Patent Application
20120149802
This disclosure generally relates to fiber reinforced resin composites, and deals more particularly with a composite having fibers coated with a distortional resin to improve the mechanical performance of a composite structure.
In fiber reinforced composites, the efficiency of load transfer between the fiber and the surrounding matrix at the micro-scale level, directly affects the overall mechanical performance of the composite at the continuum level. The region of the matrix that may be substantially affected by the presence of fibers, sometimes referred to as the “inter-phase” region, is the interfacial area of the matrix directly surrounding the fiber. In composites, it is this inter-phase region that experiences high shear strain due to the mismatch in elastic stiffness between the fibers and the surrounding matrix.
While various resin matrix formulations have been developed to maximize the distortional capability of a polymer resin, formulations demonstrating higher performance potential may have limitations such as limited fluid resistance and less than desired prepreg handling characteristics such as insufficient tack and/or prepreg handling life. These problems may be partially addressed by modifying the chemistry of the bulk polymer resin forming the matrix, however these modifications may require development of specialized monomers or additives which can add to product cost. Moreover, while these specialized formulations and additives may improve fluid resistance of the matrix resin, they may reduce other performance properties of the composite.
Adequate load transfer between the fiber and the matrix may be particularly problematic in composites using a high temperature matrix reinforced with carbon fibers because of the relatively high thermal strains generated at the resin-fiber interface. These thermal strains may enhance micro-crack susceptibility which typically may result in the cured composite having less than desired mechanical properties.
Accordingly, there is a need for a fiber reinforced polymer resin composite that exhibits improved load transfer ability between the reinforcing fibers and the surrounding resin matrix, particularly where the fibers have a relatively high modulus and the matrix is formed of a high temperature resin. There is also a need for a method of making such composites that uses conventional bulk resins and avoids the need for resin additives or special resin formulations.
Reinforcing fibers in a composite are coated with a polymeric resin having a relatively high distortional deformation capability compared to that of the surrounding bulk polymer resin forming the matrix. The coating creates an energy dissipative, distortional inter-phase region surrounding the fibers that optimizes resin-fiber load transfer across fiber discontinuities or defects, thereby improving the mechanical properties of the composite. The process of coating the fibers with a high distortion resin may be performed prior to impregnation of the fibers with the bulk matrix resin, thus allowing current commercially available fibers to be utilized in existing prepreg production processes. Substantial improvements in the mechanical performance of current composite materials may be achieved through the distortional fiber coating, such as increased strength and/or strain as well as potential improvements in delamination and micro-crack resistance. The use of fibers coated with a distortional resin may also aid in mitigating adverse effects caused by excessive thermal strain generated at the resin-fiber interface between high modulus fibers such as, without limitation, carbon fibers, and a high temperature resin matrix. Composite structures employing reinforcing fibers coated with high distortional resins may result in optimized composite designs that may reduce weight and cost.
According to one disclosed embodiment, a method is provided of making a fiber reinforced polymer resin, comprising coating reinforcing fibers with a first polymeric resin, and embedding the coated fibers in a second polymeric resin. The distortional deformation capability of the first polymeric resin is greater than that of the second polymeric resin, and the first polymeric resin may be any of various resin chemistries, such as epoxies, which are specifically designed to exhibit high deformation capability. The fibers may have a high modulus in relation to the modulus of the first polymeric resin. The method further comprises selecting the fibers from the group consisting of carbon fibers, glass fibers, organic fibers, metallic fibers and ceramic fibers. The method also comprises applying a coating of a third polymeric resin over the coating of the first polymeric resin, wherein the third polymeric resin has a distortional deformation capability greater than the first polymeric resin but less than the second polymeric resin.
According to another embodiment, a method is provided for making a fiber reinforced polymer composite, comprising providing a polymeric resin matrix and providing fibers for reinforcing the resin matrix. The method further comprises embedding the fibers in the matrix, and forming a distortional inter-phase region between the fibers and the matrix for improving load transfer between the fibers and the matrix. Forming the inter-phase region includes coating the fibers with a polymeric distortional resin having at least one property different from the resin matrix. The at least one property is selected from the group consisting of fluid resistance, increased modulus, high temperature performance, processability, and handling properties. Embedding the fibers in the matrix includes impregnating the fibers with the matrix resin, and curing the matrix. Providing fibers includes selecting the fibers from the group consisting of carbon fibers, organic fibers, metallic fibers and ceramic fibers. Providing fibers for reinforcing the resin matrix includes providing two groups of fibers respectively having different moduli, and forming the distortional inter-phase region between the fibers and the matrix includes coating the fibers in each of the groups with differing polymeric resins each having a distortional deformation capability higher than the bulk matrix resin.
According to still another disclosed embodiment, a fiber reinforced resin composite comprises a polymeric resin matrix, reinforcing fibers held in the matrix, and a coating on the fibers for improving load transfer between the fibers and the matrix. The coating includes a polymeric resin having a distortional deformation capability greater than that of the resin matrix. The coating includes first and seconds layers of polymeric resin respectively having differing distortional deformation capabilities each greater than the distortional deformation capability of the resin matrix. The fibers are impregnated with the matrix resin and may include at least two groups thereof respectively having differing stiffnesses or strengths.
According to a further embodiment, a fiber reinforced resin composite comprises a polymeric resin matrix, reinforcing fibers held in the matrix, and an inter-phase region having a high distortional deformation capability relative to the resin matrix. The inter-phase region is defined by at least a first polymeric resin coating on the fibers. The inter-phase region may be defined by a second polymeric resin coating over the first polymeric coating. The first polymeric resin coating may be a high temperature resin.
Referring to
The distortional deformation capability of the resin coating 26, which may be expressed in terms of von Mises strain performance, is high relative to the bulk resin matrix 22 in order to achieve optimum fiber-resin load transfer capability between the fibers 24 and the surrounding resin matrix 22. The von Mises strain or stress is an index derived from combinations of principle stresses at any given point in a material to determine at which point in the material, stress will cause failure. While the bulk polymer resin forming the matrix 22 may have a distortional capability lower than that of the fibers 24, exhibited by a lower von Mises strain performance, the overall mechanical performance of the composite 20 may be significantly improved due to the creation of a distinct distortional inter-phase region 25 surrounding each of the fibers 24. The inter-phase region 25 is the region in the composite 20 that experiences a high shear strain due to the mismatch between the elastic stiffness of the fibers 24 and that of the matrix 22. The distortional or deviatoric response of the polymer resin matrix 22 to an applied force may be viewed as an abrupt shear transformation or cooperative motion of a specific volume or segment of the polymer chain responding to a strain bias. The distortional resin coating 26 on the fibers 24 may also be beneficial in mitigating the effects of transverse micro-cracks created by excessive thermal strains generated in the inter-phase region 25, particularly in composites 20 using a high temperature resin in the matrix 22.
The distortional resin coating 26 may be similar to the polymeric resins described in U.S. Pat. No. 7,745,549, the entire disclosure of which patent is incorporated by reference herein. The polymeric resins disclosed in the above mentioned US Patent exhibit increased distortional deformation, and/or decreased dilatation load, as expressed within the von Mises strain relationship. As discussed in this prior US Patent, fiber performance may be limited by low matrix-critical distortional capability of the thermoset resins used in known composites. The composite polymer matrix disclosed in this prior patent exhibits improved (i.e. increased) distortional deformation and/or decreased (i.e. lower) dilatation load, increasing von Mises strain and providing enhanced composite mechanical performance.
It is hypothesized that that a resin with improved distortional capability is able to transfer load around microscale flaws in the fiber, which can be considered failure initiation sites in the fiber, along the longitudinal axis of the fiber when the fiber experiences a load. This ability to redistribute the load around the flaws may allow the fiber to continue to sustain load without failure. The molecular basis for a polymer matrix ability to undergo a distortional response to an applied force is theorized as being due to a cooperative motion of a specific volume or segment of the polymer chain. Therefore, molecular structures which are able to conformally adjust with applied force will enhance the polymer's ability to undergo and increase its distortional response.
As previously discussed, in fiber reinforced composites, the efficiency of load transfer between the reinforcing fibers 24 and the surrounding matrix 22 at the microscale level substantially affects the overall mechanical performance of the composite 20. The critical region of the matrix 20 affected by the presence of the fibers 24, is the inter-phase region 25. This inter-phase region 25 experiences relatively high shear strain due to the mismatch between the relatively high elastic stiffness of the fibers 24 and the relatively low elastic stiffness of the surrounding matrix 22.
The polymeric resin forming the matrix 22 may be any suitable commercial or custom resin system having the desired physical properties which are different from those of the distortional resin coating 26. These differences in physical properties result in the distortional resin coating 26 having a higher distortional capability than that of the matrix 22. For example and without limitation, typical physical properties of the bulk polymeric resin used in the matrix 22 which may affect its distortional capability include but are not limited to: superior fluid resistance, increased modulus, increased high temperature performance, improved process ability and/or handling properties (such as the degree of tack and tack life) relative to the distortional resin coating 26.
Where the composite 20 is produced from a prepreg, the polymeric distortional resin coating 26 may be applied to the fibers 24 prior to impregnation of the fibers 24 with the bulk resin forming the matrix 22. By impregnating the fibers 24 after the coating 26 is applied, a variety of well-known processes may be used to coat the fibers 24. Following curing, the resin impregnated, coated fibers 24 become embedded in the surrounding matrix 22. The composite 20 may also be produced by infusing a distortional resin coated fiber preform (not shown) with the matrix resin. During curing of the resin infused preform, the distortional resin coated fibers become embedded in the matrix 22.
Referring to
Attention is now directed to
In one embodiment, at step 36, the coated fibers 24 are impregnated with the bulk matrix resin, and at step 37 the impregnated, coated fibers 24 are formed into to a prepreg which may comprise prepreg tows, prepreg tape or a prepreg fabric. At 38, a composite structure is laid up and formed using the prepreg. In another embodiment, as shown in step 40, the resin coated fibers 24 are used to produce a dry or substantially dry fiber preform which, at step 42, is infused with a bulk matrix resin using, for example, a vacuum assisted resin transfer molding process. Finally, at 44, the structure is cured. During curing, the distortional resin coated fibers 24 are embedded in the surround matrix 22, resulting in the previously described inter-phase region 25 between the fibers 24 and the matrix 22.
In some applications, it may be necessary to control migration of the distortion resin coating 26 during the curing process. One solution to this problem involves formulating the distortional polymeric resin coating 26 to have a viscosity that is higher than that of the bulk resin forming the matrix 22. During curing, the distortional resin 26 is retained on the fibers' surface due to its higher viscosity and lessened ability to flow. Another solution to the problem consists of exposing the distortional coated fibers 24 to an appropriate elevated temperature after the fibers 24 are coated in order to slightly cross link (cure) the distortional resin, thereby increasing its viscosity and its adherence to the fibers 24.
Referring next to
Each of the processes of method 46 may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on.
As shown in
The apparatus embodied herein may be employed during any one or more of the stages of the production and service method 46. For example, components or subassemblies corresponding to production process 54 may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft 48 is in service. Also, one or more apparatus embodiments may be utilized during the production stages 54 and 56, for example, by substantially expediting assembly of or reducing the cost of an aircraft 48. Similarly, one or more apparatus embodiments may be utilized while the aircraft 48 is in service, for example and without limitation, to maintenance and service 62.
Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.
