Lamb Wave Propagation in Varying Thermal Environments

During flight, launch, and reentry, external surfaces on aerospace vehicles undergo extreme thermo-acoustic loads resulting in structural degradation. Structural health monitoring techniques are being devised to evaluate the health of these structures by locating and quantifying structural damage during and after flight. One such technique uses Lamb wave propagation to assess damage on the surface and through the thickness of thin materials. The objective of this study was to assess the sensitivity of piezo-generated Lamb wave propagation to isothermal and thermal gradient environments using both theoretical and experimental methods. Experimental isothermal tests were conducted over a temperature range of 0-225-F. The changes in temperature-dependent material properties were correlated to measurable differences in the response signal's waveform and propagation speed. An analysis of the experimental signal response data demonstrated that elevated temperatures delay wave propagation, although the delays are minimal at the temperatures tested in this study. Both these results and experimental group velocity dispersion curves verified theoretical predictions. Subsequent experimental testing in thermal gradient environments, with peak temperatures ranging 114-280-F, also displayed an observable yet minimal delay in wave propagation. Finally, theoretical simulations at temperatures up to 600-F revealed significantly increased delays in wave propagation.