P 603 - Determination of material behaviour and the influence of coatings using computational methods to shorten development times in vehicle construction with steel

Based on the directional dependence of the modulus of elasticity, material families could be formed for the individual steel grades. The directional dependence of the moduli of elasticity is retained with preforming. After a subsequent heat treatment, as is the case with paint baking, the modulus of elasticity of the initial state is approximately achieved again, regardless of the degree of stretching. For the numerical description of the hardening behaviour, flow curves and width change curves were recorded for different test directions and test temperatures. The level of the stress-strain curve decreases with increasing test temperature. Dynamic tensile tests were carried out at different strain rates and temperatures to characterise the strain rate dependency. The characteristic values and flow curves were determined in order to create the experimental basis for the modelling. Early necking in the tensile test only allows the determination of yield curves up to uniform elongation. In the case of higher-strength multiphase steels, however, there are special material properties that require supplementation. For this reason, a measuring device was developed for the hydraulic cupping test that enables the recording of flow curves up to material failure and practical testing under multi-axial stress conditions. When recording the forming limit diagrams, the evaluation method had an extremely large influence on the forming limit determined. The evaluation of parallel line sections (ISO design) was compared with a simple measurement of the maximum deformation of the last image before the start of cracking or necking. In the uniaxial tensile range, both methods provide similar changes in shape, in contrast to the plane-strain range, where significant deviations between the two methods occur. Cyclic material parameters are used to evaluate the service life of thin sheet structures under oscillating loads on the basis of local strains from FEM calculations. Computerised approximate solutions allow the cyclic characteristic values to be estimated from tensile test data and a short experimental procedure makes this possible in the shortest possible time. Based on the work carried out, the cyclic material properties are available or can be easily estimated for many materials and material groups. Based on the experimental investigations, a material law was developed to describe the flow curves of the analysed materials at different temperatures in a wide strain rate range. To reduce the number of parameters in the material description, the investigated materials were grouped into families and correlations were determined within these families. The transition area from isothermal to adiabatic material behaviour was localised and the dependence of the proportion of deformation work converted into heat on the strain rate was described. In a follow-up project, the verification and limits of the developed material law are to be verified and determined using further steel grades. FOSTA Research Association for Steel Application e. V.