Yielding behavior of prestrained interstitial-free steel and 70/30 brass
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INTRODUCTION
DURINGthe forming of sheet metals, the imposed stress or strain state may be abruptly changed between sequential forming operations. Such changes in loading path may cause inhomogeneous deformation and produce plastic instability, tL2'31 An accurate understanding of the effect of changes in loading path on the material behavior is necessary for proper mathematical modeling of complex forming processes. To investigate the behavior of prestrained metals, two-stage strain path experiments have been performed: after prestraining in one strain state, the material is reloaded in a second strain state. One of these strain states is often plane strain tension. A majority of the split-type failures encountered in production forming of sheet metals occur in plane strain tension, t41 and therefore, an understanding of material behavior in this strain state is essential. Wagoner, tS'6j Wagoner and co-workers, I7,81and Doucet and Wagoner t9'l~ performed multistage testing in which the metal was first prestrained in plane strain and subsequently strained in uniaxial tension. Other two-stage strain paths have been used, including combinations of balanced biaxial/uniaxial tension tm and tension/tension, t12-~5~ Similar unusual flow behavior has been observed in these works and can be classified into two types: (1) Positive transient: high subsequent yield stress followed by reduced work-hardening rate and premature load instability (ferritic-type steels), t6,a-13j (2) Negative transient: lowered subsequent yield stress followed by increased work-hardening rate and enhanced or unaffected load instability strain (brass[5"9'l~ A.B. DOUCET, Assistant Professor, Mechanical Engineering Department, and S. NATARAJAN, M.S. Candidate, Department of Computer Science, are with Louisiana State University, Baton Rouge, LA 70803. Manuscript submitted March 29, 1990. METALLURGICAL TRANSACTIONS A
Aluminum a l l o y s [12'14] and dual-phase s t e e l |7] generally show little transient behavior. Experimental observations of the transient behavior following an abrupt change from plane strain to uniaxial tension are not limited to measurements of stress; nonconstant values of the plastic anisotropy ratio, r, have also been reported during restraining, tl~ Various origins of the transient behavior have been proposed. Several authors have suggested a dislocationbased origin, and this has been investigated using transmission electron microscopy. [16,17,18] The dislocation structures formed during prestraining appeared to be unstable with respect to the subsequent strain state and evolved into more stable arrangements, thus leading to the transient flow behavior. The contribution by strain aging to the positive transient was investigated by Doucet and Wagoner. tg] They found that the transient behavior resulting from an abrupt change from plane strain to uniaxial tension occurred in a nonstrain-aging material, interstitial-free (IF) steel, thus indicating that the positive transient is a separate phenomenon. The roles of latent hardening t
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