Optimization of the physical scaling approach through the adaption of the inverted membrane stresses

Abstract

Dimensional deviations of stamped car body components caused by elastic springback still represent a significant problem for the development of the stamping tools. First, the springback causes deviations of the part in vertical direction to the part surface due to bending stresses. Second, the surface area of the part contracts (and also bends) due to elastic membrane stresses. The precise manifestation of the contraction depends both on the part stiffness and on the – usually heterogeneous – distribution of the membrane stresses. Usually, the resulting deformations cannot adequately be compensated by globally homogeneous scaling approaches. In order to carry out a locally correct compensation of the stamping tools, Birkert et al. have recently presented a physical scaling approach based on the inverted membrane stresses gained from the part in the closed die to compensate the active die surfaces. This approach delivered significantly better results but left potential for improvement. It is shown in the present paper how the existing approach and thus the scaling values can further be improved by adapting the inverted membrane stresses in an appropriate way. This is done by comparing the amount of strain changes during spring-back with those of the scaling process in a first step and adjusting the compensation stresses, in a second step, in such a way that the considered strain amounts are approximately identical.

Holistic springback compensation procedure

“…Because the part shrinks locally different as a result of its strain distribution and locally different stiffness [13], another approach is required for more accurate scaling. Such a local scaling approach was presented in [13] and developed further in [14]. The results presented there showed significant advantages compared to using the global scaling approach.…”

“…A combination of the Advanced Strategy [17] and the Improved Strategy [16] is used here as compensation strategy. The optimized Physical Scaling Approach [14] is used for scaling the drawing operation and the physical compensation method [12] is used for compensating deviations normal to the sheet surface.…”