A Robust Method to Determine True Stress–true Strain Curves for Sheet Metals at High Temperatures

Thermo-mechanical uniaxial tensile tests on sheet metals have been widely carried out to characterize the mechanical behavior of the materials at high temperatures. Because of nonuniform temperature along gauge length, however, nonuniform deformation occurs from the beginning of deformation, leading to challenges to determine stress–strain curves. In the present study, a robust method for determining true stress–true strain curves has been proposed; in this method, true strains are obtained within a local area in which fracture occurs, and true stresses are computed based on the engineering strains within this local area. Uniaxial tensile tests on aluminum alloy AA6082 sheets under hot stamping conditions have been carried out, with the strain measurement using the digital image correlation (DIC) technique. The onset of necking in those tests has been determined using the spatio-temporal method. Then the data before the necking have been analyzed and the robust method has been applied to determine the associated true stress–true strain curves. The true stresses computed using the robust method have also been benchmarked against those computed using the standard method. It concludes that this robust method is able to accurately determine the true stress–true strain curves at high temperatures.