Research On Time-Dependent Ratcheting Behaviors Of Matel Materials And Its Constitutive Models

Ratcheting is of great importance to the safety design and life assessment of structures, and has been extensively studied. In the last two decades, many experimental and theoretical results of the ratcheting have been reported for metal materials. However, the referable results are mainly obtained at the single stress rate and the effects of loading charts and peak/valley stress hold on the ratcheting have not been reasonably considered. Since the effects of loading rate, loading charts and hold time on the ratcheting are popular in the practice case, it is necessary to address such time-dependent ratcheting.Therefore, in this work, the time-dependent ratcheting behaviors of SS304 stainless steel were studied experimentally and theoretically at room temperature and 973K as follows:1) The time-dependent strain cyclic characteristics and ratcheting behavior of SS304 stainless steel were studied by experiments under uniaxial and non-proportionally multiaxial cyclic loading and at room temperature and 973K. The effects of loading path, straining/stressing rate and holding-time at peak/valley stress/strain of each cycle on the cyclic softening/hardening and ratcheting behaviors were discussed. It is shown that the ratcheting of the material presents apparent time-dependence at room temperature and 973K. Some results helpful to construct a time-dependent constitutive model of the ratcheting are obtained.2) Based on the uniaxial experimental results, three kinds of time-dependent constitutive models, i.e., a unified viscoplasticity model, a creep-plasticity superposition model and a creep-viscoplasticity superposition model were employed to describe such time-dependent ratcheting behavior of SS304 stainless steel at room temperature and 973K by using the Abdel-Karim-Ohno kinematic hardening rule. It is shown from comparing with the corresponding experimental results that the creep-visoplasticity superposition model describes the time-dependent ratcheting very well.3) Based on the framework of unified viscoplasticity, a newly time-dependent constitutive model was also introduced. The model can describe the time-dependent ratcheting by using the revised Abdel-Karim-Ohno non-linear kinematic hardening rule with a static recovery term. Furthermore, a precise and reliable method to determine the model parameters was proposed. It is shown from the comparing with corresponding experimental results that the developed model describes thetime-dependent of SS304 stainless steel very well.4) The proposed unified viscoplasticity model with static recovery term wasimplemented into the finite element software ABAQUS by user-subroutine UMAT.Based on the radial method and backward Euler's integration, a new implicit stressintegration algorithm was proposed. Simultaneously, a new expression of consistenttangent modulus was also derived. The reasonability of such implementation of themodel was verified by many numerical samples including the simulation to theratcheting of some structure components.