Stress coining tool fastened joint

Claim 1

A structural joint comprising: a workpiece having a pair of exposed opposite surfaces formed by a plurality of overlapping contiguous metal elements in face-to-face contacting relation with the next adjacent element; the metal forming the elements having the inherent physical property of being receptive to improvement in its fatigue strength via coldworking and elastically rebounding to a different size and an increased fatigue strength after such cold working; a straight bore of a first selected diameter extending coaxially through all of said elements; and a unitary metallic combined headed bolt and metal-working tool formed coaxially on a shank common to both, the bolt portion and the tool portion being joined by a breakneck groove which groove provides means for eventual frangible removal of said tool portion, the bolt portion comprising: a head; a cylindrical shank having a second predetermined diameter larger than said workpiece bore one end of which is joined to said head and an alternately ridged and grooved fastener-receiving portion having a diameter less than said bore extending from the other end of the cylindrical shank and terminating adjacent said breakneck groove, said cylindrical shank extending substantially through said workpiece bore and being engaged by the walls of said bore with an interference fit, said head engaging one of said surfaces adjacent the bore; and a fastening collar surrounding and tightly interengaged with said fastener-receiving ridges and grooves, said fastening collar having end faces one of which firmly engages with the other surface of said workpiece, the remaining face of said collar terminating adjacent said breakneck groove, said tool portion comprising: a metalworking section adjacent said breakneck groove, said metalworking section including a cylindrical shank having a diameter less than said workpiece bore along which a plurality of axially-spaced, smooth-surfaced, solid, annular, relatively hard protuberances extend radially therefrom to form hole-expanders, said expanders having successively graduated selected maximum diameters each of which is larger than the diameter of said workpiece bore with the expander of largest diameter being adjacent said breakneck groove, each expander being formed by a smoothly curved surface of revolution the maximum diameter of which is in the middle thereof, said curved surface having an initial segment facing toward said end portion and a return segment facing toward said breakneck groove, both the initial and return segments of the curved surface extending from said maximum diameter with progressively smaller diameters and generally asymptotically approach said tool shank diameter, the space between successive expanders forming recesses for a lubricant and the diameter of the largest expander section being larger than said workpiece bore by an amount substantially equal to twice said elastic rebound of said workpeice bore wall, said cylindrical shank having a diameter greater than the diameter to which the bore tends to rebound after coldworking and less than the diameter of said expander; and a tool-receiving end portion extending from the smallest of the expanders a distance sufficient for engagement by a tool and including ridge and groove means for co-operation therewith, whereby said bore has been cold worked by said expanders during insertion of said combined bolt and tool, the fatigue strength of the walls of the bore through said workpieces has been increased and thus engages said bolt shank with a stronger and more fatigue-resistant grip and thereby forming an improved structural joint.