13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- These values were comparable, or somewhat higher than, the values in air. Figure 2. A typical plot of potential drop (mV) vs. engineering stress (MPa) for NiCrMo1 tested at 0.02 MPa s-1 in acidified 0.1 M Na 2SO4, pH 2 solution at three potentials. Table 2 Yield stress in air (as an engineering stress), σy, the apparent threshold stress at which subcritical cracking appeared to initiate in solution (as an engineering stress), σth, the fracture stress (as an engineering stress), σf, the reduction of area, RA. Solution: ①0.1 M Na2SO4, pH=2. Applied stress rate (MPa/s) Sample Solution Applied potential (mVAg/AgCl) σth±5 σf±2 RA 0.02 A.02a air 0 664 763 74% 0.02 A.02b air 0 603 693 76% 0.02 A.02c air 0 631 726 75% 0.02 A.02d air 0 620 717 72% 0.002 A.002 air 0 646 740 75% 0.02 Socp.02a ① Ecorr 683 798 77% 0.02 Socp.02b ① Ecorr 625 714 79% 0.02 Socp.02c ① Ecorr 693 796 69% 0.02 S950.02 ① -950 619 719 73% 0.02 S1200.02 ① -1200 624 715 73% 0.02 S1400.02 ① -1400 664 759 73% 0.02 S1550.02 ① -1550 633 720 70% 3.2. Surface appearance in the necked region Visual examination of the surface appearance of each specimen showed that there was obvious necking for all the specimens. Black corrosion products, which were easily removed by EDTA, were produced on the specimens during tests at Ecorr. For specimens tested at negative potentials, there were cracks on the specimen surface concentrated at the fracture site. For specimens tested in air at a rate of 0.02 MPa s-1, SEM observation showed necking and shallow cracks at about 45 ° to the stress direction, and linear features parallel to the stress direction in the necked region. Specimens tested at Ecorr showed similar surface appearance except that the surface was more rough due to corrosion. Fig. 3 presents the surface appearances of specimens tested under hydrogen charging at 0.02 MPa
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