The critical importance of correctly characterising fatigue crack growth rates in the threshold regime

Abstract

The majority of the fatigue life for high strength metallic aircraft structures is often spent in the small crack near threshold region. Accurate modelling of this regime is therefore essential if we are to gain the maximum possible life from existing structures while maintaining adequate levels of safety. The threshold and very low growth rate region poses a number of challenges including small crack behaviour, Kmax effects and the apparent breakdown of similitude. Standardised test methods such as the load reduction technique in ASTM E647 introduce a range of problems including load history effects and remote closure. The end result is that typical thresholds are over estimated and rates in the threshold and near threshold regions are under estimated. This paper considers data collected by two very different methods, both of which appear to avoid the problems of the load reduction method and therefore allow for considerably improved near threshold crack growth data, which, as is shown, can improve accuracy in life predictions. Unlike the standard method, the first alternate method generates the cracks under remote compression-compression loading. The second alternate method relies on specially designed sequences to mark the fracture surface on very small/short, natural cracks such that Quantitative Fractography is possible. Excellent correlation between test and analysis are shown for 7050-T7451 aluminium alloy examples including simple coupons and cracking from a full-scale fighter aircraft centre section test.