High strain rate mechanical behaviour of uniform and hybrid metallic TPMS cellular structures

Four different geometries of uniform triply periodic minimal surface (TPMS) cellular structures (Diamond, Gyroid, IWP, and Primitive) and a hybrid Gyroid–Diamond TPMS structure were fabricated and experimentally tested under different compressive strain rates ranging from quasi-static (at 0.005 s⁻¹) to very high strain rates (at 11,000 s⁻¹). Three different testing apparatuses were used: a universal servo-hydraulic testing machine, a direct impact Hopkinson bar (DIHB), and a powder gun. Limited strain rate hardening was observed at quasi-static, low- and intermediate-dynamic deformation, while the shock deformation mode showed significant strain rate hardening of all tested structures. The computational models were developed and validated with the experimental results and then used to predict the TPMS structure's behaviour even at higher strain rates (up to 35,000 s⁻¹), not attainable by used experimental devices. The Diamond and IWP structures exhibited up to five times higher (an increase from 6.9 to 40.8 J/g) specific energy absorption (SEA) during very high strain rate loading than during the quasi-static and lower dynamic testing. The tests of hybrid samples showed that the sample orientation significantly affects the mechanical high strain rate response, which can be either progressive (less stiff structure at sample impact end) or regressive (stiffer structure at sample impact end).