Abstract:
A method for producing a fiber composite component, in particular for aerospace, includes the following method steps: introducing an elastic core sleeve into a prestressing mechanism; expanding the core sleeve that is introduced, for elastic prestressing of the same, by activating the prestressing mechanism; introducing a core body through an opening of the expanded core sleeve; releasing the core sleeve by deactivating the prestressing mechanism, for the snug enclosing of the core body by the core sleeve and thus for the forming of the molding core ( 14 ); and at least partly laying at least one semifinished fiber product on the molding core that is formed, for the shaping of the fiber composite component to be produced.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for producing a fiber composite component, in particular for aviation and spaceflight, to a molding core for producing such a fiber composite component and to a fiber composite component with at least one stringer which is produced by means of such a molding core and/or such a method. 
     BACKGROUND OF THE INVENTION 
     Although it can be applied to any desired fiber composite components, the present invention and the problems on which it is based are explained in more detail below with reference to two-dimensional stringer-stiffened carbon fiber reinforced plastic (CRP) components, for example skin shells of an aircraft. 
     It is generally known to stiffen CRP skin shells with CRP stringers in order to withstand the loads occurring in the aircraft sector with the lowest possible additional weight. In this respect, a distinction is drawn essentially between two types of stringers: T and Ω stringers. 
     The cross section of T stringers is made up of a base and a stem. The base forms the connecting surface with respect to the skin shell. The use of skin shells stiffened with T stringers is widespread in aircraft construction. 
     Ω stringers have something like a hat profile, its ends being connected to the skin shell. Ω stringers may either be adhesively attached in the cured state to the likewise cured skin shell, or be cured wet-in-wet at the same time as the shell. The latter is desired, because it is more favourable from technical aspects of the process. However, supporting or molding cores are necessary for the wet-in-wet production of skin shells stiffened with Ω stringers, in order to fix and support the dimensionally unstable semifinished fiber products in the desired Ω shape during the production process. Skin shells with Ω stringers have the advantage over T stringers that they allow better infiltration during an infusion process for introducing a matrix, for example an epoxy resin, into the semifinished fiber products. Infusion processes are less costly than other known methods for producing fiber composite components, such as the prepreg process for example, because this allows the use of lower-cost semifinished fiber products. 
     However, there is the problem with the production of Ω stringers that the material used at present for the supporting or molding core is cost-intensive and can only be removed with difficulty after the forming of the Ω stringers, with the result that the material remaining in the stringers contributes adversely to the weight of the fiber composite component, and consequently to the weight of the aircraft. Furthermore, it is problematic that the material remaining in the stringers contributes adversely to the overall weight of the aircraft. 
     SUMMARY OF THE INVENTION 
     Against this background, it is one of the objects of the present invention to provide a lower-cost and lighter fiber composite component, in particular for aviation and spaceflight. 
     Accordingly, a method for producing a fiber composite component, comprising the following method steps: introducing an elastic core sleeve into a prestressing mechanism; expanding the core sleeve that is introduced, for elastic prestressing of the same, by activating the prestressing mechanism; introducing a dimensionally stable core body through an opening of the expanded core sleeve; releasing the expanded core sleeve by deactivating the prestressing mechanism, to snugly enclose the core body with the core sleeve and to thus form the molding core; and at least partly laying at least one semifinished fiber product on the molding core that is formed, to shape the fiber composite component to be produced. 
     Also provided is a molding core for producing a fiber composite component, in particular a stringer, on a base part in aviation and spaceflight, comprising a core sleeve, which forms an outer surface of the molding core, and a core body, which is at least partially enclosed by the core sleeve. 
     Also provided is a fiber composite component with at least one stringer, in particular in aviation and spaceflight, which is produced by means of a molding core according to the invention and/or a method according to the invention. 
     Consequently, the present invention may have the advantage over the approaches mentioned at the beginning that the fiber composite component can be produced by means of a lower-cost molding core, since, instead of an expensive material, a lower-cost material is advantageously used for the molding core. 
     According to one particular embodiment, a vacuum on an outer surface of the core sleeve for expanding the same is produced by means of the prestressing mechanism when the prestressing mechanism is activated. In the relaxed state, the core sleeve may have a smaller diameter than the core body intended for pushing into the core sleeve. By means of the prestressing mechanism, the core sleeve is then expanded or stretched so far with respect to its diameter that the core body can be pushed in, in the longitudinal direction of the core sleeve. For this purpose, the vacuum produces a force acting essentially radially in relation to the core sleeve on the latter and consequently elastically prestresses the latter. If the core body is then pushed into the expanded core sleeve and then, by means of the pre-tensioning mechanism, the vacuum on the outer surface of the core sleeve is released by the deactivation of the prestressing mechanism, the core sleeve constricts in the radial direction snugly around the core body and consequently forms the molding core. 
     This consequently achieves the advantage that a core body can be surrounded with a core sleeve in a very simple way. Such a core sleeve on the one hand takes over the function of “releasing” the molding core from the CRP, so that in the subsequent removal of the molding core no adhesive attachment to the CRP wall has to be overcome. On the other hand, the function of “sealing” is provided. As a result, in the case of core materials that contain air or are porous, resin is prevented from penetrating into the core from the fiber composite component and, conversely, air is prevented from escaping from the molding core into the CRP laminate. 
     In this application, “releasing a vacuum” or “ending a vacuum” is understood as meaning a pressure equalization to an ambient pressure, for example atmospheric pressure, in the space having the vacuum. 
     In the case of one particular embodiment of the invention, the opening of the core sleeve is closed by means of welding and/or adhesive bonding after the prestressing mechanism is deactivated. In particular in the case of complete encapsulation of the molding core with semifinished fiber products, an exchange of materials between the molding core and the semifinished fiber products can consequently be reliably prevented from taking place. If the molding core is completely surrounded by semifinished fiber products, it may be necessary for the fiber composite component first to be machined to open up access to the molding core after curing of the fiber composite component. The molding core can subsequently be removed. 
     Alternatively, the core sleeve may be brought into sealing contact with a circumference of the core body, with an edge region forming the opening, by the deactivation of the prestressing mechanism. In this case, the core sleeve consequently does not completely enclose the core body. Therefore, in the case of this embodiment of the invention, for example only the part of the core body that is surrounded by the core sleeve is used for molding and supporting the semifinished fiber products for producing the fiber composite component. The portion of the core body that does not have any core sleeve in this case protrudes from the fiber composite component to be produced. Once the fiber composite component has been cured, the core body can be removed more easily from the fiber composite component, because a direction of movement of the molding core, in particular in the longitudinal direction of the molding core, has been released. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below on the basis of the exemplary embodiment represented in the schematic figures of the drawing, in which: 
         FIG. 1-4  show a number of method steps for producing the molding core according to an exemplary embodiment of the present invention; 
         FIG. 5  A-C show further method steps for producing the fiber composite component according to the exemplary embodiment; and 
         FIGS. 6 and 7  show further method steps for removing the core sleeve according to the exemplary embodiment. 
     
    
    
     In the figures, the same reference numbers refer to identical or functionally identical components unless otherwise stated. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 to 5  schematically show a number of method steps according to an exemplary embodiment of the present invention. 
     In a first method step according to the present exemplary embodiment, represented in  FIGS. 1A and 1B , a core sleeve  1  is introduced into a lower part  2  of a prestressing mechanism  3 . According to the present exemplary embodiment, the core sleeve  1  is formed from an elastic and/or dimensionally unstable material, for example a plastic, in particular a polyamide and/or a PTFE plastic. 
     The core sleeve  1  lies with its longitudinal axis essentially parallel to a longitudinal axis of the prestressing mechanism  3  in the same. The lower part  2  may be formed as a first half of a pipe cut through along its length. The core sleeve  1  is formed as an elastic flexible tube, which is formed such that it is closed at its one end la and is provided with an opening  4  at its other end  1   b . Alternatively, the lower part  2  and the upper part  5  of the prestressing mechanism  3  could also be formed as one part. 
     Subsequently, as illustrated in  FIGS. 2A and 2B , an upper part  5 , which may be formed as the second half of the pipe cut through along its length, is placed onto the lower part  2  of the prestressing mechanism  3 . 
     In the assembled state of the prestressing mechanism  3 , the cross section  5   a  of the upper part  5  and the cross section  2   a  of the lower part  2  form an essentially airtight pipe cross section  6 . The pipe cross section  6  may be adapted to the molding core cross section and formed such that it is essentially trapezoidal with rounded corners. Alternatively, the pipe cross section  6  may be formed for example such that it is triangular, oval, round and/or wavy. 
     Molded-on portions  7   a  and  7   b  on the upper part and lower part, respectively, together form, for example, a channel  9 . The channel  9  is connected to a vacuum pump (not represented). 
     In a next method step, an edge region  1   c  of the core sleeve  1  is closed in an airtight manner with respect to the pipe cross section  6 , and may be slipped over a right-hand end  3   b  of the prestressing mechanism  3 . The turned-back edge region  1   c  can be fastened by means of a clamping ring  10  on an outer circumference  3   c  of the prestressing mechanism. 
     Consequently, an adequately airtight space  11  is formed, delimited by an outer side ld of the core sleeve  1  and an inner wall  3   d  of the prestressing mechanism. The clamping ring  10  thereby prevents air from escaping between the outer side  1   d  of the core sleeve  1  and the prestressing mechanism  3  by means of its clamping action. 
     In a next method step according to the present exemplary embodiment, as shown in  FIGS. 3A and 3B , the prestressing mechanism  3  is activated, i.e. the vacuum pump is switched to evacuation of the space  11 . As a result, the outer side ld of the core sleeve  1  comes to lie snugly against the inner wall  3   d  of the prestressing mechanism  3 . This causes an expansion of the elastic material of the core sleeve  1 , whereby it is prestressed, in particular in the radial direction, i.e. perpendicularly to the longitudinal direction. 
     In a next method step, a dimensionally stable core body  13  is pushed into the opening  1   b  expanded in this way of the core sleeve  1 . 
     Subsequently, the prestressing mechanism  3  is deactivated, i.e. the vacuum in the space  11  is released, whereby the pressure in the space  11  is equalized to an outside pressure, that is to say atmospheric pressure. 
     In the relaxed state, a diameter Dl of the core sleeve  1  is smaller than a diameter D 2  of the core body  13 . The prestressed core sleeve consequently relaxes only slightly and thereby comes to lie firmly around the core body  13  in the circumferential direction. 
     After that, the clamping ring  10  can be removed and the molding core  14  consequently produced can be removed from the prestressing mechanism  3 , as represented in  FIG. 4A  and  FIG. 4B . 
     In a further method step according to the present exemplary embodiment, the edge regions  1   c  of the core sleeve may be welded together. Such welding together prevents an exchange of materials between the core body  13  and the fiber composite component to be produced (represented in  FIG. 5B ) while the core body  13  is inside the same. 
     Alternatively, the core  13  may be formed with such a length that it protrudes beyond the end  3   b  on the opening side of the prestressing mechanism  3  in the method step represented in  FIGS. 3A and 3B . If the clamping ring  10  is removed, the edge regions  1   c  cannot be welded together for geometrical reasons, but come to lie snugly around the protruding portion  13   a  (as represented in  FIG. 5B ) of the core body  13 . 
     The molding core  14  is suitable for producing a fiber composite component  22  which is intended to have geometric portions that correspond at least partly to those of the molding core  14 . 
     Generally, the molding core  14  can be used in various methods for producing a fiber composite component, such as for example manual lamination, prepreg or vacuum injection process. However, the use of the molding core in a vacuum infusion process is to be presented by way of example. 
       FIGS. 5A to 5C  illustrate further method steps for producing the fiber composite component according to the present exemplary embodiment. 
     As illustrated in  FIG. 5A , the molding cores  14  may be arranged on a base part  15  of semifinished fiber products, for example of laid fiber fabrics. Subsequently, further semifinished fiber products  16  are laid flat on the molding cores in such a way that they are at least partly in contact with the latter and assume an inner shape that corresponds to the outer shape of the molding cores. 
       FIG. 5B  shows a view along the direction of the arrow A in  FIG. 5A . In the case of this exemplary embodiment, a molding core  14  which has the protruding portion  13   a  that is not covered by the core sleeve  1  is used. In the edge region  1   d  of the core sleeve, a sealing glue  18  is introduced. Subsequently, the further semifinished fiber products  16  and the base part  15  are covered in an airtight manner with a sealing film  19 . The sealing film  19  thereby terminates with the sealing glue  18 , the sealing glue  18  sealing the space between the sealing film  19  and the core sleeve  1  in an airtight manner. Furthermore, a filling piece  2 , which supports the molding core in the region between the end of the semifinished fiber product  16  and the sealing glue, can be introduced. 
     Subsequently, a vacuum is applied to the space sealed by the sealing film and a connection to this space with the matrix  21  is provided. If the space beneath the sealing film is then evacuated, the matrix  21  is evenly distributed in the semifinished fiber products  16  and in the base part  15 . The core sleeve  1  forms here a sealing layer which prevents the matrix  21  from penetrating into the core body  13  and/or prevents substances, in particular air, from escaping from the core body  13  into the fiber composite component  22  to be produced. 
     In a further method step, the arrangement  17  is arranged in an autoclave or oven (not represented) and cured under pressure and/or heat. The core sleeve  1  may therefore be formed from a material that withstands the necessary process temperatures in the range of, for example, 180 degrees without losing its “sealing” and “releasing” functions and/or deforming outside predetermined tolerances. 
     The cured arrangement  17  then has a fiber composite component  22 , as shown in  FIG. 5C , with a hard shell  23 , which is reinforced by approximately Ω-shaped stringers  24 . 
     There are various possibilities for removing the molding cores  14 . For example, the core body  13 , which is formed for example from water-soluble material, may be flushed out by means of a water jet. For this purpose, a flushing-out device  25  is provided, having a hose  26 , by means of which water and flushed-out core body material  27  are carried away. 
     Alternatively, the core body  14  may simply be drawn out from the Ω-shaped stringer  24  in the longitudinal direction. For this purpose, the core sleeve  1  is provided on its inner side with a coating with sliding properties or is produced from a material with suitable sliding properties, i.e. the core sleeve  1  has, for example, a release layer that prevents the core body  13  from adhesively attaching itself to the core sleeve  1 . The core sleeve  1  consequently remains in the Ω-shaped stringer  24 , but only contributes slightly to the weight of the component  22 . 
     Although the core sleeve  1  could remain in the Ω-shaped stringer  24 , there is the possibility of removing the core sleeve  1  in various ways. 
       FIGS. 6 and 7  show further method steps for removing the core sleeve according to the exemplary embodiment of the present invention. 
     As illustrated in  FIG. 6 , a line  29  is fastened to a fastening point  30  on the core sleeve  1 . Subsequently, a force F is applied to the line  29  in the longitudinal direction of the stringer  24 , and consequently the core sleeve  1  is drawn out from the stringer  24 . The core sleeve  1  thereby may comprise a release layer, which prevents adhesive attachment of the fiber composite component  22  produced to the core sleeve  1 . Alternatively, a ram by means of which the core sleeve  1  is pressed out may be used. 
     In  FIG. 7 , an air pressure P is applied to the closed end  1   a  of the core sleeve  1  along the longitudinal direction of the stringer  24  and presses the core sleeve  1  out from the stringer  24  in the longitudinal direction of the latter. 
     The method explained can also be used particularly advantageously for producing molding cores  14  with a cross section that is variable in the longitudinal direction L. If, for example, a water-soluble core body  13  is used, it is then possible after the at least partial curing of the fiber composite component  22  and flushing out of the core body  13  for the molding core  14  to be removed without any problem from the stringer  24  produced, which then has a variable cross section. The core body material  27  can later be used again for forming a new core body  13 . 
     The invention is not restricted to the specific method represented in the figures for producing a fiber composite component for aviation and spaceflight. 
     Furthermore, the individual sequence of individual method steps of the production method according to the invention can be changed in various ways. The form taken by the individual method steps can also be modified. For example, extraction of the core sleeve by suction instead of it being pressed out under pressure may be carried out for removing the core sleeve from the stringer produced. 
     Furthermore, the geometry of the molding core can be modified in various ways. 
     Furthermore, it is also possible for a number of molding cores to be used to form a single molding core, around which laid fiber fabrics are placed. The aim here is to create a more complex geometry by means of the multiplicity of molding cores. Consequently, more complex fiber composite components can be produced.