Effect of microstructural length scales on spall behavior of copper
- PDF / 788,134 Bytes
- 11 Pages / 608.4 x 786 pts Page_size
- 57 Downloads / 218 Views
6/30/04
6:19 AM
Page 2663
Effect of Microstructural Length Scales on Spall Behavior of Copper ROGER W. MINICH, JAMES U. CAZAMIAS, MUKUL KUMAR, and ADAM J. SCHWARTZ A systematic study to quantify the effects of specific microstructural features on the spall behavior of 99.999 pct copper has revealed a strong dependence of the failure processes on length scale. Shock loading experiments with Cu flyer plates at velocities ranging from 300 to 2000 m/s (or impact pressures from 5 to 45 GPa) using a 35-mm single/two-stage light gas gun revealed that single crystals exhibit a higher spallation resistance than fine-grained polycrystals and internally oxidized single crystals. However, in contrast to previously reported results, the fine-grained (8-m) polycrystalline samples exhibit lower damage resistance than the coarse-grained (50- and 133-m) samples. These observations have been analyzed in the context of the length scale inherent in each of these microstructures, and modeled using an analytical model developed recently.
I. INTRODUCTION
THE spall process is a series of complex phenomena that includes nucleation, growth, coalescence/interaction of voids, and, finally, separation or spallation. When a compressive shock is introduced into a material via a high explosive or plate impact, the interaction of rarefaction waves can generate a locally stronger tensile pulse. If the magnitude of this tensile pulse locally exceeds a critical threshold stress, voids or cracks can nucleate. When such void or crack networks link together, spall failure results. If voids nucleate and grow but do not coalesce leading to full separation, then it’s termed incipient spall. It is now widely accepted that the microstructure of a material can exert a strong influence on spall characteristics.[1,2] For example, experiments on high-purity copper reported by Kanel et al.[3] indicated that the spall strength of single-crystal copper is a factor of 2 to 3 times higher than that of commercial-purity polycrystalline copper. Although details of the purity of the single-crystal and polycrystalline materials were not provided, these results clearly indicate the potential importance of grain boundaries or impurities to the nucleation, growth, and coalescence of voids in materials. Seaman and co-workers[4,5] executed one of the most comprehensive studies to date on this subject in the late 1960s and early 1970s, which resulted in spall models for ductile and brittle failures. Significant results from these studies concerned the work on various grades of aluminum, including high-purity 1145 and 2024-T81 Al. It was found that the threshold stress for void nucleation was not strictly a function of the yield strength; the highpurity Al, which has much lower yield strength, exhibited
ROGER W. MINICH, JAMES U. CAZAMIAS, MUKUL KUMAR, and ADAM J. SCHWARTZ, Staff Scientists, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Contact e-mail: [email protected] This article is based on a presentation given in the symposium “Dynamic Defo
Data Loading...
