Optimising fatigue crack growth predictions for small cracks under variable amplitude loading

Abstract

The fatigue cracks in a fighter aircraft that pose the greatest threat to structural integrity and availability are usually less than 1 mm for most of their lives. However, it has long been recognised that small cracks can grow significantly faster than long cracks at the same stress intensity range (ΔK). This means predictions for small cracks growing under realistic spectrum loading can be significantly non-conservative when using linear elastic fracture mechanics and long crack-based empirical rate data. Previous work showed empirical rate data based on small cracks grown under constant amplitude (CA) loading could significantly improve predictions. However, such data still have limitations related to an inability to predict the spectrum history effects that influence small cracks. This paper presents a method to optimise a model to predict fatigue crack growth rates for cracks loaded with realistic spectra. This method is demonstrated for small cracks in Aluminium Alloy 7050-T7451 grown under fighter wing root spectrum loading. Numerical optimisation is used to select model parameters that minimise fatigue crack growth rate prediction errors for a training dataset. This purpose-built model consistently outperformed a generic model based on the growth of small cracks under CA loading.