Anisotropic strain localization in tensile prestrained sheet steel
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INTRODUCTION
EXPERIMENTAL
I N stamping complex shapes in sheet metal, sequential forming operations are generally required. These multiple forming operations can cause local regions in the panel to undergo large changes in strain path. The effect of such changes in strain path on residual ducility are of major concern and must be measured experimentally. For example, biaxial prestraining of A-K steel and of 2036-T4 aluminum was shown to alter the residual formability of each quite differently.' This study examines the influence of tensile prestrains on residual mechanical properties of A-K steel. A similar study was reported by Hutchinson, Arthey, and Malmstrom2 by prestraining large tensile samples to different levels of tensile strain. Then, from each prestrain sample, smaller tensile samples were machined at 90 deg to the original axis and pulled to failure. They measured such properties as yield strength and residual uniform strain, as determined by the maximum load point of a tensile test, but did not report strain hardening, or strain-rate hardening, or postuniform strain behavior. Furthermore, no measurements were made at other angles to the prestrain. Therefore, the purpose of this study is to extend the earlier work of Hutchinson e t a l and measure these additional properties since their influence on formability is well documented. 3 To complement the data, strain distributions developed in the tensile specimens following prestraining are reported both as a function of prestrain and orientation to the prestrain axis.
JOSEPH V. LAUKONIS is Senior Research Physicist, Physics Department, General Motors Research Laboratories, Warren, MI 48090. Manuscript submitted October 31, 1979. METALLURGICAL TRANSACTIONS A
Material The material tested in this study is a cold-rolled and annealed, aluminum-killed sheet steel, 0.84 mm in thickness whose chemical composition is given in Table I. The Prestraining Operation The blanks for prestraining were parallel sided tensile specimens 685 mm long and 178 mm wide with the rolling direction of the sheet parallel to the tensile axis as shown in Fig. 1. Specimens were prestrained at an initial strain rate of 1.6 • l0 -3 S- | to each of several preselected strain levels. The prestrain was measured from grid circles, photoprinted along the gage section, and ranged in value from 5 to 15 pct. It should be noted that one prestrain specimen was oriented with the tensile axis lying at 90 deg to the rolling direction. This specimen was prestrained to about 8 pct to assess the effect, if any, of initial texture. Tensile Tests Standard ASTM E-8 tensile specimens with a gage length of 50.8 mm were cut from the prestrain specimens at 0, 45, and 90 deg to the original tensile axis. In
Table I. Composition of Aluminum-Killed Steel pct C pct S pct P pct Mn pct Si pct A1 N pct Fe
ISSN 0360-2133/81/0311-0467500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
0.060 0.011 ~0.004 0.33 0.019 0.052 41 ppm balance
VOLUME 12A, MARCH 1981--467
Fig. 1--Large paral
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