Strain path change effects in the local necking of aluminum sheet and in the tension of internally pressurized tubes
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INTRODUCTION
THE forming of metal sheets is a very important commercial process, to which one significant limitation is ductile failure of the material in stretching or a combination of bending and stretching. The essential feature of ductile failure is the localization of straining. This is determined to a large degree by the mechanical behavior of the material. It is not unusual to rely on data from tests such as uniaxial tension to produce the material model data for the prediction and correlation of material behavior in forming processes. The data used are from a regime where proportionality of straining components remains nominally constant, or changes slowly with strain as the preferred crystallographic orientation develops during deformation. Such data are deficient in many respects. They represent a very limited and often unrepresentative straining state in terms of the proportions and magnitudes of strain-rate components and of the actual extent of deformation. Extrapolation is then required, involving assumptions about the form o f plastic potential and the equivalence of flow-stress development in different straining states. Furthermore, changes in the proportionality of the straining components have a significant effect on the development of flow stress. [1-61 In the tensile deformation of aluminum, there is a significant effect of strain rate on the true rate of strain hardening, and useful increases in uniform elongation in tension can be achieved by accelerating the rate of stretching, tTl Such acceleration does not appear to be as effective in increasing the strain at which localized necking failure begins to occur. Previous work tS~ has indicated that a change in the proportions of straining components, or strain path, reduces the rate sensitivity of strain hardening. This may have consequences for ductility, as changes of strain path are associated with P.S. BATE, Senior Research Fellow, is with IRC in Materials for High Performance Applications, The University of Birmingham, Birmingham, B15 2TT U.K. Manuscript submitted April 5, 1993. METALLURGICAL TRANSACTIONS A
the latter stages of strain localization. Most tests have concentrated on larger changes in strain path than are appropriate to that case and usually involve a significant unloaded interval between the straining stages. The effect of nonproportional straining on forming limits is usually considered in the context of strain paths resulting directly from the processing, t9,1~ for example, the change from states near uniaxial compression to near plane strain as material is drawn inwards in the flange of a pressing, or in multistage sheet forming. In fact, the strain localization process which effectively limits stretching in ductile sheets often involves a path change because the final deformation occurs under conditions of plane strain in a band of material. Even in a simple uniaxial tensile test, the latter stages of strain localization involve a significant change in strain path. In the work presented here, the effect of accelerati
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