ultrasonics testing

8/10/2019 Ultrasonics Testing

1/91

History of Ultrasonics

Prior to World War II, sonar,the technique of sending sound waves through water and

observing the returning echoes to characterize submerged objects, inspired early

ultrasound investigators to explore ways to apply the concept to medical diagnosis. In

!"! and !#$, %o&olov studied the use of ultrasonic waves in detecting metal objects.'ulhauser, in !#, obtained a patent for using ultrasonic waves, using twotransducers

to detect flaws in solids. (irestone )!*+ and %imons )!*$ developed pulsed

ultrasonic testing using a pulse-echo technique.

%hortly after the close of World War II, researchers in apan began to explore medical

diagnostic capabilities of ultrasound. /he first ultrasonic instruments used an 0-mode

presentation with blips on an oscilloscopescreen. /hat was followed by a 1-mode

presentation with a two dimensional, gray scale imaging.

apan2s wor& in ultrasound was relatively un&nown in the 3nited %tates and 4urope

until the !$+s. /hen researchers presented their findings on the use of ultrasound to

detect gallstones, breast masses, and tumors to the international medical community.

apan was also the first country to apply 5oppler ultrasound, an application of

ultrasound that detects internal moving objects such as blood coursing through the heart

for cardiovascular investigation.

3ltrasound pioneers wor&ing in the 3nited %tates

contributed many innovations and important discoveries

to the field during the following decades. 6esearchers

learned to use ultrasound to detect potential cancer and to

visualize tumors in living subjects and in excised tissue.6eal-time imaging, another significant diagnostic tool for

physicians, presented ultrasound images directly on the

system2s 76/ screen at the time of scanning. /he

introduction of spectral 5oppler and later color 5oppler

depicted blood flow in various colors to indicate speed of

flow and direction.

/he 3nited %tates also produced the earliest hand held 8contact8 scanner for clinical

use, the second generation of 1-mode equipment, and the prototype for the first

articulated-arm hand held scanner, with "-5 images.

Beginnings of Nondestructive Evaluation (NDE)

9ondestructive testinghas been practiced for many decades, with initial rapid

developments in instrumentation spurred by the technological advances that occurred

during World War II and the subsequent defense effort. 5uring the earlier days, the

primary purpose was the detection of defects. 0s a part of 8safe life8 design, it was

intended that a structure should not develop macroscopic defects during its life, with the

detection of such defects being a cause for removal of the component from service. In

response to this need, increasingly sophisticated techniques using ultrasonics, eddy

currents, x-rays, dye penetrants, magnetic particles, and other forms of interrogating

energy emerged.


2/91

In the early !:+2s, two events occurred which caused a major change. /he continued

improvement of the technology, in particular its ability to detect small flaws, led to the

unsatisfactory situation that more and more parts had to be rejected, even though the

probability of failure had not changed. ;owever, the discipline of fracture mechanics

emerged, which enabled one to predict whether a crac& of a given size would fail under

a particular load if a material property, fracture toughness, were &nown. the (raunhofer Institute for

9ondestructive /esting in %aarbruc&en, ?ermany> and the 9ondestructive /esting

7entre in ;arwell, 4ngland can all trace their roots to those.

Present State of Ultrasonics

3ltrasonic testing )3/ has been

practiced for many decades now. Initial

rapid developments in instrumentation

spurred by the technological advances

from the !$+2s continue today. /hrough

the !@+2s and continuing into the present

computers have provided technicians

with smaller and more rugged

instruments with greater capabilities.

/hic&ness gauging is one example ofinstruments that have been refined to

reduce operator error and time on tas& by

recording readings. /his reduces the need for a 8scribe8 and allows the technician to

record as many as $*,+++ thic&ness values before downloading to a computer. %ome

instruments have the capability to capture waveforms as well as thic&ness readings. /he

waveform option allows a technician to view or review the 0-scan signal of thic&ness

readings without being present during the inspection. 'uch research and development

has gone into the understanding of sound reflected from a surfaces that contains pitting

or erosion as would be found on the inner surface of a pipe carrying product. /his has

lead to more consistent and accurate field measurements.


3/91

(or sometime ultrasonic flaw detectors have incorporated a trigonometric function

allowing for fast and accurate location of indications when performing shear wave

inspections. 7athode ray tubes, for the most part, have been replaced with A45 or A75

screens. /hese screens in most cases are extremely easy to view in a wide range of

ambient lighting. 1right or low light wor&ing conditions encountered by technicians

have little effect on the technician2s ability to view the screen. %creens can be adjustedfor brightness, contrast, and on some instruments even the color of the screen and signal

can be selected. /ransducers can be programed with predetermined instrument settings.

/he technician only has to place the transducer in contact with the instrument, the

instrument will then set variables such as range, delay, frequency, and gain as it is

directed by the 8chip8 in the transducer.

0long with computers, robotics have contributed to the advancement of ultrasonic

inspections. 4arly on the advantage of a stationary platform was recognized and used in

industry. /hese systems were produced by a number of companies. 0utomated systems

produced a system &nown as the 3ltragraph +"+0. /his system consisted of an

immersion tan&, bridge, and recording system for a printout of the scan. /he resultant7-scan provides a plan or top view of the component. %canning of components was

considerably faster than contact hand scanning, and the system provided a record of the

inspection. Aimitations included size and shape of the component, and also system cost.

Immersion systems have advanced in many directions since the the !B+2s. While

robotics, as we &now it, did not exist, the 8immersion tan&s8 provided effective and

sired inquiry into other means of inspection. /oday robotics have allowed immersion

transducers to inspect components without the need for immersing them in water.

%quirter systems allow the transducer to pass over the component transmitting sound

through a water column. 7omputers can be programed to inspect large, complex shaped

components, with one or multiple transducers collecting information. /his information

is then collected by a computer for evaluation, transmission to a customer, and finally

archival of an image that will maintain quality for years to come.

/oday quantitative theories have been developed to

describe the interaction of the interrogating fields with

flaws. 'odels incorporating the results have been

integrated with solid model descriptions of real-part

geometry2s to simulate practical inspections. 6elated tools

allow 954 to be considered during the design process on

an equal footing with other failure-related engineeringdisciplines. =uantitative descriptions of 954 performance,

such as the probability of detection )P


4/91

)preferred grain orientation, to material properties related to such failure mechanisms

as fatigue, creep, and fracture toughness. 0s technology continues to advance,

applications of ultrasound advances. /he high-resolution imaging systems in the

laboratory today will be tools of the technician tomorrow

Future Direction of Ultrasonic Inspection

Aoo&ing to the future, those in the field of 954 see an exciting new set of

opportunities. 5efense and nuclear power industries have played a major role in the

emergence of 954. Increasing global competition has led to dramatic changes in

product development and business cycles. 0t the same time aging infrastructure, from

roads to buildings and aircraft, present a new set of measurement and monitoring

challenges for engineers as well as technicians.

0mong the new applications of 954 spawned by

these changes is the increased emphasis on the use

of 954 to improve productivity of manufacturing

processes. =uantitative nondestructive evaluation

)=954 both increases the amount of information

about failure modes and the speed with which

information can be obtained and facilitates the

development of in-line measurements for process

control.

/he phrase, 8you can not inspect in quality, you

must build it in,8 exemplifies the industry2s focus on

avoiding the formation of flaws. 9evertheless, flaws and the need to identify them, bothduring manufacture and in service, will never disappear and continual development of

flaw detection and characterization techniques are necessary.

0dvanced simulation tools that are designed for inspectability and their integration into

quantitative strategies for life management will contribute to increase the number and

types of engineering applications of 954. With growth in engineering applications for

954, there will be a need to expanded the &nowledge base of technicians performing

the evaluations. 0dvanced simulation tools used in the design for inspectability may be

used to provide technical students with a greater understanding of sound behavior in

materials. 3/%I' developed at Iowa %tate 3niversity provides a glimpse into what may

be used in the technical classroom as an interactive laboratory tool.

0s globalization continues, companies will see& to develop, with ever increasing

frequency, uniform international practices. In the area of 954, this trend will drive the

emphases on standards, enhanced educational offerings, and simulations that can be

communicated electronically.

/he coming years will be exciting as 954 will continue to emerge as a full-fledged

engineering discipline


5/91

Wave Propagation

3ltrasonic testing is based on time-varying deformations or vibrations in materials,

which is generally referred to as acoustics. 0ll material substances are comprised of

atoms, which may be forced into vibrational motionabout their equilibrium positions.

'any different patterns of vibrational motion exist at the atomic level, however, mostare irrelevant to acoustics and ultrasonic testing. 0coustics is focused on particles that

contain many atoms that move in unison to produce a mechanical wave. When a

material is not stressed in tension or compression beyond its elastic limit, its individual

particles perform elastic oscillations. When the particles of a medium are displaced

from their equilibrium positions, internal )electrostatic restoration forces arise. It is

these elastic restoring forces between particles, combined with inertia of the particles,

that leads to oscillatory motionsof the medium.

In solids, sound waves can propagate in four principle modes that are based on the way

the particles oscillate. %ound can propagate as longitudinal waves, shear waves, surface

waves, and in thin materials as plate waves. Aongitudinal and shear waves are the two

modes of propagation most widely used in ultrasonic testing. /he particle movement

responsible for the propagation of longitudinal and shear waves is illustrated below.

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction

of wave propagation. %ince compressional and dilational forces are active in these

waves, they are also called pressure or compressional waves. /hey are also sometimescalled density waves because their particle density fluctuates as they move.

7ompression waves can be generated in liquids, as well as solids because the energy

travels through the atomic structure by a series of comparison and expansion

)rarefaction movements.

In the transverse or shear wave, the particles oscillate at a right angle or transverse to

the direction of propagation. %hear waves require an acoustically solid material for

effective propagation and, therefore, are not effectively propagated in materials such as

liquids or gasses. %hear waves are relatively wea& when compared to longitudinal


6/91

waves In fact, shear waves are usually generated in materials using some of the energy

from longitudinal waves.

Modes of Sound Wave Propagation

In air, sound travels by compression and rarefaction of air molecules in the direction of

travel. ;owever, in solids, molecules can support vibrations in other directions, hence, a

number of different types )modes of sound waves are possible. 0s mentioned

previously, longitudinal and transverse )shear waves are most often used in ultrasonic

inspection. ;owever, at surfaces and interfaces, various types of elliptical or complex

vibrations of the particles ma&e other waves possible. %ome of these wave modes such

as 6ayleigh and Aamb waves are also useful for ultrasonic inspection.

/he table below summarizes many, but not all, of the wave modes possible in solids.

Wave Types in Solids Particle Vibrations

Aongitudinal Parallel to wave direction

/ransverse )%hear Perpendicular to wave direction

%urface - 6ayleigh 4lliptical orbit - symmetrical mode

Plate Wave - Aamb 7omponent perpendicular to surface )extensional wave

Plate Wave - Aove Parallel to plane layer, perpendicular to wave direction

%toneley )Aea&y 6ayleigh Waves Wave guided along interface

%ezawa 0ntisymmetric mode

Aongitudinal and transverse waves were discussed on the previous page, so let2s touch

on surface and plate waves here.

%urface or 6ayleigh waves travel the surface of a relative thic& solid material

penetrating to a depth of one wavelength. /he particle movement has an elliptical orbit

as shown in the image and animation below. 6ayleigh waves are useful because they are

very sensitive to surface defects and since they will follow the surface around, curves

can also be used to inspect areas that other waves might have difficulty reaching.

Plate waves can

be propagated

only in very thin

metals. Aamb

waves are the

most commonly

used plate waves

in 95/. Aamb

waves are acomplex


7/91

vibrational wave that travels through the entire thic&ness of a material. Propagation of

Aamb waves depends on density, elastic, and material properties of a component, and

they are influenced by a great deal by selected frequency and material thic&ness. With

Aamb waves, a number of modes of particle vibration are possible, but the two most

common are symmetrical and asymmetrical. /he complex motion of the particles is

similar to the elliptical orbits for surface waves.

/he generation of waves using both piezoelectric transducers and electromagnetic

acoustic transducers )4'0/s are discussed in later sections

Properties of Acoustic Plane Wave

Wavelength !re"uency and Velocity

0mong the properties of waves propagating in isotropic solid materials are wavelength,

frequency, andvelocity. /he wavelength is directly proportional to the velocity of the

wave and inversely proportional to the frequency of the wave. /his relationship is

shown by the following equation.

/he applet below shows a longitudinal and transverse wave. /he direction of wave

propagation is from left-to-right and the movement of the lines indicate the direction of

particle oscillation. /he equation relating ultrasonic wavelength, frequency, and

propagation velocity is included at the bottom of the applet in a reorganized form. /he

values for the wavelength, frequency, and wave velocity can be adjusted in the dialog

boxes to see their effects on the wave. 9ote that the value must be set to &eep the

frequency value between +. to ';z or million cycles per second, and the wave

velocity must be between +. and +.: cmCus.

0s can be noted by the equation, a change in frequency will result in a change in

wavelength. 7hange the frequency in the applet and view the resultant wavelength. 0t a

frequency of ." and a material velocity of +.$@$ )longitudinal wave in steel note the

resulting wavelength. 0djust the material velocity to +.*@+ )longitudinal wave in cast

iron note the resulting wavelength. Increase the frequency to +.@ and note the shortened

wavelength in each material.


8/91

In ultrasonic testing, the shorter wavelength resulting from an increase in frequency will

usually provide for the detection of smaller discontinuities. /his will be discussed more

in following materials

Wavelength and Defect Detection

In ultrasonic testing the inspector must ma&e a decision about the frequency of the

transducer that will be used. 0s we learned on the previous page, changing the

frequency when the sound velocity is fixed will result in a change in the wavelength of

the sound. /he wavelength of the ultrasound used has significant affect on the

probability of detecting a discontinuity. 0 rule of thumb in industrial inspections is that

discontinuities that are larger than one-half the size of wavelength can be usually be

detected.

%ensitivity and resolution are two terms that are often used in ultrasonic inspection to

describe a technique2s ability to locate flaws. %ensitivity is the ability to locate small

discontinuities. %ensitivity generally increases with higher frequency )shorter

wavelengths. 6esolution is the ability of the system to locate discontinuities that are

close together within the material or located near the part surface. 6esolution also

generally increases as the frequency increases.

/he wave frequency can also affect the capability of an inspection in adverse ways.

/herefore, selecting the optimal inspection frequency often involves maintaining a

balance between favorable and unfavorable results of the selection. 1efore selecting an

inspection frequency, the grain structure, material thic&ness, size, type, and probable

location of the discontinuity should be considered. 0s frequency increases, sound tends

to scatter from large or course grain structure and from small imperfections within amaterial. 7ast materials often have coarse grains and other sound scatters that require

lower frequencies to be used for evaluations of these products. Wrought and forged

products with directional and refined grain structure, can usually be inspected with

higher frequency transducers.

%ince more things in a material are li&ely to scatter a portion of the sound energy at

higher frequencies, the penetrating power )or the maximum depth in a material that

flaws can be located is also reduced. (requency also has an effect on the shape of the

ultrasonic beam. 1eam spread, or the divergence of the beam from the center axis of the

transducer, and how it is affected by frequency will be discussed later.

It should be mentioned, so as not to be misleading, that a number of other variables will

also affect the ability of ultrasound to locate defects. /hese include pulse length, type

and voltage applied to the crystal, properties of the crystal, bac&ing material, transducer

diameter, and the receiver circuitry of the instrument. /hese are discussed in more detail

in the material on signal-to-noise ratio

Sound Propagation in Elastic Materials

In the previous pages, it was pointed out that sound waves

propagate due to the vibrations or oscillatory motionsof

particles within a material. 0n ultrasonic wave may be

visualized as an infinite number of oscillating masses or


9/91

particles connected by means of elastic springs. 4ach individual particle is influenced

by the motion of its nearest neighbor and both inertialand elasticrestoring forces act

upon each particle.

0 mass on a spring has a single resonant frequency determined by its spring constant #

and its mass $. /he spring constant is the restoring force of a spring per unit of length.Within the elastic limit of any material, there is a linear relationship between the

displacement of a particle and the force attempting to restore the particle to its

equilibrium position. /his linear dependency is described by %oo#e&s 'a.

In terms of the spring model, ;oo&e2s Aaw says

that the restoring force due to a spring is

proportional to the length that the spring is

stretched, and acts in the opposite direction.

'athematically, ;oo&e2s Aawis written, !

*#+, where !is the force, #is the spring

constant, and +is the amount of particledisplacement. ;oo&e2s law is represented

graphically it the right. Please note that the

spring is applying a force to the particle that is

equal and opposite to the force pulling down on the particle.

The Speed of Sound

;oo&e2s Aaw when used along with 9ewton2s %econd Aaw can explain a few things

about the speed of sound. /he speed of sound within a material is a function of the

properties of the material and is independent of the amplitude of the sound wave.

9ewton2s %econd Aaw says that the force applied to a particle will be balanced by the

particle2s mass and the acceleration of the the particle. 'athematically, 9ewton2s

%econd Aaw is written as ! $a. ;oo&e2s Aaw then says that this force will be

balanced by a force in the opposite direction that is dependent on the amount of

displacement and the spring constant )! *#+. /herefore, since the applied force and

the restoring force are equal, $a*#+ can be written, /he negative sign indicates that

the force is in the opposite direction.

%ince the mass $and the spring constant #are constants for any given material, it can

be seen that the acceleration aand the displacement +, are the only variables. It can also

be seen that they are directly proportional. %o if the displacement of the particleincreases, so does its acceleration. It turns out that the time that it ta&es a particle to

move and return to its equilibrium position is independent of the force applied. %o,

within a given material, sound always travels at the same speed no matter how much

force is applied when other variables, such as temperature, are held constant.

What properties of $aterial affect its speed of sound-


10/91

between the speed of sound in a solid and its density and elastic constants is given by

the following equationD

Where Vis the speed of sound, .is the elastic constant, andpis the material density.

/his equation may ta&e a number of different forms depending on the type of wave

)longitudinal or shear and which of the elastic constants that are used. /he typical

elastic constants of a materials includeD

Eoung2s 'odulus, ED a proportionality constant between uniaxial stress and

strain.

Poisson2s 6atio,n

D the ratio of radial strain to axial strain 1ul& modulus, /D a measure of the incompressibility of a body subjected to

hydrostatic pressure.

%hear 'odulus, 0D also called rigidity, a measure of substance2s resistance to

shear.

Aame2s 7onstants, l andm: material constants that are derived from Eoung2s

'odulus and Poisson2s 6atio.

When calculating the velocity of a longitudinal wave, Eoung2s 'odulus and Poisson2s

6atio are commonly used. When calculating the velocity of a shear wave, the shear

modulus is used. It is often most convenient to ma&e the calculations using Aame2s

7onstants, which are derived from Eoung2s 'odulus and Poisson2s 6atio.

It must also be mentioned that the subscript i1attached to .in the above equation is

used to indicate the directionality of the elastic constants with respect to the wave type

and direction of wave travel. In isotropic materials, the elastic constants are the same for

all directions within the material. ;owever, most materials are anisotropic and the

elastic constants differ with each direction. (or example, in a piece of rolled aluminum

plate, the grains are elongated in one direction and compressed in the others and the

elastic constants for the longitudinal direction are different than those for the transverse

or short transverse directions.

4xamples of approximate compressional sound velocities in materials areD

0luminum - +.B#" cmCmicrosecond

+"+ steel - +.$@! cmCmicrosecond

7ast iron - +.*@+ cmCmicrosecond.

4xamples of approximate shear sound velocities in materials areD

0luminum - +.## cmCmicrosecond

+"+ steel - +.#"* cmCmicrosecond

7ast iron - +."*+ cmCmicrosecond.


11/91

When comparing compressional and shear velocities it can be noted that shear velocity

is approximately one half that of compressional. /he sound velocities for a variety of

materials can be found in the ultrasonic properties tables in the general resources section

of this site

Attenuation of Sound Waves

When sound travels through a medium, its intensity diminishes

with distance. In idealized materials, sound pressure )signal

amplitude is only reduced by the spreading of the wave. 9atural

materials, however, all produce an effect which further wea&ens

the sound. /his further wea&ening results from two basic causes,

which are scatteringand absorption. /he combined effect of

scattering and absorption is calledattenuation.

0ttenuation of sound within a material itself is often not of intrinsic interest. ;owever,

natural properties and loading conditions can be related to attenuation. 0ttenuation often

serves as a measurement tool that leads to the formation of theories to explain physical

or chemical phenomenon, which decreases the ultrasonic intensity.

3ltrasonicattenuation is the decay rate of mechanical radiation at ultrasonic frequency

as it propagates through material. 0 decayingplanewave is expressed asD

In this expression 23is the amplitude of the propagating wave at some location. /he

amplitude 2is the reduced amplitude after the wave has traveled a distance 4from that

initial location. /he quantity is the attenuation coefficient of the wave traveling in

the z-direction. /he dimensions of are nepersClength, where a neper is a

dimensionless quantity. e is 9apier2s constant which is equal to approximately ".:@"@.

/he units of the attenuation value in nepersClength can be converted to decibelsClength

by dividing by +.$. 5ecibels is a more common unit when relating the amplitudes oftwo signals.

0ttenuation is generally proportional to the square of sound frequency. =uoted values of

attenuation are often given for a single frequency, or an attenuation value averaged over

many frequencies may be given. 0lso, the actual value of the attenuation coefficient for

a given material is highly dependent on the way in which the material was

manufactured. /hus, quoted values of attenuation only give a rough indication of the

attenuation and should not be automatically trusted. ?enerally, a reliable value of

attenuation can only be obtained by determining the attenuation experimentally for the

particular material being used.


12/91

0ttenuation can be determined by evaluating the multiple bac&wall reflections seen in a

typical 0-scan display li&e the one shown in the image above. /he number of decibels

between two adjacent signals is measured and this value is divided by the time interval

between them. /his calculation produces a attention coefficient in decibels per unit time

5t. /his value can be converted to nepersClength by the following equation.

Where vis the velocity of sound in meters per second and 5tis decibels per second

Acoustic Impedance

%ound travels through materials under the influence of sound pressure. 1ecausemolecules or atoms of a solid are bound elastically to one another, the excess pressure

results in a wave propagating through the solid.

/he acoustic i$pedance )6 of a material is defined as the product of density )p and

acoustic velocity )V of that material.

6 pV

0coustic impedance is important in

. the determination of acoustic transmission and reflection at the boundary of two

materials having different acoustic impedance

". the design of ultrasonic transducers.

#. assessing absorption of sound in a medium.

/he following figure will help you calculate the acoustic impedance for any material, so

long as you &now its density )p and acoustic velocity )F. Eou may also compare two

materials and 8see8 how they reflect and transmit sound energy. /he red arrow

represents energy of the reflected sound, while the blue arrow represents energy of the

transmitted sound. /he

reflected energy is the square of the difference divided by the sum of the acousticimpedances of the two materials.

9ote that /ransmitted %ound 4nergy G 6eflected %ound 4nergy H


13/91

eflection and !ransmission "oefficients #Pressure$

3ltrasonic waves are reflected at boundaries where there are differences in acoustic

impedance, . /his is commonly referred to as impedance mismatch. /he fraction of theincident-wave intensity in reflected waves can be derived because particle velocity and

local particle pressures are required to be continuous across the boundary between

materials.

(ormulation for acoustic reflection and transmission coefficients)pressure are shown

in the interactive figure below. 5ifferent materials may be selected or you may alter the

material velocity or density to change the acoustic impedance of one or both materials.

/he red arrowrepresents reflected sound, while theblue arrowrepresents transmitted

sound.

9ote that reflection and transmission coefficients are often expressed in decibels )d1,

where decibels are defined as "+ times the log of the reflection or transmission

coefficient.

1y multiplying the resulting numbers by ++, the energy reflected can be calculated in

percent of original energy. 3sing the above applet, note that the energy reflected at a

water steel interface is +.@@ or @@J. +." or "J is transmitted into the component. If

reflection and transmission at interfaces is followed through the component, and loss by

attenuation is ignored, a small percentage of the original energy returns to the

transducer.


14/91

0ssuming acoustic energy at the transducer is ++J and energy transmitted into a

component at a water steel interface is "J as discussed above. 0t the second interface

)bac& surface @@J or +.$BJ would be reflected and "J transmitted into the water.

/he final interface would allow only "J of +.$B or ."BJ of the original energy to be

transmitted bac& to the transducer

efraction and Snell%s &a'

When an ultrasound wave passes through an interface between two

materials at an oblique angle, and the materials have different indices

of refraction, it produces both reflected and refracted waves. /his also

occurs with light and this ma&es objects you see across an interface

appear to be shifted relative to where they really are. (or example, if

you loo& straight down at an object at the bottom of a glass of water,

it loo&s closer than it really is. 0 good way to visualize how light and

sound refract is to shine a flashlight into a bowl of slightly cloudy

water noting the refractionangle with respect to the incidence angle.

6efraction ta&es place at an interface due to the different velocities of

the acoustic waves within the two materials. /he velocity of sound in

each material is determined by the material properties )elastic

modules and density for that material. In the animation below, a

series of plane waves are shown traveling in one material and entering a second material

that has a higher acoustic velocity. /herefore, when the wave encounters the interface

between these two materials, the portion of the wave in the second material is moving

faster than the portion of the wave in the first material. It can be seen that this causes the

wave to bend.

%nell2s Aaw describes the relationship between the angles and the velocities of the

waves. %nell2s law equates the ratio of material velocities v7and v8to the ratio of the

sine&sof incident )

1

and refraction )

2

angles, as shown in the following equation.

WhereD

FAis the longitudinal wave velocity in material .

FA"is the longitudinal wave velocity in material ".

9ote that in the diagram, there is a reflected longitudinal wave )FA shown. /his wave

is reflected at the same angle as the incident wave because the two waves are traveling

in the same material and, therefore, have the same velocities. /his reflected wave is


15/91

unimportant in our explanation of %nell2s Aaw, but it should be remembered that some

of the wave energy is reflected at the interface. In the applet below, only the incident

and refracted longitudinal waves are shown. /he angle of either wave can be adjusted

by clic&ing and dragging the mouse in the region of the arrows. Falues for the angles or

acoustic velocities can also be entered in the dialog boxes so the that applet can be used

as a %nell2s Aaw calculator.

When a longitudinal wave moves from a slower to a faster material, there is an incident

angle that ma&es the angle of refraction for the wave !+K. /his is &now as the first

critical angle. /he first critical angle can be found from %nell2s law by putting in an

angle of !+K for the angle of the refracted ray. 0t the critical angle of incidence, much of

the acoustic energy is in the form of an inhomogeneous compression wave, which

travels along the interface and decays exponentially with depth from the interface. /his

wave is sometimes referred to as a 8creep wave.8 1ecause of there inhomogeneousnature and the fact that they decay rapidly, creep waves are not used as extensively as

6ayleigh surface waves in 95/. ;owever, creep waves are sometimes useful because

they suffer less from surface irregularities and coarse material microstructure, due to

their longer wavelengths, than 6ayleigh waves

Mode "onversion

When sound travels in a solid material, one form of wave energy can be transformedinto another form. (or example, when a longitudinal waves hits an interface at an angle,

some of the energy can cause particle movement in the transverse direction to start a

shear )transverse wave. 'ode conversion, occurs when a wave encounters an interface

between materials of different acoustic impedance and the incident angle is not normal

to the interface. (rom the ray tracing movie below it can be seen that since mode

conversion occurs every time a wave encountered interface at an angle, ultrasonic

signals can become confusing at times.

In the previous section it was pointed out that when sound waves pass through an

interface between materials having different acoustic velocities, refraction ta&es place at


16/91

the interface. /he larger the difference in acoustic velocities between the two materials,

the more the sound is refracted. 9otice that the shear wave is not refracted as much as

the longitudinal wave. /his occurs because shear waves travel slower than longitudinal

waves. /herefore, the velocity difference between the incident longitudinal wave and

the shear wave is not as great as it is between the incident and refracted longitudinal

waves. 0lso note that when a longitudinal wave is reflected inside the material, thereflected shear wave is reflected at a smaller angle than the reflected longitudinal wave.

/his is also due to the fact that the shear velocity is less than the longitudinal velocity

within a given material.

%nell2s Aaw holds true for shear waves as well as longitudinal waves and can be written

as follows.

WhereD

FAis the longitudinal wave velocity in material .

FA"is the longitudinal wave velocity in material ".

F%is the shear wave velocity in material .

F%"is the shear wave velocity in material ".

In the applet below, the shear )transverse wave ray path has been added. /he ray pathsof the waves can be adjusted by clic&ing and dragging in the vicinity of the arrows.

Falues for the angles or the wave velocities can also be entered into the dialog boxes. It

can be seen from the applet that when a wave moves from a slower to a faster material,

there is an incident angle which ma&es the angle of refraction for the longitudinal wave

!+ degrees. 0s mentioned on the previous page, this is &now as the first critical angle

and all of the energy from the refracted longitudinal wave is now converted to a surface

following longitudinal wave. /his surface following wave is sometime referred to as a

creep wave and it is not very useful in 95/ because it dampens out very rapidly.

1eyond the first critical angle, only the shear wave propagates down into the material.

(or this reason, most angle beam transducers use a shear wave so that the signal is not

complicated by having two wave present. In may cases, there is also an incident angle

that ma&es the angle of refraction for the shear wave !+ degrees. /his is &now as the

second critical angle and at this point, all of the wave energy is reflected or refracted

into a surface following shear wave or shear creep wave. %lightly beyond the second

critical angle, surface waves will be generated.


17/91

9ote that the applet defaults to compressional velocity in the second material. /he

refracted

compressional wave angle will be generated for given materials and angles. /o find the

angle of incidence required to generate a shear wave at a given angle complete the

followingD

. Ferify F is the longitudinal wave velocity for wedge or immersion liquid.

". 7hange F" to the shear wave velocity )approximately one-half compressional

for the

material to be inspected.

#. 7hange =" to the desired shear wave angle.

*. 6ead =, the correct angle of incidence

Signal(to()oise atio

In a previous page, the effect that frequency or wavelength has on flaw detectability was

discussed. ;owever, the detection of a defect involves many factors other than the

relationship of wavelength and flaw size. (or example, the amount of sound that reflects

from a defect is also dependent acoustic impedance mismatch between the flaw and the

surrounding material. 0 void is generally a better reflector than a metallic inclusion

because the impedance mismatch is greater between air and metal than between metal

and another metal.


18/91

/he flaw location with respect to the incident beam.

/he inherent noisiness of the metal microstructure.

/he inherent reflectivity of the flaw which is dependent on its acoustic

impedance, size, shape, and orientation.

7rac&s and volumetric defects can reflect ultrasonic waves quite differently.

'any crac&s are 8invisible8 from one direction and strong reflectors fromanother.

'ultifaceted flaws will tend to scatter sound away from the transducer.

/he following formula relates some of the variables affecting the signal-to-noise ratio

)%C9 of a defectD

6ather than go into the details of this formulation, a few fundamental relationships can

be pointed out. /he signal-to-noise ratio )%C9, and therefore the detectability of a

defect

increases with increasing flaw size )scattering amplitude. /he detectability of a

defect is directly proportional to its size.

increases with a more focused beam. In other words, flaw detectability is

inversely proportional to the transducer beam width.

increases with decreasing pulse width )delta-t. In other words, flaw detectability

is inversely proportional to the duration of the pulse produced by an ultrasonic

transducer. /he shorter the pulse )often higher frequency, the better the

detection of the defect. %horter pulses correspond to broader bandwidth

frequency response. %ee the figure below showing the waveform of a transducer

and its corresponding frequency spectrum.

decreases in materials with high density andCor a high ultrasonic velocity. /he

signal-to-noise ratio )%C9 is inversely proportional to material density and

acoustic velocity. generally increases with frequency. ;owever, in some materials, such as

titanium alloys, both the 80flaw8and the 8(igure of 'erit )(


19/91

Wave Interaction or Interference

1efore we move into the next section, the subject of wave interaction must be covered

since it is important when trying to understand the performance of an ultrasonic

transducer.


20/91

position. ;owever, anyone that has dropped something in a pool of water, can picture

the waves radiating out from the source with a circular wave front. If two objects are

dropped a short distance apart into the pool of water, their waves will radiate out from

their sources and interact with each other. 0t every point where the waves interact, the

amplitude of the particle displacement is the combined sum of the amplitude of the

particle displacement of the individual waves.

With an ultrasonic transducer, the waves propagate out from the transducer face with a

circular wave front. If it were possible to get them to propagate out from a single point

on the transducer face, the sound field would appear in the upper image to the right.

7onsider the light areas to be areas of rarefaction and the dar& areas to be areas of

compression.

;owever, as stated previously, sound waves originate from multiple points along the

face of the transducer. /he lower image to the right shows what the sound field would

loo& li&e if the waves waves originating from just two points. It can be seen that were

the waves interact, there are areas of constructive and destructive interference. /hepoints of constructive interference are often referred to as nodes.


21/91

/he curvature and the area over which the sound is being generated, the speed that the

sound waves travel within a material and the frequency of the sound all affect the sound

field. 3se the ava applet below, to experiment with these variables and see how the

sound field is affected.

Pie*oelectric !ransducers

/he conversion of electrical pulses to

mechanical vibrations and the conversion

of returned mechanical vibrations bac& into

electrical energy is the basis for ultrasonic

testing. /he active element is the heart of

the transducer as it converts the electricalenergy to acoustic energy, and vice versa.

/he active element is basically a piece

polarized material )i.e. some parts of the

molecule are positively charged, while other parts of the

molecule are negatively charged with electrodes attached

to two of its opposite faces. When an electric field is

applied across the material, the polarized molecules will align themselves with the

electric field, resulting in induced dipoles within the molecular or crystal structure of the

material. /his alignment of molecules will cause the material to change dimensions.

/his phenomenon is &nown as electrostriction. In addition, a permanently-polarized

material such as quartz )%i


22/91

active element is still sometime referred to as the crystal by old timers in the 95/ field..

When piezoelectric ceramics were introduced they soon became the dominant material

for transducers due to their good piezoelectric properties and their ease of manufacture

into a variety of shapes and sizes. /hey also operate at low voltage and are usable up to

about #++o7. /he first piezoceramic in general use was barium titanate, and that was

followed during the !B+2s by lead zirconate titanate compositions, which are now themost commonly employed ceramic for ma&ing transducers. 9ew materials such as piezo

polymers and composites are also being used in some applications.

/he thic&ness of the active element is determined by the desired frequency of the

transducer. 0 thin wafer element vibrates with a wavelength that is twice its thic&ness.

/herefore, piezoelectric crystals are cut to a thic&ness that is C" the desired radiated

wavelength. /he higher the frequency of the transducer, the thinner the active element.

/he primary reason that high frequency contact transducers are not produced in because

the element is very thin and too fragile

"haracteristics of Pie*oelectric !ransducers

/he transducer is an very important part of the ultrasonic instrumentation system. 0s

discussed on the previous page, the transducer incorporates a piezoelectric element,

which converts electrical signals into mechanical vibrations )transmit mode and

mechanical vibrations into electrical signals )receive mode. 'any factors, including

material, mechanical and electrical construction, and the external mechanical and

electrical load conditions, influence the behavior a transducer. 'echanical construction

includes parameters such as radiation surface area, mechanical damping, housing,

connector type and other variables of physical construction. 0s of this writing,

transducer manufactures are hard pressed when constructing two transducers that haveidentical performance characteristics.

0 cut away of a typical contact transducer is shown

above. It was previously learned that the piezoelectric

element is cut to C" the desired wavelength. /o get as

much energy out of the transducer as possible, animpedance matching is placed between the active


23/91

element and the face of the transducer.


24/91

piezoelectric transducer is shown below. /he intensity of the sound is indicated by

color, with lighter colors indicating higher intensity.

%ince the ultrasound originates from a number of points along the transducer face, the

ultrasound intensity along the beam is affected by constructive and destructive wave

interference as discussed in a previous page on wave interference. /hese are sometimes

also referred to as diffractioneffects in the 95/ world. /here are extensive fluctuations

near the source, &nown as the near field. /hese high and low pressure areas are

generated because the crystal is not a point source of sound pressure, but rather a series

of high and low pressure waves which are joined into a uniform front at the end of the

near zone. 1ecause of acoustic variations within a near field, it can be extremely

difficult to accurately evaluate flaws in materials when they are positioned within this

area..

/he ultrasonic beam is more uniform in the far field, where the beam spreads out in a

pattern originating from the center of the transducer. /he transition between these zones

occurs at a distance,N,and is sometimes referred to as the 8natural focus8 of a flat ) or

unfocused transducer. /he nearCfar distance,N, is significant because amplitude

variations that characterize the near field change to a smoothly declining amplitude at

this point. /his area just beyond the near field is where the sound wave is well behavedand at its maximum strength. /herefore, optimal detection results will be obtained when

flaws occur in this area.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/WaveInterference.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/WaveInterference.htm


25/91

(or a piston source transducer of radius )a, frequency )f, and velocity )V of a liquid

or solid medium, the applet below allows the calculation of the nearCfar field transition

point.

%pherical or cylindrical focusing changes the structure of a transducer field by 8pulling8

theNpoint nearer the transducer, to the focal point of the transducer. It is also important

to note that the driving excitation normally used in 95/ applications are either spi&e or

rectangular pulsars, not single frequency. /hey can significantly alter the performance

of a transducer. 9onetheless, the supporting analysis is widely used because it

represents a reasonable approximation and a good starting point

!ransducer +eam Spread

0s discussed on the previous page, round transducers are often referred to as pistonsource transducers because the sound field resembles a cylindrical mass in front of the


26/91

transducer. ;owever, the energy in the beam does not remain in a cylinder, but instead

spread out as it propagates through the material. /he phenomenon is usually referred to

as beam spread but is sometimes also called beam divergence or ultrasonic diffraction.

0lthough beam spread must be considered when performing an ultrasonic inspection, it

is important to note that in the far field, or (raunhoferzone, the maximum sound

pressure is always found along the acoustic axis )centerline of the transducer./herefore, the strongest reflection are li&ely to come from the area directly in front of

the transducer.

1eam spread occurs because the vibrating particle of the material )through which the

wave is traveling do not always transfer all of their energy in the direction of wave

propagation. 6ecall that waves propagate through that transfer of energy from one

particle to another in the medium. If the particles are not directly aligned in the direction

of wave propagation, some of the energy will get transferred off at an angle. )Picture

what happens when one ball hits another second ball slightly off center. In the near

field constructive and destructive wave interference fill the sound field with fluctuation.

0t the start of the far field, however, the beam strength is always greatest at the centerof the beam and diminishes as it spreads outward.

0s shown in the applet below, beam spread is largely determined by the frequency and

diameter of the transducer. 1eam spread is greater when using a low frequency

transducer than when using a high frequency transducer. 0s the diameter of the

transducer increases the beam spread will be reduced.

1eam angle is an important consideration in transducer selection for a couple of

reasons. (irst, beam spread lowers the amplitude of reflections since sound fields are

less concentrated and, therefore, wea&er. %econd, beam spread may result in more

difficult to interpret signals due to reflections from the lateral sides of the test object or

other features outside of the inspection area. 7haracterization of the sound field

generated by a transducer is a prerequisite to understanding observed signals.

9umerous codes exist that can be used to standardize the method used for the

characterization of beam spread. 0merican %ociety for /esting and 'aterials method

number 0%/' 4-+B$, addresses methods for ascertaining beam shapes in %ection 0B,

'easurement of %ound (ield Parameters. ;owever, these measurements are limited to

immersion probes. In fact, the methods described in 4-+B$ are primarily concernedwith the measurement of beam characteristics in water, and as such are limited to

measurements of the compression mode only. /echniques described in 4-+B$ include

pulse-echo using a ball target and hydrophone receivers, which allows the sound field of

the probe to be assessed for the entire volume in front of the probe.

(or a flat piston source transducer, an approximation of the beam shape may be

calculated as a function of radius )a, frequency )f, and velocity )V of a liquid or solid

medium. /he applet below allows the user to calculate the beam spread angle which

represents a falling of of sound pressure )intensity to the side of the acoustic axis of one

half )-B d1 as a function of transducer parameters radius and frequency and as a

function of acoustic velocity in a medium.


27/91

!ransducer !ypes

3ltrasonic transducers are manufactured for

a variety of application and can be custom

fabricated when necessary. 7areful attention

must be paid to selecting the proper

transducer for the application. /he previous

section on 0coustic Wavelength and 5efect

5etectiongave a brief overview of factors

that affect defect detectability. (rom this material, we &now that it is important to

choose transducers that have the desired frequency, bandwidth, and focusing to optimize

inspection capability. 'ost often the transducer is chosen either to enhance sensitivity

or resolution of the system.

/ransducers are classified into groups according to the application.

.ontact transducersare used for direct contact inspections, and

are generally hand manipulated. /hey have elements protected in

a rugged casing to withstand sliding contact with a variety of

materials. /hese transducers have an ergonomic design so that

they are easy to grip and move along a surface. /hey also often

have replaceable wear plates to lengthen their useful life.

7oupling materials of water, grease, oils, or commercial materials are used to

remove the air gap between the transducer and the component inspected. 9$$ersion transducersdo not contact the

component. /hese transducers are designed to

operate in a liquid environment and all

connections are watertight. Immersion

transducers usually have a impedance matching

layer that helps to get more sound energy into

the water and, in turn, into the component being

inspected. Immersion transducers can be

purchased with in a planner, cylindrically

focused or spherically focused lens. 0 focused

transducer can improve sensitivity and axialresolution by concentrating the sound energy to
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/defectdetect.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/defectdetect.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/defectdetect.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/defectdetect.htm


28/91

a smaller area. Immersion transducers are typically used inside a water tan& or as

part of a squirter or bubbler system in scanning applications.

:ore on .ontact Transducers,

7ontact transducers are available in a variety of configurations to improve theirusefulness for a variety of applications. /he flat contact transducer shown above is used

normal beam inspections of relatively flat surfaces, and where near surface resolution is

not critical. If the surface is curved, a shoe that matches the curvature of the part may

need to be added to the face of the transducer. If near surface resolution is important or

if an angle beam inspection is needed, one of the special contact transducers described

below might be used.

Dual ele$ent transducerscontain two independently

operating elements in a single housing.


29/91

2ngle bea$ transducersand wedges are typically

used to introduce a refracted shear wave into the test

material. /ransducers can be purchased in a variety of

fixed angles or in adjustable versions where the user

determines the angles of incident and refraction. In

the fixed angle versions, the angle of refraction that ismar&ed on the transducer is only accurate for a

particular material, which is usually steel. /he angled

sound path allows the sound beam to be reflected

from the bac& wall to improve detectability of flaws

in and around welded areas. /hey are also used to

generate surface waves for use in detecting defects on the surface of a component.

Nor$al incidence shear ave transducersare unique because they allow introduction

of shear waves directly into a test piece without the use of an angle beam wedge.

7areful design has enabled manufacturing of transducers with minimal longitudinal

wave contamination. /he ratio of the longitudinal to shear wave components isgenerally below -#+d1.

Paint brush transducersare used to scan wide areas. /hese long and narrow

transducers are made up of an array of small crystals that are carefully matched to

minimize variation of performance and maintain uniform sensitivity over the entire area

of the transducer. Paint brush transducers ma&e it possible to scan a larger area more

rapidly for discontinuities. %maller and more sensitive transducers are often then

required to further define the details of a discontinuity

!ransducer !esting%ome transducer manufactures have lead in the development of transducer

characterization techniques and have participated in development of the 0I3' %tandard

'ethods for /esting %ingle-4lement Pulse-4cho 3ltrasonic /ransducers as well as

0%/'-4 +B$ %tandard ?uide for 4valuating 7haracteristics of 3ltrasonic %earch

3nits.

0dditionally some manufactures perform characterizations according to 0W%, 4%I, and

many other industrial and military standards.


30/91


31/91

/ypical time and frequency domain plots provided

by transducer manufacturers

!re"uency ;esponse--/he frequency response of the above transducer has a pea& at $

';z and operates over a broad range of frequencies. Its bandwidth )*. to B.$ ';z is

measured at the -B d1 points, or :+J of the pea& frequency. /he useable bandwidth ofbroadband transducers, especially in frequency analysis measurements, is often quoted

at the -"+ d1 points. /ransducer sensitivity and bandwidth )more of one means less of

the other are chosen based on inspection needs.

.o$ple+ Electrical 9$pedance--/he complex electrical impedance may be obtained

with commercial impedance measuring instrumentation, and these measurements may

provide the magnitude and phase of impedance of the search unit over the operating

frequency range of the unit. /hese measurements are generally made under laboratory

conditions with minimum cable lengths or external accessories and in accordance with

specifications of the instrument manufacturer. /he value of magnitude of the complex

electrical impedance may also obtained using values recorded from the sinusoidal burst.

Sound !ield :easure$ents--/he objective of these measurements is to establish

parameters such as the on-axis and transverse sound beam profiles for immersion flat

and curved transducers. /hese measurements are often achieved by scanning the sound

field with a hydrophone transducer mapping the sound fields in three dimensional

space. 0n alternative approach to sound field measurements is a measure of the

transducer2s radiating surface motion using laser interferometry.

!ransducer Modeling

In high-technology manufacturing, part design and simulation of part inspection is done

in the virtual world of the computer. /ransducer modeling is necessary to ma&e accurate

predictions of how a part or component might be inspected, prior to the actual building

of that part. 7omputer modeling is also used to design ultrasonic transducers.

0s noted in the previous section, an ultrasonic transducer may be characterized by

detailed measurements of its electrical and sound radiation properties. %uch

measurements can completely determine the response of any one individual transducer.

/here is ongoing research to develop general models that relate electrical inputs

)voltage, current to mechanical outputs )force, velocity and vice-versa. /hese modelscan be very robust giving accurate prediction of transducer response, but suffer for lac&


32/91

of accurate modeling of physical variables inherent in transducer manufacturing. /hese

electrical-mechanical response models must ta&e into account physical and electrical

components in the figure below.

/he /hompson-?ray 'easurement 'odel, which ma&es very accurate predictions of

ultrasonic scattering measurements made through liquid-solid interfaces, does not

attempt to model transducer electrical-mechanical response. /he /hompson-?ray

'easurement 'odel approach ma&es use of reference data ta&en with the same

transducer)s to deconvolve out electro-physical characteristics specific to individual

transducers. %ee %ection $.# /hompson-?ray 'easurement 'odel.

/he long term goal in ultrasonic modeling is to incorporate accurate models of the

transducers themselves as well as accurate models of pulser-receivers, cables, and other

components that completely describe any given inspection setup and allow the accurate

prediction of inspection signals.

"ouplant

0 couplant is a material )usually liquid that facilitates the

transmission of ultrasonic energy from the transducer into

the test specimen. 7ouplant is generally necessarybecause the acoustic impedance mismatch between air

and solids, such as the test specimen, is large and,

therefore, nearly all of the energy is reflected and very

little is transmitted into the test material. /he couplant

displaces the air and ma&es it possible to get more sound

energy into the test specimen so that a usable ultrasonic

signal can be obtained. In contact ultrasonic testing a thin

film of oil, glycerin or water is generally used between the transducer and the test

surface.

When scanning over the part or ma&ing precise measurements, an immersion technique

is often used. In immersion ultrasonic testing both the transducer and the part are
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/thompsongray.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/thompsongray.htm


33/91

immersed in the couplant, which is typically water. /his method of coupling ma&es it

easier to maintain consistent coupling while moving and manipulating the transducer

andCor the part

Electromagnetic Acoustic !ransducers #EMA!s$

0s discussed in the previous page, one of the essential features of ultrasonic

measurements is mechanical coupling between the transducer, and the solid whose

properties or structure are to be studied. /his coupling is generally achieved in one of

two ways. In immersion measurements, energy is coupled between the transducerand

sample by placing them in a tan& filled with a fluid, generally water. In contact

measurements, the transducer is pressed directly against the sample, and coupling isachieved by the presence of a thin fluid layer inserted between the two. When shear

waves are to be transmitted, the fluid is generally selected to have a significant

viscosity.

4lectromagnetic-acoustic transducers )4'0/ acts through totally different physical

principles and do not need couplant. When a wire is placed near the surface of an

electrically conducting object and is driven by a current at the desired ultrasonic

frequency, eddy currentswill be induced in a near surface region of the object. If a static

magnetic field is also present, these eddy currents will experience Aorentz forces of the

form

F = J x B

Fis a body force per unit volume,Jis the induced dynamic current density, andBis the

static magnetic induction.

/he most important application of 4'0/s has been in nondestructive evaluation )954

applications such as flaw detection or material property characterization. 7ouplant free

transduction allows operation without contact at elevated temperatures and in remote

locations. /he coil and magnet structure can also be designed to excite complex wave

patterns and polarization2s that would be difficult to realize with fluid coupled

piezoelectric probes. In the inference of material properties from precise velocity or


34/91

attenuation measurements, use of 4'0/s can eliminate errors associated with couplant

variation, particularly in contact measurements.

0 number of practical 4'0/ configurations are shown below. In each, the biasing

magnet structure, the coil, and forces on the surface of the solid are shown in an

exploded view. /he first three configurations will excite beams propagating normal tothe surface of the half-space and produce, respectively, beams with radial, longitudinal,

and transverse polarization2s. /he final two use spatially varying stresses to excite

beams propagating at oblique angles or along the surface of a component. 0lthough a

great number of variations on these configurations have been conceived and used in

practice, consideration of these three geometry2s should suffice to introduce the

fundamentals.

7ross-sectional view of a spiral coil 4'0/ exciting radially polarized shear waves

propagating normal to the surface.

7ross-sectional view of a tangential field 4'0/ for exciting polarized longitudinal

waves propagating normal to the surface.

7ross-sectional view of a normal field 4'0/ for exciting plane polarized shear waves

propagating normal to the surface.


35/91

7ross-sectional view of a meander coil 4'0/ for exciting obliquely propagating A or

%F waves, 6ayleigh waves, or guided modes )such as Aamb waves of plates.

7ross-sectional view of a periodic permanent magnet 4'0/ for exciting grazing or

obliquely propagating horizontally polarized )%; waves or guided %; modes of plates.

Practical 4'0/ designs are relatively narrowband and require strong magnetic fields

and large currents to produce ultrasound that is often wea&er than that produced by

piezoelectric transducers. 6are-earth materials such as %amarium-7obalt and

9eodymium-Iron-1oron are often used to produce sufficiently strong magnetic fields,

which may also be generated by pulsed electromagnets.

/he 4'0/ offers many advantages based on its couplant-free operation. /hese

advantages include the abilities to operate in remote environments at elevated speeds

and temperatures, to excite polarization2s not easily excited by fluid coupled

piezoelectrics, and to produce highly consistent measurements.

/hese advantages are tempered by low efficiencies, and careful electronic design is

essential to applications.

:ore infor$ation about the use of E:2Ts can be found at the folloing lin#s,

Lamb Wave Generation With EMATsShear Wave Generation With EMATsVelocity Measurements With EMATsTexture Measurement I With EMATsTexture Measurement II With EMATsStress Measurement With EMATsComposite inspection With EMATs
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematlambwave.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematshearwave.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematvelocity.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/emattexture1.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/emattexture2.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematstess.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematcomposite.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematlambwave.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematshearwave.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematvelocity.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/emattexture1.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/emattexture2.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematstess.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/ematcomposite.htm


36/91

Pulser(eceivers

3ltrasonic pulser-receivers are well suited to general purpose ultrasonic testing. 0long

with appropriate transducers and an oscilloscope they can be used for flaw detection and

thic&ness gauging in a wide variety of metals, plastics, ceramics, and composites.

3ltrasonic pulser-receivers provide a unique, low-cost ultrasonic measurementcapability.

/he pulser section of the instrument generates short, large amplitude electric pulses of

controlled energy, which are converted into short ultrasonic pulses when applied to an

ultrasonic transducer. 'ost pulser sections have very low impedance outputs to better

drive transducers. 7ontrol function associated with the pulser circuit include

Pulse length or damping )/he amount of time the pulse is applied to the

transducer.

Pulse energy )/he voltage applied to the transducer. /ypical pulser circuits will

apply from ++ volts to @++ volts to a transducer.

In the receiver section the voltage signals produced by the transducer, which represents

the received ultrasonic pulses, are amplified. /he amplified radio frequency )6( signal

is available as output for display or capture for signal processing. 7ontrol functions

associated with the receiver circuit include

%ignal rectification )/he 6( signal can be viewed as positive half wave, negative

half wave or full wave.

(iltering to shape and smooth return signals

?ain, or signal amplification

6eject control

!one +urst ,enerators In esearch

/one burst generators often are used in high power ultrasonic applications. 'odern

computer controlled ultrasonic instrumentation, such as 6itec2s 60' ++++, is a

complete advanced measurement system designed to satisfy the needs of the acoustic

researcher in materials science or advanced 954. Its purpose is to transmit bursts of

acoustic energy into a test piece, receive signals from the piece following this burst,

then manipulate and analyze these received signals in various ways. 4xtreme versatility

is achieved through a modular approach allowing an instrument to be configured for

unique applications not previously encountered. 3nwanted modules need not bepurchased and in many cases special modules can be designed and constructed.


37/91

/he high power radio frequency )6( burst capability allows researchers to wor& with

difficult, highly attenuative materials or inefficient transducers such as 4'0/s.

0 computer interface ma&es it possible for the system to ma&e high speed complex

measurements, such as those involving multiple frequencies. 'any of these

measurements are very limited or impossible with manually controlled instruments. 0Windows or 5


38/91

transmitted into a part )structure with received signals returning from the part

)structure

Electrical Impedance Matching and !ermination

When computer systems were first introduced decades ago, they were large, slow-wor&ing devices that were incompatible with each other. /oday, national and

international networ&ing standards have established electronic control protocols that

enable different systems to 8tal&8 to each other. /he 4lectronics Industries 0ssociations

)4I0 and the Institute of 4lectrical and 4lectronics 4ngineers )I444 developed

standards that established common terminology and interface requirements, such as 4I0

6%-"#" and I444 @+".#. If a system designer builds equipment to comply with these

standards, the equipment will interface with other systems. 1ut what about analog

signals that are used in ultrasonicsL

Data Signals< 9nput versus =utput

7onsider the signal going to and from ultrasonic transducers. When you transmit data

through cable, the requirement usually simplifies into comparing what goes in one end

with what comes out the other. ;igh frequency pulses degrade or deteriorate when they

are passed through any cable. 1oth the height of the pulse )magnitude and the shape of

the pulse )wave form change dramatically, and the amount of change depends on the

data rate, transmission distance and cable electrical characteristics. %ometimes a

marginal electrical cable may perform adequately if used in only short lengths, but the

same cable with the same data in long lengths will fail. /his is why system designers

and industry standards specify precise cable criteria.

6ecommendationD


39/91

Shieldingis normally specified as a cable construction detail. (or example, the cable

may be unshielded, contain shielded pairs, have an overall aluminumCmylar tape and

drain wire, or even a double shield. 7able shields usually have two functionsD to act as a

barrier to &eep external signal from getting in and internal signals from getting out and

to be a part of the electrical circuit. %hielding effectiveness is very complex to measure

and depends on the data frequency within the cable and the precise shield design. 0shield may be very effective in one frequency range, but a different frequency may

require a completely different design. %ystem designers often test complete cable

assemblies or connected systems for shielding effectiveness.

.apacitancein cable is usually measured as picofarads per foot )pfCm. It indicates how

much charge the cable can store within itself. If a voltage signal is being transmitted by

a twisted pair, the insulation of the individual wires becomes charged by the voltage

within the circuit. %ince it ta&es a certain amount of time for the cable to reach its

charged level, this slows down and interferes with the signal being transmitted. 5igital

data pulses are a string of voltage variations that are represented by square waves. 0

cable with a high capacitance slows down these signals so that they come out of thecable loo&ing more li&e 8saw-teeth,8 rather than square waves. /he lower the

capacitance of the cable, the better it performs with high speed data

Data Presentation

3ltrasonic data can be collected and displayed in a number of different formats. /he

three most common formats are &now in the 95/ world as 2*scan, B*scanand .*scan

presentations. 4ach presentation mode provides a different way of loo&ing at and

evaluating the region of material being inspected. 'odern computerized ultrasonic

scanning systems can display data in all three presentation forms simultaneously.

2*Scan Presentation

/he 0-scan presentation displays the amount of received ultrasonic energy as a function

of time. /he relative amount of received energy is plotted along the vertical axis and

elapsed time )which may be related to the sound

energy travel time within the material is display

along the horizontal axis. 'ost instruments with

an 0-scan display allow the signal to be

displayed in its natural radio frequency form )rf,

as a fully rectified rf signal, or as either thepositive or negative half of the rf signal. In the 0-

scan presentation, relative discontinuity size can

be estimated by comparing the signal amplitude

obtained from an un&nown reflector to that from

a &nown reflector. 6eflector depth can be

determined by the position of the signal on the

horizontal sweep.

In the illustration of the 0-scan presentation to

the right, the initial pulse generated by the

transducer is represented by the signalIP, whichis near time zero. 0s the transducer is scanned


40/91

along the surface of the part, four other signals are li&ely to appear at different times on

the screen. When the transducer is in its far left position, only the IPsignal and signal

A, the sound energy reflecting from surfaceA,will be seen on the trace. 0s the

transducer is scanned to the right, a signal from the bac&wall BW will appear latter in

time showing that the sound has traveled farther to reach this surface. When the

transducer is over flawB, signalB, will appear at a point on the time scale that isapproximately halfway between theIPsignal and theBWsignal. %ince theIPsignal

corresponds to the front surface of the material, this indicates that flawBis about

halfway between the front and bac& surfaces of the sample. When the transducer is

moved over flaw C,signal Cwill appear earlier in time since the sound travel path is

shorter and signalBwill disappear since sound will no longer be reflecting from it.

B*Scan Presentation

/he 1-scan presentations is a profile )cross-sectional view of the a test specimen. In the

1-scan, the time-of-flight )travel time of the sound energy is displayed along the

vertical and the linear position of the transducer is displayed along the horizontal axis.(rom the 1-scan, the depth of the reflector and its approximate linear dimensions in the

scan direction can be determined. /he 1-scan is typically produced by establishing a

trigger gate on the 0-scan. Whenever the signal intensity is great enough to trigger the

gate, a point is produced on the 1-scan. /he gate is triggered by the sound reflecting

from the bac&wall of the specimen and by smaller reflectors within the material. In the

1-scan image above, lineAis produced as the transducer is scanned over the reduced

thic&ness portion of the specimen. When the transducer moves to the right of this

section, the bac&wall lineBWis produced. When the transducer is over flawsBand C

lines that are similar to the length of the flaws and at similar depths within the material

are drawn on the 1-scan. It should be noted that a limitation to this display technique is

that reflectors may be mas&ed by larger reflectors near the surface.

.*Scan Presentation

/he 7-scan presentation provides a plan-type view

of the location and size of test specimen features.

/he plane of the image is parallel to the scan pattern

of the transducer. 7-scan presentations are produced

with an automated data acquisition system, such as

a computer controlled immersion scanning system.

/ypically, a data collection gate is established on the

0-scan and the amplitude or the time-of-flight of

the signal is recorded at regular intervals as the

transducer is scanned over the test piece. /herelative signal amplitude or the time-of-flight is


41/91

displayed as a shade of gray or a color for each of the positions where data was

recorded. /he 7-scan presentation provides an image of the features that reflect and

scatter the sound within and on the surfaces of the test piece.

;igh resolution scan can produce very detailed images. 1elow are two ultrasonic 7-

scan images of a 3% quarter. 1oth images were produced using a pulse-echo techniqueswith the transducer scanned over the head side in an immersion scanning system. (or

the 7-scan image on the left, the gate was setup to capture the amplitude of the sound

reflecting from the front surface of the quarter. Aight areas in the image indicate area

that reflected a greater amount of energy bac& to the transducer. In the 7-scan image on

the right, the gate was moved to record the intensity of the sound reflecting from the

bac& surface of the coin. /he details on the bac& surface are clearly visible but front

surface features are also still visible since the sound energy is affected by these features

as it travels through the front surface of the coin.

Error Analysis

0ll measurement, including ultrasonic measurements, however careful and scientific, is

subject to some uncertainties. 4rror analysis is the study and evaluations of theseuncertainties> its two main functions being to allow the practitioner to estimate how

large the uncertainties are and to help him or her to reduce them when necessary.

1ecause ultrasonics depends on measurements, evaluation and minimization of

uncertainties is crucial.

In science the word 8error8 does not mean 8mista&e8 or 8blunder8 but rather the

inevitable uncertainty of all measurements. 1ecause they cannot be avoided, errors in

this context are not, strictly spea&ing, 8mista&es.8 0t best, they can be made as small as

reasonably possible, and their size can be reliably estimated.

/o illustrate the inevitable occurrence of uncertainties surrounding attempts atmeasurement, let us consider a carpenter who must measure the height of a doorway to


42/91

an M-ray vault in order to install a door. 0s a first rough measurement, she might simply

loo& at the doorway and estimate that it is "+ cm high. /his crude 8measurement8 is

certainly subject to uncertainty. If pressed, the carpenter might express this uncertainty

by admitting that the height could be as little as "+$ or as much as "$ cm.

If she wanted a more accurate measurement, she would use a tape measure, and shemight find that the height is ".# cm. /his measurement is certainly more precise than

her original estimate, but it is obviously still subject to some uncertainty, since it is

inconceivable that she could &now the height to be exactly ".#+++ rather than

".#++ cm, for example.

/here are many reasons for this remaining uncertainty. %ome of these causes of

uncertainty could be removed if she too& enough trouble. (or example, one source of

uncertainty might be that poor lighting is ma&ing it difficult to read the tape> this could

be corrected by improved lighting.

so she must estimate just where the top lies within the mar&. In either case, the carpenter

ultimately must estimate where the top of the door lies relative to the mar&ings on his

tape, and this necessity causes some uncertainty in her answer.

1y buying a better tape with closer and finer mar&ings, the carpenter can reduce her

uncertainty, but she cannot eliminate it entirely. If she becomes obsessively determined

to find the height of the door with the greatest precision that is technically possible, she

could buy an expensive laser interferometer. 1ut even the precision of an interferometer

is limited to distances of the order of the wavelength of light )about +.+++++$ meters.

0lthough she would now be able to measure the height with fantastic precision, she still

would not &now the height of the doorway exactly.

(urthermore, as the carpenter strives for greater precision, she will encounter an

important problem of principle. %he will certainly find that the height is different in

different places. 4ven in one place, she will find that the height varies if the temperature

and humidity vary, or even if she accidentally rubs off a thin layer of dirt. In otherwords, she will find that there is no such thing as one exact height of the doorway. /his

&ind of problem is called a 8problem of definition8 )the height of the door is not well-

defined and plays an important role in many scientific measurements.


43/91

8somewhere between ".$ and #.$ miles8 or 8somewhere between ".!! and #.+ miles.8

8 or v 8d>t

where dis the distance from the surface to the discontinuity in the test piece, vis the

velocity of sound waves in the material, and tis the measured round-trip transit time.

/he diagram below allows you to move a transducer over the surface of a stainless steel

test bloc& and see return echoes as they would appear on an oscilloscope. /hetransducer employed is a $ ';z broadband transducer +."$ inches in diameter. /he

signals were

ultrasonics testing

Documents