Macroscopic crack propagation is relatively well understood, allowing low-cycle-fatigue lifetime predictions. In the HCF and VHCF regime though, crack initiation and short crack evolution become lifetime governing factors.
Therefore, an experimental in-situ method has been applied, allowing to capture statistical information on damage evolution in the microstructure. A micro resonant fatigue setup is equipped with an optical system enabling quantitative in-situ damage characterization. The fatigue induced damage evolution on the sample surface can therefore be recorded. The local damage evolution is then correlated to the microstructural information from EBSD maps acquired prior to the fatigue experiment. With this scheme, information about fatigue induced damage evolution in terms of in- and extrusion forming and micro-crack initiation can be disclosed. This is possible because the sequence of in-situ images allows tracing a crack back to its initiation. Here, it becomes possible to identify critical grain configurations. Hence, also the exact time when in- and extrusions are formed can be determined as well as – due to the prior EBSD scan – in which grain orientation this happens. Moreover, when various visible fatigue indicators group together to an intergranular damage can be observed and quantified. This also enables a more precise investigation by means of SEM and FIB between the formation of in- and extrusions and micro-crack growth.
BCC stainless steel (17-4PH) was chosen since heat-treatment allows to choose between solution and precipitation strengthening as well as martensitic phase formation. In the present study the solution annealed condition will be presented.
Statistical methods will be used to analyse the data in order to understand the links among the material properties and life time observation.
Within this scheme, models for crack initiation, micro crack and short crack growth can be validated and used to develop better lifetime predictions.