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24-Feb-07 By: N.Kuppusamy 1

Theory of Ultrasonic Testing

Presented byPresented byN.KuppusamyN.Kuppusamy

Singapore Chapter

NDT HORIZONNDT HORIZON

Module-1Sound Modes


Introduction This module illustrates the Basic Modes of Sound. Ultrasonic testing uses high frequency sound energy

to conduct examinations and make measurements. Sound is produced by vibration or oscillation (Back

and forth movement).EXAMPLES OF OSCILLATION


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Displacement

Vibration is defined as the displacement of massabout its rest position. It is given by the formula:


Basic Principles of Sound

Sound is produced by avibrating body and travels in theform of a wave.

Sound waves travel throughmaterials by vibrating theparticles that make up thematerial.

Sounds

The pitch of the sound is determined by thefrequency of the wave (vibrations or cyclescompleted in a certain period of time).

Ultrasound is sound with a pitch too high to bedetected by the human ear.


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Sound Spectrum

Bat, Quartz crystalUltrasound>20,000

Speech , MusicAudible sound HumanHearing Range20-20,000

Earth QuakeInfrasound

Infrasonic0-20

ExampleDescriptionFrequencyRange, Hz


States of matter and its structureGenerally [at least as for as we are concerned] matterexists in three states

Other states include: Plasma state (ionized state of matter), Quark state (A state where theProton, & Neutron decompose to quarks)


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Wave Parts

Introduction to Waves Wave Parts

The Anatomy of aWave and online quiz


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Wave parts

Introduction toWaves

Wave Parts The Anatomy of a

Wave and onlinequiz


The measurement of sound waves from crest to crest determines itswavelength ( ).


The wavelength is the distancebetween the "crests" of two wavesthat are next to each other. Theamplitude is how high the crests are.

Wavelength and Amplitude

Transverse wave

Compression wave

Wave length = Velocity / Frequency

Wave length is determined by the following relation:


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The size of a wave (how much it is "piled up"at the high points) is its amplitude. For soundwaves, the bigger the amplitude, the louderthe sound.

Amplitude is Loudness

Since the sounds are traveling at about the samespeed, the one with the shorter wavelength will go

by more frequently; it has a higher frequency, or pitch. In other words, it sounds higher.

Strings


Basic Principles of Sound The time is takes a sound wave to travel a distance of one complete

wavelength is the same amount of time it takes the source to executeone complete vibration.

The sound wavelengthis inversely proportionalto its frequency. ( = 1/f)

The velocity of Longitudinal, shear and surface waves are fixed for agiven material. The velocity of sound in each material is determinedby the material properties (elastic modules and density) of thatmaterial.


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Several wave modes of vibrationare used in ultrasonic inspection.

The most common arelongitudinal, shear, and Rayleigh(surface) waves and Plate (Lamb)waves.

Longitudinal /CompressionWaves

Longitudinal waves are waves in which the motion of the particles in themedium is in the same (or opposite) direction to the wave propagation.In longitudinal waves, the particles of the medium move back and forthcreating regions of high and low density (or high or low pressure).

VL = E

E = Youngs modulus of elasticity

= material density

It exists in all material forms (Solid, Liquid and Air)


Longitudinal Waves

Longitudinal Waves The animationshows a one-dimensional longitudinalplane wave propagating down a tube. Theparticles do not move down the tube withthe wave; they simply oscillate back andforth about their individual equilibriumpositions.


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Wave Propagation & Particle

Water waves are an example of waves that involve a combination ofboth longitudinal and transverse motions. As a wave travels throughthe waver, the particles travel in clockwise circles .

The radius of the circles decreases as the depth into the water

increases. The movie below shows a water wave traveling from leftto right in a region where the depth of the water is greater than thewavelength of the waves.


Shear / Transverse Waves: In a transverse wave the particledisplacement is perpendicular to the direction of wave propagation.Waves on a string are transverse waves. The animation below shows aone-dimensional transverse plane wave propagating from left to right.

Wave Propagation & Particle Motion

VT = G

G = Shear modulus of material= material density

Shear wave velocity for a givenmaterial is nearly 50% of

longitudinal velocity in that material.It exists only in solid mediums.


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Wave Propagation & Particle Motion Rayleigh surface waves are the waves with both longitudinal and

transverse motion found in solids. The particles in a solid, through which a Rayleigh surface wave passes,

move in elliptical paths, with the major axis of the ellipse perpendicular tothe surface of the solid.

As the depth into the solid increases the "width" of the elliptical pathdecreases.

Rayleigh waves are different from water waves in one important way. In awater wave all particles travel in clockwise circles. However, in a Rayleighsurface wave, particles at the surface trace out a counter-clockwiseellipse, while particles at a depth of more than 1/5th of a wavelength traceout clockwise ellipses.

Its velocity is approximately 90% of shear wave in a given material

Rayleigh waves are reflected from a sharpedge or corner. But, it continues to travelaround smooth curvatures and roundedcorners.

Rayleigh wavemotion


Wave Propagation & Particle MotionRayleigh or surface waves


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Lamb waves If a surface wave is introduced into a material that has a thickness equal to

three wavelengths, or less, of the beam, a different kind of wave results. Thematerial begins to vibrate as a plate; i.e., the wave encompasses the entirethickness of the material.

When this occurs, the normal rules for wave velocity along the plate breakdown. The velocity is no longer dependent upon the type of material and thetype of wave. Instead, we get a wave velocity that is dependent on thefrequency of the wave, the angle of incidence, and, of course, the type ofmaterial

There are two general types of lamb (or plate) waves depending on the waythe particles in the material move as the wave moves along the plate.

Symmetrical & Asymmetrical Lamb Waves


Lamb waves Symmetric Each type of Lamb wave has an infinite number of modes that the wave may attain.

These modes, too, are dependent on the three factors of the frequency of the wave,the angle of incidence, and the material.

These modes are differentiated by the manner in which the particles in the materialare moving.

N.Kuppusamy


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Lamb waves Asymmetric



Ultrasonic reflections from the presence ofdiscontinuities or geometric features enables detectionand location.

The velocity of sound in a given material is constant andcan only be altered by a change in the mode of energyor change of part temperature.


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Temperature and the speed of sound Temperature is also a condition that affects the speed of sound.

Heat, like sound, is a form of kinetic energy. Molecules at highertemperatures have more energy, thus they can vibrate faster.

Since the molecules vibrate faster, sound waves can travel morequickly. The speed of sound in room temperature air is 346meters per second. This is faster than 331 meters per second,which is the speed of sound in air at freezing temperatures.

The formula to find the speed of sound in air is as follows:

v = 331m/s + .6m/s/C * T

v is the speed of sound and T is the temperature of the air. Onething to keep in mind is that this formula finds the average speedof sound for any given temperature. The speed of sound is alsoaffected by other factors such as humidity and air pressure.

Sound Temperature


Interactive sites which allow you to observe and manipulatetransverse and longitudinal waves. Each site offers its own

uniqueness

JAVA APPLET Wave Types Transverse and Longitudinal - This java applet let you visualize the difference between transverse

wave and longitudinal wave. Transverse Wave and Longitudinal Waves this interactive site

allows you to examine both types of waves Longitudinal, Transverse and Mixed Type Waves this site allows

you to examine and manipulate both types of waves and a mixtureof both waves


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Presented byPresented by

N.KuppusamyN.Kuppusamy

Singapore Chapter


Module-2

Sound Properties


Sounds can be low or high. Sounds can be low like a growlingtiger or high like a chirping bird. This characteristic of sound iscalled pitch or frequency. Objects which vibrate faster produce ahigher frequency, and objects which vibrate more slowly produce alower frequency.

The frequency of a sound is equal to how many times it vibrateseach second. Vibrations per second are measured in Hertz (Hz).

An object that vibrates 5 times each second would have afrequency of 5 Hertz (Hz).

Frequency

An object that vibrates 1 time each second would have a frequencyof 1 Hertz (Hz).


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Surfing the Waves

Imagine that you are floating on a surfboard, and wavesare going past you. As each wave passes, you rise and

fall. The frequency in this case is the number of times per

second you bob up and down. (Obviously, it will be lessthan once per second with ocean waves, so the frequencyin this case will be a less than one Hertz .)

Ocean Frequency



Ultrasonic waves are very similar to light wavesin that they can be reflected, refracted, andfocused.

Sound requires a medium to vibrate (propagate)

whereas light doesnt.

Because Electromagnetic radiation is acombination of oscillating electric andmagnetic fields moving through a mediumperpendicular to each other throughspace and carries energy from one placeto another.


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Reflection and refraction occurs when soundwaves interact with interfaces of differingacoustic properties.

In solid materials, the vibrational energy canbe split into different wave modes when thewave encounters an interface at an angleother than 90 degrees.

The angle of reflection and refraction aregoverned by Snells law .

refraction

Reflection andRefraction

Echo


Reflection and Refraction

Snells Law: Sin iSin r

= ViVr

Reflection : Angle of Reflection is equal to incident angle.

Refraction : Angle of refraction is a function of incident angle and velocity ratiobetween incident and refractive mediums.

Both reflection and refraction are governed by Snells law and it holds true for both

longitudinal and shear waves.

i = incident angler = reflected angler 1 = refracted angle

i r

r 1

Medium 1

Medium 2


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Reflection

When a longitudinal wave is reflected inside thematerial, the reflected shear wave is reflected at asmaller angle than the reflected longitudinal wave .

This is due to the fact that the shear velocity is lessthan the longitudinal velocity within a given material.

Reflection andRefraction

Mediumi1

r 1r 2i1 = r 1i1 > r 2r 2 < r 1

Sound Reflection


RefractionRefraction is the bending of waves when they enter a medium where their speed isdifferent. Refraction is an important phenomena with in ultrasound. This property isused to generate shear wave in the second medium.

Another visualization of refraction can come from the steering of various types oftractors, construction equipment, tanks and other tracked vehicle. If you apply the rightbrake, the vehicle turns right because you have slowed down one side of the vehiclewithout slowing down the other.

FastMedium As a column of

marching troopscrosses from a fastmedium to a slowmedium,the directionof marchchanges

As a toy car rolls from aHard floor onto carpet,It changes directionBecause the wheel thatHits the carpet first isSlowed down first.

SlowMedium

Concrete Swamp

Visualizationsof Reflection

Fastmedium

Slowmedium


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Refraction Refraction takes place at an interface due to the different velocities of the acoustic

waves within the two materials.

When a longitudinal wave is refracted into a material, the refracted shear waveangle is smaller than the refracted longitudinal wave.

This is due to the fact that the shear velocity is less than the longitudinal velocitywithin a given material.

Please remember that some of the wave energy is always reflected at the interface

i

r 1

Medium 1

Medium 2 r 2

r 1 < r 2

L-wave

Shear wave


Mode Conversion When sound travels in a solid material, one form of wave energy can be

transformed into another form.

For example, when a longitudinal waves hits an interface at an angle,some of the energy can cause particle movement in the transversedirection to start a shear (transverse) wave.

Mode conversion, occurs when a wave encounters an interface betweenmaterials of different acoustic impedance and the incident angle is notnormal to the interface.

Mode conversion can occur in both reflective and refractive mediums.

Mode conversion occurs every time a wave encountered interface at anangle, ultrasonic signals can become confusing at times

Mode conversion

Mode conversion1


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Diffraction, Scattering and ReflectionWhen a wave encounters a reflector within a medium, one ofthe following occurs:

Diffraction occurs when the sound wave length is larger

than the reflector size (this condition prevails at theedges of a discontinuity) [ > Reflector size].

Scattering occurs when the sound wave length is about thesame size of reflector [ Reflector size].

Reflection occurs when the sound wave length is smallerthan the reflector [ < Reflector size].


DiffractionDiffraction: the bending of waves around small* obstacles and thespreading out of waves beyond small* openings. (* small compared to thewavelength)When a wave encounters a point reflector (small in comparison to a wave-length), the reflected wave is re-radiated as a - spherical wave front.

When a plane wave encounters the edges of reflective interfaces, such asnear the tip of a fatigue crack, specular (mirror like) reflections occur along the"flat" surfaces of the crack and cylindrical wavelets are launched from theedges.

Their redirection into the path of subsequent advancing plane waves results inincident and reflected (scattered) waves interfering, i.e., forming regions ofreinforcement (constructive interference) and cancellation (destructiveinterference).

A plane wave is one in which quantities vary only with the distance along a certaindirection, and with the time.


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Diffraction

Suppose you bought aconcert ticket without lookingat the seating chart and wound upsitting behind a a large post. Youwould be able to hear the concertquite well because the wavelengthof sound are long enough to bendaround the post.

If you were several wavelengthsof sound past the post, you wouldnot be able to detect the presenceof the post from the nature of thesound.

If you were outside an opendoor, you could still hearbecause the sound wouldspread out from the smallopening as if it were a localizedsource of sound.

Soundsource

Soundwaves

Diffraction Around post

DiffractionPast smallopening


Diffraction of Sound

Important parts of our experience with sound involve diffraction. Thefact that you can hear sounds around corners and around barriersinvolves both diffraction and reflection of sound. Diffraction in suchcases helps the sound to "bend around" the obstacles. The fact thatdiffraction is more pronounced with longer wavelengths implies that

you can hear low frequencies around obstacles better than high

frequencies, as illustrated by the example of a marching band on thestreet.Another common example of diffraction is the contrast in sound froma close lightning strike and a distant one. The thunder from a closebolt of lightning will be experienced as a sharp crack, indicating thepresence of a lot of high frequency sound. The thunder from a distantstrike will be experienced as a low rumble since it is the longwavelengths which can bend around obstacles to get to you. There areother factors such as the higher air absorption of high frequenciesinvolved, but diffraction plays a part in the experience.


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Critical Angles

There is an incident angle at which the angle of refractionof the longitudinal wave is 90 degrees (i.e.,parallel tosurface). This is called First Critical Angle .

The incident angle at which the angle of refraction for theshear wave is 90 degrees, is known as the second criticalangle.

At this point, all of the wave energy is reflected orrefracted into a surface following shear wave or shearcreep wave.

Slightly beyond the second critical angle, surface waveswill be generated.


Creep Waves

This wave is sometimes referred to as a "creep wave." Theyare similar to water waves.

Because of their inhomogeneous nature and the fact thatthey decay rapidly, creep waves are not used as extensivelyas Rayleigh surface waves in NDT.

However, creep waves are sometimes useful because theysuffer less from surface irregularities and coarse materialmicrostructure, due to their longer wavelengths, thanRayleigh waves.

At the first critical angle ofincidence, much of the acousticenergy is in the form of aninhomogeneous compression wave,which travels along the interfaceand decays exponentially withdepth from the interface.


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AttenuationSound waves decrease in intensity and amplitude as theytravel away from their source, due to geometrical spreading,scattering, and absorption.

Loss of energy due to absorption and scattering is known asattenuation and it is measured in dB/m or dB/mm.

This loss is proportional to the grain volume in the materialand inversely proportional to the wavelength (1/ the beam.

It is also expressed in nepers (Np) per unit length.

1 dB/cm = 8.686 NP/cm.


Fine and coarse grained steelat the same magnification

Fine grained steel Coarse grained steel


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Grains

Grey iron Spheroidal graphite iron


Attenuation

In this expression A0 is the amplitude of the propagating wave atsome location. The amplitude A is the reduced amplitude after

the wave has traveled a distance z from that initial location. Thequantity is the attenuation coefficient of the wave traveling inthe z-direction. The dimensions of are nepers/length, where aneper is a dimensionless quantity. e is Napier's constant which isequal to approximately 2.71828.

The units of the attenuation value in nepers/length can beconverted to decibels/length by dividing by 0.1151. Decibels is amore common unit when relating the amplitudes of two signals.

A decaying plane wave is expressed as:


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AttenuationAttenuation is generally proportional to the square ofsound frequency. Quoted values of attenuation are oftengiven for a single frequency, or an attenuation valueaveraged over many frequencies may be given. Also, theactual value of the attenuation coefficient for a givenmaterial is highly dependent on the way in which thematerial was manufactured.

Thus, quoted values of attenuation only give a roughindication of the attenuation and should not beautomatically trusted. Generally, a reliable value ofattenuation can only be obtained by determining theattenuation experimentally for the particular materialbeing used.


Attenuation Generally defined as loss of amplitude over the distance

traveled in total transit time (i.e., 2T in pulse echo testing)

There are many factors which accounts for the amplitudeloss. The amplitude loss due to beam divergence has to betaken into account when calculating attenuation in the farzone.

i.e., Amplitude difference = Beam spread - Attenuation Generally in the far zone, doubling the distance reduces the

back echo by half or 6dB due to beam spread.

Attenuation in the far zone (i.e., when the NF is < thickness)

Attenuation in the near field (i.e., when the NF is > thickness)

dBdifference

T

6

2

dBdifference

T 2= dB/inch or dB/m

= dB/inch or dB/m


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AttenuationAttenuation can be determined by evaluatingthe multiple backwall reflections seen in atypical A-scan display like the one shown in

the image.The number of decibels between two adjacent signals ismeasured and this value is divided by the time interval ( or distance) between them.This calculation produces a attention coefficient in decibelsper unit time Ut ( or dB per unit distance). This value can beconverted to nepers/length by the following equation.

Where v is the velocity of sound in metersper second and Ut is decibels per secondt U v

1151.0


Geometrical SpreadingInverse Square LawAs one moves further from a source of spherical waves, the amplitude of thesound at your location gets less. The intensity I is the power W in the wavedivided by the area A over which it is spread: I = W/A or W/ 4 r 2

Where, A = 4 r 2.In the absence of absorption,the intensity of spherical soundwaves decays as 1/ r 2

The amplitude (sound pressure)of a traveling simple sphericalwave is proportional to thesquare-root of its intensity.

Therefore in the absence ofabsorption, the pressureamplitude of spherical soundwaves decays as 1/ r .


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ScatteringMicroscopic reflections in directions otherthan its original direction of propagation iscalled Scattering.

Scattering losses are greatest where thewavelength is less than one-third the grainsize.

Scattering is a more difficult problem, thanabsorption and occurs when the ultrasonicbeam encounters small, randomly orientedreflectors in the material.

These reflectors may be grain boundaries,microscopic voids or particles that scatterthe incoming wave.

Scattering

High scattering

Low scattering


ScatteringScattering can make the trace unreadable, and causediscontinuities to be missed.As scattering is caused by a multitude of small reflectors, the trace will displaya random collection of small peaks, which together may be so large as to make itdifficult to distinguish real discontinuities within this noise.

The presence of a small amount of grass at the base ofthe trace is generally an indication that the soundenergy is coupled to the test object.Once this grass exceeds about 10% full screen height(FSH), however, it is known as material noise and makesdiscrimination difficult between natural scattering anddiscontinuities. Normally, you need to have a signal tonoise (S/N ) ratio as high as possible, and at least 3:1 forreliable detection.

S/N

The ability to get a good S/N ratio is important, but should beapproached with caution.


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Absorption Absorption: The conversion of sound to other forms of energy.

Absorption occurs when the ultrasonic energy is physically convertedinto heat within the material.

Energy is taken from the beam, so of course the returning signals haveless energy, and appear smaller on the UFD screen. This can generallybe overcome by increasing amplification to compensate for the losses.

As the frequency is lowered and the wavelength becomes greater thanthe grain size, attenuation is due only to damping of the wave. Indamping losses, wave energy is lost through heat due to friction of thevibrating particles.

Absorption is used to advantage in medical ultrasonic therapy, which

intentionally produces considerable amounts of heat in human tissue toaid in recovery from injury


Approximate attenuationcharacteristics

100 mmPorous Ceramics,RocksGrey IronHigh Attenuation> 100 dB/m

0.1 1 metrePerspex,PVC

Cast Steel, SG IronWrought Copper, Brass,Lead

Medium Attenuation10 100 dB/m

1 10 metreGlass,PorcelainCast and Wrought AluminumWrought Steel

Low AttenuationUp to 10 dB/m

Max Testable ThicknessNon MetalsMetalsAttenuation Range

at 2 MHz


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Dealing with Absorption and ScatterIncreasing amplification may help to overcomeabsorption

Although the material is difficult to test, properattention to the attenuation characteristics can result ina valid test.The first reaction to dealing with attenuating materials isgenerally to increase the gain (amplification) of theinstrument to compensate for the energy loss.This will compensate for basic absorption, but will nothelp when faced with scattering. Lower frequencies also

act to reduce absorption effects.


Increasing amplification does not help with scatter

With scattering, much of the scattered beam will be sent backto the receiver and will be detected, giving rise to an

apparently random set of indications, (material noise ), oftenreferred to as grass (or hash in American terminology).

If excessive amplification is used, the grass becomesexcessive, and the screen display becomes unmanageable.

A similar effect occurs when driving in fog putting theheadlights on high beam results in the driver being dazzled

by the reflections from the fog droplets, and does not improvevisibility.

Dealing with Absorption and Scatter


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Frequency selection will increase tolerance of scattering

As attenuation is greater at short wavelengths (highfrequencies), high attenuation materials are usuallyexamined using low frequencies, typically 1 to 2 MHz.

Some experimentation may be required to find theoptimum frequency, by progressively decreasing thefrequency until a usable frequency is found.

To continue our analogy of driving in fog, using a lower

frequency is like using fog lights that operate with alower optical frequency that is, a colour closer to thered end of the visible spectrum.

Dealing with Absorption and Scatter


4001,5001,5005,0001

2501,0007503,0002

251001002005

Grey Iron(mm)

SGIron

(mm)

Coarse GrainedSteel(mm)

Fine GrainedSteel(mm)

Frequency

(MHz)

Typical maximum test ranges for compression mode

These are typical ranges. In practice, maximum range will dependon the probe design, equipment, pulse strength, probe diameter andspecific material grain structure.

For shear waves , which have approximately half the wavelength,the maximum shear wave ranges are approximately equal to acompression wave of twice the frequency in the table above. For

example 2 MHz shear has a similar test range to 4 MHzcompression.

The improved penetration at low frequencies is obtained at theexpense of reduced sensitivity to smaller discontinuities


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Acoustic Impedance Sound travels through materials under the influence of sound

pressure. Because molecules or atoms of a solid are boundelastically to one another, the excess pressure results in a wavepropagating through the solid.

The acoustic impedance (Z) of a material is defined as theproduct of density ( p) and acoustic velocity ( V) of that material.Z = pV

Acoustic impedance is important in1. the determination of acoustic transmission and reflection at

the boundary of two materials having different acousticimpedance

2.the design of ultrasonic transducers.3.assessing absorption of sound in a medium.


Amount of Energy ReflectedThe reflected energy in terms of Pressure(Amplitude) is the difference divided by thesum of the acoustic impedances of the twomaterials.

2

12

12

Z Z Z Z

R

Applet for Energy transmitted

The reflected energy in terms of intensity(power) is the square of the differencedivided by the sum of the acoustic impedancesof the two materials. Note that TransmittedSound Energy + Reflected Sound Energy = 1

12

12

Z Z Z Z

R

100% + R = Transmission

T + R = 100%only.Reflectionfor IntensityAmplitude


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Reflection and Transmission Coefficients (Pressure)

Ultrasonic waves are reflected at boundaries where there aredifferences in acoustic impedance, Z. This is commonly referred toas impedance mismatch. The fraction of the incident-wave intensityin reflected waves can be derived because particle velocity andlocal particle pressures are required to be continuous across theboundary between materials.

Formulation for acoustic reflection and transmission coefficients(pressure) are shown in the interactive figure below. Differentmaterials may be selected or you may alter the material velocity ordensity to change the acoustic impedance of one or both materials.The red arrow represents reflected sound, while the blue arrow

represents transmitted sound. Applet for energy transmitted


Amount of Energy Transmitted

The reflected energy in termsof Pressure (Amplitude) is givenby

212

12 .4)1( Z Z

Z Z RT

Applet for energy transmitted

The reflected energy in terms ofintensity is given by

12

22)1( Z Z

Z RT

The amplitude is no longer true to say that T=100-R. Under certaincircumstances there may be transmission of more than 100% and it is notimportant from which side the wave approaches the boundary because Intensityand Amplitude are still connected through Z


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Negative Reflection Coefficient(Reflection from a HARD boundary)

When R is negative (-), which indicates phase reversal afterreflection

As the wave pulse approaches the fixed rigid end,the internal restoring forces which allow the waveto propagate exert an upward force on the end ofthe string.But, since the end is clamped, it cannot move.

According to Newton's third law, the wall must beexerting an equal downward force on the end ofthe string. This new force creates a wave pulsethat propagates from right to left, with the samespeed and amplitude as the incident wave, butwith opposite polarity (upside down).

At a fixed (hard) boundary, the displacement remains zero and the reflected wavechanges its polarity (undergoes a 180o phase change)


Positive Reflection Coefficient(Reflection from a Soft boundary)

When R is positive there is no phase reversal takes placeafter reflection

When a sound wave approaches a soft boundary(metal-fluid), The soft boundary permits it to moveupward. The net vertical force at the free end is zero.

The reflected wave pulse propagates from right to left,with the same speed and amplitude as the incidentwave, and with the same polarity (right side up).

At a free (soft) boundary, the restoring force is zero and the reflected wave has thesame polarity (no phase change) as the incident wav


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Other MediumsFrom high speed to low speed

(low density to high density)From low speed to high speed

(high density to low density)

DensityTension

string aonwavesof Speed


Energy Reflected/TransmittedNote that the energy reflected at a water steel interface is 0.88 or 88%.0.12 or 12% is transmitted into the component. If reflection andtransmission at interfaces is followed through the component, and lossby attenuation is ignored, a small percentage of the original energyreturns to the transducer.

Assuming acoustic energy at the transducer is 100% and energytransmitted into a component at a water steel interface is 12% asdiscussed above. At the second interface (back surface) 88% or10.56% would be reflected and 12% transmitted into the water. Thefinal interface would allow only 12% of 10.56 or 1.26% of the originalenergy to be transmitted back to the transducer.


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Relative Amplitude

Partition of acoustic energy atwater steel interface.

The Reflection coefficient, R,is equal to 1-(L+S).

Where, L is the transmissioncoefficient of Longitudinalwave and S is the transmissioncoefficient of Shear wave.


Summary

Attenuation occurs by absorption and scattering. Absorption can often be managed by use of lower frequency,

increased pulse energy or additional amplification. Scattering is managed by using lower frequencies and minimizing

the beam path length where possible. The decibel (dB) notation is a convenient way of measuring and

comparing echo amplitude over a very wide range. Attenuation properties may be expressed as attenuation

coefficients (dB/mm), and are influenced by metallurgicalcondition, homogeneity and probe frequency.


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Test of Reasoning You are testing some forgings and you suddenly notice

that there are lots of small, apparently irrelevant indications

on the screen. Your more experienced fellow techniciansays its just grass and to turn the gain up. What is yourcolleague referring to, and should you blindly follow hisadvice?

You have been injured in a football match and yourehaving ultrasonic treatment at the physiotherapist. Are yourtissues mainly absorbing or scattering the ultrasonic

waves?


Points to Ponder

Why does attenuation increase with probe frequency? How would you expect the attenuation of compression and

shear waves of the same frequency to compare? Why is the sunset red in colour?

(Higher energy waves attenuated by the atmosphere dueto increased viewing distance)


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Phase Phase relates the vibration to time. When two vibrations are

in phase, it is called constructive phase (peak peak or valley valley). Both waves augment each other and resultant wave ismore in amplitude.

When two vibrations are in opposite phase (peak valley), theycancel out each other and the resultant amplitude is zero.

CONSTRUCTIVEINTERACTION

DESTRUCTIVEINTERACTION

DIFFENTIALINTERACTION


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By: N.Kuppusamy

Theory of Ultrasonic TestingTheory of Ultrasonic Testing


Singapore Chapter


Module-3Decibel

By: N.Kuppusamy Event Horizon

Decibel Notation

The unit of Sound is Bel, which is much biggerquantity for normal use. Therefore we use smallerunit called decibel (dB).

In ultrasonics the attenuation characteristics of agiven material are expressed in terms of anattenuation coefficient which has units of decibelsper metre or dB/mm, so we need to understanddecibel notation.

If you are not familiar with logarithms, now would be

a good time to learn about them.


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The ultrasonic flaw detector usesdecibels to measure attenuation

The most immediately obvious means of measuringthe relative pressure of the sound wave is throughits echo amplitude.

If one echo has an amplitude of 100% FSH andanother has an amplitude of 50% FSH, the firstcan be said to have twice the acoustic pressure ofthe second.


Need of Smaller Unit, dBIn ultrasonics we need to work over a very large range ofamplitudes. While it is easy to compare large screen heights, itis difficult to compare small screen heights.If we want to compare a 10% echo with a 5% echo, thereadability of the screen makes it impossible to make anaccurate comparison is it 4%, 5%, or even 6%? The inaccuracyof such a comparison is too large.To improve the useable range, most UFDs are equipped with acalibrated gain control (sometimes called an attenuator in theUS) to allow more accurate comparisons. The gain control iscalibrated in decibels ( dB).


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Decibel notation is used for comparingsignals conveniently over a wide range

The Bel is a unit forcomparing the power of twosignals by measuring theirratio.

If we measure two signalsand they have powers W1and W2 respectively, the belis a convenient way of

comparing them.

2

1log1W W

bel


Comparing signals (contd)

The result of this calculation is the relative power in Bel.

The decibel (dB) unit is one-tenth of a Bel, so anymeasurement expressed in decibels will be ten times thesame measurement expressed in Bels:

2

1log1011W W

dbdecibel


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Comparing signals (contd)

In ultrasonics we are concerned withmeasurements of sound pressure, notpower. So we need an expression ofdecibels in terms of pressure. Aspower is proportional to the pressuresquared, we have:

When comparing two amplitudes onthe screen, the amplitude is ameasure of sound pressure.

2

1

2

2

1

2

2

21

log20

log10

log10

p p

dB

p p

dB

p

pdB

To determine the dB equivalent, measure each amplitude,find the ratio, take the log, then multiply by 20.


Comparison of two amplitudes

60310000.1100

4021001100

60.322550

60.324080

120.6425100

60.3250100

2011010100

dB =20log(A 1 /A 2 )

log(A 1 /A 2 )Ratio

(A 1 /A 2 )Amplitude 2

%FSHAmplitude 1

%FSH

Some interesting points from this table are:

The dB values of any signal is not an absolute measurement it isalways relative to some other reference, eg. the response from abackwall or drilled hole.A 20 dB signal is one that is 10 times another, and is a commonlyused value in ultrasonics.A 6 dB signal is one that is twice another and is also commonlyused in ultrasonics.Many UFD units have coarse steps in 20 dB intervals, whichcorresponds to ratios of 10:1 between the coarse steps.Many UFD units have fine steps in 2 dB intervals, whichcorresponds to a ratio of 1.25:1 between the steps.Large variations in amplitude can be easily measured accurately using acalibrated gain control. If for instance, you want to accurately compare a verystrong (100% FSH) and very weak (1% FSH) signal, you can simply adjust thecalibrated gain for each signal so that the signal reaches the same screenheight. Then measure the gain difference to give an accurate comparison.


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1. The dB values of any signal is not an absolute measurement itis always relative to some other reference, eg. the response

from a backwall or drilled hole.2. A 20 dB signal is one that is 10 times another, and is a

commonly used value in ultrasonics.

3. A 6 dB signal is one that is twice another and is also commonlyused in ultrasonics.

4. Many UFD units have coarse steps in 20 dB intervals, whichcorresponds to ratios of 10:1 between the coarse steps.



5. intervals, which corresponds to a ratio of 1.25:1 between thesteps.

6. Large variations in amplitude can be easily measuredaccurately using a calibrated gain control. If for instance,

you want to accurately compare a very strong (100% FSH)and very weak (1% FSH) signal, you can simply adjust thecalibrated gain for each signal so that the signal reaches thesame screen height. Then measure the gain difference togive an accurate comparison.The use of dB is common in many other applications. Weoften see the silencers on noisy equipment being given anoise reduction rating. For instance, if it has a rating of 40dB, the noise power reduction is 100 fold; if the rating is 80dB, the noise reduction is 10,000 fold.


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Self Test1. If the noise reduction rating on a compressor is 80

dB (10,000:1) and you want to double the noisereduction to 20,000:1, how many additional decibelsof noise reduction would you need?a. 6 dBb. 20 dBc. 40 dBd. 80 dB

Answer: a20log(20,000) = 86.02 dB,therefore an extra 6 dB isneeded.


Decibel value

The decibel value of a signal is positive if greater than the reference and negative if less than the reference

When the amplitude in question is greater thanthe reference, it is said to have a positive gainrelative to the reference. When the amplitude isless than the reference, it is said to have anegative gain (or a positive attenuation) relativeto the reference.


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Example If you have a reference signal at 50% and an

unknown signal at 100%, the unknown signal is said to

have a positive gain of 6 dB. If you have a reference gain of 50% and an unknownsignal of 25%, the unknown signal is said to have anegative gain of 6 dB, or an attenuation of 6 dB.


Some typical dB ratios relative to 100% FSH

7.045

6.0505.056

4.063

3.071

2.079

1.584

1.089

0.595

0100

Attenuation orNegative Gain

(dB)

Amplitude(%)

80.00.01

60.00.1

40.01.030.03.2

20.010

18.012.5

16.016

14.020

12.025

10.032

8.040

Attenuation orNegative Gain

(dB)

Amplitude(%)


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Practice Establish an echo from a convenient back wall

and adjust the gain such that the signal is at100% FSH .

Make sure the suppression (reject) is turned off. Note the gain setting (dB). Reduce the gain a total of 20 dB in 2 dB steps

and note the screen height for each step. Compare the theoretical and actual screen

heights.


Quick Decibel Calculations

It is possible to calculate many dB equivalents if youknow that 6 dB represents a ratio of 2:1 and 20 dBrepresents 10:1. The trick is to realize that additionof decibel values corresponds to multiplication of

ratios, and subtraction of decibel values correspondsto division of ratios. For example, to determine theratio equivalent to 12 dB, we note that 12 = 6 + 6.


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Quick Decibel CalculationsChanging to ratios, the 6 dB value becomes 2, andthe addition becomes multiplication. We thereforehave a ratio equivalent of 2 times 2 = 4. That is, 12dB means a ratio of 4:1, or a quadrupling withrespect to some reference value.

422:

6612:

x Ratios

dB Decibels


Quick Decibel Calculations

Here is another example, where we find that 14 dB isequivalent to a ratio of 5:1.

5210

62014: dBdBd Decibels


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Work out the following examples

Table of decibel and ratio breakdowns

-12-8

26

30

8

14

12

RatioRatio BreakdowndB BreakdowndB


Table of decibel and ratiobreakdowns

0.25:1 2 2-6 dB - 6 dB-12

0.4:12 2 106 dB + 6 dB - 20 dB-8

202 106 dB + 20 dB26

32:12 2 2 2 26 dB + 6 dB + 6 dB + 6 dB + 6 dB30

2.5:110 2 220 dB 6 dB 6 dB8

5:110 220 dB 6 dB14

4:12 26 dB + 6 dB12

RatioRatio BreakdowndB BreakdowndB


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Readability problems with stepped gaincontrol

Most analogue UFD units have a fine stepped gaincontrol, in which the gain can be adjusted only in

steps of 2 dB. This is rarely a practical limitation, butit may make it difficult at times to measureaccurately.With experience, you will become competent ininterpolating between steps and improve youraccuracy. You should soon be able to estimate gain toan accuracy of 1 dB, and with further experience, to

read to 0.5 dB. Most digital instruments read gainwith a much greater precision.


Practical Measurement of Attenuation

It is important to makeattenuation measurementsin the far zone

We will talk about near andfar zones in the next task,

but for now: In the near zone , theultrasonic response iserratic and it is notpossible to make reliablecomparisons.

AttenuationMeasurement

In the far zone , the ultrasonic response is predictable and

sound pressure can be predicted more accurately.


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To measure the relative attenuation:

Calculate the approximate nearzone length (N) of the probe byapplying the formula:

Where: N is the near zone length in meters (mm) D is the probe crystal diameter in meters (mm) is the wavelength in meters (mm)

4

2

D N


To measure the relative attenuation..

Using either an immersion or contact set up, display two ormore backwall reflections on a parallel-sided sample of thematerial as shown. Use backwalls beyond three near zonelengths (3N), unless this is impossible due to the materialcharacteristics.

Display the first backwall at 100% screen height.

Note the extra gain required to bring the next backwall to100% screen height. Record this extra gain (g1).

Note the thickness between the backwalls (d)

Attenuation Coefficientd

g 2

1


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For a 25 mm thick test object, firstbackwall is approximately three near zones

Example For a 10 mm/2 MHz zero L-probe, calculate the near zone:

mm D

N

mmm f c

5.895.24

10104

95.200295.0102

5900

2

6

Attenuation


Example (contd)First backwall is set at 100% FSHGain is adjusted to bring the second backwall to 100% by adding (let us

say) 2 dBDifference in gain (g1) = 2 dBDistance between backwalls (d) = 25 mmAttenuation coefficient = 2 / 25 2 = 0.04 dB/mm = 40 dB/meter



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Do yourself

Measure the attenuation of your V1 block(AS2083 block1) for your probe frequency in the 25 mm direction (through thethickness of the block), then in the 100 mm direction (acrossthe width of the block).

Do you get the same answer in both directions? Discuss yourresults.

Your customer has three machined samples. One has very highattenuation, one is medium, and the third is very low. Yourcustomer thinks one was made from steel plate, one was a grey

iron casting , and the other was an SG [ Spheroidal Graphite -Ductile Iron] casting. How can you help the customer sortthem?


Points to Ponder

1. Why do we divide by 2 when calculating the attenuationcoefficient?

2. Can you see some shortcomings with this technique?

3. How could you make it more accurate?

4. Why do you get a different answer in differentdirections when testing the V1 block? (there may bemore than one reason)


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Applications of Attenuation Measurements

Measurements can tell whether a material can reasonably be examined

If a material has excessive attenuation, it may not be possible to effectivelyexamine it, particularly in thick sections. Some standards place limits on theattenuation characteristics of materials, and if the attenuation is too high,it may be necessary to carry out corrective heat treatment, or to placequalifications on the results of the examination.

Attenuation measurements can check heat treatment processes

Attenuation increases with increasing metallurgical grain size. Excessivegrain size is often an undesirable property and may be uneven through thesection. Relative attenuation measurements are quite simple and quick tomake, and can be used to check that heat treatment has been effective.

Attenuation can also be used to discriminate between SG iron and Grey Ironcastings.


Applications of Attenuation Measurements

Comparing attenuation can ensure consistent test sensitivity

Calibration blocks are generally made from ideal fine-grainedmaterials. If the test is done on a different material, theexamination may be carried out at an incorrect sensitivity dueto the attenuation difference between the calibration blockand material under test. For example, this results in a loss ofsensitivity when testing higher attenuation materials.

Standards such as AS2207 , ASW D1.1give guidance for usingattenuation measurements to compensate for losses insensitivity due to attenuation variations.


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Spot weld testing using attenuation

Resistance spot-weld testing uses attenuation to evaluate weld qualityThere are thousands of spot welds in the thin metal sheets in the averagemotor vehicle. These were traditionally tested by measuring the forcerequired to pull apart a test weld. This is not a very scientific test and hasrecently been challenged by an ultrasonic method that can determine muchmore about the weld quality.

Spot weld


Nugget Weld Examination Procedure

The examination is carried out with a very high frequency, typically 20 MHz, anda very small probe with a flexible water filled membrane to conform to the weldprofile. The display can result in four types of responses:

If there is a large weld nugget (good weld), there is a series of backwallscorresponding to two metal thicknesses. The entire beam passes through thenugget. There is, however a very steep decay in the backwall pattern, as theweld nugget is of higher attenuation than the sheet steel. At the high frequencyused, this high attenuation is quite obvious by the rapid echo decay.

If the weld nugget is undersize, there is a similar pattern to the larger weldnugget, but some intermediate echoes occur as all the sound does not travelthrough the weld, due to the unfused area around it.


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Theory of Ultrasonic TestingTheory of Ultrasonic Testing


Singapore Chapter


Module-3ACoefficients &

Couplants

1-Nov-05 N.Kuppusamy

Introduction

reflection and transmission at interfaces principles of immersion testing how to set up an immersion test specific instrumentation for immersion testing focused probes automated scanning and recording systems other applications of immersion testing.

In this section you will learn about immersion testing andunderstand all about reflection and transmission in more detail

just what does happen when an ultrasonic beam strikes aninterface? This is vital for understanding ultrasonic tests.The things you will need to know to do this task are:


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Interfaces

An interface is a boundary where two different materials meet So far, you have examined waves travelling through one medium. What

happens when a sound wave strikes an interface between differentmaterials?

In general, when sound waves come to an interface, some of the sound willbe reflected, and some will be transmitted, or pass through the interface.A similar situation occurs with light waves when you look in a shop window.You will see the objects in the shop (transmitted light) as well as your ownreflection (reflected light).

You may have noticed some offices use striped mirrors, which thecustomer cannot see through because they see a mix of reflected andtransmitted light which they cannot interpret, while staff in the officeonly see transmitted light and can see the customer quite clearly.

The most common interfaces we encounter during ultrasonic testing aremetal-to-water and metal-to-air. We also encounter Perspex-to-metalinterfaces in probe design and use. There are also applications where weexamine metal-to-metal bonds, and even vulcanised rubber bonds.


Interfaces

Some interfaces you will encounter include:the far wall of a test object (metal-to-air interface)a void in a casting (metal-to-gas-interface)

a slag inclusion in a weld (metal-to-non-metal interface)a void filled with water (metal-to-water interface)a crack filled with oil (metal-to-oil interface)a shrink fit (a mix of metal-to-metal and metal-to-air

interfaces, depending on the quality of the shrink fit)the far wall of a pipe filled with water (water-to-metal and

metal-to-air interfaces).


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An interface occurs where there is a change inacoustic impedance

An interface is formed where different materials meet, but what do wemean by different? We need a property of the materials to let usquantify how sound waves will behave at an interface. This property is

the acoustic impedance and it is a measure of the resistance to soundpropagation through a medium. The formula for calculating acoustic impedance is very simple:

Where: Z is the acoustic impedance (kg/m 2s) is the density (kg/m 3 ) c is the acoustic velocity (m/s) E is Youngs modulus (Newtons/m 2 )

E c Z


Interface

Now we can describe an interface in a much more scientific way.An interface is a zone where there is a change in acousticimpedance.

The junction between weld metal and parent metal of the sameacoustic impedance is therefore not an interface, unless the junction is discontinuous (e.g. has cracks or other physicaldefects).

An atomic junction between two dissimilar metals is an interface .Conversely, two different metals would not have an interface iftheir acoustic impedances happened to be identical.

For water, the acoustic impedance is approximately 1000 1483

= 1.48 106 kg/m2s.


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Check Your Progress

Calculate the acoustic impedance ofsteel.

Calculate the acoustic impedance ofPerspex.

Answer: 45.4 106 kg/m 2s

Answer: 3.2 106 kg/m 2s

3450.1290Air 5,46019,100Tungsten

2,3001,200Rubber-vulcanised

1,45013,600Mercury

1,4831,000Water

2,7301,180Perspex

5,9007,700Steel

6,3202,700Aluminium

Compression

Velocity (c c)(m/s)

Density ( )(kg/m 3)Material

Density and acoustic compression velocity invarious materials


Acoustic Impedance

Note that you will soon meet a concept known as attenuation .Dont confuse attenuation with acoustic impedance, as theseterms and their meanings are quite different.

Acoustic impedance is of vital importance in the reflection andtransmission of sound at interfaces. Consider an ultrasonic wavetravelling through one medium, which strikes an interface withanother medium at normal incidence. When the beam strikes theinterface, some of the energy will be transmitted across theinterface and some will be reflected back.

We can use the acoustic impedance to predict the relativeacoustic pressures and energies of the reflected sound and thetransmitted sound. But what is acoustic pressure?


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Reflection and TransmissionAcoustic pressureRelative acoustic pressure is the property we record when measuringsignal amplitude in ultrasonic testing. Compression waves propagate byfluctuations in pressure, so a wave will cause local variations in pressure

as it passes. It is these pressure variations that are detected by thepiezoelectric transducer and converted to an electrical signal, which isthen displayed on the UFD screen. When we measure the strength ofsignals in ultrasonics we are comparing their sound pressures.The acoustic pressure can be expressed as: P = Z x A

Where: P is the acoustic pressure Z is the acoustic impedance A is the amplitude of particle vibration caused by the sound wave

You will not need to actually calculate absolute acoustic pressures.Signals you will be measuring in ultrasonics are always relative measuresof the acoustic pressure, and are recorded in terms of either screenheight or decibels.


Reflection and Transmission

The reflection coefficient (R) tells us what fraction of theincoming wave pressure is reflected back from an interface .For example, if the incident sound pressure is 100 units, andthe reflection coefficient is 0.2 (20%) then the reflected wavewill have a pressure of 20 units.

The reflection coefficient can be calculated from the acousticimpedances of the two materials. We will do this now for thesimplest case of square, or normal incidence where the incomingwave strikes the interface at ninety degrees.

The reflection coefficient is a measure of reflected soundpressure


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Square incidenceThe incident wave approaches and strikes an interface at square incidence(0). It has a pressure of .

Adobe AcrobatDocument

12

12

Z Z

Z Z

12

22 Z Z

Z The transmitted wave has a pressure of Tx ,

The interface is a zone in which there is a change in acoustic impedance

The reflected wave has a pressure of Rx , R =

=T=1-R

For sound travelling from medium 1 with acousticimpedance Z 1 to medium 2 with acoustic impedance Z 2

)(12

12 IncidenceSquare Z Z Z Z

R

The transmission coefficient is a measureof transmitted sound pressure

The transmission coefficient (T) is the ratioof the transmitted wave pressure to incidentwave pressure.

IncidenceSquare Z Z

Z

12

22


The steel / water interface

A very common interface in ultrasonics is from steel to water.Lets calculate the reflection and transmission coefficients forsquare incidence.

For sound travelling from steel to water:

065.05.463

455.15.12

935.05.465.43

455.1455.1

/105.110001500

/104577005900

26)(2

26)(1

T

R

smkg Z

smkg Z

water

steel


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What do these coefficients mean?

For sound travelling from steel to water, the sound pressure ofthe wave reflected back into steel is 93.5% of the incident wave.

Dont worry about the negative sign of the reflection coefficientit signifies that positive pressures at the interface in theincident wave become negative pressure in the reflectedwave and vice versa. This is called a phase change and will bediscussed later.

For sound travelling from steel to water, the pressure of thewave transmitted across the interface into the water is 6.5% ofthe incident wave.


Check Your Progress

Calculate the reflection and transmission coefficients for soundtravelling from water to steel.

To summarise, when the beam strikes the interface, some of thesound pressure will be transmitted across the interface, andsome will be reflected back. The only time when no pressure willbe transmitted across the interface is when the other side is avacuum. For practical purposes however, a metal-to-air interfaceis an almost perfect reflector.

Answer: R = 0.935, T = 1.935


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The transmission coefficient can be greater than 1.0

In the question earlier, the transmission coefficient looks odd atfirst. How can there be a greater pressure transmitted than wasincident in the first place? This is because it is not pressurethat is conserved across the interface, but energy. It is commonto have a transmission coefficient greater than 1.0.

This situation is similar to a transformer, where we can achieve ahigher voltage at the output of a transformer, but the totalenergy output is always the same as the energy input.

There is, however, a simple relationship between reflection andtransmission coefficients. The total pressure on the incidentside is equal to the sum of the incident wave pressure and thereflected wave pressure. The incident pressure can be taken as1.0 (100%).

Thus, incident wave pressure + reflected wave pressure =transmitted wave pressure, or: 1+R = T


Energy Coefficients

So far, we have calculated thereflection and transmissioncoefficients in terms of the

pressure of the waves. It isalso possible to calculate themin terms of energy.

)(2

12

12 Energy Z Z Z Z

R

In this situation, the total energy is the same on both sidesof the interface , so we can say:

)(

42

12

12 Energy Z Z

Z Z T


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Check Your Progress

Calculate the energy reflection and transmission coefficients forsound traveling from steel to water.

Answer: R = 0.87, T = 0.12


Comparing the pressure and energy conventions

The pressure convention is like measuring the voltage across atransformer and can give a positive or negative coefficient ofreflection, as well as an increase in pressure across theinterface

The energy transmission is like measuring power across atransformer, and will always give a positive reflectioncoefficient. There is always a conservation of energy across theinterface.

You will mainly use the pressure conventions, as they relate moreto screen height as a measure of acoustic pressure.

Both can be used and you should be aware of them and be able tocalculate the coefficients for both cases.


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Couplants

Transmission coefficients explain why we need couplant

If the ultrasound wave emerges from the probe into air, there will bevery low transmission and very high reflection , meaning very little ofthe signal will enter the test piece. Remember that air has an acousticimpedance of almost zero. If you want to couple the probe to the testpiece, it is necessary to eliminate the air interface . The mostconvenient couplants are liquids such as water or oil.

For contact testing, a surface layer of couplant is used, whichdisplaces the air between the probe and test piece. Water is commonlyused as a couplant, and is often thickened with a cellulose paste to givebetter application to surfaces. Oil or grease can also be used wherethere is a risk of any adverse corrosion effect from using water basedcouplants. The couplant thickness in contact testing is usually verysmall, about 0.1 mm.

For immersion testing, the probe and the object are immersed inwater with a significant water gap in between. This is very convenientfor automating a process, and will be the key to this task.

Although we are dealing with immersion testing for which the principalcouplant is water, it is important to consider couplants generally. In ultrasonics,a couplant, as the name suggests, joins or couples the probe to the test object.


The ideal couplant has particular properties

wets the surface of the probe and test object

is non toxic and non corrosive can be applied and removed easily has an acoustic impedance somewhere between the probe

and test object, although this is not generally possible is homogeneous and free of bubbles that would scatter the

beam is sufficiently viscous to prevent flow off the test surface allows easy movement over the test surface.

A couplant can be any viscous material liquid, semi-liquid or pastethat:


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Some common liquids make good couplants

Water is the cheapest and most abundant couplant, but mayneed detergents added to wet the surface, or methyl celluloseto act as a thickening agent to retain it on the surface. It mayalso be necessary to add rust inhibitors when water is used.

Oils and greases are used where water is unsuitable they alsostay on the surface longer and do not evaporate as quickly fromwarmer surfaces

Glycerine is the most favourable liquid for acoustic impedanceproperties, and may be mixed with water if required.

Mercury is theoretically a very good couplant due to its high

acoustic impedance, but is neither practical nor safe to use.


Review

An interface is a boundary at which there is a change in acoustic impedance . Sound meeting an interface at right angles will be partly transmitted across

the interface, and partly reflected by it. The sound pressure and energy of the reflected and transmitted waves can

be calculated if the acoustic impedances are known. The greater the difference in acoustic impedance values of the two media,

the greater the amount of reflection and the lesser the amount oftransmission and vice versa.

The pressure transmission coefficient can be higher than 1.0 - that is, thetransmitted pressure can be higher than the incident pressure.

The pressure reflection coefficient can be positive or negative. A negativecoefficient signifies a change of phase. Transmission coefficients are alwayspositive. For pressure, 1 + R = T.

The energy reflection coefficient can only be positive, so does not indicateany phase change. Transmission coefficients are always positive. For energycoefficients R + T = 1.

In ultrasonic testing a liquid couplant is placed between the probe and testobject to maximise sound transmission across the interface.

Here are some important points to remember.For waves striking an interface at right angles:


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Practice

Set your zero compression probe to give a backwall reflectionfrom the 25 mm thickness of the IIW block (V1 Block). Set thisecho as close as you can to 100% full scale height ( FSH). Whilemaintaining the echo, wet your free hand with some oil or waterand dab it exactly opposite the probe. Every time you touch theopposite side, you should see the backwall dip slightly, about 5%.


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Module-4



Singapore Chapter


Flaw Detector

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The Ultrasonic Flaw Detector

The flaw detector consists of a number of key components. Theyare designed to operate in Through-Transmission/Pulse-Echomodes. In this chapter you to learn about the flaw detector andunderstand what the various controls do.

Things you will need to learn:1. the basic block diagram of the UFD2. how the flaw detector works3. the controls on a flaw detector4. enhancements to improve the performance5. comparison of digital and analogue UFDs6. matching the impedance of probes and UFD7. how to review data from various suppliers.


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Parts of the Ultrasonic Flaw Detector

1. Timer: controls the rate at which pulses are generated. The rateat which the timer operates is called the Pulse RepetitionFrequency (PRF). In some instruments the user can control this,while in others it is automatically adjusted by the UFD to suit therange.

2. Pulse generator: generates a spike of instantaneous voltage whentriggered by the timer.

3. Probe: converts the voltage spike to a mechanical sound wave. Thewave is generated at the resonant frequency of the transducer.The probe also reconverts the received mechanical sound wave toan electrical image of the sound wave.

The UFD is made up of six basic elements

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4

Parts of the Ultrasonic Flaw Detector

4. Sweep generator: sends an electron beam across the CRO(cathode ray oscilloscope ) at a constant speed, by applying avoltage between the side plates of the CRO.

5. Amplifier: amplifies the received signal from thetransducer. There may also be other processing of thesignal such as rectification.

6. CRO or Digital display: shows the received wave form. InAmerican literature, the CRO may be called the CRT (cathode ray tube ).


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The operation of a flaw detector and probe is repeatedsequence of steps

1. The pulse generator sends a spike to the transducer, around 300 V,which converts the spike to a mechanical sound wave that commencesits journey from the transducer.

2. At the same time, the sweep generator sends an electron beam on its journey across the CRO.

The timer signals the pulse generator that it is time to send a pulse. At the same time, it also signals the sweep generator that a pulse isbeing sent, and:

Block Diagram

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The operation

3. The electron beam leaves the left side of the CRO at the same instant thatthe sound wave leaves the transducer. The UFD and probe wait while thesound pulse travels through the material and is reflected back, returning tothe probe. The returning sound wave reaches the transducer, whichimmediately reconverts it to an electrical signal in the milli-volt range.

4. The weak electrical signal from the transducer is received by the amplifier

and amplified in accordance with the gain applied. Other processing, such asrectification may also be applied at this stage.5. The amplified and processed signal is applied to the top and bottom plates of

the CRO, by which time the electron beam has travelled some of the distanceacross the screen. At that point, the image of the received sound wave isdisplayed on the trace, indicating its amplitude, shape, and transit time. Notethat the transit time is the time taken to do the round trip to the reflector.

The cycle from steps 1 to 5 is occurring at a rate of around 500 times per second (500 Hz).This cycle rate is called the pulse repetition frequency (PRF).

Block Diagram


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Digital displays are becoming increasinglypopular

Many modern UFDs are now digital, and theanalogue CRO screen has been replaced by thedigital display of a computer screen. The digitaldisplay allows much greater flexibility in recordingthe trace, but loses some of the real time speed ofan analogue CRO.

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Using the UFD Controls

UFD controls

The block diagramdescribes the function ofthe components. We willnow consider how thefunctions of the variouscomponents are managedthrough the controls.

No review of controls can consider every possible control available,so we will discuss those most commonly used in the order that theywill most probably be required.


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The range button controls the sweep rate; This allows the screen to display the required test range. Obviously, if

the test requires only 100 mm of range, it is pointless displaying 1000

mm and trying to interpret signals in the left hand 10% of the display. The range is expanded or contracted by varying the rate at which thesweep generator moves the electron beam across the screen. For avery long range, e.g. 5 metres, the electron beam will, relativelyspeaking, sweep very slowly and will appear much brighter. For a shortrange, the beam will sweep very quickly across the screen, and spendmost of the time waiting for the next sweep.

Range is normally adjusted in coarse steps with the coarse range

control, and in fine steps with the fine range control. Most equipmentwill indicate the coarse step settings, e.g. 10, 100 or 1000 mm.

The Range Control

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The range button control

These apply only to compression waves in steel, so when usingshear wave probes the 100 mm setting would correspond toapproximately 50 mm. The fine range setting allowscontinuous adjustment within the coarse ranges, as well ascalibration for other ranges, modes and materials.

Analogue instruments require the range to be set by using acalibration block of known thickness. Some digitalinstruments allow the range to be keyed in by specifying therange required and the acoustic velocity, but need to beverified with a calibration block.


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Gain Control

For most applications, it is important that the amplifier canfaithfully amplify signals over the required range offrequencies used. Such amplifiers are called broadband

amplifiers. Some amplifiers can be set preferentially amplify alimited range of frequencies these are called narrow bandamplifiers and may be used in special applications.

Note that some UFDs, especially Japanese and American, usethe gain in the opposite sense, and call it an attenuator . Thereis no mystery in this, 6 dB of attenuation is just minus 6 dB ofgain and vice versa. Just be careful that you are aware of theconvention in the equipment you use.

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Suppression (Reject) Control

The suppression (reject) is used to deduct some of theamplification

Closely related to the gain control is the suppression control.The gain control allows the user to multiply and divide theamplification, applying it equally to all reflectors. Commonly,suppression operates by subtracting amplification by thesame %FSH from every indication.

Reject


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Suppression (Reject) Control

This has the result that if 10% of suppression is applied to the display, allreflections will be reduced by 10% FSH. If you have indications of 100%, 50%and 10% and apply 10% suppression, the indications will drop to 90%, 40% andzero respectively.

There is often a temptation to apply suppression when the trace is showing a highdegree of material noise when testing coarse-grained materials. If you do this,the amplifier is no longer linear, and will not amplify all indications by the sameamount, so there is a risk of missing small important indications. The presence oflow level grass on the screen is your reassurance that there is sound enteringthe test object. It is preferable to learn to work with a small amount of materialnoise on the screen to get this reassurance. The best ultrasonic professionals willalways operate with significant material noise on the screen.

Although this problem has been addressed in some later equipment designs, usesuppression only as a last resort, and do a simple linearity check each time youuse any equipment to prove to yourself that the suppression is off.

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Pulse Energy

Pulse energy can be modified slightly to combat attenuation

In some equipment, the strength of the pulse can also beincreased. This is done by either applying a stronger or longerduration pulse. For highly damped probes, a stronger input pulse

may be achieved in some equipment with a tone burstgenerator, which applies an alternating voltage to drive thetransducer harder at its resonant frequency.

This may give extra penetration range in difficult materials, butwill result in a loss of resolution . Like suppression, it should onlybe used as a last resort.


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Single /Twin Selector

The single/twin switch selects the type ofprobe to be used

The UFD needs to be set for either single or twincrystal operation. In single crystal operation, theprobe is connected to both the pulse circuit andthe amplifier. In twin crystal mode, thetransmitting crystal is connected to the pulsecircuit and the receiver crystal is connected tothe amplifier.

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Pulse Repetition Frequency (PRF)

Pulse Repetition Frequency (PRF) can be adjusted in some UFDs PRFcontrols the rate at which the pulses are generated. If the pulse

repetition frequency is too low, there are too few sweeps across thescreen, and the trace is very faint. A high pulse repetition frequencyis also needed when testing at higher speeds, or there is a risk thatthe volume of material will not be fully scanned.

If the PRF is too high, a situation can arise where one pulse has notfully died away before the next pulse is transmitted. The oscilloscopedoes not know which reflected pulse relates to which transmittedpulse, and random ghost echoes can appear on the screen.

In most portable equipment, the PRF is controlled internally by therange control.


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Signal Processing

Pulse shaping controls canmake the pulse shape moreuser friendly

The raw pulse that is received by the amplifier is an unrectifiedsine wave. Unless it is important to have an unrectified trace, mosttraces are rectified for ease of interpretation. There is also somesmoothing applied to the trace to make it easier to interpret.

Signal Processing

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Use of Monitors

a yes/no that an echo has occurred in the monitoredarea and has exceeded the set threshold to activatean alarm

the amplitude of the reflection the amplitude and range of the reflection the complete ultrasonic trace in digital form for

subsequent analysis.

Monitors (gates) can select a section of the trace for special attention A monitor is set to read a specific part of a trace that is of particular

interest, for example between zero and the first backwall echo. The limits ofthe monitor range are set, together with a threshold above which it isrequired to record. Subsequently, whenever a reflection occurs in the area ofinterest, data is exported. Depending on the instrument design, typical datamight be:

Gate


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Distance Amplitude Correction (DAC)

Distance Amplitude Correctionaccounts for attenuation variations

As the pressure of the reflected

beam decreases with distance, theamplitude of reflected echoes fromidentical reflectors in the far zonewill decrease with increasing beampath length.

Distance amplitude correction (DAC) allows this variation to be correctedby the UFD, by either drawing a DAC curve or applying additional time

corrected gain (swept gain) to echoes at various beam paths to displaythem all at a consistent screen height. The amount of DAC applied willdepend on the material and the type of reference discontinuity used.

DAC

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Point to Ponder

Would the DAC curve for a series of backwallslook similar or different to the DAC curve for aseries of small disc reflectors, such as flat-bottomed holes. Why?


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Comparison of Digital and Analogue Oscilloscopes

Analogue and digital oscilloscopes have significant differences

Traditional analogue oscilloscopes will have a green trace and engravedcalibration marks (graticules) on the screen. These are generally in multiples of 5or 10 to allow convenient calibration of the time base . The rate of response isinstantaneous, and because the frequency of sweep of the electron beam acrossthe screen is very high, typically 500 traverses per second at a PRF of 500 Hz,the path of the electron beam will look like a continuous line.

Analogue oscilloscopes will also have a number of internal controls for adjustingfocus, astigmatism, and alignment with the graticule . You will not normally berequired to adjust these controls, but be aware of them if your trace looks blurredor misaligned.

One of the main properties to watch with oscilloscopes is that they should belinear in their response. Tests for this performance will be described in the task onCalibration.

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Comparison of Digital and Analogue Oscilloscopes

Digital oscilloscopes are becoming increasingly popular. Digital displaysconstruct the trace mathematically by sampling the analogue signal andconstructing a trace from the sampled points. The more sampled points, themore the digital trace looks like an analogue trace. If the number of sampledpoints is low, (in order to speed up the sampling process), the display looks

less like an analogue display. The equipment can send its display to aconventional computer screen for viewing.

The screen markings are contained within the screen display, andexperienced users of analogue oscilloscopes will note the slower responseand update time of digital displays. They are also less able to resolve many ofthe subtleties possible with an analogue display, but no doubt as fasterequipment becomes available, these differences will narrow.


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Advantages of Digital oscilloscopes

Their principal advantages are:

The user can program settings for later use to give

greater reproducibility. The ability to store settings as a test record.

Traces can be saved for subsequent processing andreview.

The test can be rerun off site with changed settings.

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Reading the UFD Screen

Digital displays have made readingthe beam path much simpler

Reading the distance on the screen isrelatively simple with most digital

equipment. There is generally alarger choice of options forcalibration, and the screen is directlymarked with an easily readable grid.

Analogue displays need thought inselecting the range

1. Main Division2. Sub-division


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Reading the UFD Screen

Analogue instruments, however, can be a little more difficult to readaccurately. Most analogue instruments are marked with 10 major divisions,each of which has 5 minor divisions, giving a total of 50 minor divisions. Thisis quite uncomplicated when using a range of 0 - 100 mm, as each division

UT_LEVEL_2_Part_1

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