Damage leading to ductile fracture under high strain-rate conditions
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I. INTRODUCTION
DUCTILE fracture occurs within plastically deforming materials through the nucleation, growth, and coalescence of voids to form crack.[1,2] This mechanism is operative under dynamic loading conditions, such as impact, when the tensile reflections of compressive stress waves from free surfaces meet to produce tensile triaxial stress states.[3] Considerable past research has investigated the component stages of ductile fracture,[4–11] however, the nature of damage accumulation under high strain-rate dynamic loading has not been investigated extensively. Under dynamic loading, material effects such as strain-rate hardening will tend to elevate ductility,[12] whereas thermal softening due to adiabatic heating can lead to premature localization.[13,14] Inertial effects can delay necking under dynamic loading[15] and play an important role in determining the void expansion rate under very high strain-rate conditions.[16–19] Material viscosity will also delay void expansion under very high rates of strain.[18,20] This article presents results from high strain-rate mechanical testing with quantitative metallographic assessment of damage development. Leaded brass was adopted as a “model material” because it contains a dispersion of lead particles acting as well-defined void nucleation sites and has been J.P. FOWLER, formerly Research Associate with the Mechanical and Aerospace Engineering Department, Carleton University, is with the Defence Research Establishment, Suffield, P.O. Box 4000, Medicine Hat, Alberta, T1A 8K6, Canada. M.J. WORSWICK, Associate Professor, is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. A.K. PILKEY, Assistant Professor, is with the Mechanical and Aerospace Engineering Department, Carleton University, Department of Mechanical Engineering, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada. H. NAHME, Scientist, is with the Ernst Mach Institute, Ernst-Mach-Institut, Eckertrasse 4, 7800, Freibourg, Germany. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
considered in a number of previous studies of ductile fracture under quasi-static[9] and dynamic[17,21,22] conditions. Results are presented from two series of high strain-rate experiments, the tensile split Hopkinson bar (TSHB), and the planar plate impact. In the TSHB tests, uniaxial and notched tensile specimens are loaded in tension at strain rates up to 3000 s21. By varying the notch geometry, tensile triaxialities in the range of 1/3 to 1.0 are achieved. Momentum trapping techniques[21,23] were applied to arrest deformation at predetermined strain levels prior to fracture, allowing construction of damage histories from a series of interrupted TSHB experiments. In the
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