Patent Publication Number: US-10780616-B2

Title: Methods of forming composite structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional and claims priority to U.S. patent application Ser. No. 14/452,619 filed Aug. 6, 2014 for “COMPOSITE STRUCTURE AND METHOD OF FORMING THEREOF”, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The field of the present disclosure relates generally to composite structures and, more specifically, to filler materials for composite structures that facilitate restricting delamination of the composite structure. 
     Cracking of integrally stiffened composite structures, especially in aerospace applications, often initiates in a radius filler (i.e., noodle) located at an interface between a skin and integral stiffening elements of the composite structures. At least some known radius fillers are formed at least partially from an amount of resin. Cracks in the resin may form during manufacture of the composite structures as a result of improper tooling, improper handling of tools, and/or residual tensile strain. For example, residual tensile strain in composite structures may form as a result of a mismatch between the coefficients of thermal expansion creating a strain environment in the radius filler that exceeds the critical cracking strain of the resin. The geometry of the structure surrounding the radius filler creates a three-dimensional constraint to shrinkage upon cooling of the resin after it has been cured and hardened. 
     Exemplary radius filler materials include, but are not limited to, pre-impregnated (i.e., prepreg) composite materials (e.g., layered strips and/or rolled prepreg composite material), and/or pure resin. In at least some known composite structures, crack propagation in the radius filler can cause delamination of the plies in adjacent laminated joints. More specifically, crack propagation in the radius filler may initiate degradation of the laminated joints. While limiting the formation of cracks in the radius filler would ensure the integrity of the laminated joints, preventing cracks from forming entirely is generally difficult, if not impossible. Moreover, the difficulty in limiting crack formation in radius fillers increases as composite structures are fabricated in increasingly large sizes. As such, there is a need for systems and methods that ensure cracks in a radius filler do not initiate degradation of laminated joints. 
     BRIEF DESCRIPTION 
     In one aspect, a composite structure is provided. The composite structure includes a plurality of components coupled together forming a joint, wherein the plurality of components are oriented such that a gap is defined at least partially therebetween. A filler structure is positioned in the gap, and the filler structure includes a closed cell foam core. 
     In another aspect, a method of forming a composite structure is provided. The method includes coupling a plurality of components together forming a joint, wherein the plurality of components are oriented to form a radius gap therebetween. The method also includes forming a filler structure that includes a closed cell foam core, positioning the filler structure in the radius gap, and applying at least one of heat or pressure to the plurality of components and the filler structure. 
     The features, functions, and advantages that have been discussed can be achieved independently in various implementations of the present disclosure or may be combined in yet other implementations further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram of an exemplary aircraft production and service method. 
         FIG. 2  is a block diagram of an exemplary aircraft. 
         FIG. 3  is a schematic illustration of an exemplary composite structure that may be used in the aircraft shown in  FIG. 2 . 
         FIG. 4  is an enlarged schematic illustration of the composite structure shown in  FIG. 3  taken along Area  4 . 
         FIG. 5  is an enlarged schematic illustration of the closed cell foam core shown in  FIG. 4 . 
         FIG. 6  is a schematic flow diagram illustrating an exemplary sequence of process steps of fabricating a closed cell foam core that may be used in the composite structure shown in  FIG. 3 . 
         FIG. 7  is a schematic flow diagram illustrating an alternative sequence of process steps of fabricating the closed cell foam core shown in  FIG. 6 . 
         FIG. 8  is a flow diagram of an exemplary method of forming a composite structure. 
     
    
    
     DETAILED DESCRIPTION 
     The implementations described herein relate to composite structures including a radius filler structure that facilitates restricting delamination in radius gaps (i.e., noodle regions) in the composite structures. In the exemplary implementation, the filler structure includes a closed cell foam core. The closed cell foam core includes a plurality of core cells including side walls and void spaces defined by the side walls. The closed cell foam core is fabricated from a material that absorbs strain energy by enabling the side walls to yield plastically in response to hydrostatic tensile forces. Specifically, the filler structure is positioned in the radius gap of the composite structure, and the closed cell foam core has physical properties selected to ensure the structural integrity of the composite structure is substantially maintained during manufacture thereof. As such, the filler structure described herein does not include resin such that crack propagation in the resin cannot initiate degradation of the laminated joint. 
     Referring to the drawings, implementations of the disclosure may be described in the context of an aircraft manufacturing and service method  100  (shown in  FIG. 1 ) and via an aircraft  102  (shown in  FIG. 2 ). During pre-production, including specification and design  104  data of aircraft  102  may be used during the manufacturing process and other materials associated with the airframe may be procured  106 . During production, component and subassembly manufacturing  108  and system integration  110  of aircraft  102  occurs, prior to aircraft  102  entering its certification and delivery process  112 . Upon successful satisfaction and completion of airframe certification, aircraft  102  may be placed in service  114 . While in service by a customer, aircraft  102  is scheduled for periodic, routine, and scheduled maintenance and service  116 , including any modification, reconfiguration, and/or refurbishment, for example. In alternative implementations, manufacturing and service method  100  may be implemented via vehicles other than an aircraft. 
     Each portion and process associated with aircraft manufacturing and/or service  100  may be performed or completed 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 venders, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 2 , aircraft  102  produced via method  100  may include an airframe  118  having a plurality of systems  120  and an interior  122 . Examples of high-level systems  120  include one or more of a propulsion system  124 , an electrical system  126 , a hydraulic system  128 , and/or an environmental system  130 . Any number of other systems may be included. 
     Apparatus and methods embodied herein may be employed during any one or more of the stages of method  100 . For example, components or subassemblies corresponding to component production process  108  may be fabricated or manufactured in a manner similar to components or subassemblies produced while aircraft  102  is in service. Also, one or more apparatus implementations, method implementations, or a combination thereof may be utilized during the production stages  108  and  110 , for example, by substantially expediting assembly of, and/or reducing the cost of assembly of aircraft  102 . Similarly, one or more of apparatus implementations, method implementations, or a combination thereof may be utilized while aircraft  102  is being serviced or maintained, for example, during scheduled maintenance and service  116 . 
     As used herein, the term “aircraft” may include, but is not limited to only including, airplanes, unmanned aerial vehicles (UAVs), gliders, helicopters, and/or any other object that travels through airspace. Further, in an alternative implementation, the aircraft manufacturing and service method described herein may be used in any manufacturing and/or service operation. 
       FIG. 3  is a schematic illustration of an exemplary composite structure  200  that may be used in aircraft  102  (shown in  FIG. 2 ), and  FIG. 4  is an enlarged schematic illustration composite structure  200  taken along Area  4 . In the exemplary implementation, composite structure  200  includes a plurality of components  204  coupled together to form a T-joint  206 . Components  204  include a skin panel  208 , a plank  210  coupled to skin panel  208 , a first stiffener  212  coupled to plank  210 , and a second stiffener  214  coupled to plank  210 . Components  204  are oriented such that a radius gap  216  is defined between plank  210  and first and second stiffeners  212  and  214 . More specifically, radius gap  216  extends along a length L of composite structure  200  and is defined at least partially by opposing complementary bent portions  218  of first and second stiffeners  212  and  214 . In an alternative implementation, first and second stiffeners  212  and  214  may be coupled directly to skin panel  208 . Moreover, alternatively, components  204  may have any configuration such that a joint defines a radius gap therebetween. 
     Composite structure  200  also includes a filler structure  220  positioned in radius gap  216 . Referring to  FIG. 4 , filler structure  220  includes a closed cell foam core  222  including a plurality of core cells  224 . Each core cell  224  includes side walls  226 , and void spaces  228  defined by side walls  226  and substantially filled with air. As such, filler structure  220  does not include an amount of resin. Moreover, as will be described in more detail below, closed cell foam core  222  is fabricated from a material such that side walls  226  yield plastically in response to hydrostatic tensile forces  230  applied to and by composite structure  200 . Specifically, void spaces  228  facilitate enabling closed cell foam core  222  to absorb energy and facilitate defining a porosity within closed cell foam core  222 . The porosity of closed cell foam core  222  is selected such that closed cell foam core  222  has a predetermined energy absorbing capability and bulk compression stiffness. For example, in the exemplary implementation, closed cell foam core  222  includes a porosity within a range defined between about 20 percent and about 40 percent by volume of closed cell foam core  222 . 
     Closed cell foam core  222  may be fabricated from any material that enables composite structure  200  to function as described herein. The material used to fabricate closed cell foam core  222  is selected based on whether the material includes certain physical properties at predetermined levels. Exemplary physical properties include, but are not limited to, bulk modulus, surface energy, and coefficient of thermal expansion. For example, the bulk modulus is selected to facilitate limiting deformation of closed cell foam core  222  when composite structure  200  is exposed to elevated pressures during manufacture thereof, the surface energy is selected such that closed cell foam core  222  remains coupled to components  204 , and the coefficient of thermal expansion is selected such that the structural integrity of composite structure  200  is substantially maintained when exposed to changing environmental conditions. In the exemplary implementation, closed cell foam core  222  is fabricated from a silicone-based material such as RTV566 manufactured by Momentive Performance Materials Holdings, Inc. of Albany, N.Y. 
       FIG. 5  is an enlarged schematic illustration of closed cell foam core  222 . As will be described in more detail below, at least one of heat or pressure are applied to components  204  to facilitate forming composite structure  200  (each shown in  FIG. 3 ). In general, hydrostatic tensile forces  230  (shown in  FIG. 4 ) are hydrostatic due to the inability of filler structure  220  to contract upon cooling from elevated temperatures during formation of composite structure  200 . As such, side walls  226  locally stretch (i.e., are unable to contract) at narrowed regions  227  resulting in post-yield plastic deformation thereof. More specifically, side walls  226  are relatively thin membranes such that stretching side walls  226  is not hydrostatic. Rather, closed cell foam core  222  is fabricated from material that enables side walls  226  to locally yield at narrowed regions  227  to facilitate strain energy absorption therein. 
       FIG. 6  is a schematic flow diagram illustrating an exemplary sequence  232  of process steps of fabricating closed cell foam core  222 , and  FIG. 7  is a schematic flow diagram illustrating an alternative sequence  234  of process steps of fabricating closed cell foam core  222 . In the exemplary implementation, closed cell foam core  222  is fabricated by pouring a quantity of liquefied closed cell foam material (not shown) into a mold  236 . Mold  236  includes an interior cavity  238  that receives the quantity of closed cell foam material and that has a shape substantially similar to a final desired shape of closed cell foam core  222 . The quantity of closed cell foam material is then cured in a first processing step  240  such that closed cell foam core  222  has a cross-sectional shape substantially similar to radius gap  216  when it is formed. Moreover, first processing step  240  facilitates forming void spaces  228  in closed cell foam core  222 . For example, first processing step  240  facilitates forming void spaces  228  by at least one of introducing a blowing agent into the quantity of closed cell foam material, or by fabricating closed cell foam core  222  via condensation polymerization. Exemplary blowing agents include either physical or chemical blowing agents such as, but not limited to, carbon dioxide, pentane, and carbonate materials. 
     Referring to  FIG. 7 , closed cell foam core  222  is fabricated by pouring a quantity of closed cell foam material into a mold  242 . Mold  242  includes an interior cavity  244  that receives the quantity of closed cell foam material and that has any shape that enables sequence  234  to function as described herein. For example, in the exemplary implementation, interior cavity  244  has a substantially rectangular cross-sectional shape. The quantity of closed cell foam material is then cured in first processing step  240 , as described above, such that an intermediate closed cell foam core  246  is formed. A size of intermediate closed cell foam core  246  is reduced in a second processing step  248 , such as cutting, milling, or machining. As such, second processing step  248  is implemented to form intermediate closed cell foam core  246  into closed cell foam core  222  having a cross-sectional shape substantially similar to radius gap  216 . Alternatively, a plurality of intermediate closed cell foam cores  246  may be fabricated in mold  242 , coupled together, and subsequently reduced in size via second processing step  248  to form closed cell foam core  222 . Moreover, alternatively, a plurality of closed cell foam cores  246  of varying shapes may be fabricated and coupled together in a predetermined orientation to form closed cell foam core  222 . 
       FIG. 8  is a flow diagram of an exemplary method  300  of forming a composite structure, such as composite structure  200 . Method  300  includes coupling  302  components  204  together to form T-joint  206 , wherein components  204  are oriented to form radius gap  216  therebetween. A filler structure  220  is formed  304  that includes closed cell foam core  222 . Method  300  also includes positioning  306  filler structure  220  in radius gap  216 , and applying  308  at least one of heat or pressure to components  204  and filler structure  220 . The heat and/or pressure may be applied via a vacuum bagging process, and/or composite structure  200  may be placed in an autoclave (not shown). 
     In some implementations, forming  304  filler structure  220  includes pouring a quantity of closed cell foam material into mold  236  having a cross-sectional shape substantially similar to a cross-sectional shape of radius gap  216 , and curing the quantity of closed cell foam material in mold  236 . Alternatively, forming  304  includes pouring a quantity of closed cell foam material into mold  242 , curing the quantity of closed cell foam material in mold  242  to form intermediate closed cell foam core  246 , and forming intermediate closed cell foam core  246  such that filler structure  220  has a cross-sectional shape substantially similar to a cross-sectional shape of radius gap  216 . Forming intermediate closed cell foam core  246  includes at least one of cutting, milling, or machining intermediate closed cell foam core  246 . 
     Forming  304  filler structure  220  also includes fabricating closed cell foam core  222  from a closed cell foam material having a bulk modulus that facilitates limiting deformation of filler structure  220  when the pressure is applied  308 , and fabricating closed cell foam core  222  from a silicone-based material. In one implementation, forming  304  filler structure  220  includes forming closed cell foam core  222  including side walls  226  configured to yield plastically in response to hydrostatic tensile forces  230 , and forming void spaces  228  defined by side walls  226 . The void spaces  228  are substantially filled with air. 
     In some implementations, forming  304  filler structure  220  includes forming closed cell foam core  222  including a porosity within a range defined between about 20 percent and about 40 percent by volume of closed cell foam core, and forming  304  filler structure  220  that does not include an amount of resin. 
     The implementations described herein relate to filler structures that facilitate restricting delamination of a composite structure from initiating in a noodle region of the composite structure. The filler structure is fabricated from a closed cell foam core that includes side walls that yield plastically in response to hydrostatic tensile forces. For example, the side walls yield when the hydrostatic tensile forces are applied during manufacture of the composite structure, but the filler structure has a bulk modulus that facilitates limiting deformation of the closed cell foam core and maintaining a desired shape of the composite structure at increased pressures. Moreover, the filler structure does not include an amount of resin such that strain in the noodle region is absorbed by the closed cell foam core, and such that delamination caused by crack propagation in the resin is eliminated. 
     This written description uses examples to disclose various implementations, including the best mode, and also to enable any person skilled in the art to practice the various implementations, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.