A novel defect-related cyclic damage model driven by strain energy density for LPBF titanium alloy

The unavoidable defects in additive manufacturing (AM) material have an impact on the deformation behavior and fatigue lifetime. In this study, a series of experiments on Laser powder bed fusion (LPBF) Ti-6Al-4 V under symmetrical tension–compression cyclic loading were conducted, and the corresponding damage mechanism was then investigated. It is revealed that due to the nucleation, growth, and coalescence of micro-voids, LPBF Ti-6Al-4 V exhibits accelerated softening behavior after reaching cyclic stability. Based on the physical mechanism, a cyclic damage model with a critical initiation criterion was developed using fracture mechanics principles, in which plastic strain energy as the driving parameter is introduced to describe the defect evolution, and the void's closure effect during the tension–compression cycle is considered. Experimental validation has demonstrated that the damage model can accurately predict the cyclic stress–strain curves for LPBF Ti-6Al-4 V vertical and horizontal specimens, with errors in predicting crack initiation and growth lifetime falling within the ± 1.6 and ± 2.5 scatter bands.