13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- in the S-N curves as shown in Figure 4. These experimental results indicated that the fatigue life (mark Δ) of Al 2024-T4 alloy is seriously injured with the hard oxidation ceramics coating layer, but the effect of micro-arc oxidation plus sealing hole method on the fatigue life of Al 2024-T4 alloy is positive. And the fatigue life (mark {) of Al 2024-T4 alloy with the micro-arc oxidation ceramics coating layer is approximate same fatigue life (mark ) of Al 2024-T4 alloy. By means of the fatigue data with and without oxidation ceramics coating layer of Al 2024-T4 alloy compared as shown in Figure 4, we clearly know that the coating layer is able to avoid the effect of humidity or corrosion on the fatigue strength of aluminum alloy but the different coating layer technologies cause the fatigue damage mechanism change of aluminum alloy. Therefore, we should pay close attention to the issue in future to optimize and improve the coating layer technology. Figure 4 S-N curves under the different micro-arc oxidation processes of Al 2024-T4 alloy (a) (b) (c) Figure 5 Fatigue fracture surfaces for differently typical micro-arc oxidation processes of Al 2024-T4 alloy (a) micro-arc oxidation ceramics coating with sealing hole, (b) micro-arc oxidation ceramics coating, (c) hard oxidation ceramics coating. Figure 5 illustrates the fracture appearances for the different oxidation ceramics coating layer technologies including (a) the micro-arc oxidation ceramics coating layer with sealing hole, (b) the micro-arc oxidation ceramics coating layer and (c) hard oxidation ceramics coating layer. All the applied stress amplitudes are 230 MPa, respectively. These fracture surfaces have much more different flat fracture surface such as the smooth area in the center or concave-convex surface in the marginal zone as shown in Figure 5a and 5b as well as 5c. This is a fact that the different flat fracture surface reflects the different fatigue crack propagation life. That is the fatigue crack propagation life for micro-arc oxidation ceramics coating layer with sealing hole is slower than that for micro-arc oxidation ceramics coating layer. This is because the center area and the fatigue crack propagation area in the marginal zone for the micro-arc oxidation coating ceramics layer with sealing hole are larger and relative smoother than that other two oxidation technologies. It means
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