13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- as compared in Table 1 [12]. This is due to the non-uniform distribution of hydrogen concentration in the specimen from the surface to the center. 3.3 Dependence on the threshold stress intensity on defect size and hydrogen concentration In order to quantitatively characterize the hydrogen effect together with the defect size dependence on the fracture toughness, the fracture data will be analyzed in terms of the macroscopic fracture stress σf, the defect size area and the residual hydrogen content CH, R. 3.3.1. Smooth specimens Firstly, we associate the fracture stress σf with KTH for the onset of a crack from a spherical inclusion. After the interfacial debonding between inclusion and matrix, the spherical inclusion can be regarded as a traction free cavity (cf. Fig. 8). Figure 10 shows the normalized stress intensity factor KI/σ0 a for a crack emanating from an ellipsoidal cavity as a function of the parameter λ/a', Table 1. Experimental data of SAE52100 [12] t1 (min) t2 (min) Fracture origin CH, R (mass ppm) σf (MPa) (μm) KTH (MPa ) H-precharged specimens 27 76 Al, Ca, Mg, Si, S 3.29 795 15.0 2.73 49 111 Al, Ca, Si 3.17 785 19.3 3.06 27 89 Ti, C 2.46 1027 11.6 3.10 21 90 Al, Ca, Mg, Si, S 3.64 769 22.4 3.23 45 108 Al, Ca, Mg, Si, S 3.00 897 18.8 3.45 32 90 Al, Ca 3.79 779 25.0 3.45 21 89 Al, Ca, Mg, Si 3.06 943 16.3 3.37 31 94 Al, Mg 3.26 883 19.3 3.44 47 108 Al, Ca, Mg, Si 3.08 802 23.9 3.47 32 89 Al, Ca, P, Si 3.89 845 22.3 3.54 25 75 Al, Ca, Mg, Si 2.95 773 27.6 3.60 29 97 Al, Ca, Si 3.05 1001 19.8 3.95 34 94 Al, Ca, Mg, Si 2.69 1031 19.0 3.98 20 85 Ti, N, Mn, S 2.32 1041 19.3 4.05 24 105 Semi-circular slit 3.12 714 93.0 7.93 24 94 Semi-circular slit 3.20 567 186 8.90 27 94 Semi-circular slit 3.20 450 463 11.15 21 102 Drilled hole + FIB slits 3.39 657 116 7.28 28 117 Drilled hole + FIB slits 3.95 668 196 10.47 30 112 Electro discharged + FIB slits 3.77 471 513 11.67 20 102 Al, Ca, Mg, Si, S 3.03 960 12.4* 3.00 Rectangular FIB slit 35.0 Non-charged specimen ― ― Al, Ca, Mg, S 0.54 1943 23.6 8.37 ― ― Unidentified 0.54 2436 ― ―
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