Remote sensing of the thickness of hollow cylinders from optical excitation and detection of Lamb waves

The probe is a laser beam focused on a point diametrically opposite to the source. Each optical pulse launches acoustic waves that propagate in the form of a slightly dispersive symmetric mode and of a strongly dispersive antisymmetric mode (Lamb waves). At low frequencies the instantaneous-phase φ is a linear function of 1/t. The slope of the curve φ (1/t) for large times, i.e. for low frequencies, provides the thickness of the hollow cylinder. Experiments have been carried out with a YAG laser (pulse of 80 ns. 8 mJ, source length of 15 mm), a heterodyne interferometric probe, and hollow cylinders in duraluminum of inner diameter 20 mm and nominal thicknesses 0.16, 0.45, and 0.98 mm