A model of multiaxial cyclic plasticity describing yield-point phenomena and nonhomogeneous plastic deformation and its applications

In the present research, a model of cyclic viscoplasticity has been presented in order to describe the yield-point phenomena and nonhomogeneous plastic deformation of metals. This model has been constructed within a framework of the overstress theory based on the two-surface modeling. The model has been implemented to a finite element code originally developed by the present researchers. Using this FE code, the numerical simulation of some cyclic deformations (uniaxial tension with various deformation speeds, cyclic straining and ratchetting) were conducted, and the results were compared to the corresponding experimental data on mild steel. The results obtained in the present work are summarized as follows. (1) The phenomena of sharp yield point and the subsequent abrupt yield drop under uniaxial tension are well described by the model which is proposed on the premise that result from rapid dislocation multiplication and the stress-dependence of dislocation velocity. Strong plastic strain localization, as well as the Luders-band propagation, is numerically simulated. Two types of Luders-band propagation, i.e., one has only one Luders front, and the other has two fronts, can be numerically reproduced by giving different levels of geometrical imperfections of the specimen. (2) The strain-range dependent cyclic hardening behavior of materials is expressed by the concept of non-isotropic hardening region defined in the stress space. (3) The results of numerical simulation for ratchetting show good agreement with the experimental observations, such as the effect of strain rate and the shakedown of ratchetting. The kinematic hardening rule which describes the closure of the stress-strain hysteresis loop is of vital importance for accurate simulation of ratchetting.