Patent Publication Number: US-6217000-B1

Title: Composite fabrication method and tooling to improve part consolidation

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation-part of U.S. patent application Ser. No. 08/738,263, filed Oct. 25, 1996, now Pat. No. 5,817,269 and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for forming structures from composite materials. 
     BACKGROUND OF THE INVENTION 
     The use of composite materials in the manufacture of aircraft and other lightweight structures has increased steadily since the introduction of such materials. Composite materials have a high strength-to-weight ratio and high stiffness, making them attractive for use in lightweight structures. Two drawbacks to using composite materials are their high fabrication costs and low damage tolerance. It is difficult to produce composite parts for the same cost and having the same degree of damage tolerance as comparable metal parts. This cost differential is especially notable in large scale composite parts having complex contours. 
     Some contributors to the cost of composite structures are the amount of manual labor required during fabrication and the cost of the complex tools generally used to form composite parts having complex geometries. Complex parts having complex geometries, for example, aircraft wing boxes, are generally formed by manually laying up individual layers of composite material on a contoured mandrel to form the exterior skin of the wing box. The spars and ribs of the wing box are generally formed separately by manually laying up individual layers of composite material on a series of different tools in order to form individual I-beam or sine wave spars and ribs. The sine wave spars or ribs are generally vacuum bagged and precured as separate components within an autoclave under elevated temperatures and pressures. In one approach, after curing, the sine wave spars and ribs are placed on the laid up outer skin of the wing box and subsequently adhesively bonded to the skin in an autoclave operation in which the skin, spars and ribs are placed in a vacuum bag and cured at an elevated temperature and pressure. In order to insure good structural integrity between the spars, ribs and skin, “chicken fasteners” are generally used to reinforce the spar/rib-skin interface. 
     In order to decrease fabrication costs and increase structural integrity, it has been found advantageous to join the spars and ribs to the skin prior to curing the individual composite substructures. The skin and joined ribs and spars are then vacuum bagged with appropriate tooling and co-cured together in a single autoclave operation. This method of fabrication eliminates the separate curing cycles required to form individual composite subassemblies and subsequently join them to form the wing box structure. However, currently it is difficult to properly design and fabricate appropriate tools to jointly cure complex structures while maintaining structural tolerances and minimizing or eliminating areas of resin porosity or resin richness. 
     One problem that is encountered in forming spars and ribs to a skin in one autoclave operation is that the complex geometries formed by the skin and ribs are difficult to support by tooling. The spars typically are of an I-beam construction, and the webs of the spars extend substantially perpendicular to the skin. One end cap of the I-beam spar extends against the face of the skin. The other end cap of the spar extends parallel to the skin, but spaced apart from the skin. Ribs extend perpendicular to both the spars and the skin. Five-sided cavities are formed by a pair of adjacent spars, a pair of adjacent ribs, and the skin. Adjacent outer end caps on adjacent spars create a partially closed opening for the sixth side of the cavities into which tooling must be inserted to support spars during the autoclave process. 
     Inserting tooling into a cavity with such a partially closed opening can be difficult, but removing the tooling from the partially closed opening after curing of the spars and the skin is nearly impossible. One prior art manner of dealing with this problem developed by Lockheed Martin involved upper and lower cure fixtures for the skins and spars. The composite material for the skins were laid up against the lower skin cure fixture. The composite material for the spars and ribs were laid up over rubber sub-bladders having the shape of the to-be-formed cavity, and then were arranged over the skins. An upper spar cure fixture was then aligned with the top of each of the upper end caps of the spars. The upper spar cure fixture included a groove along which the upper end cap was seated. The upper spar cure fixture also included vacuum attachments and a seal that received a bladder bag. The bladder bag was shaped and configured so that it could extend into the cavity formed between adjacent spars and ribs and the skin. The soft structure of bladder bag and subbladder facilitated easy tool removal after curing. However, the inner tooling and upper spar cure fixture were expensive to produce and cumbersome to use. Moreover, close inner part tolerances were difficult to produce because the soft tooling was difficult to maintain in the proper final shape of the cavity. There is a need for a more efficient manner of providing this inner tooling. 
     Another problem associated with past tooling designs is the inability of the tooling to place appropriate consolidation pressures on all of the regions of a complex part during curing, thus leading to areas of resin richness or resin porosity. Generally, past tooling concepts have been formed of various materials such as Invar 42 or composite materials that have similar coefficients of thermal expansion as the composite material from which the part is being formed. Matching the coefficients of thermal expansion between the tooling materials and the composite material from which the part is formed is thought to reduce problems associated with thermal expansion mismatches during part curing. 
     Prior tooling concepts, even those using tools formed from materials having similar coefficients of thermal expansion as the composite materials, often fail to achieve complete part consolidation. Consolidation problems are complicated in composite parts where not only must complete consolidation be achieved, but also part dimensional tolerances must be carefully controlled. Part tolerances are increasingly important in parts having both exterior mold line surfaces and interior ribs, spars or flanges whose geometry and locations must be carefully controlled. 
     Generally, prior tooling concepts for use in complex composite parts have relied upon matched metal tooling. However, there is even small amounts of misalignment in the location of any of the surfaces of the matched metal tooling, the result is often unacceptable parts having areas of resin richness or resin porosity. Alternatively, complex metal tools incorporating indexing substructures have been used. As with matched metal tooling, such tooling often fails to produce high quality composite parts, again due to the inability of matched metal tooling to compensate for even slight errors in position. In addition, the complexity of most such prior tooling concepts has resulted in expensive tools that contribute to the overall costs of the composite parts being produced. 
     One of the contributions to resin richness and resin porosity in cured composite parts is the large difference between the uncured and cured dimensions of the composite part. Because composite parts are formed of many layers of composite material that are joined together and then, consolidated and cured, the laid up but uncured composite workpiece has a much greater volume than the fully compacted and cured composite workpiece. This changing volume and thus part thickness during curing is complicated by the fact that the magnitude of compaction-consolidation of the composite workpiece differs at different areas within the workpiece due to the varying number of layers of composite material used to form different areas of the workpiece. 
     Prior tooling concepts are configured to account for the thermal expansion of the composite tools and the composite workpiece during curing. However, such tooling concepts fail to account for the varying magnitudes of compaction/consolidation required at different areas in the composite workpiece. Thus, prior tooling has been used very successfully to form composite parts having approximately constant thicknesses, but has not been used as successfully to form composite parts having varying thicknesses and close tolerances on the surfaces of the part. 
     As can be seen from the above discussion, there remains a need in the industry for improved composite fabrication methods and apparatus. The present invention is directed toward composite fabrication methods and apparatus that fulfill part of this need. 
     SUMMARY OF THE INVENTION 
     The present invention is a tooling concept and tooling method that produces high quality, complex composite parts. In one method of the invention, a composite part is formed that has a cavity. The method includes forming a tooling concept with a plurality of rigid tools, laying up a composite material around the tooling concept so that the tooling concept is within a cavity of an eventually-formed composite part, curing the composite material with the tooling concept within the cavity; and disassembling the rigid tools of the tooling concept and removing the plurality of rigid tools out of the cavity. 
     In accordance with one aspect of the invention, the cavity of the composite part includes an opening that is smaller than a cross-section of the cavity taken parallel to the opening. Disassembly of the tool concept step involves removing the plurality of rigid tools through the opening. To aid in disassembly and removal, in one embodiment the tooling concept formed from the plurality of rigid tools is hollow. 
     The present invention is also directed to the tooling concept of the method described above. In the tooling concept, the plurality of rigid tools are preferably individually indexed relative to each other whereby small adjustments can be made relative to each other. 
     The present invention is also directed to a tooling concept for forming a composite part, the tooling concept including an inner tool formed of at least one rigid material and an outer covering formed of at least one compliant material having a high coefficient of thermal expansion in comparison to the coefficient of thermal expansion of the rigid material, the thicknesses of the compliant covering and inner tool and the materials used to form the tooling concept being selected to produce varying magnitudes of thermal expansion at different areas within the tooling concept to allow the tooling concept to produce varying levels of consolidation expansion to fully consolidate different areas of the composite part that require different magnitudes of consolidation expansion to achieve complete consolidation. 
     The present invention is also directed to a method of forming a composite wing box having at least one rib and spar and a skin. 
     In accordance with other aspects of the invention, the tooling concept is configured so that different areas of the tooling concept produce different magnitudes of thermal expansion during curing of the composite part. The different magnitudes of thermal expansion produced by the tooling concept produce the varying magnitudes of tooling movement needed to fully consolidate areas of the composite part having different unconsolidated and consolidated thicknesses. 
     In accordance with still further aspects of the invention, the tooling concept is formed of both a compliant material and a rigid material. The compliant material is used to cover the forming exterior surface of the rigid material. The compliant material has a relative high coefficient of thermal expansion as compared to the lower coefficient of thermal expansion of the rigid material. In one embodiment, the compliant material is formed at least partially of silicone rubber. 
     In accordance with still other aspects of the invention, the tooling concept is formed of multiple materials having different coefficients of thermal expansion. Various areas of the tooling concept can also be formed of varying thicknesses in order to produce varying magnitudes of thermal expansion during curing of the composite part. 
     Prior tooling designs have generally been unable to consistently produce high quality, complex composite structures free of areas of resin porosity or resin richness. Typically, prior art tooling solutions have relied upon a combination of the thermal expansion characteristics of metal tools combined with the pressures produced by the autoclave during curing to consolidate and cure the composite parts. Such prior tooling concepts rely upon the thermal expansion characteristics of tools produced from a single tooling material. Such tooling concepts have generally been successfully used to fabricate composite parts having relatively uniform thicknesses. However, such tooling concepts have not been as successful when used in the complex composite parts having multiple thicknesses. 
     The use of tooling concepts according to the present invention allow multiple tooling materials having different thermal expansion characteristics to be incorporated into a uniform tooling design. Using multiple materials having differing thermal expansion coefficients allows tools, according to the present invention, to be carefully tailored to overcome problems presented by complex composite parts. Thus, through the use of the present invention, the tooling can be designed to provide increased thermal expansion in areas of the composite part that require it and lower magnitudes of thermal expansion in areas that do not require as much tool expansion to produce good part consolidation. 
     Because both the unconsolidated and consolidated dimensions of the composite part can be approximately determined prior to curing, tooling concepts according to the present invention can be carefully designed to account for the differences in geometry in the composite part both before curing and after curing. Thus, carefully designed tooling concepts according to the present invention can produce approximately a hydrostatic force over the surface of the composite part. The production of the near hydrostatic force results in the production of consistent composite parts with reduced areas of resin richness or resin porosity. 
     Carefully configuring tooling concepts according to the present invention also allows the pressures applied by the tools to the composite workpiece to be carefully balanced on each side of the composite part being formed. Balancing the pressures produced by the tools during consolidation and curing allows tooling concepts according to the present invention to produce composite parts having the close tolerances required in today&#39;s aerospace industry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a composite wing box fabricated in accordance with the present invention; 
     FIG. 2 is a cross-section of the wing box of FIG. 1 along line  2 — 2 ; 
     FIG. 3 is an enlarged cross-section of a portion of one of the spar/skin interfaces of the wing box of FIG. 1; 
     FIG. 4 is a cross-section of a portion of the wing box of FIG. 1, along line  4 — 4 ; 
     FIG. 5 is a perspective view of a fully assembled tooling concept according to the present invention; 
     FIG. 6 is an exploded view of the tooling concept of FIG. 5; 
     FIG. 7 is a cross-section of the assembled tooling concept of FIG. 5; 
     FIG. 8 is a perspective view illustrating the application of layers of composite material to the ends of the tooling concept of FIG. 5; 
     FIG. 9 is a perspective view of two adjoining tooling concepts and the application of layers of composite materials to the bottom of the adjoining tooling concepts; 
     FIG. 10 is a perspective view of the application of layers of composite material to form portions of wing spars around the adjoining tooling concepts illustrated in FIG. 9; 
     FIG. 11 is a perspective view of a number of assembled tooling concepts placed on a composite outer skin sheet; 
     FIG. 12 is a perspective view of the completed composite workpiece and tooling used to form the wing box of FIG. 1; 
     FIG. 13 is a perspective view of the covered and vacuum bagged workpiece of FIG. 12; and 
     FIG. 14 is an enlarged cross-section of a portion of the composite workpiece and associated tooling used to form the wing box of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a wing box  20  formed in accordance with the present invention. The wing box  20  includes an outer skin panel  22 , four parallel spars  24  extending the length of the wing box, and three parallel ribs  26  extending the width of the wing box. As illustrated in FIGS. 2,  3  and  11 , the outer skin panel  22  includes a pad up or built up section  28  in the region of the attachment between the bottom of each spar  24  and the outer skin panel  22 . Each spar  24  includes a central shear web  30  (FIG.  3 ), a lower reinforced structural buildup section  32  and an upper spar cap  34 . 
     As will be described in more detail below, the structural buildup sections  32  are located on opposing sides of each shear web  30  adjoining the intersection between the bottom of each central shear web  30  and the outer skin panel  22 . The structural buildup sections  32  are formed of individual layers of either unidirectional or cloth composite prepreg materials. The shear webs  30  extend upward from the outer skin panel  22  approximately normal to the plane of the outer skin panel. The spar caps  34  are located at the top of each shear web  30  and extend outward from opposing sides of the shear web approximately normal to the plane of the shear web. 
     The area between adjacent spars  24  and adjacent ribs  26  defines a cavity  36  (FIG.  1 ). The cavity  36  has the shape of an open-ended box, with a bottom formed by the skin panel  22 , ends formed by the ribs  26 , and sides formed by the spars  24 . The top opening  38  (FIG. 2) of the cavity  36  is partially closed because the upper spar cap  34  of each of the spars  24  impinges into and over the cavity  36 . As described in detail below, the present invention provides a unique tooling concept  40  (FIG. 5) for forming the cavities  36  of the wing box  20  using a rigid panel structure. The tooling concept  40  is designed so that composite material can be built up around the tooling concept  40  so that the tooling concept  40  is within the cavity  36  after cure of the composite wing box  20 . The partially closed opening  38  (FIG. 2) at the top edge of each of the cavities  36  is formed on one surface of the tooling concept  40 . After curing of the wing box, the tooling concept  40  is disassembled and removed through the partially closed opening  38 . 
     As will be discussed below, the outer skin panel  22 , spars  24 , and ribs  26  are all formed of individual layers of composite prepreg that are joined together in a fabrication process to form a composite workpiece that is then cured to form a composite part, such as wing box  20 . The various layers of composite material used to form the composite workpiece can be formed from either cloth, composite prepreg, or unidirectional composite prepreg materials. Prepreg composite materials contain both reinforcing fibers and a matrix material such as a thermosetting polymer (e.g. epoxies) or thermoplastic polymer (e.g. polyether-lmide). 
     The specific types of composite materials used to fabricate the wing box  20 , including the type of matrix material used, the type of reinforcing fiber used, whether the materials are prepregs of unidirectional or cloth materials, whether the materials incorporate dry cloth, etc., and how many layers of composite material are used to form each section of the wing box  20 , all differ depending upon the specific application and type of composite part being formed. Therefore, the present invention is discussed below with respect to an exemplary wing box  20 . However, the present invention may be used to form various composite parts having varying geometries and being fabricated from varying composite material systems. 
     As discussed briefly in the background of the invention, depending upon the application, it is important to maintain close structural tolerances on various sections of the wing box  20 . Such close tolerances are used to achieve both proper part fit and good part quality. For example, in the wing box  20 , it is important to maintain good part consolidation throughout the structure of the wing box in order to insure good structural properties. It is also important to maintain close part tolerances on the outer surface of the outer skin panel  22  since the surface would be subject to aerodynamic considerations in a complete wing. In addition, in the wing box  20  it is important to closely maintain the positional geometry of the various spars  24  and ribs  26  in order to insure that such structures will mate correctly with additional parts of a completed wing, such as wing skins, bulkheads, etc. (not shown). The problems associated with achieving good consolidation throughout the wing box  20  are complicated by the fact that the various sections of the wing box are formed from a different number of layers of composite material and thus have varying preconsolidated and post-consolidated thicknesses. During consolidation it is important that the tooling concept used to position and cure the various sections of the composite workpiece used to form the wing box  20  be able to account for varying changes in thickness in sections of the wing box and to account for the varying magnitudes of tool movement needed to properly consolidate and cure the variable thickness sections of the wing box. 
     For example, the central shear webs  30  of the spars  24  may be formed of fourteen layers of composite prepreg having a debulked but unconsolidated thickness of approximately 0.196 inches, while the thickness of the composite workpiece in the region of the structural buildup section  32  may be formed of forty-five layers of composite material having a debulked but unconsolidated thickness of approximately 0.441 inches. However, upon consolidation, the central shear web  30  has a consolidated thickness of approximately 0.150 inches and the structural buildup section  32  has a consolidated thickness of approximately 0.410 inches. Thus, any tooling concept used to cure the composite workpiece should account for a change in thickness from unconsolidated to consolidated that is approximately 2.7 times as great in the structural buildup section  32  as in the center of the central shear web  30 . 
     As recognized by the invention, one method of reducing problems associated with part consolidation is to produce a tooling concept capable of applying approximately a hydrostatic pressure over the entire part while also maintaining part tolerances. One tooling concept according to the present invention utilized to produce varying amounts of tooling expansion that results in approximately a hydrostatic force is illustrated in FIGS. 5 and 6. The tooling concept  40  includes a base or bottom tool  42  (FIG.  6 ), opposing side tools  44 , opposing end tools  46 , and a top tool  48 . The exterior surface of each of the bottom, side, end and top tools  42 ,  44 ,  46  and  48  is contoured to serve as a forming surface to impart the desired shape to the composite workpiece during curing. 
     In the preferred embodiment, the bottom, side, end, and top tools  42 ,  44   46  and  48  are formed of a rigid material such as aluminum. In the present invention, it is advantageous to form the tools from a material that has a greater coefficient of thermal expansion than the composite material from which the part is to be formed. This allows the tooling to expand during heating more than the composite material forming the workpiece, thus allowing the expansion of the tooling to compact and help to consolidate the composite material. 
     The bottom, side, end, and top tools  42 ,  44 ,  46  and  48  are held together using two pairs of opposing side tool mounts  50  and a pair of opposing end tool mounts  52 . Both the side tool mounts  50  and the end tool mounts  52  are generally L-shaped, having the lower leg of the “L” extending inward from the inner surface of the upward extending leg of the “L.” Each of the side tool mounts  50  and end tool mounts  52  is indexed and bolted to the bottom tool  42 . As illustrated in FIG. 6, the bottom tool  42  includes two opposing end recesses  54  and two pairs of opposing side recesses  56 . The end and side recesses  54  and  56  are generally rectangular and are sized to accommodate the lower legs of the end tool mounts  52  and side tool mounts  50 . Each of the side tool mounts  50  and end tool mounts  52  is indexed and fastened into the respective recesses  54  and  56  using fasteners such as bolts (not shown). Two bolts are used for each tool mount. The bolts extend through holes such as holes  58  in the lower leg of the end tool mount  52  and are received in holes such as holes  60  within the end recess  54 . 
     When fastened in place, the upward extending legs of the side tool mounts  50  and end tool mounts  52  extend upward from the plane of the bottom tool  42  approximately perpendicular to the plane of the tool. The exterior side of each of the side tool mounts  50  and end tool mounts  52  extends slightly outward from the corresponding edges of the bottom tool  42  when the tool mounts are fastened to the bottom tool. 
     As illustrated in FIG. 6, the interior surface of each of the end tools  46  includes a rectangular recess  64  that is shaped and sized to accommodate the associated upward extending leg of the respective end tool mount  52 . Each of the end tools  46  is indexed and bolted to the respective end tool mount  52  using two fasteners such as bolts (not shown) and two indexing pins that extend through four holes such as holes  66  in end tool mount  52  and are received into holes such as holes  68  in the end tool  46 . 
     The interior surface of each of the side tools  44  includes two rectangular recesses  70  that are sized and shaped to accommodate the upward extending legs of the associated side tool mounts  50 . The side tools  44  are indexed and attached to the side tool mounts  50  using fasteners such as cap bolts and indexing pins (not shown) that extend through holes such as holes  72  in side tool mounts  50  and are received within holes such as holes  74  in the side tool  44 . The top tool  48  is fastened to the top of the side tool mounts  50  and end tool mounts  52  using fasteners such as bolts (not shown) that extend through holes such as holes  76  located along the edges of the top tool  48  and are received within holes  78  in the top of the side tool mounts  50 . Similarly, the opposing ends of the top tool  48  are fastened to the end tool mounts  52  using fasteners such as bolts (not shown) that extend through holes  80  in the ends of the top tool  48  and are received within holes  80  in the tops of the end tool mounts  52 . Location of top tool  48  is established by assembly within the boundaries created by the upper surfaces of side tools  44  and end tools  46  when assembled to their respective end tool  52  and side tool  50  mounts. 
     Importantly, the tooling concept  40  is arranged when assembled so that the side tools  44  extend against the outer edges of the top tool  48  and the bottom tool  42  (FIG.  7 ). The top tool  48  is arranged so that its ends extend slightly upward from the top surfaces of the side tools  44 , the significance of which will be discussed in detail below. 
     Also importantly, the top tool  48  is received between and slightly extends above the end tools  46 . The function of this arrangement is also described below. 
     The tooling configuration illustrated in FIG. 6 allows the fit of the bottom tool  42 , side tools  44 , end tools  46 , and top tools  48  to be individually indexed in order to insure proper dimensional tolerances. For example, the individual fasteners connecting the various tools and tool mounts may be adjusted in order to make small fit up adjustments for proper indexing. 
     As illustrated in FIG. 7, in addition to the use of the various tools and tool mounts described above, the tooling concept  40  also includes a compliant exterior cover  82 . The compliant exterior cover  82  covers the exterior surfaces of the side tools  44  and bottom tool  42  and also extends around and covers a portion of the sides and ends of the top tool  48  a sufficient amount to cover the portions of the tooling concept that are to come in contact with the composite workpiece that ultimately forms the wing box  20 . Preferably, the compliant exterior cover  82  extends around the side tools  44  and the bottom tool  42  and up to the outer side edges of the top tool  48  (FIG.  7 ). The top tool  48  preferably extends above the top surfaces of the side tools  44  an amount that is greater than the thickness of the compliant exterior cover  82 . Likewise, the compliant exterior cover  82  extends over the end tools  46  around and into abutment with the ends of the top tool  48 . The top tool  48  also extends above the top surfaces of the end tools  46  an amount that is greater than the thickness of the compliant cover  82 . 
     In the preferred embodiment, the compliant exterior cover  82  is formed of a material that has a much greater coefficient of thermal expansion than the materials from which the tools of the tooling concept  40  are formed. The greater coefficient of thermal expansion of the compliant cover assists the compliant cover in helping to compact and consolidate the composite material during curing. In addition, as described in more detail below, the expansion of the compliant cover also helps to eliminate pressure peaks and valleys produced by the tooling concept on the composite material during curing. 
     As will be described in more detail below, the cover  82  is formed of a pliable material that expands at elevated temperatures. Some exemplary materials from which to form the compliant exterior cover  82  include high temperature silicone rubber materials. In FIG. 7 the compliant cover  82  is shown to have approximately a constant cross-sectional thickness. However, as will be described in more detail below, the compliant exterior cover  82  may be tailored to have varying thicknesses at different locations in the tooling concept. 
     In the preferred embodiment, the compliant exterior cover  82  is formed by first fabricating a female tool having an interior forming surface that corresponds with the desired exterior surface of the compliant exterior cover  82 . The assembled tooling concept  40  is then lowered into the female mold and silicon rubber or other appropriate materials are then poured or injected into the mold around the tooling concept in order to form the compliant exterior cover  82 . 
     In addition to the compliant exterior cover  82 , the thicknesses of various portions of the side tools  44 , end tools  46 , bottom tool  42 , and top tool  48  may also change at different locations within the tooling concept. The various tools or portions of the tools or end and side tool mounts may also be formed of different materials having different coefficients of thermal expansion. As described below, forming the tooling concept  40  from different materials having different coefficients of thermal expansion allows the tooling concept to produce varying magnitudes of expansion at different locations in the tooling concept. 
     As can be seen in FIGS. 5-7, the exterior surfaces of the bottom, top, side and end tools and compliant exterior cover  82  are configured to produce the desired geometry of the spars  24 , ribs  26 , and outer skin panel  22 , including the structural buildup sections  32 , etc. The operation and function of the tooling concept  40  during curing of the composite workpiece used to form the wing box  20  will be described in detail below. First, the basic lay up procedure and fabrication method used to form the composite workpiece and wing box  20  will be described by reference to FIGS. 8-13. 
     FIG. 8 illustrates a schematic representation of a single tooling concept  40 . The first step in fabricating the composite workpiece is to cut and lay up individual layers of composite prepreg over both ends of the tooling concept  40  to form opposing end caps  90 . The end caps  90  are formed by first cutting individual layers of composite prepreg to size. The layers of composite prepreg are then individually placed over the compliant exterior cover  82  and end tools  46  and formed around the ends of the compliant exterior cover  82  and end tools  46  in order to form the end caps  90 . The end caps  90  are then debulked using composite fabrication techniques well known in the industry. The end caps  90  may be formed from unidirectional layers of composite prepreg, cloth layers of composite prepreg, or combinations of both unidirectional and cloth layers of prepreg. 
     The composite material used to form the end caps  90  extends up to and against but does not cover, the top tool  48 . The edge of the top tool  48  can be used as a cutting surface for trimming the composite material that forms the end cap  90 . 
     As is best seen in FIGS. 1,  4  and  11 , the end caps  90  form the ribs  26  of the wing box  20  after all the tools are assembled and the resulting workpiece is cured. In order to maintain symmetry and structural integrity, it is advantageous that each of the individual end caps  90  contains half of the layers of composite material forming each rib  26 . In addition, the portions of the end caps  90  that extend inward over the sides of the end tools  46  also form the lower portion of the flanges of the ribs  26 . 
     After the formation of the end caps  90 , two tooling concepts  40  are placed together with their ends aligned so that the two adjoining end caps  90  contact each other and are aligned with each other. The intersection  92  between the exterior edges of the two end caps  90  form ribs  26  and are filled with a rectangular radius filler  94  in a manner well known in the art. The bottom surface of the compliant exterior cover  82  and joined tooling concepts  40  is then covered with individual layers of composite material to form a bottom U-channel  100 . The bottom U-channel  100  is formed by laying up individual layers of composite material over the bottom of the joined tooling concepts and pressing the edges of the composite material around the sides of the compliant exterior cover  82  and the side tools  44  to form the legs of the U-channel. FIG. 9 illustrates the bottom U-channel  100  being formed over the joined tooling concepts  40  in an upside down position in order to assist in fabrication. After being laid up, the U-channel  100  is debulked in a manner well known in the art. As best illustrated in FIG. 3, the bottom U-channel  100  forms the interior surface of the outer skin panel  22  and also forms a portion of the structural buildup sections  32 , both on the outer skin panel  22  and on the central shear webs  30 . 
     After forming the bottom U-channel  100 , two side U-channels  102  are formed by applying individual layers of composite prepreg to both sides of the compliant exterior cover  82  and the joined tooling concepts  40 . As illustrated in FIG. 10, the side U-channels  102  are formed by applying individual layers of composite prepreg on the sides of the compliant exterior cover  82  and the side tools  46  and forming the layers of prepreg partially around the bottom tool in order to form a portion of the structural buildup sections  32 . The layers of the prepreg are formed over the top edge of the side tools  44  so that they extend to and abut the side edges of the top tool  48 . The prepreg does not extend over the top tool  48 . The side edges of the top tool  48  provide a cutting surface for trimming the prepreg. Tape (not shown) can be placed over the holes  76  to prevent flow of resin into the holes. As best illustrated in FIG. 3, each side U-channel  102  forms one half of the central shear web  30  and the upper legs of the U-channels  102  form the lower portion of the upper spar caps  34 ; and the lower legs of the U-channels  102  form the middle portion of the structural buildup sections  32 . After laying up the side U-channels  102 , they are debulked in a manner well known in the art. 
     Additional tooling concepts  40  are then joined together and layers of composite material are applied as described above. The completed tooling concepts  40  are then placed side by side on top of a prelaid up and formed outer skin panel  22  as illustrated in FIG.  11 . The outer skin panel  22  is formed by laying up individual layers of composite prepreg on a skin tool  86  having the desired contour. Additional layers of composite prepreg are added to the skin panel  22  to form the pad up sections  28  on the outer skin panel  22 . As illustrated in FIG. 11, the joined tooling concepts  40  are placed side by side on top of the outer skin panel  22  such that each pair of joined tooling concepts is placed on top of and in between two adjoining pad up sections  28 . The gaps between the upper and lower edges of the adjoining side U-channels  102  are filled with radius fillers  106 , as is well known in the art. 
     After placing all of the tooling concepts  40  on the outer skin panel  22 , side tools  108  and end tools  110  are placed around the sides and ends of the properly located tooling concepts  40  (FIG.  12 ). Each side tool  108  includes a forming surface identical to that of spars  24  (not shown) upon which side U-channels are formed in order to complete the spars  24  closest to the edges of the completed wing box  20 . Similarly, the end tools  110  also include a forming surface (not shown) that imparts a shaped surface to the ends of completed wing box  20 . The forming surface on the end tools  110  are placed adjacent the end caps  90  on the tooling concepts  40 . After positioning the side tools  108  and end tools  110 , additional layers of composite prepreg are placed over the tops of the various flanges formed by the upper flanges of the side U-channels  102  and the end caps  90  to form the cap plys  118  on the top portion of the spar caps  34  and rib caps. As shown in FIGS. 3 and 12, the cap plys  118  are placed over the top of the adjoining flanges of the adjoining side U-channels  102  and also over the top of the flanges of the end caps  90  in order to complete the composite workpiece. 
     As best illustrated in FIGS. 11-13, the entire assembly, consisting of the formed tooling concepts  40 , outer skin panel  22 , and side and end tools  108  and  110 , rest upon the skin tool  86 . After all the tools are in the proper position, caul sheets are placed over the top of the composite workpiece and cover plates are placed over the top of the side and end tools  108  and  110 . It may also be advantageous to place breather plys and release plys or other materials over the top of the composite workpiece, as is well known in the art. The resulting structure is then placed within a vacuum bag  120  (FIG. 13) which is sealed to the skin tool  86  in a manner well known in the art. The interior of the vacuum bag is connected to an autoclave vacuum pump, and the composite workpiece and associated tooling is placed within the autoclave. The processing parameters and processing conditions for curing the composite workpiece differ depending upon the composite material used to form the workpiece and are easily ascertained by those of ordinary skill in the art. 
     The fabrication and operation of the tooling concept  40  during curing of the composite workpiece will now be described in more detail by reference to FIG.  14 . As discussed briefly above, it is possible to analytically predict both the preconsolidated dimensions of the composite workpiece used to form the wing box  20  and the consolidated and cured geometry of the formed wing box. Using this information, in accordance with the present invention, the configuration of the tooling concepts  40  can be determined. Knowing the preimposed consolidated geometries of the composite workpiece, the tooling concepts  40  are configured and fabricated of appropriate materials having appropriate coefficients of thermal expansion to produce the varying magnitudes of expansion desired to result in complete consolidation of the composite workpiece used to form the composite wing box  20 . 
     For example, in the preferred method of the invention, the composite wing box  20  is designed based upon the structural loads that the wing box is intended to carry. Using this design, a geometric model of the wing box  20  is then produced based upon known material properties of the composite materials used to form the wing box. Another geometric model based upon the thicknesses of the composite materials prior to consolidation is then calculated thus creating a geometric model of the unconsolidated and uncured composite workpiece. Next, the cure cycle used for the composite materials from which the wing box is fabricated is analyzed to determine the temperature at which the resin within the composite material undergoes resin gelation. The difference between the resin gelation temperature of the composite resin and the room temperature at which the tooling concept will be used is then calculated. This provides the difference in temperature to be used along with the coefficients of thermal expansion of the various materials from which the tooling is to be formed to produce a configuration for the tooling concept. 
     Using the difference in temperature and known coefficients of thermal expansion, the tooling is configured to produce the desired expansion characteristics that will result in the desired consolidation pressures as various areas of different thickness in the composite workpiece. In addition, the thermal expansion characteristics of the compliant exterior covering  82  during expansion of the tooling concept is also taken into account. The compliant nature of the exterior covering  82  helps to even out pressure peaks or valleys to produce the desired consolidation pressures at the various areas in the composite material. The expansion characteristics of the tooling concept can be determined using either analytical calculations or, in more complex tooling configurations, through the use of 2-D or 3-D linear or nonlinear finite element analysis. 
     For example, in the preferred embodiment, the tooling concept  40  is configured using the difference in temperature and known coefficients of thermal expansion of the tooling materials and compliant exterior covering  82  to produce a tooling concept that results in the desired magnitudes of tooling expansion to properly consolidate both the thinner areas of the workpiece in the central shear webs  30  and also the thicker areas of the workpiece in the region of the structural buildup sections  32 . Thus, the tooling concept  40  in the preferred embodiment shown produces greater magnitudes of expansion to debulk and consolidate the added layers of composite material in the structural buildup sections  32  than it does in the thinner central shear webs  30 . 
     The compliant exterior covering  82  also assists the invention in the production of approximately a hydrostatic force on the various areas of the composite workpiece. As illustrated in FIG. 14, during curing and consolidation, the compliant exterior covering  82  is able to expand or be compressed at various regions in the tooling in order to produce approximately a constant consolidation force over its entire surface. The compliant nature of the silicone rubber or other material from which the compliant exterior covering  82  is formed assists the tooling concept  40  in producing approximately a hydrostatic force throughout the tooling concept. 
     In addition to carefully designing the tooling concept  40  to account for the thermal expansion characteristics desired to achieve the proper part tolerances, it is also important to balance the consolidation pressures produced by the various tooling concepts across the various portions of the composite workpiece. For example, as illustrated in FIG. 14, it is important that each of the tooling concepts  40  apply approximately the same magnitude of consolidation pressure to each side of the spars  24  as illustrated by arrows  130 . Matching the overall magnitude of consolidation force applied across the composite workpiece at various locations throughout the workpiece, ensures that the various portions of the composite workpiece, such as the spars  24 , will be maintained in the proper position throughout the consolidation and curing of the composite workpiece to form the wing box  20 . Unlike prior tooling concepts, the tooling concepts  40  are free to float and are not anchored to external frames or addition tools. The tools can be allowed to float because they are configured to produce matched forces on both sides of the various areas of the workpiece. Because the forces produced by the tooling concepts are matched on both sides of the workpiece, for example the central shear web  30 , the web remains in the proper position during consolidation and curing. 
     In order to produce the varying magnitudes of thermal expansion desired in more complex composite parts, the various tools used to form the tooling concepts  40  may have varying thicknesses and may be formed from different materials. For example, in the present invention, the lower portions of the side tools may be thicker or thinner than the rest of the tool depending upon the application to produce a greater or lesser degree of thermal expansion in the region of the structural buildup sections  32 . Alternatively, the compliant exterior covering  82  may also be formed having varying thicknesses at different locations in order to intensify or decrease the magnitude of the consolidation force provided by expansion of the compliant covering  82 . As described above, the most accurate way to predict tool expansion is through the use of a 2-D or 3-D finite element model. 
     In the preferred embodiment, the tooling concepts  40  are hollow and are formed from separate bottom, side, end and top tools  42 ,  44 ,  46  and  48 . Unlike the use of solid tools, the tooling concept  40  allows the various portions of the tools to be formed of differing materials having different thicknesses. In addition, the tooling concept  40  allows the magnitudes of expansion at different locations within the tooling to differ, thus producing some of the advantages of the present invention. The use of the compliant covering  82  also allows the expansion of the tooling concept to be tailored to account for varying levels of thermal expansion in different parts of the tooling concept. In addition, the compliant nature of the compliant exterior covering  82  helps to equalize the pressures applied by the tooling over the surface of the composite workpiece thus helping to produce a hydrostatic pressure. 
     After curing, the side tools  108  and end tools  110  are removed from the wing box  20 . The tooling concepts  40  are then ready for removal from the cavities  36  of the completed wing box  20 . Because the end caps  90  and side U-channels  102  do not extend over the top tool  48 , the ends of the cap bolts (not shown) in the holes  76  are accessible by a tool such as a hex key. If tape was placed over the holes, the tape is first removed. Each of the bolts are then removed and the top tool  48  is removed from the partially closed opening  38  of the cavity  36 . A worker then has access to the internal portions of the tooling concept  40 , and can remove the bolts holding the side tool mounts  50  and end tool mounts  52  against the end tools  46 , side tools  44 , and bottom tool  42 . As the bolts are removed, the side tool mounts  50  and end tool mounts  52 , as well as the end tools  46 , side tools  44 , and bottom tool  42  can be removed from the cavity  36  through the partially closed opening  38 . If desired, a portion of one of the side tool mounts  50  or end tool mounts  52  can remain attached to one of the side tools  44 , end tools  46 , or bottom tool  42 , so as to aid in removal of the respective tool. Because the bottom tool  42  is aligned between the end tools  46 , it is a length that permits its removal through the partially closed opening  38 . Likewise, because the side tools  44  are received within the end tools  46 , each of the side tools  44  can be removed through the partially closed opening  38 . Finally, the end tools  46  can be tilted and rotated so that they can be removed through the partially closed opening  38  of the cavity  36 . The compliant exterior cover  82  can then be removed from the cavity  36 . 
     In this manner, the entire tooling concept  40  can be removed from the cavity  36  without damage to the wing box  20 . In addition, the tools can be reassembled so as to form another tooling concept  40  for assembly of another wing box  20 . Thus, the tooling concept  40  provides a simple and efficient manner of forming a cavity with partially closed opening  38  in a composite part (the wing box  20 ). 
     The tooling concept  40  permits a cavity  36  to be formed in a composite part (in the example, a wing box  20 ). In the embodiment shown, the composite part has an opening for removing the tooling concept  40  that is smaller than a cross-section of the tooling concept parallel to the opening. Because of this configuration, the tooling concept cannot be simply removed from the cavity  36  after curing. Instead, the separate tools provided of the tooling concept  40  provide disassembly of the tooling concept and removal through the partially closed opening  38 . 
     As discussed briefly above, in order to obtain high quality composite parts and help to eliminate areas of resin richness or resin porosity, it is important to apply proper consolidation pressures throughout the surface of the workpiece. However, it is also important to carefully maintain the tolerances of the various surfaces and positions of the various parts of the composite workpiece. The tooling concept used in the present invention allows both these goals to be achieved through the use of multiple materials having differing thermal expansion characteristics. 
     While the preferred embodiment of the invention has been illustrated and described, it will be apparent that various changes can be made therein without departing from the spirit and scope of the invention. For example, various materials can be used in the fabrication of tooling concepts according to the invention. In addition, the geometry and general configuration of the tooling concepts can change depending upon the applications and geometries of the composite workpieces being produced.