COMPARISON OF SHORT AND LONG FATIGUE CRACK GROWTH IN 7075‐T6 ALUMINUM

Abstract

Abstract The fatigue crack growth rates of physically‐short cracks (0.5 ≤ a ≤ 1.0 mm), intermediate cracks (1 < a ≤ 2 mm) and long cracks (7 < a < 25 mm) were compared using SEN type tensile specimens in 7075‐T6 aluminum alloy with load ratios, R, of 0.05, − 1 and 0.5 under constant amplitude testing at room temperature. It was found that the short cracks grew much faster than long cracks based on applied δ K with d a /d N ≤ 10 −7 m/cycle. Even the intermediate cracks grew faster than the long cracks below 10 −7 m/cycle. The transition crack lengths where similitude with δ K existed was between 1 and 2 mm. Mean stress effects were similar for R = 0.05 and − t, but R = 0.5 caused increased crack growth rates. The above differences are partially attributed to crack closure effects. Based upon plastic zone sizes, LEFM was justifiable with all the experiments.