The invention relates to composite resin matrix structures and a method of making composite resin matrix structures, such as those used in aerospace applications. More particularly, the invention relates to a composite resin matrix structure having stiffness appropriate for aerospace and other applications but having improved damping characteristics that resist damage caused by acoustics and vibration.
The use of composite structures in the aerospace industry has become more and more prevalent due to the desirable properties of composites, especially low weight, high strength and stiffness, and resistance to corrosion, among other properties. Composite materials are now being used for aircraft wings, horizontal and vertical stabilizers, nose and tail cones, and elements of the aircraft""s primary structure. Composites are also being used for secondary elements of aircraft, such as internal floor panels, wall panels, and similar structural elements. The advantageous properties of composites have increased aircraft performance benefits, including increased range, decreased fuel consumption, and greater payload. Added performance benefits guide the use of composites throughout the aerospace industry.
As advancements in composite design have progressed, the composites have improved in strength and stiffness. Stiffness of the composite is important since stiffness is generally related to the strength of the material and provides better performance under aerodynamic loads. However, as composites have been successfully stiffened, problems with acoustics and vibrations have remained an issue. In fact, increased stiffness of the composites often increases the susceptibility of the composite to fatigue from vibration.
Stiffened composites often do not adequately attenuate acoustics and vibrations associated with buffet, flow-induced noise, shock/boundary layer interaction, turbulence, and mechanical noise. Acoustic forces act to fatigue composite structures over time and may lead to cracks during the service life of the composites structures.
Because premature cracking of composite materials on aerospace vehicles leads to costly repairs and maintenance schedules, there has been much research into the enhancement of damping in composite structures, particularly over the past 15 years. One technique, known as interleaving, involves co-curing viscoelastic layers into a composite laminate. Interleaving has been one of the more common approaches to increase damping, but it does require some special processing when applied to larger built-up structures.
On a macro-mechanical level, the damping can be maximized through inter-laminar stresses by variation of fiber orientation (more cross plies), increased stress (anisotropic) bending-twisting coupling, or wavy-fiber placement.
On the micro-mechanical level, damping has been increased through variation of fiber-volume-fraction, and fiber-aspect-ratio, coated fibers and woven fabric polymer matrix composites.
Despite the advances in micro-mechanical and macro-mechanical damping of composite materials, acoustic fatigue is still problematic. It is, therefore, desired to provide a composite material with integral damping characteristics capable of protecting the composite from fatigue while retaining stiffening characteristics that make the composite desirable for use in aerospace applications. It is further desired to provide a composite material that is economically fabricated and, most preferably, customizable for specific applications.
One advantageous embodiment of the invention is a composite laminate structure formed from at least one high-strength, high-stiffness fiber-resin composite structural lamina laminated to at least one fiber-resin composite damping lamina, wherein the resin matrix of the structural lamina resides below its glassification temperature (Tg) and wherein the resin matrix of the damping lamina resides above its glassification temperature during normal use temperatures, thereby providing high-strength lamina in combination with pliable, high-damping lamina that attenuates acoustics and vibrations throughout the composite structure.
The damping laminae are combined with structural laminae to form a composite laminate structure using techniques known in the art of composite laminate formation. The resulting laminate has overall strength and rigidity characteristics that are the same or insubstantially inferior to otherwise similar structural composite laminates of the art. However, the invented laminate exhibits damping characteristics that are dramatically improved compared to prior structural composite laminates. For instance, with the practice of this invention, composite laminates have been formed that provide a ten fold increase in damping over laminates that were not produced in accordance with this invention. The improved damping occurs with almost no loss of strength in the overall laminate.
The invention may alternatively be viewed as a composite fiber-resin laminate in which the resin matrix Tg of some lamina is substantially higher than the resin matrix Tg of other lamina within the same laminate.
The invention may further alternatively be viewed as a composite fiber-resin laminate in which the resin matrix of at least one lamina exhibits viscoelastic behavior while the resin matrix of the other laminae exhibits glassy behavior.
The invention is based upon the discovery that damping laminae comprising a resin matrix having a Tg below the use temperature of the laminate have dramatically enhanced acoustic attenuation characteristics compared to similar laminae having resin matrices with Tg""s above their use temperature. When the damping laminae are combined with a structural laminae (a laminae having a Tg well above its use temperature), the resulting laminate has stiffened structural properties that provide strength to the laminate as well as damping properties that attenuate acoustics and vibrations throughout the laminated structure.
Each of the lamina of the laminate comprise multiple plies of a high-strength fiber embedded within a resin matrix material. The fibers may be chopped, semi-continuous, or continuous and are typically carbon fibers but may also be other high-strength fibers, such as aramid fibers. If continuous, the fibers may have a unidirected orientation or may comprise a dual or triaxial fabric weave. The thermosetting matrix resins of the laminate may be epoxy resins, blends of epoxy resins, or other thermosetting resins that preferably have a curing temperature lower than about 400xc2x0 F., and that are thermally stable to at least about 200xc2x0 F.
According to one embodiment of the invention, the Tg of the damping lamina resin matrix is lowered by addition of a plastisizing agent to the resin prior to cure of the resin. Plastisizing agents are added to the thermosetting polymer, such as an epoxy, in an amount between about 12 wt % and about 35 wt %, and preferably about 12.5 wt % to about 17 wt %, by total weight of the resin composition. The plastisizer acts to lower the Tg of the resin. For instance, the intrinsic Tg of most epoxies is above 250xc2x0 F., but addition of plastisizer to the epoxy in an amount between about 12 wt % and 35 wt % depresses the Tg of the epoxy below room temperature.
According to another embodiment of the invention, a commercially available epoxy resin having a depressed Tg temperature is acquired and used as the matrix resin for the damping lamina. Using the commercial epoxy, a damping lamina may be prepared as known in the art. An exemplary commercial epoxy for use with the damping lamina is Duralco(trademark)4538N epoxy resin, available from Cotronics Corp, Brooklyn, N.Y.
Optimal damping has been found to occur at a temperature that is between about 50xc2x0 F. and 100xc2x0 F. above the Tg of the resin. So, plasticizers are preferably used to depress the Tg of the damping laminae resins to between 50xc2x0 F and 100xc2x0 F. below the predicted end use temperature of the laminate. Alternatively, commercial epoxies having Tg of between 50xc2x0 F. and 100xc2x0 F. below the predicted end use temperature of the laminate are used. Because the Tg may be manipulated by the amount of plastisizer used in the damping resin, laminates may be customized for optimum performance at their particular end use temperature.
Other than manipulation of the resin Tg, the laminate may be layed-up, impregnated, cured, and formed according to methods used to fabricate traditional fiber-resin laminates. Therefore, no special fabrication equipment is required to practice the invention.
The laminates of the invention are particularly suited for aerospace applications due to their combined qualities of stiffness and vibration damping. The laminates may also be useful in other applications, such as the formation of boat hulls, automotive panels, external and internal architectural components, and any other uses requiring high-strength, high-damping composite structures.