中文摘要：

通过对20钢的单轴应变控制循环和应力控制循环实验研究，揭示了20钢在室温下的循环变形特性，讨论了材料的循环软／硬化特性和材料的屈服平台以及平均应力、应力幅值和应力比对材料高周次棘轮行为的影响．研究表明：20钢表现出弱的、与应变幅值相关的循环硬化特性；其棘轮行为依赖于平均应力、应力幅值和应力比的大小，在高应力水平时的高周次循环后期，棘轮变形会出现再次增长的现象；材料的屈服平台对棘轮行为有明显的影响，在对20钢的棘轮行为进行本构描述时需要加以合理考虑．

英文摘要：

Ratcheting is a cyclic accumulation of inelastic strain occurred under the stress- controlled cyclic loading with nonzero mean stress, it is very important in assessing the fatigue life and safety of the structure subjected to cyclic loading. The existed results showed that different materials presented different ratcheting features, and the results were obtained from the cyclic tests with a small number of cycles （less than 200 cyc）. Therefore, based on the uniaxial strain-controlled and stress-controlled cyclic tests with high number of cycles （more than 2000 cyc）, the cyclic deformation features of 20 steel were observed at room temperature. The effects of the cyclic softening/hardening feature and yielding plateau of the steel and the applied mean stress, stress amplitude and stress ratio on the ratcheting of the steel with high number of cycles were studied. The results show that the 20 steel presents a weak and stain-amplitude-dependent cyclic hardening, and the ratcheting of the steel depends greatly on the applied mean stress, stress amplitude and stress ratio. The ratcheting strain increases monotonically with the increase of applied mean stress and stress amplitude, but decreases with the increase of stress ratio. A re-speeded ratcheting will occur at the end of cycling with high number of cycles when the applied stress levels are high enough. More importantly, the yielding plateau of the steel greatly influences the evolution of the ratcheting. It is concluded that the effect of yielding plateau on the ratcheting should be reasonably considered in constructing the cyclic constitutive model of the ratcheting for the 20 steel.