Abstract:
A method of forming a composite structure including passing a web that is impregnated with an uncured resin in an assembly direction. Foam is forced about opposed sides of the web with the web including corrugations at least after this step. Outer skins of a fiber mat are attached onto outer sides of the web and foam. The fiber mat is impregnated with a resin. The combination of the fiber mats, the foam and the web is thermoformed in a mold to provide a shape for a structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/718,365, which was filed Oct. 25, 2012. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates to a method of making a composite structure and structures made by the method. 
     Current designs of many structural components require load-bearing panels. These may be formed of composites or metal. 
     In composite manufacture, such elements may be used as beam elements. 
     The current methods for manufacturing such components involve multiple labor intensive processes and steps. This can result in relatively high manufacturing costs. In addition, the distinct method steps can often result in a variation across several parts. 
     SUMMARY OF THE INVENTION 
     A method of forming a composite structure includes passing a web that is impregnated with an uncured resin in an assembly direction. Foam is forced about opposed sides of the web with the web including corrugations at least after this step. Outer skins of a fiber mat are attached onto outer sides of the web and foam. The fiber mat is impregnated with a resin. The combination of the fiber mats, the foam and the web is thermoformed in a mold to provide a shape for a structure. 
     These and other features may be best understood from the following drawings and specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a manufacturing process. 
         FIG. 2A  shows a first intermediate manufactured structure. 
         FIG. 2B  shows the final manufactured structure. 
         FIG. 3A  shows an alternative intermediate structure. 
         FIG. 3B  shows the alternative final structure. 
         FIG. 4  shows an alternative method. 
         FIG. 5A  shows one embodiment shape. 
         FIG. 5B  shows a detail of the  FIG. 5A  embodiment. 
         FIG. 6A  shows another embodiment shape. 
         FIG. 6B  shows a detail of the  FIG. 6A  shape. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , a manufacturing process  100  starts with a web of material  25  from a roller  135 . The web of material may be a felt or mat. Corrugation rollers  130  form corrugations  37  in the web. As shown, the corrugated web  25  is passed through a vat  140  which may be filled with a resin to impregnate the web  25 . 
     Extrusion dies  146  extrude foam  30 , which is forced onto sides of the corrugations  37  by rollers  147  and  148 . In addition, outer skins  35  are formed onto a composite panel at  151 . Rollers  147  and  148  urge the foam cores  30  to fill the corrugation  37  and simultaneously urge the skins  35  to cover the corrugations  37  and the foam core  30 . 
     The skins  35  are formed of a fiber felt or mat and may be impregnated with a resin. Generically, the material of the skins may be called a mat, even if formed of felt. A cutting tool  150  cuts sections  155  of an intermediate product. 
     From roller  135  to cutting tool  150 , the material moves along an assembly direction A. 
     The sections  155  may be then placed in a thermoforming mold  165  and formed to a shape as shown at  167 . In the thermoforming step  165 , the impregnated resins in the web  25  and skins  35  are thermoset all in a single step. In addition, as shown at  160 , a final shaped structure is achieved. As will be understood, appropriate motors are provided to drive rollers  135 ,  130 ,  147  and  148 . 
     The structure  160  may have any number of applications, however, in one anticipated application, it will have use in the aerospace industry. As an example, material may be shaped to form nacelles, nacelle components, fuselage panels and structural components, turbine blades, propellers, other airfoils, or any number of other structures where high-load bearing and/or impact resistant performance is needed at reduced weight, with the latter requirement not being a limiting case for the disclosed applications and structures. 
       FIG. 2A  shows a first intermediate product  155  having the skins  35 , the corrugations  37  and the core foam  30 . The final structure  160  is shown at  FIG. 2B . 
       FIG. 3A  shows an alternative  20  wherein there are three stacked layers of the intermediate product  155 . Again, as shown in  FIG. 3B , those three stacked layers are formed into a final structure  260 . 
     The web  25  may be a polymer, carbon, fiberglass, quartz, or aramid-fiber composite or combinations of those several materials. The web  25  and its corrugation form a load distribution and bearing element in the final structure  160  or  260 . 
     The foam  30  may be a low density polymer foam, such as a thermoplastic polymer foam, including polyetherimide (PEI) foam, polyphenylsulfone (PPSU) foam, polysulfone (PSU) foam, polyether ether ketone (PEEK) foam, and polyethersulfone (PES) foam, among others. The foam may have the density ranging from 500-10 kg/m 3 . The foams can be unfilled or filled with a carbon or glass fibers. 
       FIG. 4  shows an alternative method embodiment  600 , shown schematically. In embodiment  600 , the foam layers  602  are formed with undulations  604 . The web  606  is shown advancing to an assembly location without any corrugations yet having been formed. When the rollers  610  force the foam layers  602  against the web  606 , the undulations  604  form the corrugations  608  in the web  606 . 
     While  FIGS. 1 and 4  both show rollers forcing the web and foam layers together, other ways of assembling the layers together may be used. Generically, the layers simply need to be positioned relative to each other. 
     The foam layers  602  may be preformed into the shape, and may be a thermoplastic or a thermoset polymer foam. Thermoset polymer foams include structural polyurethane foams, and may have densities ranging from 200-500 kg/mg 3 . 
     The web  606  may be impregnated with additional resin, as an adhesive, to secure the layers, and may be assembled in a vacuum bag or mandrel. A worker of ordinary skill in this art would recognize various alternatives given the disclosure of this application. 
     The skin  35  may be formed of a carbon fiber or organic fiber or fiberglass felt or mat. The skins  35  provide outer mechanical support to the final structures  160 ,  260 . 
     The web  25  and skins  35  will be impregnated by a polymer resin and, in one disclosed embodiment, a thermoset polymer. Upon curing in the thermoforming stage, the web  25 , skin  35  and foam core  30  are all bonded together. 
     Any number of polymer thermosetting resins can be utilized, including epoxies, phenolics, BMI (bismaleimides) and cyanates. 
     While the thermoforming step  165  is disclosed as fully curing the structure  160  or  260 , partial curing may also be performed. A final curing or post-curing stage can then be used to complete the manufacture. A sequential partial cure followed by a final cure may be beneficial to control thermal or mechanical stresses in the structure. 
     A height D or thickness across the material may be controlled as may be a spacing S between corrugations  37 . These variables can be controlled to achieve desirable characteristics for the final structure  160  and  260 . 
     The depth of the corrugation or thickness of the component D and a peak to peak distance S may be targeted for mechanical demands and can be selectively tuned to desired values, designed and optimized to the targeted final product. 
     The depth D and a profile of the web  25  can be tuned along a machine direction or a cross-direction of the web. If a machine-direction profiling is used, several full width corrugated pre-impregnated sheets may be laid into the thermoforming mold (such as shown in  FIG. 3A ) to form a desired element. 
       FIG. 5A  shows an assembly  620  with a first curved shape  626  to the corrugations in the web  625 . Again, outer skins  622  are placed outwardly of the foam layers  624 . Applicant has recognized that avoiding tight bends in the corrugations  626  is an advantage in controlling stresses in the final product. Thus, as shown in  FIG. 5B , a shortest inside radius of curvature R of the corrugation curve  626  is defined, along with a maximum thickness T in the curved section. In embodiments, a ratio of R/T is greater than or equal to 1.0. More preferably, the ratio of R/T is greater than or equal to 3.0. 
       FIG. 6A  shows another shape embodiment  630  wherein the corrugation  636  has more of a trapezoidal shape. The corrugation  636  has a flat surface contacting the skin  632 , then curved corners  637  merging back into the web at  639 . Again, there is outer skin layer  632  and foam layer  634 . 
     As shown in  FIG. 6B , the radius of curvature R of the corners  637  of the trapezoidal shape is again defined as the shortest inside radius of that curve. The ratios of R to T as defined above, hold true of this embodiment also. 
     Although an example method and product are disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.