Methodology for modelling the small crack fatigue behaviour of aluminium alloys

The objective of this study is to develop a computational modelling methodology of the small fatigue crack growth behaviour of: (1) a forged 2124 Al alloy in the T4 condition and (2) a cast 359 Al alloy in the T6 condition. In particular, the focus of this work is on correlating local crack-tip driving force conditions of an initial small crack with an experimental long crack growth rate curve, using crack closure. A defect tolerant approach is assumed. The crack tip is modelled using the finite element method, and the correlating parameter, ΔJeff (the effective range of the J-integral), is calculated. An effective crack growth rate curve is calculated, and the ΔJeff is used to obtain the crack growth increment per cycle. Small crack growth rate curves for different stress levels and initial defect sizes are presented for each alloy for a stress ratio, R, of 0.1. Predicted S–N curves are then compared with experimental results for both the Al 2124 and Al 359 alloys. A good agreement with experimental results is achieved for an appropriate choice of defect size. Finally, as a means of validating the choice of defect size, the fatigue limit vs. defect size results are compared on a Kitagawa diagram with those obtained by linear elastic fracture mechanics, for the Al 2124 alloy.