Issue 48

A. Takahashi et alii, Frattura ed Integrità Strutturale, 48 (2018) 473-480; DOI: 10.3221/IGF-ESIS.48.45 479 The categorization of the fatigue crack growth behavior and the discussion on the application of the alignment rule, in this study, is based on the visual inspection. In order to evaluate and discuss the alignment rule, the stress intensity factor of two non-coplanar embedded cracks is examined. As shown in Fig. 8, the stress intensity factor of two circular cracks is calculated. In the calculation, the horizontal separation of crack tips is zero. In order to investigate the interaction of the cracks, the stress intensity factor of a single crack with the same shape is also calculated. Fig. 8 shows an example of the stress intensity factor distribution along the crack tips. The stress intensity factor near the other crack increases as a result of the interaction. Comparing the stress intensity factor of two non-coplanar embedded cracks with that of the single crack, the strength of interaction is represented as the ratio of the maximum stress intensity factor increase to the stress intensity factor of the single crack. Fig. 9 shows the interaction map with the ratio. As shown in the figure, the boundary between the pattern B and C can be determined as the ratio of the stress intensity factor is 10%. On the other hand, as for the boundary between the pattern C and D, which is the criteria of the application of the alignment rule, the boundary can be determined as the ratio of the stress intensity factor is 4%. Thus, instead of making a decision of the fatigue crack growth pattern based on the visual inspection, the ratio of the stress intensity factor can be used, and should give more quantitative evaluation of the interaction of two non-coplanar embedded cracks. Figure 8 : Stress intensity factor distribution along the crack tip of two non-coplanar embedded cracks. The stress intensity factor increases near the other crack. The position along the crack tip is denoted with the angle theta defined in the figure (left). In order to study the interaction of two embedded cracks, the stress intensity factor of a single embedded crack is also calculated. Figure 9 : Interaction map of two non-coplanar embedded cracks. The value written in the left side of the figure shows the ratio of the maximum stress intensity factor of two non-coplanar embedded cracks to the stress intensity factor of the single crack. The horizontal and vertical axis of the figure is normalized by the initial half-length of the crack.