Studies on the life, damage evolution, and crack propagation behaviors of TC18 titanium alloy under repeated impact loading

TC18 titanium alloy has been widely used as the key structure which is often subjected to repeated impacts in service in aerospace engineering. In this study, the repeated impact responses of TC18 were investigated by conducting various repeated drop hammer impact tests. Abundant microscopic characterization technologies were used to reveal the correlation between damage evolution and the impact response. The results show that the average impact fatigue life (Nf) of the notched three-point-bend specimen decreases exponentially with the impact energy. Microscopic observation shows that cracks nucleate at the initial 40-50 % of Nf, then propagate steadily during the next 50-90 % of Nf. During the last 10 % of Nf, the cracks propagate rapidly, resulting in the failure. During crack propagation, the repeated impacts induce complex stress states in the vicinity of cracks, leading to grain refinement, phase transition, and strengthened texture. The phase transition and strengthened texture mechanisms are mainly due to the generation of the platelet-like alpha subgrains inside the refined grains. The results in this paper are crucial for understanding the microscopic damage and failure mechanism of TC18 under repeated impacts, which helps to establish the damage accumulation-based life prediction model in the future.