13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- continuous and winding. These are microscopic evidence of adhesion. At position D where a straight line was observed, there was no fretting fatigue cracks. Therefore, the formation of the small cracks is related to the local adhesion. There was a major crack leading to specimen failure at position C. The major crack started to grow from one of the small cracks that emanated from the adhered spot. Therefore, the adhesion and the subsequent formation of many small cracks are one of the root causes of the reduced fretting fatigue strength in hydrogen. Based on this observation, a model to describe the fretting fatigue failure in hydrogen is shown in Fig. 5. Pad Specimen Fretting Local adhesion Fretting Small cracks Fretting Major crack (a) Before test (b) Local adhesion (c) Crack initiation (d) Specimen failure Figure 5. Model to cause fretting fatigue failure in hydrogen 3.2.2. Effect of hydrogen on fretting fatigue crack initiation Figure 6 shows the test method used to investigate the effect of hydrogen on the initiation of a fretting fatigue crack. The specimen and contact pads were welded using a spot welding machine to mimic adhesion during fretting fatigue in hydrogen. A compressive load, which corresponds to the contact load in the fretting fatigue test, was applied. The fatigue test was interrupted at 105 cycles and identification of a small crack was then carried out. The details of the experiment are described in ref. [13]. Figure 7 shows an example of the fatigue crack in the adhesion mimic test. About 100 μm-deep cracks were found. The morphology of the cracks is similar to that observed in the fretting fatigue test in hydrogen as shown in the figure. 6 6 1 0.8 0.3 Contact surface 32 Specimen Pad Pad Welding Compressive load Fatigue load (a) Contact pad (b) Welding of specimen and pad (c) fatigue test method Figure 6. Adhesion mimic fatigue test (dimensions are in mm) Specimen Photo Pad Pad Fretting Fretting fatigue in hydrogen Small cracks Pad Specimen Adhesion mimic test Fatigue Pad Specimen (a) Small crack in adhesion mimic test (b) Morphology of small crack Figure 7. Observation of small cracks
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