Abstract:
A method for manufacturing a hollow load-bearing beam structure of fiber-reinforced composite material. The method includes preparing a first lay-up of a first plurality of layers, and a second lay-up of a second plurality of layers. The first and second lay-ups are positioned on different sides of a mandrel and formed to a beam structure by bending the lay-ups against the mandrel, such that the side edges of the respective layers of the first and second lay-ups are arranged as offset butt joints in the circumferential direction of the formed beam structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European patent application 07118133.3 filed 9 Oct. 2007. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a method for manufacture of hollow load-bearing beam structures of fiber-reinforced composite material. 
       BACKGROUND ART 
       [0003]    Load-bearing structures made of fiber composite materials exhibit advantageous strength to weight properties. A drawback of such structures is the relatively high production cost. These costs arise from the labour intensive and time consuming manufacturing process, which is associated with said structures. 
         [0004]    The fibers of such composite structures, which fibers constitute the chief part of the load-bearing capability, are to be aligned in specific configuration in order to optimize the structures. Traditionally, this alignment has been made by winding the fibers around a mandrel or similar. 
         [0005]    In modern manufacture, so-called prepreg has been used in order to speed the process up. Here, pre-impregnated mats of aligned fibers are used. The fiber mats can e.g. be impregnated in an epoxy resin, which resin thereafter is partially cured. The prepreg layers can manually or automatically, by means of a so-called tape layer, be applied onto a rigid or inflatable mandrel. Subsequently, the prepreg resin is cured by increasing the temperature and applying pressure. Generally, the pressure is applied by covering the layers with an outer membrane or by inflating the mandrel. A drawback of using an inflatable mandrel is that this procedure requires an outer female mould, which defines the outer shape of the product to be produced. On the other hand, a drawback associated with rigid mandrels is that the composite layers have a tendency to wrinkle under the applied pressure. Further, in known manufacturing methods, it is difficult to automatically, i.e. with a tape layer, accurately lay the prepreg layers up on the mandrel. 
       SUMMARY OF THE INVENTION 
       [0006]    The objective of the present invention is to provide an improved method for manufacturing beams from laminated composite layers. By the method according to the invention, high strength beam structures can be manufactured in a more automatized process. 
         [0007]    For this purpose the method of the present invention for manufacturing a hollow load-bearing structure of fiber-reinforced composite material, which load-bearing beam structure includes at least a first and a second plurality of laminated layers shaped by a rigid mandrel with an outer surface that corresponds to the inner surface of the load-bearing beam structure, comprises the steps of:
       preparing a first lay-up of the first plurality of layers, one on top the other, with the respective side edges of the plurality of layers arranged offset with respect to each other,   preparing a second lay-up of the second plurality of layers, one on top the other, with the respective side edges of the plurality of layers arranged offset with respect to each other,   positioning the first lay-up on a first side of the rigid mandrel.       
 
         [0011]    positioning the second lay-up on a second side of the rigid mandrel,
       forming the first and second lay-ups to the shape of the beam structure by bending the lay-ups against the mandrel, such that the side edges of the respective layers of the first and second lay-ups are arranged as offset butt joints in the circumferential direction of the formed beam structure,   curing the beam structure, and   removing the beam structure from the mandrel.       
 
         [0015]    In particular, a high strength structure can be obtained by the circumferentially offset butt joints. This arrangement allows for a superior force transfer between adjacent layers. 
         [0016]    The offset arrangement of the respective side edges of the plurality of layers can be achieved by sizing the first plurality of layers such that the width thereof gradually decreases from an inner layer to an outer layer, and by sizing the second plurality of layers such that the width thereof gradually increases from an inner layer to an outer layer. These layers are then positioned centrally with respect to one another. Or, alternatively, all the layers can be of the same width. These layers are then positioned with offsets in one width direction with respect to one another. 
         [0017]    By laying up the layers of the lay-ups on a substantially flat supporting surface, the use of a tape-laying apparatus is facilitated. Thereby, the method enables a higher degree of automatization. Also, this ensures a more exact mutual positioning of the layers. 
         [0018]    The step of positioning the first lay-up on the first side of the mandrel and a step of forming the first lay-up into the shape of a first portion of the beam structure on the mandrel can be carried out prior to forming the second lay-up into a second complementary portion of the beam structure on the mandrel. Thereby, the second lay-up can be accurately positioned with respect to the first lay-up. 
         [0019]    In order to simplify the positioning of the second layers, the mandrel can be turned approximately 180° after the step of forming the first lay-up into the shape of the first portion of the beam structure and before the step of positioning the second lay-up on the mandrel. 
         [0020]    The forming of the layers to the shape of the mandrel can be carried out by means of fluid pressure and an impervious membrane. Hereby, the layers are accurately formed and compressed in a desired manner. The compression entails evacuation of air from within the layers, and also ensures proper lamination of the layers. Further, fibers within the layers are appropriately aligned. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    By way of example, an embodiment of the present invention is illustrated in the enclosed schematic figures, where 
           [0022]      FIG. 1  shows a first step of a manufacturing process, where a first plurality of layers are laid up on a supporting surface; 
           [0023]      FIG. 2  illustrates how the layers are positioned on a mandrel; 
           [0024]      FIG. 3  illustrates the mandrel and the first plurality of layers after the layers have been shaped to the form of the mandrel; 
           [0025]      FIG. 4  shows how a second plurality of layers are laid up on a supporting surface; 
           [0026]      FIG. 5  illustrates the mandrel and the first and the second plurality of layers after the second plurality of layers have been shaped to the form of the mandrel; and 
           [0027]      FIG. 6  shows a hollow beam structure 
           [0028]      FIG. 7  shows an alternative lay-up for a plurality of layers on a supporting surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Below, an embodiment of the present invention is disclosed. The disclosure is not intended to limit the claimed scope of the present invention in any way. 
         [0030]    In  FIG. 1 , a first plurality of layers  1  is laid up on a substantially flat supporting surface  2 . This constitutes the initial step in the present method for manufacturing beam structures. The layer width (x-direction) of the layers  1  decreases in succession from bottom up, I.e., the lowermost inner layer  1   d  has the greatest width, and the uppermost outer layer  1   a  has the smallest width. As illustrated in the figure, the uppermost layer  1   a  is placed on the layer  1   b  below in a central manner, such that equal width differences  3   a ,  3   a  result at both ends of the layers  1   a ,  1   b . Starting from the lowermost layer  1   d , all successive layers  1   c ,  1   b ,  1   a  are arranged centrally with respect to the underlying layer  1   d ,  1   c ,  1   b . In this example, four layers  1   a ,  1   b ,  1   c ,  1   d  are used. Thus, three width differences  3   a ,  3   b ,  3   c  are indicated. 
         [0031]    The layers  1   a - 1   d  and the supporting  2  surface in  FIG. 1  are shown in cross-section; the extension of the layers in the y-direction can be adjusted in accordance with prevailing circumstances. 
         [0032]    In this embodiment, the layers  1   a - 1   d  consist of so-called prepreg, i.e. a fiber substrate impregnated in a semi-cured thermosetting epoxy resin composition. Since the layers  1  are laid up on a substantially flat supporting surface  2 , an automatic tape laying apparatus can be used. 
         [0033]      FIG. 2  shows a mandrel  4  in cross-section. The outer surface of the mandrel  4  corresponds to the inner surface of the beam structure which is to be manufactured. Here the mandrel  4  is massive, although a hollow mandrel could also be used. A solid mandrel is more cost effective in manufacture, whereas a hollow mandrel requires less material and is easier to temperature regulate. In this second step of the manufacture method, the first plurality of layers  1  is placed on a first side  5  of the mandrel  4 . In this embodiment, said first side  5  of the mandrel is the upper side of the mandrel. 
         [0034]    The present mandrel  4 , and thus the resulting beam structure  11 , is of trapezoidal cross-section. The trapezoidal shape is suitable for manufacturing e.g. the core of control surface components for aircraft. Other cross-sections, e.g. circular, oval et cetera, are also conceivable. Further, with corresponding shape of the layers  1 ,  2 , a mandrel cross-section that varies longitudinally (y-direction) can also be employed. 
         [0035]    Next, the lay-up  1  is shaped in accordance with the mandrel  4 , as illustrated in  FIG. 3 . This procedure can be realized by applying an impervious membrane (not shown) onto the first plurality of layers (lay-up  1 ), and by means of fluid pressure forcing the membrane, and thus the layers  1   a - 1   d , towards the mandrel  4 . Said fluid can be either a liquid or a gas, and said pressure can be either an overpressure or an underpressure. An overpressure must be applied from the outside of the mandrel  4 , as illustrated in  FIG. 3 , whereas an underpressure is to be applied from the inside of the mandrel  4 , for example by a so called vacuum bagging technique. This forming step can be carried out under heating, which increases the formability of the layers  1   a - 1   d.    
         [0036]      FIG. 4  illustrates how a second plurality of layers  6  is laid up on a supporting surface  2 . Compared to the arrangement of the first plurality of layers  1  in  FIG. 1 , the second plurality of layers is laid up in the reversed order. That is to say, the lowermost inner layer  6   a  has the smallest width and the uppermost layer  6   d  has the greatest width. The respective layer width differences  8   a ,  8   b ,  8   c  are equal on the left and the right side in  FIG. 4 . 
         [0037]    In a step which is not shown, the second plurality of layers  6  is placed on a second side  7  of the mandrel  4 . There, the second plurality of layers  6  is shaped in accordance with the mandrel  4 , in a manner corresponding to the procedure described in connection with  FIG. 3 . The mandrel  4  can be turned upside down, or approximately 180°, before the second plurality of layers  6  is placed on the second side  7  of the mandrel  4 . Such turning of the mandrel  4  facilitates the placing of the second plurality of layers  6  on the mandrel, since the second side  7  of the mandrel  4  is now the upper side of the mandrel. Alternatively, the second plurality of layers  6  are brought into contact with the mandrel from below. In this case, the mandrel  4  is not turned before the second plurality of layers  6  is applied. This can be accomplished by placing the second plurality of layers  6  on an impervious membrane (not shown) and lifting said membrane together with the second plurality of layers  6  up into contact with the mandrel  4 . Said membrane can also be used for forming the layers to the shape of the mandrel. In this connection, the membrane can be placed on the supporting surface  2  before the layers  6   a - 6   d  are laid up, where after the layers  6   a - 6   d  are arranged on top of each other with the membrane acting as a supporting surface. Or the layers  6   a - 6   d  can be moved from the supporting surface  2  to the membrane before application onto the mandrel  4 . 
         [0038]    The first and second plurality of layers  16  can also essentially simultaneously be laid up on the supporting surface  2 , before any of the layers  1 ,  6  are placed on the mandrel  4 . 
         [0039]    In accordance with the present embodiment, the first side  5  of the mandrel  4  is opposite to the second side  7  of the mandrel. 
         [0040]      FIG. 5  shows the mandrel  4  with both the first layers  1  and the second layers  6  applied and formed. Due to the width differences  3   a - 3   c ,  8   a - 8   c  of the first and second set of layers  1   a - 1   d ,  6   a - 6   d , the respective layers meet at circumferentially offset butt joints  9 . In each butt joint  9  there remains a gap  10  between the butting layers. In order to obtain the present arrangement of the layers  1 ,  6 , the widths of the layers are adapted to the mandrel circumference. E.g., the sum of width of the innermost layer  1   d  of the first lay-up  1  and the width of the innermost layer  6   d  of the second lay-up  6  essentially equals the circumference of the mandrel  4 . More precisely, the width of the two innermost layers  1   d ,  6   d  added by two joint gaps  10  equals the circumference of the mandrel  4 . 
         [0041]    While still positioned on the mandrel ( FIG. 5 ), the layers  1 ,  6  are cured by heat treatment. In this matter, one mandrel  4  carrying two pluralities of layers  1 ,  6  can be cured separately, which results in a single discrete load-bearing structure  11 . Or, alternatively, in order to form a more complex structure such as control surface components for aircraft, a plurality of mandrels can be brought together and group-cured as an assembly. Said assembly can be furnished with reinforcing elements and an outer embracing layer. 
         [0042]    Upon curing, the layers  1   a - 1   d ,  6   a - 6   d  can be compressed, e.g. by means of a membrane and fluid pressure, in order to closely form the layers to the mandrel  4 , and for improving the resulting beam quality by expelling air from the layers and aligning the prepreg fibers. Here, an autoclave can be used. The compression brings the respective ends of the first and second layers closer to one another, which reduces the butt joint gaps  10 . 
         [0043]      FIG. 6  illustrates a finished hollow load-bearing beam structure  11 . Four offset butt joints  9  are shown on the left and right side, respectively. Here, all joint gaps  10  are of equal size. 
         [0044]    The dimensions of the butt joint gaps  10 , i.e. the width of the layers  1 ,  6 , are optimized in accordance with prevailing circumstances. After manufacture, the butt joint gaps  10  serve no purpose. Thus, the gaps  10  are to be minimized. In this regard, the innermost butt joints  9 , between the innermost layers  1   d ,  6   d , can be of smaller dimensions than the outermost butt joints  9 . If the layers  1   a - 1   d ,  6   a - 6   d  are compressed during curing, such compression will have a greater influence on the outer layers than on the inner layers, since the circumference reduction of the layers is cumulative. Any compression obtained on the inner layers will also influence the layers outside said inner layers. As a consequence, optimization of the butt joint gaps  10  can imply that the inner butt joint gaps, before compression, are of smaller dimension than the outer butt joint gaps. 
         [0045]    Further, the width differences  3   a - 3   c ,  8   a - 8   c  can be adjusted depending on prepreg fiber and prepreg resin strength. The object of the width differences, which effect the offset arrangement of the butt joints  9 , is to facilitate force transfer between the fibers of the respective layers to neighbouring layers. The longitudinal tensile stress of the fibers in an arbitrary layer is transferred to the fibers of an adjacent layer by shear stress in the resin, in which the fibers are impregnated. Thus, the butt joints  9  must be sufficiently offset to allow the inter-layer force transfer. 
         [0046]    Above, a first and a second plurality of layers  1 ,  6  are formed around a mandrel  4  to two complementary shapes, which, joint together, constitute a hollow beam structure  11 . However, the teachings of the invention do not exclude the forming and joining of several pluralities of layers into such hollow beam structures. In this regard,  FIG. 7  shows an alternative lay-up  12 . Here, all layers  12   a - 12   d  are of the same width. The layers are arranged with an offset  13   a ,  13   b ,  13   c  with respect to each other in one width direction. Now, the different lay-ups  1 ,  6 ,  12  can be arranged around a mandrel in a desired combination. Or, alternatively, several lay-ups  12  comprising layers  12   a - 12   d  of the same width can be used exclusively. Last mentioned lay-up  12  can also be used, especially for large hollow beam structures, as an extender between the lay-ups  1 ,  6  comprising layers of different width. Then, the lay-up  12  with layers  12   a - 12   d  of equal width must be arranged with offsets  13   a ,  13   b ,  13   c  that matches the width differences  3   a - 3   c ,  8   a - 8   c  of the lay-ups  1 ,  6  comprising layers of different width.