| When the structure components are subjected to a cyclic stressing with non-zero mean stress, a cyclic accumulation of inelastic deformation will occur, which is called ratcheting. Ratcheting can shorten the fatigue life of components or induce the components not to work normally because of the exceed-limit deformation. It is important to consider ratcheting in designing engineering structures. In the last two decades, lots of experimental and theoretic research on ratcheting for metallic materials has been done. The cyclic constitutive models of ratcheting and prediction of ratcheting have advanced significantly. However, the accurate prediction of ratcheting for metallic materials is still a challenge, especially for the multiaxial ratcheting at high temperature. We should do more deep and comprehensive research on the non-proportionally multiaxial ratcheting and the relatively strain-controlled cyclic behavior, and then develop a constitutive model which can simulate them more accurately. This research is then a topic with theoretical significance for solid mechanics and other interrelated subject and application value for designing and using engineering components credibly. In the other hand, with the development of computer technology, we can simulate and predict the deformation of engineering structure by using finite element programs now. But the constitutive models used in existing programs can not simulate ratcheting accurately. So it is necessary to implement the new constitutive models to finite element programs and simulate the cyclic deformation of engineering structure well and truly. This work can accelerate the application of advanced constitutive models, and it is also of great engineering application value.In order to carry out an in-depth research on the non-proportionally multiaxialratcheting and strain-controlled cyclic behavior for cyclic hardening material at hightemperature and develop a constitutive model which can simulate them moreaccurately, this thesis is mainly concerned with the following studies:1. An experimental study was carried out on the uniaxial and non-proportionallymultiaxial cyclic deformation at high temperatures. The ratcheting behavior andcyclic hardening/softening behavior of SS304 stainless steel were experimentallystudied under several non-proportionally multiaxial loading paths at 350℃ and700'C. Some significant results are obtained by analyzing the experimental data, which are very useful to construct a corresponding constitutive.2. Based on the experimental study, a new constitutive model was developed in the unified visco-plastic frame, which can describe the non-proportionally multiaxial ratcheting and strain-controlled cyclic behavior uniformly. In this model, a new kinematic hardening equation was used;the non-proportionality, the effect of temperature and memorization for the maximum plastic strain amplitude were also considered. Correspondingly, a reliable and fairly method to determine the parameters of the model is proposed. The cyclic deformation behavior of SS304 stainless steel at 350'C and 700"C were simulated by the developed model. It is shown from the simulated result that the developed model can describe the non-proportional multiaxial deformation behavior of SS304 stainless steel at high temperature reasonably.3. The simplified constitutive model was implement into the finite element program ABAQUS by user-subroutine UMAT. Based on radial method and back Eular integration, a new implicit stress integration algorithm was proposed. Simultaneously, a new expression of consistent tangent modulus was also derived. Numerical examples and ratcheting simulation for simple structure at high temperatures were given. The computational results showed that the finite element implementation was successful, and the constitutive model has great engineering application value.
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