Fatigue crack growth transitions in Ti-6Al-4V alloy

Abstract

There are 2 transitions during fatigue crack growth in Ti-6Al-4V alloy, caused by cyclic and monotonic plastic zones when their resp. sizes become equal to the $\alpha$ lath size. The transition due to cyclic plastic zone size is a combined action of environment and the stage where cyclic plastic zone size becomes equal to lath size. Crack closure levels exhibited a transition. The crack path exhibited multiple crack branching at this transition. The cyclic plastic zone size is uniquely related with $\delta$Keff (closure-free stress intensity range) as against $\delta$K at large crack closure. A transition occurred when the monotonic plastic zone size was equal to the $\alpha$ lath size, and this is attributed to the change from continuum to single crystal behavior at the crack tip. Both the transitions were induced by $\alpha$ laths because of thick interplatelet $\beta$ layer in the microstructure.