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

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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.