13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 0 20 40 60 80 100 0 200 400 600 800 Fatigue load , F (N) Maximum range of shear stress , Δτmax (MPa) Crack initiated No crack 25 50 100 Contact pressure pc (MPa) 0 20 40 60 80 100 0 200 400 600 800 Fatigue load , F (N) Maximum range of shear stress , Δτmax (MPa) Crack initiated No crack 25 50 100 Contact pressure pc (MPa) (a) In air (b) In hydrogen Figure 8. Effect of hydrogen on crack initiation under adhesion condition The result of the local adhesion mimic fatigue test is shown in Fig. 8. The maximum range of shear stress was obtained by an elastic-plastic finite element (FE) analysis [13]. The crack initiation occurred in a significantly lower maximum shear stress range in hydrogen than in air. It was confirmed that hydrogen assisted the crack initiation in this experiment. This is one of the mechanisms other than stress concentration due to local adhesion that causes a reduction in the fretting fatigue strength in hydrogen. The maximum shear stress range was greater than the proof strength of the material. Since the regions with a higher strain attract more hydrogen [14] and mobile dislocations transport hydrogen [15], it is presumed that the local adhesion activates the hydrogen embrittlement. As further evidence, the authors confirmed the facilitating of crack initiation due to hydrogen in a low-cycle fatigue of austenitic stainless steel [16]. Furthermore, fretting wear removes the oxidized surface which may prevent the diffusion of hydrogen into the material. There is a possibility that such a higher stress causes a microstructure change from austenite to martensite. The microstructure change during the fretting fatigue will be described in the next section. 3.2.3. Microstructure change Figure 9 shows the result of the electron backscatter diffraction (EBSD) observations of the adhered part. Alpha-prime, which is considered to be strain-induced martensite, was detected at the adhered part. Martensite is vulnerable to hydrogen. Besides this, the diffusivity of hydrogen is significantly greater in the martensitic phase than in the austenitic phase [17]. As a result, the transformed martensite in the austenitic stainless steel can act as a low resistance diffusion pathway for hydrogen diffusion [18]. The transformation of the microstructure is one of the important mechanisms that cause a reduction in the fretting fatigue strength in hydrogen. (a) SEM image (b) Phase map obtained by EBSD Figure 9. Microstructure change due to local adhesion between fretting surfaces (SUS316, σa = 182MPa, Nf = 10 6)
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