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金属材料在远低于经典疲劳极限的载荷下在结构部件上的失效这一技术和科学相关问题的日益浓厚的兴趣。甚高周疲劳（VHCF）的研究主要集中在裂纹萌生机制的识别上，因为这一现象决定了整体疲劳寿命。然而，所提交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中或使用不当中产生的缺陷或裂纹。虽然用LEFM方法描述疲劳裂纹扩展行为已被正式接受，但由于晶体塑性影响的塑性区及其与微观组织特征的相互作用较小，预计会出现较大的差异。在提交的项目中，将对疲劳裂纹扩展的有效机制及其影响进行定量表征和模拟。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缓慢裂纹扩展特征。每个单独研究重点的成果总和和合作的协同效应将导致对沉淀硬化和应变硬化铝合金（每种在两种不同的微观结构条件下）相关的疲劳裂纹扩展机制的连贯理解，从而将所获得的见解转移到常见的工程实践中。

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.