Effect of initial shear stress on acoustoelastic guided waves in plates

According to acoustoelasticity, initial stress affects the propagation characteristics of ultrasonic waves. Investigation of the acoustoelastic effect on guided waves has been virtually restricted to states of axial stresses. Here, the effect of shear stress in each plane of an aluminum plate is analyzed utilizing the semi-analytical finite element method. Generally, the dependence of the dispersion curves on the shear stresses is considerably less pronounced than on axial stresses. Nonetheless, it presents a predominant quadratic behavior with the stress magnitude, which can be explained by the decomposition of the shear stress as tensile and compressive stresses. Furthermore, shear introduces coupling between guided wave modes, which is intensified at the frequencies where the dispersion curves originally intersect, and under shear veer away, presenting high sensitivity. Coupling can occur between modes of opposite or same symmetry, or between Lamb and SH modes. The latter suggests a new measuring principle which relies on the ratio of the coupled components. The ideal frequency-thickness for this end is approximately 3300 kHzmm for shear stress within the plate's horizontal plane, and approximately 2100 kHzmm for shear stress perpendicular to the propagation direction, due to the coupling of the S0 mode with the SH0 and SH1 modes, respectively.