Laser Ultrasound for the Non Contact Characterisation of the Mechanical Properties of Materials

We present our perspectives on using laser ultrasound, coupled with optical detection, to characterise the properties of plate like structures. The laser source launches a wideband signal in spatial and temporal domains exciting a Lamb wave spectrum over several octaves. Optical detection picks out the modal dispersion curves and inverting these mathematically presents values for thickness, density, modulus and Poisson ratio with confidence levels of a few per cent. Our investigations have compared approaches to dispersion curve measurement involving both 2-D and short time FFT to examine different areas of the specimen. We have compared high peak power impulse excitation with synthetic pulse generation using frequency scanned (or Pseudo Random Binary Sequence pulsed) low power sources based on semiconductor lasers or fibre amplifier based systems. A thorough analysis of the numerical inversion process has demonstrated that, with careful optimisation, the data obtained from the sample may be confidently inverted. Our demonstrations to date have been on large scale (mm thick by cm longitudinal dimensions) samples and studies of the application of the concepts to micro-systems are currently under way.