Numerical investigation of mode conversion of SH guided waves in plates reflected from discontinuities with different symmetries

Shear horizontal guided waves are commonly used in non-destructive evaluation of plates and pipes. When interacting with a thickness discontinuity, the reflected and transmitted wavefields can be composed of several modes due to mode conversion. It is known that in a plate with a symmetric discontinuity, with respect to the plate's mid-plane, mode conversion is restricted to modes that share the same symmetry as the incident mode. In this paper, we analyse mode conversion due to reflection from non-symmetric and symmetric discontinuities and investigate different types of discontinuity's symmetry through numerical analysis. We show that in a discontinuity with symmetric geometry but with one of its sides constrained fixed and the other one free, mode conversion is virtually restricted to modes with the opposite symmetry of the incident mode, acting as a symmetry inverter discontinuity. The latter can be efficiently simulated by filling one of the halves of the discontinuity with a material with at least tenfold the plate's acoustic impedance. Additionally, the scattering behaviour generally depends on whether the discontinuity is placed on the plate's surface or in the middle of the plate.