTANCE IN ALUMINUM)
1--5 ... ~ (not to scale)
11/ -6 I

7-
I Il

INTRODUCTION TO ULTRASONICTESTING I 363

PART4

ULTRASONIC TESTING EQUIPMENT

Basic Ultrasonic Test Systems generally intended for special purpose, factory floor installa-
tions operated under computer control.
Typical ultrasonic test instruments provide a number of Industrial Production Systems
basic functions. These include the generation of an elastic
wave, the reception of ultrasonic signals, signal conditioning Industrial production systems are often modular and
and processing, discontinuity signal gating and signal presen- offer multichannel capabilities. Such systems can be easily
tation. Depending on the intended application, ultrasonic optimized for a particular production environment through
instruments may incorporate other functions, including mul- the addition of plug-in modules and changes in control com-
tichannel capability, additional signal gates, filters, computer puter software (see Fig. 16).
interfaces and methods that compensate for signal loss as a
function of distance traveled. FIGURE 15. General purposeultrasonic
instrumentdesignedprimarilyfor research
After forty years of applications driven developments, applications
the ultrasonic testing instrument has evolved into several
distinct categories: portable, laboratory, industrial produc- FROM PANAMETRICS. REPRINTED WITH PERMISSION.
tion systems and special purpose instruments. A distinction
is made between portable and laboratory instruments FIGURE 16. Modular ultrasonictestingsystem
because their internal designs and external interfaces are configuredfor tests of aerospace components
considerably different. Portable instruments are generally
self contained in terms of their internal functions. Labora- FROM KRAUTKRAMER BRANSON. REPRINTED WITH PERMISSION.
tory instruments often require peripheral components not
normally found in nondestructive testing production envi-
ronments: signal sources and processors, oscilloscopes,
desktop computers and so on.

Portable and industrial production instruments are cali-
brated differently than laboratory instruments. For exam-
ple, the vertical and horizontal display sensitivities of
portable and modular instruments are generally calibrated
in relative units: decibels and millimeters (inches) of travel
in a specific material. Furthermore, portable and modular
instruments are intentionally designed to allow field adjust-
ments and calibration by a human operator using a refer-
ence standard. Laboratory instruments are typically
calibrated in absolute units (volts and microseconds) that
cannot be modified interactively by the operator.

Figure 15 shows an instrument designed primarily for
research laboratory applications. The unit has functions sim-
ilar to a medium scale portable instrument but with differ-
ent operator interfaces.

External and internal differences between portable, mod-
ular and laboratory instruments reflect different user
requirements. Portable instruments are configured to satisfy
the practical needs of an inspector whose test assignments
may vary daily. Laboratory instruments are intended primar-
ily for use by material research engineers and scientists who
require ultrasonic frequency data in terms of highly repro-
ducible engineering units. Industrial production systems are

364 I NONDESTRUCTIVETESTINGOVERVIEW

To accommodate different testing requirements, modu- Basic portable instruments are most often operated in
lar systems typically use a general purpose enclosure inter- the pulse echo mode, using the same ultrasonic transducer
nally compatible with a broad range of special function, for generating and receiving the ultrasonic signals. How-
plug-in modules. Each module is designed to perform a spe- ever, portable instruments may also be operated in the pitch
cific function. This approach offers the user the flexibility of and catch or through-transmission modes, using separate
designing a custom instrumentation package. As the transducers for generating and receiving ultrasonic signals.
requirements change, new modules can be added to the sys-
tem at incremental cost. Maintenance of modular systems is Portable Instrument Functions
facilitated by this approach. The initial cost of a basic modu-
lar system may be higher than that of a portable system. Portable test systems typically offer a minimum range of
basic functions:
Special Purpose Systems
1. pulse echo and pitch and catch modes;
The category of special purpose systems includes all 2. initial pulse generation;
instruments designed to perform a specific ultrasonic test 3. adjustment of pulse amplitude and harmonic content;
that cannot, for cost or performance reasons, be effectively 4. selection of test frequencies (typically 1 to 10 MHz);
carried out using one of the portable, laboratory or modular 5. coarse and fine receiver amplifier gain adjustment;
systems. This category includes: acoustic microscopes,27 6. signal gating (time and threshold);
bond testers, velocity determination instruments, high pow- 7. filtering and gain; and
ered drivers for air coupled and electromagnetic acoustic 8. display of ~eceived ultrasonic signals.
transducers, scanners and imaging equipment. The cost of
installing a special purpose system can vary over a large FIGURE 1 7. Ultrasonictest instrumentswith
range. Thickness gages are generally less expensive than provisionsfor interfacingto externalcomputer:
portable ultrasonic instruments while acoustic microscopes fa) portable,high resolutionflaw detector;
can be much more expensive than modular systems. fbJ microprocessorinstrumentfor testingbonds
in aerospace composites
Figure l 7a shows a portable, high resolution thickness
gage. Figure l 7b shows a microprocessor based instrument faJ
for testing bonds in aerospace composite. Both of these
instruments incorporate provisions for interfacing to an
external computer. A high power laboratory instrument can
be used with unconventional ultrasonic transducers, includ-
ing air coupled and electromagnetic acoustic transducers to
generate a variety of pulse shapes, including spike, square
wave and tone burst pulses.

In the text below, the principles of ultrasonic test instru-
ments and auxiliary equipment are explained using the
operation of a portable instrument as an example. The spe-
cial features of laboratory and modular industrial produc-
tion instruments are explained and some basic modes of
data presentation are discussed.

Portable Instruments fbJ

Portable ultrasonic instruments are battery operated and
principally used as discontinuity detectors. Some such
instruments are designed for handheld operation and offer a
limited range of functions. Other instruments are suitable
for most remote or laboratory production applications and
for procedure development. Such instruments are generally
larger and heavier than handheld discontinuity detectors
but offer many additional functions.

INTRODUCTION TO ULTRASONIC TESTING I 365

These functions typically satisfy most contact ultrasonic Internally, the basic instrument is an oscilloscope whose
test requirements. However, additional functions are horizontal deflection (sweep) voltages are synchronized with
needed in applications such as contact tests of thick sections the transmitted ultrasonic pulses. Twomodes of operation are
and immersion tests. Some of these additional functions are possible: pulse echo and pitch and catch. The pulser excites
available in portable instruments but are usually standard in an ultrasonic transducer with a high voltage pulse whose
laboratory and production systems. amplitude and shape can be controlled. The system'sreceiver
processes electrical signals detected by the receiving trans-
The portable instrument shown in Fig. 17a weighs 2. 7 kg ducer. Only the most elementary controls are provided for
(6.0 lb) and requires six flashlight batteries for power. This adjusting the bandwidth (filtering) and instrument's sensitiv-
instrument can be used to test many thick section materials ity to the received signals. In addition, such a basic instru-
and structures, up to 5.0 m (200 in.) of steel. Other func- ment has front panel controls for adjusting the sweep rate,
tions include time corrected gain and distance amplitude usually calibrated in units of length or velocity. The sweep
correction. The instrument has two flaw gates that can be rates are made adjustable because ultrasonic propagation
used in a multiecho mode to determine thickness. velocities vary with the test material. A signal gate, enabling
the operator to set an alarm level, is provided as a standard
Although nearly all portable instruments are compatible function.
with most piezoelectric ceramic transducers designed to
operate in the 1 to 10 MHz frequency region, they may not Timing and Synchronization
operate properly with special transducers. In particular,
most piezoelectric polymer and low frequency ( typically The operation of a basic instrument is timed and syn-
0.5 MHz and lower) piezoelectric ceramic transducers can- chronized by the so-called timing section, which controls
not be expected to operate properly if the equipment is not the system's pulse repetition rate. The timing section also
designed for these materials and frequencies.28 generates the internal sweep rate signals that determine the
separation between the received ultrasonic signals on the
It is not generally recommended that portable instru- instrument's display.
ments be used to drive unconventional transducers, includ-
ing air coupled transducers and electromagnetic acoustic The pulse timing signals are fed directly to the pulser,
transducers. However, specialized high power drivers29 and which drives the ultrasonic transducer through a manually
low noise preamplifiers are available for such transducers.P selectable diplexer, called a transmit/ receive switch or TIR
switch.
Operation of Portable Ultrasonic Test Equipment
Following a propagation delay corresponding to the
Figure 18 shows a low cost, portable instrument whose ultrasonic time of flight between the transducer and an
operation is representative of instruments intended primar- internal reflector, the back scattered ultrasonic signals are
ily for discontinuity detection and thickness gaging. The received by the same transducer. The electrical signals gen-
instrument uses a cathode ray tube to display the ultrasonic erated by the transducer are amplified by the receiver
signals. Thermoluminescent display tubes have replaced preamplifier. However, before the received signals can be
cathode ray tubes in many instruments. processed and displayed, additional signal processing steps
are needed.
FIGURE 1 8. Batteryoperatedultrasonic
instrumentfor discontinuitydetectionand A typical receiver uses elementary signal processing
thicknessgaging operations to prepare the ultrasonic signals for display. Pro-
cessed signals are evaluated for discontinuity indications
and then displayed. It is assumed that the propagation delay
(travel time) through the test object is sufficiently short to
prevent signals associated with prior transmitter pulses from
overlapping on the display. It is also assumed that the recip-
rocal of the pulse repetition rate is greater than the propa-
gation delay.

Receiver Gain Adjustment

After preamplification, which helps establish the best
signal-to-noise ratio, the amplitudes of the received signals
on the display can be adjusted using a combination of fixed
attenuators and multiple stages of amplification. The overall

366 I NONDESTRUCTIVETESTING OVERVIEW

gain of the amplifier can be selected by switching in two or pulse repetition can cause interference of ultrasonic signals
three 20 dB gain blocks. Generally, this selection can be generated by prior transmitter pulses, in tum producing
accomplished using controls at the front panel of the instru- undesirable fluctuations in signal amplitude. Interference
ment. In many newer designs, receiver gains can also be should be avoided because of its detrimental effect on test
adjusted through digital interfaces by an external controller. reliabilities.

After amplification, bandpass filtering and video detec- In many advanced instrument designs, fast digital sam-
tion (rectification and low pass filtering), the signals are pling, storage techniques and advanced display technologies
amplified again by the video amplifier. This is often followed are used to increase display brightness while reducing
by an adjustable low pass filter and the output of the filter is power consumption. In these designs, pulse repetition rates
then applied to the vertical plates of the display tube. The can be as low as 40 Hz. Lower frequencies could result in
final detected and filtered signal is called the video display. perceptible flicker and could make real-time scanning inad-
In some designs, it is possible to bypass video detection and visable because of the wide intervals between adjacent
to directly display radio frequency waveforms.
pulses.

Sweep, Signal Filtering and Display Pulse Amplitude and Shape Control

The horizontal plates of a cathode ray tube or a corre- Most portable ultrasonic instruments use relatively sim-
sponding component of another display device are driven by ple pulse circuitry. In the past, spike pulser designs were
the sweep signals generated in the system's timing section. commonly used. However, many designs incorporate square
Generally, the start of each sweep signal is delayed with wave pulsers.
respect to the transmitter pulse or by an interface trigger.
This delay is used to offset the start of the display to some If the instrument uses a spike pulser, then the operator
convenient interface echo. may be able to modify the pulse amplitude by adjusting the
energy of the pulse. This is accomplished by selecting the
The amplitudes of the signals displayed on the display value of the energy storage capacitor. In addition, an adjust-
tube are determined principally by the receiver gain and ment of the damping resistor value may be made to mini-
frequency filter settings. They can also be affected by the mize transducer ringing.
low pass filter in the detector circuit. In addition, the setting
of the transmitter pulse amplitude and pulse damping con- If the instrument uses a square wave pulser, the operator
trols can affect the amplitude and the appearance of the dis- is generally required to adjust pulse width individually for
played ultrasonic signals. each transducer to exactly match the frequency characteris-
tics. In addition, the value of the damping resistor should be
Signal Gating and Threshold Selection adjusted to match the impedance characteristics. To protect
the transducers from the effects of voltage overdrive, pulser
Among the essential functions of the basic instrument in voltages seldom exceed 400 V
Fig. 18 are the signal gate and the alarm threshold controls.
These functions enable the operator to isolate a specific por- Avoidance of Receiver Saturation
tion of the received signal train and to compare its peak
amplitude with a preset threshold level. Most ultrasonic testing procedures require the operator to
adjust the gain of the input amplifier and attenuator to ensure
The signal gate delay, width parameters and alarm that none of the components in the receiver amplifier chain
threshold level typically can be selected from the front are in saturation. Typically, this is accomplished by adjusting
panel. To ensure reliable results, receiver gain levels and the the maximum displayed signallevel at about 80 percent. Such
alarm threshold level within the gate interval should be an adjustment can be made using front panel controls. The
adjusted before the test using an appropriate ultrasonic ref- overall gain of a typical receiver may be adjustable over a
erence standard and an instrument calibration procedure. range of 100 dB in discrete steps of 1, 2, 6 and 10 dB.

Pulse Repetition Rate Signal Gate and Alarm Level Settings

Battery powered discontinuity detectors can be oper- The gain adjustment and signal gate functions are impor-
ated at relatively high pulse repetition rates (500 Hz and tant because they can be used with a threshold control to
higher) to ensure a bright display. Thickness gages can establish accept/reject alarm levels. If the amplitude of the
achieve even higher pulse repetition rates. However, high signal in a discontinuity gate exceeds a preestablished
pulse repetition rates result in increased power consump- threshold level, then the discontinuity alarm feature is acti-
tion, which adversely affects instrument portability. High vated. The discontinuity alarm is usually built-in and can be
audible or visual.

INTRODUCTION TO ULTRASONIC TESTING I 367

Capabilities of General Purpose generated by an external computer. Generally, modules
Ultrasonic Test Equipment designed by one manufacturer are not compatible with
instruments designed by other manufacturers.
Portable ultrasonic systems intended for general use
(field tests and nondestructive testing laboratory applica- Conceptually, the functions offered by modular instru-
tions) can incorporate more functions than basic instru- ments are similar to those of general purpose systems. How-
ments, including distance amplitude correction, interface ever, modular instruments can be configured for concurrent,
triggering, display of radio frequency waveforms, multiple multichannel operation with the parameters of each channel
signal gates, digital interfaces to external control and data optimized for a specific ultrasonic test. Thus, one channel can
logging equipment and others. be configured for pulse echo testing while another channel is
configured for pitch and catch testing. In many designs, the
Distance Amplitude Correction setup parameters can be dynamically modified under com-
puter control. This feature of modular instruments facilitates
The distance amplitude correction feature is useful for automated testing of complex structures.
controlling the gain of the instrument receiver section as a
function of sweep time. This function is often used in con- FIGURE 19. Receiver board for modular
tact testing of thick section materials where signal attenua- ultrasonic instrument (see Fig. 16}
tion as a function of depth can be severe. The use of
distance amplitude correction allows signals reflected from fa)
similar discontinuities at different depths to be displayed
with comparable amplitudes.

Interface Triggering fb)

The interface triggering function is typically needed
when the transducer separation from the front surface of
the test object cannot be precisely controlled. This situation
often occurs in pulse echo, water immersion testing of thin,
flexible laminates. If the time delay of the signal gate is syn-
chronized with the initial pulse, the ultrasonic signals
reflected from the interior of the laminate cannot be
expected to arrive within the gate. This is highly undesirable
because many potential discontinuities may be missed.

To ensure that the ultrasonic signals returned from the
interior of the laminate always arrive within the signal gate,
the beginning of each signal gate and display sweep wave-
form can be synchronized to the signal reflected from the
front face of the laminate. This procedure is called interface
triggering.

ModularUltrasonic Equipment FROM KRAUTKRAMERBRANSON. REPRINTEDWITH PERMISSION.

Modules that can be used to configure an ultrasonic
instrument for specialized application include a multichan-
nel preamplifier, a high energy pulser, a receiver module
,(Fig. 19) and a gate. Integration is accomplished using elec-
tronics that can accommodate both analog and digital sig-
nals. The operation of the instrument can be controlled by
a dedicated module that interprets digital control signals

368 I NONDESTRUCTIVETESTINGOVERVIEW

Special Purpose Ultrasonic FIGURE 20. Interfaceand back surface echoes
Equipment from thin test object: (a) test setup; (bJ cathode
ray tube display
Special purpose ultrasonic instruments have typically
been developed to satisfyspecific requirements of a particu- fa)
lar industry segment. Examples of such instruments include
thickness gages, acoustic microscopes, bond testers and oth- TRANSDUCER INTERFACE ECHO
ers. With the exception of thickness gages, special purpose CHO
instruments account for a relatively small percentage of the
ultrasonic test equipment market. The market is dominated tfb) BACK SURFACE
by discontinuity detection equipment.
ui INTERFACE
Thickness Gages ECHO
0 :0:)
Thickness gages are widely used by the petrochemical,
aerospace and other industries.31 Because they serve a sin- IUw!o-:.:.l.~.
gle purpose, thickness gages feature few panel controls. <( '
Typically, thickness gages can operate only with special
transducers supplied with the instrument by the manufac- ECHO ROUND TRIP TIME ~
turer. Contemporary thickness gages are highly integrated
and very small. FIGURE 21 . lnterface and back surface echoes
from thick test object:(a) test setup;
Thickness gages use pulsers that excite specially (bJ cathoderay tube display
designed transducers with waveforms whose rise times are
generally shorter than 10 ns. They also use broad band fa)
receivers and special signal processing circuits for low
amplitude, repetitive signals. The display is typically a digi- TRANSDUCER INTERFACE ECHO
tal readout. Sometimes, a supplemental A-scan liquid crys-
tal display is also provided. BACK SURFACE

Principle of Operation tfb)
INTERFACE
The pulse of a thickness gage excites a special highly w ECHO BACK SURFACE
damped piezoelectric transducer with a short electrical 0 ECHO
pulse. This pulse is known as the initial pulse. A portion of
the ultrasonic signal excited by the initial pulse is reflected ::)
from the material interface. The remainder of the pulse f:-J
propagates through the material and reflects from the oppo-
site parallel surface. The resulting signal is known as the Q..
back surface echo.
~
The time period separating the initial pulse and back
surface echoes represents twice the travel time of the ultra- 0
sonic signal through the test object. The back surface echo uI
travel time (a direct function of the test object thickness)
and material ultrasonic velocity can then be determined. To ui
obtain correct readings, the operator must calibrate the
instrument using appropriate procedures. ECHO ROUND TRIP TIME ~

When the transducer incorporates a delay line to sepa-
rate the initial pulse from the material interface, a separate
interface signal occurs. Figures 20 and 21 show interface
and back surface echoes on a display screen for thin and
thick test objects. The back surface echoes are typically
smaller than interface echoes because of material attenua-
tion effects. Generally, these echoes cannot be seen by the

INTRODUCTION TO ULTRASONIC TESTING I 369

operator bec~use most instruments use digital readout and Continuous wave bond testers operate by setting up a 50
do not have analog signal displays. to 500 kHz standing wave within the test object. In principle,
such instruments are sensitive to changes in the acoustic
Contact Thickness Gaging impedance of the laminate at preselected test frequencies.33
The results, in the form of complex impedance values, are
Thickness gages operate in two ways: contact mode and displayed on the cathode ray tube using a polar presentation
delay line mode. The two differ by the method used to cou- appropriate for continuous wave measurements.
ple the ultrasonic energy between the transducer and mate-
rial surface. The major limitation of continuous wave bond testers
arises from their insensitivity to delaminations near the free
Special transducers are needed to operate in the contact surfaces of the test objects. However, such discontinuities
mode. Such transducers use thin plates made of special can generally be detected using other methods.
wear resistant materials. Contact transducers are coupled
directly to the material using a film of liquid. This procedure Operation in Large Testing Systems
and the resulting test signals are shown in Fig. 22.
Many ultrasonic instruments, particularly modular
During contact gaging, material thickness is determined industrial production systems, are used as components of
by measuring the time period separating the interface echo large, automated quality control systems. In such environ-
from the back surface echo. The thickness gage accomplishes ments (see Fig. 23), the ultrasonic instruments must func-
this automatically using built-in signal processing circuits. tion reliably in close proximity to motion control
mechanisms and other industrial equipment.
Bond Testers
Most discontinuity detectors use pulses to excite the To ensure reliable operation, extreme care must be taken
not to degrade the quality of the ultrasonic signals and to
ultrasonic transducers. Bond testers use continuous wave prevent noise.34 In particular, transducers should not be
excitation.32 Bond testers are principally intended for lami- mounted on scanning devices requiring long cable lengths.
nates that cannot be adequately tested using conventional If this is unavoidable, then specially designed remote pulser
discontinuity detection equipment. For example, they can preamplifier modules should be used. Such devices are gen-
be used for single sided contact testing of aerospace honey- erally small enough to allow positioning close to the trans-
comb structures. ducer, minimizing the undesirable effects of excessive cable
lengths.
FIGURE22. Contacttest usingthin film of
liquidcouplant:(a) test setup; (b) cathoderay FIGURE23. Large automatedqualitycontrol
tube display systemcontainingmodularultrasonictesting
component
(a)
I CARRIAGE
COUP LANT
SCANNING
FIRST BACK SURFACE ECHO BRIDGE

(b) PRINTER

FIRST BACK ELECTRONIC _,
SURFACE ECHO CABINET

WATER SYSTEM

FROM McDONNEU AIRCRAFT COMPANY. REPRINTED WITH PERMISSION.

370 I NONDESTRUCTIVE TESTING OVERVIEW

PART 5

OTHER ULTRASONIC TECHNIQUES

Optical Generation and Detection Optical generation and detection of ultrasound do have
of Ultrasound some limitations that primarily affect detection sensitivity.
Elastic waves generated by optical sources often have rela-
Optical means for generating and detecting ultrasonic tively low amplitude. Using higher laser intensities to
signals represent an area of intensive research and develop- increase acoustic amplitude may cause damage to the test
ment on the threshold of industrial application. These tech- object surface. This is generally prohibited except in special
niques are also useful for material characterization and can cases such as probing of hot metals during processing.
be used to characterize piezoelectric transducers.
Optical Generation of Elastic
Noncontact Tests Waves

A major advantage of optical techniques is that there is When light radiation is absorbed by the irradiated por-
no mechanical contact made with the test object surface. In tion of a test object, thermal expansion results, producing
fact, these techniques are not simply noncontacting but may elastic ultrasonic waves. A contribution might also come
be used for remote sensing - a clear distinction and poten- from the momentum transfer of the reflected light but these
tial advantage over electromagnetic acoustic transducers radiation pressure effects are extremely small compared to
and capacitance transducer techniques. those associated with light absorption.

Because transduction of light energy to acoustic energy With increasing incident optical intensity, the tempera-
is performed by the test material, no intervening couplant is ture rise at the object surface can be so great that vaporiza-
needed. Likewise, material surface vibrations are directly tion of the material may occur. The momentum transfer of
encoded onto a light beam, also without couplant. These the ablated material leaving the surface results in a force
techniques make possible ultrasonic testing in conditions normal to the surface that also gives rise to elastic waves.
difficult for other techniques - probing hot materials, test- This subject has been reviewed in the literature and its main
ing in vacuum (in space) and probing moving objects either features are outlined below.35,36
transversely or toward the transducer. Important for indus-
trial applications is the ease of. testing objects with con- Optical Detection of Ultrasound
toured surfaces and nonplanar shapes.
Optical methods for ultrasonic wave detection can be
With laser ultrasonic techniques, there is no require- grouped into two categories. The first includes those meth-
ment for precise transducer orientation, such as that found ods that permit real time detection of ultrasonic distur-
in conventional piezoelectric techniques. bances at a single point or over a single zone on a test object
surface. The second category includes full field methods
Other Advantages that provide maps of the acoustic energy distribution over
an entire field of view at one instant in time.37
Also notable is the optical resolution obtained with laser
ultrasonics. For example, detector spot diameters down to Full field methods are limited in use by insufficient sen-
10 µm (4 x 1Q-4 in.) are used routinely in experimental sys- sitivity and this text focuses on single spot detection tech-
tems. In addition, the spectral content of laser generated niques. Such techniques have been widely reviewed in the
elastic waves may be extremely broad band. Using optical literature, 38 except for the recent methods described below.
pulses of picosecond duration, acoustic signals of only 2 ns Generally, in the case of laser generated ultrasound, the
have been generated. This provides a potential for disconti- ultrasonic displacements (on the surface of generation and
nuity detection in materials as thin as a razor blade. any other surface) have a nonvanishing normal component.
Research continues into means by which optical sources
may be used to generate narrow band signals as well.

INTRODUCTION TO ULTRASONIC TESTING I 371

Therefore, it is generally sufficient to detect this compo- pressure) gases. This loss depends on the thermal conduc-
nent, althou~h in-plane motion can be detected by optical tivity and the square of the frequency. Vibrational losses are
tochniques." For detecting normal displacement, two inter- caused by coupling the sound wave into resonances of the
ferometric methods can be used. constituent molecules of air. These losses occur at specific
frequencies and are negligible elsewhere in the frequency
Future Developments in Laser domain.41-43
Ultrasonics
At frequencies over 20 kHz this loss mechanism is not
Laser ultrasonic technology is sufficiently mature for use important and viscous and thermal losses dominate with
in several fabrication and inservice nondestructive tests. their frequency square dependence. At a frequency of
Sensitivity is still an important limitation for a number of 1 MHz, the attenuation is calculated to be 101 dB·m-1 but
applications and research continues to improve it. The aim actual measurements yield 165 dB·m-1.43 This loss limits a
is to expand the application of laser ultrasonics to tests now pulse echo measurement in air at a frequency of 1 MHz to
identified as difficult, such as the tests of materials that about 250 mm (10 in.) for a system with a reasonable signal-
severely attenuate ultrasound. The ways being evaluated to-noise ratio. It is useful to compare the attenuation in air
include increasing the laser's power, improving the effi- to that of water (0.22 dB-m-1 at 1 MHz) to understand the
ciency of reception (larger optics) and finding a more effi- difficulty in using high frequency ultrasonics in air. It is also
cient generation scheme, particularly one tailored to a important to remember that because of the lower velocity
reduced detection bandwidth.t" and resulting shorter wavelength, a 5x better resolution
occurs while operating in air at the same frequency.
Although laser ultrasonics is not likely to replace tradi-
tional techniques (because of sensitivity, cost and complex- Low Frequency Transducers
ity), it does have its own range of applications, likely to
include metrology, specialized or research laboratory testing The earliest air transducers operated under 100 kHz and
and hot or complex products. The absence of a coupling typically belonged to one of the following classes: modu-
medium for generation and detection allows the use of laser lated airflow units, mechanical vibrating sources (whistles),
ultrasonic tests in outer space and it could be speculated electroacoustic transducers (a piezoelectric tube operating
that applications will multiply with increases in low gravity in its length resonant mode), flexurally vibrating transducers
activities. (cantilevers clamped on one or two ends), electrostatic
transducers (including the automatic focus transducer
Air Coupled Transducers described below) and microphones.44 All these devices
operate at low frequency with large displacements to gener-
Ultrasonic transducers operating in liquids and solids ate large power density in the air.
have many applications in signal processing devices such as
delay lines, resonators, convolvers and correlators and in Electrostatic Transducer
systems for medical imaging, nondestructive testing and
underwater sensing. In air, ultrasonic transducers are used An electrostatic transducer has been developed and is
for robotic and metrology applications. They have a more widely used in the range finder of an autofocusing commer-
limited potential at high frequencies (over 500 kHz) cial film camera.45 The transducer operates as a conven-
because of the low impedance of air and the high attenua- tional electrostatic transducer. A special foil is stretched
tion of sound waves. over a grooved plate, forming a moving element that trans-
forms electrical energy into sound waves. The returning
Sound Attenuation in Air echo is transformed into electrical energy.

There are three sources of sound attenuation in air: vis- A grooved metallic back plate is in contact with the foil
cous, thermal and vibrational losses. Viscous losses are and forms a capacitor. When charged, the capacitor exerts
caused by frictional damping and are proportional to the electrostatic force on the foil.45 The frequency response of
coefficient of viscosity and the square of the frequency. the transducer has a broad bandwidth essential for opera-
Thermal losses result from the conversion to heat of some tion of the range finder.
energy in the sound wave and its conduction from elevated
temperature (high pressure) gases to low temperature (low Air Coupled System

A 250 kHz air coupled ultrasonic system has been devel-
oped and used for robotic range sensing and wind velocity

372 I NONDESTRUCTIVE TESTING OVERVIEW

measurements.vv'" The transducer for this work is similar to insertion loss of the device is about 35 dB with a fractional
conventional immersion transducers and consists of a reso- bandwidth of 3 percent. Inanother application, a mixture of
nant disk of piezoelectric ceramic. Because of the small silicone rubber and glass microbubbles is used to obtain a
diameter-to-thickness ratio (2:1) used for the piezoelectric matching layer with an impedance of 0.3 x 106 kg·m-2-s-1.
ceramic, particular care must be taken to dampen the lateral This material is used in transducers with single and double
modes that interact with the casing and reduce the band- matching layers, for the purpose of imaging threads in
width of the transducer. cloth.50

A matching layer of epoxy resin is used to improve cou- The insertion loss of a device with two matching layers is
pling into the air. Though the impedance of the matching 50 dB, with a large 6 dB fractional bandwidth of 38 percent
layer is not ideal, the device is functional and a successful at a center frequency of 1 MHz. The insertion loss is 15 dB
metrology system has been demonstrated. Such devices are higher than expected because of errors in controlling the
typically narrow band and their two-way insertion loss is in thickness of the matching layer and, more importantly,
the range of 40 to 50 dB. because of attenuation in the second matching layer.

Ceramic Transducer Mismatch and Attenuation

A device has been developed that operates at 200 kHz A study of mismatch and attenuation in matching layers
with good bandwidth.48 The transducer has a compact has demonstrated that attenuation controls the insertion
impulse response that reflects the large bandwidth at the loss and the bandwidth of the device.51 The limit on the
center frequency of 200 kHz. This good performance is the bandwidth is set because the mechanical bandwidth (due to
result of two novel ideas in the design of the transducer. attenuation) is' far narrower than the electrical bandwidth
First, it uses a composite material with a ceramic volume caused by the impedance mismatch between air and the
fraction of about 1 percent. Second, it uses ceramics reso- ceramic.
nant in the cross polarization direction.
Attenuation also controls the insertion loss - because of
Composite ceramics are popular for water immersion the impedance mismatch, large stresses are set up in the
applications because their impedance, dielectric constant matching layer leading to much attenuation. A design with
and coupling coefficient can be tailored for the application. multiple matching layers is preferred because the stress
In present applications, a reduction in the impedance fields set up in the matching layers are weaker and the influ-
improves the match into air with no decrease in the elec- ence of attenuation in each matching layer is reduced.
tromechanical coupling coefficient. The ceramic is damped
along its length in the resonant direction, improving the FIGURE 24. Single elementof electromagnetic
bandwidth of the transducer. A focused, higher frequency, acoustic transducer, showingapplied currentI,
version of this device is of use in imaging and nondestruc-
tive testing. inducededdy currents Iw magneticbias
induction80 and body forces FL
High Frequency Transducers

High frequency transducers operate above 500 kHz and
are limited to short distances because of the high attenua-
tion of sound waves in air. The traditional plane piston,
water immersion transducer for air applications has been
well documented.PP? In this design, the main problem is
that of matching the impedances of the ceramic and air,
which differ by six orders of magnitude. Conventional
design rules require matching layers with impedances on
the order of 0.01 x 106 kg·m-2-s-1.

Impedance Matching Layers

In one high frequency application, a silicone rubber
matching layer with an impedance of 1 x 106 kg·m-2-s-1 is
used to match a ceramic transducer to air.49 The two-way

INTRODUCTION TO ULTRASONIC TESTING I 373

Electromagnetic Acoustic Physical Principles
Transducers
The physical principles of electromagnetic acoustic
The electromagnetic acoustic transducer (EMAT) is a
device for the excitation and detection of ultrasonic waves in transducer operation52 are shown in Fig. 24. Suppose that a
conductive or magnetic materials. No physical contact is wire is placed adjacent to a metal surface and driven by a
required with the test object because the coupling occurs
through electromagnetic forces. The working distances are Jcwurarreentindatucthede desired ultrasonic frequency. Eddy currents
typically less than 1 mm (0.04 in.) and the probe often is within the metal and, if a static magnetic bias
allowed to rest on the surface of the test object. This ability
to provide reproducible signals with no couplant is often induction B0 is also present, the eddy currents experience
more important than noncontact operation.
periodic Lorentz forces FL given by:

(Eq. 3)

The Lorentz forces on the eddy currents are transmitted to
the solid by collisions with the lattice or other microscopic

FIGURE 25. Crosssectionalview of practical electromagneticacoustic transducer(EMATJ
configurations:(a) spiral coil EMAT excitingradiallypolarizedshear wave propagatingnormalto
surface; (bJ tangential field EMAT for excitingplane polarizedlongitudinalwaves propagatingnormal
to surface; (cJ normalfield EMAT for excitingplane polarizedshear waves propagatingnormalto
surface; (dJ meandercoil EMAT for excitingobliquelongitudinalor verticallypolarizedshear waves,
Rayleigh waves or guided modes of plates; (eJ periodicpermanentmagnet EMAT for exciting
obliquelypropagatinghorizontallypolarizedshear waves or guided horizontallypolarized shear

modes of plates

CJfa) fcJ [~~~
Q®®®®0000 ®®®®0000

~

fbJ (dJ

I NI
0®0®0®0®

~

LEGEND

® = CROSS SECTION OF WIRE CARRYING CURRENT AWAY FROM VIEWER
0 = CROSS SECTION OF WIRE CARRYING CURRENT TOWARD VIEWER

374 I NONDESTRUCTIVETESTING OVERVIEW

processes. These forces on the solid are alternating at the 5. horizontallypolarized shear horizontal beams propa-
frequency of the driving current and act as a source of ultra- gating at oblique angles.
sonic waves. The process is in many ways similar to that
which creates motion in an electrical motor. Reciprocal By virtue of the spatially periodic stresses that it excites,
mechanisms also exist whereby waves can be detected, a the meander coil electromagnetic acoustic transducer
process analogous to operation of an electrical generator. shown in Fig. 25d can also excite Rayleigh waves on surfaces
or Lamb modes in plates. The periodic permanent magnet
If the material is ferromagnetic, additional coupling transducer in Fig. 25e can also excite horizontallypolarized
mechanisms are found. Direct interactions occur between shear modes of plates.
the magnetization of the material and the dynamic magnetic
fields associated with the eddy currents. Magnetostrictive Advantages of Electromagnetic Acoustic Transducers
processes are the tendency of a material to change length
when magnetized. These processes can also play a major role The major motivation for using electromagnetic acoustic
in generating ultrasound. Again, reciprocal processes exist transducers is their ability to operate without couplant or
whereby these mechanisms can contribute to detection. contact. Important consequences of this include operation
on moving objects, in remote or hazardous locations, at ele-
Probe Configurations vated temperatures, in vacuum and on oily or rough sur-
faces. Moreover, alignment problems may be reduced
Practical electromagnetic probes consist of much more because the direction in which the wave is launched is pri-
than a single wire. It is usually necessary to wind a coil and marily determined by the orientation of the test object sur-
design a bias magnet structure so that the distribution of face rather than the probe. Finally, electromagnetic acoustic
forces predicted by Eq. 3 couples to a particular wave type. transducers have the ability to conveniently excite horizon-
Figure 25 shows the cross sections of five coil types: tally polarized shear waves or other special wave types that
provide test advantages in certain applications.
1. radially polarized shear beams propagating normal to
the surface; It must be noted that the cost of realizing these advan-
tages is a relatively low operating efficiency. This is over-
2. longitudinal polarized beams propagating normal to come by the use of high transmitter currents, low noise
the surface; receivers and careful electrical matching. In ferromagnetic
materials, the magnetization or magnetostrictive mecha-
3. shear plane polarized beams propagating normal to nisms of coupling can often be used to enhance signal levels.
the surface;

4. oblique longitudinal or vertically polarized shear
beams; and

INTRODUCTION TO ULTRASONIC TESTING I 375

REFERENCES

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2. Trost, A. "Detection of Material Separation in
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5. Van Valkenburg, H.E., F.H. Hardie and R.C. Good-
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(April 1942). Company ( 1945): p 131.

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20. Ultrasonic Flaw Detector. United States Patent
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376 I NONDESTRUCTIVETESTINGOVERVIEW

24. Howry, D.H. "Techniques Used in Ultrasonic Visu- 36. Hutchins, D.A. "Ultrasonic Generation by Pulsed
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Columbus, OH: American Society for Nondestruc- quency Control. UFFC-33. New York,NY: Institute
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27. Burton,N.J. "NDT Applications of Scanning Acous-
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40. McKie, A.D.W, J.W. Wagner, J.B. Spicer and C.M.
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31. Lynnworth, L.C. Ultrasonic Measurements for Pro­ 43. Fox, John D. Sensing with High­Frequency Ultra­
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32. Botsco, R.J. and R.T. Anderson. "Ultrasonic 44. Massa, Frank. "Ultrasonic Transducers for Use in
Impedance-Plane Analysis of Aerospace Lami- Air." Proceedings of the IEEE. Vol. 53, No. 10. New
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45. Ultrasonic Ranging System. Document Pl834B.
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p 263-296. and T. Taroni. "An Ultrasonic Proximity Sensor
Operating in Air." Sensors and Actuators. Vol. 2,
34. Ott, H.W. Noise Reduction Techniques in Electronic No. 1. Lausanne, Switzerland: Elsevier Sequoia
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47. Canali, C., G. DeCicco, B. Morten, M. Prudenziati
35. Scruby, C.B., R.J. Dewhurst, D.A. Hutchins and and T. Taroni. "A Temperature Compensated Ultra-
S.B. Palmer. Research Techniques in Nondestruc­ sonic Sensor Operating in Air for Distance and
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INTRODUCTION TO ULTRASONIC TESTING I 377

48. Klenschmidt, P. and V Magori. "Ultrasonic Robotic 50. Tone, M., T. Yano and A. Fukomoto. "High Fre-
Sensors for Exact Short Range Distance Measure- quency Ultrasonic Transducers Operating in Air."
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Ultrasonics Symposium. B.R. McAvoy, ed. Vol. 1. Japanese Journal of Applied Physics. Vol. 23, No. 6.
85CH2209-5SU. New York, NY: Institute of Elec- Tokyo, Japan: Oyo Butsurigaku Obunshi Kankokai
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51. Khuri-Yakub, B.T., J.H. Kim, C.H. Chou, P. Parent
49. Fox, J.D., B.T. Khuri-Yakub and G.S. Kino. and G.S. Kino. "A New Design for Air Transduc-
"High-Frequency Acoustic Wave Measurements in
Air." Proceedings of the Ultrasonics Symposium. ers." Proceedings of the Ultrasonics Symposium.
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Institute of Electrical and Electronics Engineers Electrical and Electronics Engineers (1988): p 503.
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ments with EMAT Transducers." Physical Acous­
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New York, NY: Academic Press (1990): p 157-200.

378 I NONDESTRUCTIVE TESTING OVERVIEW

BIBLIOGRAPHY

1. Nondestructive Testing Handbook, second edition: 4. Bray, D.E. and R.K. Stanley. "Ultrasonic Tech-
Vol. 7, Ultrasonic Testing. Columbus, OH: Ameri- niques in Nondestructive Evaluation." Nondestruc­
can Society for Nondestructive Testing (1991). tive Evaluation: A Tool in Design, Manufacturing,
and Service. New York, NY: McGraw-Hill (1989):
2. "Ultrasonic Inspection." Metals Handbook, ninth
edition: Vol. 17, Nondestructive Evaluation and p 45-172.
Quality Control. Materials Park, OH: ASM Interna- 5. Ensminger, D. Ultrasonics: Fundamentals, Technol­
tional (1978): p 231-277.
ogy, Applications, second edition. New York, NY:
3. Bray, D.E. and D. Mcbride, ed. "Ultrasonic Test- Marcel Dekker, Incorporated (1988).
ing of Materials." Nondestructive Testing Tech­ 6. Krautkramer, J. and H. Krautkramer. Ultrasonic
niques. New York, NY: John Wiley & Sons (1992): Testing of Materials, fourth edition. Berlin, Ger-
p 253-343. many: Springer Verlag (1990).

11SECTION

ULTRASONIC PULSE ECHO
TECHNIQUES

Yoseph Bar-Cohen, Jet Propulsion Laboratory, Pasadena, California (Parts 1,6 and 7)
Albert S. Birks, AKZO Nobel Chemicals, Axis, Alabama
Francis H. Chang, Lockheed Martin Technical Aircraft Systems, Fort Worth, Texas (Parts 2, 3 and 4)

380 I NONDESTRUCTIVETESTINGOVERVIEW

PART 1

ULTRASONIC TESTING TECHNIQUES

In contact pulse echo ultrasonic testing, the transducer Ultrasonic systems can be used with three forms of ultra-
touches the test object surface, usually with an intervening sonic scanning: the A-scan, B-scan or C-scan (Fig. 1). These
film of couplant. A single transducer is typically used to designations are used to indicate the information provided
transmit and receive the ultrasound; dual-element transduc- by the ultrasonic scan. Some computer based test systems
ers may be used when a pitch-catch arrangement is desired. can display the results of all three scanning methods.
The presence and location of a reflecting discontinuity is
indicated by the echo signal amplitude and the moment the The A-Scan Method
echo signal arrives at the transducer.
The ultrasonic A-scan presents one-dimensional data
The major advantage of the pulse echo contact tech- showing the response along the beam path at a specific loca-
nique is its adaptability to large and irregularly shaped tion of the test object. Such scans can produce detailed
objects. The test object does not need to be placed in a information about discontinuities in the scanned material.
water tank, as in the immersion method, or in a large fixture,
as in the water jet method. The contact method can be used The depth of discontinuities is indicated by the time-of-
directly on a large stationary object or on confined areas of a flight as measured from the time base of the display screen.
complex structure. The size of discontinuities can be estimated from the ampli-
tude of the reflected signal. The type of discontinuity can be
The pulse echo contact technique offers high sensitivity determined by analysis of the amplitude and phase informa-
to small discontinuities and permits accurate determina- tion. The A-scan method is the most widely used and can be
tions of discontinuity depth beneath the entry surface. In displayed on most standard ultrasonic instruments.
many cases, pulse echo tests also permit determination of
the size and orientation of internal discontinuities, particu- The 8-Scan Method
larly laminar discontinuities oriented perpendicular to the
path of the incident ultrasonic beam. With rough surface With the ultrasonic B-scan, the test object is scanned
discontinuities or nearly spherical small discontinuities along one axis to produce a presentation of its cross section.
(such as porosities in composites), the ultrasonic beam is The location along the scanning path is shown on the X axis
often scattered and the echo signals are low in magnitude. and time-of-flight values are shown along the Y axis.
In such cases, it is possible to recognize the presence of the Because a cross section is produced, the B-scan is not used
discontinuity by the loss of the echo from the back surface where large volumes of material must be inspected.
of the test object if this surface is nearly parallel to the test
The B-scan is popular for medical diagnosis where cross-
object's front surface. sectional views are very useful. In medical applications, the
The instrumentation for the pulse echo technique can be angular manipulation of the transducer is monitored to pre-
vent image distortion and the display is adjusted to account
quite simple. A single transducer, a pulser/receiver unit and for changes in the beam angle along the cross section of the
a display screen are sufficient for tests with no need of per- examined area.
manent records. If permanent records are required, there
must also be equipment for indicating the amplitude of the The C-ScanMethod
signal and the transducer position on the test object.
The ultrasonic C-scan is applied to the test object in a
The major disadvantage of the pulse echo contact tech- raster pattern and presents a view of the discontinuity's area
nique is its reduced near surface resolution caused by a as seen from above. Discontinuity location and size data are
wider interface signal on the screen. This is caused by the available from changes in amplitude as a function of posi-
direct contact of the transducer face and the test object and tion. These are displayed on a screen or recorded on paper.
the presence of a large excitation pulse. This can be some-
what offset by introducing a solid delay line at the end of the
transducer or by placing a partitioning column ofliquid cou-
plant between the transducer face and the test object.

Following is a discussion of the pulse echo contact tech-
nique. It should be emphasized that much of the discussion
is equally applicable to noncontact techniques, such as
immersion nondestructive testing.

ULTRASONIC PULSE ECHO TECHNIQUES I 381

Modem C-scan systems use computers to control the Multiple transducers can be used in C-scan tests. A mul-
transducer position and to acquire, display, document and tiplexer is used to sequentially trigger the transducers in a
store the test results. The computer synchronously acquires predetermined order during scanning. As an alternative to
the digitized position of the transducer and the associated mechanical scanning, these transducer arrays are operated
reflected signal or the value of a specific ultrasonic parame- synchronously and their scanning location is indicated by
ter. The position can be obtained by various means, includ- the cursor position on the computer monitor.
ing optical encoders or sonic digitizers.
Ultrasonic C-scan systems are large in size and most are
Computerized C-scan systems can acquire several ultra- limited to on-site testing conditions. With the increasing
sonic parameters as a function of position. In most cases, the availability of inexpensive microprocessors, scanners have
parameter is time of flight or the amplitude of reflection or also been used for field applications (Fig. 2). The limitations
transmission amplitude at a certain time range. The param- of this generation of field systems are: (1) high cost,
eters are digitized with aid of an analog-to-digital converter. (2) small scanning area and (3) reliability for testing objects

In immersion ultrasonic testing, the C-scan systems can with simple geometries.
scan at speeds up to 500 mm-s! (20 in-s ") or higher. Crawlers have been developed to perform field scanning
Speeds must be kept at a level that does not induce water
turbulence, which introduces noise and degrades the relia- of structures. Current crawlers are tethered to provide
bility of the test. power. Air coupled systems make it possible to test parts of

complex geometry.

FIGURE 1 . Comparisonof scanning FIGURE 2. Manual ultrasonicC-scansystem for
techniques:(a) laminationin plate; (bJ A-scan field use
of discontinuity;(cJ B-scanof discontinuity;
(dJ plan view of C-scan (entiresurface of plate
must be scanned to produceplan view)

(a)

(b) FRONT SURFACE
LAMINATION
(c) ....__ BACK SURFACE
(d)
TOP OF PLATE
BOTTOM OF PLATE

LEGEND
I . COMPUTER AND PULSER/RECEIVER
2. ULTRASONIC TRANSDUCER

3. POSITION DIGITIZER

382 I NONDESTRUCTIVE TESTING OVERVIEW

PART 2

STRAIGHTBEAM PULSE ECHO TESTS

Instrumentation for Straight Beam A-scan mode as the only means of data presentation. There
Tests are computerized pulse echo contact systems that analyze
test results, store them in magnetic media and output the
To meet mobility requirements of contact testing, smaller real time C-scan on a graphic display. Separate discussions
portable ultrasonic test systems often are preferred over of these systems are presented later.
larger, more sophisticated units. In a typical system, the
dynamic gain range required for the pulser/receiver depends Straight Beam Test Procedures
on the required penetration, usually a minimum of 20 dB.
For accessible areas of a test object, pulse echo contact
Because the pulse echo contact method is often per- methods are straightforward. As shown in Fig. 4 for a direct
formed in the field, the operation of the pulser/receiver contact transducer, ultrasound enters the test object and is
should be as simple as possible - recent developments in reflected off its back surface. On the screen display, the
pulser/receiver technology have produced digital control of interface or front surface signal coincides with the initial
gain, sweep delay and gate setting. pulse. The back surface reflection signal occurs at a later
time, corresponding to the thickness of the test object. If
Transducers for Straight Beam Tests the ultrasound encounters a discontinuity, its reflected sig-
nal occurs before the back surface signal. A gate can be set
In most cases, a single-element contact transducer is between the front and back surface signals to monitor
used in the pulse echo mode. A wear plate is placed on the responses from the material during scanning. Any signal in
face of the transducer to reduce damage from contact with the gate and exceeding a preset level can set off an audible
test object surfaces. A wear plate with an acoustic alarm or transmit a computer message.
impedance midway between that of the element and the
test material optimizes contact. For a transducer with a delay line, the interface signal or
front surface signal is separated from the initial pulse. The
To improve the near surface resolution of the test, a near surface resolution can be improved by this separation. It
delay line can be attached to the face of the transducer to is important to set the length of the delay line so that multiple
separate the excitation pulse from the incident surface reflections from the delay line do not fall within the gate.
response and to better match the acoustic impedance of the
test material. The length of the delay line should be such For test objects with lateral dimensions small compared
that the multiple reflections from the delay line fall well out- to the path length of the ultrasound, the beam spread of the
side the back surface reflection. Many commercial contact sound pattern may create interference from side walls. If
transducers are designed with interchangeable delay lines.
It is important to place a thin layer of gel or other liquid FIGURE 3. Transducers for ultrasonic testing
between the transducer face and the delay line for good applications: (a) direct contact transducers;
coupling. Examples of direct contact transducers and coax- (bJ transducer with delay line; (cJ dual element
ial cables are shown in Fig. 3. transducers

The disadvantage of using a delay line is that its attendant (a) (b) (c)
attenuation and impedance mismatch (with metals) can
diminish the penetration power of the ultrasound. Some-
times a dual-element transducer can be used instead of a
single-element transducer with delay line. Separate elements
for transmitting and receiving transducers are arranged side
by side to eliminate interface signal interference. Multiple
transducers arranged in a paint brush fashion are also used so
that a wider path can be covered in a single scan.

Displaying Pulse Echo Test Results

Most pulse echo contact testing is performed visually
using a pulser/receiver cathode ray tube's display in the