Strain-rate sensitivity of zinc sheet

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I.

INTRODUCTION

FLOWstress depends appreciably on applied strain rate for many commercial alloys. This dependence contributes greatly to the resistance to strain localization (necking). 1,2,3 During necking, a material element undergoes continuous changes in strain and strain rate that are not represented well by standard tensile tests carried out along constant strain or constant "structure" paths. Standard tests are also open to variation in the representations of strain-rate sensitivity parameters because of interpretation and analysis of experimental results. 4-7 In particular, various extrapolation techniques may be used to compensate for machine or material transient effects following jumps in strain rate. A method for measuring m along a typical necking path by analytically deconvoluting work hardening and strain-rate hardening has been presented. 8 In an attempt to determine the measurement method most appropriate to tensile necking, Wagoner and Wang~ have modeled a sheet tensile test using various rate-sensitivity formulations. Aluminum-killed steel tested continuously and in jumprate sequences exhibits different strain-rate behavior. 8 The jump tests show an m-value increasing with rate ~ such that a multiple-parameter rate law ll-14 must be used. The origin of the different behavior under jump or continuous testing conditions is not clear. Wagoner proposed ~~ a post-jump transient model similar to that observed following stressstate changes ~5'16 while Hart proposed ~2-~7 a state variable formulation to account for these differences. Kocks, however, has pointed OUt 18'19 that the state-variable approach cannot account for the entire range of material behavior following a jump in strain rate. In this study, zinc sheet was chosen for testing because of its high strain-rate sensitivity8 at room temperature. Many strain rate effects in ferrous and aluminum alloys are minor compared with overall stress levels in a tensile test even though the post-uniform elongation is controlled largely by strain-rate sensitivity. The use of zinc sheet should allow a more definite separation of rate effects from experimental uncertainty. R.H. WAGONER, formerly Staff Research Scientist, General Motors Research Laboratories, is now Associate Professor, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted November 1, 1983. METALLURGICALTRANSACTIONS A

II.

EXPERIMENTAL PROCEDURE

The material used in the experiments was provided by the Ball Corporation2~ and is designated "Alloy 101." It is a zinc-based rolled product, 1.4 mm thick, and is alloyed (in weight percent) as follows: Cd (0.04 to 0.06), Pb (0.06 to 0.08), Fe (-

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