A new combined impact fatigue damage model and its application of influencing factors analysis on impact fatigue of TC18 titanium alloy

Based on the theory of continuum damage mechanics, a new elastic damage evolution model taking into account the effect of strain hardening is proposed. It is then combined with the previously proposed elastic-plastic damage evolution model considering the strain rate effect to calculate impact fatigue damage evolution and predict fatigue life for specimens undergoing various impact loading conditions. The impact fatigue test was conducted by a hammer impact testing machine and the experimental data was used to verify the proposed damage model. Afterwards, the model was applied to investigate the influences of loading condition, geometric configuration, and specimen size on the impact fatigue damage of TC18 titanium alloy. The calculation results reveal some different characteristics from conventional fatigue. First, the impact energy cannot be regarded as the only index of loading level, because the damage evolution rate increases with the decrease of impact velocity even under the same impact energy. Second, the impact fatigue life becomes longer as the stress concentration coefficient increases under the same loading conditions, which is contrary to the cases of conventional fatigue. Third, the effect of specimen size on impact fatigue life presents a new trend. The larger the specimen size, the shorter the impact fatigue life for notched specimens, while the longer the impact fatigue life for smooth specimens.