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 crac k growth in unidirectional SiC/Ti-15-3 composite were investigated at room temperature. The effect of fiber volume fraction on the fiber bri dging mechanism was studied to support development of physically-based crack growth models. While each fiber volume fraction exhibits simila r 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. Thes e different behaviors occur because of differences in the amount of fi ber bridging during the post-arrest regime. Metallography of interrupt ed tests revealed the extent of fiber bridging in the crack wake and m atrix plasticity ahead of the crack tip. Models for predicting the eff ective matrix stress intensities were evaluated and compared to experi mental data. A fiber pressure model and finite element studies were us ed to estimate the condition of the bridged fiber zone and associated fiber stresses. Since the vast majority of useful life for these mater ials experiences fatigue crack growth, these results assist in discern ing an optimum fiber volume fraction for structural applications.