Abstract:
Method for producing a fiber composite component, in particular for aerospace, having the following method steps: forming a moulding core that comprises a predetermined number of hollow bodies for defining an external geometry of the moulding core, the hollow bodies being designed such that they extend in the longitudinal direction of the moulding core and are expandable elastically at least in their lateral direction, and are fixed to one another, the formed moulding core having a flexible outer layer for smoothing the contour of its outer surfaces, and a core sleeve surrounding the moulding core, for sealing the moulding core from the fiber composite component to be produced; placing at least one fiber semi-finished product, at least in places, on the moulding core (that has been formed, in order to shape at least one moulding section of the fiber composite component to be produced; and applying heat and/or pressure to the at least one moulding section in order to produce the fiber composite component.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for producing a fibre composite component, in particular for aerospace, to a moulding core for producing such a fibre composite component, and to a fibre composite component having at least one stringer, which is produced by such moulding core and/or such a method. 
     Although it can be applied to any desired fibre composite components, the present invention and the problem on which it based will be explained in more detail in the following text with reference to flat, stringer-reinforced carbon fibre plastic (CFP) components, for example skin shells for an aircraft. 
     BACKGROUND OF THE INVENTION 
     It is generally known that CFP skin shells can be reinforced with CFP stringers in order to withstand high loads in aircraft, with as little additional weight as possible. In this case, a distinction is essentially drawn between two types of stringers: T stringers and Ω stringers. 
     T stringers have a cross section composed of a base and a stem. The base forms the connection surface to the skin shell. The use of skin shells reinforced by T stringers is widespread in aircraft construction. 
     Ω stringers have an approximately top-hat-shaped profile, whose ends are connected to the skin shell. Ω stringers can either be adhesively bonded to the likewise cured shell when in the cured state themselves, or they can be cured at the same time as the shell using a wet-in-wet process. The latter is desirable because this is better from the process engineering point of view. However, supporting cores or moulding cores are required for wet-in-wet production of skin shells reinforced with Ω stringers, in order to fix and support the dimensionally unstable fibre semi-finished products in the desired Ω shape during the production process. Skin shells with Ω stringers have the advantage over T stringers that the infiltration capability during an infusion process for introduction of a matrix, for example of an epoxy resin, into the fibre semi-finished products is better. Infusion processes may be more cost-effective than other known methods for producing fibre composite components, such as the prepreg process, because this allows the use of lower-cost fibre semi-finished products. 
     However, one problem that arises in the production of Ω stringers is that the material used at present for the supporting core or moulding core is costly and can be removed only with difficulty after the formation of the Ω stringers, so that the material which remains in the stringers disadvantageously contributes to the total 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-weight fibre composite component, in particular for aerospace. 
     Accordingly, a method for producing a fibre composite component, in particular for aerospace, is provided that includes:
         forming a moulding core that comprises a predetermined number of hollow bodies for defining an external geometry of the moulding core, the hollow bodies being designed such that they extend in the longitudinal direction of the moulding core and are expandable elastically at least in their lateral direction; and are fixed to one another, the formed moulding core having a flexible outer layer for smoothing the contour of its outer surfaces, and a core sleeve surrounding the moulding core, for sealing the moulding core from the fibre composite component to be produced;   placing at least one fibre semi-finished product, at least in places, on the moulding core that has been formed, in order to shape at least one moulding section of the fibre composite component to be produced; and   applying heat and/or pressure to the at least one moulding section in order to produce the fibre composite component.       

     When the moulding core is used in an appropriate manner, these hollow bodies have an internal pressure applied to them, as a result of which they expand in the lateral direction and form the moulding core with the external geometry. For removal from the mould, the internal pressure is changed such that, for example, the cross section of the moulding core is reduced again. This allows the moulding core to be removed easily. A further advantage is may be that the moulding core is reusable. 
     Furthermore, a moulding core for producing a fibre composite component, such as a stringer on a base component for aerospace, is provided that has a predetermined number of hollow bodies, the hollow bodies extending in the longitudinal direction of the moulding core and being expandable elastically at least in their lateral direction, the hollow bodies being fixed to one another, and the moulding core comprising a flexible outer layer in order to smooth the contour of its outer surfaces, and a core sleeve surrounding the moulding core, in order to seal the moulding core from the fibre composite component to be produced. 
     Furthermore, a fibre composite component is provided having at least one stringer, in particular for aviation and spaceflight, which is produced by means of the moulding core according to the invention and/or the method according to the invention. 
     The present invention therefore may provide the advantage over the initially mentioned approaches that the fibre composite component can be produced by means of a low-cost moulding core. Instead of a conventional costly core material which remains in the component, it is advantageously possible to use a reusable moulding core, whose shape can be varied in a simple manner by application of an internal pressure, such that it can easily be removed from the mould. In consequence, the moulding core no longer contributes to the component weight. 
     With a moulding core such as this, the hollow bodies may be fixed to one another, and the moulding core formed in this way may be provided with a flexible outer layer in order to smooth the contour of the outer surfaces of the moulding core. This may smooth the outer surfaces of the moulding core. The flexibility of the outer layer means that the hollow bodies can expand and contract flexibly in the lateral direction, in a simple manner. 
     One alternative embodiment provides for the outer layer to be applied such that it completely surrounds the hollow bodies  7 , and forms their fixing to one another. 
     In one embodiment, the moulding core is completely surrounded by a core sleeve which, on removal from the mould, results in advantageous separation between the moulding section and the surfaces of the moulding core. Furthermore, the core sleeve prevents the pressure medium, for example air, from being able to escape into the fibre composite component in the event of any leakage from a small tube. At the same time, this prevents damage and adverse effects to the moulding core, thus ensuring that it can be reused. 
     According to one embodiment of the invention, reinforcing means are arranged inside or outside the core sleeve in the area of transitions, which have to be formed with sharp edges, in the external geometry of the moulding core to be formed. These reinforcing means, in particular corner profile parts, have the advantage that they form the sharp edges and corners, and the moulding core can be provided with rounded areas, which can be produced easily, in this region. 
     A separating layer can be applied to the core sleeve and prevents adhesion of the fibre semi-finished product and/or of a matrix to the core sleeve. This makes it easier to remove the core sleeve after the at least partial curing of the section of the fibre composite component created by means of the moulding core. 
     Fibre semi-finished products include fabric, tissue and fibre mats. These are provided with a matrix, for example an epoxy resin, and are then cured, for example in an autoclave. 
     According to a further embodiment of the invention, the moulding core is arranged on a base part composed of fibre composite semi-finished products, and/or is at least partially surrounded by fibre semi-finished products in order to form at least one moulding section of the fibre composite component. This advantageously allows base parts, for example skin shells, pressure cups etc., to be formed with Ω stringers. Alternatively or additionally, separate fibre composite components may also be produced, which are produced with their shape being defined entirely by the moulding core. 
     During the production of an Ω stringer, for example, the core sleeve is removed from the mould in the longitudinal direction of the stringer when removing it from the mould. In consequence, the core then no longer contributes to the total weight of the aircraft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is explained in more detail below using preferred exemplary embodiments and with reference to the attached figures of the drawings, in which: 
         FIG. 1  shows a schematic perspective view of a first exemplary embodiment of a fibre composite component during production using one method according to the invention; 
         FIG. 2  shows a schematic general section illustration of a moulding core for the fibre composite component shown in  FIG. 1 ; 
         FIG. 3  shows a schematic section illustration of a first moulding core according to the invention for the fibre composite component as shown in  FIG. 1 ; 
         FIG. 4  shows a schematic section illustration of a second moulding core according to the invention for the fibre composite component as shown in  FIG. 1 ; 
         FIG. 5  shows a schematic side view of the first or second moulding core according to the invention as shown in  FIG. 3  or  4 ; and 
         FIG. 6  shows a schematic perspective view of the finished fibre composite component as shown in  FIG. 1 , after removal of the moulding cores. 
       In the figures, like reference numerals refer to identical or functionally identical components unless otherwise stated. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic perspective view of a first exemplary embodiment of a fibre composite component  1  during production using a method according to the invention. 
     This example has two moulding cores  4 , although the number is not restricted to this. The two moulding cores  4 , whose production will be explained further below, are provided with an approximately trapezoidal cross section, with their base  5  resting on a base component  2 . 
     Fibre semi-finished products  3  are placed down on the moulding cores  4 . The fibre semi-finished products  3  in this case rest with a central section on the outer surface of the moulding cores  4 , and with their ends on the base component  2 , for example on an aircraft skin. Two moulding sections  14  of the fibre composite component  1  are thus formed. 
     Various manufacturing methods can be used to process the fibre composite material. The so-called infusion process may be chosen in order to introduce a matrix, that is to say for example epoxy resin, into the fibre semi-finished products. The prepreg process may, however, be used just as well in this case. 
     A further step that of curing the base component  2  together with the moulding cores  4  and the fibre semi-finished product in an autoclave or oven under the influence of heat and pressure, using a curing cycle which will not be explained in any more detail, thus resulting in the production of a complete fibre composite component  1 . 
     First of all, the production of the moulding cores  4  will be described with reference to  FIGS. 2 to 4 . 
       FIG. 2  shows a schematic, general section illustration of a moulding core  4  according to the invention for the fibre composite component  1  as shown in  FIG. 1 , in the form of a cross section. 
     The moulding core  4 , whose construction will be described in detail further below, has a cross section  6  which is introduced into a mould  8  and is changed in this mould to the desired shape, in this case an approximately trapezoidal shape. In this example, the moulding core  4  is surrounded by a core sleeve  9 , which completely surrounds the moulding core  4  and is suitable, in terms of temperature and pressure, for its production process and its further treatment and processing. The core sleeve  9  is manufactured from a plastic, in particular a polyamide and/or a PTFE plastic. Its inside  11  rests directly on the surfaces of the moulding core  4 , with its outside  10  in this example being coated with a separating (mold release) layer (not shown), which may also comprise an additional sleeve. The separating layer is used to correctly separate the moulding core  4  from the moulding section  14  during removal from the mould. 
     The moulding core  4  is composed of individual hollow bodies  7 , as is illustrated in  FIG. 3 . In this example, the hollow bodies  7  are individual small tubes, for example in the form of thin films, with an approximately circular cross section, which are flexible in the lateral direction, that is to say in the radial direction. The hollow bodies  7  form an initially somewhat coarse structure for the cross section  6  of the moulding core  4 . They are fixed to one another on their touching lines in the longitudinal direction, for example with the aid of suitable adhesive bonding. In this case, intermediate spaces  15  remain between them. The number of small tubes alongside one another and one above the other, as well as their diameters, may be chosen freely within certain limits, in order to achieve a desired cross section. In this example, the only feature is that the corner angle is set to 60° . 
     The outside of the moulding core  4  that is formed from the individual hollow bodies  7  is then provided with an outer layer  16 , which contributes to smoothing its contour, and forms smooth surfaces for the moulding core  4 , for example using the mould  8  ( FIG. 2 ). This outer layer  16  fills external grooves between the hollow bodies  7 . 
     In  FIG. 3 , reinforcing means  13  in the form of profiles, for example strips composed of metal or plastic, are used at the lower corners. The moulding core  4  can in this way be provided with particularly well-formed corner areas, by manufacturing the reinforcing means  13  in a separate mould. They may be arranged within the core sleeve  9  (not shown in  FIG. 3 ), or else outside it. 
     The outer layer  16  is composed of a sufficiently flexible material, for example a suitable elastic plastic, which will reliably withstand the process temperatures that occur. 
     The outer layer can also be applied thicker in subareas, such that an otherwise fixed corner angle of 60° can be varied with certain limits. 
     The outer layer  16  can also be applied such that it completely surrounds the hollow bodies  7 , and forms their fixing to one another. One example of a core such as this, which at the same time has a corner angle other than 60°, is illustrated in  FIG. 4 . 
     The moulding core  4  produced in this way is removed from the mould  8  and is applied to the base component  2  as described above. This state is illustrated in  FIGS. 3 and 4 . In this unpressurized state, the moulding core initially has a slightly smaller cross section. Internal pressure is then applied to the hollow bodies  7  such that the moulding core  4  assumes the desired external geometry. This will be explained in more detail further below. The moulding core  4  is then coated with the fibre semi-finished product  3  in order to form the moulding section  14 , as explained above. 
     The fibre composite component  1  produced after a curing cycle, which will not be explained in any more detail, is illustrated in  FIG. 6  in the form of a respective view after removal of the moulding cores  4  from the mould, with moulding sections  14  in the form of stringers  20 . 
       FIG. 5  shows a side view of the moulding section  14 , illustrated schematically. The moulding core  4  is arranged such that the core sleeve  9  projects at both ends, with its openings  12 , out of the moulding section  14 . This has the advantage that, if there are any leaks from the hollow bodies  7 , the escaping air cannot enter the fibre semi-finished product  3  to be cured, but is passed to the exterior through the core sleeve  9 . All of the ends of the hollow bodies  7  likewise project at both ends of the moulding section  14 . 
     In this example, the right-hand ends of the moulding bodies  7  are closed by closures  19 , and the other, opposite ends of the hollow bodies  7  are each connected to a connecting apparatus  17 . This connection may be configured such that the connections are plugged onto the ends of the hollow bodies  7  or, as is shown in  FIG. 5 , are plugged into them. In this example, the connecting apparatus  17  comprises a line  18 , which has branch ends that lead to each hollow body  7  and are connected to the hollow bodies  7 . 
     The core sleeve  9 , which has a circumference that is slightly larger than the circumference of the moulding core  4 , is then pushed over it. 
     An internal pressure is applied via the connecting apparatus to the hollow bodies  7 , expanding the hollow bodies  7  in their lateral direction, thus forming the desired external geometry of the moulding core  4 . This is done before production of the moulding section  14 , as explained above. The resultant enlargement of the circumference results in it being located tightly around the moulding core  4 . The core sleeve  9  is clamped by the internal pressure applied during expansion of the moulding core  4 . 
     The operation of this method is governed by the correct combination of a high internal pressure of, for example, 10 bar, which, however, still allows elastic, reversible, lateral expansion of the hollow bodies  7  at the curing temperature of, for example, 180° C., and a sufficiently small wall thickness for the hollow bodies  7  of, for example, 0.05 mm. Another critical factor in this case is the material, with a suitable compromise of the modulus of elasticity, low creepage tendency at the curing temperature with good resistance properties over time, and sufficiently good ductility. Possible materials in this case include, in particular, thermoplastics from the group of aromatic polysulphides and polysulphones, such as PSU, PPS or PES. These offer good long-term temperature resistance, with little creepage tendency, at temperatures such as these. 
     In this example, 35 hollow bodies  7  were used, each having a diameter of 5 mm. Reversible expansions of between 2 and 3% in the height and the width of the moulding core can be achieved with the data example quoted above. Each hollow body  7  has a pressure connection and a closure. Pressure connections (connecting apparatuses  17 ) are also possible on both sides. 
       FIG. 6  shows a schematic perspective view of the finished fibre composite component  1  shown in  FIG. 1 , with moulding sections  14  in the form of stringers  20 , after removal of the moulding cores  4 . 
     During removal from the mould, the hollow bodies  7  have an internal pressure applied to them via the connecting apparatus such that the hollow bodies  7  return to their original extent, that is to say they contract and are detached from the wall and from the separating flexible tube. A negative pressure can also be applied, which leads to further contraction of the core, with the individual small tubes collapsing even further and, for example, collapsing such that they are flat. This makes it possible to very greatly reduce the cross section. 
     The moulding core  4  which has been shrunk down in this way can be removed from the cured moulding section  14 , and can be reused. The core sleeve  9  can then likewise be removed, and this can particularly advantageously be done easily and also when a separating layer is present. The fibre composite component  1  can now be processed further. If reinforcing means  13  are present, these are likewise also removed, or remain in the fibre composite component  1 . 
     A method is therefore provided for producing a fibre composite component, a corresponding moulding core and a corresponding fibre composite component, which can achieve a considerable material cost reduction in comparison to conventional methods with remaining core materials. The moulding core can be removed completely, thus making it possible to reduce the weight of the fibre composite component, in comparison to conventional moulding cores. 
     The invention is not restricted to the specific method illustrated in the figures for producing a fibre composite component for aerospace. 
     For example, the idea of the present invention is also applicable to fibre composite components in the field of sporting equipment and motor sports. 
     Furthermore, the geometry of the moulding core, in particular the wall thickness and diameter of the small tubes as well as their arrangement, can be modified in many ways. 
     Furthermore, a plurality of moulding cores can also be used in order to form a moulding core which is surrounded by fibre composite mats. The aim in this case is to provide a more complex geometry by means of the multiplicity of moulding cores. This allows more complex fibre composite components to be produced. 
     One alternative embodiment provides for the outer layer to be applied thicker in subareas, so that it is possible to vary the otherwise fixed core angle of 60° within certain limits. 
     The cross sections of the hollow bodies  7  need not necessarily be circular, when in the unpressurized state. Their diameters may likewise be different, in order in this way to create different corner radii. 
     The internal pressure must be regulated in order to ensure reproducible cross-sectional expansion. 
     The wall thickness of the hollow bodies  7  may also differ.