Specialist Inspection Services and Equipment from Silverwing

Shear wavesat various angles of refraction between 35 o and 80 o are used to locate defects whose orientation is not suitable for detection by com pression wave techniques. Some defects, of course, have volume and their shape enables them to be detected by both compression and angled shear waves. In this chapter, however, we will be dealing with planar defects whose orientation is such that only angled shear waves can be used.

Because the beam is travelling through the test piece at a refracted angle other than perpendicular, we need to distinguish between the beam path length to a discontinuity and its depth below the test surface. When we encounter a signal, we can measure the beam path length (range) from the timebase, but we may want to calculate how far in front of the probe (horizontal distance) and how far below the surface the reflector is located. It is also important when using shear waves to know where along your probe the beam enters the specimen (beam index). Knowing the beam index position relative to some datum on the specimen, and the exact beam angle allows you to calculate the horizontal and vertical distances. There are standard terms for various distances when using shear waves and these are illustrated in figure 1.

Full skip and half skip distances are measured along the top surface and beam path length (BPL), along the beam centre. To calculate these, knowing specimen thickness (t) and probe angle ( q ) use the following formulae: -

Calculate the Full skip distance for a 40^o shear wave beam in a 20mm thick steel plate.

We need to find the exact beam index for any shear wave probe before measuring the beam angle. This is because the beam index may not be the one marked on a new probe – it may be a millimetre or so before or after the marked index. Manufacturers use a standard drawing to make probes, but the velocity of sound in Perspex varies from batch to batch, and with temperature. Also the beam index and probe angle change as the probe wedge or ‘shoe’ becomes worn with use. So, the establishing of beam index and angle will be routine checks throughout the life of the probe. Finding the beam index is a simple procedure carried out on the A2 or A4 calibration block.

The probe is positioned close to the edge of the calibration block and beaming towards the 100mm radius (A2 block) or the 25mm or 50mm radius (A4 block) as shown in figure 2 (A2 block) and figure 3 (A4block).

The probe is moved backwards and forwards about the centre mark of the radius with the probe kept parallel to the edge of the block. As the probe moves, the signal will rise to a maximum and then fall again as shown in figure 4. When the signal reaches the maximum amplitude, the beam centre is meeting the tangent to the radius at right angles. This happens when the beam centre is entering the block at the centre of the radius. The true beam index is now in line with the centre mark of the radius being scanned. If this does not coincide with the beam index marked on the probe, you would then either mark the true index on the probe body, or, if the probe body has a millimetre scale, make a note of the true position in front or behind the marked index.

Once the true beam index is known, the true beam angle can be measured on the A2 or A4 calibration block. The nominal probe angle is marked on the probe and is the refracted angle for steel unless identified for another material. A 45 o shear wave probe made for aluminium would be marked ‘45AL’ and for copper ‘45CU’. The actual angle for a new probe may be plus or minus two degrees from the nominal angle because of the batch velocity variations in Perspex, and will change with wear. Most of us have an inherent tendency to wear the probe in a particular way, just as we do for shoes. We may wear the heel of the probe down and so increase the actual angle, or wear the toe and decrease the actual angle. For this reason the beam angle measurement is also a routine probe check. If the probe is worn down towards one edge, the beam will be thrown off towards that side and the condition is called ‘squint’.

Beam angle is measured on the A2 block by aiming the beam at the 45mm diameter hole and on the A4 block at the small hole. The probe is positioned on the block at a point near the nominal angle and the gain adjusted to give a signal amplitude of about 50% full screen height. As the probe is moved forward and back, the signal rises and falls just as it did when finding the beam index. When the signal reaches its maximum amplitude, the beam centre is aimed at the centre of the hole and the beam is hitting the tangent to the hole at right angles. The true beam angle can be read against the true beam index from the graticule on the calibration block. The example shown in figure 5 has the beam index opposite an angle of about 43 o and the nominal angle is 45 o. With this probe, we would have to use 43 o in our distance calculations and for defect sizing.

The method of calibration of the timebase for shear waves depends on the purpose of the inspection. If the inspection were to be volumetric, looking for any discontinuities within the scanned volume of the test piece, then we would calibrate for a suitable timebase range at shear wave velocity. On the other hand, if the purpose is to look for a specific discontinuity such as a fatigue crack, in a predicted location, we may well use a ‘Skip’ method or a ‘Reference Block’ method. The calibration for a known range will be dealt with first, using the A2 block and then the A4 block.

Sometimes you may see part of the initial (transmission) pulse around the zero, this will depend on the pulse length and gain setting as shown in figure 7.

The slot that marks the 100mm radius on the A2 block is about 4mm deep so that when the probe is aligned with the edge of the block, the slot makes a corner for the returning echo to reflect part of the energy back to the 100mm radius. This is why it is possible to obtain repeat echoes from the radius. If the slot were not there, the reflected energy from the first returned signal would reflect to the rear of the probe. Figure 8 shows an exaggerated view of the beam path to illustrate the ‘corner’ effect.

The time base is now calibrated for 100mm at shear wave velocity, and zero represents the top surface entry point below the beam index.

The sound path in figure 9 shows the first echo from the 25mm radius and then the echo from the 50mm radius after reflecting at the scanning surface down to the 25mm radius and back. The total return path is: -

Facing the 25mm radius on the A4 block, signals will arrive at 25, 100, 175, 250mm and so on, incrementing by 75mm each time. If the probe is turned around to face the 50mm radius, signals will arrive at 50, 125, 200, 275mm and so on, again incrementing by 75mm.

If the purpose of the inspection is to detect surface breaking flaws at the bottom surface or top surface (typical of fatigue cracks), we know that the echoes will arrive at exactly the half skip or full skip beam path lengths. We could calibrate the timebase for an exact range using one of the methods described above and calculate the beam path lengths for half and full skip using the formulae. We would then know exactly where to look on the timebase for the two conditions. We do use this method to carry out the critical root scan in weld inspection.

The signal will rise to a maximum as the centre of the beam moves into the corner. We can adjust the timebase and gain to make sure that we can see that maximum point. As the maximum is reached, we would adjust the timebase range to position the signal at some convenient part of the trace, usually about ’4’. We would then continue moving the probe backwards until the top corner reflection is seen (position 3). As this signal maximises, we note its position along the timebase. Figure 11 shows a trace with the half skip and full skip positions marked, and in this example, gates positioned over the two critical locations so that the operator can listen for the alarm rather than watch the display all the time.

Another point to note from figure 11 is that the position for full skip is at ‘9’ on the timebase and not at ‘8’ (twice ‘4’). This means that timebase zero is not the top surface, and furthermore, we don’t know the exact timebase range. However, for this inspection it doesn’t matter because we are only interested to find out whether or not there is a bottom or top surface breaking ‘corner’.

If the plate to be inspected has accessible edges, you don’t need a calibration plate because you can use the corners on the test piece to set up the two positions on the timebase. However, you have to be sure that there are no laminations in the beam path because these might reflect the beam back to the top without reaching the bottom. It is easy to check whether the signal is coming from the anticipated corner because a shear wave meeting an interface at an oblique angle is easy to ‘damp’. If you put an oily finger on the expected reflecting corner, the signal will be seen to reduce in amplitude significantly. In figure 10, if you ‘damp’ the bottom corner when the beam is at the half skip position (2), the signal will fall and when the beam is at the full skip position (3) you can damp the signal at the top corner and at the reflecting point on the bottom surface.

If you are going to scan a pipe wall in the longitudinal direction, then you can use any of the above calibration procedures. However, if you are scanning circumferentially the calculation of beam path length, and skip distances is more complicated. If you have a segment of pipe of the same outside diameter and wall thickness as a reference block, you can use the ‘skip’ method for finding the critical half and full skip positions on the timebase. If you also need to look for discontinuities in the volume of the object, you calibrate the timebase on the A2 or A4 block for an exact range, and then put the probe on the ‘reference’ pipe segment and note the half and full skip ranges.

The wall thickness for any given outside diameter is important because t he normal range of angled shear wave probes (45 o, 60 o,and 70 o), when used on thick wall pipe may cut across to the outside surface again without touching the bore. An example is shown in figure 12 where a 45 o shear wave only reaches about half way through the wall. In other words, for this outside diameter, the thickest pipe wall we could test with a 45 o probe is only half that shown in the diagram. It follows that, when you are presented with an unusually thick pipe wall for a particular outside diameter, you need to choose your probe angle carefully in order to inspect the bore properly. For a given angle, the maximum wall thickness that allows the centre of the beam to reach the bore of the pipe can be calculated from: -

This formula can be turned round so that you can calculate the best angle given for a wall thickness, the formula becoming: -

Table 1 shows maximum wall thickness that can be tested for three standard angles and a range of pipe diameters

Pipe OD	Maximum pipe wall thickness for probe angles 35^o 45^o 60^o
4” (100mm)	21mm	14mm	7mm
6” (150mm)	32mm	22mm	10mm
8” (200mm)	43mm	29mm	13mm
10” (250mm)	53mm	36mm	17mm
12” (300mm)	64mm	44mm	20mm
14” (350mm)	74mm	51mm	23mm
16” (400mm)	85mm	58mm	27mm
18” (450mm)	96mm	66mm	30mm
20” (500mm)	106mm	73mm	33mm

Table 1

Once the correct angle for the pipe size and wall thickness has been chosen, you can establish the skip and half skip positions using a section of pipe with a drilled hole to produce the required ‘corner’ reflectors – as shown in figure 13.

Fig. 13

CALIBRATION OF THE GAIN

This is often called "setting the sen sitivity", and it means that we adjust the gain so that a significant discontinuity will give a signal that is large enough to see, but small surface scratches will not. Very often, we use a reference block, similar in shape and material to the specimen, and con taining either a drilled hole or an artificial (machined) crack. The probe is aimed at this reference hole or crack, to obtain an echo, this is then maximised by probe movement, and then, the gain is adjusted to give the required signal height known as the ‘reference level’ and the gain is then said to be ‘calibrated’. This reference level may be 50% or 75% of full screen height, and is often used as the basis for getting acceptance standards for the inspection. Hence, you may find that you are working to a specification that says that any signal equal to, or greater than the amplitude of the reference level is cause for rejection of the component, whereas any signal lower than the reference level may be ignored.

TESTING FOR OUTSIDE DIAMETER SURFACE FLAWS

Discontinuities that break the top surface such as the crack show in figure 14 will cause a reflection to occur at exactly the beam path distance for the full skip if a suitable angle that will reach the bore is used. However, as you can see from figure 15, if you are testing a thick wall tube or a solid bar, the beam may reach the top surface without first reflec ting from any other surface. The beam path length at which a top surface defect will appear in that case can be calculated from the formula: -

Where: - D is the outside diameter and q is the probe angle.

In the sort of application illustrated in figure 15, if there is no crack, the sound will carry on around the bar or pipe as shown in figure 16. Provided there is enough sensitivity, you may only need to scan from position ‘A’ to position ‘B’. The beam will sweep the entire circumference during the short scan and as long as you have enough timebase and gain, echoes from any discontinuities breaking the surface will appear at predictable positions.

‘Ultrasonic Flaw Detection for Technicians’ - Third Edition, June 2004 by J. C. Drury


	This is the tenth chapter of an eleven part article on Ultrasonics by John Drury, the Author of Ultrasonic Flaw Detection for Technicians. This article was first published in INSIGHT magazine throughout 2004/5. The chapters can be downloaded in PDF for you to build into a complete series. To access the other chapters please use the navigation at the bottom of this page.
	For more comprehensive information on Ultrasonics, purchase Ultrasonic Flaw Detection for Technicians - 3rd Edition. Written by John Drury. This is widely regarded as the most complete UT book ever written. This link will take you to the Silverwing UK site.


	J.C. Drury ' BACK TO BASICS - ULTRASONICS' Chapter10. SHEAR WAVE TECHNIQUES	Download this document as a PDF

J.C. Drury ' BACK TO BASICS - ULTRASONICS'

Chapter10. SHEAR WAVE TECHNIQUES

Chapter10.
SHEAR WAVE TECHNIQUES