Semi-tubular rivets and method of using
BACKGROUND OF THE INVENTION 
     Assembly of fiber reinforced plastic laminates is accomplished by adhesive bonding or by joining with fasteners. In installations where fasteners are required and weight savings are a consideration it would be desirable to join the laminates with rivets instead of threaded fasteners, however, rivets create a problem as expansion of the rivet during the forming operation causes the laminate to be weakened as is evidenced by crazing in the area of the matrix around the formed rivet. 
     In U.S. Pat. No. 4,022,310 to Gennes a rivet tubular throughout the length of the shank is shown in combination with an adhesive for joining clutch friction discs. 
     In U.S. Pat. No. 1,591,478 to Eaton a rivet tubular throughout part of the length of the shank is used to join and compress a dust guard of fiber board or leather-board covering a compressible absorbent body portion. 
     It was found that a special shaped semi-tubular rivet can be used to fasten together fiber reinforced plastic laminates without creating a weakness in the laminates. 
     SUMMARY OF THE INVENTION 
     A rivet of titanium columbium alloy is tubular for a distance in from the end away from the head, and the inside wall of the tubular section is tapered outwardly at an angle of about 15 degrees. 
     It is an object of this invention to provide a rivet for joining plastic laminates without the main shank of the rivet expanding against the bore of the laminate. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an exploded side elevational partially sectional view of the rivet of this invention in combination with a fragmented sectional part to be joined. 
     FIG. 2 shows the rivet and part of FIG. 1 with the rivet inserted and ready to be formed. 
     FIG. 3 shows the rivet being formed with forming dies. 
     FIG. 4 shows the rivet joined part. 
    
    
     DETAILED DESCRIPTION 
     In rivet 10 the head 12 of the rivet is of the flush type and is at an angle of 100° to fit into a 100° countersink. The shank 14 is of a constant diameter and is open ended opposite the head to form a tubular end 16. The inside walls 18 of the tubular section are tapered outward at an angle of about 15° and terminate on a radius at 20. The rivets are used to join fiber reinforced laminates or fiber reinforced laminates to metals. It is imperative the rivet firmly hold the laminates without having the main part of the shank of the rivet expand during the forming process as the expanding shank sets up cracks in the laminate which causes structural failure. These rivets are made of an alloy of titanium and columbium to be compatible with laminates using graphite fibers as the reinforcement, but are not restricted to such use as the rivets are also used to join laminates of other materials. The alloy is preferably about 55% titanium and about 45% columbium. Straight walled tubular rivets and tubular rivets with inside walls tapered at 30° were completely unsatisfactory. Aluminum rivets may not be used to join laminates containing graphite as aluminum reacts chemically with the graphite. 
     In the figures an airplane elevator 22 is shown which is made up of a pair of laminates 24 and 26 joined with titanium columbium rivets 14. The laminates are of graphite fibers in an epoxy resin, are drilled through with bore 28 and are countersunk to 100° at each side at 30 and 32. 
     When joining the laminates the rivet is selected of a diameter to provide a net to a clearance fit into the bore, the rivet inserted and forming dies 34 and 36 located in a riveting machine not shown, are advanced to form a button 38 on tubular end 16 with the button pressing against the countersink 30. In forming the button the forming die uses contacting surface 40 that is at an angle of about 130° instead of a conventional angle for a die; which would be at about the same angle as the countersink. Preferably the depth of the tubular section of the shank is greater than the depth of the countersink against which the button is formed. 
     These figures show a flush head rivet, however it is not intended to limit the rivet to a flush head as other types of manufactured heads may be used as long as the opposite end of the rivet is tubular as shown.