Measurement of in-plane and out-of-plane displacements for ultrasonic flaw detection

The simultaneous detection of in-plane and out-of-plane ultrasound displacements is crucial for the interpretation of Lamb wave mode pattern and their interaction with defects within the plate. In this report we present our preliminary experiments for the simultaneous measurement of these two components. A two-channel fibre optic interferometer system has been built which enables the measurements of in-plane and out-of-plane displacements separately and simultaneously. One channel is a Michelson and the other a modified Michelson interferometer or a Mach-Zehnder interferometer. The Michelson interferometer allowed direct measurement of the absolute out-of-plane displacement, while the modified Michelson interferometer measured the in-plane displacement. The Mach-Zehnder interferometer measured both components. This two-channel fibre optic interferometer allows a directly calibrated measurement of the two components of displacement simultaneously and offers a great insight into ultrasonic flaw interrogation in plate-like materials.