Influence of Loading Rate on the Material Deformation Behaviour under Bi-axial Loading

In recent years, great effort has been made in all branches of industries to put into practise new methods and procedures using numerical support for technological processes, digitization and robotization of the production operations. The complex of these measures is known as Industry 4.0, which aims to reduce production costs, while increasing the flexibility of companies. The concept of the so-called virtual factory makes possible to predict possible problems and problematic production points already in the design phase by means of the numerical modelling. A prerequisite for the proposed virtual concept functioning is necessity to master the individual partial solutions with the sufficient accuracy of the whole computation with the subsequent data sharing in case of their need in other stages of the virtual solution. With the development of the computer technology (both in hardware and software), there is obvious effort to use still more and more complex mathematical computation models that allow taking into account a higher number of input process parameters and thus to achieve a higher computation accuracy. The paper is focused on the issue of technological processes numerical modelling at thin sheets drawing. As a needed prerequisite for obtaining the result having required accuracy, there is the correct choice of proper deformation model and definition of boundary conditions, which take into account the monitored technological process. In order to define advanced mathematical models of thin steel sheets deformation behaviour, it is necessary to perform (beside standard tests as e.g. static tensile test) also tests under biaxial loading. One of the most common tests for determining mechanical properties under biaxial loading is the hydrostatic bulge test (HBT). In the experimental part, the paper evaluates the influence of loading rates on the resulting mechanical properties of tested material. The required strain rate is affected by the time change of pressure increase. There were used three loading rates with linear pressure increase and two loading rates with holding time to enable stress relaxation of tested material. The contact-less optical system Mercury RT was used for data acquisition and deformation analysis. For the selected test parameters, stress-strain curves were determined from the measured values. Based upon these stress-strain curves, there was subsequently evaluated influence of the strain rate on the deformation behaviour of tested material and stress relaxation during the tests. As a testing material was used deep-drawing material specified for drawing stampings in the automotive industry.