An Investigation of the Extent of Crack Closure for Crack Growth in an Aluminium Alloy

The corrections incorporated in fatigue crack growth prediction programs for crack closure are usually tested by their ability to predict retardation following an overload and for the accuracy of their prediction lives for long cracks greater than about 1mm. They should, however, be examined on their ability to predict the life of cracks that grow from small sizes, such as small inherent material discontinuities, to failure, which is more typical of service situations and the growth produced by small cycles as well as the larger cycles. To examine the extent of crack closure in aluminium alloy 7050-T7451 and the prediction of that growth, quantitative fractography measurements of short periods of fatigue crack growth produced with a specially engineered spectrum were conducted and are reported here. The spectrum contained bands of constant amplitude loads with diminishing mean stress designed to examine the extent of closure. The quantitative fractography results are compared to predictions by the common analytical programs FASTRAN and AFGROW and further with a basic effective stress intensity calculation method at a crack depth of about 1mm. The results showed that the analytical programs were able to predict the presence of closure; however, the extent of the closure was not accurately predicted.