120 S. Mates et al. Fig. 21.1 Fabrication drawing for alumina dumbbell compression specimens. Dimensions in inches joint on both rotation and apparent fracture stress is determined by comparing the results against similar experiments without the spherical joint. Experimental Specimens provided by the U.S. Army RDECOM were manufactured according to the specifications shown in Fig. 21.1. The test material was Coorstek1 CAP3 alumina with a nominal Young’s modulus of 370 GPa and a nominal compressive fracture strength of 5 GPa. The NIST compression Kolsky Bar is pictured in Fig. 21.2 along with the results of a bar-to-bar test showing a small first reflection, indicative of imperfect interface alignment. Dimensions of the NIST Kolsky bar are given in Table 21.1 along with bar material properties and the longitudinal elastic wave speed in the bars. The Kolsky bar test was designed to fracture the samples in a single strike with a constant strain rate. For test design purposes, the breaking stress of this material was assumed to be 5 GPa, such that the fracture load of the specimens is estimated to be 17.8 kN. The fracture strain is estimated to be 0.0135 based on this fracture load and the nominal Young’s modulus of 370 GPa. To break the sample, the striker length and impact velocity is selected to exceed both the peak load and the fracture strain. Based on a limited number of trials, a 203 mm long maraging steel striker bar (15 mm diameter) with an impact velocity of 9.9 m/s ±0.1m/s (±here is 2 standard deviations) were found to produce an adequate loading pulse. To achieve a constant strain rate, a ramped input pulse is created, following the recommendation in [1], with an annealed copper pulse shaper measuring 4.64 mm in diameter 1 Certain commercial equipment, instruments, or materials are identified in this paper to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.
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