Transient thermal cycle damage of thermal barrier type of functionally gradient material

Functionally Gradient Materials (FGM) promise to be used for advanced thermal barrier coatings in gas turbine engines and for heat insulation systems in structural components in hypersonic aircraft. The damage mode associated with inelastic deformation of FGM in elevated thermal cycling conditions, however, must be clarified for the practical service environment use. The work reported herein describes the results of numerical experiments on the thermomechanical response of the FGM using Finite Element Methods (FEM) wherein the nonlinear inelasticity of Hastelloy X was utilized for the analysis model. Further experiments were conducted to illustrate the effects of thermal and mechanical properties of FGM constituent materials on the inelastic behavior in thermal cycling environments. The compressive inelastic strain at the hot side surface of FGM in the heat-up cycle causes a significant residual tensile stress generation after the cool-down cycle. Using ceramics with a low thermal expansion coefficient enables the residual tensile stress reduction owing to the less inelastic compressive deformation at the hot side surface of FGM in the heat-up cycle. FGM compositional distribution is less effective for controlling the residual stress. The effects of elastic modulus of the FGM constituent materials were also investigated.