Effect of mesoscale inhomogeneities on planar shock response of materials

Abstract

In all previous spall models, the source of spall failure in metals either comes from damage at the grain boundary or from void nucleation, growth, and coalescence. However, it has been observed in experiments that both phenomena occur in Aluminum 6061-T6, which is termed “combined failure” for the purposes of this thesis. Thus, the challenge undertaken in this thesis is to use a computational study to determine the role that each source of spall plays separately, and then in tandem to determine the traditional failure parameters for each source. The results of determining each failure model’s ideal parameters, which are representative of that source’s role in combined failure, is compared with data gathered from plate-flyer experiments to determine the accuracy of the model in both 1D and in 2D simulations. Sand is a heterogeneous granular material that has the capability of allowing a shock wave to propagate through it. The computational model and study presented in this thesis is phenomenologically similar, yet easier to conduct than a spall study on granular Aluminum. The study of sand using the same computational LS-DYNA method shows both an introduction to the process for completing the spall study on granular Aluminum, and it also yields interesting results in the wave phenomena as well as the effect of porosity on the average stress on the sand grains. With the conclusion of the sand study, the same process of creating the grain structure is applied to create the Aluminum grain structure for spall simulations, which are carried out in LS-DYNA using 2D cohesive elements. The results of the LS-DYNA Aluminum simulation are compared to both the 1D spall results as well as to the experimental data to determine model accuracy. The main findings from this thesis show that, first, a mutually exclusive combined failure linear relationship can be shown with the 1D simulation results, which gives insight into a method that could be used to choose a set of optimal failure parameters. Second, the 2D LS-DYNA homogeneous results had excellent agreement with the 1D homogeneous results, which gave confidence to the notion that the parametric studies in 1D simulations could be used to find parameter values that could be applied in the 2D models. Lastly, LS-DYNA was shown to be an effective way to simulate grain structure response to shock wave propagation and showed spall modeling was possible with 2D cohesive elements, which lays the groundwork for combined failure studies in 2D.