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5. CALIBRATION OF THE TESTING SYSTEM

5.1 PURPOSE OF CALIBRATION

Ultrasonic test equipment provides a number of basic functions. These include thegeneration of an elastic wave, the reception of ultrasonic signals, signal conditioning and

processing, discontinuity signal, gating and signal presentation. Codes usually specify therequired instrument capability. To ensure that this can be achieved, equipment shall always

be properly calibrated. Calibration of equipment is most important if accurate testing andreliable results are to be obtained.

n ultrasonic testing calibration means the verification and ad!usting of ultrasonic equipmentcharacteristics so that reliable and reproducible test results are obtained. The calibration

procedure used in ultrasonic testing can be classified into"

#i$ %quipment characteristics verification.

#ii$ &ange calibration.

#iii$ &eference level or sensitivity setting.

5.2 STANDARD TEST BLOCKS

5.2.1 Calibrati ! a!" r#$#r#!%# bl %&'

n ultrasonic pulse echo testing test bloc's containing notches, slots or drilled holes are usedto"

#i$ (etermine the operating characteristics of the flaw detector and probes.

#ii$ %stablish reproducible test conditions.

#iii$ Compare the height or location of the echo from a flaw in the test specimen to thatfrom an artificial flaw in the test bloc'.

The bloc's used for the first two purposes are termed calibration bloc's, while test bloc'sused for the third purpose, are 'nown as reference bloc's. The same test bloc' may be usedas a calibration or reference bloc'. Test bloc's whose dimensions have been established and

sanctioned by any of the various groups concerned with material testing standards are calledstandard test bloc's. )ome of the commonly used test bloc's along with their uses are asfollows"

5.2.2 I.I.(. %alibrati ! bl %&

The most versatile calibration bloc' is the bloc' made from medium carbon ferritic andnormali*ed steel described by the nternational nstitute of +elding # . .+$ and proposed bythe nternational )tandard rgani*ation # .). .$. This bloc', called the . .+. 1 bloc', is asshown in igure 5.1.

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igure 5.1 " Test bloc' . .+. # 1$ for calibration of an equipment with normaland angle probes #all dimensions are in millimetres$.

This bloc' is generally used for"

#i$ The calibration of the time base using /5 mm, 100 mm and /00 mm thic'ness withthe normal beam probes and 100 mm quadrant for angle beam probes.

#ii$ The determination of probe inde using 100 mm quadrant.

#iii$ The determination of the probe angle using plastic wedge and degrees stamped onthe side of the bloc'. The angle beam transducer is sub!ect to wear in normal use.This wear can change the probe inde and the probe angle.

#iv$ The chec'ing of performance characteristics of the ultrasonic flaw detector such as"

- Time base linearity.- )creen height linearity.- 2mplitude control linearity.- &esolving power.- 3enetrative power.- (ead *one chec'.- 3ulse length.

#v$ The setting of sensitivity.

#vi$ The calibration of time base and sensitivity setting for (4) method.

#vii$ The comparison of various materials due to their different acoustic velocities.

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5.2.) *2 %alibrati ! bl %&

This is a more compact form of the 1 bloc' suitable for site use, although somewhat lessversatile in its function. t is also described by nternational nstitute of +elding # . .+$. Thelatest version of this bloc' is shown in igure 5./. t is particularly suitable for short near

field lengths and the time base calibration of small diameter normal and angle probes.

This calibration bloc' is made of steel with the dimensions in millimetres as shown inigure 5./. The tolerances are ±0.1 mm e cept on the length of the engraved scale where

they are ±0.5 mm. This calibration bloc' contains two quadrants having radii of /5 mm and50 mm. 3oint 627 is the common focal point for both quadrants. The uses of the bloc' are asfollows"

igure 5./ " . .+ / calibration bloc'.

5.2.+ ASME r#$#r#!%# bl %&

The bloc' is used to construct a distance-amplitude-correction #(2C$ curve on the C&T

screen by noting the changes in echo amplitude from the hole with change in scanningdistance #multiple s'ip$. This bloc' is made from the same material as that of the testspecimen and contains side drilled holes. Thic'ness of the bloc' and the diameter of the sidedrilled holes depend on the thic'ness of the test specimen. 2 typical 2)8% reference bloc' called the basic calibration bloc' #9C9$ used for the inspection of welds is shown in

igure 5. . )pecified calibration reflectors for different weld thic'nesses are as shown inTable 5.1.

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igure 5. " 2)8% basic calibration bloc' #9C9$.

T29;% 5.1 " )3%C %( C2; 9&2T < &% ;%CT &)

+eld thic'ness #t$ 9asic calibration bloc' thic'ness T

=ole diameter <otch si*e

1 in #/5.: mm$ or less >: in #1?.0 mm$ or t

> / in #/. @ mm$ +idth A 1>@ in # .1B mm$to 1>: in # . 5 mm$

ver 1 in #/5.: mm$

through / in #50.@ mm$

1-1>/ in # @.1 mm$

or t

1>@ in # .1B mm$ --

ver / in #50.@ mm$through : in #101. mm$

in #B ./ mm$or t

>1 in #:.B mm$ (epth A / D T

ver : in #101. mm$through in #15/.: mm$

5 in #1/B.0 mm$or t

1>: in # . 5 mm$ --

ver in #15/.: mm$through @ in #/0 ./ mm$

B in #1BB.@ mm$ ort

5>1 in #B.?: mm$ ;ength A / in#50.@ mm$

ver @ in #/0 ./ mm$through 10 in #/5: mm$

? in #//@. mm$or t

>@ in #?.5/ mm$ --

ver 10 in #/5:.0 mm$ -- -- --

4eneral notes"

#a$ =oles shall be drilled and reamed a minimum of 1-1>/ in # @.1 mm$ deep andessentially parallel to the e amination surface.

#b$ <otches may be provided as required.

#c$ The tolerance for hole diameter shall be ±1> / in #0.B? mm$. The tolerance on notchdepth shall be E10 and -/0D. The tolerance on hole location through the thic'nessshall be ±1>@ in # .1B mm$.

#d$ or each increase in thic'ness of / in #50.@ mm$ or fraction thereof, the holediameter shall increase 1>1 in #1.5@ mm$.

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5.2.5 Ar#a,a- lit/"# bl %&'

2rea-amplitude bloc's provide artificial flaws of different si*es at the same depth. %ight bloc's made from the same #50 mm$ diameter round stoc', each - >:F #?5./5 mm$ inheight, constitute a set of area-amplitude bloc's. The bloc' material must have the same

acoustic properties as the test piece material. %ach bloc' has a >:F #1?. mm$ deep flat bottom hole drilled in the centre of the bottom surface # igure 5.:$. The hole diameters varyfrom 1> :F #0.: mm$ to @> :F # .1B mm$. The bloc's are numbered to correspond with thediameter of the holes, that is bloc' <o. 1 has a 1> :F #0.: mm$ diameter hole, and so on upto <o. @, which has an @> :F # .1B mm$ diameter hole. )imilar area-amplitude bloc's madefrom 1-15>1 F #:?./ mm$ square stoc' are sometimes 'nown as 2lcoa series-2 bloc's.

igure 5.: " 2rea-amplitude bloc'.The amplitude of the echo from a flat bottom hole in the far field of a normal beam probe is

proportional to the area of the bottom of the hole. Therefore these bloc's can be used tochec' linearity of a pulse echo inspection system and to relate echo amplitude to the area#or, in other words, the si*e of a flaw$. 9ecause a flat bottom hole is an ideal reflector andmost natural flaws are less than ideal in reflective properties, an area-amplitude bloc' defines a lower limit for the si*e of a flaw that yields a given height of indication on theC&T screen. or instance if the height of indication from a flaw in a test piece is si scaleunits and this is also the height of the indication from a 5> :F #1.?@ mm$ diameter flat

bottom hole at the same depth as the flaw, it is not possible to determine accurately how

much larger than the reference hole the flaw actually is. 9ut the flaw is at least as large asthe diameter of flat bottom hole.

5.2.0 Di'ta!%#,a- lit/"# bl %&'

(istance-amplitude bloc's provide artificial flaws of a given si*e at various depths #metaldistance$. rom ultrasonic wave theory it is 'nown that the decrease in echo amplitude froma flat bottom hole using a circular probe is inversely proportional to the square of thedistance to the hole bottom. (istance-amplitude bloc's #also 'nown as 2lcoa series-9 or =itt bloc's$ can be used to chec' actual variations of amplitude with distance for normal

beam inspection in a given material. They also serve as reference for setting or standardi*ing

the sensitivity of the inspection system so that readable indications will be displayed on theC&T screen for flaws of a given si*e and larger, but the screen will not be flooded withindications of smaller discontinuities that are of no interest. n instruments so equipped,these bloc's are used for distance-amplitude correction so that a flaw of a given si*e will

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produce an indication on the C&T screen that is of a predetermined height regardless of distance from the entry surface.

There are 1? bloc's in an 2lcoa series-9 set. 2ll are /F #50 mm$ diameter bloc's of the samematerial as that being inspected, and all have a >:F # 1?. mm$ deep flat bottom hole drilled

in the centre of the bottom surface # igure 5.5$. The hole diameter is the same in all the bloc's of a set. )ets can be made with hole diameters of > :F #1.1? mm$, 5> :F#1.?@ mm$ and @> :F # .1B mm$. The bloc's vary in length to provide metal distances of 1>1 F #1.5? mm$, 1>@F through 1F #/5.: mm$ in 1>@F # .1B mm$ increments and 1-1>:F # 1.Bmm$ through 5- >:F #1: mm$ in 1>/F #1/.B mm$ increments #see Table 5./$.

igure 5.5 " (istance-amplitude bloc'.

%ach 2lcoa series-9 bloc' is identified by a code number consisting of a digit, a dash andfour more digits. The first digit is the diameter of the hole in one si ty fourths of an inch.The four other digits are the metal distance from the top surface to the hole bottom in onehundredth of an inch. or instance, as illustrated in the Table 5./ a bloc' mar'ed -00B5 hasa > :F #1.1? mm$ diameter hole and a >:F #1?. mm$ metal distance.

5.2. ASTM bl %&'

2)T8 bloc's can be combined into various sets of area-amplitude and distance-amplitude bloc's. The 2)T8 basic set of distance-amplitude bloc's consists of ten /F #50 mm$diameter bloc's, each with a >:F #1? mm$ deep flat bottom hole drilled in the centre of the

bottom surface. ne bloc' has a > :F #1.1? mm$ diameter hole at a F #B ./ mm$ metaldistance. )even bloc's have 5> :F #1.?@ mm$ diameter holes at metal distances of1>@F # .1B mm$, 1>:F # . 5 mm$, 1>/F #1/.B mm$, >:F #1? mm$, 1-1>/F # @.1 mm$,

F #B ./ mm$ and F #15/.: mm$. The remaining bloc's have @> :F # .1B mm$ diameter holes at F #B ./ mm$ and F #15/.: mm$ metal distances. The 2)T8 basic set of area-amplitude bloc's consists of three bloc's with a F #B ./ mm$ metal distance and holes withdiameter of > :F #1.1? mm$, 5> :F #1.?@ mm$, and @> :F # .1B mm$. The remainingamplitude bloc's with the > :F #1.1? mm$ diameter holes form the set of distance-amplitude bloc's.

n addition to the basic set, 2)T8 lists five more area-amplitude standard reference bloc's

and @0 more distance-amplitude bloc's. %ach 2)T8 bloc' is identified by a code number,using the same system as that used for the 2lcoa series-9 set.

T29;% 5./ " ( )T2<C%-283; TU(% )%T

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9loc' identification

8etal distance verall length

number nches mm nches mm

-000 0.0 / #1>1 $ 1.5B 0.@B5 //./

-001/ 0.1/5 #1>@$ ./ 0.@B5 //./

-00/5 0./50 #/>@$ .: 1.000 /5.:

-00 B 0. B5 # >@$ ?.5 1.1/5 /@.

-0050 0.500 #:>@$ 1/.B 1./50 1.@

-00 / 0. /5 #5>@$ 15.? 1. B5 :.?

-00B5 0.B50 # >@$ 1?.1 1.500 .1

-00@B 0.@B5 #B>@$ //./ 1. /5 :1.

-0100 1.000 /5.: 1.B50 ::.5

-01/5 1./50 #1-1>:$ 1.@ /.000 50.@

-01B5 1.B50 #1- >:$ ::.5 /.500 .5

-0//5 /./50 #/-1>:$ 5B./ .000 B ./

-0/B5 /.B50 #/- >:$ ?.? .500 @@.?

-0 /5 ./50 # -1>:$ @/. :.000 101.

-0 B5 .B50 # - >:$ ?5. :.500 11:.

-0:/5 :./50 #:-1>:$ [email protected] 5.000 1/B.0-0:B5 :.B50 #:- >:$ 1/0.B 5.500 1 ?.B

-05/5 5./50 #5-1>:$ 1 .: .000 15/.:

-05B5 5.B50 #5- >:$ 1: .1 .500 1 5.1

5.2. IO( b#a- r $il# bl %&

This bloc' # igure 5. $ is designed primarily for beam profile measurement and offers eight

different direct scan approaches to the target holes for :5 °, 0° and B0 ° probes, as shown inigure 5.B. There are eight more approaches by indirect #one bounce$ scan for :5 ° probes

and si for 0 ° probes but none for B0 ° probes. The total number of usable probe positionsin any case is determined by the si*e of the probe and the probe angle.

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igure 5. " nstitute of +elding # . .+$ beam profile bloc'.

igure 5.B " )ites for probes for plotting beam profile using . .+. bloc'G#a$ :5°, #b$ 0°, #c$ B0°.

5.) E3UIPMENT CHARACTERISTICS

The most important equipment characteristics to be verified are hori*ontal linearity, screenheight linearity, amplitude control linearity, resolution of equipment, dead *one estimationand penetrative power.

5.).1 H ri4 !tal li!#arit

The hori*ontal linearity or time base linearity is a measure of the degree of difference

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between an actual distance and a distance read out on the C&T. The . .+, ( < 5:1// or any bloc' of similar material and finish may be used to measure hori*ontal linearity. The choiceof thic'ness is determined by the requirement that a longitudinal wave probe placed on the

bloc' produces several bac'wall echoes #usually four or five$ within the chosen range. or chec'ing the linearity two of the bac'wall echoes #say, the first and fourth in a five echo

display$ should be set to coincide with appropriate scale divisions. The position of each of the remaining echoes is then carefully noted. The ma imum tolerance is 1D for the rangechosen. <on-linearity of the time base is seldom a real problem with modern flaw detectorsand the most common cause of apparent non-linearity is the poor calibration of time base*ero by the operator.

2n important precaution to ta'e during the assessment of time base linearity is that time base readings are ta'en as each signal is brought to a common amplitude. This is usuallyabout 1>/ screen height.

5.).2 S%r##! 6#i76t li!#arit

To verify the ability of the ultrasonic instrument to meet the linearity requirement, positionan angle beam search unit as shown in igure 5.@ so that indications can be observed from

both the H and I T holes in a basic calibration bloc'. 2d!ust the search unit position to givea /"1 ratio of amplitudes between the two indications, with the larger set at @0D of fullscreen height. +ithout moving the search unit, ad!ust sensitivity #gain$ to successively setthe larger indication from 100D to /0D of full screen height, in 10D increments #or / d9steps if a fine control is not available$, and read the smaller indication at each setting. Thereading must be 50D of the larger amplitude, within 5D of full screen height. The settingsand readings must be estimated to the nearest 1D of full screen. 2lternatively, a straight

beam search unit may be used on any calibration bloc' which will provide amplitudedifferences, with sufficient signal separation to prevent overlapping of the two signals.

igure 5.@" )creen height linearity

5.).) A- lit/"# % !tr l li!#arit

or the chec'ing of amplitude control linearity, the time base is calibrated for a desiredrange and an echo about midway along the time base is obtained. The echo amplitude is setto a desired height and the attenuator reading is noted. The attenuator setting is then reduced

by d9, four or five times in succession and the decrease in echo amplitude is noted everytime. f it decreases to half the value of the previous setting then the gain control of theequipment is properly calibrated.

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5.).+ R#' l/ti !

The resolution of a flaw detector is the ability to resolve minor difference in distance anddirection. To determine the resolution of a flaw detector . .+ 1 bloc' is used with normal

beam probes. This bloc' has three target reflectors at ranges of @5 mm, ?1 mm and

100 mm. 2 probe is placed on the bloc' as shown in igure 5.? #a$ and echoes from thethree reflectors are obtained. The separation of the echoes from each other indicates thedegree of resolution of the flaw detector for that particular probe. igures 5.? #b$ J #c$ showthe degree of resolution for a flaw detector using two different normal beam probes.

igure 5.? #a$ " 3lacement of probe on . .+ 1 bloc' to determine the resolutionof flaw detector and probe system.

#b$ #c$

igure 5.? #b J c$ " C&T display showing resolving power of the flaw detector using twodifferent normal beam probesG #b$ shows better resolution, while,#c$ shows a poor resolution.

2 rough estimate of the length of the dead *one beneath a compressional wave probe isobtainable using the 1.5 mm hole and the plastic insert of the 1 bloc'. 2ny attempt to addholes would limit its usefulness, and the use of a special bloc' of the 'ind shown in

igure 5.10 #described in 9) ?/ " 3art " 1?B/$ is, therefore, recommended. +ith this

bloc' the resolution is determined by the minimum distance apart that flaws can be indicatedclearly and separately. n use the probe is placed on the centre line of the bloc' over thechange in radius from one step to the ne t. ts position is ad!usted so that echoes from thetwo radii are of the same height and appro imately 1>/ full screen height. The steps are saidto be resolved when their echoes are clearly separated at half ma imum echo height or less.

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igure 5.10 " 9ritish standard test bloc' for measuring resolution ofthe probe and the flaw detector system.

5.).5 Ma8i-/- #!#trati9# :#r

This is the term used in 9ritish )tandard 9) : 1. t describes a chec' which is used tocompare the energy output for a particular set and probe with its past performance or withsimilar equipment. The chec' is carried out as follows"

2 longitudinal wave probe is placed on the plastic insert #methyl polymethacrylate cylinder$of the . .+ 1 bloc' # igure 5.11$ having a thic'ness of / mm which is equal to 50 mm of steel and the gain for the instrument is set to its ma imum. The number of multiple echoesand the amplitude of the last echo are noted and are used to e press the ma imum

penetrative power of the set and the probe.

igure 5.11 #a$ " 3lacement of normal beam probe to determine penetrative power of the system.

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igure 5.11 #b$ " C&T display illustrating penetrative power of the system.

5.).0 D#t#r-i!i!7 t6# /l'# l#!7t6

5.3.6.1 Normal probe

n flaw detectors with a rectified display, place the probe at position ; # igure 5.1/ a$ andcalibrate the test range using the mm step #equivalent to 1 µs transit time in steel$ to ashort time range. 3lace the probe on a suitable surface of the bloc' to produce a bac' echoand ad!ust the delay and amplification to display the bac' echo at 100D full screen height# )=$. %stimate the pulse length as the distance between the points on the rising and fallingflan's of the displayed pulse which are at 10D of the pea' amplitude. The pulse length ise pressed in mm or as a time interval in microseconds.

#a$ #b$

igure 5.1/ #a J b$ " 3robe position on 1 bloc' with normal beam and angle beam probes for determination of pulse length.

5.3.6.2 Angle probe

3osition the angle probe at position ; # igure 5.1/ b$ and obtain a bac' echo from the100 mm radius after calibrating the time base to a short range. The delay is ad!usted to bringthe 100 mm bac' echo into the calibrated range. )et the bac'wall echo to 100D )=. The

pulse length can be estimated as the distance between the points on the rising and fallingflan's of the displayed pulse which are at 10D of the pea' amplitude # igure 5.1 $.

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igure 5.1 " Typical pulse length display.

5.+ CALIBRATION (ITH NORMAL PROBES

5.+.1 Calibrati ! $ ti-# ba'#

5.4.1.1 Using V1 block

or calibration of the time base with a normal beam probe for a range of up to /50 mm, the probe is placed at position C # igure 5.1:$ and multiple bac'wall echoes are obtained andad!usted to the appropriate scale division of the C&T screen using the delay and fine materialtesting range controls. igure 5.15 shows the C&T screen display for an 100 mm calibratedC&T screen. The points where the rising bac'wall echoes leave the base line have beenad!usted to the appropriate scale divisions to give the time base calibration.

or time base calibration of more than /50 mm with normal beam probe, the probe is placedat position 2 or 9 # igure 5.1:$ and multiple bac'wall echoes are obtained and ad!usted to

the appropriate scale divisions. igure 5.1 shows the C&T screen display for a one metrerange. or time base calibration of ?1, 1@/, /B ,........ normal beam probe is placed at position (.

8ultiple bac'wall echoes are used for time base calibration because the distance betweenthe transmission pulse and the first bac'wall echo is some what larger than the distance

between two consecutive multiple echoes. This *ero error is caused by the ultrasoundtravelling in the transducer, probe protective layer #if any$ and the layer of the couplant

before entering the specimen.

igure 5.1:" 3robe positions on . .+. # 1$ calibration bloc' for different thic'ness ranges.

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T29;% 5. " &%;2T <)= 3 9%T+%%< T=% 3& 9% 3 ) T <) 2<( T=%T= CK<%)) &2<4%) & C2; 9&2T <

3robe position on calibration bloc' Thic'ness ranges #mm$

2 /00, :00, 00, -------------------------------------9 100, /00, 00, :00, 500,--------------------------

C /5, 50, B5, 100, 1/5, 150, 1B5, /00, //5, /50

( ?1, 1@/, /B ,---------------------------------------

igure 5.15 " C&T screen display for 100 mm test range calibration #when the probe is placed at position 6C7$.

igure 5.1 " C&T screen display for one metre test range calibration #when the probe is placed at position 697$.


2 normal probe is placed on the bloc' as shown in igure 5.1B #a$ and multiple bac'wallechoes are obtained. These echoes are ad!usted using the test range and delay controls.

igure 5.1B #b$ shows the screen display for a 50 mm range calibration.

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#a$

#b$

igure 5.1B #a J b$ " 3robe position on . .+ / calibration bloc' for test range of50 mm and C&T screen display.

5.5 CALIBRATION (ITH ANGLE PROBES

5.5.1 Ra!7# %alibrati !


or a range of 100 mm or more the most direct method is to get multiple bac'wall echoesfrom the 100 mm quadrant by placing the probe at position 6%7 # igure 5.1@ a$. 2 C&Tscreen display for a range of /00 mm is shown in igure 5.1@ b.

igure 5.1@ #a$ " 3robe position for the test range calibration of 100 mm and abovewith angle probes.

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igure 5.1@ #b$ " C&T screen display for /00 mm test range calibration #when the probe is positioned at 6%7$.

2nother method of calibrating the time base for angle beam probes is to position a normalwave probe at 6(7 in igure 5.1: at which the distance of ?1 mm for a longitudinal wavecorresponds to 50 mm for shear waves. igure 5.1? shows the C&T display for a range of /50 mm for an angle probe which has been calibrated with a normal probe and the ?1 mmlength of the calibration bloc'.

igure 5.1? " C&T screen display for /50 mm test range for an angle beam probe which has been calibrated with a normal beam probe and the ?1 mm length of thecalibration bloc'.

2fter calibrating with the normal probe replace the normal probe with an angle beam probe, position the probe at 6%7 # igure 5.1@ a$ so that a ma imum response is obtained from the100 mm radius face. This coincides with the 100 mm reflection previously obtained with thenormal probe thereby correcting for the delay which occurs in the probe shoe.

To calibrate the time base for a 100 mm range with an angle beam probe, the procedureused is e plained in igure 5./0 a, b J c.

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igure 5./0 #a$ " 3ea' 9 is set provisionally at or near 10 using the sweep length control.

igure 5./0 #b$ " 3ea' 9 is set at 0 and 10 using the delay and sweep length controlsrespectively.

igure 5./0 #c$ " )et pea' 9 to 10 using the delay control. The *ero is automatically correct.


The time base calibration for an angle beam probe for range up to /50 mm can be done byone of the following two methods. n both these methods, the probe is moved to and frountil a ma imum echo is obtained.

n the first method the probe faces the /5 mm radius quadrant as shown in igure 5./1 #a$.9y this method the screen can be calibrated for 100 mm, 1B5 mm, /00 mm and /50 mmranges. or 100 mm test range calibration, facing the probe crystal to /5 mm quadrant of /

bloc', first echo is obtained from /5 mm quadrant, the same wave is then reflected from probe inde towards 50 mm quadrant. This wave is reflected bac' to the probe crystal whichis not received by the crystal due to the orientation of the crystal as it is towards /5 mmquadrant. 2gain it is reflected to the /5 mm quadrant, this reflected wave from /5 mmquadrant is received by the crystal. The echo obtained now is at 100 mm on C&T, which

means that after first echo obtained at /5 mm on C&T the other multiple echoes will beobtained at an interval of B5 mm. The echo pattern for a /00 mm range is as shown in igure5./1 #b$. The echoes appear at /5 mm, 100 mm, and 1B5 mm. or a /50 mm range theechoes appear at /5 mm, 100 mm, 1B5 mm and /50 mm.

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#a$ #b$

igure 5./1 #a J b$ " Calibration of time base up to /00 mm using / bloc' andangle probe facing the /5 mm quadrant.

n the second method the probe faces the 50 mm radius quadrant as shown inigure 5.// #a$. The C&T screen in this case can be calibrated for ranges of 1/5 mm and /00

mm. The C&T screen pattern for a /00 mm range is as illustrated in igure 5.// #b$. Theechoes appear at 50 mm, 1/5 mm and /00 mm.

#a$ #b$

igure 5.// #a J b$ " Calibration of time base up to /00 mm using / bloc' and angle probe facing the 50 mm quadrant.

n this method the echo from the 50 mm quadrant is set at 10th scale division of C&T screenusing the sweep control or range control. The probe is then reversed so that the echo fromthe /5 mm quadrant in obtained. This echo is set at 5th scale division of C&T screen usingthe delay control. The procedure is repeated until the echoes from /5 mm and 50 mmquadrants respectively coincide with 5 and 10 scale divisions of C&T. The calibration for50 mm range is then said to have been achieved.

5.5.2 D#t#r-i!ati ! $ t6# r b# i!"#8


The probe is placed at position ; on the calibration bloc' # igure 5./ $ and a bac'wall echofrom the 100 mm quadrant is obtained. The ma imum amplitude of this bac'wall echo isdetermined by moving the probe to and fro about the position ;. +hen the ma imumamplitude is found then the point on the probe which coincides with the point 0 #or cutmar'$ on the bloc' is the probe inde .

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igure 5./ " (etermination of probe inde using 1 bloc'.


The probe is placed either facing the /5 mm quadrant or the 50 mm quadrant to obtainechoes at /5 mm or 50 mm on the C&T screen. The probe is moved to and fro to ma imi*ethe echo. +hen the echo amplitude is a ma imum, the probe inde is obtained by e tendingthe centre mar' of the millimetre scale on the bloc' on to the probe.

5.5.) D#t#r-i!ati ! a!" %6#%&i!7 t6# r b# a!7l#


To determine the probe angle, the probe is moved to and fro according to its angle either at

position LaM # 5° to 0 ° $, LbM # 0° to B5° $ or LcM #B5° to @0° $ as shown in igure 5./:until the amplitude of the echo from the perspe insert or 1.5 mm diameter hole isma imum. The angle of the probe is the one at which the inde of the probe meets the anglescale on the bloc' when the echo amplitude is ma imum.

Cut mar'

0

3robe inde;

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igure 5./: " (etermination of probe angle using 1 bloc'.


To determine the actual probe angle, the probe inde is placed against the appropriate probeangle inscribed on the bloc' with the beam directed towards the 5 mm diameter hole# igure 5./5$. The probe is moved to and fro until the echo is a ma imum. 2n estimate of the probe angle is then made by noting the probe inde position with respect to the anglesinscribed on the bloc'.

igure 5./5 " Chec'ing of probe angles using / bloc'.

5.0 CALIBRATION IN CUR*ED (ORK PIECES

5.0.1 S#!'iti9it

Two factors contribute to the reduction in sensitivity when a test specimen with a curvedsurface is ultrasonically tested. ne is the widening or divergence of the transmitted beam

because of refraction and the other is the reduction in the contact area between the probe andthe test specimen. 9oth of these effects are shown in igure 5./ .

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igure 5./ " 2dditional sound beam divergence caused by refraction between couplant andwor' piece surface.

The contact area can be increased, and thus the contribution of this factor in reducingsensitivity can be minimi*ed, by the use of an adapter bloc' or shoe. These bloc's are madeto fit the surface curvature of the test specimen.

5.0.2 S&i "i'ta!%# a!" b#a- at6 l#!7t6 % rr#%ti !

The range calibration for a test with angle probes is usually made with . .+ 1 calibration bloc'. The s'ip-distances and beam-path-length respectively are increased by factors 6f p7 and6f s7, which depend on the probe angle θ and the ratio of wall thic'ness 6d7 to the outsidediameter 6(7 #i.e. d>($ and can be obtained from igures 5./B J 5./@ respectively. The

procedure is as follows"

#i$ (etermine the d>( ratio.

#ii$ (raw a line perpendicular to the d>( a is at the value calculated in #i$, and determineits point of intersection with the curve of the probe to be used.

#iii$ (raw a line parallel to the d>( a is at this point and determine its point of intersection with f p a is.

#iv$ &ead the values of factor f p.

#v$ &epeat the above steps for determination of the factor f s.

#vi$ Using the following formulae, calculate 3 r and ) r .

3 r A f p x 3 e ----------------------------------------- #5./$

and

) r A f s x ) e ---------------------------------------- #5. $

+here 3 r is the increased full s'ip distance for curved surface and it is determined bymultiplying the full s'ip distance in a plate of same thic'ness, 3 e ,with the factor f p , and ) r isthe increased beam path length, i.e. range due to surface curvature, and ) r is obtained bymultiplying the beam path length, ) e , in a flat material of the same thic'ness with the factor f s. igure 5./? illustrates the concepts.

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igure 5./B " )'ip distance multiplying factor for pipes and curved surfaces.

igure 5./@ " 9eam path multiplying factor for pipes and curved surfaces.

igure 5./? " Correction for curved surfaces.

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5. CONSTRUCTION OF DAC USING REFERENCE BLOCKS

(istance 2mplitude Correction #(2C$ curves are produced using a reference bloc' with aside drilled hole as a reference in the case of angle beam probes and flat bottom holes inseries of bloc's as references for normal probes. The 2)8% code uses this method to set

3&% level sensitivity.

The primary reference is set for an angle probe by ad!usting the signal from the drilledreference target, scanned from a beam path length !ust into the far field to an amplitude of B5D of full screen height and mar'ing the position of the echo pea' on the C&T screen. The

probe position is as shown position 1 in igure 5. 0 and the screen presentation is as shownin igure 5. 1. The probe is then moved to other locations #positions /, , and :, in igure5. 0$ and the signal amplitude is mar'ed on the C&T for each position # igure 5. 1$. 2curve is drawn !oining these points. This is the (istance 2mplitude Correction #(2C$ curve.This line, represents the reference level at various depths in the specimen. ;ines may also bedrawn at 50D or /0D of this reference level. Transfer loss is then calculated between the

reference bloc' and the wor' piece and is added to the (2C gain. or initial scanning thesensitivity is then set at twice #i.e. E d9 $ the primary reference level plus transfer loss. Theevaluation of flaws for acceptance or re!ection is however carried out with the gain controlset at the 3&% level plus the transfer loss.

The transfer loss may also be determined by another method by noting the difference between the response received from the reference reflector in the basic calibration bloc' andthe same reflector drilled in the test specimen.

igure 5. 0 " (ifferent probe positions on basic calibration bloc' for drawing (2C curvewith angle probes.

igure 5. 1 " C&T screen presentation for 100D, 50D and /0D (2C.

or normal beam probes the distance amplitude correction curve need not be constructedwhen the thic'ness of material is less than / inches #50 mm$. This correction is only needed

for thic'nesses greater than / inches #50 mm$. To construct the (2C curve, the ma imi*edecho height from the drilled hole at 1>: T distance is set to 50D of full screen height and ista'en as the 3&% level. +ithout changing the gain set for the 3&%, the probe is positionedfor ma imum response from the drilled hole at >: T distance, igure 5. / #a$. The heights

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of the 3&% and the ma imi*ed echo at >: T distance are mar'ed on the C&T screen. Therequired (2C is obtained by !oining these two points with a straight line and e tending theline to cover the required testing range as shown in igure 5. / #b$.

The scanning sensitivity level for normal beam probes, if possible, is then set at twice the

3&% level, i.e. the gain control of the flaw detector is set at 3&% level gain value plus d9.The evaluation of flaws is, however, carried out at the 3&% level gain setting plus transfer loss and attenuation correction.

igure 5. / #a$ " 3robe positions for 1>: T and >: T.

igure 5. / #b$ " C&T screen presentation for 1>: T and >: T positions.

5. .1. Att#!/ati ! l '' % rr#%ti !

The intensity of an ultrasonic beam that is sensed by a receiving transducer is considerablyless than the intensity of the initial transmission because of the reflection and scattering of

the sound beam at the grain boundries with in the material. This energy loss is defined asattenuation. 2ttenuation is different in different materials. 2coustic energy loss per unitdistance is called as attenuation coefficient usually denoted by letter 6 ∝ 6 and is e pressedas d9>mm or d9>in. The procedure for the measurement of transverse wave attenuation ina specimen is as follows"

i$ )elect two identical angle beam probes one to be used as transmitter and the other as a receiver of ultrasonic waves.

ii$ Calculate the s'ip distance and half-s'ip beam path length.

iii$ Calibrate the time base for sufficient range for at least one full s'ip beam pathlength.

iv$ Use a guide to align the transmitter and the receiver probes.

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v$ 3osition the probes at one s'ip distance apart #3ositions T and & 1 in igure 5. $.

vi$ 2d!ust the gain-control to bring the received echo to @0D of screen height. <otethe gain control reading. ;et it be 2 1 d9.

vii$ 8ove the receiver to two s'ip distances #position & / $.

T & 1 & /

igure 5. " 3ositions of receiver and transmitter probes for transverse waveattenuation measurement

viii$ 9ring the height of the received echo to @0D of screen height and note the gaincontrol reading. ;et it be 2 / d9.

i $ Calculated the difference #2 / - 2 1 $ d9.

$ Calculate 2 , energy loss to beam spread #from (4) diagram of the probe$.

i$ Calculate the attenuation in d9>mm from"2ttenuation Co-efficient A #2 / -2 1$-2 > =alf-s'ip beam path length #d9>mm$

5. .2. Tra!'$#r l '' % rr#%ti !

The reference bloc' is generally made of a material acoustically equivalent to the testspecimen and the contact surface should have similar roughness and degree of curvaturethus eliminating the effect of transfer loss at the contact surface. n this case transfer correction is not required. =owever if there is difference in the surface roughness of testspecimen and the reference bloc', the transfer correction must be ta'en into consideration.The procedure for the determination of transfer loss is as follows"

1. Calibrate the screen for a suitable range using a probe of the same angle andfrequency as will be used for the test.

/. Use two probes of same crystal si*e, angle and frequency operating as separatetransmitter and receiver.

. 3lace the two probes at one s'ip distance apart # ig. 1 of 2ppendi - :$ on the9C9 and bring the echo to @0D of full screen height. <ote the reading of the gaincontrol. ;et it be 627 d9.

:. &epeat step- but this time with the test specimen # ig. 1 of 2ppendi - :$ and notethe gain control reading. ;et it be 697 d9.

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5. 9y using the (4) diagram of the probe used, determine the difference in the echoheights due to the beam spread as follows"

a$ (etermine the d9 value using the half-s'ip beam path length for the 9C9and the bac'wall curve in the (4) diagram. ;et it be 6a7 d9.

b$ (etermine the d9 value using the half-s'ip beam path length for the testspecimen and the bac'wall curve in the (4) diagram. ;et it be 6b7 d9.

c$ (etermine the difference C A #a-b$ d9.

. a$ (etermine the ) 1 α 1 where ) 1 is the half-s'ip distance in specimen and α 1

is the attenuation co-efficient of specimen.

b$ (etermine ) / α / , where ) / is the half-s'ip distance and α / is the attenuationco-efficient of 9C9.

c$ (etermine ( A ) 1 α 1 - ) / α /

B. (etermine the transfer loss #T.;$ as follows"

T.; A #2 - 9$ - C - ( d9

5. DGS ;DISTANCE,GAIN,SI<E= DIAGRAM

2nother method which is used to set the sensitivity, i.e. ad!usting the gain control of theultrasonic flaw detection system for testing, is the (4) diagram method. This methodma'es use of the so-called (4) diagrams, developed by Kraut'rNmer in 1?5@.

rom a study of the geometry and intensity distribution of a sound beam, following facts can be concluded"

#i$ or small reflectors the amount of sound energy reflected bac' to the probe is a ratioof the reflector area and the cross-sectional area of the sound beam at the location of the reflector. The ma imum echo amplitude occurs at one near field length becauseof focusing of the sound beam. or distances shorter than one near field length theecho amplitudes decrease slightlyG

#ii$ n the far field and for large reflectors, such as a bac'wall, the echo amplitude isinversely proportional to the distance from the probe. f the distance is doubled, theecho amplitude decreases by 50D, i.e. - d9G

#iii$ or small reflectors, such as small inclusions, blowholes and flat bottom holes in thefar field, the echo amplitude is inversely proportional to square of the distance fromthe probe. f the distance is doubled, the echo amplitude is reduced to /5D or by-1/ d9G

#iv$ or small reflectors in the far field, the echo amplitude also depends upon the area of the reflecting surface and is proportional to the square of the diameter of thereflecting surface, such as a flat bottom hole. f the diameter is doubled, the echoamplitude is increased by four times or by E1/ d9.

t is on the basis of these facts that (4) diagrams are drawn by comparing the echoes fromsmall reflectors, namely different diameter flat bottom holes located at various distancesfrom the probe, with the echo of a large reflector such as a bac'wall, also at different

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)ince in the case of angle beam probes some of the near field length is contained within the perspe path length and this varies for different designs and si*es of probe, individual (4)diagrams are drawn for each design, si*e and frequency of angle beam probe. or thisreason the scale used in the angle beam probe (4) diagrams is simplified" the (-scale iscalibrated in beam path lengths, the 4-scale in decibels as beforeG and the )-scale

representing flat bottom hole or disc shaped reflector diameters in mm. igure 5. 5 shows atypical (4) diagram for a particular angle beam probe.

igure 5. 5 " Typical (4) diagram for an angle beam probe.

5.> COUPLING MEDIUM

3roper coupling medium or couplant should be used between the probe and the testspecimen to improve the transmission of ultrasonic energy by eliminating air between thetwo. Commonly used couplants in ultrasonic testing are glycerine, water, oils, petroleumgreases, silicon grease, wall-paper paste and various commercial paste li'e substances. or

the selection of a suitable couplant for a particular ultrasonic inspection tas' the following points should be ta'en into consideration"

#i$ )urface finish of the test specimen.

#ii$ Temperature of the test specimen.

#iii$ 3ossibility of chemical reaction between the test specimen and the couplant.

#iv$ Cleaning requirements #some couplants are difficult to remove$.

igure 5. shows a comparison of different couplants. This comparison is made by thee perimental arrangement shown in the figure. t is obvious from this figure that couplant

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used during calibration of the equipment and during the testing of the test specimen should be of same type for reliable results. The figure shows the variation of echo amplitude withvariations in surface roughness and for different types of couplants. )imilar studies can beunderta'en for other variables such as temperature of the test surfaces. 2 simple comparisonfor the efficiency of coupling between a test bloc' and the test specimen can be made by

measuring the echo heights in d9 respectively on each using the same couplant.

+ater is a suitable couplant for use on a relatively smooth surfaceG however, a wetting agentshould be added. t is sometimes appropriate to add glycerine to increase viscosityG however,glycerine tends to induce corrosion in aluminium and therefore is not recommended inaerospace applications.

=eavy oil or grease should be used on hot or vertical surfaces or on rough surfaces whereirregularities need to be filled.

igure 5. " ariations of signal amplitude with types of couplants and different surfaceroughness.

+allpaper paste is especially useful on rough surfaces when good coupling is needed tominimi*e bac'ground noise and yield an adequate signal-to-noise ratio.

+ater is not a good couplant to use with carbon steel testpieces, because it can promotesurface corrosion. ils, greases, and proprietary pastes of a non-corrosive nature can beused.=eavy oil, grease, or wallpaper paste may not be good choices when water will suffice,

because these substances are more difficult to remove. +allpaper paste li'e some proprietary couplants, will harden and may fla'e off if allowed to stand e posed to air.+hen dry and hard, wallpaper paste can be easily removed by blasting or wire brushing. ilor grease often must be removed with solvents.

Couplants used in contact inspection should be applied as a uniform, thin coating to obtainuniform and consistent inspection results. The necessity for a couplant is one of thedrawbac's of ultrasonic inspection and may be a limitation, such as with high temperaturesurfaces. +hen the si*e and shape of the part being inspected permit, immersion inspection

is often done. This practice satisfies the requirement for uniform coupling.

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