Determination of stresses in slightly orthotropic plates using off-axis horizontally polarized ultrasonic shear waves

Many structural materials display a slight orthotropy when they are rolled into plate form. A theory is developed for propagation of horizontally polarized ultrasonic shear waves in the planes of (orthotropic) material symmetry of these plates. The waves propagate at an angle θ to the plate normal. If E represents a measure of the shear stress field, then pure-mode waves can be propagated if both cos² θ and sin² θ E; the polarization directions are the material symmetry axes.

The theory can be applied to ultrasonic stress measurements. The (normalized) velocity difference (acoustic birefringence) between two pure-mode SH-waves propagating in orthogonal material symmetry planes is related to stresses in the plate, and to the (initial) birefringence in the unstressed state. By making measurements of birefringence at two or more values of θ, the dependence upon the initial birefringence can be removed and information about the stresses obtained.