Robustness of the sheet metal springback cup test
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RINGBACK is the elastic shape change of a sheet metal part after forming and removal from the die. In the automotive industry, the poor predictability of springback has always been a problem, but it was somewhat alleviated by the use of rather thick walled, low strength steels and by the accumulation of empirical methods of compensation during die design and rework. Due to the need for weight reduction, there is a major shift underway to use thinner high-strength steels and aluminum alloys. In both cases, these materials exhibit larger and unpredictable springback magnitudes, thus making an accurate prediction all the more necessary in order to avoid a costly redesign of the stamping tool. Modeling efforts in die design are beginning to treat springback in the same way that other phenomena such as forming limits and friction have been handled, through the modeling of a standard test geometry to verify theoretical predictions. There have been attempts to measure springback deflections in near-production parts,[1] but often these measurements are difficult to perform because the displacements can be small and body forces (gravity) often distort the part, so that fixturing becomes very important and is difficult to do in a similar manner for each differently shaped part. There is a need for a standard test that will characterize a material’s springback properties in a quick and consistent way. For a standard test, it is desirable that it be easy to perform, repeatable and in the case of this particular need, have a large springback component that can be easily measured. A test that has received considerable attention[2–6] consists of a ring sample taken from the sidewall of a flat bottom, deep drawn cup. The test geometry is presented graphically below in Figure 1. This geometry provides an axisymmetric sample with a large residual hoop stress that, when the ring is split, provides a large opening displacement. Residual stresses in the cup exist because different T. FOECKE is with the Metallurgy Division, National Institute of Standards and Technology, Technology Administration, United States Department of Commerce, Gaithersburg, MD 20899-8553. Contact e-mail: [email protected] T. GNAEUPEL-HEROLD is with the Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115, and the Center for Neutron Research, National Institute of Standards and Technology, Technology Administration, United States Department of Commerce. Manuscript submitted March 21, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
locations in the cup have accumulated different magnitudes of plastic strain during the draw-bend-unbend process. These stresses are key for the modeling of the springback because, integrated over the thickness, they create a bending moment in the split ring, and thus the shape change. In preparation for submitting this test to the standardization process, a variational analysis has been performed to study the robustness of the springback measurement to experimental errors. A number of different
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