Photoelastic Images of ultrasonic pulses
1. TOFD
5MHz 13mm diameter probe using a “TOFD” wedge producing a nominal 70° L wave in glass. Glass is 25mm high with a 3x1mm square notch milled such that the upper tip is 16mm down from the upper surface. The dark line on the left side of the notch going from top to bottom is the line of “optical fusion”. This provides an acoustically transparent fusion line and now reflections are visible. The image is capture at the time that the bulk shear wave has hit the opposite surface and the compression wave has passed the notch forming the upper and lower tip diffractions.
2. Angled Phased Array
The probe used is a 60 element linear array with a nominal frequency of 7.5MHz. Using 24 elements and a focal law used that produces a 60° compression wave the probe was placed directly on the glass surface. However, since there is no intermediary “refracting wedge” the shear component is formed as well.
3. Focused Phased Array
A 32 element 10MHz probe is used to make a focused beam. Focus occurs at 25mm depth for the compression mode but strong shear wave components can also be seem forming above the focal spot.
4. Immersion UT
An unfocused immersion probe, 9mm diameter 5MHz, is used to show the effects of wavelength change. Illustrated on the left is the pulse having just reflected from the glass. A portion of the pulse energy has entered the glass and the 2 cycle pulse can be seen to have a noticeably longer wavelength than the pulse in water. The image on the right is taken after the pulse in the glass has returned to the top surface and re-transmits into the water. Two slower moving pulses are therefore seen moving back towards the probe and a fainter reflection remains in the glass.
5. So-called Creeping Wave
The so-called creeping wave is better defined as the subsurface compression mode. It is generally made using a probe on a wedge with an incident angle that produces a high angle compression mode (e.g. 80°) and has a short range of effective detection. The image sequence shows a pulse configured to produce a nominal 80° L wave (Arrow #1). A refracted compression mode produces a shear mode bulk wave (Arrow #2). The origin of the shear mode is mode conversion by the compression mode. However, since the compression wave is spherically shaped it makes a glancing incidence at the surface boundary and a shear wave is generated that connects the compression wave at the surface to the bulk shear wave. The wavefront that forms is a straight line (Arrow #3). As the compression mode passes the surface notch (Arrow 4) a reflection occurs (Arrow #5). The initial compression wave continues past the notch (Arrow #6) and the reflecting compression wave returning just under surface (Arrow #7) is very weak but again generates a shear headwave (Arrow #8). The reflecting compression mode being spherical also generates a mode converted shear wave (Arrow #9).
6. Pulse in Thin Wall
When an angled pulse enters a thin section the size of the element used may result in the back of the pulse (lower part of the probe) reaching the back of the plate before the front of the pulse has left the wedge. This, and divergent effects of the beam, make for a variety of bounce positions in the thin wall material at the same time.
The upper image shows the probe in place (6mm diameter 5MHz element) and the image stopped in the second half skip.
The images below are clips of just the pulse in the 6mm thick glass at the midpoints of the 1st, 2nd and 3rd half skips. Note that by the third half skip the beam is so wide that it is seen on three legs of the skip at the same time.
