Investigation of stress intensity factor for overloaded holes and cold expanded holes

Abstract

An understanding of how residual stresses due to overloads effect stress intensity factors for fatigue cracks at common stress concentrators, such as holes, is important. In this paper a weight function approach is used to determine stress intensity factors for cracks in residual stress fields, due to three types of overloads for holes in metallic plates with and without subsequent remote loading. The cases resulting in compressive residual stresses are remote tension overload and hole cold expansion. Also considered is the case of a tensile residual stress field due to a compressive overload. Initially generic cases for a D6ac steel plate are considered. Stress intensity solutions are given for different crack sizes for different levels of overload which produce yield zones of different sizes. For both remote overload cases it is shown that once the crack length is the same or larger than the initial yield zone, the stress intensity factors are the same as for the case without the initial overload. However, for the cold expanded hole case the beneficial reduction in stress intensity factor extends some distance outside the initial yield zone. Then the application of cold expansion to an Aluminium alloy fatigue test coupon representing a lower wing skin location in the C-130 aircraft is considered. Here the significant reduction in stress intensity factor is investigated. Some potential inaccuracies in the weight function method for compressive fields, due to possible crack closure are also discussed.