Effect of interfacial debonding and sliding on matrix crack initiation during isothermal fatigue of SCS-6/Ti-15-3 composites

Direct observation of initial damage-evolution processes occurring during cyclic testing of an unnotched SCS-6 fiber-reinforced Ti-15-3 composite has been carried out. The aligned fibers break at an early stage, followed by debonding and subsequent sliding along the interface between the reaction layer (RL) and Ti-15-3 alloy matrix. Matrix cracking initiation from the initial broken fiber and RL was avoided. This fracture behavior during cyclic loading is modeled and analyzed by the finite-element method, with plastic deformation of the matrix being considered. The plastic strain in the matrix at the initial crack and at the deflected crack tips, when the interface crack is deflected into the RL after extensive interface debonding propagation, is characterized. The effects of interfacial debond lengths and test temperatures on the matrix cracking mechanism are discussed, based on a fatigue-damage summation rule under low-cycle fatigue conditions. The numerical results provide a rationale for experimental observations regarding the avoidance and occurrence of the matrix cracking found in fiber-reinforced titanium composites.