A laser/EMAT system for thickness monitoring applications using shear and L-S mode-converted waves

A laser/EMAT system is described for non-contact monitoring of metal thickness. In a B-scan imaging configuration, quantitative time-of-flight analysis of laser-generated shear waves and longitudinal-shear mode-converted waves can be used as an effective method for thickness monitoring. Q-switched Nd:YAG laser pulses with energies of similar to 18 mJ were delivered to the material surface via an optical fibre and focused to a line source by a cylindrical tens. Transient acoustic waves propagated through the plate to be reflected from the far surface. Waves returning to the surface, including L-S mode-converted waves, were detected using an EMAT sensitive to in-plane motion. B-scans were generated as the sensor head was moved along the material's surface, forming a 2D intensity profile that made changes in plate thickness easy to visualize. The L-S and S-L mode-converted waves provided a method of simultaneously monitoring two different points on the far surface, enabling any changes in the material thickness to be clearly identified.