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.

Keywords:

• Weight functions
• crack growth
• stress intensity factors
• finite element analysis

Additional information

Notes on contributors

R J Callinan

Richard Callinan graduated from the Royal Melbourne Institute of Technology (Aeronautical Engineering) in 1969 and from Monash University in 1971 (Civil Engineering) and completed a MEngSc in 1981 at Melbourne University. He commenced work at the Aeronautical Research Laboratories in 1972. His work has been in the areas of finite element analysis, fracture mechanics and structural mechanics of composites and bonded repairs, and also military aircraft accident investigations. He has been involved with design studies of low radar cross-section battlefield surveillance Unmanned Aerial Vehicles. In 1985 he was seconded to the USAF at Eglin Air Force Base for 18 months, to carry out vulnerability studies on composite structures. More recently he has been involved in a specific program on validation of bonded repairs to RAAF aicraft, and bonded repairs subject to acoustic fatigue. He has over 100 publications including three book chapters on repair of cracked aircraft structures. He is currently a Senior Research Engineer in the Air Vehicles Division of the Defence Science and Technology Organisation.

R Kaye

Robert Kaye joined the Air Vehicles Division of Defence Science and Technology Organisation (DSTO) in 1990 as a structural engineer with a background in full scale structural testing. The first three years at DSTO were spent in evaluation of bonded repairs primarily using finite element methods. Included was the analysis of repairs to fuselage skin lap-joints, wing skin planks and bulkhead frames. More recently he has been involved with structural and mechanical aspects of full scale fatigue test installations. In particular he played a key role in the development of a low stiffness air-spring for the application of load to a vibrating air-frame. This work was followed by a period of several years doing research and development in the alleviation of stress concentrations by way of adaptive shape optimisation. This has been applied to concave metallic free boundaries and to the adhesive layer and end tapering of boron patches bonded to metallic structure.

M Heller

Manfred Heller completed a BEng (Hons) in Aeronautical Engineering at the University of New South Wales in 1981. He commenced employment in Structures Division at the Aeronautical Research Laboratory in 1982. He was awarded a Department of Defence Postgraduate Cadetship in 1986, completing a PhD at Melbourne University in 1989. He is currently a Principal Research Scientist and Functional Head for Structural Mechanics in the Air Vehicles Division of the Defence Science and Technology Organisation. His research contributions have focused on the areas of stress analysis, structural shape optimisation, and bonded repair technology, in the context of airframe life extension. Since 1992 he has led tasks which develop and evaluate techniques for extending the fatigue life of Australian Defence Force aircraft components. He is a Corresponding Member, Committee on Stress Analysis and Strength of Components, Engineering Science Data Units, UK.