Loading effect of a metallic parabolic tip on S₀ and A₀ lamb waves

摘要

The loading effect induced by the contact between a parabolic duralumin tip and a free glass plate is investigated using Lamb waves and an optical heterodyne interferometric probe. The instrument detects 1-MHz impulse symmetric S0 and antisymmetric A₀ Lamb wave trains launched in 1-mmthick B270-type glass. Strain-optic modeling is carried out to explain optical measurement through the transparent medium and the loading effect of the tip. Three-wave optical interference modeling is also developed to explain the presence of fringes of equal thickness in C-scans of both modes propagating in a plate that has a 1.1-mrad wedge. Results show that through-glass probing inverts by a factor of ??3.1 the signal that is normally returned by the interferometer at a free-air surface for the S₀ Lamb mode. Fringes of equal thickness reveal the spatial extension of the mechanical loading. Through-glass probing on A₀ produces about the same signal as in a free-air measurement configuration. This mode appears to be more appropriate for the evaluation of the loading effect of the tip. For this parabolic tip, we observe an A0 attenuation of about 50% in the contact area.