Plane strain and axisymmetric spallation of graded coatings under thermal loading

In this study the failure mechanics of thermal barrier coatings under thermally induced in-plane compression is considered. The layered medium studied is assumed to consist of a substrate (the main structural component), a bond coat (the oxygen diffusion barrier), a thin layer of thermally grown oxide that develops during operation, and a homogeneous ceramic or graded metal/ceramic top coat (the primary heat shield). After presenting some results from the plane strain solution, the axisymmetric problem is considered in some detail. Noting that for the geometry and loading studied, namely a layered medium with an interface crack that is subjected to in-plane compression, the small deformation theory would give only a trivial result, and in order to assess the limitation of the plate theory that is often used to study such instability problems, a finite deformation continuum theory is used to solve the instability and the postbuckling problems. A nonlinear finite element method incorporating special enriched and transition elements is used to analyze the problem. The primary objective of the study is the investigation of the effect of property grading parameters, kinematic nonlinearity, plate approximation, and component curvature on the crack-tip stress intensity factors, the strain energy release rate, and the crack-opening displacements.