13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- tests. As cracks were found both on the basal plane and on prismatic planes in αp-grains, the respective systems are considered in the model. In reality a lamellar colony consists of parallel alpha plates in a beta matrix and the orientation relation between both phases is defined by the Burgers relationship. This means that there are two parallel slip systems in both phases, and therefore slip can be easily transferred over the interface between alpha plate and beta matrix so that the effective slip length is equal to the colony diameter. As the effective slip length is the crucial parameter for crack initiation and crack growth, a lamellar colony is modeled as one grain in the virtual microstructure without resolving each single lamella. 6. Model capability First, the model is verified by comparing simulation results to real fatigue crack propagation behavior. For this purpose the geometry of the crack shown in Fig. 7b is given as an input into the simulation model. Then, the simulation is started from the initial crack length until the crack tip reaches the end of the slip band. In Fig. 7a the projected crack length of the left and the right half is plotted over the number of cycles. It can be seen that the crack propagates fast on the left hand side on a slip band favorably oriented for slip. However, when the crack tip approaches the grain boundary the propagation rate decreases. On the right hand side the crack grows significantly slower on a grain boundary. The comparison of simulation and experimental data shows satisfying agreement. Figure 7. (a) Comparison between simulation and experimentally observed fatigue crack and (b) crack geometry. Figure 8. Crack growth simulation through several grains in virtual microstructure.
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