13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- its marked texture. This can be explained by considering that the angle between the propagation direction of the shear mode crack and the load axis, 35°, is related to the angle between the composed plane (100) of fracture surface with the plane (111) of texture. On the other hand, the effect of humidity on crack propagation behavior may be explained as follows. A crack whose growth was accelerated in high humidity propagated in a ductile manner, which means that fatigue deformation was assisted by hydrogen because the hydrogen enhanced localized plasticity (HELP) [9] is assumed as the propagation mechanism of a fatigue crack in high humidity. Moreover, the reason for the indistinct effect of humidity on crack propagation behavior in the region II may be related to the high crack growth rate under ultrasonic loading or in other words, atmosphere is difficult to reach at crack tips so that the environment at the crack tips is similar to vacuum. As a result, crack propagation mode changed and the effect of humidity disappeared. In order to confirm this assumption, fatigue tests under conventional loading frequency (50Hz) were carried out in RH25% and in nitrogen gas as well to simulate vacuum condition. Figure 7 shows fracture surfaces in the region II in both RH25% and N2 gas after rotating bending fatigue. In both alloys, fracture surfaces of specimens subjected to rotating bending in N2 gas are similar to those under ultrasonic loading, though definite striations characteristic of general propagation mechanism are observed in RH25%. 4. Conclusions Fatigue tests under ultrasonic loading frequency (20kHz) were carried out for two kinds of age-hardened Al alloys, extruded and drawn alloys of 2017-T4 (Al-Cu-Mg alloy) with nearly the same static strengths in relative humidity of 25% and 85%, to investigate the effects of microstructure and humidity on propagation behavior of a fatigue crack under ultrasonic loading. The extruded Al alloy has a marked texture with plane (111) in the cross section, but the drawn alloy has not any specified orientation. In the both alloys, the propagation of small cracks shorter than a specific length, e.g. about 1 mm, which is stress level dependent, was accelerated in high humidity, causing large decrease in fatigue strength, though there was no or little influence of humidity on the propagation of a fatigue crack beyond that length. In the extruded Al alloy, fatigue cracks propagated in tensile mode macroscopically, and then changed to grow in shear mode in low humidity, in comparison with that all of fatigue crack propagation process was occupied by the shear mode crack growth in high humidity. On the other hand, in the drawn alloy, a fatigue crack propagated in tensile mode to final fracture in low humidity and in a combined mode of tensile and shear in high humidity. Fracture surfaces caused by the propagation of a shear mode crack were covered with many slip planes. The difference in fatigue crack propagation behavior between the two alloys was mainly caused by the texture. The effect of humidity on crack propagation was yielded through the acceleration to the propagation of a ductile crack. References [1] QY. Wang, C. Bathias, N. Kawagoishi, Q. Chen , Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength, International Journal of Fatigue, 24 (2002) 1269–1274. [2] K. Komai, K. Yamaji, K. Endo, Effect of atmosphere on fatigue crack propagation of aluminum alloy,
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