Ratcheting of cyclically hardening and softening materials: II. Multiaxial behavior

Abstract Part II of this study is concerned with ratcheting phenomena of cyclically hardening and softening materials under biaxial, cyclic loading. Two sets of biaxial experiments were performed on carbon steel 1018 and stainless steel 304 thin-walled tubes. In the first tyoe of experiment, a constant internal pressure was prescribed while the tubes were cycled axially in a strain-symmetric fashion. This causes ratcheting in the circumferential direction. In the second type of experiment, the axial cycling was carried out under stress control. This loading history results in simultaneous ratcheting in the axial and circumferential directions. In the case of stainless steel 304, the nonproportionality of these loading histories was found to induce significant hardening in addition to that recorded in unaxial loading. Cyclic hardening was found to reduce the rate of ratcheting. In the case of carbon steel 1018, the nonproportionality of the loading paths was found not to influence the induced softening. Cyclic softening in the axial and circumferential directions were found to be uncoupled. The time-independent cyclic plasticity models developed in Part I, suitably extended to multiaxial loading, were used to simulate the biaxial ratcheting experiments. Two methods for modeling the additional hardening/softening of the material due to nonproportional loading, developed by previous investigators, were incorporated in the models. The prediction of circumferential ratcheting is shown again to be sensitive to the kinematic hardening rule of the yield surface incorporated in the models. The performance of the models in predicting the biaxial ratcheting results was found to be rather poor. Several reasons for this poor performance are identified and suggestions for future improvements are made.

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