Characterization of the very early and very reduced fatigue crack growth in Al-alloys under very low loading conditions

最近的研究活动反映了人们对应用于承受远低于经典疲劳极限的载荷制度的结构部件中的金属材料的失效的技术和科学相关问题的日益增长的兴趣。超高周疲劳（VHCF）的研究主要集中在裂纹萌生机制的识别，因为整个疲劳寿命是由这种现象占主导地位。然而，提交的项目的关键问题是在非常低的循环应力幅下裂纹扩展行为的表征，因为在实践中，产品很少没有在制造过程或误用期间产生的缺陷或裂纹。虽然用LEFM描述疲劳裂纹扩展行为已被正式接受，但预计会有很大的差异，这主要是由于塑性区相当小，受晶体塑性影响，并与显微组织特征相互作用。在提交的项目中，将定量表征和模拟有效机制及其对疲劳裂纹扩展的影响。因此，基于三个工作组的专业知识的实验和模拟方法的范围得到了巩固，目的是在非常低的应力幅下开发基于模型的非常缓慢的裂纹扩展特性。每个单独的研究重点和合作的协同效应的结果的总和应导致相关的沉淀硬化和应变硬化铝合金（在两个不同的微观结构条件下）的疲劳裂纹扩展机制的一致理解，允许获得的见解转移到共同的工程实践。

Recent research activities reflect the growing interest in the technologically as well as scientifically relevant problem of the failure of metallic materials applied in structural parts subjected to load regimes well below the classical fatigue limit. Very high cycle fatigue (VHCF) investigations primarily focus on the identification of crack initiation mechanisms, since overall fatigue life is dominated by this phenomenon. However, the key issue of the project submitted is a characterization of the crack growth behavior at very low cyclic stress amplitudes since in practice products are seldom free from flaws or cracks initiated during manufacturing processes or misuse. Although the description of the fatigue crack growth behavior by means of LEFM is formally accepted, drastic discrepancies are expected, which are mainly due to the rather small and by the crystal plasticity influenced plastic zone and its interaction with microstructural features. The effective mechanisms and their influences on the fatigue crack growth will be quantitatively characterized and simulated in the project submitted. The scope of experimental and simulation methods based on the expertise of three workgroups are therefore consolidated with the aim to develop a model-based characterization of the very slow crack growth at very low stress amplitudes. The sum of the outcomes of each individual research focus and the synergistic effect of the cooperation shall result in a coherent understanding of the fatigue crack growth mechanisms relevant for a precipitation hardening and a strain hardening aluminum alloy (each in two different microstructural conditions) allowing a transfer of the insights gained into common engineering practice.