Residual stresses and fatigue performances at cold expanded fastener holes

• Main Author: Ozdemir, A. T.
• Published: Open University 1993
• Subjects: 620.110287
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.499966

This work presents an investigation of the effect of cold-expansion methods on both residual stress distributions and fatigue life improvements in a high strength 7050 T76 aluminium alloy. Cold-expansion is usually achieved by the FTI split-sleeve process using commercially available equipment and prescribed levels of mandrel interference. A cold-expansion system has also been developed at the Open University which allows the degree of cold-expansion to be infinitely varied in a simple manner. The nature of residual stress distributions after the FTI split-sleeve and the Open University processes are evaluated by a mechanical (destructive) method based on Sachs boring-out technique. Since the Sachs method requires cylindrical specimens, 40mm diameter washers concentric with expanded holes have been produced from original rectangular specimens. To allow for any stress relaxation caused by washer manufacture, relevant compensation equations have been developed and are used to compensate the evaluated residual stresses after each Sachs experiment. It is revealed that residual hoop stresses produced by the FTI split-sleeve method are more compressive than that are produced by the Open University method. Residual hoop stress distributions are found to be unaxisymmetric and orientation dependent. In the FTI split-sleeve process, the presence of the split increases the magnitude of the compressive hoop stress adjacent to the hole. In addition, the residual hoop stresses in the transverse direction are more compressive than the residual hoop stresses in the rolling direction of rectangular specimens. This has been attributed to the geometry of the specimen as differences in material constraints in the transverse and rolling directions which influence the extents of plastic deformations and the resultant residual stress distributions. Residual stresses are also found to be nonuniform in the thickness direction. At FTI split sleeve expanded holes, the mandrel entrance side (inlet) has a lower residual compressive hoop stress.