[2B2] Propagation distance independent directivity for shear waves thermoelastically excited by laser
X L Tu, P D Wilcox, A M Gambaruto and J Zhang
University of Bristol, UK
Shear waves are commonly used for defect detection in the thermoelastic regime of laser ultrasound inspection, as they are more efficiently excited than longitudinal waves in most materials. Obtaining an accurate measure of directivity, the angle-dependent amplitude of the shear waves, is important to inform optimal sensor positions and maximise signal-to-noise ratios. However, the shear wave directivity is found to be dependent on the propagation distance due to the inference from the head wave, which decays faster than the shear wave. This work proposes a method to subtract the head wave so that the true shear wave directivity can be obtained in finite element (FE) simulations, without using very large spatial domains. The subtraction method is first benchmarked with the analytical directivity, which assumes the laser source is a force dipole acting on the free surface of a homogeneous, isotropic half-space. Then it is applied to a more realistic multiphysics model, taking optical penetration depth and heat transfer into account, and also a model with anisotropic material, such as unidirectional carbon fibre-reinforced polymers. Experimental measurements for those two cases are provided to validate the simulation results.
