Hemispherical joint for composite material joining

An assembly includes a fiber-reinforced composite structure, a bushing, and a fastener. The composite structure includes a composite panel on a first side, where the composite panel defines a hemispherical well. The structure further includes a plurality of stiffening ribs on a second side, which are affixed to the composite panel. The composite panel defines a first bore at the center of the hemispherical well, which extends through the structure. The bushing has a hemispherical end and a flat end, and defines a second bore from the center of the hemispherical end to the center of the flat end. The hemispherical end of the bushing is configured to nest within the hemispherical well defined by the composite structure such that the flat end of the bushing is flush with the surrounding composite panel. The fastener is configured to extend within both of the first bore and the second bore.

TECHNICAL FIELD

The present invention relates generally to fasteners for use with composite materials.

BACKGROUND

Composite materials are typically formed by suspending a high-tensile strength fibrous material within a solidified epoxy/resin matrix. Composite materials are favored in certain manufacturing settings for their high strength and light weight properties. In particular they are notably strong when the embedded fibers are placed in tension. When non-tensile forces are applied, however, the often-brittle epoxy/resin is prone to cracking at even low stresses. This brittle nature presents challenges when attempting to fasten the composite to a rigid structure, since the fastening load is typically a compressive load applied in a direction normal to the material.

SUMMARY

A joint assembly includes a fiber-reinforced composite structure, a bushing, and a fastener. The fiber-reinforced composite structure has a first side and a second side opposite the first side, and includes a composite panel disposed on the first side of the structure, and a plurality of stiffening ribs disposed on the second side of the structure and affixed to the composite panel. The composite panel defines a rounded well extending toward the second side of the composite structure, which has a a first radius of curvature. The composite panel further defines a first bore at the center of the rounded well, which extends through the composite structure.

The bushing generally has a rounded end and a flat end opposite the rounded end. The rounded end has a second radius of curvature, which may equal the first radius of curvature. The bushing defines a second bore centered within the rounded end and extending through the center of the flat end.

The rounded end of the bushing is configured to nest within the rounded well defined by the composite structure such that the flat end of the bushing is flush with the surrounding composite panel. The fastener is then configured to extend within both of the first bore and the second bore, and is configured to apply a compressive load against the flat end of the bushing.

In one configuration, each of the plurality of stiffening ribs are oriented such that they extend radially outward from the first bore. Likewise, an annular rib may be disposed about, and may partially define the first bore.

The fastener may include a body and a head, wherein the body is configured to extend within both the first bore and the second bore. Once installed, the head is configured to contact the flat surface of the bushing.

In one configuration, the flat end of the bushing defines a receiving portion configured to receive the head of the fastener such that an outward surface of the fastener is flush with the flat end of the bushing. In this manner, the head of the fastener may not excessively protrude from the flat end of the bushing.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numerals are used to identify like or identical components in the various views,FIG. 1schematically illustrates an exploded perspective view of a joint assembly10for securing a composite structure12to a secondary member (not shown). In an automotive context, for example, the joint assembly may be useful in securing the composite structure12to a portion of the vehicle frame.

The joint assembly10may include the fiber-reinforced composite structure12, a bushing14, and a fastener16that may all cooperate to secure the composite structure12to a secondary element (not shown). The present design is intended to minimize stresses in the composite structure12that would otherwise exist immediately proximate to the fastener16when securing the composite structure12, or during subsequent use.

The composite structure12may include a first side20, and a second side22that is opposite the first side20. The structure12may further include a panel portion24that may be disposed at or proximate to the first side20, and a rib portion26that may be disposed at or proximate to the second side22. The panel portion24may be defined by a composite panel28that has a generally constant outward facing surface. The rib portion26may include a plurality of stiffening ribs30that may be affixed to the composite panel28, and may provide the panel28with an enhanced resistance to transverse bending.

One or both of the panel portion24and rib portion26may be formed from a fiber-reinforced composite material that may include a plurality of uni- or multi-directional fibers disposed within an epoxy or resinous matrix/substrate. For example, each rib30of the rib structure may include a plurality of fibers disposed lengthwise along the rib30. In one configuration, the fibers may include spun glass fibers, carbon fibers, graphite fibers or other suitable high-tensile strength fiber materials. Each individual fiber may have a thickness/diameter of, for example approximately 5-10 μm. In other configurations, however, fibers having larger or smaller thicknesses may likewise be used.

The composite panel28may define a rounded well32extending toward the second side22of the composite structure12. The rounded well32may have a generally uniform radius of curvature34that originates from a single center point36. In one configuration, the center point36may lie on a plane defined by the outer surface38of the composite panel28. As such, the rounded well32may approximate a hemisphere. In another configuration, the center point36may lie above the plane defined by the outer surface38of the composite panel28. In this instance, the rounded well32may resemble a portion of a sphere.

The composite panel28may further define a bore40at the center of the rounded well32. The bore40may extend through the entire composite structure12(i.e., from the first side20to the second side22), and may be dimensioned to receive the fastener16. In one embodiment, the rib portion26may cooperate with the composite panel28to define the bore40. For example, an annular rib42may be disposed about, and may partially define the bore40. Additionally, a plurality of stiffening ribs44may be configured to extend radially outward from the bore. Each rib44may be specifically positioned along a predetermined load path within the structure12. Alternatively, each rib44may extend outward in a sunburst-type arrangement, or in some other concentric, radially-extending-type arrangement.

The bushing14may be configured to nest within the rounded well32defined by the composite structure12. As used herein, the term “nest” is intended to mean that when the bushing14is inserted into the rounded well32, a surface of the bushing makes surface contact (as opposed to point contact) with a surface defining the well32. In this manner, the bushing may have a rounded end52and a flat end54. The rounded end52may have a radius of curvature that is substantially equal to the radius of curvature34of the well32. When inserted into the well, in one configuration, the flat end54of the bushing14may be substantially flush with the outer surface38of the composite panel12.

The bushing14may define a second bore56that is centered within the rounded end52and extends through the center of the flat end54. The second bore56may be positioned such that when the bushing14is inserted within the rounded well32, the bore56of the bushing14may axially align with the bore40of the composite structure12. Similar to the first bore40, the second bore56may also be dimensioned to receive the fastener16. When fully assembled, such as generally illustrated in the cross-sectional view60provided inFIG. 3, the fastener16may pass through each of the first bore40and the second bore56, and a portion62of the fastener16may protrude from the second side22(i.e., the “protruding portion62”).

The fastener16may generally include a head portion64and a body portion66. The body portion66may be dimensioned to extend within the first bore40and second bore56, while the head portion64may be relatively wider than the body portion66(i.e., such that it may not pass through the second bore56). When fully assembled, the head portion64of the fastener16may contact and may apply a compressive load to the flat end54of the bushing14. This load may be sufficient to ensure continued contact between the round end54of the bushing14and the rounded well32of the composite structure12. The compressive load may further aid in securing the composite structure to a secondary member that may ultimately be disposed about the protruding portion62of the fastener16.

As generally illustrated inFIG. 3, in one configuration, the fastener16may be a rivet that further defines a central bore68. As may be appreciated, to fasten the rivet to the secondary member, a portion of the protruding portion62may be radially deformed outward to prevent the rivet from withdrawing back through the first bore40. In other configurations, the fastener may be, for example, a screw, bolt, or pin.

The flat end54of the bushing14may further define a receiving portion70portion that may be dimensioned to receive the head portion64of the fastener16. Once assembled, a portion or the entire head portion64of the fastener16may be disposed in the receiving portion70and below the outward surface of the flat end54of the bushing14. In one configuration, the receiving portion70may be dimensioned such that upon insertion of the fastener into the second bore56, an outward surface of the head portion64of the fastener16would be flush with the surrounding surface of the flat end54of the bushing14.

In one configuration, such as schematically shown inFIGS. 1 and 2, the head portion64of the fastener16may include a profiled (i.e., non-circular) outer perimeter72. Example profiles may include square-tooth or saw-tooth projections around the outer perimeter, and/or oval, triangle, or square shaped outer perimeters. Likewise, an outer perimeter74of the receiving portion70may have an inverse profile of the fastener perimeter. In this manner, the fastener16may interlock with the bushing14upon assembly. This keyed nature may prevent rotation of the fastener16relative to the bushing14during assembly and use.

As mentioned above, the composite structure12may include a high tensile-strength fiber suspended within a hardened resin/epoxy. The fibers may be disposed within both the panel portion24and the rib portion26, and may include both uni- and multi-directional fibers. In one configuration, the fastener16may be formed from either a high strength polymer, or a metal (e.g. aluminum or steel), and the bushing14may be formed from a polymeric material (e.g., a nylon material, a polyvinyl chloride (PVC) material, or a Polytetrafluoroethylene (PTFE) material). As such, the bushing may aid in electrically isolating the fastener from the composite structure, and may correspondingly resist the occurrence of a galvanic couple/corrosion between the fastener16and the fiber in the structure12.

One benefit of using such a rounded bushing14is that any lateral loads that may be imparted to either the fastener or the composite structure during use would be transferred across a larger surface area than if the fastener was merely inserted through the composite alone. This increase in surface area will minimize any localized stresses, which would have otherwise occurred absent the bushing. A lower and more even stress distribution through the composite structure12may preserve the integrity of the resin/epoxy matrix and prolong the life of the composite.

While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not as limiting.