13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- One of the causes of the increase in the fretting fatigue strength in the oxygen-hydrogen mixture is the reduction of the tangential force between the contacting surfaces due to generation of oxidized fretting wear particles. Another cause is the increase in the crack initiation limit. Based on the adhesion mimic test (section 3.2.2), the critical maximum shear stress range to crack initiation in the hydrogen-oxygen mixture was between that in hydrogen and air [20]. 104 105 106 107 108 100 150 200 250 300 SUS316 Solution heat-treated f =15Hz R =-1 pc=100MPa Ra = 0.008 μm Number of cycles to failure , Nf Stress amplitude , σa (MPa) In air CH (mass ppm) 1.2 22 30 105 106 107 100 150 200 250 Stress amplitude, σa (MPa) Number of cycles to failure, Nf In Air In 0.2MPa H2 In 0.2MPa oxygen-hydrogen mixture (100 vol ppm O2) SUS304 R = -1, f = 20 Hz Figure 14. Effect of hydrogen content on fretting Figure 15. Effect of addition of oxygen on fretting fatigue strength of SUS316 fatigue strength of SUS304 in hydrogen 8. Conclusions Fretting fatigue is one of the major concerns in various engineering fields such as railway, energy, aviation, automobile, etc., because fretting fatigue strength is significantly lower than the fatigue strength of a smooth specimen. This study showed the additional reduction in fretting fatigue strength due to hydrogen. The major results can be summarized as: • S-N curves: the fretting fatigue strength of SUS304 and SUS316L is reduced by gaseous hydrogen. When the fretting fatigue test of the hydrogen-charged material, such as SUS304 was done in hydrogen, the reduction in the fretting fatigue strength was more affected due to the synergistic effect of gaseous hydrogen and internal hydrogen; • Mechanisms that cause the reduction in fretting fatigue strength in gaseous hydrogen: The first mechanism is explained by the action of hydrogen as the cause for local adhesion between contacting surfaces in fretting-fatigue samples of both SUS304 and SUS316L. The second one is the microstructure change to martensite in the local adhesion part. Both mechanisms will act together in lowering the fretting fatigue strength of these stainless steels in contact with gaseous hydrogen; • Reduction in fatigue threshold by absorbed hydrogen: The reduction in ΔKth is one of the causes of the reduced fretting fatigue strength by a hydrogen; • Effect of surface roughness: The effect of hydrogen was significant in the rougher surface specimen, but not so in the smoother surface specimen. Rough surfaces present more stress concentrators to attract hydrogen; • Effect of hydrogen content: The fretting fatigue strength of the SUS316 decreased with an increase in the hydrogen content; • Effect of hydrogen-containing oxygen as minor impurity: The oxygen content of 100vol ppm in gaseous hydrogen had a beneficial effect by improving fretting fatigue strength of SUS304. Even at this low impurity level, oxygen causes the reduction of the tangential force between the contacting surfaces due to generation of oxidized fretting wear particles, and the increase in the crack initiation limit. Despite all these findings, additional work is needed to better explore the mitigating effects of oxidizing impurities in hydrogen environments.
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