The influence of residual compressive stresses on the fatigue strength under cyclic fatigue loading of steels is a central topic of materials research and also of particular interest in mechanical engineering. The aim of this work is to establish a relationship between the microstructural conditions and the macroscopic material behavior in order to produce optimized components by means of appropriate manufacturing and post-processing methods with tailored materials. In this context, surface hardening by shot peening is one of the most important methods for introducing residual compressive stresses into component surfaces, especially since oscillating stressed components are usually subjected to bending and/or torsional loads. For these two types of loading, especially in the case of superposition, the highest stresses occur at the outer surface. Various design concepts have been established that realize a match between the component dimensioning and the necessary fatigue life for the specific application. However, the underlying microstructure mechanisms that have an influence on fatigue life have not yet been definitely clarified.

In the relevant literature, numerous studies for a cyclic uniaxial loading case have already been used to understand the microstructural fatigue mechanisms. In contrast, the microstructural influences in cyclic torsional fatigue have been rarely studied, especially in the presence of residual stresses in the component surface. Most of the available studies on cyclic torsional loading focus on long cracks and thus on the fatigue life limit, while the literature on short cracks, especially the microstructure influence on short crack initiation and propagation, is limited.

Therefore, this work investigates the influence of residual stresses on crack initiation and short crack propagation on martensitic spring steel under cyclic torsional loading. To achieve this objective, non-shot-peened and shot-peened martensitic steel specimens were subjected to cyclic torsional fatigue loading using a miniature torsion testing machine which was developed in advance. Furthermore, a method was developed to map the stress states at the crack tip using numerical calculations. In order to quantify the microstructural influence, electron backscatter diffraction (EBSD) measurements were carried out along the crack path. These were imported into the crystallography toolbox MTEX using scripts that were adapted for the problem of this work. The stress tensors obtained from the numerical calculations were given as input parameters for the prevailing stress state at the crack tip. This enabled to determine values for the resulting Schmid factors, slip transmission values as well as the analysis of the active slip systems for the given stress state. In addition, the stress tensors were used to determine a cyclic equivalent stress intensity factor to establish comparability with the cyclic uniaxial stress case.