An experimental investigation of deformationinduced heating during tensile testing
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INTRODUCTION
INrecent years, there has been a wide interest in the effects of temperature on the formability of metals. Control of temperature can reduce flow localization and enhance formability of workpieces. This improvement can reduce failure problems in many forming operations such as bending, stretching, and drawing; operations which involve deformation heating, large strain gradients, or locally high strain rates. Recent forming processes, such as "Armco Cold Forming ''l and the "flood" lubrication process, z have used temperature control to reduce thermal gradients caused by deformation-induced heating (or to impose thermal gradients opposite to strain gradients) to improve formability. Other types of processes, such as "line heating ''3 or "cooled die/heated punch superplastic forming", 4 exploit artificial temperature gradients to redistribute strain and promote formability. When a metal is plastically deformed, most of the absorbed strain energy is converted to heat. 5 Recent measurements revealed local temperature rises approaching 100 ~ during the deformation of steel sheet. 6'7 At higher strain rates, less time is available for heat exchange and therefore higher temperature increases will occur. 8 Effects of temperature and strain rate may dominate the influence of standard mechanical properties in flow and failure processes, 9'1~ particularly after necking. ~2-~7In the necked region, the thermal gradients soften the material and increase the local strain rate, which in turn further enhances higher thermal gradients .8 Although thermal gradients and strain rates are probably higher in practical forming processes than those in tensile tests and therefore have a higher influence on mechanical properties, constitutive equations MUH-REN LIN, formerly Graduate Student at The Ohio State University, is Postdoctoral Researcher, Department of Mechanical Engineering, University of Massachusetts, Amherst, MA 01003. R.H. WAGONER is Professor, Department of Metallurgical Engineering, The Ohio State University, 116 West 19th Avenue, Columbus, OH 43210. Manuscript submitted June 11, 1986.
METALLURGICALTRANSACTIONS A
incorporating temperature, strain-rate, and work-hardening are seldom generated for commercial alloys. Pertinent constitutive equations for the'studied materials ~8.~9were generated to delineate the flow properties at different temperatures and different strain rates. In studying the role of deformation-induced heating in strain localization, three thermal constraints (i.e., surface boundary conditions) may be considered: the extreme cases of the adiabatic condition and isothermal condition, and an intermediate heat flow condition such as free convection into air with end conduction to bodies in contact with the deforming specimens. Many forming operations are carried out under neither adiabatic nor isothermal constraints such that heat flow conditions will play a role in the deformation problem. The most important factors to affect the deformation heating in this environment are mechanical and th
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