Additively Manufactured Lattice Structures

The specific objective of this research is to understand the underlying deformation mechanisms that occur at multiple length scale, which control yielding, post-yielding, plateau region, failure mechanism and give rise to widely disparate local responses in metallic AMLS (Fig. 2). The focus is on understanding the specific contribution and interplay between the microstructurally driven mechanisms (e.g., grain structures, orientation, porosity) and geometrically driven events (e.g., unit-cell buckling, node fracturing, macroscopic shear) on the dominant deformation mechanisms, as well as under what condition one mechanism becomes dominant. Once the governing mechanisms that controls strength and failure modes are understood, we will investigate whether they remain the same when the rate of the deformation increases. Doing so will reveal preferred materials' microstructures and topologies for a specific application and exactly what can be done to control unexpected failure.

Figure 6 shows the numerical results for 4 mm unit cell Octet truss, Diamond (D), Rhombic dodecahedron (RD), Dode-medium (DM). From Figure 6a and 6d unexpected softening after 1 mm and 0.5 mm displacement for OT and D respectively can be noticed. The simulation shows damage of the internal struts is the major cause of the drops in force-displacement behavior in all of the topologies studied here. In contrast, Figure 6b and Figure 6c show hardening during compression deformation. In all unit cells, damage tends to initiate from the junctions and propagate into the struts. This is due to the fact that the aspect ratio of the struts are rather large and they are not expected to experience high bending stresses due to their high flexural stiffness. When damage initiates from multiple junction sites, such as OT structure, a more or less steady plateau is observed in the stress-strain curves. That plateau is an indication of homogeneous propagation of damage. Diamond unit cell shows the least number of damage initiation sites as in Figure 6d, so further loading will cause rapid propagation of damage in the junction that effectively cleaves the struts and causes a significant drop in the load. All structures show eventual densification and internal contact but depending on the geometry, the initiation of densification and its extent are may vary. Unit cells with RD and DM topologies, Figure 6b & c, show a secondary hardening region due to self contact as a result of initial damage in Rhombic Dodecahedron and Dode Medium structures. This secondary hardening curve will lead to a secondary softening due to failure of newly contacted struts. Therefore, considering a single unit cell as the structural element, they can be classified as softening-based and hardening-based structures. Generally, the more diffused initial damage sites are, the more uniform load-displacement response is expected.