Fatigue crack growth under variable amplitude loading and negative stress ratios

为了防止意外故障并确保安全使用寿命，在损伤容限设计或安全相关部件的情况下，需要定期进行非破坏性检查。检查间隔的估计是通过基于裂纹计算剩余寿命来完成的，裂纹可以通过无损检查方法检测到。服务加载导致负载交互作用的影响，这要么增加或减少剩余寿命。与正应力比下的变幅载荷相反，负应力比下的工作载荷的影响研究得很少。此外，剩余寿命计算的分析概念几乎完全基于正应力比下的研究。这意味着基于现有寿命概念的预测具有很高的不确定性。因此，本研究项目的总体目标是研究变幅载荷对负应力比下裂纹扩展速率的影响，作为进一步发展载荷相互作用模型的基础。因此，可以确保部件、机器和运输工具的安全设计。在这项研究工作的框架下，压缩载荷对裂纹扩展速率的影响在变幅载荷下的考虑sigmatip作为衡量裂纹尖端加载在压缩的实验和数值研究。研究的重点是两个主要的影响因素：a）载荷历史和压缩载荷的强度以及B）材料特性和约束效应。为了研究载荷历史和载荷水平的影响，将研究不同的过载/欠载、过载/欠载序列、块载荷和标准载荷序列。所研究的材料的选择的基础上，一方面的运动硬化量，另一方面的循环硬化和软化。为了解释实验观察到的现象，通过二维和三维有限元分析进行了数值模拟。模拟研究了裂纹闭合、残余应力和接触对裂纹尖端变形和载荷的影响。通过对裂纹侧翼的实验测量验证了模拟结果。实验和数值计算结果都用于进一步的发展和现有的分析方法，特别是带材屈服模型的修改。

In order to prevent unexpected failures and to assure a safe life, regular non-destructive inspections are required in cases of a damage tolerant design or safety-relevant components. The estimation of the inspection intervals is done by the calculation of the residual lifetime based on a crack, which can be detected with non-destructive inspection methods. A service loading leads to load interaction effects, which either increase or decrease the residual lifetime. In contrast to variable amplitude loadings at positive stress ratios the effect of service loadings at negative stress ratios is only poorly investigated. Furthermore, the analytical concepts for the calculation of residual lifetimes are almost exclusively based on investigations at positive stress ratios. This means that predictions, which are based on the available lifetime concepts include a high potential of uncertainty. Therefore, the global objective of this research project is to investigate the effect of variable amplitude loading on the crack growth rates at negative stress ratios as basis for further developments of load interaction models. Therewith, a safe design of components, machines and means of transport can be assured. In the framework of this research work the influence of compressive loads on the crack growth rate at variable amplitude loadings under the consideration of sigmatip as measure for the crack tip loading at compression is experimentally and numerically investigated. The emphasis of the investigations is on two main influencing factors: a) the load history und the intensity of compressive loads as well as b) the material properties and the constraint effect. For the investigation of the influence of the load history and the load level different over-/ underloads, over-/underload sequences, block loads and standard load sequences will be investigated. The investigated materials are chosen on the basis on the one hand of the amount of kinematic hardening and on the other hand of the cyclic hardening and softening. For the explanation of experimentally observed phenomena numerical simulations are performed by 2D and 3D finite element analyses. The simulations are performed to investigate the influence of crack closure, residual stresses and contact on the crack tip deformation and load. The results of the simulation are validated with experimental measurements of the crack flanks. The experimental and numerical results are used both for further developments and for modifications of the available analytical approaches, especially the strip yield model.