Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
1999-12-30
2001-09-18
Copenheaver, Blaine (Department: 1773)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S339000, C428S413000, C428S423100
Reexamination Certificate
active
06291071
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to composite structures, and, more particularly, to the fabrication of a structure having a laminated skin structure bonded to a substrate.
Multilayer laminated composite structures are used in a variety of applications requiring high strength and low weight. For low and moderate temperature applications, the structures are typically made of light-weight composites of fibers embedded in an organic matrix. In some cases, substrates such as structural foams are bonded into the structures as well. Through careful selection of materials and processing, this composite-design approach offers the opportunity to optimize many of the properties of the structure.
Such structures are generally fabricated from a number of individual elements carefully selected to achieve the required performance objectives, and thereafter bonded together. Some or all of the components may initially be in an uncured state, so that curing and post-curing steps are used in the fabrication procedure. The curing and post-curing steps are performed by heating the structure according to a temperature-time schedule specified to cure and, optionally, post-cure the organic components of the composite material.
Although the final fabricated structure may have exceptional performance, the fabrication operation may present challenging problems. One of the ongoing obstacles to the fabrication of laminated composite structures is a consequence of the differing coefficients of thermal expansion of the constituents of the composite material. For example, if two components having differing coefficients of thermal expansion are bonded together and then heated to elevated temperature for curing, thermal strains and stresses are created within the cured structure upon cooling. When there are multiple components with anisotropic coefficients of thermal expansion, the internal strains and stresses are even more complex. Internal strains and stresses can arise in other ways as well.
Whatever their origin, the internal strains and stresses usually have adverse effects on the performance of the composite material. They often cause deterioration of the bonds between the components and laminates. The deterioration is manifested in lower measured property values than would be otherwise expected, and/or by observed bond line failure mechanisms. There may also be a shifting of the ultimate stress between the two dissimilar materials to a lower strength, more remote surface.
In a specific case of interest to the inventors, an aircraft structural member is fabricated by preparing a skin structure sub-assembly made of a precured quartz fiber/cyanate ester resin laminate and bonding the skin structure sub-assembly to a substrate sub-assembly formed of a low density (less than 25 pounds per cubic foot), surface-sealed syntactic foam, using an epoxy structural adhesive. This composite structure is observed to preferentially fail at the quartz fiber/cyanate ester resin interface. In many instances, failure occurred as the structure was cooled from the fabrication temperature to room temperature. Other bonded assemblies survived for a time at room temperature, but later failed catastrophically at the quartz fiber/cyanate ester resin interface. This failure mechanism indicates that the full strength potential of the skin structure and the syntactic foam is not realized because of the high stress developed as a result of the differences in the coefficients of thermal expansion of the two bonded sub-assemblies.
There is a need for an improved fabrication technique for such laminated composite structures. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a fabrication technique for preparing a composite structure, and a composite structure prepared by the method. The strength of the structure according the invention is superior to that made by prior techniques. Failure of the structure in shear occurs in the syntactic foam, so that the strength potentials of the components are more nearly achieved. The finished composite structure differs from that according to the prior approach, but the difference is compatible with the overall requirements of the structure and also compatible with the processing required to otherwise achieve optimal properties of the structure.
In accordance with the invention, a method for making a composite structure includes furnishing a skin layer having a skin-layer surface and comprising a composite material of quartz fibers embedded in an uncured cyanate ester-resin matrix. The cyanate ester-resin matrix is curable at a first temperature and post-curable at a second temperature greater than the first temperature. The method further includes applying to the uncured skin-layer surface a transition layer of a first epoxy resin adhesive system, leaving an exposed transition-layer bonding surface. The skin layer and the transition layer together comprise a skin structure. The first epoxy resin adhesive system is curable at the first temperature and post-curable at the second temperature. The skin structure is cured at the first temperature and post-cured at the second temperature. Next, a bonding layer of a second epoxy resin adhesive system is applied to the exposed transition-layer bonding surface of the skin structure, leaving an exposed face of the bonding layer. The second epoxy resin adhesive system is curable at a third temperature no greater than the first temperature, and which is preferably the same as the first temperature. The method includes contacting a substrate to the exposed face of the bonding layer, and thereafter curing the second epoxy resin adhesive system at the third temperature.
More generally, a method for making a composite structure comprises the steps of furnishing a skin layer having a skin-layer surface and comprising a composite material of fibers embedded in an uncured organic matrix, wherein the matrix is curable at a first temperature and post-curable at a second temperature greater than the first temperature. A transition layer of a first curable resin is applied to the skin-layer surface, leaving an exposed transition-layer bonding surface. The first curable resin is curable at the first temperature and post-curable at the second temperature. The skin layer and the transition layer together comprise a skin structure. The skin structure is cured at the first temperature and post-cured at the second temperature. Next, a bonding layer of a second curable resin, curable at a third temperature no greater than the first temperature, is applied to the exposed transition-layer bonding surface of the skin structure, leaving an exposed face of the bonding layer. A substrate is contacted to the exposed face of the bonding layer, and thereafter the second curable resin is cured at the third temperature.
A composite structure according to the present invention comprises a skin layer having a skin-layer surface and comprising a composite material of fibers embedded in a matrix. The matrix is a cured form of a matrix precursor curable at a first temperature and post-curable at a second temperature greater than the first temperature. A transition layer of a co-cured and interdiffused first adhesive resin and the skin-layer matrix contacts the skin-layer surface. The first adhesive resin is a cured form of a first adhesive resin precursor curable at the first temperature and post-curable at the second temperature. A bonding layer of a second adhesive resin contacts and is bonded to the transition layer at a bonding surface, but is not substantially interdiffused therewith. The second adhesive resin is a cured form of a second adhesive resin precursor curable at a third temperature that is no greater than the first temperature. A substrate is bonded to the bonding layer.
Common to all of these approaches is the application of a transition layer of a first curable resin to the skin layer, prior to its curing. In the preferred approach, the transition layer is an epoxy r
Bridges Don J.
Cesena Rudy
Harrison Edward S.
Kuhl Paul C.
Melquist James L.
Alston & Bird LLP
Copenheaver Blaine
McDonnell Douglas Helicopter Co.
Paulraj Christopher
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