13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 4: Crack length vs. the number of cycles for the left and right crack front. The crack shown in Figure 5 initiated at the phase boundary between the austenite grain 3 and the ferrite grain 1. The specimen was loaded with a stress amplitude of 400 MPa up to 1.7·106 cycles until the test was stopped manually to prevent the breaking of the specimen. Figure 5: Crack path of a fatigue sample loaded at Pa, 1.7·10 6 cycles. By analyzing the micrographs which have been taken every thousand cycles during the fatigue test, it was possible to reconstruct the crack propagation path. In this case, the crack spends about 1.3·106 cycles to cross the ferrite grain 1, starting from its initiation site and propagating in the direction of austenite grain 2 at an angle of about 45°with respect to the stress axis. This first part of the crack propagation process is quantified in Figure 6. After reaching the austenite grains 2 and 3, the crack is changing its propagation direction perpendicular to the stress axis by changing its slip mechanism into double slip.
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