INTERFACE EFFECTS ON CRACK DEFLECTION AND BRIDGING DURING FATIGUE-CRACK GROWTH OF TITANIUM MATRIX COMPOSITES

The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites ha s been investigated using three interfaces with significantly differen t mechanical characteristics. Each of these composites exhibited stres s ranges in which fiber bridging was present and stress ranges in whic h stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber brid ging, was significantly below that of the matrix. This phenomenon, bel ieved to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface Failure ahead of the crack tip and its influence on the local stress intensit y factor is believed to be responsible for the decrease in the shieldi ng effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the cra ck tip was seen in two of the three composites. A stress-based criteri on for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflec tion even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improv ed transverse properties without compromising the fatigue life. (C) 19 97 Acta Metallurgica Inc.