13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 5. Determination of crack opening stress (magnet, a = 25.93 mm) 3.2. The result of crack opening stress measurement The effective stress range, eff, which is calculated using the maximum stress, max, has one to one correlation to the crack growth rate. Here, eff is calculated by the following equation 1. op max eff (1) Elber[8] shows in full detail that The rate of crack propagation can be evaluated uniformly by using the effective stress range. So the crack growth rate can be predicted by crack opening stress. To use the effective stress range, we have to accurately measure the crack opening stress of FSW panel. There are some studies calculating crack opening stress of base metal by using empirical equations. Some of them, we have used empirical equations for 2024-T3 proposed by schijive[9] as eq. 2 and eq. 3. 2 055 03 01 . . R . R U , (2) 2 055 03 01 . . R . R max op . (3) Figure 6 shows the result of crack opening stress measurement in each crack length. In addition, the calculated values obtained by the Schijive’s equation and the experimental result of base metal ( = 75 MPa) are shown together. In case of =75 MPa, the unti-buckling pate which suppress the out of plane deformation was used and measured with bolt jig. However, in case of = 25 MPa, the unti-buckling plate was not used and measured with magnet jig. Therefore, the width of the variability of the measurement results is reduced by about 58%. Moreover, the result of measurement is same as the calculated values obtained by Schijive equation. For that reason, the validity of this measurement was confirmed and the test results indicate that the accuracy of crack opening stress measurement is improved. Next, focus on the result of FSW panel as shown part A in figure 6. After crack tip invasion into weld line, crack opening stress is reduced substantially. Then, crack opening stress is asymptotic to the result of base metal as go away from weld line. From this result, the effect of residual stress is reduced when a position of crack tip is taken enough away from the weld line. 0 5 10 15 20 25 30 -20 0 20 40 60 80 100 stress MPa Relative compliance offset % 20-29 cycle average 30-39 cycle average 40-49 cycle average
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