13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- humidity (RH) of 25% and 85%. In addition, rotating bending fatigue tests (50Hz) were carried out in nitrogen gas (N2>99.995%, O2<5ppm, H2O<10ppm) and RH25% to investigate the effects of oxygen and humidity. In the ultrasonic fatigue, pulse-pause tests were employed with a pulse length of 1s and a pause length of 5s to reduce temperature rise under high frequency cycling. By this method, temperature rise during ultrasonic fatigue was controlled below 3K. The humidity conditions were selected by considering the daily humidity and the significant influence of humidity on fatigue strength [4]. The deviation of humidity was controlled in the range of RH±5% with the temperature at 298±3K in each humidity condition. 3 Experimental Results and Discussion Figure 3 shows S-N curves of both alloys in RH25% and RH85%. In Fig. 3, results under rotating bending fatigue tests were also displayed by lines only [5]. Fatigue strength is larger in ultrasonic loading than in rotating bending in both alloys. Moreover, fatigue strengths of both alloys in both fatigue tests were largely decreased in high humidity with the drop in fatigue strength is larger under ultrasonic loading than under rotating bending, though the time consumed at given fatigue life was much shorter in ultrasonic fatigue. The effect of humidity on fatigue strength was larger in the extruded alloy than in the drawn one. For example, fatigue strength at 107 cycles of the extruded alloy in RH85% was decreased to about 30% of that in RH25%. Figure 4 shows crack propagation curves of both alloys. In both alloys, a crack initiated at the early stage of stress repetitions, so most of fatigue life was occupied by the growth life of a crack in both humidity. Moreover, the propagation of cracks shorter than a specified length ℓ0, e.g. about 1 mm, was accelerated in high humidity, and there was no or little influence of humidity on the propagation of a fatigue crack over ℓ0. The existence of crack length ℓ0 is clearly confirmed in RH25%, though the value of ℓ0 changed with stress level. Optical feature of surface cracks and SEM morphology of fracture at different stage of crack growth are shown in Fig. 5, in which the results of Fig. 4 are rearranged into two groups so as to better understand whether or not and how the humidity affects crack growth behavior in the two alloys. It is found that the crack growth in each alloy can be classified into two regions, i.e. region I and II. In the region I (ℓ <ℓ0), crack growth is accelerated by high humidity, while in the region II (ℓ>ℓ0), no distinct effect of humidity is recognized. In the extruded Al alloy, cracks propagated in tensile mode Figure 2. Shape and dimensions of specimens (mm) (a) Ultrasonic fatigue (b) Rotating bending fatigue
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