25. High-Rate Mechanical Response of Aluminum Using Miniature Kolsky Bar Techniques

The compressive stress-strain response of 99.999% pure aluminum at strain-rates ranging from 5 k/s to 1 M/s is presented. Experiments were conducted using optically instrumented miniature Kolsky bars. Samples were machined from 25 um thick rolled annealed foils using a laser mill and a focused ion beam. They were oriented such that the compression axis of each sample is along the direction of the foil thickness so that the final dimensions of each cylindrical sample was nominally 25 um long and 50 um in diameter. The grain size of the material, as well as its texture, was determined by EBSD. The results are compared to published pressure shear plate impact data on the same material at similar strain-rates. It is found that the mini-Kolsky bar data contains a large amount of scatter, which is attributed to the relatively large grain size compared to the small sample volume. In spite of this scatter, it is observed that the flow stresses measured using the miniature Kolsky bar methods are substantially lower than those measured with pressure shear plate impact even at similar strain-rates. Potential reasons for this discrepancy include texture effects in the samples and pressure hardening that is present in the pressure shear experiment but absent in the Kolsky bar.