PREDICTION OF FCG BEHAVIOR UNDER VARIABLE AMPLITUDE LOADING IN MMCS

A model which describes fatigue crack propagation in fibre-reinforced, metal-matrix composites is further developed in this paper in order t o characterise crack propagation under variable amplitude loading. The crack system is divided into three zones: the crack, the plastic zone and the fibre zone. Crack tip plasticity is constrained by the fibres and remains so until certain conditions are met. The solution of the equilibrium equation of all the forces (internal and external) acting on the crack system, determines two important relationships, first, th e stress distribution throughout the system and second, the crack open ing displacement. The conditions for crack propagation are such that w hen the stresses at the fibre zone attain the level required to debond the fibre from the matrix, the fibre constraint is overcome. Crack ti p plasticity increases and the crack is then able to propagate round t he fibre. If the fibres remained unbroken they would introduce a frict ion or clamping stress on the crack hanks which would be a function of the local GOD. Crack propagation rate is assumed to be proportional t o crack tip plastic displacement, which in turn depends on the level o f fibre bridging, the degree of matrix cyclic hardening, fibre spacing , the debonding strength, the fibre strength and the applied load. The effect of overloads of various intensities, and applied at different stages of fatigue life, are assessed in terms of the magnitude of the damage generated. The damage is mainly in the form of fibre-matrix deb onding and fibre failure.