Experimental and microstructure-sensitive fatigue modeling of the effects of periodic dwell and overload on additively manufactured Ti-6Al-4V

Traditional microstructure-sensitive fatigue life prediction studies to date have focused on constant amplitude loading (CAL), while most real-world service conditions often involve variable amplitude loading (VAL). To address this limitation, the present study investigates the fatigue response of Ti-6Al-4V produced via laser powder bed fusion under two representative VAL conditions, i.e., periodic dwell holds and periodic overloads. Given the growing adoption of additive manufacturing techniques and the susceptibility of Ti-6Al-4V to cold dwell fatigue, this alloy provides a platform for studying microstructure-sensitive fatigue behavior under a wider range of loading conditions. Initially, experimental fatigue tests were conducted on specimens subjected to CAL, periodic dwell holds, and periodic overloads to quantify the impact of these VAL events. The results revealed that periodic dwell caused a reduction in life compared to CAL, whereas no significant decrease in life was observed in the case of periodic overload. Further, crystal plasticity finite element modeling was performed on statistically equivalent virtual microstructures with explicitly modeled prior $\beta$ grain boundaries, to provide insights into the mechanism behind the experimentally observed differences in damage. The evolution of slip system activity following the application of the VAL events was found to govern the difference in fatigue life and also the resulting accumulated plastic strain energy density, which is used as a damage metric in this study. The microstructure-sensitive modeling provided an agreement with the experimentally observed fatigue trends and a mechanistic understanding of the underlying deformation mechanics leading to fatigue damage.