Abstract
BackgroundThe split sleeve cold expansion process is widely used to improve the fatigue life of fastener holes in the civil and military aircraft industry. The process introduces beneficial compressive residual stresses around the processed hole, but uncertainties remain about the character of the stress field immediately adjacent to the bore of the hole.ObjectiveThe primary objective of this study was to implement the contour method with minimising error associated with cutting-induced plasticity to provide detailed and reliable characterisation of the residual stress introduced by the split sleeve cold expansion process.MethodsA systematic FE study of plasticity effects by simulating different contour cutting strategies (a single cut, two sequential cuts and a 6-cut sequence) for a cold expanded hole in an aluminium alloy coupon was conducted. The identified ‘optimum’ cutting strategy was then applied experimentally on coupons containing a hole that had been processed to 3.16% applied expansion.ResultsThe FE study of different cutting simulations show that the locations of the stress error is consistent with the location where cutting-induced plasticity accumulated and that the magnitude and locations of stress re-distribution plasticity can be controlled by an optimised cutting strategy. In order to validate this hypothesis a high quality contour measurement was performed, showing that accurate near bore stress results can be achieved by the proposed 6-cut approach that controls cutting induced plasticity.ConclusionsThe present work has demonstrated that detailed FE simulation analysis can be a very effective tool in supporting the development of an optimum cutting sequence and in making correct choices of boundary conditions. Through optimizing these key aspects of the cutting sequence one is much more likely to have a successful, low error contour residual stress result.
Highlights
Fastener holes are essential in aircraft structures to allow for the mating of structural items
In an effort to increase the length of time before an initial inspection, and between recurring inspections, the Split Sleeve Cold ExpansionTM (SSCxTM ) process was developed in the early 1970s by the Boeing Company, later patented and marketed by Fatigue Technology Incorporation (FTI) [1, 2]
The SSCxTM process is performed by hydraulically pulling a tapered mandrel through a lubricated sleeve which has been placed between the mandrel and the fastener hole
Summary
Fastener holes are essential in aircraft structures to allow for the mating of structural items. The introduction of a fastener hole into a critical structural member introduces a local stress concentration under far field operational loading. Objective The primary objective of this study was to implement the contour method with minimising error associated with cutting-induced plasticity to provide detailed and reliable characterisation of the residual stress introduced by the split sleeve cold expansion process. Conclusions The present work has demonstrated that detailed FE simulation analysis can be a very effective tool in supporting the development of an optimum cutting sequence and in making correct choices of boundary conditions Through optimizing these key aspects of the cutting sequence one is much more likely to have a successful, low error contour residual stress result
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