This is the last 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.

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

Chapter11.
SURFACE & TOFD TECHNIQUES

SURFACE WAVE TECHNIQUES

Surface waves have been used very successfully for a great number of applications, particularly in the Aircraft Industry. However, it is not so common in the Steel Industry because surface finishes are often less smooth, and magnetic particle inspection will find most defects detectable by surface waves. Nevertheless, there are occasions when the use of surface wave techniques can give the simplest and most positive results and so, in this section we will discuss some general principles that can be applied when considering a surface wave technique.

ADVANTAGES OF SURFACE WAVES

Surface waves will follow gentle contours without reflection, but will reflect sharply from a sudden change in contour. Figure 1 shows a typical example of a component having a complex shape that would make the use of shear or compression waves difficult, if not impossible. Cracks may develop anywhere along the leading or trailing edge of the blade out to about two thirds of the blade length, or in the root radius. A surface wave probe placed at the end of the blade, and directed towards the root will send a beam along the surface, round the radius and reflect from the edge of the root as shown. Cracks in the suspect areas will give reflections at an earlier time than the root.

The fact that surface waves only penetrate to a depth of about one wavelength can be used to advantage when testing relatively thin wall sections. Figure 2 shows a pipe with a change of section. We are told that cracks may occur on the inside or outside diameter of the pipe in the necked region. An angled shear wave probe might be used, but it would be difficult to predict the skip points as the beam bounces around the section change. However, if we choose a surface wave at a frequency for which the wavelength is approximately equal to the wall thickness, then the surface wave will fill the wall thickness, and follow the section change, reflecting for a defect breaking either surface.

LIMITATIONS OF SURFACE WAVES

The main limitation of the surface wave technique is that the beam is almost immediately attenuated if the surface finish is rough, covered in scale, or a liquid (such as the couplant), or has any pressure applied by another object (such as your hand). For this reason it is normal to use grease as the couplant for surface wave probes (it doesn't run!), and to apply the grease to the probe, place the probe on the job and scan forward (away from your own grease trail). Ridges left in the couplant during scanning, and objects resting on the test surface, often give spurious signals that might be taken to originate from defects so it is normal to test such indication by rub­bing a cloth over the area indicated by the signal. If after this ‘cleaning’ operation, the signal disappears, then it was a spurious indication.

CALIBRATION & DEFECT LOCATION

It is not usual to calibrate the timebase for surface waves in the way we would do for shear or compression waves. This is because we can normally run a finger along the surface in front of the probe, when we find a defect indication, until the signal is no longer ‘dam­ ped’. This happens as we pass over the defect with our finger, however, there are occasions when access is limited and we are directing surface waves to a region that is out of sight and cannot be reached with the hand. In these cases, the timebase can be calibrated using the same procedure as for shear waves on an A2 or similar block.

The sensitivity can be set from drilled holes or spark-eroded slits in suitable reference blocks. In the aircraft industry, these reference blocks are usually sections of an actual component with a spark-eroded slit in the critical location.

DEFECT SIZING USING TOFD

The TOFD technique, first used by Silk in 1977, uses tip diffraction to identify the top, bottom and ends of a discontinuity in one pass. Silk chose to use an angled compression wave for the TOFD technique rather than a shear wave, for two reasons. Firstly, the tip diffraction signal is stronger than a shear wave diffraction signal, and secondly, a lateral wave is produced which can be used to measure the horizontal distance between the transmitter and receiver.

The tip diffraction signal is generated at the tip of the discontinuity – effectively a ‘Point’ source. According to Huyghens, a point source produces a spherical beam. Figure 3 shows a typical TOFD transducer set-up on a component with a vertical discontinuity. There are four sound paths from the transmitter to the receiver. Path ‘A’ is the lateral wave path travelling just below the surface. Path ‘B’ is the tip diffraction path from the top of the discontinuity. Path ‘C’ is the tip diffraction path from the bottom of the discontinuity and path ‘D’ is the backwall echo path.

Figure 4 shows a typical unrectified trace for the four signals. Note the phase relationships, A and C are in opposite phase to B and D. The important difference to note is between B and C – the top and bottom diffraction signals are in opposite phase. This phase difference allows the practitioner to identify those points.

Fig. 4

Assuming that the diffracting tip is centred between the two transducers, the depth of the tip below the surface can be calculated from: -

Where:

BPL = Beam path length for the signal in question

HD = Beam path length for the lateral wave.

The distance measurements taken from the ultrasonic trace must be made from the same part of each waveform. In the trace shown in Figure 4, the largest half cycle would be selected. For signals A & C this is negative and for signal B positive. The advances in computer technology have made it possible to carry out all the calculations and plotting to be handled automatically and stored for subsequent evaluation. The method that has been chosen to display this TOFD data presents the information in a special ‘B-scan’ form that is easy to assimilate. The way in which the positive and negative half cycles are displayed needs explaining.

In a conventional B-scan image, the ‘slice’ is taken across the weld perpendicular to the centre line. In the TOFD display, the ‘slice’ is taken along the weld (figure 5). However, whereas the conventional B-scan is a relatively thin slice, the TOFD image represents the volume between the probes as they scan along the weld. The presentation is known as a ‘D-scan’.

An echo arriving at the receiver is a pulse of a certain pulse width and amplitude. In conventional B-scan displays, this pulse is displayed as a bright spot whose diameter is proportional to the pulse width and whose brightness is proportional to the signal amplitude. In some ways, it is like a broad pencil tip that can be used to draw pictures in light or bold broad strokes. The pulse is really a short burst of a few cycles of alternating waveform. In the TOFD system, the waveform is depicted in greyscale with positive going half cycles tending towards white, and negative going half cycles tending towards black (see figure 6). This type of display will allow us to identify phase change so that we can discriminate between he lateral wave, top and bottom defect signals and backwall.

This allows particular half cycles to be identified for measurement purposes, and phase changes to be recognized for determination of top or bottom echo. Figure 7 shows a typical computer screen for a TOFD inspection. The image shows details of the component (in this case, a weld) as well as the TOFD D-scan image and an A-scan trace. In this image, left to right represents the component thickness, and the vertical dimension represents scan length.

The A-scan trace shown corresponds to a slice through the weld at the location indicated by the ‘cross hairs’ of the cursor. The striped band on the left of the TOFD image represents the lateral wave, and the bold striped band to the right of the image represents the backwall echo. The difference in boldness is due to the different signal amplitudes. Following the horizontal ‘cross hairs’ and about half way between the lateral wave and backwall ‘stripes’, a series of feint ‘horse shoe’ shaped stripes can be seen. These are diffraction signals from a small discontinuity. The A-scan trace shows the signal clearly.

In this example, the discontinuity has a very small dimension in the through-thickness dimension, but close study of the A-scan shows a small phase shift in the last half-cycle of the discontinuity signal. This tells the practitioner that the distance from top to bottom of the discontinuity is about the same as the pulse length for this particular discontinuity.

A much bolder indication can be seen towards the top of the lateral wave line suggesting a discontinuity at, or just below the surface. In figure 8, the cursor has been moved to this location. The lateral wave signal can be seen to be longer and stronger than at the previous location. The fact that the wave shape stays in phase suggests that the diffraction echo, which is extending the signal, has the same phase as the lateral wave. In other words, it is a bottom tip signal. However, it is not possible in this case to see where the lateral wave ends and the bottom tip begins, and so it is not possible to say how deep the discontinuity extends below the surface. The TOFD method is limited in its ability to size near surface discontinuities when the arrival time difference between the lateral wave and the diffraction signal is similar to pulse length. Near surface resolution when using TOFD can be a bit confusing if you look at it from a conventional ultrasonics point of view. Imagine a top surface crack 4mm deep. At 5MHz, it represents more than two wavelengths at compression wave velocity and with a reasonably short pulse of two cycles; you might expect to resolve the bottom of the defect. However, the path difference between the lateral wave and the tip diffraction signals for a probe separation of 80mm is only 0.4mm and this is about the same as the wavelength for 15MHz (See figure 9). You would need a 15MHz transmitter with only one cycle in the pulse to resolve the crack.

The transducers used in TOFD techniques are angled compression wave transducers. The common angles used are 60 o and 70 o, although other angles may be used if the component thickness makes it necessary. The design and construction of the transducer is important in order to promote a good lateral wave. Previous theory has suggested that a shear wave should also exist in the component and this is true, it does. Figure 10 shows a little more of the trace for the above example. On the extreme right of both the A-scan and TOFD D-scan, the shear wave can be seen. Since it arrives well after the other signals, it does not present a problem in this application.

Scanning with the TOFD system is fast and many scanning systems are motorized. They all require distance encoders so that the D-scan image can be constructed. The vertical extent of those defects that can be resolved is many times more accurate than other sizing systems.

References: -

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