Fatigue Design in Long Life Region under Variable Amplitude Loading

为了研究变幅载荷作用下载荷顺序对长寿命区疲劳裂纹扩展行为的影响，对非载荷角焊试件进行了疲劳试验。并对疲劳裂纹扩展分析进行了对比研究。然后，对中心裂纹拉伸试件在恒幅、单次超载、周期超载、块载和变幅加载下的疲劳裂纹扩展速率进行了测量。最后，建立了基于裂纹闭合概念的疲劳裂纹扩展预测模型。研究结果总结如下：(L)变幅载荷作用下存在一个变幅疲劳极限。它位于恒幅疲劳极限以下，其系数等于谱的最高应力范围与等效应力范围的比率。(2)Miner法则或等效应力范围概念很好地预测了在VA载荷下的短寿命区域的疲劳寿命，例如，无论应力范围谱的类型是什么，在VA载荷下的疲劳寿命都小于200万次。在长寿命区域，例如超过500万次循环，计算的疲劳裂纹扩展寿命很大程度上依赖于VA载荷谱模式。(3)在疲劳裂纹扩展测量中，发现单个过载、周期性过载、块载荷谱存在载荷序列效应。然而，对于当前加载条件下的变幅加载，虽然应力比效应显著，但在加载过程中没有发现载荷序列效应。(4)利用所建立的疲劳裂纹扩展模型，对疲劳裂纹扩展测量中使用的所有载荷谱，实验结果与预测结果吻合较好。

In order to investigate the load sequence effect on fatigue crack growth behavior in the long life region under variable amplitude loading, fatigue tests were carried out on non-load carrying fillet welded specimens. A comparative study of fatigue crack growth analysis was also carried out. And then, fatigue crack growth rates were measured under constant amplitude, single overload, periodic overload, block loading and variable amplitude loading on center-cracked tension specimens. At last, a fatigue crack growth prediction model based on crack closure concept was established. A summary of the research results is given as follows.(l)There exists a variable amplitude fatigue limit under variable amplitude loading. It is located below the constant amplitude fatigue limit by a factor equal to the ratio of the highest to the equivalent stress ranges of the spectrum.(2)Miner's rule or equivalent stress range concept predict well the fatigue life under VA loading in the short life region, for example, less than two million cycles, whatever the type of stress range spectrum is. In the long life region, for example, over five million cycles, the computed fatigue crack propagation life is much dependent on the VA loading spectrum patterns.(3)In the fatigue crack growth measurements, load sequence effect was found for single overload, periodic overload, block loading spectra. However, for variable amplitude loading under the current loading conditions, though the stress ratio effect was proved to be significant, no load sequence effect was found during the loading procedure.(4)By using the established fatigue crack growth model, good agreements between experiments and predicted results are achieved for all of the loading spectra used in the fatigue crack growth measurements.