13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 4.2. Observations of CCG Path To investigate the three-dimensional CCG behavior for C(T) specimen of P92 steel, the CCG path at the center of the thickness direction (hereinafter referred to as “center”) and near the side-groove (hereinafter referred to as “side”) were observed using laser microscope. The observational results of the center and the side are shown in Fig. 7 and Fig. 8, respectively. From Fig. 7, the CCG behavior showed the periodic convexo-concave manner at the center. This crack path is called “fracture unit area (FA) cracking” [19-21]. On the other hand, the creep crack grew in a linear manner at the side as shown in Fig. 8. Creep crack length of each interrupted times were measured. At 52.5h (t/tf = 0.17), the creep crack did not initiate both at the center and at the side. After the middle of total life (t/tf > 0.50), the creep crack has already initiated both at the center and at the side. And the creep crack length of the side was longer than that of the center. This tendency became marked at 226h (t/tf = 0.75). This result indicates that the creep crack was preferential growth at the side from 50% to 75% of total life and accelerated at the center after 75% of total life. Therefore, it seems that the creep crack of C(T) specimen with side-grooves grew as shown in Fig. 9. The characteristic of CCG without the steady-state CCG rate region is thought to be due to the preferential CCG at the side. 52.5h 150h 226h 301h notch crack notch crack notch notch crack xy-plane (center of the thickness) observational area x y z crack length 0.433 mm crack length 0.575 mm crack length 2.719mm crack length 0 mm (t/tf=0.17) (t/tf=0.50) (t/tf=0.75) (t/tf=0.99) notch notch crack crack notch crack xy-plane (near the S.G.) observational area x y z 52.5h 150h 226h 301h crack length 0.736 mm crack length 3.700 mm crack length 3.014 mm crack length 0 mm (t/tf=0.17) (t/tf=0.50) (t/tf=0.75) (t/tf=0.99) Figure 7. CCG path of center Figure 8. CCG path of side crack tip x z Figure 9. Schematic illustrations of CCG process on the xz-plane
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