Functionally graded materials (FGM) are a new range of composite materials having a gradual and continuous variation of the volume fractions of each of the constituents (in general, metal and ceramic) in thickness, which accordingly causes changes in the overall thermomechanical properties of the structural elements they constitute. The interest of this work is the use of a high-order plate theory for the study of thermal buckling of FGM plates resting on Winkler-Pasternak type elastic foundation. The present method leads to a system of differential equations, where the number of unknowns is five. The material properties of FGM plate such as Young's modulus and coefficient of thermal expansion are assumed to be variable through the thickness according to the Mori-Tanaka distribution model. The thermal loading is assumed to be uniform, linear and nonlinear through the thickness of the plate. A parametric study is thus developed to see the influence of the geometric and mechanical characteristics, in particular, the geometric ratio (a/b), thickness ratio (a/h) and the material index (k), as well as the impact of the Winkler and Pasternak parameters on the critical buckling load.

Using a high order theory (HSDT), this work presents a study of thermal buckling of functionally graded (FG) sandwich plates subjected to various temperature rises across their thickness and resting on a twoparameter elastic foundation. The mechanical properties of the FG sandwich plates are supposed to change gradually through the thickness according to a power law (P-FGM). The intermediate layer is homogeneous and made of a purely ceramic material. The principle of virtual works is used to obtain stability equations, and their solutions are obtained based on Navier's solution technique. The obtained results are compared with other studies in the literature. Then, a parametric study is conducted to investigate the influence of geometric and mechanical characteristics such as the ratios of dimensions (width, length, and thickness), the material index (k), and the effect of the elastic foundation on the critical buckling temperature.

Our work is to study the bending behavior of sandwich beams with functional gradient by constituting an isotropic material whose material properties vary smoothly in the z direction only (FGM), where the central layer presents purely a homogeneous and isotropic ceramic. The mechanical properties of FG sandwich beams are assumed to be progressive in thickness according to a power law (P-FGM). Generally, the principle of virtual works is used to obtain the equilibrium equations, and their solutions are obtained based on Navier's solution technique. The present model is based on a shear deformation theory of 2D and 3D beams which contains four unknowns to extract the equilibrium equations of FG sandwich beams. In addition, analytical solutions for bending are used and numerical models are presented to verify the accuracy of the present theory. All the results obtained show that the stiffness of the FG beam decreases as a function of the increase in the volume fraction index k, leading to an increase in the deflections. However, FG beams become flexible by increasing the proportion of the metal to the ceramic part. Furthermore, the influences of material volume fraction index, layer thickness ratio, side-to-height ratio, and the effect of the phase contrast, on the deflections, normal and shear stress of simply supported sandwich FG beams are taken into investigation and discussed in detail. Finally, all our results obtained are in agreement with other previous theoretical works.