METAL FORMING PROCESSES CONDITIONED BY CYCLIC LOADING. A NEW CHALLENGE FOR THE THEORY OF PLASTICITY

Summary Since early works of cyclic plasticity, it was observed that stress-strain cycles of large amplitude imposed on the cold-worked specimens of metallic materials produce cyclic softening. Korbel and Bochniak proposed the improvement of a method for plastic forming operations conditioned by cyclic loading. The experimental investigations led to the identification of shear banding as the basic mechanism responsible for plastic softening. A new approach to the theory of plasticity describing the discussed above phenomena is proposed and an example illustrating its predictive power is presented. In particular, the parameters controlling cyclic loading under torsion: the amplitude of the cycle and its frequency are related with the microscopic model of shear banding. Since early works of cyclic plasticity, it was observed that stress-strain cycles of large amplitude imposed on the cold-worked specimens of copper, steel, and aluminum alloy produce cyclic softening [1]. It was also reported that the length of the specimen increases during cyclic loading while subjected to combined tension and torsion, with the axial load maintained constant and with the alternating torque [2]. One of first attempts of the theoretical description of this phenomenon, called recently ratchetting, was proposed in [3]. Due to the importance of the latter phenomenon for the prediction of the risk of failure of structures subjected to repeated loads the cyclic plasticity theory was developed. On the other hand, the search for the applications of controlled cyclic torsion [2] for the reduction in load during the processes of drawing of copper and extrusion of lead wa