Enhancement of the fatigue limit at HCF- and VHCF-loadings with the help of thermomechanical treatment at the temperature of maximum dynamic strain ageing

在过去，疲劳试验只进行到106或107的极限循环次数，因为人们认为在更长的寿命下循环强度不会进一步降低。如今，我们知道，许多特别高强度的材料甚至在超过107个载荷循环后也会失效。有几个想法，为什么失败可能会发生，即使在非常高的循环疲劳（VHCF）。提出者的观点是，夹杂物处的应力集中和相应的局部塑性是局部晶粒细化的原因，这导致了所谓的细晶区（FGA），以及随后在VHCF期间的裂纹萌生。文献中没有提供改善VHCF行为或避免上述失效机制的方法。因此，这是目前建议的主要问题。为了提高VHCF疲劳极限，必须通过在最大动态应变时效温度下的热机械处理（TMT）来实现具有增加的位错密度的稳定位错结构。以前的研究表明，这种TMT可以提高高强度轴承钢100Cr6的HCF疲劳极限，但从未研究过这种TMT是否对其他钢也有用，以避免VHCF失效。因此，该项目的目标是提高高强度钢在VHCF载荷下的承载能力。因此，必须从材料科学和技术的角度来理解负责任的潜在机制。因此，实验必须表明TMT是否增加了VHCF疲劳极限，以及它是否修改和稳定了位错结构。为此，在测试HCF和VHCF载荷可能增加之前，有必要澄清42CrMo4和100Cr6钢的TMT边界条件。TMT是否引起失效机制的改变将通过对断裂表面进行详细的断口分析来确定。因此，重点将放在裂纹萌生区的非金属夹杂物，以检测典型结构FGA的VHCF加载后的断裂表面。如果在TMT之后在断裂表面上没有识别出这样的FGA，则这将强烈地指向位错结构稳定的观点，这意味着疲劳极限由于微观结构原因而增加。

In the past fatigue experiments were carried out only until an ultimate number of cycles of 106 or 107 because people thought that there is no further decrease of the cyclic strength at longer lifetimes. Nowadays, we know that a lot of especially high-strength materials can fail even after more than 107 loading cycles. There are several ideas why a failure might happen even after very high cycle fatigue (VHCF). The proposer’s opinion is that the stress concentration at inclusions and the corresponding localized plasticity are the reason for a local grain refinement, which leads to a so called fine grained area (FGA), and the subsequent crack initiation during VHCF. Approaches to improve the VHCF-behaviour or to avoid the said failure mechanisms are not available in literature. So, this is the main question of the current proposal.To increase the VHCF-fatigue limit a stabilized dislocation structure with increased dislocation density has to be realized by a thermomechanical treatment (TMT) at the temperature of maximum dynamic strain ageing. Former investigations of the proposers showed that such a TMT can improve the HCF-fatigue limit of the high-strength bearing steel 100Cr6 but it was never investigated whether this TMT is also useful for other steels and for avoiding VHCF-failure.The goal of the project consequently is the increase of the loadability of high-strength steels at VHCF-loadings. Thereby the responsible underlying mechanisms have to be understood from a materials science and technology point of view.Therefore, the experiments have to show whether the TMT increases the VHCF-fatigue limit and whether or not it modifies and stabilizes dislocation structure. To that end, before testing the possible increase by HCF and VHCF loadings, it is necessary to clarify the boundary conditions of the TMT for the steels 42CrMo4 and 100Cr6. Whether or not TMT causes a change of the failure mechanism will be determined with the means of detailed fractographic investigations of the fracture surfaces. Thereby, the focus will be on the crack initiation region at non-metallic inclusions in order to detect the typical structure FGA on fracture surfaces after VHCF loading. If no such FGA on the fracture surfaces are identified after TMT this would strongly point to the view that the dislocation structure is stabilized, which means that the fatigue limit has increased by microstructural reasons.