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The propagation behaviour of short cracks under cyclic loading issimulated. Short cracks determine up to 90% of fatigue life and exhibit substantiallynon-uniform propagation kinetics as compared to the growth of long cracks due to theirstrong interactions with microstructural features. Experimental investigations on aduplex steel have been performed to characterise the different barrier effects of grainand phase boundaries on short crack propagation and to determine the mechanicalproperties of the individual components of the two-phase material [1]. The findingswere implemented into a mechanism-based model for two-dimensional crackpropagation in stage I (operating by single slip), which is capable to take the realmicrostructure into account. Crack growth simulations performed with the model haveshown good agreement with experimental data. Based on this method, an algorithm forthe transition of stage I crack growth to crack propagation on multiple slip systems ispresented. Thereby, the crack changes its propagation direction from approximately45° in stage I to a path perpendicular to the loading axis, which is the direction of crackgrowth in stage II. For propagation on multiple slip systems, the closure behaviour ofthe crack has been simulated and the findings were compared to experimental results.By means of using virtual microstructures based on Voronoi diagrams, it is possible tosimulate the overall fatigue-crack propagation process starting from amicrostructurally short crack in a single grain until the crack has crossed several (10-20) grains with just one model. It was shown that the propagation mechanism changesfrom stage I crack growth on single slip systems to the growth on alternating slipsystems, which is the preliminary step to stage II crack propagation.