13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- has increased in the inclination angle ψ from 50° to 130° and decreased from 130° to 150°, when the micro crack 1 and micro crack 2 are completely non-oriented. For original micro crack tips 1 and 2, there aren't any noticeable effects with a change in the micro crack inclination angle ψ. As it was shown in Figs. 6 and 7, the most changes were in original micro crack tips 3 and 4, when the original tips 3 and 4 are close to the other of original micro crack tips, both of modes I and mode II stress intensity factors, Ik and II k are increased. In nearly vertical micro cracks, 11 k plays a more important role than Ik . Fig. 8 shows the wing crack initiation angle of four tips versus the changes of inclination angle ψ. This figure shows that in tips 3, 4, wing crack initiation angle has increased in the inclination angle ψ from 60° to 90° and decreased from 90° to 150°. The maximum of wing crack initiation angle occurs at ψ= 90°. And also for tips 1 and 2, the changes of ψ are not noticeable due to non-orientation of micro cracks and it is assumed that there is zero interaction. Figure 6. Treatment of mode I SIFs versus the locations of two micro cracks, under uniform tension (parallel to the y axis), for L/b=0.1 and 360+6 nodes for each micro crack
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