Strain rate sensitivity of DC06 for high strains under biaxial stress in hydraulic bulge test and under uniaxial stress in tensile test

An accurate description of the material behaviour is the most important part to generate precise results in sheet metal forming simulation. The main parameters for the plastic material behaviour are the yield curve, the anisotropy and the strain rate sensitivity, which can be determined by various tests, e.g. tensile test, hydraulic bulge test or the compression test. For sheet metal forming the tensile test is the most common procedure to determine the yield curve, simple yield loci and the strain rate sensitivity. The main advantages of the tensile test are the frictionless behaviour and the simple control of the strain rate in analogy to the test velocity, nevertheless only uniaxial material behaviour can be described. For the determination of yield curves under biaxial stress and up to high strains the hydraulic bulge test, which is standardized in ISO-16808 (ISO copyright office 2013 [2]), is applied. In this paper the dependence of the strain rate sensitivity upon the applied stress state for a mild steel is investigated. To realize constant strain rates up to 0.1 s⁻¹ in the hydraulic bulge test the punch velocity was adjusted instantaneously depending on the forming state. For prediction of the required velocity curve an inverse method using FE simulations is established. The results show a decrease of the strain rate sensitivity for increasing plastic equivalent strain up to 0.7 for uniaxial and biaxial stress conditions. However a high deviation of the level of strain rate sensitivity, which depends on the stress state, was observed. These dependencies are taken into account in a yield curve approach.