cylindrical. Its determination is based on a procedure involving the relative stress gradient to determine an inflexion point on the notch tip opening stress distribution [8]. If Xef is smaller than Xef, H, the corresponding value after hydrogen embrittlement, the effective stress in the fracture process volume is greater and then the work done for fracture is higher. There is the tendency of monotonic decreasing of HC values with increasing of yield stress Y or ultimate stress U steel (Fig. 10). This dependency can be described by power function, like to: 1n H 1 Y C B or 2n H 2 U C B , (6) where 1 2 1 2 B ,B ,n ,n are the some constants of material’s properties and testing conditions. Here, the standard deviation 2R = 0.98-0.99 that is fully acceptable. As first approximation, it can be concluded that HC ~ 2 U 1 . (7) 3.2 fatigue initiation The fatigue resistance to initiation of the API 5L X52 and X70 steels has been measured in radial direction at room temperature using non-standard curved notched specimens, namely, “Roman tile” specimens because the pipe dimensions do not permit to measure through thickness mechanical characteristics. The applied load, frequency and the fatigue cycle (sinusoidal) were monitored on the control panel. Hydrogen charging was made using the same cell filled with NS4 solution. Tests conditions are given in Table 6. Table 6 : Fatigue test conditions Shape of the cycle used Sinus: Frequency : 0.05 Hz Load ratio 0.5 Working potential - 1 Vsce Electrolytic solution New Solution 4 (NS4) Solution pH between 6.66 and 6.74 Wöhler curves were drawn at both initiation and failure. Results are presented in a bi logarithmic graph of stress amplitude versus number of cycles to failure. The classical power fit of the stress range versus the number of cycles to failure is in accordance with Basquin’s law: (8) where ' f is the fatigue resistance and b the Basquin’s exponent. Results are presented in Table 7. Table 7: Fatigue resistance parameters with and without hydrogen charging for X 52and X70 steels Steel, environment ’f fatigue resistance (MPa) exponent R2 X70 air 436 -0.023 0.94 X70 Hydrogen 395 -0.015 0.94 X52 air 343 -0.022 0.89 X52 Hydrogen 301 -0.012 0.95 NR b f '
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