PSI - Issue 66

Miu Hayashi et al. / Procedia Structural Integrity 66 (2024) 161–166 Author name / Structural Integrity Procedia 00 (2025) 000–000

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382 MPa. Therefore, the anodising treatment had little effect on the tensile strength. The anodised film is composed of amorphous alumina, and cracking can easily occur. Because cracking affects not only the corrosion and wear resistance but also the fatigue strength, the stress value at which cracking occurs in the anodised film was investigated. An-7 was subjected to a tensile stress of 40 MPa from the unloaded and unloaded surfaces, and the surface properties of the anodised films were observed by SEM. This operation was performed once, and the loading and unloading tests were conducted by increasing the loading stress by 40 MPa until cracking occurred. Consequently, numerous cracks were observed in the direction perpendicular to the load axis at σ = 160 MPa. Therefore, the loading stress boundary at which cracking occurred in the anodised film was 160 MPa. This is defined as the critical stress value in this study.

3.2 Fatigue test results Fig. 3 shows the fatigue test results of the Virgin , An-7 , and An-20 . By investigating the effect of anodisation treatment on fatigue strength, it was confirmed that the anodised film had little effect on the high- and low-stress amplitude ranges compared with Virgin and An-7 . On the other hand, the fatigue life of An-7 showed a variation in σ a = 140–180 MPa (as shown in the blue area). This result is consistent with the critical stress value at which cracking occurs in anodised films. Therefore, it can be inferred that the fatigue life of the specimens tested at σ a = 140–180 MPa is on the low fatigue life side if cracking occurs early, while it is comparable to that of the Virgin if no cracking occurs. Next, An-7 and An-20 were compared to investigate the effect of the anodised layer thickness on the fatigue strength. The fatigue strength decreased as the film thickness increased, and this tendency was more significant in the low stress amplitude range. In addition, the variation in fatigue life observed for An-7 was not confirmed. Therefore, An-20 exhibited many cracks on its surface during the anodising treatment, which became a source of stress concentration, resulting in a decrease in the fatigue life. 3.3 Fracture surface observation To elucidate the above factors and fatigue fracture mechanism, fracture surface observations were conducted on Virgin , An-7 , and An-20 . First, Fig. 4(a) shows the results of fracture surface observation of the Virgin ( σ a =120 MPa, N f =5.7×10 6 cycles). The figure shows that the Virgin specimen exhibited a surface-origin fracture, and a similar fracture morphology was confirmed for all Virgin specimens used in this study. Fig. 4(b) and (c) show the results of the fracture surface observations of An-7 . Fig. 4(b) and (c) show the fracture surfaces of An-7 at σ a = 220 MPa and N f = 3.1×10 4 cycles, which are higher than the critical stress value, and at σ a = 140 MPa and N f = 1.3×10 6 cycles, which are lower than the critical stress value. As shown in the white image in (b), multiple step shapes are observed in the side view. Because the step shape is linear, it is assumed to be brittle cracking, which is considered cracking