Abstract
Fatigue crack growth behavior under constant and non-stationary variable amplitude load sequences was investigated using compact tension i specimens of Ti-6Al-4V alloy with bimodal microstructure. Fatigue crack growth rate and macroscopic crack closure were measured by means of an unloading elastic compliance method. The da/dn-ΔKeff relationship under constant amplitude exhibited a trilinear form even in so-called Paris region corresponding to the change of growth mechanism from the intergranular fracture in the low ΔK region to the transgranular one in the high ΔK region. The fatigue crack growth rate under two-step block loading was found to be higher than that estimated by da/dn-ΔKeff relation under constant amplitude loading, indicating that the load variation resulted in the crack growth acceleration terms of ΔKeff. The load variation induced the transgranular fracture in the low level loading where the intergranular fracture type was dominant under constant amplitude loading. It is considered that the transition of fracture type in the low level loading resulted in the acceleration. Under single over loading, however, the crack growth deceleration in terms of ΔKeff was observed due to the blunted crack tip.
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