Experimental And Theoretical Study On Multiaxial Ratchetting-Fatigue Interaction For Metal Materials

The fatigue of material and structure components is always concerned by scholars and engineers. In the area of aerospace, nuclear industry, chemical industry and railway, many metal components are subjected to a complex loading condition. In order to evaluate the reliability, security and fatigue life of such componetnts reasonablely, it is necessary to obtain an accurate constitutive model to describe the material deformation behavior of materials under the complex loading conditions, and a damage evolution quation to describe the performance deterioration of materials during the loading, as well as a fatigue failure criterion to predict the failure life of materials. At present, the constituitve modelling for materials subjected to complex loading conditions and the construction of relative fatigue failure model are one of the key issues for solid mechanics. In the last two decades, many experimental and theoretical studies on the cyclic deformation of metal have been achieved. The cyclic plastic and visco-plastic constitutive models have advanced significantly. However, the constitutive modelling to the ratchetting of the metals has not been perfectely solved yet, and much further effort is necessary. For the low cycle fatigue of materials, most of referable literatures concern the cases under the strain-controlled loading, while few literatures discusss the low cycle faituge under the stress-controlled loading. Ratchetting, a cyclic accumulation of inelastic deformation, will occur under asymmetrical cyclic stressing. It is extremely necessary to consider the ratchetting, fatigue and their interaction simultaneously in order to simulate the cyclic deformation and fatigue failure of the materials subjected to the stress cyclic loading. Therefore, it is very significant to discusss the ratchetting -fatigue interaction experimentally and theoretically.In order to realize the ratchetting-fatigue interaction of metal materials and develop a damage-coupled constitutive model and fatigue failure model, this thesis has carried out the studies as follows:1. An experimental study was performed to the uniaxial and non-proportionally multiaxial ratchetting deformation, fatigue failure and damage evolution of the materials (i.e., SS304 stainlesss steel, annealed 42CrMo steel and tempered 42CrMo steel) under asymmetrical stress cyclic loading and at room temperature. Some significant results of ratchetting-fatigue interaciton are obtained by analyzing the experimental data, which are very useful to construct corresponding constitutive model and fatigue failure model.2. Based on the framework of unified visco-plasticity and continuum damage mechanics, a damage-coupled visco-plastic cyclic constitutive model is proposed to simulate the whole-life ratchetting. Combined with the corresponding fatigue criterion, the model can predict the fatigue failure life too. In the proposed model, the damage is introduced, and the damage is divided into two parts, i.e., macroscopic elastic damage and plastic damage, which were described by different evolution equations, respectively. The threshold value of damage and maximum strain are adopted as failure criterion to reflect fatigue failure and ductile failure respectively. It is shown that the model simulates the whole-life ratchetting behavior and fatigue life of tempered 42CrMo steel reasonably.3. Based on the systemic experimental study, a stress-based simplified fatigue failure models were proposed to predict the fatigue life of the materials by addressing the ratchetting-fatigue interaction. In the model, the stress paramters are adopted as basic parameter, and the effects of ratchetting deformation and multiaxial loading path on the fatigue life are also included. The fatigue lives of the materials under various cyclic stressing cases can be predicted directly by the simplified failure model.