Effect of twist on indentation resistance

With mechanical metamaterials that display force-torque coupling receiving recent attention, we tested the effect of twist on indentation resistance. We hypothesised that the force required to indent a twisting lattice to a set depth would increase with the amount of transverse deformation caused by twist. Based on previous work, various chiral lattices were designed to twist by up to 1.3° per 1 % compression. These lattice designs were 3D-printed in Nylon-12, then tested, with the experiments replicated in finite element simulations. Indentation resistance increased with twist; the lattice with maximum twist required ∼70 % higher indentation force (when normalised to compressive stiffness) than the non-twisting (antichiral) one during spherical indentations to 1 % of sample thickness. Further, we calculate the expected effects of twist on indentation resistance by combining the established micropolar and Willis’ plane stress moduli with classical Hertzian indentation equations. We found reasonable agreement (within 10 %) between 2D calculation methods use here and previous 3D calculation methods. The indentation resistance calculated using the micropolar plane stress modulus followed the same trends as the simulations and experiments. The calculated indentation force was within 10 % of the simulations and experiments.