Influence of Loading Mode on the Biaxial Stress–Strain Curve at Hydraulic Bulge Test

Stress-strain curves are generally a very important material characteristic. For example, in numerical simulations, especially in sheet metal forming, stress-strain curves represent one of the most important data inputs. However, there is quite a wide range of parameters that influence their outline under the chosen technological conditions and, therefore, must always be taken into account. Among them, the influence of stress state and loading history is also relevant. In addition to that, to properly define the advanced yield conditions used in numerical simulations, it is also necessary to perform material tests under multi-axial stress states. For the above reasons, the present paper deals with the influence of the loading mode on the resulting outline of stress-strain curves under the equi-biaxial stress state at hydraulic bulge test (HBT). In light of the different loading modes, the classical continuous increase in pressure in accordance with ISO 16808 was compared with the so-called ramp test, where holding times for a duration of 90 s were applied. Two materials were selected for experiments, namely, a dual-phase steel (DP steel) with UTS of 500 MPa and an interstitial-free steel (IF steel) with a yield strength of 150 MPa. The results revealed totally different deformation behaviour of the tested materials depending on the used loading mode. Moreover, an evaluation of the microstructure was performed as well to clarify the measured results. The contactless optical system GOM Correlate Pro was used to evaluate the results of the HBT.