Methodology for modelling the small crack fatigue behaviour of aluminium alloys

Abstract

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, Δ J eff (the effective range of the J-integral), is calculated. An effective crack growth rate curve is calculated, and the Δ J eff 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.