SHEATHED COMPOSITE BLIND RIVET

Abstract: A composite blind rivet wherein a ductile sheath (34) fully covers the blind head forming portion of the rivet before and after deformation thereof so as to preclude protrusion of said fibers upon formation of said blind head.

Description: SHEATHED CO?tPOSITE BLIND RIVET BACKGROUND OF THE INVENTION The blind rivet of the instant invention is an improvement on the composite rivet disclosed in U. S. Patent No. 4,478,544 issued October 23, 1984, for Composite Rivet, and assigned to the assignee hereof. Carbon fiber reinforced materials are now widely used in the aircraft industry for airframe structural components. However, the use of carbon fiber reinforced resins in blind rivets has been limited by the difficulty of properly forming the blind head. One characteristic of composite materials utilizing carbon fibers is that the material often exhibits a rough finish after reforming due to protrusion or breakage of the carbon fibers. While structural integrity of the material may not be compromised, the end product is abrasive and aesthetically unsatisfactory. When such material is employed in the reformable head of a blind rivet, the problem becomes acute since the blind head is exposed on the rear surface of the workpiece. Another problem that must be addressed is that plastic cold flow of the rivet tends to reduce clamp-up forces on a workpiece. Thus, pretensioning of the rivet is highly desirable to offset ultimate relaxation of the tensile forces. SUMMARY OF THE INVENTlIX The aforesaid problems are solved by the rivet of the present invention by encapsulating the carbon fibers and resin matrix of the blind head thereof in a tensioned sheath. The sheath trols deflection and projection of the carbon fibers Upon formation of the rivet head aids in creating and intaining pretensioft on the rivet. In both disclosed embodiments of the invention, compression Of the resin matrix, which is softened due to the appliation of heat, forms a radially extending rivet h@@@ that is integral with the shear portion of the rivet and which, when fully polymerized, forms a riveted connection that exhibits relatively high shear strength. Ithe configuration of the sheath and the mandrel of the rivet aids the maintenance of clamp-up forces. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an elevational view, partially in cross section, of one embodiment of the blind rivet of the instant invention; Figure 2 is a view of the rivet of Fig. 1 after tensioning of the rivet mandrel to form a blind head on the rivet; Figure 3 is a view similar to Fig. 1 of another embodiment of the instant invention; and Figure 4 is a view of the rivet of Fig. 3 after tensioning of the mandrel thereof to for. a blind head on the rivet. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION As best seen in Fig. 1 of the drawings, a rivet 10 in accordance with a constructed embodiment of the instant invention comprises a carbon fiberreinforced resin matrix, preferably a wB"-stage thermoset resin preform 11 having a head portion 12, a shear portion 14, and a blind head forming portion 16. A mandrel 18 is disposed centrally of the preform 11 and has an anvil 20 at one end thereof and a tensioning stem 22 at the other end thereof. The mandrel 18 is journaled in a complementary aperture 24 that extends through the head, shear and head forming portions 12, 14 and 16, respectively, of the preform 11. The shear portion 14 of the rivet 10 is coextensive with the cumulative thickness of a pair of workpieces 30 and 32. As seen in Fig. 2 of the drawings, the mandrel 18 is adapted to be pulled, after heating of the preform 11, by a tool (not shown) of conventional design resulting in deformation of head forming portion 16 of the rivet 10 into the mushroom configuration shown. In accordance with one feature of the instant invention, the head forming portion 16, of the rivet 10 is encapsulated by a sheath 34, preferably Titanium or suitable plastic, which expands to the configuration of the blind head and completely covers said head eliminating protrusion of carbon fibers 36 that are impregnated or encapsulated in the resin matrix. The sheath 34 has a uniform external diameter prior to deformation but has an upper end portion 37 of reduced internal diameter that controls collapse and maintains a bias on the reformed resin matrix after deformation. It is to be noted that the thickened wall section 37 commences well above the workpiece 30 whereby the relatively thin section thereof accommodates radial deflection of the sheath 34 and helps define the ultimate truncated conical cross section of the upper portion 37 thereof. From the foregoing it should be apparent that a smooth exterior surface is presented on the blind head which is not abrasive and is aesthetically pleasing. The sheath 34 is mechanically collapsed by the anvil portion 20 of the mandrel 18 in a manner that maintains the clamp-up force of the rivet 10 in the event of plastic cold flow of the shear portion 14 thereof. In accordance with another feature of the instant invention, the mandrel 18 is provided with an annular groove 40 in the stern 22 thereof into which resin flows thereby tensioning the mandrel and locking it within the head forming portion 16 of the rivet 10. A breakoff groove 44 is provided in the stem 22 to facilitate removal of excess stem portion after setting of the rivet 10. As seen in Figs. 3 and 4 of the drawings, a rivet 50 comprises a carbon fiber reinforced preform 51, preferably a wB"-stage thermoset resin, having a head portion 52 of any desired configuration, a shear portion 54 and a blind head forming portion 56. A centrally disposed mandrel 58 has a head or anvil 60 at one end thereof and a tensioning stem 62 at the other end thereof. The mandrel 58 is journaled in a complementary aperture 64 that extends through the head, shear and head forming portions 52, 54 and 56, respectively, of the rivet 50. The shear portion 54 of the rivet 50 is coextensive with the cumulative thickness of a pair of workpieces 70 and 72. As seen in Fig. 4 of the drawings, the mandrel 58 is adapted to be pulled, after heating of the preform 51, by a tool (not shown) resulting in deformation of the head forming portion 56 of the rivet 50 into the mushroom configuration shown. The head forming portion 56, of the rivet 50, is encapsulated by a plastic sheath 74 which expands to the configuration of the blind head eliminating protrusion of carbon fibers 76 that are impregnated or encapsulated in the resin matrix. The sheath 74 has a thickened upper end portion 78 that effects controlled deformation thereof and is reformed into a conical configuration that maintains pretension on the rivet 50. It is to be noted that the sheath 74 extends entirely through the workpiece 70 and into a counterbore 80 in the workpiece 72. Thus, plastic cold flow of the shear portion 54 of the rivet 50 will not effect a reduction in clamp-up force of the rivet 50 since tension induced by deformation of the sheath 74 will continue to pinch the workpieces 70 and 72. A breakoff groove 82 is provided in the stem 62 to facilitate removal of the excess stem portion after setting of the rivet 50.

Claims: CLAIMS 1. A composite blind rivet comprising a tubular body portion, a blind head forming portion, a mandrel extending centrally of said rivet body and having a stem portion at one end and an anvil at the other end overlying said blind head forming portion, said mandrel being movable axially relative to the body and blind head forming portions of said rivet to form a blind head on said rivet, said body and head forming portions comprising a plurality of substantially continuous carbon fibers encapsulated in a resin matrix, the head forming portion of said rivet being deformable upon heating thereof, and a ductile sheath fully covering the blind head forming portion of said rivet before and after deformation thereof so as to preclude protrusion of said fibers upon formation of said blind head, said sheath being deformable from a cylindrical configuration to a truncated conical configuration and tensioned so as to exert a radially inward bias on the deformed head of said rivet. 2. A rivet in accordance with claim 1 wherein said sheath is metal. 3. A rivet in accordance with claim 1 wherein said sheath is plastic.

Inventors: BERECZ, IMRE, HINTON, DENNIS L., SCHULTZ, DENNIS
Applicants: MICRODOT INC.
Application year: 1986
Main classification: F16B19
IPC codes: F16B19