Untersuchungen zum Einfluss innerer Grenzflächen auf das Ermüdungsverhalten metallischer Konstruktionswerkstoffe

This thesis covers the fatigue behaviour of two-phased construction materials at high and very high numbers of cycles (up to 10 8 load reversals) using the example of a duplex steel and a titanium alloy. Main emphases are put to the early stage of damage initiation and the growth of short fatigue cracks. Both phenomena are strongly dependant on the local microstructure. It becomes apparent that the phase boundaries present in both alloys determine the fatigue strength.
In case of the duplex steel fatigue damage starts by slip in the softer austenite predominantly in grains exhibiting a high Schmid factor, whereas most of the cracks developing afterwards initiate at phase boundaries due to the elastic and plastic incompatibility. A definition of the physical fatigue limit results from the barrier effect of phase boundaries, where dislocations and short cracks are stopped in case of low load amplitudes. Grain boundaries, in particular twin grain boundaries present in the duplex steel, exhibit only a minor influence on the fatigue strength due to the missing twist angle. The rarely observed exception of crack initiation at non-metallic inclusions, which result from production, is used to establish a technological fatigue limit in addition.
In case of the titanium alloy, which was investigated in two different microstructural modifications, crack initiation predominantly is observed in the secondary alpha phase. The crack propagation is mainly controlled by those primary slip systems (basal and prismatic), which exhibit a high Schmid factor with respect to the external load. Moreover, it can be shown that some cracks partially grow by an intercrystalline mechanism in the lamellar part of the microstructure controlled by the maximum normal stress.
Furthermore, the results are used to establish two different methods for predicting the fatigue lifetime. In case of the duplex steel, this is the phenomenologically based concept of PHYBAL LSV , which uses three fatigue experiments in order to determine a reliable and conservative estimation of the fatigue strength. A mechanism-based approach is chosen in case of the titanium alloy, which relies on the simulation of short fatigue crack growth in two-dimensional virtual microstructures. Since this model takes into account the identified characteristics of the crack growth which is depending on the microstructure, lifetimes based on synthetic Wöhler curves can be calculated using also virtual microstructures. Finally, a technologically and economically attractive combination of both concepts is proposed by using the short crack model for the optimisation of virtual microstructures followed by the experimental verification of the calculated lifetimes