No abstract

A method for quantitative analysis of fatigue fracture surfaces has been developed in our laboratory. Firstly, the salient features of the proposed method are presented based on the results obtained under constant amplitude (CA) tests at five R ratios for the 2024 T351 aluminum alloy. Secondly, the proposed method is applied to fatigue crack growth under variable amplitude loading. Two kinds of tests were studied, simple block loading (BL) consisting of four mean load levels with different number of cycles at the highest R ratio. These tests were also carried out on the 2024 T351 alloy. Finally, variable amplitude loading (VAL) tests using a transport aircraft wing spectrum were carried out on three different aluminum alloys, including an aluminum lithium alloy. In the case of BL tests the spatial distribution of striations and dimples are quantified with respect to the maximum stress intensity factor. After comparison with CA results, it is shown that the peak to peak load ratio and the highest load ratio in a block are important parameters in determining the fracture surface appearance. For VAL conditions, the results obtained for the 2024 T351 alloy show that the spatial distribution of the significant fractographic features is quite similar to that obtained at R ≃ O under CA conditions at Kmax levels corresponding to the maximum load in the spectrum. Comparison of fracture aspects under VA conditions for the different aluminum alloys shows that a larger amount of striation like features rather than dimples produces a better crack growth resistance. It is also shown that the presence of secondary cracks may also have a beneficial effect on the crack growth resistance. In the discussion, a technique for the determination of Kmax and equivalent R ratio for VAL conditions is proposed.