The Effect of Microstructure on Fatigue Crack Growth and Crack Closure Behavior in Ti-6AI-4V Alloys under Variable Amplitude Loading

摘要

Fatigue crack growth test were carried out on two kinds of annealed (α+β) Ti-6Al-4V alloys with different microstructures, which were classified into type A and B, under constant amplitude and repeated two-step loading. The microstructure of type A was characterized by equiaxed primary α-phase grains and that of type B was the large grain and needle bar α-phase grains. Crack closure behavior as well as crack growth rates of those materials was investigated. The results obtained are as follows; (1) Fatigue crack growth mechanism under constant amplitude loading varies depending on the load level. In type A, transgranular fracture occurred at low growth rate and a crack grew preferentially within α-phase at high growth rate. In type B, microstructurally sensitive mode of fatigue crack growth was observed at low growth rate. (2) The crack growth retardation was observed in terms of stress intensity factor under repeated two-step loading in both types. The crack opening point was found strongly affected by the amplitude of low-level load and became higher than that predicted by constant amplitude data having the identical maximum stress intensity with the low-level stress intensity factor, K_L, because both plasticity induced crack closure by K_H and roughness induced crack closure caused by rough surface operated simultaneously. (3) In type A, the fracture surface under repeated two-step loading was very rough compared to the fracture surface morphology under constant amplitude loading, especially in lower K_L conditions, while the roughness of fracture surface was almost same between repeated two-step and constant amplitude loading in type B. (4) It was found that the fatigue crack growth rate under variable amplitude loading could be well predicted in terms of effective stress intensity range taking account of crack closure behavior in both materials.