1 1 max,H expA C B H ef ; 2 2 ef,H expA X C B X H ef (13) B 0,0442 5 10 9 10 B 0,1182 2 4 2 1 5 1 A A So the Notch Stress Intensity factor under hydrogen can be expressed as a function of K, air , , , H air H K K f C (14) With 2 1 2 1 2 1 2 1 exp B B A A C f C H H The prameter a is introduced in order to get air H K K , , when 0 HC Figure 7 : Evolution of ratios ef max,H and ef ef H X X , with average hydrogen concentration. 4. CONCLUSION It is important to note that hydrogen concentration provides a fracture resistance transition .similarly to temperature, loading rate and notch acuity in pipe steels X52 and X. This phenomenon is describes for the first time (according to our knowledge) in this paper. Some critical hydrogen concentration HC exists for tested steels, which causes the significant loss of local fracture resistance of material at notches. The tendency of monotonic decreasing of HC values with increasing of yield stress Y or ultimate stress U of steels has been derived. Critical hydrogen concentration HC may be considered as an important engineering parameter for forecasting of strength and reliability of exploited pipelines and under designing of new pipelines for hydrogen transportation as well.
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