Contrasting the Microstructural and Mechanical Response to Shock Loading of Cold-Rolled Copper with Annealed Copper
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NTRODUCTION
IT is well known that many materials display a strain rate dependence on their response to loading.[1–4] As a result, multiple materials have been subjected to shock loading experiments over the years. Of these, one of the most common is Cu, which has undergone shock loading using a variety of experimental methods, over a large range of peak shock pressures.[3,5] Cu responds to shock loading by generating a dislocation cell structure.[3] Under mechanical testing, Cu loaded at a higher strain rate (104 to 105 s 1) produces a greater increase in yield strength than Cu loaded at a lower strain rate (0.01 to 1 s 1).[3] These two factors are linked to the existence of dislocation tangles acting as a barrier to further plastic strain. Cu represents a relatively inexpensive, easily obtainable material, whose response to shock loading has been thoroughly investigated in the literature. This provides a good knowledge base from which to investigate changes to the traditional shock experiments, in this case, the introduction of plastic strain into the material (or cold working) prior to shock loading. Little literature exists regarding shock loading cold-worked Cu. II.
EXPERIMENTAL DETAILS
The term ‘‘specimen’’ will be used to refer to the recovered shock loaded material and ‘‘sample’’ for the
DANIEL L. HIGGINS and BO PANG, Postgraduate Students, IAN P. JONES, Professor, and YU-LUNG CHIU, Lecturer, are with the University of Birmingham, Birmingham B15 2TT, U.K. Contact e-mail: [email protected] JEREMY C.F. MILLETT and GLENN WHITEMAN, Senior Scientists, are with the AWE, Aldermaston RG7 4PR, U.K. Manuscript submitted April 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
section of the specimen that is prepared for analysis in a microscope, or for hardness analysis. Polycrystalline specimens of annealed and coldworked Cu were subjected to plate impact in the AWE 70 cm bore 1 m single stage gas gun. The cold-worked Cu had seen a reduction in thickness by rolling of around 20 pct. The specimens were disks, 5 mm thick, with a diameter of 35 mm, with the sides sloped at a 7 deg angle. Two sets of momentum trapping apparatus were used during the impact, hereafter called the sequential plate apparatus[1] (Figure 1(a)) and the cup apparatus[3,6] (Figure 1(b)). Both sets contained a cover plate to mitigate the additional deformation experienced at the impact face of the specimen[7]; a series of lateral momentum traps to prevent lateral release waves interacting within the specimen causing deformation, and spall plates to prevent spallation damage to the specimen. The cup apparatus differed from the sequential plate apparatus in that it had one fewer spall plate and included a cup structure around the specimen. The annealed sample was shock loaded to 5 GPa (sequential plate apparatus) and the cold-worked specimens were shock loaded to 6 GPa (sequential plate apparatus) and 10 GPa (cup apparatus). The peak pressures were calculated from the impact velocities measured during the impact experiment. The purpose of the two
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