Many technical components are subjected to uniaxial cyclic loading in the high cycle fatigue regime. The resulting fatigue damage evolution is commonly subdivided into four stages, i.e., crack initiation, short crack propagation, long crack propagation, and eventually failure. Especially, the stages of crack initiation and short crack propagation can account for up to 90% of the fatigue life in the high cycle fatigue regime, leading to a particular interest in investigating these stages of fatigue damage evolution. [1] Furthermore, it is extremely important to understand the mechanisms of crack initiation and short crack propagation for lifetime predictions of structures which are not fail safe, as their failure can cause a catastrophic damage.Many studies have shown, that crack initiation often occurs at slip bands (see for example, the studies by Christ, McEvily, Efthymiadis et al., and Hong et al. [1][2][3][4] ) and that short crack propagation is strongly influenced by the local microstructure, leading to an oscillating crack propagation rate (see for example, the studies by Christ, McEvily, Miller, and Yang et al. [1,2,5,6] ). However, until now, just a few studies have investigated the fatigue damage evolution in a martensitic steel and the corresponding impact of the complex martensitic microstructure on the crack initiation and the short crack propagation. The crack initiation in martensitic steels seems to occur often at slip bands, [7][8][9][10][11] which mainly arise at microstructural interfaces and are oriented parallel to martensitic laths. [11][12][13] It seems that those slip bands do not cross microstructural interfaces with a high-angle boundary like packet boundaries or prior austenite grain boundaries (PAGBs). [10,13,14] Hence, the crack initiation and early short crack propagation in martensitic steels seem to occur mainly intergranularly at PAGBs [11,12,14,15] or block boundaries, [11,16] although in some studies, transgranular crack initiation and early short crack propagation were also observed. [9,13] In many studies, the preferential alignment of carbides along microstructural interfaces is cited as a possible reason for intergranular crack initiation and early short crack propagation. [7,14,17,18] In addition, intergranular crack initiation can be explained by the impingement of slip bands on grain boundaries [14,19,20] or the anisotropic elastic and plastic properties of the grains. [11,12,15,[21][22][23] The number of slip bands formed and cracks initiated rises with increasing number of cycles and with increasing stress amplitude. [4,8,9,13,24] The subsequent short crack propagation seems to be strongly influenced by the local microstructure, whereby a short crack propagation parallel to the martensitic laths is often observed. [8,24,25] PAGBs [6,8,14,24,26] and block boundaries [6,7,26] seem to act as obstacles for short crack propagation, leading to an oscillating short crack propagation rate. An often referred explanation for the barrier effect of microstructural interfaces is the...

The crack initiation and short crack propagation in a martensitic spring steel were investigated by means of in-situ fatigue testing. Shot peened samples as well as untreated samples were exposed to uniaxial alternating stress to analyze the impact of compressive residual stresses. The early fatigue damage started in both sample conditions with the formation of slip bands, which subsequently served as crack initiation sites. Most of the slip bands and, correspondingly, most of the short fatigue cracks initiated at or close to prior austenite grain boundaries. The observed crack density of the emerging network of short cracks increased with the number of cycles and with increasing applied stress amplitudes. Furthermore, the prior austenite grain boundaries acted as obstacles to short crack propagation in both sample conditions. Compressive residual stresses enhanced the fatigue strength, and it is assumed that this beneficial effect was due to a delayed transition from short crack propagation to long crack propagation and a shift of the crack initiation site from the sample surface to the sample interior.

X-ray diffraction with high spatial resolution is commonly used to characterize (poly)crystalline samples with, for example, respect to local strain, residual stress, grain boundaries and texture. However, the investigation of highly absorbing samples or the simultaneous assessment of high-Z materials by X-ray fluorescence have been limited due to the utilization of low photon energies. Here, a goniometer-based setup implemented at the P06 beamline of PETRA III that allows for micrometre spatial resolution with a photon energy of 35 keV and above is reported. A highly focused beam was achieved by using compound refractive lenses, and high-precision sample manipulation was enabled by a goniometer that allows up to 5D scans (three rotations and two translations). As experimental examples, the determination of local strain variations in martensitic steel samples with micrometre spatial resolution, as well as the simultaneous elemental distribution for high-Z materials in a thin-film solar cell, are demonstrated. The proposed approach allows users from the materials-science community to determine micro-structural properties even in highly absorbing samples.