Photoelastic Visualisation _ Phased Array Sound Fields Part 20 S-scan using a refracting wedge 40 to 70 transverse wave

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1 Vol.20 No.3 (Mar 2015) - The e-journal of Nondestructive Testing - ISSN Photoelastic Visualisation _ Phased Array Sound Fields Part 20 S-scan using a refracting wedge 40 to 70 transverse wave Ed GINZEL 1 1 Materials Research Institute, Waterloo, Ontario, Canada eginzel@mri.on.ca Keywords: Photoelastic visualisation, ultrasonic, phased array The video to this article can be seen here: 1. Introduction This technical note is Part 20 of a series in NDT.net. In Part 17 of this series, an S-scan was shown using a linear array probe in direct contact with the glass block. This produced a compression mode from -30 to +30. Because the range of angles was below the first critical angle a transverse mode was also seen. Ultrasonic weld inspection is typically done using angled transverse waves that can be directed into the weld volume either directly or after reflection off the far surface. In order to avoid confusing signals from the compression mode, weld inspections are typically carried out using a refracting wedge that eliminates the compression mode by internal reflection inside the wedge. When using phased-array generated pulses, the range of angles that produces useful inspection results is now considered to be approximately 40 to 70. By starting at 40 refracted beam is approximately 6.5 away from the first critical angle. As seen in previous videos, the arc shape of the pulse results in the ends of the wavefront having angles slightly less than the nominal so it is a good idea to not use angles near 35 refracted because that could be within 1 of the critical angle for some steels. At the other end of the sweep range, 70 provides a reasonably high angle that is sufficiently below the second critical angle (90 refracted transverse mode) that the surface Rayleigh wave is avoided. Also, the concern for potential grating lobe formation is increased as the refracted angle increases. The range of 40 to 70 refracted transverse mode therefore represents a practical inspection range for general weld inspections. Figure 1 illustrates the sort of coverage provided by a 40 to 70 sweep when this angular range is used in an S-scan. Page 1 of 5

2 Figure 1 Weld volume coverage using a 40 to 70 S-scan Note how the lower angles (from 40 to approximately 45 ) are used after the reflection off the far surface to cover the heat affected zone and upper weld edge, whereas the higher angles (approximately 65 to 70 ) are used to ensure that the root and heat affected zone on the opposite side of the weld are covered. The remaining angular range (45 to 65 ) provides coverage of the weld volume including the heat affected zone. The same range of angles can be repeated at a slightly different standoff (start element) to improve detection of potential nonfusion conditions along the weld bevel. This 40 to 70 condition is modelled using a set of delay laws configured by activating 16 elements starting at element 10 and using 2 angular increments. Figure 2 illustrates the relative for 3 of the angles. (The length of the red bars over the active elements indicates the relative in firing). The conditions calculated are for glass with a transverse velocity of 3450m/s and a crosslinked polystyrene wedge machined at 36.5 (natural incident angle). For the 40 refracted beam element 25 is fired first (0ns delay) and element 10 is fired last with a 689ns delay. Since the are linear for a specific angle the time difference between each element is 45.93ns. Delays for the 60 beam start with element 25 firing at 0ns delay and element 10 firing at 12ns. Delays for the 70 beam start with element 10 firing at 0ns delay and element 25 firing at 223ns Figure 2 Relative element firing for 40, 60 and 70 angles These provide a range of incident angles from about 26 to 39.8 as indicated in the images in Figure 3. Page 2 of 5

3 Figure 3 Incident angles for 40 and 70 refracted angles in glass 2. Comments on the Video The video begins with the beam for the 40 refracted angle imaged in the wedge. A graphic overlay indicates a line parallel to the smallest incident angle and it is measured to be close to the calculated 26 angle. This is illustrated here in Figure 4. Figure 4 Incident compression mode imaged in wedge at 26 The video then advances through the incident steps to the maximum incident angle and another overlay indicates the angle that will provide the 70 refracted beam (39.8 ). This is seen in Figure 5. Page 3 of 5

4 Figure 5 Incident compression mode imaged in wedge at 39.8 The strobe delay is progressively increased to see the sweep of beams enter the glass. As the beams enter the glass it can be seen that a weak compression mode still exists that is more noticeable for the lower refracted angles. Mode converted transverse Weak compression mode Figure 6 Weak compression mode seen at lower refracted angles Towards the end of the video a protractor is overlaid and the range of angles calculated is confirmed as seen in Figure 7. Page 4 of 5

5 40 70 Figure 7 Confirmation of 40 and 70 refracted angles in transverse mode Note that the protractor images in Figure 7 are approximate as the exit point of the beam migrates over a distance of approximately 7mm (ass illustrated in Figure 8). Figure 8 Exit point migration of 40 and 70 refracted angles As the refracted transverse mode progresses through the glass, the internally reflected compression mode can be seen in the wedge. For more information about the photoelastic system see For information on the Image Intensity Analysis software see The video to this article can be seen here: Page 5 of 5