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，部件承受非常高的载荷循环次数。由于疲劳强度并不总是在这种非常高的循环疲劳(VHCF)状态下给出，因此，尽管应力低于常规的疲劳强度，部件仍可能在相应的高循环载荷下失效。假设损伤机制在从疲劳强度到VHCF区的转变过程中发生了变化。因此，对于所谓的第二类材料，在VHCF制度下，裂纹主要在试件内部开始。在裂纹萌生位置周围，通常可以观察到一个显著的区域，通常被称为细粒区(FGA)。在文献中，人们普遍认为，90%以上的疲劳寿命是在这一区域的形成过程中花费的。然而，导致这一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。因此，本课题的研究将有助于了解这一显著区域形成的基本机制，以及相应的疲劳寿命部位。然而，由于固体结构中裂纹的萌生位置并不是预先确定的，因此很难使用传统的VHCF试件来观察和评估FGA的形成以及不同阶段的寿命部分。因此，不同平均应力的疲劳试验将用附加制造的Ti6Al4V试件进行，其中单个人工主缺陷的形式是在内部插入一个空洞。目标是在明确的位置围绕空腔形成可重现的FGA。借助于多样品技术以及FIB制备或金属切片，可以研究人工缺陷周围的FGA的形成点以及微观结构、形状和程度。此外，疲劳寿命阶段将通过以规定的循环次数反复中断现场实验(使用快速射线照相术)以执行计算机断层扫描(XCT)来监测。随后，可以通过适当的微观结构研究来评估断裂试件上缺陷周围的FGA的形成。基于XCT数据，将使用定量图像分析技术(包括数字体积关联)来可视化裂纹的萌生和扩展。此外，还将进行重复的两步块载荷试验，通过跟踪止裂标记来研究裂纹的扩展。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，目的是开发一种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.