Cracking stress in continuous fiber reinforced ceramic matrix composites

Abstract:

To elucidate the micro-mechanisms of matrix crack evolution in continuous fiber reinforced ceramic-matrix composites (FRCMCs), this study systematically investigates the influences of the initial matrix crack length, interfacial properties, thermal residual stresses, and radial stress on the matrix cracking stress. First, a simplified Coulomb-friction model that comprehensively accounts for Coulomb friction, Poisson’s effect, residual stresses and radial stress, is introduced to derive the analytical formula between the fiber bridging stress and the crack opening displacement. Then, a theoretical model for the matrix cracking stress is established by combining this bridging law with linear-elastic fracture mechanics. By this theoretical model, a quantitative relationship between the cracking stress and the initial crack length is obtained. Subsequently, the roles of constituent properties and the external factors including the ambient temperature and the radial stress are studied. The results show that long initial cracks propagate at low stress levels, whereas short initial cracks require high stress levels to grow, and the distribution of initial crack lengths thus governs the evolution of matrix cracking. Moreover, in addition to increasing matrix fracture toughness, interface friction coefficient and interface debonding energy, the application of radial compressive stress can significantly raise the matrix cracking stress. Therefore, the effect of radial stress must be fully considered in the structural design of continuous-fiber-reinforced ceramic matrix composites, and a radial compressive state should be maintained as far as possible to suppress crack propagation and enhance structural performance.