Finite element prediction of high cycle fatigue life of aluminum alloys

A new method is described for calculating the long fatigue life (>10⁵ cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy. The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength (at 10⁸ cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast, the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue.