Study On Ratchetting And Fatigue Of Reactor Materials

The ratchetting phenomenon corresponds to materials' progressive inelastic deformation, when subjected to cyclic loading. This problem greatly challenges the design of components and has been paid much attention to by students at home and abroad. Lots of experiments about this problem have been made. Now, it is well known that not only the mean stress, the stress amplitude and their loading rate, but the cyclic stress history and the cyclic strain history and etc. also affect the ratchetting behavior. By studying the non-linear kinematic hardening of the back stress, many models have been presented, based on the assumption of Von-Mises materials. Another two-surface model, concerned with the yield surface and the bounding surface and their coupled translation and deformation, was presented, defined in the term of plasticity inner variables. Most of the models mentioned above are hard to develop and complex in application. Cai Lixun and his fellow workers, develop a methodological system and the universal ratchetting model (URM) with unitary parameter according to the experimental results and the discovery of maximum stress controlling the saturated ratchetting strain. Their model, which is less complex when used on 304 stainless steel, is proved to have satisfying simulation results.On the basis of the methodological system with unitary parameter, series uniaxial experiments of !Crl8Ni9Ti were conducted at room temperature. It is showed that this system has its restriction when used under low mean stress conditions. In that case, the ratchetting strain is affected not by only the maximum stress but also the mean stress. By simplifying the saturated ratchetting model and considering the mean stress' effects in the ratchetting evolution model, the earlier URM model is modified. The modified model gives better simulation results when compared with experiments.In order to study the uniaxial ratchetting behavior of T42NG and T225NG titanium alloys, some experiments with high mean stress andlow stress amplitude were carried out. It can be observed that shakedown takes place for either of the alloys if shakedown is defined by the rate of the ratchetting strain if below a given value. According to the features of ratchetting strain of the T225NG alloy, an exponential model is represented. When studying the effects of the cyclic creep on the ratchetting behavior, more obvious effects could be observed in experiments with lower stress amplitude.As for the T42NG alloy, nothing could be found conclusive.The LCF (low cycle fatigue) feasibility test of the Zr-4 alloy slice specimen with 1~2 thickness is studied and then the LCF experiments are conducted. Based on the shape factor and the effective Poisson ratio an effective method to transform from lateral strain of funnel-like specimen to axial strain of standard specimen is developed. Simulation results of M-C model and a simplified A.-M-C model are provided, in which X stands for the factor of temperature effect on the fatigue life.