Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor – Longitudinally progressive helical winding means
Reexamination Certificate
2001-12-28
2004-11-30
Nolan, Sandra M. (Department: 1772)
Adhesive bonding and miscellaneous chemical manufacture
Surface bonding means and/or assembly means therefor
Longitudinally progressive helical winding means
C156S431000, C156S449000, C156S598000
Reexamination Certificate
active
06823918
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a composite sandwich structure having outer fiber reinforced composite layers separated by a lightweight core. More particularly, the present invention relates to integrally reinforcing a section of the composite sandwich structure, e.g., by changing the thickness or proportions of one or more layers of the sandwich structure, to provide a stiffened section for a structural interface. The invention has particular advantages for constructing sidewalls of launch vehicles or other aerospace applications.
BACKGROUND OF THE INVENTION
Reinforced composite sandwich structures typically have outer fiber reinforced composite layers separated by a lightweight core made up of metallic or non-metallic honeycomb, structural foams and/or wooden fibers. The outer fiber reinforced composite layers, or face sheets, are generally separated by and connected to the core, which is usually less stiff and less dense than the face sheets. These composite sandwich structures are widely used today in aerospace applications due to their high stiffness-to-weight (i.e., specific stiffness) and strength-to-weight (i.e., specific strength) ratios. The face sheets generally comprise a fiber reinforced resin matrix composite that incorporates strong stiff fibers, such as carbon fiber, into a softer, more ductile resin matrix. The resin matrix material transmits forces to the fibers and provides ductility and toughness while the fibers carry most of the applied force. In the case of composite sandwich structures, the behavior of the face sheets is analogous to the flange of a structural I-beam while the behavior of the core is analogous to the web of the I-beam. In this regard, the face sheets carry the applied loads and the core transfers the load from one face sheet to the other.
Though composite sandwich structures provide increased strength-to-weight ratios compared to, for example, metallic structures, there are several important limitations to use of such composite structures. Composite structures depend primarily on the fiber reinforcement in the resin matrix for their high specific strength and stiffness. These composite structures generally have limited in-plane compressive strength (bearing strength) and may not have the strength to absorb highly localized stress loads, especially when those loads are applied substantially perpendicular to the composite structure. For example at a structural interface a fastener, such as a bolt, passing through the cross sectional area of a composite sandwich structure may provide a localized stress concentration and/or a point load on one or both of the face sheets. In this regard, the composite sandwich structure must provide adequate bearing strength and compressive strength to resist tearing of the face sheets and/or crushing of the core while providing required structural properties to distribute the point load across the structure's surface without failing.
In order to provide the necessary structural integrity necessary at, for example, structural interfaces, additional composite material layers are typically added to the face sheets of the composite sandwich structure. These additional layers, or doublers, provide increased stiffness and bearing strength to the structural interface. Generally, to provide the necessary structural integrity, both face sheets are reinforced with doublers. The additional layers increase the weight of the composite structure, thus reducing the specific strength and stiffness benefits provided by the composite sandwich structure. Therefore, only the region surrounding the structural interface is “doubled”, allowing the rest of the composite structure to maintain its high strength-to-weight and stiffness-to-weight ratios.
In the case of large tubular composite structures, as are used for various components of space launch vehicles, doublers may be applied in one or more ways. For example, the doublers may be co-cured on the outside of composite face sheets, which requires the doublers be applied during the initial composite structure “lay-up.” As will be appreciated, tubular composite structures are generally formed or laid-up on a mandrel that is removed after the structure is cured. In the case of tubular composite sandwich structures, adding doublers during lay-up requires a stepped mandrel having a varied diameter along its length. The stepped mandrel allows a doubling layer to be wound about the mandrel and then the normal face sheet layer wound on top of the doubling layer. As will be appreciated, if the double layer on the inside surface of the tubular structure is in any position other than the end of the mandrel, or if two doubling layers are utilized along the length of the mandrel, the mandrel cannot slide out of the composite sandwich structure upon curing. In this regard, a collapsible mandrel must be used. However, collapsible mandrels increase the cost, weight, and internal structure required of the mandrel, creating difficulties in maintaining mandrel stiffness and tolerances and further creating difficulties in the machinery utilized to apply the materials to the mandrel.
A second method for adding doublers to a section of a tubular composite sandwich structure involves post-bonding the doublers onto a pre-cured structure's face sheets. This allows a mandrel to be removed from a tubular composite structure prior to application of the doublers. However, adding the doublers, especially to the inside surface of a tubular structure, requires extensive tooling and costs. Further, care must be taken to assure the secondary bonding of the doublers to the face sheets provides good mechanical conformance. As will be appreciated, if the doublers do not properly adhere to the surface of the pre-cured face sheets such that, for example, internal voids exist, the entire composite sandwich structure may be irreparably damaged.
Finally, another method for providing a structural interface for a composite sandwich structure is to pan down the ends of the composite sandwich structure such that it transitions from a sandwich construction having two face sheets and an internal core to a monocoque construction where there is no core and the face sheets are now in direct contact. However, this eliminates many of the benefits of utilizing a composite sandwich structure, e.g., I-beam behavior and increased moment of inertia. Further, monocoque transition requires additional tooling and fabrication steps.
All of the above noted methods for providing doublers to stiffen a section of a tubular composite sandwich structure require substantial tooling and manufacturing steps, increasing the cost of the composite structure. Further, each of the above noted methods changes the external geometry of the composite structure (i.e., the spatial envelope within which the structure is contained defined by its exposed surfaces, which may define interior or exterior walls of an aerospace structure) relative to an un-reinforced geometry, which may be problematic in space launch vehicles.
SUMMARY OF THE INVENTION
It is therefore an objective of the present invention to provide an integrally reinforced section in a composite sandwich structure for use as a structural interface.
It is a further objective of the present invention to provide a process to produce an enhanced structural interface in a composite sandwich structure that does not require the use of specialized mandrels or tooling in the lay-up process.
It is a yet further objective of the present invention to provide an enhanced structural interface in a section of composite sandwich structure without altering the exterior dimensions of that composite structure
It is a yet further objective of the present invention to provide a method for allowing selective alteration of the structural properties of a composite sandwich structure for a given exterior geometry limitation.
One or more of the above-noted objectives, as well as additional advantages, are provided by the present invention, which includes a composite sandwich str
Grosskrueger Duane D.
Hora Keith Y.
Lockheed Martin Corporation
Marsh & Fischmann & Breyfogle LLP
Nolan Sandra M.
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