Prediction of property degradation and fatigue life of fiber-reinforced titanium alloy composites

The damage mechanisms of a SCS-6 Silicon Carbide fiber-reinforced Ti-15 at%A1-3 at%Cr-3 at%Sn-3 at%Nb alloy (ductile matrix) and Ti-22 at%Al-23 at%Nb orthorhombic alloy (brittle matrix) under tension-tension cyclic loading were carefully investigated. The accumulation of fatigue damages including matrix cracking density, matrix cracking length, and interfacial debonding length as a function of fatigue cycles was characterized. The degradation of mechanical properties as a result of microstructural damage during fatigue loading was also measured. It was found that matrix cracking, interfacial debonding, and fiber breakage are the three fatigue damage modes of these composites. For the SCS-6/Ti-15Al-3Cr-3Sn-3Nb composite, matrix cracks initiated from the damaged interfacial reaction layers and propagated perpendicularly with respect to the loading axis, while the interfacial debonding also initiated from the damaged interfacial reaction layer and propagated along the fiber/matrix interface. Most of the fibers remained intact and bridged the matrix cracks when the maximum applied stress levels were below 750 MPa. However, in SCS-6/Ti-22Al-23Nb composite, matrix cracks initiated from the free edge of the samples and propagated perpendicularly with respect to the loading axis.; Micromechanical models incorporating empirical formulas were developed to simulate the evolution of matrix crack density and matrix crack length. The residual stress was successfully predicted by using the shear-lag model and rule of mixture. The residual strength of the composites was predicted by using the load carrying capacity of the composites as the failure criterion. The distribution of fatigue lives of the composites under various applied stress levels were determined by Weibull statistics simulation. The effect of fiber strength and applied stress levels on the distribution of fatigue lives was also discussed. Good correlation between the experimental results and theoretical prediction was achieved.