Simple method of measuring thicknesses of surface-hardened layers by laser ultrasonic technique

We proposed a simple non-contact inspection method using a laser ultrasonic technique to measure the thickness of a surface-hardened layer. This measurement is based on the dependence of a surface-wave velocity on the thickness of the hardened layers. In this case, it is essential to measure the surface-wave velocity with a wavelength comparable to the thickness of a hardened layer. However, it is not easy to selectively generate the surface waves with a desired wavelength. Thus we proposed the method where the surface waves with a desired wavelength are extracted using bandpass electrical filters. This method is simpler than the conventional one based on dispersion because the surface-wave velocity is directly obtained from measured temporal waveforms. We applied the method to measure the thickness of hardened layers and showed that the results were in good agreement with that obtained by the conventional method and numerical results.