Patent Application
20130119191
The present invention generally relates to load-bearing structures and to processes for their production. More particularly, this invention is directed to the use of composite materials in the fabrication of load-bearing structures, as an example, brackets used in aircraft engines.
The maturation of composite technologies has increased the opportunities for the use of composite materials in a wide variety of applications, including but not limited to aircraft engines such as the GE90® and GEnx® commercial engines manufactured by the General Electric Company. Historically, the fabrication of components from composite materials has been driven by the desire to reduce weight, though increases in metal costs have also become a driving factor for some applications.
Composite materials generally comprise a fibrous reinforcement material embedded in a matrix material, such as a polymer or ceramic material. The reinforcement material serves as the load-bearing constituent of the composite material, while the matrix material protects the reinforcement material, maintains the orientation of its fibers and serves to dissipate loads to the reinforcement material. Polymer matrix composite (PMC) materials are typically fabricated by impregnating a fabric with a resin, followed by curing or solidification of the resin. Resins for matrix materials of PMCs can be generally classified as thermosets or thermoplastics. Thermoplastic resins are generally categorized as polymers that can be repeatedly softened and flowed when heated and hardened when sufficiently cooled due to a physical rather than chemical change. Notable example classes of thermoplastic resins include nylons, thermoplastic polyesters, polyaryletherketones, and polycarbonate resins. Specific examples of high performance thermoplastic resins that have been contemplated for use in aerospace applications include, polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI) and polyphenylene sulfide (PPS). In contrast, once fully cured into a hard rigid solid, thermoset resins do not undergo significant softening when heated, but instead thermally decompose when sufficiently heated. Notable examples of thermoset resins include epoxy and polyester resins. A variety of fibrous reinforcement materials have been used in PMCs, for example, carbon (e.g., AS4), glass (e.g., S2), polymer (e.g., Kevlar®), ceramic (e.g., Nextel®) and metal fibers. Fibrous reinforcement materials can be used in the form of relatively short chopped fibers or long continuous fibers, the latter of which are often used to produce a “dry” fabric or mat. PMC materials can be produced by dispersing short fibers in a matrix material, or impregnating one or more fiber layers (plies) of dry fabrics with a matrix material.
Whether a PMC material is suitable for a given application depends on its matrix and reinforcement materials, the requirements of the particular application, and the feasibility of fabricating a PMC article having the required geometry. Due to their considerable potential for weight savings, various applications have been explored for PMCs in aircraft gas turbine engines. However, a challenge has been the identification of material systems that have acceptable properties yet can be produced by manufacturing methods to yield a cost-effective PMC component. In particular, it is well known that aircraft engine applications have high performance mechanical requirements, for example, strength and fatigue properties (necessitated by vibrations in the engine environment), as well as high temperature properties, chemical/fluid resistance, etc. Though considerable weight savings could be realized by fabricating aircraft engine brackets from PMC materials, performance requirements as well as the size, variability and complexity of such brackets have complicated the ability to cost-effectively produce brackets from these materials. For example, the use of traditional thermoset resins to produce PMC brackets has been generally viewed as cost prohibitive due to the labor-intensive process and long manufacturing cycle times involved with thermosets, as well as the large number of relatively small brackets having many different part configurations. On the other hand, PMCs formed with thermoplastic matrix materials are limited by their tendency to soften and lose strength at elevated temperatures.
Another complication is the type of reinforcement system required by PMC materials in aircraft engine applications. Generally, to realize a significant level of weight savings through the use of thermoset or thermoplastic PMC materials, brackets would require the use of continuous fiber-reinforced PMC materials to enable their cross-sections to be minimized while simultaneously achieving the high performance mechanical requirements (particularly strength and fatigue properties) dictated by aircraft engine applications. However, hand lay-up processes involved in the use of continuous fiber reinforcement materials further complicate the ability to produce a wide variety of relatively small brackets having complex shapes. On the other hand, chopped fiber reinforcement systems, whether in a thermoplastic or thermoset resin matrix, are not an ideal solution due to their lower mechanical performance. In particular, the lower strength of PMC components reinforced with chopped fibers necessitates the fabrication of a relatively thick and heavy bracket. Furthermore, chopped fiber systems are often processed using net shape molding methods, which enable complex shapes to be formed. However, because there is a large number of brackets that have different shapes on aircraft engines, the tooling cost associated with an individual mold being required for each unique bracket generally prohibits this manufacturing approach.
The present invention provides load-bearing structures constructed from PMC materials, and processes for their production. Notable but nonlimiting examples of such structures include the various types of brackets used in aircraft engines that can have relatively complex shapes.
According to a first aspect of the invention, a process of fabricating a load-bearing structure includes producing at least a first shaped panel that has a substantially constant cross-sectional thickness and has at least first and second portions that lie in different planes and are interconnected by at least a first bend therebetween. The first shaped panel is formed by thermoforming a polymer matrix composite material comprising a thermoplastic resin reinforced with a continuous fiber reinforcement material. The first shaped panel is then machined to alter its shape. The machining step may directly produce the load-bearing bracket from the first shaped panel. Alternatively, the machining step may produce at least a first subcomponent from the first shaped panel, and the process further entails a joining operation with the result that the first subcomponent forms part of the load-bearing bracket. Yet another alternative is for the machining step to produce multiple separate subcomponents from the first shaped panel, at least some of which then undergo a joining operation to form the load-bearing bracket. The resulting bracket can then be installed on an aircraft engine to secure a component to the aircraft engine.
A second aspect of the invention is a process that includes producing at least first and second flat panels of a polymer matrix composite material comprising a thermoplastic resin reinforced with a continuous fiber reinforcement material, in which each of the flat panels has a substantially constant cross-sectional thickness and is flat so as to lie in a single plane. At least one of the flat panels is then thermoformed to form at least a first shaped panel having a substantially constant cross-sectional thickness and having at least first and second portions that lie in different planes and are interconnected by at least a first bend therebetween. The first shaped panel is then machined to alter its shape and produce at least a first subcomponent therefrom. A load-bearing bracket is then produced by joining the first subcomponent to a second subcomponent defined by the second flat panel or a second shaped panel produced by thermoforming the second flat panel, after which the load-bearing bracket can be installed on an aircraft engine to secure a component to the aircraft engine.
Additional aspects of the invention include load-bearing brackets that are produced by the steps of one of the processes described above. However, more generally, the invention broadly encompasses aircraft engine brackets that are formed of a polymer matrix composite material that comprises a continuous fiber reinforcement material in a thermoplastic resin matrix material. As a more particular example, such an aircraft engine bracket includes at least first and second subcomponents that are joined together to form the bracket. Each subcomponent is formed of a polymer matrix material comprising a continuous fiber reinforcement material in a thermoplastic resin matrix material, and each subcomponent has a substantially constant cross-sectional thickness. At least one of the subcomponents is machined from at least one shaped panel that was thermoformed to have at least first and second portions that lie in different planes and are interconnected by at least a first bend therebetween.
A significant advantage of this invention is the ability to produce and utilize a load-bearing structure in applications such as aircraft engines, which greatly benefit from weight savings but simultaneously have demanding mechanical and environmental conditions. The invention enables the fabrication and use of thermoplastic PMC materials in a manner that manufacturing and materials costs and/or weight can be minimized without compromising the load-bearing functionality of the structure.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
The present invention will be described in terms of composite load-bearing structures that, though capable of being adapted for use in a wide range of applications, are particularly well suited as brackets whose primary purpose is to support or secure various components of aircraft engines, for example, components within the fan sections of high-bypass gas turbine engines. Particularly notable examples are brackets that are mounted on the exterior of the fan case and support components such as tubes, wiring harnesses, oil tanks, etc. However, various other load-bearing structures and various other applications to which the present invention could be applied are also within the scope of the invention.
The present invention provides a process by which brackets that exhibit mechanical, chemical and thermal properties (including strength, fatigue resistance, maximum temperature capability, chemical/fluid resistance, etc.) that are suitable for aircraft engine applications and yet can be produced in a cost-effective manner. The invention involves producing components and/or subcomponents that are fabricated from PMC materials and undergo thermoforming to produce shaped panels that have what will be referred to as simple shapes. As used herein, a “simple shape” refers to a shape that can be formed from a single flat panel to have one or more bends that are present between portions of the shaped panel, and the shaped panel has a substantially constant cross-sectional thickness throughout its portions and bends. Three representative but nonlimiting examples of shaped panels that can be produced with this invention are shown in
Preferred PMC materials for use with this invention have a thermoplastic matrix material that is reinforced with continuous fibers, which may be individual fibers or fiber tows arranged parallel (unidirectional) within the matrix material, or individual fibers or fiber tows arranged to have multiple different orientations (e.g., multiple layers of unidirectional fibers or fiber tows to form a biaxial or triaxial architecture) within the matrix material, or individual fibers or fiber tows woven to form a mesh or fabric within the matrix material. The fibers, tows, meshes or fabrics can be arranged to define a single ply within the PMC or any suitable number of plies. Particularly suitable thermoplastic matrix materials include polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI) and polyphenylene sulfide (PPS), and particularly suitable continuous fiber materials include carbon (e.g., AS4), glass (e.g., S2), polymer (e.g., aramid, such as Kevlar®), ceramic and metal fibers. A preferred thermoplastic matrix material is believed to be PEEK, and a preferred reinforcement material is believed to be continuous carbon fibers. However, it is foreseeable that other suitable matrix and reinforcement materials could be used or later developed for use with the present invention. Suitable fiber contents for the PMC materials of this invention can vary widely, though it is believed that the fiber content should be at least 35 percent by volume and not more than 75 percent by volume, with a preferred range believed to be about 50 to about 65 percent by volume.
As noted above, processes of this invention generally start with a flat panel of the desired PMC material, for example, the flat panel 22 represented in
The flat panel 22 is then thermoformed to define a shaped panel that has a simple shape (for example, as represented in
If the process of producing a shaped panel involves the creation of a ply stack as described above, it is also within the scope of the invention to simultaneously consolidate and shape the ply stack to produce a shaped panel. For example, the ply stack can be fed into a thermoforming press, where the ply stack is simultaneously consolidated and thermoformed to yield the desired shape of the shaped panel (e.g., the shaped panels of
As should be evident from
While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.
