加载历史和裂纹闭合对疲劳裂纹扩展行为影响的数值模拟

NUMERICAL SIMULATION OF THE EFFECT OF LOADING HISTORY AND CRACK CLOSURE ON CRACK PROPAGATION BEHAVIOUR

采用不同应力比条件下的16MnR钢紧凑拉伸试样,设计了三种有限元分析模型,即不考虑加载历史效应的静态裂纹扩展模型,同时考虑加载历史和裂纹闭合的动态裂纹扩展模型以及仅考虑加载历史的伪动态裂纹扩展模型,对疲劳裂纹闭合过程、裂纹尖端的应力-应变迟滞环、疲劳损伤和裂纹扩展速率进行了数值模拟与分析,进而着重探讨了加载历史和裂纹闭合影响疲劳裂纹扩展行为的交互作用机制。结果表明:对于同类分析模型,应力比越大越不容易产生裂纹闭合;而在应力比相同的情况下,加载历史引起的残余压应力对裂纹闭合有明显的促进作用。裂纹闭合效应阻碍了平均应力的松弛,减小了裂纹尖端附近的应力-应变场强度、疲劳损伤和裂纹扩展速率,而加载历史引起的残余压应力则加快了平均应力的松弛和抑制了棘轮效应。与实验结果比较发现,只有同时考虑了裂纹闭合效应和加载历史影响的动态裂纹扩展模型,才能对疲劳裂纹扩展行为进行准确、定量的模拟。

Three kinds of finite element models .were used to study the fatigue crack growth behavior of 16MnR steel under different stress ratios. The stationary crack propagation model considers only the effect of a crack closure, and the fake dynamic crack propagation model considers only the effect of loading history. However, the loading history and crack closure were considered in the dynamic crack propagation model. Based on the outputted elastic-plastic stress-strain results from the FE analysis, the crack closure process, the stress-strain hysteresis loop, the fatigue damage distribution and the fatigue crack growth rate were studied. The interactive mechanism between the crack closure and loading history for the fatigue crack growth behavior was discussed further. For an identical finite element model, it is difficult to find the appearance of a crack closure for the loading condition with a higher stress ratio. On the other hand, the residual compressive stress induced by loading history can promote the crack closure obviously for the identical stress ratio. The crack closure slows down the mean stress relaxation and decreases the stress-strain magnitude, which leads to the reduction of a fatigue crack growth rate. The residual compressive stress induced by loading history speeds up the mean stress relaxation and brings down the strain ratcheting. The predicted fatigue crack growth rate by the dynamic crack propagation model, in which the interactive influence of loading history and a crack closure was incorporated, agrees with the benchmark experimental data very well.