Ermüdungsmechanismen eines Duplexstahls bei niedrigen Spannungsamplituden : experimentelle Charakterisierung und modellmäßige Beschreibung des Einflusses der Mikrostruktur auf die Lebensdauer

This work contributes to the elucidation of those fatigue mechanisms in two phase metallic materials which are relevant for low stress amplitudes up to very high numbers of load cycles. The investigations were carried out on an austenitic-ferritic duplex stainless steel and revealed that plastic deformation occurs in form of dislocation glide along slip bands mainly in the austenitic phase leading to slip traces at the surface. Frequently, stress concentrations are in-duced by dislocations piling up in such slip bands against phase boundaries, possibly causing the formation of fatigue crack nuclei at intersection points between austenite slip traces and phase boundaries. Resulting short fatigue cracks often permanently stop in the middle of the first grain without interacting with microstructural barriers, such as a grain or phase boundary. The reason for this crack stop lies in the inhomogeneous stress distribution in the grains due to anisotropic elastic properties and residual stresses. Obviously, the shear stress at the crack tip decreases stronger along the anticipated crack path than the stress field in front of the crack increases with crack extension. Under these conditions, the irreversible fraction of cyclic dislocation motion at the crack tip vanishes and, hence, the driving force for the short fatigue crack propagation no longer exists. In addition, grain or phase boundaries (i) retard short fatigue cracks, (ii) block them temporarily or even (iii) stop them permanently. The obstacle strength against crack growth depends on the crystallographic orientation difference between the grain before and behind the grain or phase boundary. The experimentally revealed mechanisms of fatigue crack nucleation and microstructure-governed short fatigue crack growth were simulated taking into account crystal plasticity, anisotropic elasticity as well as residual stresses due to the heat treatment and manufacturing process of the material. It was clearly shown by means of the developed simulation model that in addition to the anisotropic elasticity of the grains and the residual stresses, the three-dimensional geometry of the microstructure has a significant impact on the fatigue damage development.