Improved Pulse Shaping to Achieve Constant Strain Rate and Stress Equilibrium in Split-Hopkinson Pressure Bar Testing
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INTRODUCTION
HOPKINSON bar techniques have been widely employed to quantify the dynamic mechanical behavior of solid materials at high strain rates in the range of 102 to 104/s.[1–12] The split Hopkinson pressure bar (SHPB) (dynamic compressive test) is most commonly used in practice compared to tension or torsion loading due to the simplicity and convenience of both the compression test technique and sample designs, in addition to the reduced effects of inertia during compressive loading. However, when testing some high-work-hardening materials, such as tungsten alloys, superalloys, and stainless steel, or materials that exhibit phase transformation during deformation, such as shape memory alloys, or materials exhibiting complex hardening, such as textured hexagonal metals zirconium and magnesium deforming via a mixture of slip and twinning, it is difficult to achieve a constant strain rate in a conventional SHPB test. The strain rate in a typical SHPB test is derived from the reflected pulse measured in the KENNETH S. VECCHIO, Professor of Materials Science and Engineering, and FENGCHUN JIANG, Project Scientist, are with the NanoEngineering Department, University of California, San Diego, La Jolla, CA 92093-0411, USA. Contact e-mail: [email protected] 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 20, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
incident bar. Therefore, as an example, when testing a 321-type stainless steel sample, a stain rate variation from 3000/s to 100/s was achieved in a conventional SHPB test[3] due to a trapezoid-shape reflected pulse with a large change in pulse amplitude. For a NiTi shape-memory alloy, the compression strain rate of the sample may vary dramatically in a conventional SHPB test, due to the highly nonlinear response associated with the formation of stress-induced martensite during plastic deformation. A considerable variation in the applied strain rate may significantly influence the measured mechanical response of materials that are strain rate sensitive. Furthermore, when testing some brittle materials, such as ceramics, certain composites such as cermets, and bone, or soft materials with low wave speeds or low impedance, such as polymers, rubber, or lead, an incident pulse with a relatively long rise time is required to yield stress equilibrium in the sample.[10–13] Therefore, in order to achieve constant strain rate and dynamic stress equilibrium in samples to obtain reliable mechanical properties of the material, various pulse-shaping methods have been proposed.[1–12] In a torsion Hopkinson bar test, Duffy et al.[5] were probably the first authors to propose the pulse shaping technique, while this method was initially termed as
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