CRACK PROBLEMS IN FGM LAYERS UNDER THERMAL-STRESSES

In this study an unconstrained elastic layer under statically self-equ ilibrating thermal or residual stresses is considered. The layer is as sumed to be a functionally graded material (FGM), meaning that its the rmo-mechanical properties are assumed to be continuous functions of th e thickness coordinate. The layer contains an embedded or a surface cr ack perpendicular to its boundaries. Using superposition the problem i s reduced to a perturbation problem in which the crack surface tractio ns are the only external forces. The dimensions, geometry, and loading conditions of the original problem are such that the perturbation pro blem may be approximated by a plane strain mode I crack problem for an infinite layer. After a general discussion of the thermal stress prob lem, the crack problem in the nonhomogeneous medium is formulated. Wit h the application to graded coatings and interfacial zones in mind, th e thickness variation of the thermo-mechanical properties is assumed t o be monotonous. Thus, the functions such as Young's modulus, the ther mal expansion coefficient, and thermal conductivity may be expressed b y appropriate exponential functions through a two-parameter curve fit. The crack problem is reduced to art integral equation with a generali zed Cauchy kernel and solved numerically. After giving some sample res ults regarding the distribution of thermal stresses, stress intensity factors for embedded and surface cracks are presented. Also included a re the results for a crack/contact problem in a FGM layer that is unde r compression near and at the surface and tension in the interior regi on.