Influence of Microstructure on the Spall Failure of Aluminum Materials
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INTRODUCTION
SPALL is a category of dynamic tensile fracture characterized by a planar failure surface formed from multiple initiation sites and instigated by the constructive interference of rarefaction waves.[1] Spall can occur under shock loading of a material specimen with finite thickness and is associated with fast rates of local tensile straining, typically in the range of 104 to 108 s–1. Two rarefaction waves, also called release waves, are required to cause spall. Typically, the first is the rarefaction wave trailing a compressive wave caused by an impulsive shock loading to the front face of the specimen. The second rarefaction wave propagates in the opposite direction of the first and is caused by interaction of the initial compressive wave with the back face of the specimen. These two rarefaction waves interact at a distance from the back face to create a locally intense plane tensile wave, which initiates and propagates spall fracture. Spall can result in the separation of a region of material from the back face of a specimen, leaving either JONATHAN L. BREWER, Engineer, is with the Stress Engineering Services, Inc., Houston, TX 77041, USA. D. ALLEN DALTON, and WILL GRIGSBY, Graduate Students, and TODD DITMIRE, Professor, Department of Physics, EVAN D. JACKSON, Undergraduate Student, and ERIC M. TALEFF, Associate Professor, Department of Mechanical Engineering, and AARON C. BERNSTEIN, Research Associate, are with the Fusion Research Center, University of Texas at Austin, Austin, TX 78712, USA. Contact e-mail: taleff@mail.utexas.edu This article is based on a presentation made in the symposium entitled ‘‘Dynamic Behavior of Materials,’’ which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee. Article published online July 11, 2007 2666—VOLUME 38A, NOVEMBER 2007
an attached blister or completely removing a section of material from the specimen. When material separation by fracture is not complete, this is referred to as incipient spall, which is a condition particularly useful in the study of spall failure initiation. Both ductile and brittle modes of spall have been observed.[2,3] In ductile spall, voids nucleate throughout the spall plane, and then grow and coalesce into a fracture. In brittle spall, microcracks nucleate and grow along the spall plane, eventually interlinking to cause fracture. Hopkinson, in 1914, first investigated spall of nickel and steel plates by detonating gun cotton to cause shock loadings.[4] Other methods of controlled shock loading used to study spall include mechanical impact and irradiation. A typical mechanical impact method is the flyer plate, which allows for shock loading to strain rates from 104 to 106 s–1.[5] High-intensity, pulsed lasers can induce shock loading at strain rates of 106 s–1 and faster. Laser-induced shock results from ablation of material by a laser on
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