Abstract:
A method for production of fiber composite components, particularly suitable for the production of profiles of complicated shape with varying profile cross-section and/or with at least parts with curved lines, such as used for the production of aircraft, for example. The method (a) produces plural core components, (b) separates application of a first fiber material to each of the core components, (c) assembles the core components provided with the first fiber material to form a sequence of core components, (d) applies a second fiber material common to the core component sequence along at least one side of the core component sequence, provided with the first fiber material, (e) impregnates and hardens the first and second fiber materials to form a fiber-reinforced component body, and (f) separates the component body into plural sections as represented by the fiber composite components.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for production of fiber-composite structural elements. 
     2. Discussion of the Background 
     The use of fiber-composite structural elements is interesting for many areas of application, in particular because of their high specific strength (ratio of strength to weight). A fiber-composite material is a mixed material that is generally composed of two main components, namely a matrix and fibers embedded therein. Mutual interactions of these components endow the material with higher-performance characteristics than those of the two individual components involved. 
     In particular, the present invention relates to the production of highly stressed profile sections having more or less complicated geometry. According to prior art based on internal industrial know-how of the Applicant, carbon-fiber-reinforced plastic profile sections, for example, are mostly produced at present either in prepreg technology or by draping semifinished textile products (woven and nonwoven fabrics, fiber mats, etc.) of carbon fibers. However, this requires a relatively large amount of manual labor. In the production of curved profile sections, the cutting loss is typically as high as 50%. 
     The only manufacturing process known to date to be more streamlined with a small percentage of waste for the production of carbon-fiber-reinforced plastic profile sections is pultrusion. However, only straight profile sections with constant cross section can be manufactured with this process. Local thick zones, partly optimized fiber angle or even modifications of the shape are not possible. For practical purposes, therefore, such structural elements often have to be provided with thick zones (for stiffening and/or subsequent force transmission) by laborious post-processing. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a simple method for the production of fiber-composite structural elements, which method is also suitable in particular for the production of fiber-composite structural elements having complex geometry, such as profile sections with varying profile cross section and/or with curved shape in at least some portions. 
     This object is achieved according to the invention by a method for production of fiber-composite structural elements comprising the steps of:
         a) providing a plurality of core parts,   b) separately applying a first fiber material on each of the core parts,   c) joining the core parts lined with the first fiber material to one another to form a core-part row,   d) applying a second fiber material common to the core-part row along at least one side of the core-part row on which the core parts are lined with the first fiber material,   e) infiltrating and curing the first and second fiber materials to form a fiber-reinforced structural-element block, and   f) subdividing the structural-element block into a plurality of portions, which represent the fiber-composite structural elements.       

     With this method it is possible in simple manner to produce even fiber-composite structural elements of complex shape, especially, for example, even elongated curved profiled structural elements with irregular radius of curvature. A particular advantage of the invention is that a plurality of fiber-composite structural elements, especially a plurality of identical fiber-composite structural elements, can be produced simultaneously and therefore inexpensively with the method (“package manufacture”). In this respect, the simultaneous production of at least three, especially at least five or even at least ten structural elements is preferred. 
     In principle, the inventive method is not subject to any special restrictions on the type of fiber material to be used (such as individual fibers, rovings, flat semifinished fiber products, etc.) or on the type of matrix material to be used. In one embodiment, the use of carbon fibers is provided. Alternatively or additionally, however, it is possible without difficulty to use even other fibers, such as glass fibers, synthetic plastic fibers, steel fibers or natural fibers. Interesting in particular as matrix materials are plastics, such as thermosetting plastics (synthetic resins). However, the items in these lists are to be understood only as examples. Moreover, fillers or additives may be incorporated in ways known in themselves if necessary. 
     The terms “first fiber material” and “second fiber material” used here are intended to convey the fact that fiber material is used in two stages in the inventive method, namely first in step b), in which a fiber material is applied separately on each of the core parts, and later in step d), in which a fiber material is applied on the previously formed core-part row. These terms are not intended to give the impression that the first fiber material must be different from the second fiber material. This can indeed be provided, but is by no means imperative. In this sense, it is also possible to use a plurality of different first fiber materials in step b) and/or a plurality of different second fiber materials in step d). The term “application” (of the fiber material in question) is to be understood very broadly within the scope of the invention. As an example, individual fibers can be applied by a circular braiding or wrapping process. Alternatively or additionally, application may also take place by laying (especially in the case of a flat semifinished textile product), fixed if necessary by an adhesive layer. 
     In a preferred embodiment, the core parts are formed as profiled parts, whose profile cross section varies over the longitudinal extent of the profile and/or whose longitudinal extent of the profile has curved shape in at least portions. Since the core parts being used define the shape for the manufactured structural elements, it is therefore possible to produce, with this measure, profiled structural elements wherein the profile cross section varies over the longitudinal extent of the profile and/or wherein the longitudinal extent of the profile has curved shape (including “kinked” shape) in at least portions. In a preferred embodiment, the produced profiled structural elements are elongated, and in particular the longitudinal extent of the profile is larger by a factor of at least 5, especially by a factor of at least 10 than the maximum longitudinal extent of the profile. 
     To produce a plurality of elongated profiled structural elements, it is possible to provide, in step a) for example, a plurality of identical elongated profiled core parts, which, after application of the fiber material in step b), are joined to one another along their longitudinal sides to form a core-part row, before the fiber material common to the core-part row is applied in step d), for example by being laid. The core-part row then has a length that corresponds to the length of the individual profiled core parts and a width that corresponds to the sum of the widths of the individual profiled core parts plus the thicknesses of the fiber material in the direction in which the core parts are joined to one another or arranged in a row. 
     The core parts used in the method can be provided as reusable or non-reusable parts. In the case of reusable core parts, which may be made of metal (such as aluminum), for example, it is merely necessary to ensure that these are not damaged while the structural-element block is being subdivided in step f), if at that time the core parts are still located inside the structural-element block. Non-reusable core parts can be formed in simple manner, for example from plastic, especially foamed plastic, and can be cut apart from one another and therefore destroyed if necessary during subdivision of the structural-element block. 
     In one embodiment it is provided that the core parts have at least one local recess, which is filled with fiber material in step b). In this way local thick zones for the finished structural element can be created in simple manner. An alternative or additional possibility is to form such local thick zones by laying the first fiber material in step b) and/or the second fiber material in step d) in a thickness that is greater at one or more locations. 
     If necessary, local thick zones can be formed by a special material, such as fiber material, which may be different from the first fiber material and/or the second fiber material. If the core part is provided with one or more of the aforesaid local recesses for this purpose, it is possible, for example, to lay one or more cut-to-size pieces of a semifinished textile product in each of these recesses (integration of reinforcing plies). In a more special embodiment, it is provided that a local recess at the surface of the core part in question is filled to level condition and subsequently completely covered during application of the first fiber material. 
     In a preferred embodiment it is provided that the application of the first fiber material in step b) comprises circular braiding and/or wrapping of the individual core parts. This method of applying the first fiber material can be automated very readily, for example by using a method or an apparatus such as described in German Unexamined Patent Application DE 102004017311 A1 (for the production of semifinished fiber-composite products). Accordingly, the application of the first fiber material in step b) can be accomplished in particular by means of circular braiding techniques, wherein the core part in question is braided with braiding threads wound over lace bobbins revolving concentrically in different directions around the core part. In this case it can be provided in particular that the lace bobbins of one direction of revolution are filled with reinforcing threads and the lace bobbins of the opposite direction of revolution are filled at least partly with support threads, which are able to hold the reinforcing threads in position and which can consist at least partly of thermoplastic threads. In a preferred improvement, the core part is braided several times, in each case by laying unidirectional individual plies of the first fiber material onto the core part. By virtue of automated application of the first fiber material by a circular braiding technique, it is advantageously possible to achieve high reproducibility and well-defined fiber orientation (unidirectional or multidirectional). Moreover, additional strips of fiber material can be interposed in simple manner during the circular braiding process. In a preferred embodiment of circular braiding, an even braid, or in other words a non-wavy braid, is formed. 
     The local thick zones explained in the foregoing can be formed directly during circular braiding and/or wrapping as early as step b), or else they can be formed by locally repeated circular braiding or wrapping processes or even by flat semifinished fiber-material products (cut-to-size pieces, strips, etc.) to be additionally inserted. 
     In principle, another conceivable alternative to circular braiding or wrapping in step b) is in particular draping of fiber-material plies, although this usually involves considerably more manual labor in practice. In one embodiment, it is therefore provided that a large part of the first fiber material will be laid on the core part in question by circular braiding or wrapping, while if need be a smaller proportion of the first fiber material is laid in the form of a flat semifinished fiber composite, which if necessary is cut to size. The latter application of a ply of fiber material may be accompanied in particular (before and/or after) by circular braiding or wrapping. 
     In one embodiment it is provided that the core parts lined and joined together are fixed in step c) by means of a holding device, which is part of an infiltration unit used for step e). The infiltration unit can in turn be part of a mold, in which both infiltration of the fiber material with the matrix material in question and at least partial curing of the fiber-matrix composite are performed. Depending on the number of pieces of fiber-composite structural elements to be produced, either an open mold or a closed mold may be used during joining together (stacking) of the lined core parts and/or during application of the second fiber material (for example, cover plies of multiaxial nonwoven fabrics, woven fabrics, etc. on one or two sides) and/or during infiltration or curing. 
     In a preferred embodiment it is provided that the application of the second fiber material in step d) comprises the application of at least one sheet-like ply of fiber material (such as semifinished textile), which may be in particular a cut-to-size piece or strip of multiaxial nonwoven fabric, woven fabric or similar material. 
     The second fiber material may be laid if necessary on both sides of the core-part row, using the same or different material type and the same or different material thickness (and also in a plurality of layers). 
     The infiltration and curing provided in step e) can be advantageously achieved with all methods known in themselves from fiber-composite manufacturing practice (such as VAP, RTM, etc.). In the case of infiltration with an epoxy resin, it is possible to achieve curing thermally, for example, in a temperature range from room temperature to approximately 180° C., depending on the nature of the resin. 
     In one embodiment it is provided that the structural-element block is subdivided in step f) by a plurality of parting cuts that take place respectively in the region of one of the core parts, wherein each parting cut causes the fiber material (and possibly the core part) adjoining the core part in question to be split and thus associated with several of the resulting fiber-composite structural elements. 
     Core parts not scheduled for reuse (such as foamed-material cores) may be destroyed if necessary during removal from the mold. 
     The fiber-composite structural elements formed by subdivision of the structural-element block in step f) may be post-machined if necessary before being used. 
     The invention will be described further hereinafter on the basis of exemplary embodiments with reference to the attached drawings, wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a diagram for illustration of the production of a plurality of fiber-reinforced profile sections, wherein the profile cross section varies over the longitudinal extent of the profile, 
         FIG. 2  shows a diagram for illustration of the production of a plurality of fiber-reinforced profile sections, wherein the longitudinal extent of the profile has a curved shape, 
         FIG. 3  shows a detail from  FIG. 2 , 
         FIG. 4  shows a perspective view of a core part according to a further embodiment, 
         FIG. 5  shows a diagram for illustration of examples of the geometries of several core-parts or fiber-composite structural elements, 
         FIG. 6  shows a schematic diagram of the automated application of a fiber material on a core part, and 
         FIG. 7  shows a schematic diagram of the application of a multiaxial fiber material on individual core parts, on the one hand for core parts with rectilinear extent and on the other hand for core parts with curved extent. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates, from left to right, various stages during the simultaneous production of six fiber-composite structural elements  10  in the form of I-beam sections (see extreme right in  FIG. 1 ). 
     This “package manufacture” of profile sections  10 , which will be subsequently usable as crossbeams in the floor of an aircraft fuselage, for example, comprises the following steps:
     a) Firstly there are provided core parts  12  of the type illustrated at the extreme left in  FIG. 1  (in  FIG. 7  there are illustrated seven such core parts, which in the illustrated exemplary embodiment are formed as core-part profile sections, which are elongated in one direction and whose profile cross section varies over the longitudinal extent of the profile). In the illustrated example the profile cross section is rectangular, with visible indentations  14 , where the cross-sectional area is correspondingly reduced, at two positions along the extent of the profile. As an example, core parts  12  can be produced by milling a commercial hard foamed material or else a metal material. Their nature is of secondary importance, since they are used only as shaping aids in the production of the actual structural elements  10 .   b) In the next step, a first fiber material  16  is applied separately on each of core parts  12 . In the illustrated exemplary embodiment, this step comprises circular braiding of core parts  12  with the fiber material. In view of the subsequent use of structural elements  10 , a favorable fiber orientation (or plurality of fiber orientations) can be provided in a manner known in itself for this purpose. Such core parts  12  are used as “braided cores”.   c) Core parts  12  lined with first fiber material  16  are then joined to one another to form a core-part row  18 . In the illustrated exemplary embodiment, each two immediately neighboring core parts  12  enveloped with fiber material  16  (completely, except for the end faces) are in contact along a plane boundary face, so that the individual lined core parts  12  bear against one another with their entire surface as viewed in stacking direction.   d) A second fiber material  20  is then applied along at least one side of core-part row  18 . In the illustrated exemplary embodiment, this fiber material  20  is applied both on the bottom side and top side of illustrated core-part row  18 .   e) The structure created in this way and comprising shaping core parts  12  lined up against one another and also fiber materials  16 ,  20  is then infiltrated with a suitable matrix material (such as epoxy resin) and thermally cured. In a preferred embodiment, the curable matrix material is added by using a vacuum infusion process, for example by means of a standard infusion process such as VAP, VARI, etc. For this purpose it is possible to use, for example, an infiltration system with membrane structure, wherein appropriate inlets and outlets for a resin matrix are provided in a covering film and/or in a mold. Such a vacuum system may also be used if necessary for compaction of core-part row  18  lined with fiber material. When the lined core-part row is covered with an airtight film and then the space under the film is more or less evacuated, the system is subjected to the atmospheric ambient pressure. Alternatively or in addition to evacuation of the space bounded by the film, an elevated ambient pressure may be applied, for example by introducing lined core-part row  18  together with film in an autoclave. Complete curing or even only partial curing may be provided in this step.   f) Finally, the structural-element block created in the previous step by infiltration and curing is subdivided into a plurality of portions, which (after final post-machining if necessary) represent fiber-composite structural elements  10 . In general, depending on the geometry of core parts  12  and the thickness of the binding of fiber material at the surface of core parts  12 , it will not be possible to remove these core parts before the structural-element block is subdivided. In one embodiment, it is therefore provided that both fiber material  16 ,  20  and also core parts  12  will be severed by parting cuts (core parts  12  not reusable). In another embodiment, it is provided that parting cuts will be made in such a way that only fiber material  16 ,  20  is severed thereby and that core parts  12  released as a result are reusable.   

     By means of the described production method, six fiber-reinforced I-beams  10  are produced simultaneously by using seven core parts  12 , in the manner illustrated in  FIG. 1 . In this case, the structural-element block is subdivided by seven parting cuts (vertical in  FIG. 1 ), each passing through the region of one of core parts  12 , wherein each parting cut causes fiber material  16 ,  20  adjoining core part  12  in question to be split and thus associated with several of the resulting fiber-composite structural elements  10 . 
     In a deviation from the illustrated exemplary embodiment, individual beams  10  could be endowed in simple manner with an approximate C-shaped profile, by positioning the planes of the vertical parting cuts somewhat offset relative to the illustrated exemplary embodiment (by approximately half the width of a core part  12 ). 
     Starting from the structural-element block illustrated in  FIG. 1 , it would also be possible to produce (twice as many) T-beams or L-beams, for example, by the subdivision step, for example by providing a horizontal parting cut in addition to the vertical parting cuts. 
     As an example, the I-beams or C-beams originally obtained by separating the structural-element block are each split one more time horizontally. 
     In the exemplary embodiment according to  FIG. 1 , core parts  12  are each elongated and have identical shape, and the longitudinal extent of their profile is rectilinear (and orthogonal to the subsequent stacking direction). However, the profile cross section varies over the longitudinal extent of the profile (see indentations  14 ). Profiled core parts  12  are arranged with identical orientation in core-part stack  18 . 
     In the description hereinafter of further exemplary embodiments, like reference numerals are used for components having like effect, but in each case they are supplemented by a lower-case letter to distinguish the embodiment. Thus substantially only the differences relative to the already described exemplary embodiment or embodiments will be pointed out and otherwise the description of the preceding exemplary embodiments will be expressly applicable by reference. 
       FIGS. 2 and 3  illustrate an exemplary embodiment of a method for simultaneous production of a plurality of fiber-composite structural elements, which are again formed as I-beam sections as in the example described hereinabove with reference to  FIG. 1 . In a deviation from the foregoing example, however, the longitudinal extent of the produced profiled structural elements has curved shape. 
     In a diagram similar to  FIG. 1 ,  FIG. 2  shows, at the extreme left, a core part  12   a , which is again provided several times in identical form for the method described hereinafter. Further to the right in  FIG. 2  there are again illustrated further intermediate production stages, namely
         a core part  12   a  lined (for example, wrapped) with a first fiber material  16   a,      a core-part row  18   a  formed by joining core parts  12   a  lined with the first fiber material  16   a  to one another,   core-part row  18   a  lined additionally on its bottom side and top side with a second fiber material  20   a , and   a fiber-composite structural element  10   a  obtained after infiltration, curing and subdivision of the structural-element block.       

       FIG. 3  once more illustrates, in an enlarged detail diagram, the arrangement of fiber materials  16   a ,  20   a  along the longitudinal sides of core-part row  18   a . As an example, it is shown that fiber-material layers  20   a  on the two opposite flat sides (top side and bottom side) of core-part row  18   a  may also have different material thicknesses. Such different layer thicknesses of second fiber material  20   a  as well as the ratio of these layer thicknesses to the layer thickness of first fiber material  16   a  can be advantageously adapted to the mechanical stresses expected on finished structural element  10   a . In this respect it is also favorable under some circumstances to provide—viewed in the section plane of  FIG. 3  and/or viewed in longitudinal direction of the core-part row—a non-uniform thicknesses and/or a non-uniform material for at least one of fiber materials  16   a ,  20   a , thus deviating from the illustrated exemplary embodiment. 
     The parting cuts made at the end of the production process in order to separate structural elements  10   a  (into a plurality of I-beam sections) are indicated by broken lines in  FIG. 3 . 
       FIG. 4  is a detail diagram for illustration of the manner in which local thick zones can be created on the finished fiber-composite structural element in a production method of the type explained in the foregoing. 
     At the top of  FIG. 4  there is illustrated a portion of a core part  12   c  used in the production method and having a stepped recess  22   c  in a curved portion. Otherwise core part  12   c  has, for example, a rectangular cross-sectional contour. 
     In this exemplary embodiment it is provided that, during laying of a first fiber material on each of core parts  12   c , recess  22   c  is first filled completely with “additional first fiber material” before core part  12   c  is also lined (for example wrapped and/or circularly braided) with first fiber material in the regions adjacent to recess  22   c . Thus a local thick zone, which is “inwardly directed”, so to speak, is produced in the region of recess  22   c  of the finished structural element (not illustrated). Alternatively or additionally, it would obviously also be possible to provide “outwardly” directed thick zones, by forming corresponding thick zones during application of the first fiber material. 
     The fiber material to be introduced into the illustrated recess  22   c  could be composed, for example, of two cut-to-size pieces of a fiber mat laid successively (into the two illustrated recess regions). 
     The provision of recess  22   c  in a curved portion of core part  12   c  in order to form a reinforcement on the finished structural element is usually advantageous from the practical viewpoint, because structural elements of the type of interest here are usually subjected to greater stresses in the curved portions. 
     The special feature illustrated in  FIG. 4 , namely the formation of one or more local recesses for integration of additional fiber material, can be provided without difficulty for each of the exemplary embodiments described in the foregoing in connection with  FIG. 1  or  FIGS. 2 and 3  (or for a combination thereof). 
     A special advantage of the thick zone created by a recess of the core part being used is that the first fiber material additionally applied locally on the core part does not interfere with formation of a core-part row of core parts abutting one another with their entire surface even if the recess is oriented “in stacking direction”, or in other words is facing a neighboring core part in the core-part row. On the other hand, in the case of creation of local thick zones by “fiber material protruding outwardly on the core part”, it is usually advisable to produce additional fiber material on a side of the core part that is not directly facing a neighboring core part in the core-part row. In core-part rows  18  or  18   a  illustrated in  FIGS. 1 and 2 , these are the top sides and bottom sides of the core-part row in question. 
     As regards the preferred use of the fiber-composite structural elements as a structural member in aircraft construction, local thick zones may be particularly expedient, especially in portions of a profiled structural element that are curved or have reduced contour area. 
     As already explained, the described production method is suitable in particular for “package manufacture” of a plurality of identical fiber-composite structural elements, which resemble profiled sections on the whole but in which the profile cross section varies over the longitudinal extent of the profile and/or the longitudinal extent of the profile is curved in one or more regions. 
       FIG. 5  shows longitudinal sections through several fiber-composite structural elements  10   d ,  10   e ,  10   f  and  10   g  that can be produced with the described method. Obviously the illustrated longitudinal-section shapes are to be understood merely as examples and are intended to illustrate the great flexibility of the method as regards the geometry of the fiber-composite structural elements that can be produced. 
     Many working steps in the described method can be advantageously performed in at least partly automated manner. This will be illustrated hereinafter with reference to FIGS.  6  and  7 , taking the step of separate laying of the first fiber material on each of the plurality of core parts as an example. 
       FIG. 6  schematically shows an apparatus  40   h , by means of which part of the first fiber material to be laid on a core part  12   h  in step b) is applied in automated manner. 
     Each core part  12   h  is first provided on portions of two opposite longitudinal sides with cut-to-size pieces  42   h  of a semifinished textile product (such as cut-to-size pieces glued on in the manner of labels). 
     Core part  12   h  already provided in portions with the first fiber material (cut-to-size piece  42   h ) is then conveyed in the direction of arrow  44   h  lengthwise through apparatus  40   h.  In the process, the opposite top and bottom sides of core part  12   h  shown in  FIG. 6  are each provided with a continuous strip  46   h  of fiber material (such as semifinished textile product), which is unwound from respective supply rolls. Finally, an enveloping fiber-material layer  48   h  is formed by apparatus  40   h , also in automated manner, by circular braiding in the illustrated exemplary embodiment. 
     Several of these core parts  12   h  lined with first fiber material  42   h ,  46   h  and  48   h  are then joined to one another to form a core-part row, and are lined with a second fiber material common to all core parts, as already described in the foregoing examples according to  FIG. 1  or  FIGS. 2 and 3 . Moreover, the second fiber material can also be laid (not illustrated) in automated manner. 
     The factors important for the mechanical characteristics of the fiber-composite structural elements produced by using apparatus  40   h  include the layer thickness and fiber orientation of the individual fiber materials, in this case fiber materials  42   h ,  46   h  and  48   h . In an improvement of the enveloping method illustrated in  FIG. 6 , there is provided an apparatus having a plurality of circular braiding stations disposed in succession in the direction of travel of the core part. Such an improvement will be described hereinafter with reference to  FIG. 7 . 
     At the top of  FIG. 7  there is schematically illustrated an apparatus  40   i , which comprises a core-feed unit  50   i , a plurality (four in this case) of coating stations (such as circular braiding units, and possibly also including devices for integration of additional fiber materials)  52   i  and one cutting unit  54   i  for severing the fiber material. 
     A stock of core parts  12   i  to be conveyed successively through apparatus  40   i  is denoted by  56   i . After coating of core parts  12   i  by means of apparatus  40   i , a stock  58   i  of core parts lined with fiber material is obtained. In this example, the circular braiding stations apply circular braiding having different fiber orientations (such as +45°, −45°, −45°, +45°) around core parts  12   i.    
     At the bottom of  FIG. 7  there is illustrated a slightly modified apparatus  40   j , in which the individual processing stations  50   j ,  52   j  and  54   j  are disposed along a curved path  44   j.  This processing path is chosen such that it is adapted to the curved shape of core parts  12   j  to be coated.