Numerical investigation of sub-critical crack growth in α-titanium due to hydride formation and fracture

Hydride-induced embrittlement is one type of material deterioration caused by hydrogen in metals such as zirconium, titanium, magnesium, niobium and vanadium. According to in-situ electron microscopy studies, embrittlement may occur by the precipitation of brittle hydrides at stress concentration locations. The fracture of the hydrides could lead to delayed hydride cracking, which is a sub-critical crack growth mechanism operating by the repetition of hydride precipitation and fracture near the crack tip. In the present work, steady-state sub-critical crack growth in α-titanium is investigated. It is confirmed that hydride precipitation has a strong effect on the hydrostatic stress level ahead of the propagating crack and leads to significant deviations from the well-known near-tip stress distributions in elastic-plastic materials. The threshold stress intensity factor for sustained load cracking is predicted by considering the distribution of the hoop stress on the crack plane. The extent of the hydride-rich zone near the crack tip is also calculated and used for the estimation of the velocity of the crack within stage-II regime. Thus a simplified diagram of crack velocity vs. applied stress intensity factor is constructed.