Theoretical study of elastic wave propagation through a functionally graded micro-structured plate base on the modified couple-stress theory

In this paper, two different methods, Legendre orthogonal polynomial method (LOPM) and Global matrix method (GMM), are combined with the couple-stress theory to investigate the reflection and transmission of P wave in a functionally graded (FG) couple-stressed plate immersed in liquid. The GMM needs to divide the FG plate into the multilayered model, with each layer containing six partial waves. However, the LOPM only expands the displacement vectors in the FG couple-stressed plate into the Legendre orthogonal polynomial series. The paper shows that both methods are in good agreement. Nevertheless, the proposed LOPM is more efficient for the FG microstructure because it does not require plate delamination and calculation of the displacements of each partial wave. It also shows that the couple-stress effect affects the resonant frequencies of the FG plate, decreasing the number of resonant frequencies. By controlling the gradient field, some resonances can be avoided.