Fiber volume fraction effects on fatigue crack growth in SiC/Ti-15-3 composite

The effects of fiber volume fraction (15, 37, and 41%) on fatigue crack growth in unidirectional SiC/Ti-15-3 composite were investigated at room temperature. The effect of fiber volume fraction on the fiber bridging mechanism was studied to support development of physically-based crack growth models. While each fiber volume fraction exhibits similar decreasing crack growth rates prior to fiber bridging induced crack arrest, post-arrest behavior (observed after incrementally increasing the applied stress level) is quite different. After crack arrest, the 15% (37 and 41%) material exhibited higher (lower) crack growth rates and lower (higher) toughness values than the unreinforced matrix. These different behaviors occur because of differences in the amount of fiber bridging during the post-arrest regime. Metallography of interrupted tests revealed the extent of fiber bridging in the crack wake and matrix plasticity ahead of the crack tip. Models for predicting the effective matrix stress intensities were evaluated and compared to experimental data. A fiber pressure model and finite element studies were used to estimate the condition of the bridged fiber zone and associated fiber stresses. Since the vast majority of useful life for these materials experiences fatigue crack growth, these results assist in discerning an optimum fiber volume fraction for structural applications.