13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- tension mode dominated crack propagation prevails, if a threshold value is exceeded, transition mode dominated crack propagation is a reasonable crack growth pattern. The threshold K*II th is a function of several parameters, such as material property, the R ratio or the mode I threshold value and so on. Further research is needed to complete the concept. K*II ＞ K*II th (4) Stress intensity factorsl Time KI KII tmax K*I K*II KI KII 0 △KI △KII Figure 7. Variation of stress intensity factors Figure 8. K*II values Only for specimen S7 transition and shear modes did not appear. However, the K*II increased in the same way as for the other specimens. This implies that the crack growth path behaviour is not just controlled by K*II. Considering one loading cycle, the stress intensity range is defined as: (see Figure 7) △Ki = Ki max (1-R) (i=I, II) (5) The ratios of △KII to △KI in one loading cycle are shown in Figure 9. Similar with K*II versus crack growth step curve, an ascending tendency is revealed in the ratio of mode II range to mode I range especially for specimen A7 and A8. The transition mode started at step 13 for specimen A7 and at step 15 for specimen A8, the ratio of stress intensity range corresponding to these two steps are larger than 0.4, which could be considered as an assumed threshold value. Although the ratios from step 8 to 11 are also larger than 0.4 in specimen A7, the K*II values are too small to induce transition mode crack growth. In specimen A7 and A8 shear mode was subsequent from step 18,
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