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Description
Elastic waves in plates are commonly classified by their polarization. Shear-horizontal (SH) waves exhibit in-plane displacement orthogonal to the wave vector, whereas Lamb waves involve the remaining two displacement components. In isotropic plates, these polarizations are fully decoupled, and Lamb waves alone suffice to characterize the material completely. This contribution examines how far this picture holds for anisotropic media. The analysis is complicated by the fact that SH and Lamb polarizations generally couple in anisotropic plates, forming a single family of guided waves. While this suggests that measuring any such wave should in principle allow full material characterization, the conclusion is not straightforward once measurement selectivity is taken into account. Our laser-ultrasonic system has a polarization bias: the interferometric detection is sensitive only to out-of-plane displacement, while the laser excitation on a metallic sample is dominated by in-plane forcing. As a result, only Lamb-like waves are readily observed. To quantify this bias, we introduce the polarization angle — defined as the angle between a wave's polarization vector and the SH direction — so that 0° denotes a pure SH wave and 90° a pure Lamb wave. Using this metric, we perform a sensitivity analysis of laser-ultrasonic measurements with respect to the stiffness parameters of an orthotropic medium. The analysis demonstrates that SH-like waves carry information that is not accessible from Lamb-like waves alone and that their acquisition is essential for unique identification of the full orthotropic stiffness tensor. These findings are illustrated through a two-dimensional wavefield scan on a cold-rolled steel plate.