Abstract:
A method of manufacture of articles formed of composite materials including providing a plurality of elements, each of which is formed of a plurality of layers of composite material prepregs, assembling the plurality of elements in a desired mutual arrangement and applying heat and pressure to the plurality of elements following the assembling, thereby at least generally simultaneously to join the elements together and to cure at least some of the layers of composite materials.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the manufacture of articles formed of composite materials. 
       BACKGROUND OF THE INVENTION 
       [0002]    The following publications are believed to represent the current state of the art: 
         [0003]    U.S. Pat. Nos. 4,591,400; 4,780,262; 4,693,678; 5,059,377; 5,087,187; 5,454,895; 5,772,950; 6,319,346; 6,561,459; 6,896,841; 7,676,923 and 7,681,835; and 
         [0004]    U.S. Published Patent Application No. 2010/0166988. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention seeks to provide an improved method for manufacture of articles formed of composite materials. 
         [0006]    There is thus provided in accordance with a preferred embodiment of the present invention a method of manufacture of articles formed of composite materials including providing a plurality of elements, each of which is formed of a plurality of layers of composite material prepregs, assembling the plurality of elements in a desired mutual arrangement and applying heat and pressure to the plurality of elements following the assembling, thereby at least generally simultaneously to join the elements together and to cure at least some of the layers of composite materials. 
         [0007]    Preferably, the method also includes inserting at least one inflatable element between at least some of the plurality of elements prior to the applying heat and pressure. Additionally or alternatively, the plurality of elements include at least some elements which extend in mutually disparate directions. In accordance with a preferred embodiment of the present invention the plurality of elements include at least some elements which extend in at least nearly perpendicular directions. 
         [0008]    There is also provided in accordance with another preferred embodiment of the present invention an article of manufacture including a plurality of elements, each formed of a plurality of layers of composite material prepregs, arranged in a desired mutual arrangement, the plurality of elements being joined together and cured by the application of heat and pressure. 
         [0009]    In accordance with a preferred embodiment of the present invention the plurality of elements include at least some elements which extend in mutually disparate directions. Preferably, the plurality of elements include at least some elements which extend in at least nearly perpendicular directions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
           [0011]      FIG. 1  is a simplified illustration of an integral composite article constructed and operative in accordance with a preferred embodiment of the present invention; 
           [0012]      FIGS. 2A ,  2 B and  2 C are simplified illustrations of a method of manufacture of the integral composite article of  FIG. 1  in accordance with an embodiment of the present invention; 
           [0013]      FIG. 3  is a simplified illustration of another integral composite article constructed and operative in accordance with a preferred embodiment of the present invention; and 
           [0014]      FIGS. 4A ,  4 B and  4 C are simplified illustrations of a method of manufacture of the integral composite article of  FIG. 3  in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    Reference is now made to  FIG. 1 , which is a simplified illustration of an integral composite article constructed and operative in accordance with a preferred embodiment of the present invention. As seen in  FIG. 1 , an integral composite article  100 , here a control surface for an aircraft, such as an elevator, a rudder or an aileron, is formed with a spar  102 , which may have any suitable configuration, and typically includes a web  104 , integrally formed with flanges  106  and  108  as shown. Spar  102  is preferably prepared by conventional lay-up techniques used for composite materials but is preferably not cured prior to assembly in integral composite article  100 . Spar  102  may be formed as solid laminate or as sandwich structure. 
         [0016]    In accordance with a preferred embodiment of the present invention, a plurality of ribs  110  extend transversely and preferably perpendicularly to spar  102  and preferably include end ribs  112  and internal ribs  114 . Ribs  110  are preferably prepared by conventional lay-up techniques used for composite materials but are preferably not cured prior to assembly in integral composite article  100 . Ribs  110  may be foamed as solid laminates or as sandwich structures. Typically, the ribs are not cured until assembly together with the spar  102 , but alternatively, they may include one or more cured portions. 
         [0017]    As shown in an enlargement of part of  FIG. 1 , the ribs  110  preferably have an overall triangular configuration and include a generally triangular web  116  optionally having a sandwich construction, an end flange  118  and a pair of converging flanges  120 . 
         [0018]    An outer skin  126  extends over ribs  110  as well as spar flanges  106  and  108  to define an exterior configuration of article  100 . Alternatively, spar  102  may be obviated and outer skin  126  is folded to replace web  104 . 
         [0019]    Outer skin  126  preferably includes a layup of pre-preg layers, which may or may not include a core and thus may be either a solid laminate or a sandwich. The typical overall thickness of outer skin  126  is approximately 1-4 mm for a solid laminate and approximately 5-15 mm for a sandwich. Outer skin  126  is preferably prepared by conventional lay-up techniques used for composite materials but is preferably not cured prior to assembly in integral composite article  100 . 
         [0020]    Reference is now made to  FIGS. 2A-2C , which are simplified illustrations of a method of manufacture of an integral composite article, such as article  100  ( FIG. 1 ) in accordance with an embodiment of the present invention. For convenience, the reference numerals used in  FIG. 1  are also used in  FIGS. 2A-2C , as appropriate. 
         [0021]    As seen in  FIG. 2A , the outer skin  126  is preferably produced in a conventional manner, by laying up multiple prepreg layers  130  on a wedge-shaped male tool  132 . Following standard compaction, the outer skin  126  on tool  132  is placed in an article shape defining tool  200 , having an open top and an inner configuration corresponding to the outer configuration of article  100 . The term “compaction” is used throughout to refer to the application of pressure with or without heat and is also referred to as “debulking”. The wedge-shaped male tool  132  is subsequently removed from tool  200 , leaving skin  126  inside tool  200 , as shown. 
         [0022]    Alternatively, wedge shaped tool  132  may be obviated and outer skin  126  may be layed up on a flat tool and subsequently folded to define a wedge shaped configuration. Outer skin  126  may be formed as a solid laminate or as a sandwich structure having a core. If a sandwich structure is employed, a multiple piece wedge shaped tool  132  may be required. 
         [0023]    Thereafter, a plurality of ribs  110 , including end ribs  112  and internal ribs  114 , are placed in engagement with the outer skin  126  in tool  200 . 
         [0024]    Ribs  110  are preferably prepared using conventional prepreg layup techniques on shaped tools, followed by a conventional compaction process. It is appreciated that, while in the illustrated embodiment shown in  FIGS. 1-2C , ribs  110  are removed from the shaped tools prior to being placed in outer skin  126 , ribs  110  may be retained in the shaped tools until they are placed in outer skin  126  and subsequently the shaped tools are removed after each of ribs  110  is located in place. 
         [0025]    In accordance with a preferred embodiment of the present invention, a plurality of inter-rib transverse volumes  210  are defined between adjacent ribs  110 . 
         [0026]    In accordance with a preferred embodiment of the present invention, as seen in  FIG. 2B , a specifically configured inflatable element  212  is disposed in each of inter-rib transverse volumes  210 . Each inflatable element  212  preferably includes an inflation tube  214 . 
         [0027]    It is appreciated that, while in the illustrated embodiment shown in  FIGS. 1-2C , end flange  118  is formed in a direction transverse to web  116 , in order to facilitate insertion of inflatable elements  212 , end flange  118  may alternatively be formed of two side portions folded together, extending from web  116  in a generally parallel orientation thereto and including a separation layer, and, subsequent to the insertion of inflatable elements  212 , folding back the side portions of end flange  118  to lie transversely to web  116 . 
         [0028]    Spar  102 , together with a rigid spar shape defining tool  216  is then placed in tool  200  over ribs  110  and inflatable elements  212 . Spar  102  is formed with apertures  218  for accommodating inflation tubes  214 . Tool  216  is formed with apertures  220  which correspond in size and placement to apertures  218 . 
         [0029]    Turning now to  FIG. 2C , it is seen that the inflatable elements  212  are inflated and vacuum is preferably applied to the volume between the outside of the inflatable elements  212  and the inside surface of outer skin  126 , ribs  110  and spar  102 , when located inside tool  200 , and heat is applied. Typically, to ensure that the vacuum evacuates the air in tool  200  outside of the inflatable elements  212 , conventional methods, such as including a breather layer, may be used. 
         [0030]    It is a particular feature of the present invention that the resulting heat and pressure applied to spar  102 , ribs  110  and outer skin  126  is sufficient not only to cure these elements but to close gaps therebetween and to create a positive pressure on respective mating surfaces that bonds the respective mating surfaces together, thereby integrating the structural parts into a unified structure. Typical pressures and temperatures applied are between 1 and 7 bar of pressure and between 100 degrees Centigrade and 190 degrees Centigrade. 
         [0031]    This application of pressure, heat and vacuum may be realized by surrounding tool  200  with a vacuum bag and placing the tool and surrounding vacuum bag in an autoclave. In this embodiment using an autoclave, the pressure differential on external tool  200  during curing is relatively low compared to the pressure differential on tool  200  when not using an autoclave, so that in the embodiment using an autoclave, tool  200  may be of relatively lighter construction than necessary when not using an autoclave. Alternatively, the tool  200  may have integral heating elements and may be constructed to withstand the applied pressure of the inflatable elements  212 . In such a case, the autoclave may be obviated. In another alternative embodiment, prepregs that cure at low pressures and do not require an autoclave are utilized to form composite article  100 . 
         [0032]    It is appreciated that the vacuum bag may be placed over tool  200  while tool  200  is lying on a flat tool, as shown in  FIG. 2C . Alternatively, the vacuum bag may be placed over external tool  200  while tool  200  is placed in tool supports, such as the tool supports shown in  FIG. 2B , thus obviating the need for a flat tool. 
         [0033]    Following suitable curing and joining of spar  102 , ribs  110  and outer skin  126 , the article  100  inside tool  200  is allowed to cool in the autoclave. Alternatively, article  100  may be removed from the autoclave and allowed to cool at ambient temperature and pressure. The article  100  may then be removed from tool  200 . Optionally inflatable elements  212  may be removed from the article via apertures  218  in spar  102 . Alternatively, inflatable elements  212  may be retained in article  100 , as shown, bonded to spar  102 , ribs  110  and skin  126 . 
         [0034]    In an alternative embodiment, top and bottom portions of outer skin  126  may each be formed separately on a flat tool. In this embodiment, the bottom portion of outer skin  126  is then placed on a flat tool, followed by placing ribs  112  and  114 , inflatable elements  212  and spar  102 , together with a rigid spar shape defining tool  216 , on the bottom portion of outer skin  126 . The top portion of outer skin  126  is then placed over the bottom portion of outer skin  126 , ribs  112  and  114 , inflatable elements  212  and spar  102 , while adding prepreg layers to splice top and bottom portions of outer skin  126  according to conventional splicing methods. The top portion of outer skin  126  is then covered with a top part of an article shape defining tool, effectively reaching the assembly shown in the final stage of  FIG. 2B . Subsequently inflatable elements  212  are inflated and vacuum is applied as described hereinabove. 
         [0035]    The composite article  100  may include a rounded leading edge portion (not shown) forward of spar  102 , which may be assembled to the spar in a conventional manner by employing an inflatable element extending the length of the leading edge, which is inserted between the spar and the leading edge during curing of composite article  100 . Additionally or alternatively, a wedge shaped portion may be included at the trailing edge of composite article  100 . 
         [0036]    It is appreciated that integral composite article  100  may also include ‘pad-ups’, which are local regions having increased thickness typically for providing increased local strength at points of attachment of associated components, such as supports, hinges and actuators. One realization of pad-ups employs discrete elements, which may be precured, but preferably are not cured and are thus assembled as part of the integral composite article  100  in accordance with an embodiment of the present invention. Alternatively, discrete metallic inserts may be included for pad-ups. 
         [0037]    Reference is now made to  FIG. 3 , which is a simplified illustration of an integral composite article constructed and operative in accordance with another preferred embodiment of the present invention. As seen in  FIG. 3 , an integral composite article  300 , here an aerodynamic surface for an aircraft, such as a wing, a horizontal stabilizer or a vertical stabilizer, is preferably formed with a top surface  302  and a bottom surface  304 , having the external geometry of the main part of an aerodynamic contour, and typically includes a front spar  306  and a rear spar  308 . It is appreciated that composite article  300  may have either a constant cross section or a varying cross section, in both vertical and transverse directions. 
         [0038]    In the illustrated embodiment shown in  FIG. 3 , spars  306  and  308  are integrally formed as portions of top and bottom surfaces  302  and  304 , and are attached as indicated by reference number  305 . Alternatively, spars may be attached at any suitable location. Alternatively, spars  306  and  308  may be formed separately using conventional lay-up techniques used for composite materials, but are preferably not cured prior to assembly in integral composite article  300 . At least one of spars  306  or  308  includes apertures  309  for the insertion of inflation tubes. 
         [0039]    In accordance with a preferred embodiment of the present invention, a plurality of ribs  310  extend transversely and preferably perpendicularly to spars  306  and  308 . Ribs  310  include internal ribs  314  and may also include end ribs  312 . Ribs  310  are preferably prepared by conventional lay-up techniques used for composite materials. Ribs  310  may be formed as solid laminates or as sandwich structures. Typically the ribs  310  are not cured until assembly together with integral composite article  300 , but alternatively, they may include one or more cured portions. 
         [0040]    As shown in enlargements C and D of  FIG. 3 , ribs  310  preferably have an overall configuration designed to support the aerodynamic contour of surfaces  302  and  304 , and include a generally oval shaped web  316 , optionally having a sandwich construction, end flanges  318  and top and bottom flanges  320  and  322 . As seen in respective enlargements C and D, flanges  320  and  322  of internal ribs  314  may be formed with or without cutouts  324 . It is appreciated that flanges  320  and  322  of end ribs  312  are typically formed without cutouts, and are typically formed on only one side of web  316 . 
         [0041]    It is appreciated that, in integral composite article  300 , end flanges  318  are joined to spars  306  and  308  and top and bottom flanges  320  and  322  are respectively joined to top surface  302  and bottom surface  304 . 
         [0042]    In accordance with a preferred embodiment of the present invention, as seen in enlargement A, integral composite article  300  also includes stiffening elements  330 , such as stringers, to prevent buckling of surfaces  302  and  304  when subject to compressive and/or shear loads. Alternatively, as seen in enlargement B, surfaces  302  and  304  have a sandwich construction and stiffening elements  330  are obviated. 
         [0043]    Top and bottom surfaces  302  and  304  extend over ribs  310  to define, together with spars  306  and  308 , an exterior configuration of article  300 . As described hereinabove, spars  306  and  308  may be integrally formed with top and bottom surfaces  302  and  304 . Alternatively, spars  306  and  308  may be formed as separate parts from top and bottom surfaces  302  and  304 . 
         [0044]    Top and bottom surfaces  302  and  304  each preferably include a layup of pre-preg layers, which may or may not include a core and thus may be either a solid laminate or a sandwich. The typical overall thickness of top and bottom surfaces  302  and  304  is approximately 1-10 mm for a solid laminate and approximately 5-25 mm for a sandwich. Top and bottom surfaces  302  and  304  are preferably prepared by conventional lay-up techniques used for composite materials but are preferably not cured prior to assembly in integral composite article  300 . 
         [0045]    Reference is now made to  FIGS. 4A-4C , which are simplified illustrations of a method of manufacture of an integral composite article, such as article  300  ( FIG. 3 ) in accordance with an embodiment of the present invention. For convenience, the reference numerals used in  FIG. 3  are also used in  FIGS. 4A-4C , as appropriate. 
         [0046]    As seen in  FIG. 4A , bottom surface  304  is preferably produced in a conventional manner, by laying up multiple prepreg layers on a male tool (not shown) that has the required external aerodynamic contour. Following standard compaction, the bottom surface  304  on the male tool is placed in a bottom half of a composite article shape defining tool  400 , having an open top and an inner configuration corresponding to the outer configuration of composite article  300 . The male tool is subsequently removed from bottom half shape defining tool  400 , leaving surface  304  generally inside bottom half shape defining tool  400 , as shown. 
         [0047]    Alternatively, male shaped tool may be obviated and bottom surface  304  may be directly laid up in bottom half shape defining tool  400 . Alternatively, bottom surface  304  may be formed on a flat tool and subsequently folded to obtain the required shape including the spars  306  and  308 . Bottom surface  304  may be formed as a solid laminate or as a sandwich structure having a core. 
         [0048]    In the illustrated embodiment shown in  FIG. 4A , the bottom portion of spars  306  and  308  are integrally formed with bottom surface  304 . Spar  306  preferably also includes apertures  309 . Alternatively, apertures may be in spar  308 . 
         [0049]    Thereafter, a plurality of stiffening elements  330  are placed on bottom surface  304 . In a preferred embodiment, stiffening elements  330  are formed and precured prior to placement on bottom surface  304 . The size and cross section of stiffening elements  330  are configured so that the pressure caused by inflation of the inflatable elements will not cause the stiffening elements  330  to collapse, and are also configured to ensure that stiffening elements  330  will maintain sufficient pressure on bottom surface  304  during the curing process. While in the illustrated embodiment trapezoidal shaped stiffening elements  330  are shown, stiffening elements  330  may be any other suitable shape, such as semi-circular or triangular. Additionally or alternatively, foam filled stiffening elements  330  with suitable properties may be provided. 
         [0050]    Alternatively, as shown in enlargement B of  FIG. 3 , bottom surface  304  may be formed with a sandwich construction, and stiffening elements  330  are obviated. 
         [0051]    Thereafter, a plurality of ribs  310 , including end ribs  312  and internal ribs  314 , are placed in engagement with the bottom surface  304  and bottom portions of spars  306  and  308  in bottom half shape defining tool  400 . As seen in  FIG. 3 , flanges  320  and  322  of internal ribs  314  include cutouts  324  to allow passage of stiffening elements  330  through cutouts  324 . 
         [0052]    Ribs  310  are preferably prepared using conventional prepreg layup techniques on shaped tools, followed by a conventional compaction process. 
         [0053]    As described hereinabove, in the alternative embodiment shown in enlargement B of  FIG. 3 , in which top and bottom surfaces  302  and  304  have a sandwich construction and stiffening elements  330  are obviated, internal ribs  314  are formed without cutouts  324 , as shown in enlargement D of  FIG. 3 . 
         [0054]    In accordance with a preferred embodiment of the present invention, a plurality of inter-rib transverse volumes  410  are defined between adjacent ribs  310 . 
         [0055]    In accordance with a preferred embodiment of the present invention, as seen in  FIG. 4B , a specifically configured inflatable element  412  is disposed in each of inter-rib transverse volumes  410 . Each inflatable element  412  preferably includes an inflation tube  414 . Inflation tubes  414  are accommodated by apertures  309  of front spar  306 . Bottom half shape defining tool  400  is formed with cutouts  420  to accommodate inflation tubes  414 . 
         [0056]    It is appreciated that, in the embodiment illustrated in  FIGS. 4A-4C , where bottom portion of spar  306  is integrally formed with bottom surface  304 , apertures  309  may be formed as cutouts in bottom portion of spar  306  to facilitate placement of inflation tubes  414 , and top portions of apertures  309  are formed in top portion of spar  306  integrally formed with top surface  302 . 
         [0057]    Top surface  302 , preferably also including top portions of spars  306  and  308 , is preferably formed in a manner similar to bottom surface  304  and placed in a top half of a composite article shape defining tool  430 . Shape defining tool  430  is formed with cutouts  432  to accommodate inflation tubes  414 . Cutouts  432  are located to correspond to apertures  309  in spar  306 . 
         [0058]    Thereafter, a plurality of stiffening elements  330  are placed on top surface  302  in top half shape defining tool  430 , and held in place by performing standard compaction to top surface  302  and stiffening elements  330 . In a preferred embodiment, stiffening elements  330  are conventional stiffening elements. The size and cross section of stiffening elements  330  are configured so that the pressure caused by inflation of inflatable elements  412  will not cause the stiffening elements  330  to collapse, and are also configured to ensure that stiffening elements  330  will maintain sufficient pressure on top surface  306  during the curing process. While in the illustrated embodiment trapezoidal shaped stiffening elements  330  are shown, stiffening elements  330  may be any other suitable shape, such as semi-circular or triangular. Alternatively, as shown in enlargement B of  FIG. 3 , top surface  306  may be formed with a sandwich construction, and stiffening elements  330  are obviated. 
         [0059]    Subsequently, top surface  302 , including top portions of spars  306  and  308 , with stiffening elements  330  and top half shape defining tool  430  are placed over ribs  310  and inflatable elements  412  and bottom portions of spars  306 ,  308  in bottom half shape defining tool  400 . Alternatively, as shown in enlargement B of  FIG. 3 , top surface  302  may have sandwich construction and stiffening elements  330  are obviated. 
         [0060]    Turning now to  FIG. 4C , it is seen that the inflatable elements  412  are inflated and vacuum is preferably applied to the volume between the outside of the inflatable elements  412  and the inside surface of top and bottom surfaces  302  and  304 , ribs  310  and integral spars  306  and  308 , when located inside tools  400  and  430 , and heat is applied. 
         [0061]    It is a particular feature of the present invention that the resulting heat and pressure applied to surfaces  302  and  304 , spars  306  and  308  and ribs  310  is sufficient not only to cure these elements but to close gaps therebetween and create a positive pressure on respective mating surfaces that bonds the mating surfaces together and also bonds stiffening elements  330  to surfaces  302  and  304 . Thus, the structural parts are integrated into a unified structure. Typical pressures and temperatures applied are between 1 and 7 bar of pressure and between 100 degrees Centigrade and 190 degrees Centigrade. 
         [0062]    This application of pressure, heat and vacuum may be realized by surrounding tools  400  and  430  with a vacuum bag and placing the tool and surrounding vacuum bag in an autoclave. Alternatively, tools  400  and  430  may have integral heating elements and may be constructed to withstand the applied pressure of the inflatable elements  412 . In such a case, the autoclave may be obviated. In another alternative embodiment, prepregs that cure at low pressures and do not require an autoclave are utilized to form composite article  300 . 
         [0063]    Following suitable curing and joining of surfaces  302  and  304 , spars  306 ,  308 , ribs  310  and stiffening elements  330 , composite article  300  inside tools  400  and  430  is allowed to cool in the autoclave. Alternatively, composite article  300  may be removed from the autoclave and allowed to cool at ambient temperature and pressure. Composite article  300  may then be removed from tools  400  and  430 . Optionally, inflatable elements  412  may be removed from the article via apertures  309  in spar  306 . Alternatively, inflatable elements  412  may be retained in composite article  300 , as shown, bonded to surfaces  302  and  304 , integral spars  306  and  308 , ribs  310  and stiffening elements  330 . 
         [0064]    In another alternative embodiment, composite article  300  includes a rounded leading edge portion forward of spar  306  and/or a trailing edge portion rearward of spar  308 . In this embodiment, the required leading edge layup or trailing edge layup is added contiguously with spar  306  and/or spar  308 , respectively, and an inflatable element extending the length of the leading edge and/or the trailing edge is then inserted during curing of composite article  300 . 
         [0065]    It is appreciated that integral composite article  300  may also include ‘pad-ups’, which are local increases in the thickness of the components of composite article  300 , typically providing increased local strength at attachment points such as joints, hinges and actuator attachment points. Alternatively, local increases in strength may be provided by adding separate local strengthening elements. The local strengthening elements may be precured, but preferably are not cured prior to assembly in integral composite article  300 . Alternatively, discrete metallic inserts may be included for pad-ups. 
         [0066]    It is appreciated that stiffening elements, such as stiffening elements  330  shown in the embodiment of  FIGS. 3-4C  or other suitable stiffening elements, may also be utilized in the formation of composite article  100 , arranged in either a longitudinal or a transverse direction. 
         [0067]    The present invention is applicable in various additional industries, such as building construction and automotive manufacturing. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove as well as modifications and variations thereof which are not in the prior art.