On Thermal Shock Behavior of Functionally Graded Materials

Abstract

Cracking is a common phenomenon for many media subjected to a rapid temperature change. Cracks in a medium may be in the form of either a single dominant crack or an array of multiple cracks. In this article a review is given of our work on this subject. Finite element method is used to obtain the solution of the transient temperature field in functionally graded materials (FGMs). A multiple surface cracking analysis is performed to study the thermal shock resistance behavior of a FGM, which is a ceramic/metal composite with its gradation characterized by the local volume fractions of the metal and ceramic phrases. Crack initiation behavior is discussed; and the crack tip field intensity factors and stresses are computed as functions of crack spacing, normalized time and crack depth. It is found that the thermal shock resistance of the FGM is significantly enhanced by multiple cracking. That is, thermal shock resistance is a monotonously increasing function of crack density, which is defined as the number of cracks per unit length and it increases with decreasing crack spacing. The single crack case represents the lower bound solution for the thermal shock resistance of FGMs. It is also found that the gradation of the FGM has little influence on the crack tip thermal stress intensity factors but has a strong influence on the thermal shock resistance. Multi-scale thermal shock resistance behavior of FGMs is discussed. Two critical size parameters, which control the applicability of the stress-based criterion and the fracture mechanics-based criterion for determination of thermal shock resistance of FGMs are investigated.