Filling-fraction-dependent deformation and fatigue behaviour in interpenetrating Ta/PMMA lattice composites

Interpenetrating metal–polymer lattice structures enable mechanical tailoring through geometric constraint in addition to relative-density increase. In this study, rhombic-dodecahedron tantalum lattices fabricated by selective laser melting were infiltrated with PMMA at filling fractions of 0%, 50%, and 100% to investigate filling-dependent compressive and fatigue behaviour. Quasi-static compression tests, compression–compression fatigue experiments, and finite-element simulations were performed to characterise deformation evolution and damage progression. The results show that PMMA infiltration increases elastic modulus, yield strength, and plateau stress beyond what would be expected from relative-density increase alone, which is consistent with the reduced deformation localisation and nodal stress concentration observed in the simulations. Fatigue behaviour exhibits a non-monotonic dependence on filling fraction: full infiltration produces the longest fatigue lives and more stable cyclic deformation response, whereas partial infiltration shows improved plateau stability with limited ratcheting and damage accumulation. The combined experimental and numerical observations suggest that PMMA infiltration alters the dominant deformation mode of the lattice and promotes a more spatially distributed load-transfer behaviour. These findings provide a mechanism-guided interpretation of how filling fraction influences deformation stability and fatigue response in interpenetrating lattice composites and indicate that filling fraction can be used as a design variable for balancing stiffness, strength, and fatigue durability.