13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- step 19, the ratio values are larger than 1.3. It implies that transition mode is not a stable crack growth pattern; a critical ratio of stress intensity factor range is proposed between transition mode and shear mode. Once the proportion of △KII in one loading cycle is large enough, shear mode crack growth occurred. (△KII/△KI)th＜△KII/△KI＜(△KII/△KI)cr (6) The curves for specimen S13 and S7 are much smoother compared to the curve of A7 and A8. The different loading type may be a main reason to explain this phenomenon. Even so, it can be observed that from step 15 curve S13 increased persistently and the values are larger than 0.25. The curve of S7 does not show a rising trend but a relative downward tendency. It means that the mode I stress intensity range increased faster than the mode II range in this specimen. Transition and shear mode crack propagation is suppressed by this rapidly growing mode I component. The assumed transition mode threshold ratios for specimen A7 and A8, S13 are 0.6, 0.45 and 0.25 respectively. Beside the different material property, plasticity plays an important role in reducing the threshold value for specimen S13. Depending on the loading level, more plasticity compared to A7, A8 is displayed in specimen S13, accelerating the transformation of tension mode to transition mode or even to shear mode. Figure 9. Ratio of stress intensity factors range 5. Conclusions In the present paper, 3-dimensional crack growth simulation is implemented by using the LEFM-based algorithm. The simulation for two different materials AlMg4.5Mn, S460N, two phase angles 45° and 90°, different loading levels and MT/F ratio have been presented. The MT/F ratio has a great influence on the number of cracks, 4 cracks will occur when torsion loading MT reaches a certain percentage of the total loading. The loading phase angle affects the crack growth path behaviour and also numbers of initiated cracks. Compared to the phase angle of 45°, specimens under out of phase loading with a phase angle of 90°, demonstrate more variations in crack initiation and crack growth path, depending on different load levels and MT/F ratios. Under non-proportional mixed mode loading, three distinct features of the crack growth path behaviour are observed which are tension mode, transition mode and shear mode. Based on the
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