Characterization and modeling of stiffness reduction in SCS-6-Ti composites under low cycle fatigue loading

Abstract The stiffness reduction and evolution of microstructural damage of a unidirectional silicon carbide fiber reinforced titanium matrix composite under tension-tension fatigue were investigated. Tests were conducted under load control with maximum applied stresses ranging from 750 to 945 MPa. The crack density of the interfacial reaction layer and matrix, matrix crack length, and interfacial debonding length as a function of fatigue cycles and applied stress levels were measured. The results showed that the composites exhibited an initial regime with slow stiffness reduction, followed by a rapid stiffness drop regime and a plateau regime with minimal change in stiffness for the applied stress levels used in this study. The residual stiffness at N = 10 6 cycles is independent of the applied stress levels, while the microstructural damage accumulation varied with the applied stresses. A partial crack shear-lag model was also developed to predict the residual stiffness as a function of fatigue damage accumulation. Analytical simulation indicated that the profile of the stiffness reduction curves was dominated by the matrix crack density, while the extent of stiffness reduction was dominated by the matrix crack length.