Flaw tolerance performance of TC4 alloy used in transmission system

Flaw tolerance performance of TC4 alloy was investigated for helicopter transmission systems design. The impact pit flaws and scratch flaws were investigated by fatigue specimens with same flaw depth for flaw tolerance design, and high-cycle fatigue S-N curve tests were conducted. The results show that both scratch flaws and impact pit flaws significantly reduce the fatigue limit of the TC4 titanium alloy. The scratch flaw has a higher flaw influence factor（K_flaw） of 2.29 compared to the impact pit flaw’s K_flaw value of 1.75 under the same flaw depth conditions. Fracture surface analysis provided laws of the fatigue crack initiation and propagation associated with these flaws. Scratch flaws mainly exhibit multiple sources, with crack initiated at the root of the scratch, while the characteristics of fatigue crack initiation for impact pit flaws vary with the applied stress. Multiple sources are observed under high stress lever, with crack initiated on the surface of the pit, while under low stress, the crack initiates at a subsurface of the pit.