Thermal shock resistance enhancement of functionally graded materials by multiple cracking

Multiple cracking is a common phenomenon for a medium subjected to a sudden temperature change. In this paper, a multiple surface cracking analysis is carried out to investigate the thermal shock resistance behavior of functionally graded materials (FGMs). The FGM is taken to be a ceramic/metal composite with its gradation characterized by the local volume fractions of metal and ceramic phases. The finite element method is used to obtain the solution of the crack problem. The crack tip field intensity factors as well as the stress are computed as functions of the crack spacing, the normalized time, and the crack depth. Crack initiation behavior is discussed. It is found that the thermal shock resistance of the FGM is significantly enhanced by multiple cracking. That is, the thermal shock resistance is a monotonically increasing function of the crack density (crack density is the number of cracks per unit length; crack density increases with decreasing crack spacing). A single crack 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. Multiscale behavior of the thermal shock resistance of FGMs is discussed. Two critical size parameters, which control the applicability of the stress-based criterion and the fracture mechanics-based criterion to the determination of the thermal shock resistance of FGMs, are explored.