Simulation der mikrostrukturbestimmten Kurzrissausbreitung unter dem Einfluss lokaler Phasenumwandlung

In various practical situations, structures and components are exposed to high-cycle fatigue loading. In such conditions, their lifetime is determined by the growth of microstructurally short fatigue cracks. In order to increase the resistance of materials against short crack propagation, understanding its mechanisms is an important basis. Hence, this thesis presents a new two-dimensional model representing short crack growth in a metastable austenitic steel. It is influenced by phase transformation, provided two intersecting slip systems are activated at the crack tip. Due to the increased volume of the transformed material, the crack is closed for a longer part of the load cycle compared to transformation-free crack growth. This reduces the cyclic crack tip slide displacement determining the crack propagation rate and results in an increased lifetime of components.
Based on experimental investigations, crack propagation and phase transformation – both occurring at the same time – are modelled performing the following main steps:
• The stress field in the vicinity of the crack tip is evaluated during one load cycle. If it activates two slip systems, slip planes with the determined orientations are introduced at the crack tip. They represent elastic ideal-plastic material behaviour.
• Crack propagation and the increase of transformed material at the crack tip are determined resulting from slip which has been computed along the slip planes.
• Crack extension and the increased transformed regions are included into the model and subsequently, a new load cycle is simulated.
In order to solve the model numerically, a boundary element method is used, which couples two types of boundary elements: Those with absolute displacements enclose the martensite regions which allows to assign volume increase to these areas. Elements with relative displacements discretise the crack and the slip planes.
The model is applied to simulate the growth of microstructrually short cracks. Besides of phase transformation, barrier effects of grain boundaries and individual isotropic stiffnesses of the grains of a microstructure are considered. Effects of short crack propagation resulting from these features are highlighted and parameter studies are performed. Besides of other results, reduction of cyclic crack tip slide displacement due to phase transformation is reproduced. Exemplarily, the propagation of a real crack is simulated yielding a good agreement with the experiment.