Time-Dependent Nonproportinally Multiaxial Ratcheting Of 1Cr18Ni9 Stainless Steel And Its Constitutive Model

Ratcheting is a cyclic accumulation of inelastic strain occurred during the asymmetrical stress-controlled cyclic loading of materials. It may reduce the fatigue life of components or interfere the functions of structures due to the resulted excessive deformation, and should be considered in the reliable design and life estimation of engineering structures. Existing tests and theoretical studies for the ratcheting behavior of metals were mostly carried out at a single constant loading rate and with a fixed loading wave, and the studies on the influences of time-dependent factors such as loading rates, loading waves and loading-holds on the ratcheting are not sufficient yet. The time-dependent tests and corresponding constitutive models have not been performed to the non-proportionally multiaxial ratcheting behavior either. So it is of theoretical importance and engineering value to obtain the experimental observations to the uniaxial and non-proportionally multiaxial ratcheting of metals at different loading rates, with different hold times and loading waves, and at different temperatures; and then to develop a reasonable time-dependent constitutive model.Therefore, the studies carried out in this thesis are as follows:(1) Uniaxial and non-proportionally multiaxial ratcheting tests of 1Cr18Ni9 stainless steel are conducted systematically at room temperature,250℃ and 650℃, which reveals the characteristics of the time-dependent ratcheting of the material at different temperatures and with different non-proportionally multiaxial loading paths, and demonstrates the dependence of ratcheting upon the stress rate, peak stress hold-time and loading wave. Such observations provide a solid experimental foundation for the construction of corresponding constitutive model.(2) In the framework of unified viscoplasticity, by introducing a static recovery term and a non-proportionality defined by Tanaka into the Abdel-Karim-Ohno nonlinear kinematic hardening rule and isotropic hardening rule, respectively, and considering the influence of temperature, the existing viscoplastic cyclic constitutive model is revised to describe the time-dependent nonproportionally multiaxial ratcheting of 1Cr18Ni9 stainless steel.(3) Based on the features of the revised model, a physically reliable method is proposed to determine the material parameters to be used in this model first, and then the numerical implementation of the model is carried out with the aid of radial return mapping and implicit integration methods. Comparisons between the tests and simulations show that the revised model can simulate the time-dependent uniaxial and multiaxial ratcheting of the stainless steel at the three prescribed temperatures well and describe the dependence of the ratcheting upon stress rate, peak stress hold-time and loading wave reasonably.