13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figs. 6e and f. In this case, the main crack was finally formed by propagating the short branch crack and the non-propagation of the original main crack (the lower side branch crack in Fig. 6e). With the crack length increasing, as indicated in Fig. 6g, the slanted crack was again prone to return to the pure mode I trajectory, producing multiple bifurcations along the curved crack path. The bifurcated cracks were often located at grain boundaries and martensite lath interfaces. It is worth noting that the overall crack path propagated with serious local deviations, showing a ‘zigzag’ mode. It is inferred that the significant changes of crack orientation is due to the highly anisotropic microstructures at the crack tip and the gradient strength distribution in the HAZ. It is generally accepted that the crack path is governed by the plastic behavior of the crack tip [21]. Accordingly, the cracks would orient themselves into the materials with lower strength values. Therefore, it is reasonable that the crack path is tended to deflect into the weld metal, which has lower strength than the PQTZ in the HAZ. Figure 6. Fatigue crack growth morphology in pure mode I loading Fig. 7 presents the FCG path at a loading angle of 30°. In Fig. 7a, the crack direction changed immediately from the initial pre-crack orientation with an angle of 27°. Crack bifurcations and shear cracks coexisted along with the main crack, which was resulted from the contribution of mode I and II components. As a result, the mode I component would drive the branch crack for a long length to a mode I crack direction. This was evidenced by formation of main crack based on the original branch crack in Fig. 7b. Tong et al. [22] reported that the formation and propagation of mode I branch cracks were of decisive importance for fatigue failures under combined loadings. It is also observed that the crack tip in Fig. 7a is branched along the grain boundary when N is increased to 622 cycles. On the other hand, the mode II component also contributed to FCG. With the formation of the grain boundary cracks at crack tip in Fig. 7b, a damage area containing many micro-cracks whose direction were almost parallel to the main crack was also formed, as shown in Fig. 7c. This was arisen from the shear deformation under mode II component in the mixed mode
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