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In times of highest significance of process modelling and numerical simulation characterisation of material properties is of special importance for tools’ and components’ dimensioning. But in general material properties depend on many different influencing variables, e.g. temperature, humidity and many others. Especially in fields of sheet metal forming the mechanical behaviour of components highly differs according to real stress condition. In particular yield loci combine the information of beginning of yielding with a biaxial stress condition, but nevertheless for many materials they have not been determined yet. For all others the existing values are available only at room temperature. In this paper a novel concept of the experimental setup is shown, with which plastic yielding of sheet metal can be examined also at elevated temperatures. In usual biaxial tension tests cruciform specimen are drawn in plane. The new machine-concept, which is presented in this paper, is based on a punch-load moving perpendicular to the sheet. By clamping the specimen restoring forces are induced, which cause in dependence of special developed tool and work piece geometries defined stress conditions. Using an optical measurement system for determination of strains with CCDcameras of very high frame rate allows exact identification of starting plastification by offline analysis. Experiments at elevated temperatures are realised by local heating with a diode laser and a special optical system to reach a homogenous distribution of temperatures in the forming zone. On the one hand these investigations are necessary for many materials to achieve further information on characteristic properties in warm forming, because their data are only known at room temperature. On the other hand some materials, e.g. magnesium wrought alloys, are mostly formed at elevated temperatures (here in the range of 200°C to 250°C), because of its significant higher formability. Thus, material behaviour must be characterised at these temperatures.

The finite element analysis (FEA) has become one of the most relevant and most important tools in fields of sheet metal forming for designing processes and dimensioning parts. However, reliability and quality of the numerical results strongly depend on the whole FE-model and especially on the modeling of the material behavior, which shows wide impact on calculated stresses and strains of sheet metal parts. Therefore, the experimental determination of characteristic material data concerning anisotropic and temperatureeffects is essential. In this paper the influence of temperature on the yielding and the hardening behavior of the magnesium sheet metal alloy AZ31 are investigated for different uniaxial and biaxial stress conditions. For that purpose an experimental setup has been developed at the Chair of Manufacturing Technology (LFT) which enables biaxial tensile testing of sheet metal. Yield loci of AZ31 are determined as a function of temperature and they are based on solely measurement data of the forming process itself.

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