Influence of a Hydrostatic Pressure Shift on the Flow Stress in Sheet Metal

The stack compression test (SCT) and the strain-rate controlled hydraulic bulge test (HBT) enable to determine the large strain flow curve of sheet metal under an identical deformation mode and balanced biaxial tension. This equivalence should lead to identical flow behavior if plastic yielding is independent of the hydrostatic stress. However, a discrepancy is observed in the flow curves of DP600 steel sheet determined by the SCT and the HBT. In order to avoid uncertainty with respect to dissimilar test conditions, the average strain-rate in both material tests is carefully controlled. Additionally, evidence is provided that friction can be sufficiently minimized yielding a homogeneous SCT up to a true plastic strain of 0.3. Assuming reliable experimental data, the hypothesis that the hydrostatic pressure shift between the stress states in the SCT and the HBT causes the observed difference in flow behavior is scrutinized. Theoretical considerations regarding the effect of a superimposed hydrostatic pressure (i.e. putting a material under a pressure environment for a certain stress state) on the flow stress, as suggested by Spitzig et al. [1] and Spitzig and Richmond [2], are used to understand the effect of a pressure shift between two stress states on the flow stress. Finally, the theoretical considerations are experimentally validated using the discrepancy in flow behavior of DP600 measured by the SCT and the HBT.