Free vibration and wave propagation of thick plates using the generalized nonlocal strain gradient theory

In this paper, a size-dependent first-order shear deformation plate model is formulated in the framework of the higher-order generalized nonlocal strain-gradient (GNSG) theory. This modelemploys two nonlocal parameters and a strain-gradient coefficient to capture the both higher-order nonlocal stress-gradient and strain-gradient effects in nanostructures. The presence of these different scale parameters renders a unified model, which is able to predict both increase and reduction of stiffness in nanoplates. The governing equations are developed for freevibration of first-order shear deformation plates using Ritz method. The dispersion relations for the GNSG plate model is also derived. Several numerical examples are studied to show the efficiency, competence and accuracy of the proposed model. To ensure the applicability of the presented GNSG plate model, the results are compared with the experimental data available in the scientific literature. It is found that the effects of scale parameters on the wave frequencies are significant at high wavenumbers and ratio of any pair of these parameters is the main criterion for the correct study of size effects. The results show that the reduced nonlocal strain-gradient (RNSG) model and the GNSG model diverge in higher vibration modes.