13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Figure 2 also shows the fretting fatigue strength of SUS316L. A failure of the hydrogen-charged specimen occurred at the fretting fatigue limit of the uncharged material. There was a trend that the finite life of the hydrogen-charged material was shorter than that of the uncharged material. Although the amount of the reduction in the fretting fatigue was small compared to SUS304, hydrogen reduced the fretting fatigue strength of the SUS316L. Since this material is recognized as a hydrogen compatible material [4, 5], the result which shows the reduction in fatigue strength is important for the design of hydrogen equipment. The hydrogen embrittlement during the fatigue of SUS316L was also reported by Murakami et al [6]. 104 105 106 107 108 0 50 100 150 200 250 300 350 Number of cycles to failure, Nf Stress amplitude, σa (MPa) Solution heat-treated SUS304 In air Uncharge H charge In H2 Uncharge H charge 104 105 106 107 108 0 50 100 150 200 250 300 350 In air Uncharge H charge Solution heat-treated SUS316L Stress amplitude, σa (MPa) Number of cycles to failure, Nf (a) Solution heat-treated SUS304 (b) Solution heat-treated SUS316L Figure 2. Effect of hydrogen on fretting fatigue strength 3.2. Mechanism that causes the reduction in fretting fatigue strength due to hydrogen 3.2.1. Local adhesion between contacting surfaces Figure 3 shows the characteristic phenomenon that occurs during fretting fatigue in hydrogen. The specimen and contact pad adhered to each other during the fretting fatigue in hydrogen. During the fretting in air, oxidized fretting wear particles separate the specimen and pad. In air, the fretting damage is produced by an oxidation dominant process. On the other hand, during the fretting in non-oxidative environments, such as a vacuum [7] or nitrogen [8], the fretting damage mechanism changes to an adhesion dominant process. In hydrogen, a similar mechanism may occur. Figure 3 also shows the tangential force coefficient in each environment. An increase in the tangential force in hydrogen is clearly shown. The adhesion between the contacting surfaces is the cause of the increased tangential force in hydrogen. The stress conditions on the fretted surface are determined by the fatigue stress, contact stress and tangential stress due to friction [9]. The tangential force is a dominant factor of the fretting fatigue strength [10]. The increase in the tangential force in hydrogen causes an increase in the mechanical stresses on the contact surface. Consequently, the increase in the tangential force due to adhesion is one of the possible reasons for the reduced fretting fatigue strength in hydrogen. Figure 4 shows the section along the specimen axis of the adhered specimen and contact pad during the fretting fatigue test in hydrogen. There were many small cracks at the interface between the specimen and pad. The small cracks propagated in two directions at which the small cracks made angles of approximately 45 or 135 degrees to the contact surface. During the fretting fatigue in air, small oblique cracks and multiple small cracks are the typical characteristics [11]. However, the angle of the oblique small cracks is constant [12]. Furthermore, the small cracks observed in this experiment propagated into both the specimen and the contact pad. Fretting fatigue cracks are Tension and compression Loading frequency f = 20Hz Contact pressure pc = 100MPa Surface finish: buffing Tension and compression Loading frequency f = 20Hz Contact pressure pc = 100MPa Surface finish: buffing Hydrogen content at surface Uncharged: 2.8mass ppm Hydrogen charged: 35ppm Hydrogen content at surface Uncharged: 1.2mass ppm Hydrogen charged: 62ppm
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