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 Copyright imagineNDE - 2011 Technique 1 - Beam Spread Sizing:  If the ultrasonic beam were like a laser beam we would have a method of sizing flaws based on the extent of probe movement and the distance along the beam where the flaw is found. By simple plotting of coordinates we could determine with a high degree of accuracy the size or vertical extent, position in the structure and o rientation. Becaus e the beam spreads, the ability of the system to resolve reflectors in close proximity to each other, adds a significant error factor to sizing. The benefit of the ‘phased arr ay’ method, allows manipulation of the beam width at selected locations along the beam, thus improving flaw sizing. You tell the instrument where you would like to have the beam focused, and the ‘focal law’ will be calculated by manipulation of the active aperture (number of elements firing) and where you want the beam to focus. Focus is not what you might think, and there are limitations t o where and to what degree you can have focus. The limit of focus is the near zone (or natural focus) manipulated by the active aperture (changing element size in conventional UT). For general interrogation of t he weld, a long foc us is selected which gives a nominally focused beam somewhere at 0.7 of one near zone, and the active aperture, 16 to 24 elements firing. The natural focus of the probe is extended out by utilizing higher frequencies (one near zone is proportional to the element size squared and the frequency). Thus we now have probe design factoring into sizing techniques. Criteria for developing sizing techniques:  Critical sizing of flaws requires that you develop an engineering specification which specifies the criteria for minimum flaw size, flaw type, and depending whether the flaw is ID or OD connected, and if not connected, what is the allowable ligament length between the flaw and the ID or OD surface. The ASME Code requires that any sizing technique be qualified and demonstrated to show that the technique can reliably size the flaw within the engineering specification’s criteria, and what are the allowable error li mits. Test samples are fabricated with known flaws, and the samples scanned using the ‘inspection procedure’  to collect the UT data. Based on this written procedure, the flaws are sized and the results evaluated based on radiography or actual sectioning through the flaw and physical measurement. Following a successful qualification of the inspection/ sizing procedure, technician training will follow, and each technician will complete a written and practical test of his/her ability to size the flaws within the error limits. Phased array lends itself well to this qualification process, and can be adjusted where accuracy can be improved, and with the superior computer processing power, and the use of Tomoview software, removes much of the subjectivity and error due to manual manipulation of the probe. However there are basic principles that you must follow if you are to use ‘beam spread sizing. Basic criteria for meeting resolution requirements: The IOW calibration block was designed in the early 60’s to provide beam qualification and sizing. Specifically, the minimum frequency required to resolve the 1/16” diameter holes, see figure 1.
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Technique 1 - Beam Spread Sizing:

If the ultrasonic beam were like a laser beam we would have a method of sizing flaws based on the extent of probemovement and the distance along the beam where the flaw is found. By simple plotting of coordinates we coulddetermine with a high degree of accuracy the size or vertical extent, position in the structure and orientation. Becausethe beam spreads, the ability of the system to resolve reflectors in close proximity to each other, adds a significant errorfactor to sizing. The benefit of the ‘phased array’ method, allows manipulation of the beam width at selected locationsalong the beam, thus improving flaw sizing. You tell the instrument where you would like to have the beam focused,and the ‘focal law’ will be calculated by manipulation of the active aperture (number of elements firing) and where youwant the beam to focus. Focus is not what you might think, and there are limitations to where and to what degree youcan have focus. The limit of focus is the near zone (or natural focus) manipulated by the active aperture (changingelement size in conventional UT). For general interrogation of the weld, a long focus is selected which gives a nominallyfocused beam somewhere at 0.7 of one near zone, and the active aperture, 16 to 24 elements firing. The natural focusof the probe is extended out by utilizing higher frequencies (one near zone is proportional to the element size squaredand the frequency). Thus we now have probe design factoring into sizing techniques.

Criteria for developing sizing techniques: Critical sizing of flaws requires that you develop an ‘engineering specification ’which specifies the criteria for minimum flaw size, flaw type, and depending whether the flaw is ID or OD connected,and if not connected, what is the allowable ligament length between the flaw and the ID or OD surface. The ASME Coderequires that any sizing technique be qualified and demonstrated to show that the technique can reliably size the flawwithin the engineering specification ’s criteria, and what are the allowable error limits. Test samples are fabricated withknown flaws, and the samples scanned u sing the ‘inspection procedure’ to collect the UT data. Based on this writtenprocedure, the flaws are sized and the results evaluated based on radiography or actual sectioning through the flaw andphysical measurement. Following a successful qualification of the inspection/ sizing procedure, technician training will

follow, and each technician will complete a written and practical test of his/her ability to size the flaws within the errorlimits.

Phased array lends itself well to this qualification process, and can be adjusted where accuracy can be improved, andwith the superior computer processing power, and the use of Tomoview software, removes much of the subjectivity anderror due to manual manipulation of the probe. However there are basic principles that you must follow if you are touse ‘beam spread sizing.

Basic criteria for meeting resolution requirements: The ‘IOW calibration block ’ was designed in the early 60’s toprovide beam qualification and sizing. Specifically, the minimum frequency required to resolve the 1/16” diameterholes, see figure 1.

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Resolution Test Holes

Figure 1 – IOW Calibration Block

The top three holes shown spaced 1/8” must be resolved for the transducer to be qualified for use with this technique.The closely spaced holes (1/16” spacing) using standard frequencies and pulse width (damping) cannot be expected tobe resolved. The definition of resolution is usually any two signals that can be shown to be clearly separated on the Ascan timebase above the noise (grass) level. The one I like is that if the highest of the two indications is set at 80% fullscreen height (FSH), the second comparing indication is seen to be distinctly separated at the 20% level. 2.25MHz

probes at standard ‘medium damping’ cannot be shown to pass this resolution test. A minimum of 4MHz isrecommended.

Much of the controversy that has given this technique a poor rating on accuracy, is the application of low frequencyprobes. Procedure; find the flaw, qualify the flaw as having an amplitude requiring sizing for vertical extent, then applythe beam spread sizing technique. What you will produce in most outcomes, is a point plot of the peak indication. Asfar as the author is aware, the beam spread sizing technique was introduced to the welding fabrication of butt welds,described in a specification titled: Ultrasonic Testing of Butt Welds, a UK specification. In this specification, itrecommended that frequencies 4MHz and higher were to be used. The beam spread plot was to -20dB, whichrepresents a drop in amplitude from the center of the beam to the extremities of 100% down to 10%. At -20dB, the

spread of 2.25MHz probes was too large to be effective, and thus low frequency probes only required a drop of -6dB(50%) to the beam extremity. Because of this, the plot of the ends of the flaw at the beam extremity, was premature,thus a significant sizing error is introduced.

Another consideration required in this specification, was the requirement to interrogate the flaw and observe the riseand fall of the signal envelope taking into account the lower amplitude but connected (unresolved) indications. This isnot a requirement in amplitude based systems, and requires that you only take the highest peak and drop 6dB.Invariably you will plot a point and no accurate measurement of vertical extent will be available. Worse, due to operatorerror you can have short probe travel which is travel from front to back of the beam, shorter than the beam width.

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When this occurs, the operator must be aware of this and if the desired amount of travel to at least cover the beamwidth is not obtained, he/she should enter to the r eport, ‘no measureable vertical extent’.

We have featured in this paper, a flaw that has significant vertical extent and is close enough to the ID surface that weneed to accurately determine its vertical extent and the ligament spacing between the bottom of the flaw and the ID

surface. In addition, the flaw has two parts (A and B) to show how we consider the two parts resolved and unresolvedbased on ‘pulse frequency’. We have not considered ‘pulse damping’, which can also be a function of resolution.

Figure 2 – Animated flaw sizing

Sizing using ‘Maximum Peak’: In the simulated flaw in the animations, the maximum peak has been determined to be+4dB to reference. The following graphic is taken from an animation showing, a plot of the flaw using 2.25MHz and a6dB beam spread. The flaw is plotted as a single point with ‘no measureable vertical extent’. This is the expectedoutcome when using low frequency interrogation of the flaw with 6dB beam spread.

The flaw, by virtue of its high amplitude would have significant vertical extent and in addition would be very close to theback surface.

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Figure 3

Animation 1: To view this animation, click on the following link: Animation 1

The second scenario we have used is one with a 7MHz probe that meets the resolution criteria of the IOW Block. In thisanimation you will see the process of interrogating the flaw and determining with a high degree of accuracy the

resolution of the flaws two parts, and the extremities of the flaw. We have used a 6dB beam spread to determine theextent of the flaw. The choice between a 6dB beam spread and a 20dB beam spread will be based on your provenaccuracy of the performance demonstration. The difference is not likely to be significant.

Animation 2: To view this animation, click on the following link: Animation 2

Animation 3: We have summarized videos 1 and 2. Click on the following link: Animation 3

Last significant echo (LSE): This technique is very similar to the technique discussed above except that beam spread isnot used, and the proponents of this technique do not see the benefit of using beam spread to determine the

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extremities of the flaw, only the ‘center of beam’ location of all associated (connected) peaks. Again, the decision to usebeam spread or not, will be based on the findings of the ‘performance demonstration’ for accuracy. The application ofPahsed Array uses this methodology.

ASME Sizing : The performance demonstration procedure previously discussed, is a requirement of the ASME Code. The

ASME Code Section V, Article 4 provides an example of a sizing technique that would yield a result very close to theabove plot, and uses terminology such as toward and away, for back and front of beam. The concept is very simple: Youinterrogate the flaw and determine its max peak relative to DAC Reference. Add 14dB of gain to your reference gain,and record probe position and position on the A scan timebase. Move the transducer toward the flaw until theamplitude drops to 20% of DAC (at +14dB indication is brought to DAC). Record the probe position, and the reading onthe A scan time base. We will call this CT . Move transducer away from flaw and observe the indication rise then dropto 20% of DAC. Record the probe position, and the reading on the A scan time base. We will call this CA. The verticalextent of the flaw is then calculated as:

Cosine of the probe angle x (CA-CT)

Flaw position relative to the weld would be plotted off the data on probe position and A scan timebase, for maximumpeak. This technique does not provide information on flaw orientation as would be provided in a ‘beam profile plot’.For plotting length, the procedure would be the same except that instead of toward and away, you are manipulating theprobe left and right of the flaw.

If you were to use this technique, as part of your inspection procedure, you would be required to complete the‘performance demonstration ’ process, and operator training and qualification.

Some of you would be asking ‘why the mirror image drawing of the weld?’. If you are plotting a flaw in the ‘secondhHalf skip’, you would simply plot as you would in the first half skip, then transfer your plot to th e top image. Just one of

the little tricks you learn along the way.

Sizing for length: The previous sizing method using beam profiles, works for flaws which are smaller than the beamwidth, and if larger than the beam width such as our crack, are sized by plotting the peak location of all unresolved andassociated indications, then sizing using beam spread at the extremities of the flaw. Sizing for length can be carried outthis way, but a simpler method is to size with what we sometimes term, ‘the half on half off technique’. This techniqueworks on the basis that you determine the maximum peak, then move the transducer first left (sideways) until theamplitude drops to -6dD of the maximum peak, marking the center of the transducer, then moving right through theflaw until the amplitude drops to -6dB of the maximum peak, marking the center of the transducer. The two marksrepresent the extremities of the flaw or the flaws length.

Animation 4: Click on the following link to watch a video on sizing for flaw length. Animation 4

Determining type of flaw: In this description on how to determine a type of flaw, I am speaking more to weld flaws.Being able to determine the type of weld flaw with some degree of certainty is fundamental to flaw sizing. With respectto type of flaw, some flaws such as inclusions or porosity, are confined to one weld pass, and therefore cannot beexpected to have significant vertical extent. If we have good confidence we are looking at an inclusion or a gas pore, wecan be sure that the flaw is confined to between weld passes and has no significant vertical extent.

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Flaws which can have significant vertical extent are ‘side wall lack of fusion ’, cracking at any location in the weld, and ‘in-complete penetration ’ of the root pass in ‘single vee butt welds’, and at the juncture of a ‘double vee prep’. The ASMECode designates these types of flaw as being unacceptable regardless of length and must be repaired. It is vital that theoperator performing the inspection must be able to recognize with a high degree of certainty, these types of flaws.

Simple is good, and a simple way of determining if a flaw has significant vertical extent, as can be the case for cracking, isto observe the dynamics of the indication (A scan signal envelope) while pivoting the transducer about its axis. Thebeam will trace an arc across an imaginary vertical face. Therefore flaws that have significant vertical extent will betouched by this arc at all locations about its vertical face, as the probe pivots. The visual result is a signal envelope thatappears to roll across the screen. With flaws that are confined within weld passes, the arc will touch the flaw at only thetop of the arc, and the signal envelope will appear to simply rise and fall without this characteristic roll.

Animation 5: Click on the following link to watch an animation of this pivot technique. Animation 5

Some who write about this theory of determining flaw type, consider the echo dynamics or the shape of the A scansignal, as being a function of flaw type. There is some truth to this especially with porosity which exhibits a low

amplitude cluster of indications, lack of fusion which exhibits a slim uncomplicated signal typical of the signal youobserve when you calibrate with an IIW block, inclusions which can look like anything and cracking, some report asrecognizing a Christmas tree like signal. This author puts no credibility on signal recognition and if you find this works foryou, good luck! The sectorial scan with Phased Array offers promise that we can reliably determine type of flaw usingsignal recognition, but it does require considerable experience examining welds.


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