Relative-density-dependent flexural and fatigue behaviour of LPBF-fabricated 316L stainless steel TPMS sandwich structures

Laser powder bed fusion (LPBF)-fabricated triply periodic minimal surface (TPMS) sandwich structures are promising for lightweight load-bearing and energy-absorbing applications, but their flexural response and bending fatigue behaviour remain less understood than their compressive behaviour. It remains unclear whether increasing relative density (RD) can be consistently converted into improved flexural strength, stiffness and energy absorption across different TPMS topologies. To address this issue, 316L stainless steel (SS 316L) Diamond, Gyroid and Primitive TPMS sandwich structures with relative densities of 20%, 30%, 40% and 50% were fabricated by LPBF, and their three-point bending response, deformation behaviour and fatigue failure were investigated. All printable specimens retained clear periodic pore-channel features, whereas the 20% relative-density Diamond specimen (D20) exhibited pinch-off and failed to form a continuous load-bearing core. Increasing relative density generally improved flexural strength and stiffness, but did not consistently enhance energy absorption. From 40% to 50% relative density, the flexural strength, stiffness and energy absorption of the Diamond structure increased by 16.6%, 14.7% and 20.8%, respectively, whereas the energy absorption of Gyroid and Primitive structures decreased by 3.9% and 0.1%, respectively. The 50% relative-density Diamond specimen (D50) achieved the highest flexural strength of 446.48 MPa and energy absorption of 127.04 J. Under cyclic bending, D50 fractured after approximately 3.99 × 10 4 cycles at 310 MPa and reached 2 × 10 6 -cycle run-out at 150 MPa.