Quasi-static and dynamic behavior of additively manufactured lattice structures with hybrid topologies

When different unit cell topologies with distinct mechanical behavior (e.g. bending vs stretching dominated) are incorporated into a single hybrid lattice structure (LS), questions arise about the resolution of local stresses within the struts and how localized states of strain as a result govern the global response of the structure. To understand the mechanics of hybrid LS, this study uses a combination of experimental and modeling data to investigate the relationship between localized states of stress with the global behavior of hybrid additive manufactured lattice structures (AMLS) under different loading directions and strain rates. The hybrid AMLS in this study consist of two different unit cell topologies stacked in alternating rows, with loading directions identified with respect to this topology stacking. It is shown that the loading direction influences the mechanical behavior, as the flow stress of the hybrid AMLS is 7%-10% lower when loaded in the stacking direction than when loaded in the transverse direction. This flow stress decrease is due to a smaller number of structural elements supporting the loading and tensile failure of horizontally-manufactured struts in the stacking direction. The strain rate also influenced the mechanical behavior of the AMLS, as irrespective to the loading direction, for all hybrid AMLS, the first peak stress after static equilibrium is 5%-10% higher under dynamic loading compared to quasi-static loading. Additionally, it is shown that the collapse mechanisms are influenced by the order of the topology stacking. Structural shear band formation, which leads to up to a 60% drop in flow stress under dynamic loading of the hybrid AMLS, can be inhibited by separating adjacent rows of shear band-forming topologies with a row of unit cells of a topology which does not form shear bands. Ultimately, it was determined that the performance of these layered structures is limited by the weakest topology. Even under transverse loading, where the first peak stress approaches that of the stronger topology, the magnitude of the subsequent decrease in flow stress is generally more in line with that of the weaker topology.