Environmentally Influenced Mixed Mode Fatigue Crack Propagation of Titanium Metal Matrix Composites

Effect of humid air environments on the mixed mode fatigue crack propagation behavior of B₄C-B and BORSIC reinforced Ti-6A1-4V metal matrix composites was studied. Humid environments enhanced the mixed mode fatigue crack propagation rates in the as-received titanium matrix composites atR = 0.1. The effect was more pronounced in the transverse and 45 deg specimens. A transition in failure modes from fiber splitting in humid air to interfacial splitting in dry environments was observed at a load ratio of 0.1. The transition took place at around 100 Pa water vapor pressure, where a steep rise in fatigue crack propagation rate was noticed. AtR = 0.5, however, no fiber splitting was observed in humid air. Fatigue crack closure load measurements revealed that closure loads were higher in humid air than in dry environments. The fiber splitting is suggested as an environmentally induced crack closure effect, where plastically deformed matrix sets up stress fields (radial and mode III stresses) on the brittle boron fibers weakened by the humidity.