Rate-dependent energy absorption and design optimization of STG-filled functionally graded TPMS metastructures

Developing lightweight protective structures capable of adapting to varying impact intensities remains a critical challenge in engineering. This study proposes a novel bio-inspired mechanical metastructure by incorporating shear thickening gel (STG) into primitive triply periodic minimal surface (TPMS-P) shellular lattices with functionally graded designs. A comprehensive investigation combining quasi-static compression, low-velocity drop-weight tests (1–2.5 m/s), and high-fidelity coupled Eulerian-Lagrangian (CEL) finite element simulations (200 m/s) was conducted to elucidate the fluid-structure interaction mechanisms. Experimental results demonstrate that STG filling significantly alters the deformation mode from localized buckling to global hydraulic bulging. A pronounced rate sensitivity is observed: the STG acts as viscous damping at low velocities but transitions to a stiffening reinforcement at higher velocities, resulting in a rate-adaptive energy absorption response. Furthermore, the topological hierarchy study reveals that a denser cell array (3×3) enhances fluid confinement, outperforming lower-order counterparts. Crucially, numerical investigations under high-velocity impact uncover a conflict in gradient orientation requirements. While a negative gradient (P70–30) provides improved cushioning under low-speed loading, a positive gradient (P30–70), featuring a compliant impact layer, is essential for mitigating high-velocity impact. The P30–70 configuration effectively suppresses the initial inertial shock, reducing the loading mean crushing stress (MCS) by approximately 11% and creating a prolonged zero-stress buffering window at the support. This work establishes a systematic design strategy for rate-adaptive, reusable, and efficient impact-resistant metamaterials, with strong potential for advanced protective applications.