DOE OSTI2019
Architected structures are gaining prominence for structural applications as the additive manufacturing technologies that can enable such materials mature. With the expanded design space, such mesoscopic structures occupy the middle ground between random foams and bulk materials. Studies into the deformation response of such structures are limited, particularly under dynamic compression. Early studies indicate the presence of an elastic precursor wave running ahead of the compaction wave. Additional studies also have shown that jetting is known to occur under shock loading conditions due to the presence of geometric perturbations introduced in the long-range periodic structures. Given that starting point, the motivation for this research was to understand the origin of the observed jetting and extend our understanding of how this comes about across a broad spectrum of lattice types, material compositions, length scales, and dynamic loading conditions. The jetting behavior due to lattice structures was studied through a series of dynamic compression plate impact experiments with in-situ x-ray imaging. This work was enabled by the NNSA-funded Dynamic Compression Sector at the Advanced Photon Source, which is housed at Argonne National Laboratory. Prior studies, and also evident in this work, have shown that deformation mechanisms in architected structures are difficult to study using our traditional diagnostics, and the role of in situ observations in this regard cannot be overstated. The role of the impact conditions, the lattice spacing, the lattice architecture, and the lattice base material was explored in the context of promoting or suppressing jet formation. A transition from latticeled to impactor-led jetting is observed above a certain impact threshold. Complementary direct numerical simulations (DNS) were also performed to compare with the experiments, study the underlying stress state giving rise to jetting, and provide insight for conditions not studied experimentally. Such simulations suggest certain trends for promoting or suppressing jetting akin to traditional Richtmyer-Meshkov instability studies. We present a geometric argument to explain the competitive process leading to lattice and/or impactor jetting which incorporates base material properties, the periodicity of the lattice, and basic tunable length scales of the lattice. Our results are consistent with previous observations on such lattices. Expanding on that work, the emergence of a transition of the jet material composition must depends solely on the base material properties and the geometric arrangement of the lattice structure. The fact that we have obtained reasonable qualitative and quantitative agreement regarding the jet formation and propagation behavior leads us to trust the underlying materials models. We can speculate that this and potentially other emergent phenomena might also be possible from a unique combination of base material properties and geometric arrangement of lattice structures. While the transition from polymer-led to flyer-led jetting might seem counterintuitive at first, it is a natural extension of the impact conditions and the arrangement of the materials. Better understanding of how the base material properties are coupled to the lattice structure to give rise to such phenomena will likely improve design for such structures under dynamic compression conditions. As such, the direct numerical simulations provide a pathway to design of architectures in future applications.