Multiaxial fatigue behavior of triply periodic minimal surface lattice structures in Ti6Al4V fabricated by laser powder bed fusion under combined axial–torsional loading

Multiaxial fatigue resistance is fundamental to the durability of porous metallic implants subjected to physiological stress states; however, limited data are available to guide lattice design under such conditions. This study investigated the low-cycle fatigue (LCF) response of Ti6Al4V lattice structures—gyroid, primitive, and diamond—fabricated via laser powder bed fusion (LPBF). Cylindrical specimens were tested under strain-controlled axial–torsional loading, with an axial strain ratio of R = 0.1 and a torsional strain ratio of R = –1. Quasi-static testing revealed topology-dependent behavior, with the diamond lattice exhibiting the highest axial strength, whereas the primitive structure demonstrated enhanced shear capacity. Under cyclic loading, the gyroid lattice achieved superior fatigue performance, with fatigue lives ranging in the decades 10 1 -10 2 cycles, depending on the strain amplitude and geometry. Fatigue behavior was predicted using the Brown-Miller and Fatemi–Socie parameters, with the latter achieving the highest correlation (R 2 > 0.92). An energy-based approach using the average plastic strain energy density further distinguished the structural performance. Fractographic analysis revealed fatigue-driven crack initiation at manufacturing-induced surface defects, characterized by striations, shear bands, and ductile rupture zones. These results underscore the importance of geometric continuity and strain localization in governing multiaxial fatigue life and positioning gyroid lattices as optimal candidates for fatigue-critical biomedical implants.