13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- the anodic dissolution. The same discussion could be made from the fracture surface observations, i.e., the fracture surface of AZ31 under the cathodic potential of -1.4 V at KI of 8.6 MPa m1/2 was covered with corrosion products. However, under the same cathodic potential of -1.4 V, corrosion products were less on the fracture surface at comparatively higher KI of 14 MPa m1/2. This is attributed to the difference of exposure time to the NaCl solution which is the corrosive environment. In the SCC tests conducted in the present study, the load was sustained once after a crack initiated to propagate. So KI corresponds to the crack length. The fracture surface at low KI was the area near the notch root and was exposed to the NaCl solution for longer time until a CT specimen was lead to final fracture than that at high KI. So in the fracture surface at low KI, the effect of the corrosive environment is large and the fracture surface became covered with corrosion products. The reason that almost all of the fracture surfaces of AZ61 and AZ61-T5, which are more insensitive to corrosion than AZ31, were covered with corrosion products is also explained by the exposure time to the corrosive environment. The crack growth rates of AZ61 and AZ61-T5 were slower and it took longer time until the final fractures than AZ31. Although AZ61 is insensitive to corrosion, the effect of longer exposure to the corrosive environment becomes large and so the fracture surfaces of AZ61 were also covered with corrosion products. (a) AZ31, E=-1.4 V, KI=8.6 MPa m1/2 (b) AZ31, E=-1.4 V, KI=14 MPa m1/2 (c) AZ31, E=-2.5 V, KI=19MPa m1/2 (d) AZ31, E=-3.0 V, KI=14MPa m1/2 (e) AZ61, E=-1.4 V, KI=19 MPa m1/2 (f) AZ61, E=-4.0 V, KI=30MPa m1/2 Figure 9. SEM micrographs showing fracture surfaces 50m 50m 50m 50m 50m 50m
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