Taper headed fastener to increase work fatigue life

In a fastening assembly capable of displacing work material between first and second locations, the first location being proximate the junction between a workbore and a conical countersink surface in the work, the second location being at the work surface adjacent and closely bounding the countersink surface, the combination that comprises:

• a. a fastener head and shank, the head being conical to closely interfit the work countersink surface, the conical head defining an included conical angle of at least about 100.degree.,
• b. the shank defining first, second and third sections in a lengthwise axially forward sequence away from the head, the third section being threaded to receive a tightening nut, the second section being cylndrical and having an outer diameter to closely fit the workbore diameter,
• c. the first shank section being conically tapered forwardly away from the head and toward the second section and being sufficiently oversized in relation to the workbore that interference force will develop in response to forward displacement and seating of the fastener into the work, thereby to effect said displacement of work material between said locations, said first section taper being less than about 4.degree. and greater than about 1/2.degree. , the work to include first and second panels, the combined axial extents of said conical head and said shank first section being less than the thickness of said first panel, whereby said first section will be located entirely within the first panel,
• d. the work to have a rear side face into which said countersink is sunk, whereby when said fastener is seated in said work said work rear side face will form a rearward bulge extending annularly about the rearward facing side of the head at said second location, the entirety of the bulge being immediately proximate to the radially outermost periphery of the head.