Measurement of Stress Profiles by Phase Contrast Techniques

An acoustic wave pissing through a material has its velocity changed when stress is applied. This is due to changes in the third order elastic constant and the density of the material. By using a small diameter beam or a focused beam Incident on a metal and reflected from both its front and back surfaces, It is possible to measure the difference in phase of the two reflected waves; the beam Itself can be scanned over the surface of the material. Three kinds of measurements will be shown. The first relates the change of velocity of a compressional wave to the applied stress taken ' in an MTS testing system. The second shows a scan of the profile of the velocity change around a circular defect. The third is an image of the stressed region around a circular defect obtained with a scanned electronically focused system operating in a phase contrast mode. A system has been implemented to make ultrasonic measurements of the state of stress In a metal sample. The basic technique determines the change in velocity of an acoustic wave passing through a sample when it is stressed. Scanned acoustic beams of small diameter are used also to determine the variation of the stress in an inhomogeneously stressed sample. One system uses mechanical scanning and Is extremely accurate but slow in operation, the second system uses an array of transducers and is electronically scanned in one direction and mechanically scanned in the other. It is fast in operation but less accurate. A block diagram for the accurate system is shown in Fig. 1. An acoustic transducer approximately 3 IIID diameter is used to excite two rf tone bursts in water which are reflected from the front and back faces, respectively, of the sample so that the two received echoes from the front and back faces coincide. We measure the phase difference between the carriers of these tone bursts to determine the change in velocity of waves In the material. The advantage of this technique is that It eliminates errors due to the time delay of the wave In the water path between the transducer and the sample, for only the difference in time delays between waves reflected from the front and back face of the sample is determined. The output is gated so that only the coincident pulses are observed. The phase relationship of the two echoes is a function of transit time through the sample and the carrier frequency of the tone burst. Thus, the frequency may be adjusted to create the minimum condition. It can be shown that If the transit time through the sample changes by bT, the required change In frequency 6f is given by the relation: