Strain rate dependent mechanical performance of 3D-printed elastically isotropic TPMS-based lattices in thermoplastic polyurethane

Soft lattice structures with direction-independent mechanical properties are desirable for protective applications associated with unpredictable loading directions and deformation rates, yet the rate dependent viscoelastic behaviour of flexible thermoplastic polyurethane (TPU)-based triply periodic minimal surface (TPMS) lattices under large strain deformation remains largely unexplored. This study established a systematic framework for designing elastically isotropic TPU-based TPMS lattices through data-driven optimisation, followed by comprehensive experimental characterisation and finite element analysis across strain rates from 0.001–0.1 s−1. Three optimised structures (D-iso, IWP-iso, and P-iso) demonstrate exceptional energy absorption capacity (specific energy absorption of 0.61–1.42 J/g). Systematic optimisation achieved elastic isotropy whilst revealing a progressive decoupling of directional uniformity across deformation stages: all structures achieved strong elastic isotropy (coefficient of variation (CV) < 7%), with moderate directional variation in plateau stress (CV = 4.2–23.1%) and energy absorption (CV = 12.9–22.9%). The revealed stage-dependent isotropy characteristics provide essential criteria for selecting and tailoring soft TPMS lattices to meet application-specific requirements in protective structures across the investigated strain rate range.