Internal crack initiation characteristics and early growth behaviors for very-high-cycle fatigue of a titanium alloy electron beam welded joints

The very-high-cycle fatigue has become an important issue in recent years. The crack initiation mechanism and early crack growth behaviors of electron beam welded joints of TC21 titanium alloy were discussed. In order for the subsurface crack growth to be observed, a repeated two-step fatigue test was performed for the beach mark formation. Two kinds of initiation modes of microcracks could exist. One is the initiation from microvoids and the other results from the dislocation accumulation. During early growth, the microcrack path could deflect when it passed through the boundaries, leading to the FGA region formation. The crack closure could also occur during growth. By the Paris' law integration, the crack propagation life was discovered to occupy only a small number of the fatigue life.