13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- t = 1.3 hours (CH, R = 4.0 mass ppm) t = 45 hours (CH, R = 1.8 mass ppm) t = 117 hours (CH, R = 0.8 mass ppm) Temperature (K) t : Total hold time in ambient air CH, R: Residual hydrogen content 4.5 mm 2 mm Heating rate: 0.5 K/s Intensity ( 108 A) 0 100 200 300 400 500 600 700 800 1 2 3 4 Total hold time in ambient air, t (hours) Residual hydrogen content, CH, R (mass ppm) 4.5 mm 2 mm + Non-charged specimen: CH, R = 0.54 (mass ppm) Least square fitting with Demarez et al.'s solution [19] D = 8.6 1013 (m2/s) CH, R = 4.7 (mass ppm) at t = 0 SAE52100 HV = 712 0.1 1 10 100 1000 0 1 2 3 4 5 6 Figure 5. Hydrogen thermal desorption spectra for the hydrogen-charged specimen [12] defect significantly decreased the nominal stress at the final fracture of specimen, σf, which was decreased with an increase in the defect size. Table 1 lists the chemical compositions of all the inclusions at fracture origin together with a series of experimental data. The inclusion size area ranged from about 10 to 30 m. The fracture processes of specimens originated from two types of inclusions, i.e. Al2O3·(CaO)x and TiN, has been reported in a separate paper [6]. Figure 8 illustrates crack initiation processes from the two kinds of nonmetallic inclusions in the non-charged and H-precharged specimens [6]. In an early stage of the fracture process from the Al2O3·(CaO)x type inclusion, an interfacial cracking occurs at the pole of the inclusion/matrix interface where the tensile stress becomes maximum owing to a large modulus of the inclusion. Subsequently, the complete separation of the interface between the inclusion and matrix creates a spherical-like cavity, which shifts the location of the highest tensile stress from the pole to the equator of the cavity. On the other hand, the fracture process from the TiN type inclusion is initiated by the cracking of inclusion itself. These processes for two types of inclusions take place while the overall deformation of the specimen is elastic as shown in [6]. Figure 9 [12] shows the statistics of extremes distribution of nonmetallic inclusions at the fracture origin of the H-precharged tensile specimens, where y' is the reduced variant, F is the cumulative distribution fraction and T ' is the return period. The distribution of the extreme value max area shows a good linearity and provides a justification for the use of the distribution of extremes. In the H-precharged specimens having artificial defect, the fracture was originated from the defect. It is worth noting that there was an exception, i.e. the fracture of the specimen having the smallest artificial defect (cf. Fig. 3(a)) was not originated from the artificial defect but from a nonmetallic inclusion, though the size of the inclusion was definitely smaller than that of the artificial defect, Figure 6. Residual hydrogen content as a function of total hold time in ambient air [12] 0 500 1000 1500 2000 2500 0 1 2 3 Nominal strain (%) Nominal stress (MPa) H-precharged, Type B defect, = 93.0 m, CH, R = 3.12 ppm H-precharged, Type B defect, = 186 m, CH, R = 3.20 ppm H-precharged, Type B defect, = 463 m, CH, R = 3.20 ppm H-precharged, Non-metallic inclusion, = 19.3 m, CH, R = 3.26 ppm Non-charged, Non-metallic inclusion, = 23.6 m, CH, R = 0.54 ppm Non-charged, Unidentified, CH, R = 0.54 ppm Figure 7. Example of the stress-strain curve and fracture points
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