Investigations of the FGA formation and lifetime phases in the VHCF regime starting from artificial defects based on in-situ and ex-situ imaging methods

在许多应用中，组件承受非常高的负载循环次数。由于疲劳强度并不总是在这种高周疲劳（VHCF）状态下给出，因此即使应力低于常规疲劳强度，部件也可能在相应的高负载循环下失效。假设在从疲劳强度到VHCF状态的过渡过程中，损伤机制发生了变化。因此，在VHCF状态下，裂纹主要在所谓的II型材料的试样内部开始。在裂纹起裂位置周围，通常可以观察到一个显著的区域，这个区域通常被称为细颗粒区（FGA）。在文献中，人们普遍认为90%以上的疲劳寿命是为这一区域的形成而消耗的。然而，导致该地区形成的机制尚不清楚，因此存在不同的模式。然而，到目前为止，还没有一个模型能够描述所有的实验观察结果。因此，提出的研究项目将有助于了解导致这一显著区域形成的基本机制以及相应的VHCF状态下的疲劳寿命部件。然而，由于在固体结构中裂纹起裂位置不是预先确定的，使用常规制造的VHCF试样很难观察和评估FGA的形成以及不同阶段的寿命部件。因此，不同平均应力下的疲劳试验将采用增材制造的Ti6Al4V试样进行，该试样在内部插入一个空洞形式的单一人工主要缺陷。目的是在一个明确的位置上，在一个腔周围可重复地形成FGA。借助多样品技术以及FIB制备或金属切片，可以研究人工缺陷周围FGA的形成点、微观结构、形状和范围。此外，疲劳寿命阶段将通过重复中断原位实验（使用快速射线照相）进行计算机断层扫描（XCT）来监测。随后，可以通过适当的显微组织研究来评估断裂试样上缺陷周围FGA的形成。基于XCT数据，将使用定量图像分析技术（包括数字体积相关）可视化裂缝的起裂和扩展。此外，将进行重复的两步块加载试验，通过跟踪停止标志来研究裂纹扩展。实验研究将伴随着相应的复杂线弹性和弹塑性有限元模拟和分析考虑，目的是建立一个新的基于断裂力学的寿命概念。

In many applications, components are subjected to very high number of load cycles. Since the fatigue strength is not always given in this very high cycle fatigue (VHCF) regime, components can fail at correspondingly high load cycles despite the stresses are below the conventional fatigue strength. It is assumed that a change in the damage mechanism occurs during the transition from the fatigue strength to the VHCF regime. Thus, in the VHCF regime cracks initiate mostly in the interior of a specimen for so-called type II materials. Surrounding the crack initiation location, a remarkable area generally can be observed, which is often referred to as fine granular area (FGA). In literature, it is widely accepted that more than 90% of the fatigue life is spent for the formation of this area. However, the mechanism leading to the formation of this area is not clear, so that different models exist. So far, however, no model is able to describe all experimental observations. Therefore, the proposed research project will contribute to understand the basic mechanisms leading to the formation of this remarkable area and the corresponding fatigue life parts in the VHCF regime. However, because in solid structures the crack initiation location is not predetermined, it is difficult to observe and to evaluate the formation of the FGA as well as the lifetime parts in the different stages using conventionally fabricated VHCF specimens. Therefore, fatigue tests with different mean stresses will be carried out with additively manufactured Ti6Al4V specimens with a single artificial main defect in the form of a cavity inserted in the interior. The objective is the reproducible formation of the FGA around a cavity at a well-defined position. With the help of the multi-sample technique as well as FIB preparations or metallic sections, the point of formation as well as the microstructure, shape and extent of the FGA around the artificial defect can be investigated. In addition, the fatigue life stages will be monitored by repeatedly interrupting in-situ experiments (using fast radiography) at defined numbers of cycles to perform computed tomographic (XCT) scans. Subsequently, the formation of FGA around the flaw on the fractured specimens can be assessed by appropriate microstructural investigations. Based on the XCT data, crack initiation and propagation will be visualized using quantitative image analysis techniques (including digital volume correlation). In addition, repeated two-step block load tests will be performed to investigate the crack propagation by following arrest marks. The experimental investigations will be accompanied by corresponding complex linear-elastic as well as elastic-plastic finite element simulations and analytical considerations, with the aim of developing a new fracture mechanical based lifetime concept.