13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3. Results and discussion 3.1 Effect of the relative humidity on fatigue life of Al 7075-T6 alloy Figure 3 illustrates the S-N curves under the different relative humidity scopes of Al 7075-T6 alloy. The high-cycle fatigue (HCF) behavior of Al 7075-T6 alloy indicated that the effect of the relative humidity on the fatigue life can not be ignored. Especially when the relative humidity is over than 75% and the fatigue cycles is over than 2×106, the effect of the relative humidity on fatigue life of aluminum alloy is very obvious. At the same time, there is a difference about the fatigue behavior in the two relative humidity states of both 75% and 100% (in water), in which with the increasing of relative humidity, the fatigue life of Al 7075-T6 alloy observably decreases no matter what is the relative humidity of over than 75% and the low- or high- cycle fatigue life of aluminum alloy. Of course, the difference of fatigue life is much obvious when the cycles are over than 106. However, the decreasing trend of fatigue life was restrained at lower relative humidity scopes from 75% to 50% and low-cycle or N<106. The effect of the relative humidity on the fatigue life of aluminum alloy can be even ignored. It has previously been reported that the influence mechanism was expressed by the different fatigue fracture models [5,10]. One of main reasons is because the shear stress action occupies more than part in fatigue fracture process although the applied stress is only a compressive and tensile stress at the surface damage point of specimen. The shear stress at the fatigue fracture surface might be caused by the coupling action both stress state and hydrogen or oxygen on the fatigue crack initiation and growth mechanism of aluminum alloys. Therefore, the fatigue crack growth driven force of Al 7075-T6 alloy comes from the interaction both shear and normal stresses. With the corrosion effect of hydrogen or oxygen increasing, the shear stress occupied the proportion to increase. However, the composite damage behavior of humidity effect needs to be carefully discussed in further. Figure 3 S-N curves under the different relative humidity scopes of Al 7075-T6 alloy 3.2 Effect of different micro-arc oxidation processes on fatigue life of Al 2024-T4 alloy Considering the countermeasures against the effect of relative humidity on the fatigue life of aluminum alloy, the oxidation ceramics coating layer were carried out on the surface of Al 2024-T4 alloy specimens by using the micro-arc oxidation, micro-arc oxidation plus the sealing hole and hard oxidation technologies, respectively. All coating layers have an approximate same thickness of about 0.15 mm. in addition, to compare with the differently typical coating layer how to influence on the fatigue life of Al 2024-T4 alloy, the fatigue data without coating layer has been also plotted
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