Nondestructive Material Testingwith Ultrasonics

Introduction to the Basic Principles

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Nondestructive Material Testing with Ultrasonics

Introduction to the Basic Principles

Michael Berke

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41. Why use ultrasonics for nondestructive material testing? . . . . . . . . . . . . . 52. Ultrasonic testing tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53. Detection of discontinuities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64. Method of testing and instrument technology . . . . . . . . . . . . . . . . . . . . . 104.1 The ultrasonic flaw detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2 Near resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.3 The probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.4 Refraction and mode conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.5 Characteristics of angle-beam probes . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.6 The TR probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205. Locating discontinuities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.1 Calibration of the instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.1.1 Calibration with a straight-beam probe . . . . . . . . . . . . . . . . . . . . . . . . . . 225.1.2 Calibration with a TR probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.1.3 Calibration with an angel-beam probe . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.1.4 Locating reflectors with an angle-beam probe . . . . . . . . . . . . . . . . . . . . 286. Evaluation of discontinuities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.1 Scanning method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.2 Evaluation of small discontinuities: The DGS method . . . . . . . . . . . . . . . 306.3 Sound attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346.4 The reference block method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346.4.1 Comparison of echo amplitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346.4.2 Distance amplitude curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357. Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378. Diagnosis of indications (outlook) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Reference list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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Introduction

Nondestructive material testing with ultrasonics is more than 40 years old.From the very first examinations, using ultrasonic oscillations for detection of flawsin different materials, it has become a classical test method based on measu-rements with due regard to all the impor-tant influencing factors. Today it is expec-ted that ultrasonic testing, supported bygreat advances in instrument technology,give reproducible test results within narrowtolerances. This assumes exact know-ledge of the influencing factors and theability to apply these in testing technology.

Not all influences have to be seriously regarded by the operator. In many casessome of the influences can be neglectedwithout exceeding the permitted measu-rement tolerances. Due to this, the test se-quence is simplified and the testing timereduced. Despite this, the future belongsto the qualified operator who carries outhis task responsibly and who continuouslyendeavours to keep his knowledge at the latest state of the art

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1. Why use ultrasonics fornondestructive materialtesting?

At the beginning of the fifties the technicianonly knew radiography (x-ray or radioac-tive isotopes) as a method for detection ofinternal flaws in addition to the methodsfor nondestructive testing of material sur-faces, e.g. the dye penetrant and ma-gnetic particle method. After the SecondWorld War the ultrasonic method, as de-scribed by Sokolov in 1935 and applied byFirestone in 1940, was further developedso that very soon instruments were avail-able for ultrasonic testing of materials.

The ultrasonic principle is based on thefact that solid materials are good conduc-tors of sound waves. Whereby the wavesare not only reflected at the interfaces butalso by internal flaws (material separa-tions, inclusions etc.). The interaction ef-fect of sound waves with the material isstronger the smaller the wave length, thismeans the higher the frequency of thewave.

c = Sound velocity [km/s]f = Frequency [MHz]λ = Wave lenght [mm]

This means that ultrasonic waves must beused in a frequency range between about0.5 MHz and 25 MHz and that the resultingwave length is in mm. With lower frequen-cies, the interaction effect of the waveswith internal flaws would be so small thatdetection becomes questionable. Both testmethods, radiography and ultrasonic test-ing, are the most frequently used methodsof testing different test pieces for internalflaws, partly covering the applicationrange and partly extending it.

This means that today many volume testsare possible with the more economical andnon-risk ultrasonic test method, on the

other hand special test problems are sol-ved, the same as before, using radiogra-phy. In cases where the highest safety re-quirements are demanded (e.g. nuclearpower plants, aerospace industry) bothmethods are used.

2. Ultrasonic testing tasks

Is there a primary classification of tasksassigned to the ultrasonic operator? If welimit ourselves to testing objects for possible material flaws then the classifica-tion is as follows:

1. Detection of reflectors2. Location of reflectors3. Evaluation of reflectors4. Diagnosis of reflectors

(reflector type,orientation, etc.)

Instead of using the word "reflector", theultrasonic operator very often uses theterm "discontinuity". This is defined asbeing an "irregularity in the test objectwhich is suspected as being a flaw". Inreality, only after location, evaluation anddiagnosis has been made, can it be deter-mined whether or not there is a flaw whicheffects the purpose of the test object. Theterm "discontinuity" is therefore alwaysused as long as it is not certain whether itconcerns a flaw which means a non-per-missible irregularity.

λ = cf

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3. Detection of discontinuities

The essential "tool" for the ultrasonic ope-rator is the probe, Figs. 1a + 1b.

The piezoelectric element, excited by anextremely short electrical discharge, trans-mits an ultrasonic pulse. The same ele-ment on the other hand generates an electrical signal when it receives an ultra-sonic signal thus causing it to oscillate.The probe is coupled to the surface of thetest object with a liquid or coupling paste sothat the sound waves from the probe areable to be transmitted into the test object.

The operator then scans the test object,i.e. he moves the probe evenly to and froacross the surface. In doing this, he obser-ves an instrument display for any signalscaused by reflections from internal discon-tinuities, Fig. 2.

Every probe has a certain directivity, i.e.the ultrasonic waves only cover a certainsection of the test object. The area effec-tive for the ultrasonic test is called the"sound beam" which is characteristic forthe applied probe and material in whichsound waves propagate.

A sound beam can be roughly divided intoa convergent (focusing) area, the near-field, and a divergent (spreading) part, thefar field, Fig. 3.

The length N of the near-field (near-fieldlength) and the divergence angle is de-pendent on the diameter of the element, its

Fig. 1a Straight-beam probe (section)

Fig. 1b Angle-beam probe (section)

Fig. 2a Plane flaw – straight-beam probe

Fig. 2b Plane flaw – angle-beam probe

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frequency and the sound velocity of thematerial to be tested. The center beam istermed the acoustic axis. The shape ofthe sound beam plays an important part inthe selection of a probe for solving a testproblem. It is often sufficient to draw theacoustic axis in order to show what the solution to a test task looks like. A volu-metric discontinuity (hollow space, foreignmaterial) reflects the sound waves in dif-ferent directions, Figs. 4a + 4b.

The portion of sound wave which comesback to the probe after being reflected bythe discontinuity is mainly dependent onthe direction of the sound wave; i.e. it doesnot matter whether scanning is made witha straight-beam probe or an angle-beamprobe or whether it is carried out from different surfaces on the test object, Fig. 5.

If the received portion of the reflectedsound wave from the probe is sufficientthen the detection of the existing volume-tric discontinuity is not critical, this meansthat the operator is able to detect it by

Fig. 3 Sound field

Fig. 4b Volumetric discontinuity – angle-beam probe

Fig. 4a Volumetric discontinuity – straight-beam probe Fig. 5 Volumetric flaw – detection form different directions

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scanning from different directions. A plane(two-dimensional) discontinuity (e.g. ma-terial separation, crack) reflects the ultra-sonic waves mostly in a certain direction,Fig. 6.

If the reflected portion of the sound waveis not received by the probe then it is unli-kely that the discontinuity will be detected.The possibilities of detection only increasewhen the plane discontinuity is hit vertical-ly by the sound beam. This applies to dis-continuities which are isolated within thetest object.

With plane discontinuities which are opento the surface of the test object, e.g. a crackrunning vertically from the surface into thetest object, a vertical scan of the crackdoes not always produce the required suc-cess. In this case wave overlapping occurs(interferences) due to sound wave reflec-tion on the side wall of the test object whichseems as if the sound wave bends awayfrom the corresponding side wall, Fig. 7.

In such cases, the probability of crack de-tection is very good if the angle reflectioneffect is used, Fig. 8a. At the 90˚ edge,between the crack and the surface of thetest object, the sound waves are reflectedback within themselves due to a double reflection, Fig. 8b.

Use of the angle reflection effect is ofteneven possible when a plane discontinuity,which is vertical to the surface, does notextend to the surface and under the condi-tion that the sound wave reflections at thediscontinuity and the surface are receivedby the probe, Fig. 9.

Fig. 6 Reflection on angled plane discontinuity

Fig. 7 Apparent deformation of the sound beam on a side wall Fig. 8a Crack detection with 45˚ scanning

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Often in thick-walled test objects, in whichthere are vertical discontinuities, this con-dition cannot be fulfilled so that the re-flected sound waves from the discontinuityand the surface of the test object do notreturn to the probe. In this case, a secondprobe is used for receiving the reflectedportions of sound thus enabling detectionof the discontinuity.

With this type of testing, the TandemTechnique, one probe is used as a trans-mitter, and the other probe is used as thereceiver. Both probes are moved over thesurface of the test object and are spacedapart at a fixed distance. Scanning is madefor vertically positioned discontinuities atdifferent depths of the test object, depen-ding on the probe spacing, Figs. 10a, 10band 10c.

Although, with angle scanning in thin testobjects, there is a possibility that plane di-scontinuities cannot be vertically hit, Fig.11 a, the detection sensitivity is much bet-ter, especially by suitable selection of thescanning angle and the test frequency sothat the user favours the single probe test

Fig. 8b Angle reflection effect Fig. 10a Angle reflection effect

Fig. 9 Plane, vertical reflector near the surface

Fig. 10b Tandem testing: center zone

Fig. 10c Tamden testing: lower zone

Fig. 11a 70˚ scanning: unfavourable angle

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as opposed to the more complicated tan-dem method. This is normally the casewhen testing welds up to a thickness ofabout 30 mm.

Of course the possibility of detecting dis-continuities which are not vertically hit isreduced. However, this deficiency is oftencompensated by an additional test withanother angle of incidence, Fig. 11 b, or byusing a probe with a lower frequency, Fig.11 c. A typical procedure can be found inthe corresponding specifications (test in-structions) for weld testing.

Fig. 11b 45˚ scanning: favourable angle

4. Method of testing and instrument technology

4.1 The ultrasonic flaw detector

Before we concern ourselves with furthertest tasks and their solutions, we mustfirstly acquire more detailed knowledgeabout the most frequently applied ultra-sonic technique, including test instrumentsand probes. Based on what has alreadybeen stated concerning the location of dis-continuities, we must transmit short soundpulses into the test object in order to measure the sound pulse's time of flightfrom the probe to the reflector and back.This is only possible when there is a clearlydefined start time and target time. As longas the test object's sound velocity is knownit is then possible to determine, using simple calculation, the distance of the reflector and thus its exact position in thetest object, Fig. 12.

Sound reflections in the audio range arecalled echoes (think of the yodeler in themountains). Therefore why should we notuse this short appropriate term for the re-flection of an ultrasonic pulse? Thus thename of the method came into being whichis applied in most areas of application for

Fig. 11c 70˚ scanning with 2 MHz; detection by largedivergence of the sound beam

Fig. 12 The priciple of time of flight measurement

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material testing with ultrasonics: the PulseEcho Method, Fig. 13.

The time measurement starts with the electrical transmission pulse, the initialpulse. This is an extremely short electricaldischarge which triggers a sound pulse atthe probe crystal. This pulse travelsthrough the material and is reflected by adiscontinuity or the opposing wall and re-turns back to the probe. The received os-cillations are converted into an electricalpulse which stops the time measurement.The distance to the reflector can now beinstantly determined by the following for-mula:

s = sound path [mm]c = sound velocity [km/s]t = time of flight [µs)

If the time of flight is graphically displayedthen we are not far from the universal Ultrasonic Flaw Detector, Fig. 14.

In order to evaluate the visual signals(echoes) on the screen there is a grid onthe inside of the CRT. The exchangeableattachment scale, which has a horizontalscale with 10 graduations is called the display scale, Fig. 15.

Using this scale, the ultrasonic operator isable to measure echoes on the display.How is this done? As already stated, theelectrical transmission pulse triggers thesound pulse at the probe crystal. At thesame time this voltage pulse is feed to theinput of the amplifier so that the high vol-tage causes a vertical deflection of the dis-play sweep, this is called the initial pulse,Fig. 16a.

With this initial pulse, the sweep starts inthe lower left corner of the display syn-chronous to the start of the sound pulse inthe test object and moves along the baseline at a constant speed to the right,

Fig. 13 Block diagram: Pulse Echo Method

Fig. 14 Ultrasonic Testing in practice

Fig. 15 The Display scales = ct

2

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Fig. 16b. The speed of the pulse is depend-ent on the material of the test object (soundvelocity = material constant). The sweepspeed of the instrument's display can bevaried within wide limits.

Thus the speed of the display sweep canbe exactly matched to the sound velocity.In our example the electron beam reachesscale division 4 while the pulse is at the opposing side of the test object, Fig. 17 a,then it will of course need the same time toreturn, i.e. the beam spot will be at the 8thscale graduation, Fig. 17 b.

The part of the sound pulse, which is trans-mitted through the couplant and into theprobe, generates a small electrical recep-tion signal at the crystal which, via the amplifier, causes vertical deflection of thebeam spot, this is the backwall echo Fig.18. The deflection takes place quickly be-cause the sound pulse is short, thereforecan only trigger a short voltage pulse at theprobe crystal. The electron beam returnsquickly back to the base line and continuesto the right, whilst the largest part of thesound pulse is reflected at the coupling

Fig. 17a Beam spot at the 4th scale graduationFig. 16a Initial pulse = Start

Fig. 16b after 10 µs

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surface and travels through the test objecta second time.

The display indications can now be allo-cated into two measurement values:

1. Horizontal position.left flank of the echo at the 8th scale graduation

2. Vertical amplitude:70% screen height

At the moment this does not tell us verymuch, however, later we will see that nearly all usable results which we obtainfrom ultrasonic testing are based on thesetwo readings. Let us take a look more closely at the current result: The high initialpulse starts at the left in front of the scalezero point. The rising flank corresponds tothe time at which the electrical signal is onthe crystal and starts the sound pulse. However, before it is fed to the surface ofthe test object it must travel through theprotection layer of the probe (probedelay). Although it is relatively thin, a shortperiod of time is required. The initial pulseis exactly shifted to the left by this periodof time, Fig. 19a.

Fig. 17b Beam spot at the 8th scale graduation

Fig. 18 Backwall echo at the 8th scale graduation Fig. 19a Straight-beam probe: initial pulse delay

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With angle-beam probes the sound pulsein the probe must travel through a muchlonger delay path made of perspex beforeit is transmitted into the test object. De-pending on the type of probe, the initialpulse delay can be so large that it no longerappears on the display, Fig. 19 b.

We already explained the echo at the 8thscale graduation before: It is the pulse reflected at the opposite wall of the testobject, the backwall echo. Now it is not toodifficult to guess how the display changeswhen there is another reflector within thesound beam, e.g. a material separation:

Fig. 19b Angle-beam probe: initial pulse delay

Fig. 20 Test object with discontinuity, display with flaw echo Fig. 21b Discontinuity near the surface

Fig. 21a Discontinuity in front of the backwall

between the initial pulse and the backwallecho another echo will appear, caused bypartial reflection of the sound wave on a discontinuity, Fig. 20.

Such an echo is called an intermediateecho. It is easy to foresee the positionchanges of the intermediate echo on thedisplay if the reflector is at different depths.Fig. 21 a+b: the position of the intermedi-ate echo on the display in relation to theposition of the backwall echo behaves thesame as the distance of the discontinuityrelated to the total thickness of the test

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object. We already know a method of determining the distance of an internalflaw; the ultrasonic tester speaks of loca-tion of the discontinuity.

4.2 Near resolution

So, what can we do when a small discon-tinuity is just below the surface of the testobject, i.e. directly in front of the probe?Can this discontinuity still be detected?The answer is no, because the inter-mediate echo is now within the initial pulse,it is therefore covered by it. Probably thereare also no further indications that there isa near-to-surface discontinuity here, Fig.22.

Or do we perhaps have a clue which willlead us to the unseen intermediate echo (anear-to-surface discontinuity)? The answeris yes, when the discontinuity is large enough and shadows a noticeablepart of the sound beam so that the back-wall echo becomes smaller, Fig. 23. If thenear-to-surface discontinuity is alsosmooth and parallel to the surface, thenthere is an echo sequence which is moreor less well formed because the pulses are

reflected many times between the surfaceand the discontinuity, Fig. 24.

In this case, the amplitudes of the echoesbecome smaller as the distance increases.The more dense the flat echoes advanceto the surface, the more the echoes of theecho sequence disappear into the initialpulse, this causes the echoes to becomeeven more dense. In such cases there is alimit to detection.

Fig. 22 A non-detectable near-to-surface discontinuity

Fig. 23 Shadowing of the backwall echo by a largernear-to-surface reflector

Fig. 24 Echo sequence of a near-to-surface discontinuity

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From everything, we see that the initialpulse is not welcome on the display, how-ever it is a technical necessity: it limits thedetectability of near-to-surface disconti-nuities. Reflectors in the dead zone, thenon-testable area immediately beneaththe surface, can no longer be detected,Fig. 25. The dead zone is dependent onthe test setup, this means from the probeand the test instrument. However, it can beminimized by suitable selection of the test-ing device.

4.3 The probe

Probes whose beams are normal to thesurface are called straight-beam probes,Figs. 1a and 26.

Most standard straight-beam probestransmit and receive longitudinal waves(pressure waves). The oscillations of sucha wave can be described by compressionand decompression of the atoms propa-gating through the material (gas, liquid andsolid), Fig 27. There is a large selection ofstraight-beam probes in various sizes andrange from frequencies of approximately0.5 MHz to 25 MHz. Distances of over10†m can be obtained thus enabling largetest objects to be tested.

The wide range enables individual match-ing of probe characteristics to every testtask, even under difficult testing condi-tions. We have already mentioned a disad-vantage of straight-beam probes which,under certain conditions, can be decisive:the poor recognition of near-to-surface dis-

Fig. 25 Dead zone: display, test object

Fig. 26 Straight beam probe

Fig. 26 Straight beam probe

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continuities due to the width of the initialpulse.

Probes whose beams enter at an angle arecalled angle-beam probes because theytransmit and receive the sound waves atan angle to the surface of the test object,Figs. 1b and 28.

Most standard angle-beam probes trans-mit and receive, due to technical reasons,transverse waves or shear waves.

With a transverse wave the atoms or mole-cules oscillate vertical to the wave's direc-tion of propagation, Fig. 29, due to the factthat excitation is made by shear force(transverse to the propagation's directiveforces). Transverse waves only occur insolid materials never in liquids or gasesbecause these do not have a shear mo-dulus and therefore do not effect any shearforces. In addition to this, they propagatemuch slower than longitudinal waves in thesame material. There is no quick reply to

the question about why angle-beam probes do not transmit longitudinal waves.In this case a detailed examination is re-quired.

4.4 Refraction and mode conversion

Inclined sound waves are almost exclu-sively generated so that they occur at anangle to the probe/test object interface,Fig. 1b. This is simply achieved by cement-ing the element onto a wedge shapeddelay path which is normally made of per-spex. If a longitudinal wave, at a fixedangle of incidence (the wedge angle), hitsthe perspex/steel interface then this waveis firstly split-up into a reflected and a trans-mitted wave, Fig. 30a.

Reflected waves obey the reflection law(angle of incidence = angle of reflection)and transmitted waves the refraction law(Snell's law):

α = angle of incidenceβ = angle of refractionc1= sound velocity in medium 1c2= sound velocity in medium 2

sin α

sin β=

c1

c2

Fig. 28 Angel-beam probes

Fig. 29 Transverse wave

Fig. 30a Refraction and reflection without transverse waves

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Moreover something strange happens: Inaddition transverse waves are created atthe sound beam's point of impact, Fig. 30b.This happens with reflection as well as withrefraction! Due to the fact that the trans-verse waves propagate at around only halfthe sound velocity of longitudinal waves,other propagation directions are automat-ically produced due to the refraction law,i.e. reflection and refraction angles.

If, with inclined scanning, this wave con-version is not taken into consideration,then location and evaluation of disconti-nuities is not possible in many cases, evendetection becomes questionable becauseone echo on the display leads to two differ-ent reflector locations depending onwhether one takes longitudinal waves ortransverse waves as a basis, Fig.31.

But where is the discontinuity? A clear answer can only be given by the operatorwhen one of the wave modes does notoccur. That is undoubtedly the precondi-tion for the universal application of angle-beam probes. This precondition can bederived from the refraction law: firstly werecognize that the refraction angle of lon-gitudinal waves is for steel approximatelytwice as large as that of the transversewaves, Fig. 30b.

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With further enlargement of the angle of incidence the angle of refraction β also in-creases until finally, at an angle of in-cidence of α = 27.5˚ (1st critical angle),the longitudinal wave, with an angle β of90˚, is refracted. This means that it runsalong the interface whilst the transversewave is still transmitted into the test object,Fig 32a.

Fig. 30b Refraction and reflection with transverse waves

Fig. 31 Evaluation: one echo – two possible reflector locations

Fig. 32a Refraction: 1st critical angle

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Our precondition for clear reflector evalua-tion is fulfilled: now only one sound waveoccurs in the test object, this is the trans-verse wave with a refraction angle of 33.3˚(for perspex/steel). With further enlarge-ment of the angle of incidence various refraction angles of the transverse wave(= beam angle) can be set, e.g. exactly45˚, Fig. 32 b. Finally, with an angle of incidence of about 57˚ (2nd critical angle), the transverse wave, with an angleof 90˚, is refracted and propagates alongthe surface of the test object, it then becomes a surface wave, Fig. 32 c.

That is the limit over which no more soundwaves are transmitted into the test object.Total reflection starts from here, Fig. 32d.

The area in which an angle of incidence ispresent between the 1st and 2nd criticalangle (27.5˚ - 57˚) gives us a clear evalu-

able sound wave in the test object (madeof steel), namely the transverse wave bet-ween 33.3˚ and 90˚, Fig. 33.

4.5 Characteristics of angle-beam probes

Due to the fact that steel is tested in mostapplications, the angle-beam probes aredesigned so that suitable angles of inci-dence are produced in steel. To achieveclear evaluation there are angle-beam pro-bes with angles of 35˚, 45˚, 60˚, 70˚, 80˚ and90˚ (surface waves), Fig. 33.

Angles of 45˚, 60˚ and 70˚ are mostly used. With regard to frequency, angle-

Fig. 32b Refraction: transverse wave under 45˚

Fig. 32d Total reflection

Fig. 33 Usable range for angle-beam probes in steel

Fig. 32c Refraction: 2nd critical angle, surface wave

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beam probes do not have such a wideselection as straight-beam probes. This isprimarily due to the fact that high frequencytransverse waves in non-alloyed fine grainsteels are subjected to high attenuation.As the sound energy of the waves travelsthrough the material it is so strongly ab-sorbed and scattered that only relativelysmall test objects can be tested with suffi-cient sensitivity.

If discontinuities have to be detected overlarger distances (in thicker test objects)then angle-beam probes with larger crys-tals and lower frequencies are to be used;e.g. a reflector with a size of 2 mm in lowalloyed fine grain steel with a 2 MHz angle-beam probe with a large crystal can be detected up to a distance of 700 mm. Thenear resolution of angle-beam probes isoften better than with straight-beam probes because the initial pulse is shiftedfar to the left due to the relatively large perspex delay path. The falling flank of theinitial pulse could sometimes still covernear-to-surface discontinuities. Figs.34a+b show, when using an angle-beamprobe, how a near-to-surface drilled hole(1 mm deep) can be reliably detected.

Fig. 36 TR probe on the test object: CRT with backwall echo

Fig. 35 TR probe: section

4.6 The TR probe

If you want to obtain a similarly good nearresolution with straight-beam scanningyou should use a TR probe, Fig. 35.

This technique uses two crystal elementswhich are acoustically and electrically se-parated from each other in the same hou-sing. In addition to this, both elements arestuck to a relatively long delay path (madeof perspex) and are slightly inclined to-wards each other. Connection of the TRprobe on the instrument is made in the TRor dual mode, i.e. one element is con-nected to the transmitter and the otherwith the input of the receiver amplifier. The initial pulse is positioned far left of the display due to the long delay path, Fig. 36.

Fig. 34a Scanning a 1 mm transverse hole at a depth of 1 mm

Fig. 34b Detection of a hole with a MWB 70-4E

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Multi-reflections within the delay path ofthe transmitter do not interfer because thetransmitter element does not have any re-ception function. Only when the soundpulses come out of the test object and intothe receiver element of the TR probe doevaluatable echoes appear on the display.

The electrical and acoustic separation is,due to technical reasons, not completelypossible. Especially high gain adjustmentsand rough test object surfaces cause por-tions of sound to be directly transferredfrom the transmitter to the receiver.

This generates an interference echo on thedisplay which is called the cross-talkecho. The cross-talk echo can exactlycover the near-surface area of the test object and once again there is a loss in detection sensitivity, especially of small discontinuities. However, most cross-talkechoes are so small, or even negligible,that they can be clearly distinguished frompossible discontinuity echoes, Fig. 37.

TR probes are therefore ideally suited forthe detection of near-to-surface disconti-nuities and for thickness measurementson thin test objects. The TR probe reactsconsiderably less sensitive to coupling variations which may be caused by roughor curved material surfaces. This charac-

teristic explains why TR probes play a valuable part in the chemical and energygenerating industries: they are ideal fortesting all types of tubes and containers,for the detection of discontinuities in tubewalls, and for measurements of inside cor-rosion and remaining wall thicknesses.

Special high temperature probes are evenable to measure the wall thickness on testobject surfaces up to about 550˚C so thatinstallations can be tested during oper-ation.

Fig. 37 TR probe on the test object: discontinuity echo in thecross-talk echo

Fig. 38 Wall thickness measurement with a digital thicknessgauge in practice

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5. Locating discontinuities

5.1 Calibration of the instrument

The location of a discontinuity can be in-stantly determined using its echo if the instrument is correctly calibrated. Calibra-tion means, linear display, from the zeropoint on the scale, of a certain distancerange of the object to be tested. The zeropoint on the scale corresponds to the sur-face of the test object and the 10th scalegraduation the maximum distance, e.g.100 mm steel, 10 mm aluminum, 25 mmbrass etc. When specifying the calibrationrange the naming of the material is alsoimportant because the displayed distanceof the echo, sound path s, is always de-duced from the time of flight t of the pulseand the sound velocity c according to theequation:

s = sound path [mm]c = sound velocity [km/s]t = transit time [µs]

This relationship is not unimportant for theultrasonic operator but it is not required forthe sequence of calibration. The rule sim-ply says:

Use a work piece of the same material asthe test object whose dimensions areknown.

By coupling the probe onto an object ofknown thickness t an echo sequence ap-pears on the display, Fig. 39. The associ-ated sound paths correspond logically tothe paths being travelled in the test object,for example with a straight-beam probe itis the multiple of the test object's thicknesst, therefore:

1st Echo = t,2nd Echo = 2t,3rd Echo = 3t, etc.

Fig. 39 USK 7: Backwall echo sequence with a straight-beamprobe

s =c · t

2

We must now adjust 2 of these echoes onthe corresponding scale graduation to therequired calibration range.

The instrument is then calibrated, i.e. byreading off the scale position T the soundpath s (distance) of the associated reflec-tor can be determined (location of reflec-tors, wall thickness measurement).

5.1.1 Calibration with a straight-beam probe

The reference piece used for calibration iscalled the Calibration Block, or StandardCalibration Block, if the block used isstandardized. The Standard CalibrationBlock 1, also simply referred to as V1block (according to BS 2704 - A2), has athickness of exactly 25 mm and is made oflow-alloyed fine grained steel so that it canbe used for nearly all types of calibrationwhen similar steels are to be tested.

Example 1: Calibration range 100 mmsteel (longitudinal waves)

The 10 scale graduations on the horizontaldisplay scale are to have a range of 0 to100†mm steel, Fig. 40. One scale gradua-tion therefore corresponds to 10 mm in thetest object. We say: the scale factor k

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(display scale) is 10 mm per scale gradua-tion.

We couple the straight-beam probe to theV1 block (laying flatwise), Fig. 39. Thebackwall echo sequence now comes fromthe 25 mm steel path. The allocation ofsound paths si to the corresponding scalepositions Ti is carried out using the calibra-tion table:

The corresponding scale position Ti is cal-culated by using the formula:

si = sound path of umpteenth echoesTi = scale position of the umpteenth echok = scale factor

The exact adjustment of echoes from thecalibration block, as in Fig. 41, is made withanalog ultrasonic flaw detectors using thecontrols pulse shift (or delay) as well ascoarse and fine ranges.

In doing this, the adjustments must be alternately carried out at these points untilthe echo flanks are at the correct scale positions. With modern digital instrumentsthe calibration range of 100 mm and thesound velocity of 5920 m/s are firstly en-tered. After coupling the probe to the cali-bration block, the function delay or probedelay is changed until the echoes are cor-rectly positioned, Fig. 42.

Fig. 40 Calibration range: 0-100 mm

Fig. 42 USK 7 D: Consideration of the probe delay

Fig. 41 USK 7: Calibration in the 100 mm range

Echo-No Sound Scale Skalen-i path si factor k position Ti

[mm] [mm/scale [scalegrad.] grad.]

1 25 10 2.5

2 50 10 5.0

3 75 10 7.5

4 100 10 10.0

sik

Ti =

23

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Example 2: Calibration range of 250 mm in aluminum

10 scale graduations correspond to 250mm in aluminum: k = 25 mm/graduation.We couple the straight-beam probe to analuminum test block which is 80 mm thick,i.e. a backwall echo sequence is producedfrom this thickness (t = 80 mm), Fig. 43.

The calibration table now looks like this:

Exact reflector location is only possibleafter correct calibration of a test instru-ment. The ultrasonic operator moves theprobe over the test object. In a normalcase, i.e. when a discontinuity does notexist, only the initial pulse and the backwallecho are visible on the display. As soon asa discontinuity is within the area of thesound beam, an additional echo appears

between the initial pulse and the backwallecho, Fig. 44, e.g. an echo at scale grad-uation 1.4. With calibration in the 250 mmrange the distance to the reflector s is therefore 1.4 x 25 = 35 mm.

5.1.2 Calibration with a TR probe

For technical reasons, the calibration witha TR probe can only be made to a certainextent using a backwall echo sequencefrom a comparison object. Due to the slightangular beaming, Fig. 35, transversewaves occur with the TR probe whichcause strong interference behind the 1stbackwall echo so that the 2nd backwallecho is often unable to be identified. Therefore, a stepped calibration block isused for the adjustment of both echoes,alternately going between two steps (2point calibration).

Example 3: Calibration range for 10 mmsteel

Step block VW (steel: 1 - 8 mm). The 3 mm and6 mm steps should be used for calibration.The step selection depends on the depthrange of the expected reflectors. Here the echo from 3 mm must be adjusted to the 3rd scale graduation and the echo

Echo-No Sound Scale Skalen-i path si factor k position Ti

[mm] [mm/scale [scalegrad.] grad.]

1 80 25 3.2

2 160 25 6.4

3 240 25 9.6

Fig. 43 USK 7 D: Calibration of a 250 mm range with an80 mm aluminum path

Fig. 44 USK 7 D: Sound path measurement

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from 6 mm to scale graduation 6, Fig.45a+b.

a) Firstly, we couple the TR probe to the3†mm step and use the delay control foradjusting the echo flank to the 3rd scalegraduation.

b) Now we couple the probe to the 6 mmstep and bring the echo to the 6th scalegraduation with the range control.

c) Steps a) and b) are alternately repeateduntil both echo flanks are exactly on the3rd and 6th scale graduations, Fig.45a+b.

The main application of TR probes are wallthickness measurements, but they are alsosuitable for the detection of near-to-surfa-ce discontinuities, Fig. 46a+b.

Fig. 45a The two positions (3 mm and 6 mm step) of the TRprobe on the stepped calibration block VW

Fig. 45b Calibration echo at the 3rd graduation (top)Calibration echo at the 6th graduation (bottom)

Fig. 46 b Detection of the drill hole from Fig. 46 a

Fig. 46a Probe DA 312 on a speciemen with a side drilled holein a depth of 1 mm

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5.1.3 Calibration with an angle-beam probe

For calibration of the test instrument withan angle-beam probe the standard calibra-tion block 1, Fig. 47a, and the calibrationblock V2 (according to BS 2704 - A4), Fig. 47b, are almost exclusively used because no backwall echo sequence is received due to the angular beaming froma plane-parallel calibration block.

The advantage with echoes from the circlesegment of the calibration block is that thesame sound path is always given inde-pendent of the probe angle, Fig. 48.

When the angle-beam probe is exactlycoupled in the center of the circle segment,a first echo is exactly received from 100mm out of V1 block. According to the re-flection law, the sound waves coming outof the arc are reflected away from the

Fig. 47a WB 60-2E on Calibration Block 1

Fig. 48 Different probe angels at V1 block

Fig. 47b MWB 45-4E on Calibration Block 2coupling surface to the back, this meansaway from the arc, Fig. 49a.

A second echo out of the arc, needed forthe calibration sequence, cannot thereforebe produced. For this, there are two sawcuts made in the center of the quartercircle: in the edges, which these saw cutsform with the surfaces, the sound wavesare reflected back within themselves dueto double reflection (angle reflection ef-fect) so that they go back to the arc, Fig.49b.

Fig. 49a Sound path in the V1 block without angle reflection

Fig. 49b Sound path in the V1 block with angle reflection

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Because the radius of the circle segment is exactly 100 mm we will regularly receivean echo sequence with distances of 100mm, 200 mm, 300 mm etc. with which weare able to carry out calibration of the testinstrument the same way as the straight-beam probe. Fig. 50 shows calibration ofthe 250 mm range.

For the miniature angle-beam probe oneuses the considerably smaller and lighterStandard Calibration Block 2 (V2 block).This has, as opposed to the V1 block, twocircle segments with a common centerpoint, however it does not have saw cuts.The required echo sequence is producedhere by the alternating reflection of thesound waves, Fig. 51a+b.

The corresponding echo sequence is pro-duced according to whether the probebeams into the 25 mm radius or the 50 mmradius. No echoes appear with soundpaths by which the sound pulses from the"wrong" direction meet at the center pointbecause these pulses are absorbed by thefront damping element of the probe.

Fig. 52 shows calibration of the 100 mmrange by scanning into the 25 mm radiusof Standard Calibration Block V2.

Fig. 50 Range: 250 mm with a WB 60-2 on V1 block Fig. 51b Path of a sound wave in a V2 block, radius 50 mm

Fig. 51b Path of a sound wave in a V2 block, radius 25 mm

Fig. 52 Range: 100 mm calibrated on V2, radius 25 mm

practical reasons, the reduced surfacedistance is used because this is measured from the front edge of the probe.The difference between the surface dis-tance and the reduced surface distancecorresponds to the x-value of the probe,this is the distance of the sound exit pointto the front edge of the probe, Fig. 54b.

With ultrasonic instruments having digitalecho evaluation these calculations are naturally carried out by an integratedmicroprocessor and immediately dis-played so that the operator does not needto make any more time-consuming calcu-lations, Fig. 55.

This is of great help with weld testing be-cause with the calculation of the flaw depthan additional factor must be taken into account, namely: whether the sound pulses were reflected from the opposingwall. If this is the case then an apparent

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5.1.4 Locating reflectors with anangle-beam probe

The echo of a discontinuity on the instru-ment display does not now give us any direct information about its position in the material. The only available information for determination of the reflector position is the scale position and therefore the soundpath s, this means the distance of the discontinuity from the index point (sound exit point) of the probe, Fig. 53.

The mathematics of the right-angled triangle helps us to evaluate the Surface Distance and the Depth of a reflector which are both important for the ultrasonictest, Fig. 54a.

We therefore now have the possibility to in-stantly mark a detected flaw's position onthe surface of the test object by measu-rement of the surface distance from thesound exit point and to give the depth. For

Fig. 53 Scanning a reflector using an angle beam probe

Fig. 54b Reduced surface distances and x-value

Fig. 55 USN 50: A hole being scanned with the probeMWB 60-4E

Fig. 54a The flaw triangle

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depth of the reflector is produced by usingthe depth formula which is greater than thethickness T of the test object.

The ultrasonic operator must acertainwhether a reflection comes from the oppo-site wall and then proceed with calculatingthe reflector depth, Fig. 56b.

6. Evaluation of discontinuities

Of course, a discontinuity is best evaluatedwhen its size (extension) is known. Theoperator's wish to accurately know the"real reflector size" is understandable therefore it is expected that an nondestructive testing method, such as ultrasonic testing, give this information.However, due to the fact that on the displayonly the echo can be interpreted, thismeans the reflected sound coming fromthe discontinuity, it is very often difficult,and in some cases even impossible, to reliably assert the size of the reflector. Infact, the echo height plays the decisive partwhen evaluating discontinuities during ma-nual Ultrasonic Testing.

6.1 Scanning method

In ultrasonic evaluation one is frequentlyable to come near to the true reflector sizeas long as the discontinuity is large com-pared to the diameter of the sound field.The discontinuity then reflects the com-plete impacting energy back, Fig. 57.

By scanning the boundaries of the discon-tinuity, reliable information can be ob-tained about its extension. The ultrasonicoperator normally observes the height ofthe discontinuity echo. The probe positionon the test object at which the echo dropsby exactly half indicates that the disconti-nuity is only being hit by half the soundbeam, Fig. 58a.

This means that the acoustic axis is exactlyon the boundary of the discontinuity. Theprobe position is marked and the operatordetermines further boundry points until acontour of the discontinuity is formed byjoining the marked points together, Fig.58b.

Fig. 56a The apparent depth

Fig. 56b The real reflector depth after sound reflection

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Location of the reflector boundry becomesmore exact the smaller the diameter of thesound beam is at the reflector position.Therefore, if the reflector extension is to beexactly measured it is recommended thata probe be selected which has its focalpoint at the same distance as the reflector.TR probes are especially suited whichhave a hose-shaped sound beam with asmall diameter (1 - 3 mm) at their mostsensitive depth range.√

6.2 Evaluation of smalldiscontinuities:The DGS method

A reflector which is completely containedwithin the sound beam is regarded as asmall reflector. If such a reflector is evalu-ated by scanning then it is not the size ofthe reflector which is obtained as a resultbut the diameter of the sound beam! Therefore, the scanning method is notpractical in this case. We have noticed previously that the height of a reflectorecho will become greater the larger thesound beam area is which covers the re-flector. This feasible behavior can be usedon small reflectors: their echo heights in-crease with their areas, Fig. 59.

Fig. 57 A large reflector in the sound beam

Fig. 58a Straight beam probe on the reflector boundry

Fig. 58b Top view with reflector for extension

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Under optimal conditions, e.g. drill holeswith flat bottoms and at equal depths, thislaw can be confirmed:

Example: The flat-bottom hole with a dia-meter of 2 mm has an echo which is 4times that of a 1 mm flat-bottom hole be-cause the area has quadrupled.

However, if the echoes from two drill holesat different depths are compared then anadditional distance dependence of theecho heights is established, Fig. 60.

Fig. 59 Reflectors with different areas and their echoes

Fig. 60 Reflectors at different depths and their echoes

The echo heights are propor-tional to their area orThe echo heights are propor-tional to the square of theirdiameter

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With accurate tests using flat-bottom holesat different depths a simple law can befound, at least in the far field of the appliedsound beam:

This does not normally apply to the near-field of the sound beam! Here, the test results show that the echo heights withinthe focus reach their highest amplitudeand are reduced again at shorter dis-tances, Fig. 61.

If such curves are put on transparent scales having the CRT format then we immediately have the possibility to com-paratively evaluate echoes from unknownreflectors and those from natural reflec-tors, i.e. the echo height of the disconti-nuity is compared to that of a circular disk.The discontinuity in Fig. 62 reflects thesound waves the same as a circular diskhaving a diameter of 4 mm.

Due to the fact that we can only assess thesound reflected from the discontinuities wemust of course not equate the diameter of4 mm with the "true size" of the disconti-nuity. We therefore refer to them as anequivalent disk-shaped reflector or asequivalent reflector size (ERS). The equivalent reflector size only corresponds

The echo heights inversely reduce to the square of their distance

to the true reflector size of a discontinuityin an ideal case which is when it is circularand exactly hit vertical to the acoustic axis.

In practise this almost never occurs whichmeans that the true size of a discontinuityis normally larger than the equivalent re-flector size. A law for this cannot be derivedbecause the echo height is strongly de-pendent on the characteristics of the dis-continuity, this means its geometry, orientation to the sound beam and the surface quality. For example, a pore (spherically shaped gas inclusion) with adiameter of 2 mm has an equivalent reflec-tor size of 1 mm; an angled flat reflector 5mm long gives, according to orientation, aresult of ERS 0 (not detectable) to perhapsERS 2.

This uncertainty in the evaluation of the discontinuity is however neutralized whenother possibilities and techniques in ultra-sonic testing are used to inspect detecteddiscontinuities closer. An experienced ultrasonic operator can, without additionalexpense, accurately give informationabout the discontinuity which he has de-tected. Scanning the discontinuity from dif-ferent directions, assessing the echoshape and the behavior of the displaywhen moving the probe (echo dynamics)are just a few techniques which can besuccessfully applied.

Despite the remaining uncertainty withevaluation of natural discontinuities the

Fig. 61 Distance amplitude curve of a 2 mm – disk reflector

Fig. 62 Evaluation of a discontinuity (F) using evaluationcurves

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above method of discontinuity evaluationis applied in many countries due to the factthat the method is based on well provenlaws in the sound field. It is therefore re-producible, i.e. the evaluation results areindependent of testing device and operator.

The socalled DGS scales or discontinuityevaluation can be obtained from the probemanufacturer for many probes and variouscalibration ranges. DGS means that thescale is allocated an echo at the Distance,with correctly set Gain and (equivalent re-flector) Size. However, the modern versionof the DGS scale would need some expla-nation because it was developed to fulfillthe requirements of the most common spe-cifications in practical testing: If, on a cer-tain test object whose purpose and therefore stress values are known, an ultrasonic test is to be carried out thenfirstly, if necessary with destructive testing,it should be established how large the per-mitted material flaw should be. Of course,the position of such a flaw in the materialand its rate of occurance play a part.

If a permitted flaw size has been deter-mined then this size is multiplied with thesafety factor which, amongst others, alsotakes the evaluation uncertainty of the ultrasonic test into account. The cor-responding echo amplitude curve for thissize is now of importance for the ultrasonictest. The ultrasonic operator scans the testobject with the probe and only needs to record the indications which exceed thisrecording curve, Fig. 63.

Consequently, only one curve is necess-ary for the evaluation. Due to the fact that,depending on the application, different recording limits occur, it must be possibleto allocate other equivalent reflector sizesto this curve. This allocation is shown by atable positioned at the top right of thescale: starting from a defined default set-ting of the instrument, the auxiliary gainis taken from the table which belongs to therequired recording value and added to thegain controls. If the correct range calibra-

tion has been made then test object scan-ning can now begin. When an indicationfrom the test object exceeds the recordingcurve then this result is to be recorded inwriting and evaluated. If required, the testinstructions provide the following measures: rejection, repair or further testsfor exact assessment of the discontinuity(diagnosis).

Fig. 64 shows testing of a forged part. Therecording curve corresponds to EquivalentReflector Size 3. The detected disconti-nuity, at a depth of 110 mm, exceeds thecurve, i.e. all reflector data must now berecorded into a predetermined form.

Fig. 64 Discontinuity evaluation with a DGS scale

Fig. 63 DGS scale for the probe B 4 S

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6.3 Sound attenuation

In addition to the laws which establish thebehaviour of disk shaped reflectors withinthe sound beam of a probe (distance andsize laws) another effect can be observed:The sound attenuation. The sound atte-nuation is caused by the structure of thetest object but is also strongly dependenton the frequency and the wave mode of theapplied probe. Only when these effects areknown can they be considered by the dis-continuity evaluation. However, the eva-luation becomes more difficult, time-consuming and more unreliable so thatDGS evaluation can be burdened with tolerances which are too great.

6.4 The reference block method

These uncertainties in evaluation can bereduced when there is a socalled refer-ence block available which is made of thesame material as the object to be testedand which also contains artificial reflectorswhose echoes can be directly compared tothe discontinuity echoes from the test ob-ject. The application of the referenceblock method is, in practise, made in twodifferent ways:

6.4.1 Comparison of echo amplitudes

The test object is tested with a high gainsetting by which the smallest detectablereflector is displayed. An echo indication ispeaked, i.e. the maximum echo indicationis achieved by careful movement of theprobe and the echo peak set by adjustmentof the gain to a predetermined height, e.g.80% CRT screen height (referenceheight), Fig. 65.

Fig. 65 Test object with a flaw: echo at 80% (reference height)

Fig. 66 Reference block: reference echo at 30%

Using the same settings, the reflector fromthe reference block is scanned which isapproximately positioned at the same dis-tance as the discontinuity, Fig. 66.

The quantative unit for evaluation is nowthe gain change of the ultrasonic instru-ment which is necessary to set the refe-rence echo to the reference height, Fig. 67.

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Result: The discontinuity echo is 8 dB higher than the reference echo becausethe gain must be increased by 8 dB (from34 dB to 42 dB).

The recording limit normally correspondsto the echo height of the reference reflectorwhose size is to be determined, the sameas the DGS method, before the ultrasonictest.

6.4.2 Distance amplitude curve

All reflectors in the reference block arescanned before the test, their maximumecho heights marked on the attachmentscale of the display and joined by a curve,Fig. 68.

The curve produced is called the DistanceAmplitude Curve, or DAC for short. Whena discontinuity echo appears, an immedia-te assessment can be made whether ornot the discontinuity echo exceeds theDAC. In addition to this a determination ismade, by a corresponding gain change, to

Fig. 67 References block: reference echo to reference height

Fig. 68 Reference block wiht side drilled holes and resultingechoes

see by how many dBs an echo exceedsthe curve. This excess recording echoheight (EREH) is our reproducible measure for the evaluation and reportingof the discontinuity.

The advantages of the reference block method with a DAC are:

1. that it is no longer necessary to compareeach discontinuity echo with the corre-sponding reference echo from the refe-rence block but to directly make theevaluation with the DAC.

2. that the heavy reference block need notbe transported to the testing location.

3. that the recording of a DAC for certainapplications is only required once be-cause the curve is documented on atransparency or in the memory of a modern ultrasonic test instrument.

By recording the curve using reflectors in a test object comparable to the work piece,this curve contains all the influences in thetest object (distance law, sound attenua-tion, surface losses). Corresponding cor-rections are therefore not necessary. Regarding the evaluation results, we must

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Fig. 70a Weld testing with the USD 10

understand here that the effect of the discontinuity (geometry, orientation andsurface quality) is not taken a great dealinto account the same as the DGS method.Therefore, the result of a discontinuity evaluation with the reference block method has the same uncertainty as theDGS method.

The preference regarding which method touse is subjective. The corresponding na-tional test specifications normally state thetest method to be used so that the operatoris not able to make his own decision. If nodata is available, the test situation shouldbe analyzed in order to decide which method be best used:

Firstly, it must be established whether a reference block exists which correspondsto the test object. If yes, then the test canbe carried out simply and reproducibly withthe reference block method. If no referen-ce block is available then the DGS methodcan be used, or a reference block must besubsequently produced comparable to thetest object.

However, in many cases the DGS methodcan be used without difficulty, namelywhen the test object is made of low alloysteel, has a simple geometry, a low soundattenuation and an even surface quality.The test should be carried out with a nar-row band standard probe with a frequencybetween 1 MHz and 6 MHz for which thereis a DGS diagram or a DGS scale.

The new computer controlled instrumentsnormally support the program controlledrecording of DACs. With the USD 10 therecorded DAC is automatically convertedto a horizontal line. This is known as timecorrected gain (TCG), Fig. 69.

The recording curve is therefore an hori-zontal line so that the evaluation can be visually and acoustically supported usinga monitor gate (flaw alarm), Fig. 70a-c. Atthe same time for each echo, the excessrecording echo height is displayed in dB(DBR value in the measurement line of the

USD 10) in addition to the data for discon-tinuity location.

Of course, the recorded curves, includingthe complete instrument settings, can bestored. It is therefore guaranteed that anylater tests can be carried out with the samesettings. By storing the instrument set-tings, including the active A-Scans withthe discontinuity echo, the operator has allthe data available for producing a test reportat any time.

Fig. 69 DAC of the reference echoes (top) and with timecorrected gain (bottom)

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7. Documentation

The higher the value of a test object or thegreater the importance of a componentwith regard to the safety requirements ofthe complete object, e.g. an aircraft bolt,then the more important the required ul-trasonic documentation becomes (pro-duct liability). On the one hand this documentation proves that the test wascompletely and correctly carried out, andon the other hand enables the test to be repeated at any time under the same con-ditions (test reproducibilty).

The documentation, the test report of anultrasonic test, roughly consists of 3 parts:

a. Data concerning the test object and thetest method.

b. Data concerning the testing device.

c. Results of the test:Typical tabular list of all detected andevaluated discontinuities, refer to theexample report, if necessary, a drawingof the test object with flaw positions.

The operator must record data during thetest, especially data of the detected dis-continuity. Creation of the actual report isnormally made later in the office. The rec-ordings must be frequently supplementedby calculations before the operator trans-fers them into the report. The creation of atest report very often takes as much timeas the actual test itself and should be takeninto account when determining test expen-diture. Even when working conscien-tiously, transfer or calculation errors cannot be excluded, especially with largeamounts of data.

Modern ultrasonic instruments with mem-ory and data transfer functions greatly im-prove recording of the adjustment data andtest results during the test as well as cre-ation of the test report and therefore easethe operator of a burden, who can thenconcentrate on the test task. By directly

Fig. 70 b A discontinuity to be recorded, DBR 14,4 dB

Fig. 70c A discontinuity not to be recorded; DBR -9.2 dB

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storing of all discontinuity data in the instru-ment, transfer errors can be excluded, position coordinates of the detected discontinuities are entered into the instru-ment and are therefore contained in thestored data set along with the A-Scan. Ifrequired, the documentation can be printed at the test location, Fig. 71.

Individual documentation matched to thetest task is carried out with a PC. The possibilities for measurement data pro-cessing are as versatile as the programsoffered on the market. In order to make theapplication of data communication with adigital ultrasonic instrument flexible, a utility program is very often used. This isnormally a program which is easy to ope-rate and enables reliable data transfer be-tween PC and test instrument.

In addition to this, all stored instrumentsettings (= Data set), the correspondingadjustment parameters (= Function lists),the display contents (= A-Scans) as wellas the individual parameters can be re-called and stored on a floppy disk or harddisk. Function lists are filed into an ASCIIformat, A-Scans and LCD menus in normalgraphic formats (PCX, IMG). A data setcorresponds to a complete instrument ad-justment and is saved in a hexadecimalformat. This guarantees that by returntransfer of the data set to the test instru-ment exactly the same instrument settingsare available. The function lists and graphics are now available to the operatorfor further use.

Now he is able to use his own program(word processing, database) for individualdesign of his own test documentation. In-strument settings and display graphics aresimply read into his program. This routinework is easily made by a so called Macro.A macro is nothing more than a list of com-mands belonging to the user programbeing used and runs automatically thuscreating the required documentation. Thenames of the files to be processed are re-quested, in dialog, by the operator.

The following report is an example of do-cumentation automatically produced for aweld test. The test was made with the USK7 D and the indications from the disconti-nuities stored in the instrument. Finally, alldata (function lists and A-Scans) were sto-red on a disk using the program UltraDoc.The test report was made with a macrofrom the program WordPerfect 5.1:

Fig. 71 USD 10: My Choice table with flaw data

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Ultrasonic test report(example referring to AD-Merkblatt HP 5/3)

Manufacturer SLV-Duisburg Weld test Specimen No. 1Weld type: V Weld preparation: 30˚Welding method: Covered electrode Parent material: ST 52Added material: unknown Test volume: Weld + HAZTest surface: 1,2 (see drawing) Test surface condition: evenly roughWeld thickness: 25 mm Weld width 32 mmWeld length 200 mm Reference points: see drawingMisc.: Evaluation with DGS

scaleTest instrument: USK 7 D Calibration range: 200 mmSensitivity setting: ERS 3 + 6 dB Corrections: + 14 dB transfer lossProbe: MWB 70 4E Frequency: 4 MHzAngle of incidence: 70˚ Scanning position: see drawingStandard Calibration V2 Reference block: R25/Cblock:Couplant: ZG 5 Pre-test: nothing foundTest class: b Recording threshold: ERS 3

Indications to be recorded:

No. Sec. l1 lmax l2 t dt d Dir dB Findings / remarks1 1 0 20 30 0 4 21 2 + 18 Root crack2 1 0 30 32 - 1 3 23 1 + 6 Root crack3 1 166 191 201 0 2 21 2 + 6 Root crack4 1 163 168 201 - 1 2 22 1 + 2 Root crack5 1 – 126 – 2 – 19 1 - 8 Pore/small inclusion near

edge6 1 – 50 – 0 – 22 6 - 6 Transverse crack

Test location: Cologne Date: 12.3.1991Test result: Repair necessary

Operator: J. Smith Supervisor: H. MüllerSignature:

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8. Diagnosis of indications (outlook)

As opposed to the tasks of an ultrasonicoperator, dealt with up until now, the diag-nosis of indications is not only based onmeasured ascertainable parameters. Theinterpretation of the echo is an additionaltask. This interpretation normally requiresmany years of practical experience includ-ing carrying out comparsion tests of ultra-sonic findings with the results of a destructive test on a test object which isno longer to be used.

Methods for diagnosis of indications aswell as signal analysis techniques usingauxiliary equipment are outside the bounds of this introduction to ultrasonictesting. For this, there is a range of lit-erature available as well as many specialpublications which we would like to pointout to the reader.

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Literature list

M. Berke, U. Hoppenkamps: "Testing ma-terials ultrasonically"Krautkrämer Training System, Level 13rd edition (1990)

M. Berke, U. Hoppenkamps: "Testing ma-terials ultrasonically"Krautkrämer Training System, Level 23rd edition (1986)

M. Berke, U. Hoppenkamps: "Practicaltraining with digital ultrasonic instruments".Krautkrämer Training System, Part 4 3rd edition (1992)

M. Berke: "Thickness measurement withultrasonics"Krautkrämer Training System, Part 5 2nd edition (1992)This edition is only available in German.

H.-W. Corsepius: "Nondestructive testingof materials using ultrasonics, introductionto basics"Special Issue 218 (1990)Krautkrämer GmbH & Co.

J. and H. Krautkrämer:"Ultrasonic testing of materials"4th edition (1990)Springer-Verlag