The fatigue crack growth in the austenite X5CrNi18-10 and the ferrite C45E was analysed under constant (single-level) loading for variable R-ratios and stress intensities as well as under single and multiple overloads. The experiments were compared with those ones carried out by Rödling and Broll on aluminium alloys. All experiments were carried out with a special test-equipment specifically designed for fatigue crack propagation experiments. The high-resolution potential drop method of the machine enables to measure the crack growth cycle by cycle with a measuring accuracy of approximatly one micrometer. The experiments showed that the crack growth rate da/dN is influenced by two quantities, the cyclic stess intensity ΔK as well as the maximum stress intensity Kmax. To plot the experiments in reference to this dependence a 3D-presentation was introduced. It was found that the plot of da/dN vs. R (the R-ratio) and Kmax is more informative than the plot of da/dN vs. ΔK and Kmax but as ΔK = Kmax • (1-R) of coarse both presentations are possible. Therefore, the aim has been developed to find a global function for da/dN(R,Kmax) for stress intensities starting form threshold level Kth up to K-levels of the upper Paris range. Consequently, different crack growth models were discussed. As a first result, a new basic approach for the threshold behaviour Kmax,th(R) was developed to express its behaviour for steels and aluminium alloys for all R-ratios in the light of micro-structural aspects. Furthermore, the concept for the prediction of crack propagation curves for variable R-ratios has been improved. The new concept works very well for both steels and the aluminium alloy 6013. A successful prediction of the crack growth rate for any R-ratio and Kmax could be stated. The single overload tests have shown that the crack propagates some micrometers during the cycle of the overload ΔaOL. Thereafter, the crack grows for some cycles faster than before the overload and then develops the well-known effect of retardation. The crack propagation, ΔaOL, is in steel as well as in aluminium approximatly 100 times greater than the expected one, taking the cyclic crack growth rate of the crack propagation curves at an adequate stress intensity into account. This difference cannot be explained by the linear elastic fracture mechanics and is attributed to the dependence of crack propagation during the overload on the crack tip opening displacement, whereas under cyclic loading the crack propagation is controlled by the microstructure and dislocation emission as well as dislocation movement at the crack-tip, respectively.     «

The fatigue crack growth in the austenite X5CrNi18-10 and the ferrite C45E was analysed under constant (single-level) loading for variable R-ratios and stress intensities as well as under single and multiple overloads. The experiments were compared with those ones carried out by Rödling and Broll on aluminium alloys. All experiments were carried out with a special test-equipment specifically designed for fatigue crack propagation experiments. The high-resolution potential drop method of the mach...    »