Study On Time Dependence Uniaxial Ratcheting-Fatigue Interaction For Metal Materials

For the materials subjected to a cyclic stressing with non-zero mean stress, a cyclic accumulation of inelastic deformation will occur, which is called ratchetting. Ratchetting will shorten the fatigue life of components or induce the components not to work normally because of the exceed-limit deformation. So the ratchetting is very important and should be addressed in the security or reliability assessment and fatigue life estimation of the structure components. In many cases, ratcheting is influenced by the applied stress rates and peak/valley stress hold time, that is to say, ratcheting is time-dependence. So it is necessary to consider time dependence ratchetting-fatigue interaction in the estimation of fatigue life for the structure components made of such materials. Therefore, in this paper, the detailed experimental observations for the uniaxial time-dependence ratchetting-fatigue interaction of tempered42CrMo steel and SS304stainless steel have been carried out, and the whole-life ratchetting behaviour of the two materials are simulated by an improved version of the exist constitutive model. This work is of great engineering reference value for reasonable applications and reliability assessment of structures. The following issues have been addressed in this thesis:(1) An experimental study was carried out on the cyclic hardening/softening behavior and uniaxial ratchetting-fatigue interaction of SS304stainless steel at room and high temperature (100℃and300℃). The effects of mean stress, stress amplitude, loading rates and holding-time at peak/valley stress of each cycle on the uniaxial ratchetting and fatigue life of SS304stainless steel were discussed.(2) Systematic experimental study for the ratchetting-fatigue interaction of tempered42CrMo steel (a kind of cyclic softening material) was carried out at room temperature. And the influence of different peak stress hold and stress rates on the rachtetting and fatigue life of tempered42CrMo steel was discussed.(3) Based on the experimental observations, continuous damage mechanics and the framework of cyclic visco-plasticity, a newly time-dependent damage-coupled constitutive model was introduced by employing a static recovery term in the Abdel-Karim-Ohno non-linear kinematic hardening rule. Comparing with the corresponding experimental results, it is shown that the developed model can describe the whole-life ratchetting behaviour of the tempered42CrMo steel well. (4) The whole-life ratcheting behaviour of the cyclic hardening SS304stainless steel was reasonably simulated by the employing the AF+OW II model at room and high temperature (100℃).