Microscopic stress- and strain analysis to investigate the influence of hardening on crack retardation mechanisms under cyclic loading with variable amplitudes (overloads)

在过载和变幅载荷影响下的裂纹扩展行为已经研究了40多年，其预测模型只取得了部分成功。这些模型通常基于相应的裂纹扩展曲线和对裂纹张开行为的宏观(因此不是局部)测量。由于微观应力场和应变场测量技术的不可获得性，对影响裂纹扩展行为的机制：裂纹侧面的塑性闭合和裂纹尖端前方的残余应力进行了讨论。在我们的工作中，我们可以表明，由于现代测量技术的结合--磁巴克豪森噪声和扫描电子显微镜的数字图像相关--我们能够成像、分离和定量评估超载效应的机制。基于这些结果，使用一个简单的模型，我们进一步能够预测由于超载效应引起的裂纹扩展行为。有些材料表现出强烈的超载敏感性，另一些材料超载只有很小的影响。我们过去的结果是使用弹性-理想塑料材料获得的，因此不能解释这种材料行为。这种敏感性的可能原因是静态和动态应变硬化的不同以及加载方向的改变(包辛格效应)。通过我们的分析方法，我们可以研究硬化效应对超载机制的影响。这些新的见解将改进预测裂缝扩展的模型，这将提高安全性，或者--尤其与轻质建筑相关--一种不那么保守但仍然安全的建筑。我们的数据还可以用作模拟的物理输入数据，甚至允许采用新的模拟方法，这些方法不仅基于裂纹扩展曲线，而且还基于微观力学效应。此外，研究结果可用于有目的地选择变幅加载的材料，不仅要考虑强度，还要考虑硬化行为。

Crack growth behaviour under the influence of overloads and variable amplitude loading has been examined for over 40 years and its predictive modelling is only partly successful. The models are usually based on the corresponding crack growth curves and macroscopic (therefore not local) measurements of the crack opening behaviour. Due to the inaccessibility of measurement techniques for microscopic stress and strain fields, there have been discussions on the mechanisms that influences to the crack growth behaviour: the plasticity induced crack closure on the crack flanks and the residual stresses in front of the crack tip. In our work we could show that due to the combination of modern measurement techniques - magnetic Barkhausen noise and digital image correlation in scanning electron microscope - we are able to image, separate and evaluate quantitatively the mechanisms of the overload effect. Using a simple model based on these results, we were furthermore able to predict the crack growth behaviour due to the overload effect. There are materials that show a strong overload sensitivity and others on which overloads have only a minor effect. Our past results were achieved using an elastic-ideal plastic material and can therefore not explain this material behaviour. Possible reasons for this sensitivity can be found in differences in the strain hardening, both in static and in dynamic case as well as in change of loading direction (Bauschinger Effect). By using our analysis methods, we caninvestigate the influence of the hardening effects on the overload mechanisms. These new insights will improve models that predict crack growth, which would improve safety or - especially relevant for lightweight construction - a less conservative, but still safe construction. Our data can also be used as physically based input data for simulations or even allow new simulation approaches that will not only be based on crack growth curves but also on micromechanic effects. Furthermore, findings can be used for purposeful material selection regarding variable amplitude loadings considering not only strength but also the hardening behaviour.