The purpose of this study is to investigate the yield behavior under a large deformation based on the Natural Strain theory. In this study, using the test pieces made from high purity tough pitch copper, the experiments for cyclic loading of tension and compression are carried out after applying the large uniaxial tension. As a method for determining the yield stress in each cycle, the slope of the tangent at yielding in the deviatoric stress- deviatoric strain curve is used instead of the conventional proof stress. In order to reveal the change of yield stress in cyclic loads after applying large pre-deformations, the experiments are conducted with the different sizes of pre- deformation and the different strain amplitudes. Moreover, the yield behavior under cyclic loads is revealed by comparing with the results from conventional proof stress.
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