13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- high pressure hydrogen environment (PH2:24.5MPa,733K) [7].The low cycle fatigue life was50% lower than that in nitrogen environment. The low cycle fatigue life in hydrogen environment was dependent on loading frequency and the hydrogen effect on crack initiation was observed. In order to ensure the safety of pipe and vessel used in high pressure hydrogen gaseous environment Nakamura et al. conducted cyclic pressurization fatigue tests for plane and pre-cracked austenitic stainless steels such as SUS304 steel, SUS316 steel, 32% cold worked SUS316 steel, precipitation hardening A286 steel and SCM435 low alloy steel with different tensile strength in hydrogen environment (PH2:35MPa) [8]. The stable austenitic stainless steel SUS316 showed no reduction of fatigue life in high pressure hydrogen gaseous environment. In contrast significant reduction of fatigue life was observed on metastable SUS304 steel, precipitation hardening A-286 steel and SCM 435 steels. They concluded that hydrogen condensation at austenitic phase for metastable 304 steel and hydrogen trapping at cementite for A286 steel is the cause of fatigue life reduction in high pressure hydrogen embrittlement. Fatigue crack propagation process in structural materials is significantly affected by hydrogen. It has been reported that fatigue crack propagation rate was significantly accelerated by hydrogen for various kinds of structural materials such as Nickel base alloy, Inconel 718 and Waspaloy for space shuttle engine[9], 21/4Cr-1Mo steel[10],mild steel,HT790 steel[11],low alloy steel, super dual phase stainless steel,super ferritic stainless steel[12,13] and BS4360 steel[14]. R.P.Wei proposed hydrogen embrittlement mechanism at local stressed area ahead of crack tip [15]. Hydrogen by transportation process diffused into the local processed area (fracture zone) where embrittlement reaction occurs. Quantitative relation of plastic zone size during fatigue crack propagation and hydrogen embrittlement fracture zone size is recommended to evaluate. The potential hydrogen trap site might be martensite laths, carbides, sulfides or oxy sulfides[16].Certain gaseous activities to a hydrogen atmosphere can stop a running crack in 4340 steel[17].The hydrogen-metal interactions was prevented by the addition of gaseous specie such as O2,CO2 and N2O[18]. In the results of accelerated fatigue crack propagation rate by hydrogen fracture surfaces such as intergranular, cleavage and quasi cleavage are predominantly observed [18,19 and 20]. Hydrogen effect on fatigue crack initiation in structural materials seems not to be prominent as compared with that on fatigue crack propagation. Nagumo explained that hydrogen embrittlement fracture surface is characterized by hydrogen assisted nano scale failure such as vacancy defect and dislocation interaction in ductile fracture process [21]. Hydrogen embrittlement is deeply related to plastic slip [22, 23]. The microscopic mechanism related to dislocation motion is recommended to investigate in order to understand hydrogen effect on fatigue crack initiation of structural materials more in detail. 3. Hydrogen related corrosion fatigue crack initiation behavior of structural materials In order to determine the design stress of machinery components in oil and energy related industries it is indispensable to consider the effect of gaseous environment such as H2, H2S, SO2 and CO2, and sour crude oil on fatigue strength of structural materials. However, these environments are aggressive and injurious to human health. Therefor corrosion fatigue data under such aggressive environments are very few to find because of difficulty to conduct corrosion fatigue testing. 3.1 Effect of aggressive gas in steam and humid air
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