Load ratio effects on fatigue response of titanium matrix composite at elevated temperature

Fatigue tests were conducted to characterize the effects of loading ratio (i.e. stress and strain ratios) on the fatigue life and damage mechanisms of a unidirectional, ceramic fiber reinforced titanium matrix composite (SCS-6/Ti-15-3) at elevated temperature (427°C). Trends in the stress–strain response, fatigue life, and damage mechanisms were examined in detail for each control mode and loading ratio. A micromechanical analysis was also conducted to supplement the experimental observations. For the strain-controlled mode, it was observed that the most severe damage and shortest fatigue life occurred under the fully reversible condition when compared on the maximum strain basis. The fatigue life curves showed a layered effect (i.e. slopes were the same but they were shifted along the abscissa) when plotted on the maximum strain basis, with the fatigue life increasing as the strain ratio increased (i.e. as strain range decreased). Similar trends in damage and fatigue life were observed under the load-controlled mode. The fatigue life data from both loading modes collapsed on to a single curve when compared on the strain range basis for an applied loading ratio less than or equal to zero. However, at a loading ratio of 0.5, the fatigue life curves of the two loading modes diverged from each other as the strain range increased. In this case, the fatigue life under the strain-controlled mode was longer than under the load-controlled mode.