RATCHETTING IN CYCLIC PLASTICITY .2. MULTIAXIAL BEHAVIOR

Part II is concerned with time-independent ratchetting of materials under biaxial, cyclic loading histories. Carbon steel 1026 (heat-treated) tubes were tested under constant internal pressure and two axial loading histories. In the first set of experiments, the tubes were axially cycled in a strain symmetric fashion. For this loading, the material exhibits ratchetting in the circumferential direction. The second set involved stress controlled axial cycling in the presence of internal pressure. This loading results in strain ratchetting in the axial as well as the circumferential directions. Also, even though the material was cyclically stabilized prior to the ratchetting experiments, the biaxial loading was found to induce significant additional changes to the shapes of the hysteresis loops traced. The three cyclic plasticity models discussed in Part I are used to simulate the two sets of experiments using the material parameters selected for the modeling of the uniaxial ratchetting experiments. Prediction of the correct rate of ratchetting in the first set of experiments was found to be very sensitive to the hardening rule incorporated in the models. An explanation of this sensitivity is given. A hardening rule proposed by Armstrong and Frederick, with suitably selected parameters, was found to provide reasonably good predictions of the six experiments conducted. Prediction of the second set of experiments was found to require modeling of the changes induced to the shape of the basic hysteresis loop by this cyclic history.