Fatigue Properties of a High Strength Aluminium Alloy

Abstract

The initiation and propagation of fatigue cracks from notches under constant amplitude loading have been studied in a high strength aluminium alloy 7010-T736. The root radii of the notches varied from 0.11 mm to5 mm with the depth varying from 5 mm to 10 mm. The various approaches to prediction of fatigue crack initiation from notches were compared. It was concluded that the modified stress intensity factor could be described by introducing a function of microstructural characteristic of material, notch geometry and crack length. This was applied to the prediction of fatigue crack initiation of notches and fatigue crack growth within the notch stress field. The correlation between fatigue crack growth rates and fatigue striation spacings was established. An empirical equation relating striation spacings and the normalised4K was derived and used to predict failure life times. The effects of shot peening on fatigue crack initiation and propagation from notches were evaluated and it was found that the beneficial effect of shot peening on crack initiation could be expected at low nominal loading level but the reverse was true at high loading levels. The behaviour of alloy 7010-1736 under variable amplitude loading was investigated. The results suggested that the retardation in fatigue crack growth rates following multiple overloads mainly occurred in the near surface region and the crack growth rates could be retarded by af actor of 8 by a Hi-Lo multiple overload even if the overloads ratio was only 1.17. The fatigue crack propagation rate for alloy 7010-1736 under block spectrum loading was tested and the results were represented by the characteristic stress intensity approach.