Abstract

The invention is concerned with fasteners which are used to secure a plurality of plates, or other structural members, together in an interference fit between the fastener and the plates to impart a residual hoop stress within the structural members and around the hole through which the fastener extends so as to obviate fatigue failure in the structural members in the vicinity of the hole. The fastener assembly of the invention comprises the combination of a flared sleeve of relatively soft material, and a tapered mandrel of relatively hard material. The mandrel is first driven into the sleeve at the factory in a preliminary interference fit with the sleeve, and the sleeve is then finish ground to the final tolerances, so that each mandrel and sleeve becomes a matched pair. When inserted into the hole in the structural members to be joined thereby, the mandrel is then driven further into the sleeve causing the sleeve to expand radially outwardly to assume a final diameter greater than the original diameter of the hole. In this way the fastener assembly imparts a residual tension stress into the portions of the structural members surrounding the hole; this being achieved without burnishing or galling the bore of the hole due to the fact that the expansion of the sleeve is purely in the radial direction and there is no axial movement thereof.