13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- deformation between the specimen and contact pad. The shape of the contact pad is the so-called bridge type in which there are two contact parts. As shown in Fig. 1, a strain gage was cemented at the midpoint of the contact parts to measure the friction force. The nominal contact pressure was 100MPa. The tangential force coefficient, φ, was defined as the ratio of the friction force and the contact force. The fretting fatigue test was carried out with a stress ratio, R, of -1 at a loading frequency, f, of 20Hz. The test environment was hydrogen and laboratory air. The hydrogen pressure was 0.2MPa. The test temperature was room temperature. The fretting fatigue test in this study was terminated at 107 cycles if no specimen failure occurred. Contact load Strain gage (for nominal stress measurement) Strain gage (for tangential force measurement) Fatigue specimen Fatigue load Contact pads Contact surfaces Bar springs Figure 1. Fretting fatigue test method 2.2. Material The test materials were three kinds of austenitic stainless steels, JIS SUS304, SUS316 and SUS316L. The chemical compositions of the materials are listed in Table 1. A solution heat treatment was done to the materials by quenching following heating at 1303K for 3.9ks. Since fretting is a surface phenomenon, hydrogen diffusion into the material is one of the important issues. Therefore, hydrogen pre-charged materials were used in a part of the fretting fatigue test. The method of hydrogen pre-charging was thermal hydrogen charging. Table 1. Chemical composition Material C Si Mn P S Ni Cr Mo Fe SUS 304 0.06 0.51 0.92 0.033 0.004 8.08 18.8 - Bal. SUS 316 0.05 0.49 1.31 0.030 0.027 10.22 17.0 2.04 Bal. SUS 316L 0.012 0.19 1.64 0.031 0.012 12.19 16.6 2.22 Bal. 3. Effect of hydrogen on fretting fatigue strength 3.1. S-N curves Figure 2 shows the fretting fatigue S-N diagrams. For the SUS304, the fretting fatigue strength of the uncharged material was significantly lower in the hydrogen than in air (□ and ○). The effect of hydrogen pre-charging is also shown in Fig. 2. The details of the hydrogen pre-charging are found in ref. [1]. In air, the fretting fatigue strength was significantly reduced by the hydrogen pre-charging (● and ○). The fretting fatigue strength of SUS304 is reduced by not only gaseous hydrogen, but also internal hydrogen. When the fretting fatigue test of the hydrogen-charged material was done in hydrogen, the reduction in the fretting fatigue strength was very significant (■). The gaseous hydrogen and internal hydrogen synergistically works in decreasing the fretting fatigue strength. The mechanisms will be discussed in the latter part of this paper. Bridge pad
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