Strength and energy absorption characteristics of Ti6Al4V auxetic 3D anti-tetrachiral metamaterials

Abstract

The strength and energy absorption properties of the auxetic 3D anti-tetrachiral (3ATC) lattice, a member of the newly formalized joint-rotation-dominated cellular topology, were studied for the first time, using analytical modelling, simulations and experiments employing additively manufactured titanium alloy (Ti6Al4V) lattices. A limited ductility failure model was employed in the simulations to accurately account for the enhanced brittleness of 3D printed Ti6Al4V. The 3ATC lattices exhibited 2 distinct relative strength vs. relative density relationships. If the relative density was varied through changes in strut length, the relationship was linear, a result that has, until now, been a distinguishing trait of rigid stretch-dominated lattices, rather than auxetic lattices, which normally experience strut bending. On the other hand, if the relative density was varied through changes in strut width, the relationship was highly nonlinear and does not follow the power law trend that is typical of many cellular solids. The strength of 3ATC lattices can be higher than that of stochastic foams, but were lower than those of stretch-dominated designs. In addition, 3ATC bulk lattices exhibited highly uniform properties across the three different orthogonal loading directions, despite individual unit cells being highly orthotropic. The failure of 3ATC lattices was characterized by a series of bumps in the plateau regime, which corresponded to the sequential failure and compaction of individual unit cell layers in the lattice. The specific energy absorbed during the failure process was found to be linearly related to the failure strength. The mechanical characteristics of the 3ATC lattices are dependent on 3 normalized design parameters: i) strut length, ii) strut width, and iii) eccentricity, offering enhanced flexibility for design optimization over conventional lattice designs. When manufactured with a material of high specific strength like Ti6Al4V, it was shown that 3ATC lattices can be useful as lightweight and high strength auxetic architected metamaterials that provide good energy absorption capabilities.