13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 3. Result and discussion 3.1. Improvement of the jig We need to mount an extensometer on specimens when we measure the crack opening stress. In case of = 50 and 75 MPa, the extensometer was installed on specimen with bolt jig as shown in Figure 3(a) and measure the crack opening displacement. However, if tightening torque of bolts is not appropriate, the extensometer is not fixed on specimen and it is drifted. Therefore, we improved jig to mount the extensometer to the specimen tightly with magnets as shown in Figure 3(b). To verify that the magnet jig can measure the crack opening stress, we measured the crack opening stress of base metal and compare the result of both jigs. In this study, we decided that crack is opening completely when the relative compliance offset become 5 %. This method is based on Chung’s research[7]. In case of using bolt jig, the result of relative compliance offset is unstable and become greater than 5 % when the crack tip is not opening. So judgment of crack opening had been difficult. In case of using magnet jig, the relative compliance offset is stable and become less than 5 % when the crack tip is not opening. Therefore, the accuracy of crack opening stress measurement is improved by the magnet jig. (a) Bolt jig (b) Magnet jig Figure 3. Installing extensometer Figure 4. Determination of crack opening stress (bolt, a = 25.93 mm) Extensometer Bolt Specimen Hole 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 magnet Specimen Extensometer Knife edge
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