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The problem of determining true stress-strain curves from flat tensile specimens beyond the onset of necking has been investigated based on finite element analyses under consideration of experimental accessible data using digital image correlation (DIC). The displacement field on the specimen surface is determined by in-situ deformation field measurement. A three-dimensional finite element study with different stress-strain-curves has been carried out to develop a formula, with which it is possible to calculate the true stress subject to the strain in the necking region. The method has been used to evaluate the true stress-strain curve with a so-called micro flat tensile specimen, which is normally used to determine the material properties in the material gradient around thin weldments.