Half amplitude / 6 dB drop technique :Half amplitude technique for defining the ends of a discontinuity is

used when it is longer than the crystal size of the probe. This is themost popular technique for discontinuity length measurement.

After maximizing the discontinuity indication, set this signal to aconvenient height [ ~ 80 % ]. The probe is then moved towards right,parallel to the discontinuity or the weld seam as appropriate, up tothe position where the signal drops to half the set amplitude. A pointis marked with a marker above the discontinuity locationcorresponding to the centerline of the probe crystal. A similar point isfound by moving the probe towards the left. The distance betweenthese two points is the length of the discontinuity.

The discontinuity length measured by this technique always resultsin an over estimation of the actual size. This is because the halfintensity line is located at an angle to the main axis of the beam[ centerline of the crystal ].

The error will depend on the size and frequency of the probe andthe beam path distance to the discontinuity. The shape at the endsof the discontinuity will also influence the measurement. Small crystalprobes [ 4 MHz, 8 X 9, 10 mm ] should be preferred for themeasurements.

This measurement error can be minimized by plotting the halfamplitude beam boundary in the horizontal plane using the IOWbeam profile block and using the graph thus obtained for correctingthe length at the applicable flaw depth.Edge approach technique :When the discontinuity size is much smaller than the size of thecrystal, half amplitude technique of measurement does not producesatisfactory result. A different method called edge approach techniqueis used.

In this method the edge of the ultrasoundbeam is used to find the starting edge of thediscontinuity.

Once the presence of the discontinuity isconfirmed, the scanning starts from a positionwhere the discontinuity signal is not visible onthe CRT screen.

The probe is now slowly moved towards thediscontinuity up to the position where thediscontinuity indication just begins to appear onthe CRT screen. A point above the discontinuitylocation is marked which coincides with the linepassing through the sidewall of the probe,facing the discontinuity.

A similar point is to be found by scanning thediscontinuity by probe movement from the otherend. The distance between these two points isthe length of the discontinuity.

Angle probe DAC with side drilled hole block :Side drilled hole blocks are widely used for plotting the DAC

curve for weld testing. This method compares the reflectivity of aflaw with that of the tangential area of the horizontal hole at therelevant test distance and is thus echo amplitude dependent whenassessing flaw acceptability. These blocks contain one inch or moredeep drilled hole, with the hole axis parallel to the scanningsurface and perpendicular to the edge. The side of the holereflects ultrasound energy incident at any angle. The diameter ofthe hole normally increases with block thickness. A 1.6 mmdiameter reference hole provides good test sensitivity for most ofthe weld testing applications.

Depending on the block thickness T, holes are drilled at 1/4 T,1/2 T, 3/4 T on the edge of the block.Plotting the DAC curve :

A block is selected whose thickness [ T ] most closely matchesthe part thickness [ equal to or within 10% of test thickness ] andcontains the hole, specified in the test procedure in use.

The maximized signal from the hole which produces shortestbeam path, [ Pos 1 or 2 ] is set to 80 % screen height. The probeshould be at least one half inch away from the edge of the blockto avoid corner effect. The peak of the signal is marked on theface of the CRT screen. Without changing the gain control,maximized signals from other positions [ Pos 2, 3, 4 ] are markedand a curve is drawn through all these marks to complete theDAC curve. A 50 % curve is also drawn by 6 dB reduction method.Minimum three signal peaks are required to plot the DAC.

For a 1/4 T hole, signal from position 2 should be preferred asthe first reference point of the DAC curve. If the weld crown isground flush and the probe can move over the weld surface,signal from position 1 may be used as the first reference point. Inangle beam testing of unground welds, the sound beam enters theweld zone after traveling a certain distance in the test materialand depends on the location of the exit point on the probe body.The DAC curve level falls sharply for higher frequency miniature[ 4 MHz, 10 mm ] probes.

Flaws producing, > 100% DAC signal is normally rejected or acertain length is permitted by the code. A certain length is alsopermitted, for 50 % or more DAC signals. These signals are to berecorded in the report for future reference.

For ASME pressure vessels, > 20 % DAC signals are to beevaluated for crack, lack of fusion and lack of penetration. Thesedefects are not acceptable regardless of echo amplitude.

Important : The sound attenuation, surface condition and contact area

of the block should be similar to the test part. If possible, the block is to be prepared from the test

material itself. Transfer correction of the reference test sensitivity will be

required if the block is different from the test material. It is preferable to setup the test sensitivity on a block

equal in thickness to the test part. This will produce aDAC curve with sufficient test length and internalreflections similar to the test part. Thickness difference of10 % is acceptable.

The DAC curve [ level ] must be at least 20 % screenheight up to the full test distance. If required more thanone curve should be plotted for flaw evaluation.

Side drilled hole block thickness and holesize in inches for weld testing as per ASME Sec V.plate block hole holethk, T thk size locationup to 1 T or 3 / 4 3 / 32 1/4, 1/2, 3/4T>1 to 2 T or 1.5 1 / 8 “>2 to 4 T or 3 3 / 16 “>4 to 6 T or 5 1 / 4 “>6 to 8 T or 7 5 / 16 “>8 to 10 T or 9 3 / 8 “

Angle beam probe :Angle probes are used for detecting flaws oriented at an angle to

the material surface. They will miss flat reflectors, parallel orperpendicular to the scanning surface. The angled beam must hit thereflecting plane of the discontinuity at 900 for maximum reflection.Angle beam reflects well from corners but may undergo wave modeconversion.

Usually shear wave probes are used in angle beam testing, becausethe refracted longitudinal wave probe will have a shear wavecomponent of weaker intensity in the test material. Longitudinal waveangle probes are specially useful for testing austenitic stainless steelwelds and inside surface of highly curved hollow parts and welds.Shear wave probes are used for testing welds, material defects inpipes, tubes, plates and sheets and in irregular shapes where fullcontact testing area is not available.

Angle beam probes use the principle of refraction and modeconversion to produce refracted longitudinal and shear waves in thetest part. In an angle beam probe, longitudinal wave is converted torefracted longitudinal or shear wave by means of an angled plasticwedge. The wedge is cut at an angle to provide an incident beamangle to produce refracted longitudinal or shear wave at the desiredangle in Steel, because steel is tested in most applications. Fordesigning small crystal low frequency probes, incident angle calculationbased on snell’s law fails because of wider beam spread associatedwith these probes. The refracted angles will change in other materialsbecause of velocity difference. A probe of 450 in steel will equal 430

in aluminum. The wedges have serrations cut on the front surfaceand fitted with absorbing medium to prevent internal reflections fromproducing unwanted echoes but some spurious echoes are oftendetectable just after the initial pulse. The wedge can be an integralpart or can be detachable. In detachable system, a single transducercan be configured to different angles by changing the wedge and iseconomical. The detachable wedges are bigger and absorbs moreenergy. The detachable type requires couplant between the transducerand the wedge to permit transmission of ultrasound into the wedge.

Standard angles are 35, 45, 60, 70 and 800 in steel. 800 and 350

probes have limited applications due to prevalence of surface wavesfrom 800 and presence of longitudinal waves from 350 probes. Crystalsizes around 10 mm for miniature and 20 mm for bigger probes aremost popular. Probes with 12.5 mm crystal size is normallyrecommended for standard test applications. Rectangular crystals areoften used. Bigger probes have higher sensitivity and sharply focusedsound beam and permit flaw detection at higher depth but cannot beplaced very close to the weld edge. 4 MHz, 8 X 9 / 10 mm probes arevery popular for their small contact area, high resolution and sharpechoes and are used up to a test distance of 200 mm in steel. Largerprobes are mostly used for thick welds and for applications wheresufficient probe contact area is available and a large area is to bescanned. A 20 X 22 mm, 2 MHz probe can detect a 2 mm reflector upto a distance of 700 mm.

Twin crystal angle probes, longitudinal and shear, are available fordirect scanning on ground weld surfaces and low thickness materials.These probes are more suitable for testing stainless steel and soundabsorbing materials.With use, the beam exit point and the refracted beam angle will

change with acrylic wear and should be checked before each use.The beam angle change should be controlled within + / - 20. Newacrylic soles can be pasted with araldite to repair the wear. The frontcorner of the sole should be shaped to reduce spurious echoes.Performance :

A good angle probe with a standard flaw detector, should produce aminimum 75% signal from the 100 mm radius of a standardIIW - V1 block with a minimum of 40 dB gain reserve.

The probe should resolve at least three of the five holes [ clearlydetectable peaks ] in the IOW beam profile block.

Noise from internal reflections should not exceed 5% screen height,at all the working test sensitivity levels.

Beam axis abnormality should be checked by maximizing a signaland moving the probe forward and backward and rotating left andright, while monitoring the falling signal. The signal should fallcontinuously without any sudden rise in signal height.

Pulse echo A- scan test method :

Pulse echo test method uses reflected ultrasound as a means of collectingtest information. A single crystal probe is normally used for ultrasoundgeneration as well as reception.

The transmitter circuit of the flaw detector supplies short excitation pulsesof few hundred volts at regular interval to the probe crystal. The excitationpulse oscillates the crystal to generate short burst of ultrasound such thatthe arrival of each returning echo may be identifiable as a discrete event.During the interval between two successive pulses, the crystal is at rest anddetects any return echo such as from the back wall. A large percentage ofthe sound is reflected from the front surface of the test part and theremainder is reflected by the back surface or discontinuities. The flawdetector’s CRT screen displays the whole operation by producing separatedsignals of transmission and the time of arrival of defect echo and the backwall echo. The transmission pulse and subsequent echoes appear as peaksrising out of the CRT’s base line. The distance between the peaks is ameasure of the defect’s location or the part’s thickness.Transmission of high frequency ultrasound cannot takes place in air. It iscarried out through an intermediate liquid, in bulk or as a thin layer. Oilysubstances or water are generally used. They are called couplants.

The initial or transmitter pulse appears first in time and represents theelectrical zero. This is the exact start time of crystal excitation. The exactpoint in time when ultrasound enters the test material is called acousticalzero. Acoustical zero is superimposed within the initial pulse and is notdistinguishable. The next pulse represents the total elapsed time for soundto travel from the entry surface to the reflector and back to the entrysurface again.

At the instant the electrical pulse is removed the oscillations of the crystaldo not cease immediately but decreases in an exponential manner until theyreach zero. A dead zone is produced, starting immediately after entry into thetest surface, in which echoes can not be detected.

One single test cycle is so fast that it is not physically visible in thedetector’s screen. Hence the flaw detector repeats the test cycle severaltimes per second by supplying successive excitation pulses to the crystaland make the event appear as constant due to persistence of vision. Thenumber of times, the crystal is electrically pulsed per second is called thepulse repetition rate.

A sufficient amount of time betweensuccessive pulses is necessary to allowultrasound to travel through the materialunder examination. Higher pulse repetitionrate produces brighter screen display. Veryhigh pulse repetition rate producesspurious signals [ ghost echoes ] on theCRT screen.

The ultrasonic pulses used by the flawdetector are radio frequency type andhave a serrated look. The pulses arefiltered and rectified to smooth lookingshapes by the flaw detector beforedisplay.

Pulse echo A-scan method displaysdistance along the horizontal scale calledthe baseline and amplitude of thereflection along the vertical scale. Becauseof similar return path, the screen iscalibrated to display one way travel only.

A scan test method can accurately locatea discontinuity. The amplitude of the returnsignal is a relative measure of theamount of reflected energy and dependson the area and orientation of thereflecting surface. Amplitude of the signalcan be used for accept / reject decision.

UlllllWeld discontinuity evaluation :Discontinuity evaluation consists of various steps to assess the

type or character, orientation, location, length and width of thereflector.

To reach a reasonable conclusion regarding identification of thediscontinuity, the technician must be thoroughly familiar with thewelding process, degree of perfection in welding fit up and shouldaccurately locate the reflector within the weld cross section.Combining this information with the basic shape of the reflectoridentified by beam manipulations, the identification of the welddiscontinuity may be possible.

The basic weld discontinuity shapes which can be identified byultrasonic beam manipulations are Spherical, Cylindrical and Planar.Discontinuity types :Spherical : single and widely spaced pores [ P ],

non elongated slag [ SL ].Cylindrical : elongated slag, wormholes, aligned pores,

hollow bead, unfused slug, concave root [ C].Planar : cracks, side wall lack of fusion [ LF ], unfused

root faces, Lack of penetration [ LP ], fusion lineslag, burn through, undercut [ UC ], cold lap,misalignment.

Angle beam probes can be used to obtain information about thetype of the reflector detected during scanning. Use of all the threeangles is recommended. The reflector is to be scanned by lateral,orbital and swivel movement of the probe. Orbital scanning is madearound the discontinuity in a circular path with the discontinuity atthe center of the circle. Swivel scanning involves rotation of theprobe around an axis through the center of the probe body, andperpendicular to the scanning surface.

The signal from a spherical reflector remains basically unchangedas the probe is moved around the reflector in orbital fashion. Thereflector does not respond to swivel scan and also shows no lengthwith the lateral movement of the probe. The signal from this type ofreflector is normally small with narrow base because of only asmall area of the discontinuity actually reflects the ultrasonic beam.

A single pore produces a sharp indication and a cluster willproduce multiple or a broad based indication with many peaks.

A planar or a cylindrical reflector will produce weak or noindication if orbital and swivel scanning is performed. With the lateralmovement of the probe, a varying or constant signal may result, butthe continuity of the signal indicates length of the reflector.

To differentiate a cylindrical reflector from the planar, the reflectorshould be scanned with several different angle probes. A cylindricalreflector will produce equivalent or significant signal at all angles ofincidence [ assuming equal sensitivity calibration and adjustment forattenuation ]. A significantly greater amplitude signal from a particularangle probe, indicates a planar reflector. Sound is reflected at itsmaximum from only one angle of interception, i.e. around 900.

Indications from opened cracks typically shows multiple peaks andof wider shape because of the many discontinuity facets usuallypresent. When the probe is rotated or orbited, an angle of some 20to 300 can be maintained before the echo is totally reflected away.

Side wall Lack of Fusion produces sharp indication and usuallydetectable from one side. The maximum signal remains constant forsome distance of the probe movement perpendicular to the weldaxis.

discontinuity type relative sensitivityPlanar HighestLinear IntermediateSpherical [ cluster ] IntermediateSpherical [ isolated ] Lowest[ assuming favorable orientation ]

UlllllEquipment performance check :The calibration and performance of manual ultrasonic test

system can be checked using the IIW - V1 block.Normal Probe :S : System Sensitivity check, with the hole signal set to 75%screen height, minimum 40 dB reserved gain required.R : Resolution check [ ability to produce separate indication ]The signals from the 85, 91 and 100 mm distances should bedisplayed on the screen without overlapping.D2 : Dead zone, signal from hole indicates 10 mm or less.D1 : Dead zone, signal from hole indicates, 5 mm or less.P : Checks sound generating ability of the system. With thegain at maximum, 5 full screen signals from the 23 mmPerspex insert, using a 2 MHz probe should be obtained.R1 and R2 : Range calibration and Horizontal Linearity.Angle Probe :E: Beam exit point, when the signal from the radiusbecomes maximum, the exit point of the probe coincideswith the center mark of the scale on the face of the block.S: Sensitivity check. with the signal from the radius set to100%, minimum 40 dB reserved gain required.R: Range calibration for angle probe.A1: and A2: Angle check, when the signal from the holebecomes maximum, the exit point of the probe may coincidewith one of the marks on the face of the block to indicatethe refracted beam angle of the probe.Horizontal Linearity : For accurately locating reflectors, a lineardistance scale is essential.

A 100 mm range is accurately calibrated from location R1.With the probe at position R2, signals should appear exactlyat 12.5, 25, 37.5 and 50 th division on the screen. A signalposition deviation by more than 1% indicates, non lineardistance scale. This check should be carried out over themaximum range used for actual testing.

The ultrasonic instrument must provide linear verticalpresentation within +/- 5% of the full screen height.Screen Height Linearity : Using a viscous couplant, a normalbeam probe is positioned at a suitable location of the blockto give a 2 : 1 ratio of amplitudes between two steadysignals. When the attenuator is changed in 2 dB steps, thesmaller amplitude signal must remain 50% of the largeramplitude within +/- 5% of full screen height.Amplitude Control Linearity : The accuracy of the amplitudecontrol of the ultrasonic equipment is also essential.

Using a viscous couplant, a normal beam probe ispositioned on the block to produce a 80% steady signal.With the attenuator changing by 2 dB steps, the signalamplitude shall change corresponding to the figure givenbelow. A deviation of +/- 5% is considered acceptable.

A 50% signal, when reduced by –24 dB, should beclearly detectable. ( Dynamic range )Signal to Noise ratio : After setting a signal to 20% screenheight, the gain is further increased till the base line noiseequals 20%. The difference in gain is the signal to noiseratio and indicates the quality of the amplifier.

Beam exit point and effective angle :Exit point is the location which marks the entry of the central

ray of the ultrasonic beam into the test specimen. A smallerdiscontinuity produces maximum indication, when it is on thiscentral ray of the beam. Hence, the exact angle of the axis ofthe beam can be determined by triangulation after maximizingsignals from known small reflectors. During angle probeexamination, the probe is moved backward and forward to hitthe discontinuity with the axis of the beam so that the locationof the reflector can be determined. All measurements for flawlocation utilize the exit point.

To locate the exit point, move the angle probe over the scalemarked on the IIW - V1 block to obtain the maximum signalfrom the 100 mm radius curved surface. Mark the point on theprobe body which coincides with the central line of the scalemarked on the face of the block. This is the exit point of theaxis of the ultrasonic beam.

This check can be performed on the V2 calibration blockalso. The 50 mm radius side should be used to avoid nearzone effect.Refracted beam angle :

Place the exit point of the angle probe in the approximateposition of the angle marked on the face of the IIW - V1calibration block, that corresponds to the nominal angle markedon the probe faceplate. Move the probe to maximize the signalobtained from the reflector hole. Correlate the exit point to thecorresponding angle marked on the calibration block todetermine the angle of refraction of the ultrasonic beam axis,i. e. the exit point of a 600 probe should be on the 600 markwhen the signal becomes maximum. The angle thus obtained isvalid for steel only.

Angle can be determined with the V2 block also.Accurate method :

Refracted angle of the axis of the beam can be accuratelydetermined by analyzing the maximum reflection signal obtainedfrom a small diameter side drilled hole at known depth in areference calibration block. Smaller hole size at higher depthprovides better accuracy. For deep holes, the maximum signalshould be carefully detected.

Refracted Beam angle is calculated as,Cos inverse [ hole depth / ( beam path + hole radius )]

Effective beam angle in other materials:Material Velocity Effective

mtrs/sec angles--------------------------------------------------------------------4340 Steel 3240 45 60 70347,Aust Stainless 3090 42 56 64Aluminum 3130 43.5 57 65Inconel 3020 41 54 61Titanium, 105A 3120 43 57 65

Hole depth up to center = 30mmBeam path up to hole center = 56 + 0.75

or 56.75 mm.[ Radius of hole, 0.75mm added to beam path ]Probe angle = Inv Cosine [ 30 / 56.75 ]

= Inv Cosine 0.5286 or 580

Locating discontinuities :Ultrasound beam is a diverging cone with maximum sensitivity along its axis. Consequently, a flaw will produce a

signal even when the beam axis does not pass directly through it. It is therefore necessary to shift the probe till theflaw signal becomes maximum. The flaw is then on the axis of the beam. The angle marked on the probe is theangle of this beam axis for steel testing. The location of the flaw is then calculated by recording the sound travelpath displayed on the CRT screen and using trigonometric relations.

The CRT screen is to be marked at half skip and full skip beam paths [ Thickness X Sec A, 2 X Thickness X Sec A ]to separate Leg 1 and Leg 2 region.

The defects in a test part usually produce a clearly defined echo envelope [ rise and fall of the echo signal ]. Suchindications should be analyzed thoroughly.Weld flaw signals :Single Pore : produces low amplitude narrow based sharp echo. When the probe is orbited around the pore, the echois maintained with small amplitude variations. They can only be detected as points.Porosity cluster : Produces low amplitude broad based indications with many peaks. When the probe is orbited aroundthe cluster, the echoes will be held with amplitude and range variations.Isolated Slag : produces forked and broad based echo. When the probe is orbited, the echo is maintained with someamplitude variation and retains the forked shape.Slag lines : echo falls rapidly when the probe is rotated or orbited and held when moved lateral to the weld.Cracks and bonding flaws along the weld seam : produce a sharply defined large echo when beamed at right angles.Due to multi - faceted nature of an open crack, the echo will have multiple peaks and wide envelope.

Flaw Calculations :

Distance to reflector fromThe probe exit point :Beam path X Sin A

Depth of reflector from thescanning surface level :In Leg 1[ before reflection from theundersurface ]beam path X Cos A

In Leg 2[ after reflection from theUndersurface ]2 Thickness - ( Beam path X Cos A )

A is effective Angle of the Probe.

Angle 800 700 600 450

----------------------------------------------Sin A .98 .94 .86 .7Cos A .17 .34 .5 .7Tan A 5.67 2.74 1.73 1.0Sec A 5.88 2.94 2.0 1.4[ Sec A = 1 / Cos A ]

Graphical plot of discontinuity :Plotting a discontinuity on the actual image of the weld not

only gives information about the possible type of thediscontinuity but also reduces error of misinterpretation.Indications caused by misalignment, excess penetration andfrom the surfaces of the weld can be easily identified. Whenthe scanning cross section is an irregular curve, such asscanning a nozzle from the shell body, graphical plotting is themost appropriate method.When a suspected discontinuity signal appears on the CRT

screen, the indication is to be maximized by probemanipulation. The distance between the probe exit point andthe centerline of the weld or any other selected reference pointis recorded. The sound beam path displayed on the CRTscreen at maximum amplitude is also recorded.Drawing the weld cross-section :An accurate cross section of the weld is to be drawn on a

paper, at 1 : 1 scale, with the help of a profile gauge, mound ofmodeling clay or a soft soldering wire. The weld drawing mustreproduce, the root face, root gap and the fusion lines.Accuracy of flaw locating depends on the ability to reproducethe weld cross section as accurate as possible. It is essentialto know, the weld process used, how it was fabricated and withwhich type of weld preparation and materials.

It is the responsibility of the UT operator to physically see allcomplicated weld joint preparations [ fit up ] before welding. Thiswill help him to estimate the root gap and locate the fusionlines during flaw plotting.

Plotting :The plotting card is positioned on the weld image such that

the scale point representing the distance, which is equal to thedistance between the exit point and the weld centerline,coincides with the centerline of the weld image. [ see fig ]The horizontal line in the card shall overlay the scanningsurface line. The reflector is then marked on the probescanning line, at the beam path distance indicated on the CRTscreen.

The reflector position is then considered for possible existenceof a discontinuity.

Flaw detection by diverging sound beam oflower frequency probe.

Root bead echoes : For a flat root bead, 600 and700 probes will hardly pick up any contourechoes. When the root bead penetration is 1 mmor more, both the 450 and 600 probes will receiveroot bead response. The response from the beadis more for a 450 probe and less when a 600

probe is used and no response when a 700

probe is used at normal test sensitivity. The rootbead echoes are broad based echoes. Theechoes will plot slightly below the plate level.Misalignment : large echoes detected from oneside only at the root usually indicatemisalignment.Backing bar : Where the backing bar is welded tothe parent material, the inherent gap known as the‘ nose ‘ tends to give a reflected echo which isnot classified as a defect.Weld cap echoes : The irregular surface contour ofthe weld cap produces lower amplitude broadbased indications when the probe is located nearthe full skip distance. The response is maximumwith a 450 probe. The echoes will plot outside thetop level of the plate. The signal may be strong ifthe capping is excessive and a 450 probe is inuse. The echoes will respond well to fingerdamping techniques.

Graphical plotting card :Graphical plotting card is used for locating an ultrasound reflectorwithin a test specimen. It shows the sound beam path in thespecimen and provide all details required for a triangulationsolution of reflector location. The card is extremely useful duringthe weld examination with as welded condition and welds withcurved surfaces.The graphical plotting card together with the drawing of the weldcross section helps to identify the possible type of thediscontinuity. By showing actual reflection location within the weldvolume, signal indications from misalignment of members andweld surfaces are correctly indicated.Drawing a graphical plotting card :

Draw two lines perpendicular to each other as shown in thepicture above.

The horizontal line represents the scanning surface and showsthe distance from exit point to the reflector. The vertical lineshows depth of the reflector from the scanning surface. Theintersection point represents the probe exit point.

Draw a line parallel to and at a depth of 20 mm from thehorizontal line Draw another line starting from the intersection ofthe perpendicular lines such that it meets the 20 mm deep lineat a distance of 59 mm. [ 20 X probe factor of 700 ]

This line represents the scanning line of a 700 probe.

Draw two more lines starting from theintersection of the perpendicular lines, such thatthey meet the 20 mm line at a distance of 40mm and 28 mm respectively.

The former line represents the scanning lineof a 600 probe and the later line representsthat of a 450 probe.

Now draw linear scale with 5 mm interval oneach of the lines on the card. Mark allnecessary identification numbers.The graphical plotting card is now ready foruse.

If the 20 dB beam profile is available for theprobe, then the same can be plotted on thecard. In this case, prepare individual plottingcard for each angle probe.

DAC curve with normal beam Probe :As sound energy propagates through a medium, energy conversion

takes place and the sound pressure decreases with distance fromthe source. In addition, sound spreads out from a true parallel beamand the intensity per unit area also decreases. Ultrasound energyalso gets scattered by grain boundaries. All these causes the signalamplitude from a constant reflector to vary with distance. Therefore,to evaluate reflected signals from discontinuities, Distance AmplitudeCorrection [ DAC ] curve is used.

DAC is plotted from signals from reflectors of equal area atdifferent distances in the same material. Flat bottomed hole blocksor side drilled hole blocks are used to plot the DAC curve.Procedure :

At least three flat bottom hole blocks or three scanning locationsin a side drilled hole block are chosen, such that the length of thecurve is sufficient to evaluate discontinuities at any depth in thematerial thickness to be tested. The size of the reference hole inthe DAC blocks must be as per the requirements of the testprocedure in use.

Maximum signal from the hole nearest to the scanning surface ispicked up [ pos 1 ] and amplitude of this signal is set to 80 %screen height. The tip of this signal is marked on the face of theCRT screen with a marker pen. The gain used in this condition isthe reference gain setting.Without changing the gain control, the maximum signals from the

hole ( s ) at position 2, 3 etc… are picked up and marked on theCRT screen. The marked points are then joined with a smoothcurve to complete the DAC curve.

To draw a 50 % reference level, the gain setting is reduced by6 dB from the reference gain setting at each maximized signallocation and the peaks of these reduced signals are marked andjoined to obtain the 50 % reference level.

Transfer correction for the reference gain setting should be madeif the sound attenuation in the block [s] are different from the testmaterial. Compare back wall signals from the test part and theblock which is nearest to the part thickness and correct the gaindifference as necessary.When a flaw signal equals the DAC curve, the flaw is generally

considered to be larger than the reference reflector’s activereflection area.

ASTM Flat Bottom Hole set of 10 / 19 blocks are used fordistance amplitude correction and ultrasonic flaw detector’s amplitudelinearity check.

Flat bottom and Side drilled hole reference reflectors :The flat bottom hole [ FBH ] is drilled to have a flat reflecting

surface at the bottom of the hole. The hole bottom is parallel tothe scanning surface for normal probe and at a desired angle foran angle probe. These holes are useful for comparing equivalentreflecting area of small unknown reflectors. FBH bears norelationship to the actual size of a welded flaw. These blocks areused for evaluating Cast, Rolled and Forged materials.

Side drilled hole [ SDH ] is a drilled hole, the side wall of whichis used as the reflecting surface. The hole bottom is not used.Side drilled hole reflects equally to ultrasound energy incident atany angle. Side drilled hole reflection is comparable to manynatural flaws in the weld and these blocks are generally used forweld evaluation.

Mat FBHDist dia1 / 8 5 X 1 / 641 / 4 51 / 2 53 / 4 51.5 53.0 3, 5 , 86.0 5 , 8

DAC Plot.

ASTM FBH blocks.

Normal beam probe for contact testing :Normal beam probes are useful for detecting discontinuities with

reflecting plane parallel to the scanning surface. These probes areused on relatively flat and smooth surfaces to introduce longitudinalwave into the test material. The common applications are ; testing ofplates, forged and rolled products, castings, thickness and velocitymeasurements.

Frequencies between 0.5 to 6 MHz is normally used in contacttesting, because the high frequency crystals are thin and fragile.High frequency contact probes in smaller crystal sizes are availablewith some additional protection. Standard test frequency is 2 - 2.25MHz. Crystal size of 6.4, 10, 12.7, 19 and 24 mm are mostcommon. Lower frequencies and larger crystals, are used on longparts and coarse grained materials. High frequencies and smallercrystals, are used on thin or fine grained materials. Smaller, highfrequency probes are highly sensitive for short range flawdetection.

A single crystal probe has a dead zone [ ringing time of theactive element / width of the initial echo on the CRT screen ] andcan not detect discontinuities located within this distance. Scanningfrom the opposite parallel surface overcomes this problem.

The main components in the probe are,Active element : The active element [ or the crystal ] is a piezoelectric ceramic which converts electrical energy received from theflaw detector’s pulser to mechanical vibrations for generatingultrasound in the test material. It also converts mechanicalvibrations to electrical charge for detecting ultrasound reflections.Silver conductive coating on the active faces serve as electrodes.Backing medium : Backing medium is a highly attenuative, highdensity material, attached to the back of the crystal to control thevibration by absorbing the energy radiated from the back surface.Tungsten powder / Epoxy mixture is often used. The purpose is toeliminate unwanted and prolong vibration of the crystal. This iscalled damping. Careful consideration of the characteristics[ acoustic impedance ] of this backing material enable the designerto produce either a very wide band, but lower sensitivity or anarrow band, but higher sensitivity probe. High damping reducesdead zone, improves resolution but reduces the overall sensitivityand thickness penetrating power of the probe.Wear plate : The wear plate protects the soft silver coated surfaceof the crystal from corrosion and wear by friction during contacttesting. A permanently attached ceramic disc or a coating ofabrasive material on the front face is used as the rubbing face.This offers only limited protection. Replaceable plastic membrane ornylon wear cap is most popular. Delay line attachments are alsoused to reduce dead zone and improve the ability to measure thinmaterials and small flaws. Delay line can be contoured to matchcurved test surfaces to improve sound transmission. Dry couplingand short duration testing at high temperature [ 10 sec scan / 60 secoff, 9000 F ] delay line options are available. When using a delayline, there will be multiple back reflections from the end of it.Performance : The probe should produce a 75 % signal indicationfrom the 1.5 mm diameter drilled hole in a standard IIW -V1 blockwith a reserve gain of 40 dB or more, when used with a standardflaw detector. [ sensitivity ]

The probe, with a standard flaw detector, should produce clearlyseparated 75% height indications from the three distances[ 85, 91, 100 mm ] in the IIW-V1 block. [ resolution ]

The probe should have low dead zone and low noise.Probe applications [ in metals ] :.5 / 1 MHz cast Iron, brass, mill rolls, copper, stainless steel,[ 24 dia ] sound scattering materials, long parts.2 / 2.5 MHz standard test applications.4 MHz thinner parts, aluminium, thk chk, flaw measurements.5 / 10 MHz fine grained materials, inclusions, thin matetrials, bonding

thickness measurement.High frequency probe will completely miss a disoriented reflector

because the reflected beam is highly directional.

Crystal Size mm :3 / 5 [ HF ] applications on small contact area.6.4 [ HF ] curves, corrosion, de-lamination, thk, tubing

[ with delay line applications ]10 mm complex shapes, small parts, flat / round

bars, tubing, thickness, flaw sizing.19 / 24 mm plate, ingot, slab, billet, axels, rolls, big

wheels and gears, large forgings andcastings etc.

Paint brush [ multiple element ] for plate testing.Wheel type probe mounted in liquid filled rubber

Tire with adjustment for normal, angularand surface wave scanning of largeparts such as plates, billets etc.

Note : For curved surface, it is recommended to usea suitably designed contoured acrylic adapter. Thiswill improve transmission of energy, reduce deadzone. Length of the adapter to be used such thatthe part’s back reflection appear before the adapter’sback reflection.

Notches :For evaluating incomplete penetration in welds or discontinuities

forming a corner, such as a surface crack or weld undercut,machined surface notches may be employed for sensitivitycalibration and size comparison, since they more nearly representthe actual discontinuity. A Vee shaped notch is an useful referencefor evaluating side wall lack of fusion in vee welds. Squarenotches are useful for evaluating Lack of penetration.Reflection amplitude from Vee, square and U – shaped notches of

comparable dimensions may vary widely depending on the angle,frequency and vibrational mode of the interrogating sound beam.These notches are best produced by Electric Discharge Machining[ spark eroded ] process.

The notches are usually at least one inch long [ min two timesthe probe crystal width is recommended ], with a depth[ d ] of 2 to 10 % of thickness of the part for weld inspection.The width [ w ] of the cut is usually 1/16". For base materialexamination of tubular products, axial and circumferential notches ofthe required dimensions on the outside and the inside surfaces, ina calibration block made from the product being examined is used.For plates, 3% of thickness depth notch is recommended.[ always refer applicable test procedure for notch details ]

Sound beam path in curved part :While testing curved parts, increase in beam path due to part

curvature must be considered.Two similar angle probes can be used to find the beam path

to hit the undersurface of a curved part. Using a calibratedrange, the equipment is set to dual probe mode. The probesare positioned on the part, facing each other, and atapproximately one skip distance apart. Keeping one probe fixed,the other probe is moved to produce a maximized signal. Thedistance indicated on the CRT is the effective beam path forthe angle probe in use to hit the under surface of the part.

The maximum incident sound beam angle, [ grazing incidence ]which can still hit the undersurface [ irradiation depth ] of acurved part can be found from the relation ;Probe angle A = Inv Sin [1- ( 2 t / D )]where, t is wall thickness and D is outside diameter of pipe.Depth of irradiation can be calculated from ;Irradiation depth = Irradiation factor X Diameter

Irradiation factor : 350 450 600 700

.5 [ 1 - sin A ] 0.213 0.146 0.067 0.030

Piezo-electric effect :Piezo electric effect is a phenomenon, whereby electric charges

appear on the surfaces of certain solid materials, when it issubjected to mechanical stress or pressure.

Conversely, when a piezoelectric crystal is placed in an electricfield, the crystal exhibits deformation, i.e. the dimensions of thecrystal change. When the direction of the applied electric field isreversed, the direction of the deformation is also reversed. This iscalled the reverse piezoelectric effect.

If, instead of DC field, an alternating field is applied, the crystalwill vibrate at the frequency of the AC field. If this drivingfrequency corresponds with the frequency where the thickness ofthe crystal represents half a wavelength, [ Crystal thk = / 2, wave length in crystal ] the amplitude of the vibration will be

much greater. This is the crystal’s fundamental resonance frequency.The crystal will also have frequencies of large amplitude wheneverthe thickness of the crystal is equal to an odd multiple of half awavelength. The largest amplitude occurs only at fundamentalfrequency and as the harmonic number increases the amplitude ofvibration decreases.

The ability of a material to exhibit piezoelectric effect is due to itsatomic structure. An unstressed piezoelectric crystal will have equaldistances between its positive and negative charges. As stress isapplied, deformation occurs which changes this distancerelationship. This change in distance between atoms results inpolarization of the crystal. Detectable electrical charges appear onthe surfaces. As the applied stress is relived, the positive andnegative charges attract each other and the crystal comes back toits unstressed condition.

In some crystalline materials piezoelectricity occurs naturally, suchas crystals of quartz or tourmaline. In some ceramic materials,piezoelectricity can be induced artificially by reorienting their crystaldomains. This is performed by slowly cooling the material fromcurie temperature under intense electric field. This is known aspolling. These materials are known as polarized ceramics and arewidely used for the construction of ultrasonic probes for their goodefficiency of ultrasound generation with lower voltage.

Polarized ceramic crystals of Barium Titanate and Lead ZirconateTitanate [ PZT ] are most common for the construction of standardprobes. Barium Titanates are produced by baking together bariumcarbonate and titanium dioxide at 12500C. The crystals aresubjected to an intense 2kV / mm electric field at 1400C [ the curiepoint ] and allowed to cool. The crystals become polarized, andafter loosing 50% activity in 24 hours become fairly constant. Thesecrystals suffer from aging and are not suitable for testing at hightemperature.Solid solution of lead zirconate and lead titanate [ or PZT ] offersgood overall performance. This has a much higher curie point [ 320to 3500 C ] and its piezoelectric properties can be tailored to out -perform barium titanate. Lead zirconate titanate is commerciallyavailable with many added minor constituents, which are notdisclosed by the manufactures.

Lithium Niobate, Lithium Sulphate, Lead Meta Niobate, LeadTitanate, Quartz are some of the other active elements in probesand are utilized for their specific properties.

Ultrasonic Probes :Probe uses a piezoelectric crystal

to generate and receive ultrasonicsignals. It is the most critical componentin any ultrasonic test system. Instrumentcharacteristics, settings, materialproperties and coupling conditions alsoplay a major role in overall system performance.

Selection of an ultrasonic probe depends on therequirements of good sensitivity or high resolution[ depends on probe bandwidth ] and desiredthickness penetration.

A system with high sensitivity [ with narrowbandwidth probe ] has the ability to producedetectable signals from small reflectors at a givendepth. Such system has low resolution.

A system with high resolution [ with broadbandwidth probe ] has the ability to produce clearlydetectable separate signals from reflectors lying atnearly the same depth and position with respect tothe sound beam. A highly damped probe isselected for such applications. This system willhave lesser flaw detection sensitivity.

Larger crystals produces less beam spread andmore sound pressure. With good surface and finegrained material, a 2 MHz probe can penetratemore than 10 meters. Lower test frequencies allowinspection of long test parts and sound scatteringmaterials because of less attenuation but has lowsensitivity to small discontinuities. High frequencywaves are heavily scattered in coarse grainedmaterials. They do not penetrate far and producestrong interference echoes. High frequency probesare suitable for fine grained materials. Highfrequency, miniature crystal probes can detect verysmall flaws within its usable short range and havegood resolution because of sharper echoes. Highfrequency is highly sensitive to flaw orientation.When using lower frequencies, probes with larger

crystal diameters are to be used to reduce theeffect of beam spread.

Use of higher test frequencies require goodsurface finish for better energy transmission. Onrough surfaces, lower frequencies with a viscouscouplant must be used.

Newly developed 1 : 3 composite materials offerimproved performance. These are materials in whichparallel orientated piezo rods [ active ] areembedded into a polymer matrix [ passive ] to lowerthe overall acoustic impedance of the crystal. Thisimproves transmission of energy into the testmaterial. Composite probes have superiorcombination of mechanical flexibility, scan width,sensitivity, resolution, and good penetration in highlyattenuative materials. They are specially useful forapplications where low signal to noise ratio isrequired. These probes are costly.

Reflection and Refraction of Ultrasound at Interface :In some ways, the behavior of ultrasound at an interface of two

mediums is similar to that of light. Snell's law is applicable.When ultrasound strikes an interface of two different mediums,

depending on the ratio of the specific acoustic impedances, someof the energy will be transmitted through the interface and theremaining energy will be reflected back with an angle equal to theangle of incidence. This property of ultrasound is utilized fordetecting reflectors or defects in materials.% of energy reflection [ for 00 angle of incidence ] at an interface is,

]Z 1 and Z 2 are the acoustic impedances of the two mediums.

Acoustic impedance of a material is the opposition to displacementof its particles by sound energy. Acoustic impedance for,longitudinal wave = material density X long wave velocity.During ultrasonic testing of steel using oil as couplant, only 10%energy enters steel and on its return journey again only 10% istransmitted to the probe. Hence, only 1% of the generated energy isavailable for amplification and display on the CRT.When the incidence of a longitudinal wave is perpendicular to the

interface, the transmitted wave mode does not change.However, If the longitudinal wave hits the interface at an angle,

then mode conversion takes place and both the longitudinal andthe shear wave modes will be produced in the second medium.The refracted wave is predominantly longitudinal. The refractedangle of the longitudinal wave is always more than the refractedshear wave because of higher velocity. With increase in incidentangle, the refracted angle of the waves also increases.When the first medium is Perspex and the second medium is

steel, at 27.60 angle of incidence, the refracted angle of thelongitudinal wave becomes 900 [ along the interface ] and onlyshear wave will be present inside steel at 330. This is the firstCritical angle for Perspex to Steel transmission.With further increase in incident angle, at 57.20, the angle of

refraction of the shear wave also becomes 900 and the shearwaves converts to surface waves which propagates along theinterface, [ provided a gas or only a very thin layer of Couplant ispresent above the second medium ] otherwise shear waves will bereflected back into the first medium [ total reflection ]. This anglewhen the shear wave is refracted at 900 is known as the secondCritical angle of the mediums. The value of these critical anglesare different for different combination of mediums.If the first medium is a liquid, then there would be no reflectedshear wave in the liquid.In common shear wave probes, which are designed for testingsteel and eliminates the refracted longitudinal wave in steel, theincident angles usable are between 27.50 and 570 for refractedshear wave angles between 33.30 and below 900 in steel.The incident, reflected and refracted angles are related as,

Sin 1 is incident angle, C 1 velocity in first medium, Sin 2 isangle of reflection / refraction. C 2 velocity in second medium.

Calculate the angle of refraction of longitudinal and shear wavesin steel when a longitudinal wave hits the interface of acrylicand steel at an angle of 200 ;Long wave velocity in acrylic = 2730 mtrs / secLong wave velocity in steel = 5900 mtrs / secShear wave velocity in steel = 3230 mtrs / sec

Sin 200 = .3420refracted Longitudinal, = Inv Sin ( V2 / V1 X Sin 20 )

= Inv Sin ( 5900 / 2730 X .3420 )= Inv Sin .7391 or 47.650

refracted Shear, = Inv Sin ( 3230 / 2730 X .3420 )= Inv Sin .4046 or 23.860

Sound reflection at water / steel interface,Z1 steel = 4.5, Z2 water = .15 in gms / cm2 / sec.% of reflection ;

[(4.5 - .15) / ( 4.5 + .15)]2 X 100 = 87.5i.e. only 12.5 % ultrasound energy will entersteel. At water / aluminium interface, 30 % ofincident energy will enter aluminium.at metal / gas interface, reflection is ~ 100%.

Sound field :The ultrasound intensity along the beam is not

uniform but varies due to the size of the sourcethat gives rise to interference effects. The faceof the crystal does not vibrate uniformly underthe influence of the triggering electrical pulse.The crystal acts as a mosaic of large numberof tiny individual crystals, each vibrating in thesame direction but slightly out of phase with itsneighbors. Each element in the mosaic acts likea point source and radiates a spherical wave,outward from the plane of the crystal face. Asthese spherical waves with different phasesencounter one another in the region near thecrystal face, interference effect sets up a seriesof maximum and minimum intensity locations.This interference zone ends at the lastmaximum, at a distance N from the crystal andis known as the near field [ Fresnel zone ] of thesound beam. The location of the last maximumis the natural focus of the transducer and thesound field converges to half the source size atthis location. Because of this intensity variations,echo amplitude from a constant reflector in thenear zone will vary and it can be extremelydifficult to accurately evaluate the size of thereflector.

Calculation for the near field length and the beamspread for a 2 MHz, 24 mm Ø, longitudinal waveprobe in steel and in water;wavelength in steel [ ] is 5.9 / 2 or 2.95 mm.wavelength in water [ ] is 1.5 / 2 or .75 mm.crystal size [ D ] is 24 mm.in steelnear field length, 24 2 / ( 4 X 2.95 )

or 48.81 mmHalfbeam spread, Inv Sin ( 1.22 X 2.95 ) / 24 or 8.62 0

full spread, 2 X 8.62 ~ 170.

In waternear field length, 24 2 / ( 4 X .75 )

or 192 mm.Halfbeam spread, Inv Sin ( 1.22 X .75 ) / 24 or 2.15 0

full spread, 2 X 2.15 0 or 4.30 0.

Testing in the near zone is limited to :Thickness measurements, Detection of defects and Sizing oflarge defects only.

Intensity variations [ interference ] in the near zone is lesserfor rectangular shaped crystals.

The near field distance depends on the size and frequency ofthe transducer and the effective wave length in the testmaterial. Near field for a circular element with a single operatingfrequency in a single medium is,

N = D2 / 4 D is element size and is effective wavelength.At distance greater than N, known as the far field of the

ultrasonic beam [ Fraunhoper zone ], there are no interferenceeffects. In this zone the sound field diverge in the shape of acone and the sound pressure is inversely proportional todistance and follows an exponential decay curve.Half angle of divergence [ to the boundary of sound field ],

/ 2 = Sin -- 1.22 / DIn the far zone the signal from a large reflector [ larger thanthe beam cross section at that distance ] follows the inverseproportional law. After a distance of three near zones from thecrystal, a double distant echo from a large reflector will causethe echo intensity to reduce by 6 dB. The signal from a smallreflector causes greater directional change and a smaller amountof the reflected energy reaches the probe. The signal amplitudedecreases to one fourth when the distance is doubled [ - 12 dB ]

Beam spread decreases with increase in frequency and Crystalsize. Consideration of beam spread is important when measuringthe size of a discontinuity and inspecting for flaws near asidewall or corner or small round parts, where the divergingbeam may produce spurious echoes.

The near field and far field effects also occur when ultrasonicwaves are reflected from an interface.

Transfer Correction :If the reference block used for setting up the

test sensitivity is not fabricated from the materialto be tested, the sound attenuation characteristicof the block material may be different from thatof the test material. The differences in surfaceconditions will also change sound attenuation.

A correction for test sensitivity is required tocompensate for the differences between thereference blocks and the test specimen forsurface roughness, contact area and internalsound attenuation. This is known as transfercorrection and must be considered while using aDAC curve for flaw evaluation.

Correction for normal beam testing :Transfer correction with normal probe can be determined by

comparing the instrument gain required to produce equal amplitudeback reflections at same material distance from the referenceblock and the test part.

Select a region on the test part that has parallel walls and thesurface condition similar to most of the remaining scanning areaas a transfer correction measurement point. Select the referenceblock, being used for DAC set up or from the same set ofblocks whose overall material distance most closely matches thethickness of the part at the measurement point.

Set the back reflection through the block thickness to80 % screen height and record the gain setting.

Place the probe at the measurement point of the part, andproduce a back reflection of 80 % screen height and record anychange in the gain setting.

The difference between the new and the old gain setting [ for theblock signal ] is the transfer correction.Correction for angle beam testing :

Plot the DAC curve from the recommended reference block. Usingthe probe to be used for testing as a transmitter and a similarangle probe as a receiver, position both on the reference block,facing each other at one full skip distance apart. Set the flawdetector in dual probe mode. Keep the transmitter probe still andmove the receiver probe to maximize the received signal. Adjustthe gain control to peak this signal on the line of the DAC curve.Record the gain used for this condition.

Repeat the same technique as mentioned above on thecomponent to be tested. The probe position should be in thesame direction as to be used for the actual examination. Recordthe new gain setting.The difference in gain value is the transfer correction.

Transfer correction may be positive or negative and theadjustments in the DAC gain setting will be made accordingly. Thiscorrected gain setting will be used as the discontinuity evaluationsensitivity for the examination.

The transfer correction value should not be more than 6 dB. Thevalue of transfer correction increases with probe angle. Forreliability, the transfer correction value should be determined in atleast three different locations of the test part. To ensure thatuseful transfer correction factor is obtained, signal comparisonshould be made in the far zone of the ultrasonic beam.

Twin Crystal Probe :The twin crystal probe is designed to eliminate the dead zone

problem of a single crystal probe. But actually, a twin crystal probealso has a dead zone of few millimeters.

Twin crystal probe contains two independent crystals in a singlehousing. The crystals are mounted on plastic delay lines that areusually cut at an angle to the horizontal plane [ forms the roofangle ], so that the transmitting and receiving beam paths crossbeneath the surface of the test piece. The dead zone is where thetransmitting and receiving beams have not converged. In thisarrangement, one of the crystals transmits ultrasound and the otherreceives the reflected signal. The crystal assemblies are separatedby some form of acoustic barrier [ usually Cork ] to prevent crosstalk noise.A highly penetrating couplant should not be used, otherwisedamage to this cross talk barrier may take place. Dual crystalconfiguration almost eliminates dead zone, improves near surfaceand lateral resolution and performs well on corroded back wallsand rough entry surfaces. The crossed beam design acts aspseudo focus and increases sensitivity for short range flawdetection. In general, a decrease in the roof angle or an increasein the crystal size will result in a longer pseudo -focal distance andan increase in the useful test range of the probe.Range calibration :

Angled crystal arrangement in dual probe produces a V-path inthe test specimen and hence the sound travel path is more thanthe actual thickness of the part. Because of this condition, multipleback reflections are not used for range calibration when accuracyof calibration is desired.

For accurate range calibration two independent thickness sections,one near the start and the other near the end of the required testrange are used. The lower thickness echo is set by the delaycontrol and the higher thickness echo by the fine range control.Echo positioning requires many repetitions. Calibrated range will beaccurate between the two selected calibration block thickness.

The calibrated screen does not contain the initial echo becauseof the long delay path in the plastic delay line and permits flawdetection and measurement near the surface. Multiple reflections inthe transmitter delay line is not detected because the transmittercrystal does not have any reception function. A small cross-talkecho may appear on the screen and its height will decrease whenthe probe is coupled to the test material.

Accuracy of calibration may be checked on a step wedge blockfor thickness measurement applications.

Dual crystal probes are useful for detecting discontinuities closerto the scanning surface. The probe exhibits high sensitivity andproduces good signal amplitude from small discontinuities locatedwithin short distance. After the pseudo focus, the sensitivity dropsrapidly and the useful range for detecting smaller flaws is around50 mm. [ - 6 dB sensitivity ]Dual crystal probes are excellent for thickness measurements in

thinner sections, inspection of cladding and bonding, laminationtesting in thinner plates, evaluation of castings and other soundscattering materials and for measuring corrosion in low thicknessplates and tubing. When measuring very thin sections with largerprobes, sound may bounce twice within the part before reachingthe receiving crystal and record twice the actual thickness.

Ultrasonic wave :Ultrasound is transmission of energy through an elastic medium,

by means of vibrations of the particles. The vibrating particlestransfers some of the vibrational [ mechanical ] energy on toneighboring particles and force them to vibrate. The energy thuspropagates through particles. Because of this, sound cannotpropagate in vacuum.

Sound generated above 20,000 Hz is called ultrasound. Ultrasoundpropagates more easily through solids than through liquids orgasses. Ultrasound with frequencies of 1 MHz and above isdirectional, has short wavelength, and gets reflected from smalldiscontinuities in materials. This property makes ultrasound usefulfor detecting and locating defects in materials.Wave length : Ultrasonic vibrations travel in the form of waves. Thedistance, measured along the line of propagation, between two wavesurfaces in which the phase differs by one complete period iswavelength. It is not the material's particles that moves through thethickness, it is the vibrational [ mechanical ] energy that istransferred from one particle to another.Frequency : The number of wave lengths [ vibration cycles of aparticle ] completed in one second is frequency.

Unit of frequency is Hertz [ Hz ]1000 Hz is equal to 1 KHz, 1000 KHz is 1 MHz.If a 2 megahertz probe is used for an inspection, that means the

part’s particles will vibrate 2000000 times per second. Ultrasoundtravels in Steel, 5960000 mm in 1 second, i. e. 2000000 waves willoccupy 5960000 mm. So wave length in this case is 2.98 mm.Frequency depends on the probe and does not change with testmaterial.Cycle : The particles are displaced, first in the forward directionand then in the opposite direction. These two displacements equalone cycle.Period : The time required to complete a full cycle of vibration of aparticle is period.Period is one second divided by frequency. [ T = 1 / F ]Velocity : Velocity is the speed of energy transfer between twopoints. The distance of propagation of the wave [ energy ] in onesecond is the velocity of the wave. Velocity of ultrasound in aperfectly elastic material at a given temperature and pressure isconstant. Velocity depends on the density, elasticity and rigidity ofthe test material.

Velocity, Frequency and Wavelength are related as,Velocity = Frequency X Wavelength.

Acoustic pressure [ P ] is the amplitude of alternating stresses onthe material by a propagating ultrasonic wave.P = acoustic impedance X amplitude of particle vibration.Ultrasonic Intensity [ I ] is the transmission of mechanical energy,through an unit cross- section area, which is perpendicular to thedirection of the wave propagation.I = ½ [ acoustic pressure X amplitude of particle vibration ]Important : The minimum size of a detectable reflector is generally

considered to be half of the test wavelength. Smaller than this,reflectors scatter sound energy [ reflects energy in random fashion ]without producing flaw indications. Discontinuity, with thickness ofquarter wavelength or more reflects ultrasound very well. In additionto the reflection ultrasonic waves get deflected [ spreads out ] fromthe edges of a discontinuity.[ known as tip Diffraction and is useful for measuring the size ofcracks ]For satisfactory transmission and reception of ultrasound betweenthe probe and the part, the part surface should be sufficientlysmooth. The roughness of the surface should be less than1/10 of the wavelength being used for the examination.

Calculate wave length of ultrasound in steel andwater for a 2 MHz, 10 mm dia longitudinal waveprobe, Longitudinal velocity, insteel 5900 Mtrs / sec, water 1480 Mtrs / sec.Velocity = Frequency X Wavelength.or, Wavelength = Velocity / Frequency.steel = 5900 Mtrs / sec or 5.9 X 106 mm / sec.water = 1480 Mtrs / sec or 1.48 X 106 mm / secFrequency = 2 MHz or 2 X 106 cycles / sec

( each cycle produces one wavelength )Hence, Wavelength = 5.9 / 2 = 1.48 / 2

= 2.95 mm. = .74 mmin steel in water

Ultrasound velocity in meters / sec and acousticimpedance in gms / cm2 / sec :Material Longitudinal Shear ImpdSteel 5900 3230 4.54Cast Iron 5600 3220 4.0Inconel 5820 3020 4.94Aluminum 6320 3130 1.7Acrylic 2730 1430 .32Water 1480 ------ .148Brass 3830 2050Titanium 6100 3120 2.76

Range calibration with the IIW – V1 block :The main objective of range calibration is to make the CRT

screen represent a desired material thickness. The CRT screen isgraduated in 50 small equal divisions, divided into 10 major groups.By positioning the leading edge of the echoes of known materialdistances to appropriate scale divisions, different test ranges arecalibrated.100 mm full scale :

Set the coarse range control to 50 mm. In this position, the rangecan be calibrated between 50 mm full scale minimum to 250 mmrange full scale maximum. Now Place the probe on the broad faceof the IIW - V1 block, i.e. Pos 1. Multiple echoes of the blockthickness will appear on the screen with distances of 25, 50, 75,100 mm etc…[ use the fine range control to compress or expandthe screen display and the bring the required calibration echoeswithin the display screen ]

Now, using the delay control, set the leading edge of the 25 mm[ first back reflection ] echo at 12.5 th small scale division. Positionthe leading edge of the 100 mm echo [ Fourth back reflection ] at50 th small scale division with the fine range control. Repeat theseadjustments till both the echoes are accurately positioned [ fixed ]on the scale. To check the calibration, place the probe at pos 2,the first back wall echo shall appear at 50 th scale division.The calibrated range is valid for similar material only.Minimum calibration range possible is equal to one block thickness.Alternate Method :

Place the probe at Pos 2. Back wall echoes with 100 mm intervalwill appear along with the side wall reflection echoes between them[ use fine range control ]. Use lower gain setting. Correctly identifythe first and second back wall reflections. Side wall echoes aresmaller and appear after each back wall echo. Position the firstback reflection at 0 division of the scale with the delay control. Setthe second back reflection at 50 th small scale division with thefine range control.

Repeat these adjustments till the echoes are accurately positionedon the scale. The screen now displays from 100 mm to 200 mm.Shift the first back reflection to 50 th small scale division with thedelay control only to bring the first 100 mm range within the displayscreen.Full Scale 200 mm :With the probe at pos 2, set the first back reflection to 25 th

division with delay control and the second back reflection to the50 th division with the fine range control.Repeat till positioning is accurate.Check the calibration accuracy from pos 3.

Required range 100 mm :Number of small divisions on the scale = 501 small scale division = 100 / 50 or 2 mmdistance to 1 st indication = 25 mm [ Probe position 1 ]location on scale = 25 / 2

= 12.5 th small scale division.distance to 2 nd indication = 50 mmlocation on scale = 50 / 2

= 25 th small scale division.Set the 25 mm indication at 12.5 th small scale division withdelay and the 50 mm indication at 25 th small scale divisionwith the fine range control.For accuracy of calibration, the first and the last possible echo[ within the calibrating range ] should be positioned, to reducethe flaw detector’s horizontal linearity error.

IIW block :for calibration oftest ranges,probe and flawdetector’sperformancecheck.

Calibration of 100 mm full scale for shear wave probe usinga longitudinal wave probe and V1 block :

Angle probe range calibration with the IIW – V1 block normallyprovides 150 and 250 mm minimum calibrated distances. A 100mm distance calibration is possible by separately scanning the25 and the 100 mm radius.

The 91 mm step in the IIW block is also useful for calibrating100 mm full scale for shear wave probes. The travel time for 91mm steel by longitudinal wave equals travel time of 50 mm steelby shear wave. This is because the velocity of shear wave isnearly 50 % of longitudinal wave. Hence, this 91 mm thickness isconsidered to be 50 mm equivalent for a shear wave probe.Procedure : With the coarse range at 50 mm, pick up two backreflections from the 91 mm step using a normal probe.

Position the 1st back reflection at half scale with the delaycontrol and the 2 nd back reflection at full scale location withthe range control. Repeat till the echoes are positioned.

Be careful to correctly identify the side wall reflectionsappearing after each back reflection.

The range thus obtained is theoretically equivalent to 100 mmfor a shear wave probe.Delay correction for the Shear wave probe : A delay correctionfor the plastic wedge is required to correctly set the scale to100 mm with each shear wave probe.

Scan the 100 mm radius with the shear wave probe for whichthe range is to be calibrated and maximize the signal obtainedfrom the radius. Observe the location of this indication on thescale and bring it to the full scale location using the delaycontrol only.

Maximizing the 100 mm radius signal, produces indicationswith distances of 100, 225, 350 etc. By positioning the 100and 225 mm indications at 4th and 9th division respectively,250 mm range is obtained.If the 25 mm radius is scanned the indications will appear at25 mm, 150 mm, 275 mm etc. [ echo separation for thisblock is 125 mm ]

Calibration of shear wave test ranges with theV2 calibration block :

Position the exit point [ as found by scanning the 25 mm radiusof the block ] of the angle probe on the center of thescale marked on the face of the V2 calibration block to obtainthe maximized signal from the 25 mm radius.Alternatively, move the probe above the scale area to obtain themaximized signal from the 25 mm radius.

Set the coarse range control to 50mm.The echoes with distances of 25 and 100 mm will appear on the

screen [ use the fine range control to compress the screen andbring the echoes within view ] Set the gain control to display clearinterpretable signals.Full scale 100 mm :

Position the 25 mm echo at 12.5 th small scale division with thedelay control.

Adjust the fine range control to position the 100 mm echo at50 th scale division.

Repeat these adjustments till the echoes are accuratelypositioned.

To confirm the accuracy of calibration, scan the 50 mm radius ofthe block, the maximized indication shall form on the 25 th divisionon the CRT screen [ 2 X 12.5 divisions ].Full scale 125 mm :

To calibrate 125 mm full scale, position the 25 mm echo at10 th small scale division with the delay control and the 100 mmecho at 40 th small scale division with the fine range control.Full scale 200 mm :

To calibrate 200 mm full scale, set the coarse range control to250 mm position, because shear wave velocity is nearly 50 % oflongitudinal wave.

Bring the 25, 100 and 175 mm echoes within the screen [ usefine range control ].

Now position the 25 mm echo at 6.25 th small scale divisionwith the delay control and the 100 mm echo at 25 th small scaledivision with the fine range control, repeat adjustments till theechoes are accurately positioned. Check accuracy of calibration.

Only reflections from 25 mm radius reach crystal.

Probe angle check using the 5 mm dia through drilled hole.

Calibration of 125 and 200 mm using V2 block :Move the probe over the scaled area to obtain the maximized

signal from the 50 mm radius. The beam index point of the probecan be found by marking the probe body, above the central lineof the scale divisions.

Set the coarse range control to 50 mm.Using the fine range control, compress the scale and bring the

50 and 125 mm distance signals within the CRT screen.Full scale 125 mm :

Position the 50 mm signal at 20 th small scale division with thedelay control. Adjust the fine range control to position the 125 mmsignal at 50 th small scale division. Repeat these adjustments tillthe echo signals stand at desired positions with no furtheradjustments.

Check calibration using the 25 mm radius, maximum signal shallappear at 10 th division.Full scale 200 mm :

Set the coarse range switch to 250 mm position.Compress the scale using fine range and bring 50, 125 and 200

mm echo signals within the screen.Position the 50 mm [ 1 st echo ] at 12.5 th small scale division

with the delay control and the 200 mm [ 3 rd echo ] at 50 th smallscale division with the fine range control. Repeat these adjustmentstill the echo signals stand at desired positions with no furtheradjustments.

Maximizing the indication from the 50 mmradius of the block, produces signals at 50,125 and 200 mm etc…

Range calibration procedure :Required range 125 mm ;Number of small divisions in the scale = 501 small scale division = 125 / 50 or 2.5 mmdistance to 1 st indication = 50 mmlocation on scale = 50 / 2.5

= 20 th small scale division[ 4th mark on the screen ]

distance to 2 nd indication = 125 mmlocation on scale = 125 / 2.5

= 50 th small scale division.[ 10th mark on the screen ]

Set the 50 mm indication at 20 th small scale division withdelay control and the 125 mm indication at 50 th small scaledivision with the fine range control. Repeat the adjustments tillthe indications are accurately positioned.

AWS - DC type block designed for distance andbeam index calibration of angle beam probes. Thisis a simple block for calibrating a shear waveprobe. The block has 1" radius overlying a 2" radiuson 180 degree half circle. The block produces 1and 2 inch echoes and calibration is easy. Thisblock can be easily machined and is useful whenstandard blocks are not available for non ferrousmaterial testing. Block thickness is 1 inch.

Ultrasound wave modes :All material substances are composed of atoms, which may beforced into vibrational motion about their equilibrium positions.When the particles are displaced from their equilibrium positionsby any applied force, internal stress acts to restore the particlesto their original positions. Because of this inter-atomic forcesbetween adjacent particles, a displacement at one point inducesdisplacement at neighboring points and so on, thus propagatinga stress - strain wave. The actual displacement of matter thatoccur in ultrasonic wave is very small.

The amplitude, mode of vibration and the velocity of the wavediffer in solids, liquids and gasses because of the largedifferences in the mean distances between particles in theseform of matters. These differences influence the forces ofattraction between particles and the elastic behavior of thematerials. In air, sound travels by compressions and rarefactionsof the air molecules in the direction of sound propagation andgenerate longitudinal wave. In solids, the molecules can supportvibrations in other directions, hence, a number of different typesof sound wave modes are possible.

Longitudinal or Compressoinal wave is generated when theexternal applied force produces vibration of the particles in thedirection of the wave propagation. The wave produces alternatezones of compressions and rarefactions. Longitudinal wave canpropagate by simply pushing the particles of the medium andhence can be generated in gasses, liquids and solids. Becauseof its easy generation and detection, longitudinal wave is mostwidely used in ultrasonic testing. Almost all of the ultrasonicenergy used for material testing is generated in this mode andis then converted to other wave modes for different testingapplications. Straight beam examination utilize these waves.

Shear waves can have the vibration at any angle with respectto the direction of wave propagation and can therefore bepolarized. Shear wave is commonly described as a wave withparticle vibration perpendicular to wave propagation. These wavespropagate with a velocity which is about 55 % of longitudinalwaves. Shear waves will not propagate in liquids and gassessince these mediums do not support shear forces. Shear wavehas shorter wavelength, can detect smaller flaws and has higherattenuation compared to equivalent longitudinal wave. Anglebeam examination mostly utilize shear waves.

Surface waves propagate along the surface of a metal with anelliptical particle motion. To sustain the waves, the medium abovethe metal surface must be gas or a very thin layer of liquidcouplant. The waves propagate along the test surface through athickness of only one wavelength. At one wavelength deep, thewave energy drops to only 4 %. The waves have a velocity ofapproximately 92 % of equivalent shear wave.Surface wave follows curved surfaces and gets reflected fromsharp corners, surface and very near surface discontinuities.Surface wave has low attenuation and high sensitivity fordetection of surface defects.

Lamb wave or plate waves are produced by an angulatedcompression wave launched into a thin plate [ few wavelengthsthick ]. Lamb waves are zig zag reflected longitudinal andtransverse waves, but bounded by the sheet or plate surfaces

causing a wave guide effect. Some particles oscillate900 to the plate surfaces and others at variousangles. They propagate by flexing the plate surfacesand saturate the plate thickness. Lamb wave testingis usually done by mechanized scanning systems.Mode conversion : Ultrasonic wave when reflectedmay change from one form to another, i.e. fromlongitudinal to shear, shear to surface etc. If a 600

shear wave hits a vertical plane at 300 angle ofincidence, the wave changes to longitudinal modewith nearly 20 dB loss of energy. The mode changesare accompanied by the appropriate change invelocity. 600 probes are not suitable for verticaldefects, tandem technique, weld root face.

Ultrasound propagates bymeans of the oscillatorymotion of the atomic ormolecular particles in themedium.

Welding defects :Crack : Cracks are rupture of metal caused by severe heat and stress.Crack can occur anywhere in the weld metal, heat affected zone andin the base metal. Longitudinal cracks propagate along the weld length.Transverse cracks are perpendicular to the weld seam. Crater cracksform at the weld surface and radiate out in many directions [ visible atsurface ]. Base metal cracks may be in any orientation to the weld.Tight crack produces small indication or no indication at all. Openedcracks usually produce recognizable high amplitude broad basedindication with wide signal envelope.Lack of penetration : Incomplete filling and not bridging the gap of theweld root opening. Weld metal does not extend entirely through thejoint thickness. For welding from one side, this discontinuity is open tothe surface. Produces high amplitude sharp indication from the cornerof the root face, detectable from both the sides of the weld. Plottingdoes not cross over the root centerline. Cross over indicates probabilityof root lack of fusion or root undercut.Lack of fusion / Cold lap : Failure of the weld metal to fuse along theedges of the base metal [ bevel ] or lack of bond between adjacentweld passes [ an area of the parent metal or already solidified weldmetal does not get melted to fuse with the weld metal ]. Usually causedby improper heat or poorly prepared weld surfaces. Lack of fusion mayhave slag associated with it. Produces high amplitude sharp indication,only when intercepted at 900, usually detectable from one side of theweld only. When slag is present, the defect is detectable from boththe sides of the weld.Slag / Inclusions : Entrapment of foreign material in the weld metal. Slagmay be small irregular fragments or elongated. Slag lines follow thedirection of welding and will be located along the weld groove edge orbetween passes following a valley left by weld passes. Producesforked, broad based, lower height indication, Usually detectable from boththe sides of the weld.Porosity : Porosity occurs when gasses in the molten weld metal failsto escape before solidification of the weld material. Occurs anywherewithin the weld. Isolated pore is a single spherical gas pocket.Scattered porosity is random distribution of single pores. Clusterporosity is a group of pores. Wormhole is an elongated[ tunneling ] pore. Hollow bead is an elongated gas pocket that tunnelsdown the root pass in the direction of welding.Single pore produces a narrow base sharp small amplitude indication.Single and dispersed / scattered pores are difficult to detect.Cluster will produce broad base indication with multiple peaks.Root concavity : The root of the weld is fused but the center of theroot weld pass is below the surface of the adjacent base material. Thisdefect occurs on joints that are welded from one side only, whereexcessive melting of the underside occurs. This discontinuity is open tothe surface. Detectable as low amplitude signal from both the sides ofthe weld. Plotting short of plate thickness with no crossover. Difficult todetect if wide and shallow. [ a 450 probe is preferred ]Undercut : Undercut is a groove cut along the edge of the weld,caused by excessive melting of base material and left unfilled by theweld metal. Undercut forms on any or all the four edges of the weld. Itis open to the surface. Produces sharp indication at half or full skipbeam path, amplitude depends on depth of cut [ 45 0 probe preferred ].May produce twin peaked signal due to beam spread hitting both thedefect and the root bead. Plots short of plate thickness with no crossover.Excess Penetration : Excess metal accumulation at root, occurs mostlyafter root repair. Low amplitude signal from the root bead, detectablefrom both the sides, distinguishing feature is ringing of the falling edgeof the signal. Beam path just longer than half skip beam path. Plotsdeeper than plate thickness level with cross over.Misalignment with / without penetration : Misalignment of the members isvisible at the surface. Produces high amplitude root signal detectablefrom the member which is lower in height. Scanning from the othermember does not produce any indication.Backing Bar : The inherent gap between the backing bar and the basematerial and the edges of a narrow backing strip produce indications.Other defects : Irregular weld surface, Excessive capping, Under fill,Irregular root penetration, Burn through, Tungsten [ GTAW ],

Weld scanning with angle probes :In angle beam weld testing, the probe is moved in a Zig –

Zag path, perpendicular to the axis of the weld, from theedge of the weld to full skip distance. [ 2 Thk X Tan A ]

The distance [ along the test surface ] between the center /root of the weld to probe exit point, when the sound beamhits the root of the weld is known as Half skip distance.The distance, where the beam hits the top [ plate level ] ofthe weld is Full skip distance.

First, the angle probe movement area should be scannedwith a suitable single / twin normal probe to detect presenceof laminar flaws which may prevent full body weld testing.For sensitivity, second backwall echo should be set to fullscreen height. If the weld surface condition permits, then theweld volume should be scanned with a twin crystal normalprobe from the surface of the weld to detect internal flaws.This is a good technique for detecting slag, porosity androot penetration.

To scan the full body of the weld and to detectdiscontinuities along the fusion lines, the weld must bescanned with different angle probes and from both the sidesof the weld [ scan 1, 2 ].Thick welds [ > 50 mm ] should be scanned from all the four

surfaces [ up to half skip distance from each surface ].Attempt should be made to hit the defect with shorter beampath to produce good signal amplitude for evaluation. Careshould be taken to detect fusion line discontinuities with asuitable angle probe [ perpendicular hit ]. The weld should bescanned for transverse defects also by directing the soundbeam nearly parallel to the weld seam [ probe body nearlyparallel to the weld seam, scan 3a ]. Weld scanning sensitivitymust be at least 6 to 12 dB above the flaw evaluation gainsetting. Slag fragments and dispersed porosity is difficult todetect at DAC gain setting. Ideal sensitivity standard is thesmallest flaw placed in the test block at maximum testdistance. The flaw detector / probe combination shall be ableto produce a clear signal from the smallest flaw to bedetected throughout the scanning range. Maximum sensitivitycan be achieved by adjusting the gain to produce grass up.

to the maximum test distance.A fixed root scan should be carried out from bothsides of the weld using a guide strip. For weld rootexamination, a 450 angle probe exhibits good sensitivity.The probe will also produce indications from irregular[ corroded / pitted ] undersurface. A 600 probe may hitthe root with 300 angle of incidence. This will causemode conversion and loss of signal amplitude. A 700

probe does not produce good signal from rootdiscontinuities with rounded reflecting surface such as aconcave root or shallow undercut. Misaligned root mayproduce strong signal, detectable from one side of theweld. Misalignment may produce Lack of penetration.

For evaluation of root defects reference notches maybe given consideration. Unnecessary root repair shouldbe avoided.Wherever possible, reflections, located near the surfaces

should be checked by finger damping.Generally selected angle probes for weld scanning,

4 to 6 mm -- 80 0

up to 10 mm -- 70 0

up to 25 mm -- 70 0 and 60 0

more than 25 mm -- 60 0 and 45 0

For curved surfaces / welds 45 0 and 60 0 probes aremore suitable because of lesser increase in beam path.

Weld Scanning calculations :[ for flat plates ]

Half skip distanceThickness X Tan A

Half skip Beam pathThickness / Cos A

or, Thickness X SFFull skip distance

2 Thickness X Tan AFull skip Beam path

2 Thickness / Cos Aor, 2 Thickness X SFA is effective probe angle,SF [ Slant factor = 1 / Cos A ]

A SFTan 80 0 - 5.67 80 0 - 5.75Tan 70 0 - 2.74 70 0 - 2.94Tan 60 0 - 1.73 60 0 - 2.0Tan 45 0 - 1.0 45 0 - 1.41Cos 80 0 - 0.17Cos 70 0 - 0.34Cos 60 0 - 0.50Cos 45 0 - 0.70