13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- 104 105 106 107 108 0 50 100 150 200 250 300 350 Number of cycles to failure, Nf Stress amplitude, σa (MPa) 40% pre-strained SUS304 In air Uncharge H charge In H2 Uncharge H charge Figure 11. Effects of work-hardening and hydrogen Figure 12. Effects of surface roughness and hydrogen on fretting fatigue strength of SUS304. on fretting fatigue strength of SUS316. different contact surface roughnesses. The smoother surface specimen showed a considerably lower fretting fatigue strength than the rougher surface one (○ and □). The effect of hydrogen was significant in the rougher surface specimen (□ and ■), but not so in the smoother surface specimen (○ and ●). These results can be interpreted by the crack nucleation mechanism. Figure 13 shows the major crack in the smoother surface specimen used for the fretting fatigue test in air. The major crack started to grow from the locally adhered spot similar to that observed in hydrogen. That is, when the contact surface has an extremely low surface roughness, adhesion occurred even in air and the adhesion played a dominant role in causing the fretting fatigue failure. (a) Adhered specimen and pad (b) Major crack emanated from adhered spot Figure 13. Fretting fatigue test of smooth contact surface specimen in air 6. Effect of hydrogen content Figure 14 shows the fretting fatigue strength of the SUS316 for specimens having different hydrogen contents. The hydrogen content in the material was increased by the thermal hydrogen charging at 573K for 200h. The fretting fatigue strength of the SUS316 decreased with an increase in the hydrogen content. 7. Effect of hydrogen-containing oxygen as minor impurity The purity of hydrogen used in hydrogen equipment is one of the most important concerns in terms of the performance of hydrogen equipment and the cost of hydrogen. Figure 15 shows the effect of oxygen addition on the fretting fatigue strength of the SUS304 in hydrogen. The oxygen content was 100vol ppm. The fretting fatigue strength in the oxygen-hydrogen mixture was between that in the pure hydrogen and in air.
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