Abstract:
There is described a method of producing fibre reinforced laminated structures by layering a number of dry fibre reinforcement layers by placing them on top of each other in a mould, infusing a curable viscous or liquid polymer into the mould after the fibre reinforcement layers have been layered in the mould and curing the polymer, wherein a flow enhancing layer for enhancing the polymer flow during infusion of the polymer is placed between two fibre reinforcement layers when layering the number of dry fibre reinforcement layers and wherein a pre-cured solid layer is used as the flow enhancing layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 07009187.1 EP filed May 7, 2007, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a method of producing fibre reinforced laminated structures, such as wind turbine rotor blades, by placing a number of dry fibre reinforcement layers on top of each other in a mould, injecting a curable viscose or liquid polymer into the mould after the fibre reinforcement layers have been placed in the mould and curing the polymer. 
       BACKGROUND OF INVENTION 
       [0003]    In such methods of producing fibre reinforced laminated structures it is of importance to ensure sufficient wetting of the dry reinforcement layers after they have been placed in the mould. If the wetting is insufficient, this may lead to delamination and air pockets within the laminate structure and, as a consequence, to wrinkles in the laminate structure which constitute weak points of the structure. This deficiency is in particular an issue in case of thick fibre reinforced laminated structures such as, e.g. spar caps of wind turbine rotor blades. 
         [0004]    In order to address this issue it has been proposed in US 2007/0040294 A1 to use a number of prefabricated laminate structures which are placed on top of each other to form the spar caps as well as the front and rear ends of a wind turbine rotor blade. The overall construction of the blade disclosed in US 2007/0040294 A1 comprises sandwiched structures which are formed conventionally and laminated structures which are formed by parts of the prefabricated laminate structures. Thus, the upper and lower shells of the blade are each composed of sections made of different material. The contact points between the sections made of different material can constitute weak points of the blade. 
         [0005]    WO 2007/038930 A1 describes a method for producing a fibre-reinforced product in which one or more layers of reinforcing fibres are placed in a mould together with at least one porous layer and a resin for distribution through the porous member to the fibre layers is introduced. The porous member may, in particular, be placed so as to form an inner layer (see page 9, lines 21 and 22) and may have a sheet structure in the form of a knitted, woven, needled a crocheted foamed or filter-like material. The product may in particular be a part of a shell of a wind turbine blade. 
         [0006]    US 2005/0037678 A1 describes open grid fabric resin infusion media and reinforcing composite lamina. The open grid fabrics serve as an interlamina infusion medium that significantly improves the speed, uniformity and ability to quality-control the transfer, delivery and distribution of matrix resin (plastic) throughout the laminates stack. The open grid fabric refers to knitted or woven fabrics. The open grid fabric can be sandwiched in the middle and/or placed on either or both sides of the laminate schedule. 
         [0007]    US 2003/0077965 A1 describes three-dimensional spacer fabric resin infusion media and reinforcing composite lamina. The spaceer of fabric infusion media can be sandwiched in the middle and/or placed on either or both ends of the laminate schedule to promote rapid and uniform distribution on all sides of the dry laminate. 
         [0008]    US 2004/0017020 A1 describes a process for fibreglass moulding using a vacuum. A non-absorbent porous layer is positioned between fibre glass resin absorber layers and is woven to provide a wave which defines passages for the resin to travel. The non-absorbent layer may be a weave of non-absorbent fibres or solid core including wood having grooves channels or holes throughout the surface and foam having grooves, channels or holes throughout the surface. Furthermore, metal woven from fibres or having grooves, channels or holes throughout the surface is also mentioned, as are plastic woven fibres. 
         [0009]    WO 2005/121430 A2 describes a multi layer construction that can be used a reinforcement in a part obtained by resin transfer moulding. It comprises a core layer consisting of an open work structure having a special undulation and is made of high tenacity yarns. The yarns can be made from aramide, carbon, glass, or metal fibres. 
         [0010]    FR 2 870 861 A1 describes a textile laminate to be integrated in the structure of a moulded article realized by infusing of resin. The textile laminate combines at least one layer of reinforcing textile construction and at least one drainage layer formed by an open work construction capable of forming a preferential passage area for the resin during the infusion. The reinforcing layer and the drainage layer a mechanically joined by means of a bonding interface of the type that permits the laminate to remain deformable. 
         [0011]    U.S. Pat. No. 5,484,642, which corresponds to FR 2 605 929 A1 describes a textile material useful for producing composite laminated articles by injection moulding. The injection moulding technique involves arranging a stack of layers of textile reinforcement in the mould having a shape that corresponds to that of the article to be obtained and after the mould has been closed, and injecting a resin into it. At least one layer of the stack of textile reinforcement has a structure in which ducts extend in at least one direction in the stack for improving the flow of the resin flowing injection. 
         [0012]    GB 2 381 493 A describes composite materials in which a flow medium for enhancing the flow of a liquid through the composite, is present between carbon fabrics. 
         [0013]    WO 02/058915 A1 describes a core material for fibre reinforced resin composite structure that has slits on its surface and through holes which pass through it in thickness direction. 
       SUMMARY OF INVENTION 
       [0014]    It is therefore an objective of the present invention to provide a method of producing fibre reinforced laminated structures which overcome the above-mentioned problems. 
         [0015]    This objective is solved by a method of producing fibre reinforced laminated structures as claimed in an independent claim. The depending claims define further developments of the invention. 
         [0016]    The inventive method of producing fibre reinforced laminated structures comprises the steps of layering a number of dry fibre reinforcement layers in a mould by placing them on top of each other, infusing a curable viscous or liquid polymer into the mould after the fibre reinforcement layers have been layered in the mould and curing the polymer. A flow enhancing layer for enhancing the polymer flow during infusion of the polymer is placed between two fibre reinforcement layers when layering the number of dry fibre reinforcement layers. The flow enhancing layer is a pre-cured solid layer. 
         [0017]    By using such flow enhancing layers it is possible to ensure sufficient wetting of all fibre reinforcement layers even in thick layer stacks. Therefore, the risk of delamination and air pockets, which would lead to wrinkles, is highly reduced. Moreover, the laminated structure can be made of a continuous structure without points where different structures abut each other. 
         [0018]    As a flow-enhancing layer, a layer may be used which is made of a layer material having a higher permeability with respect to the polymer than a layer made of the material of the fibre reinforcement layers and having the same thickness as the flow-enhancing layer. For example, the higher permeability could be achieved by a lower fibre density of the layer material as compared to the material of the fibre reinforcement layers. However, it is even more advantageous if a meshed or woven layer or a perforated layer is used as a flow-enhancing layer. In this case, the flow-enhancing layer may be made from the same material as the fibre reinforcement layers. However, the meshed or woven layer or the perforated layer may also be implemented as a pre-cured solid layer. This ensures that its flow-enhancing property is maintained even if pressure is applied to the layers during infusion of the polymer or a vacuum is present during infusion. If the flow-enhancing layer has too high a compressibility, there could be a risk of reducing its flow-enhancing properties too much when applying pressure or vacuum. 
         [0019]    Although the same material may be used for the flow-enhancing layers as is used for the fibre reinforcement layers, it may be advantageous if a different material is used for forming the flow-enhancing layers. This offers the possibility of providing a desired stiffness ratio of the flow-enhancing layers to the fibre reinforcement layers after curing the resin. 
         [0020]    In addition, in particular the perforated layers may be corrugated to increase the space available for polymer flow. 
         [0021]    In particular, if thick fibre reinforced laminated structures are to be produced, one may create a stack comprising a number of fibre reinforcement layers and then lay at least one flow-enhancing layer on top of the stack when layering the number of dry fibre reinforcement layers. This offers the possibility of forming stacks of fibre reinforcement layers without a flow-enhancing layer up to a thickness for which sufficient wetting can be ensured without a flow-enhancing layer so that the overall number of flow-enhancing layers can be kept small. In particular, a number of stacks and a number of flow-enhancing layers can be layered such that stacks and flow-enhancing layers alternate. The flow-enhancing layers then ensure that a sufficient amount of polymer can flow between different stacks of fibre reinforcement layers for sufficiently wetting all fibre reinforcement layers of the stacks. 
         [0022]    In addition, a flow-enhancing layer may be placed as the lowermost layer and/or a flow-enhancing layer may be placed as the uppermost layer of the laminated structure to facilitate polymer flow at the bottom and the top of the layered structure in the mould. 
         [0023]    The inventive method may, in particular, be used to produce wind turbine rotor blades as fibre reinforced laminated structures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Further features, properties and advantages of the present invention will become clear from the following description of an embodiment of the invention in conjunction with the accompanying drawings. 
           [0025]      FIG. 1  schematically shows a section through a laminated wind turbine rotor blade. 
           [0026]      FIG. 2  shows a detail of  FIG. 1 . 
           [0027]      FIG. 3  schematically shows a first stage in the process of producing a rotor blade according to  FIG. 1 . 
           [0028]      FIG. 4  shows a second stage in the process of producing a rotor blade according to  FIG. 1 . 
           [0029]      FIG. 5  shows a third stage in the process of producing a rotor blade according to  FIG. 1 . 
           [0030]      FIG. 6  shows a first example of a flow-enhancing layer used in the process of producing a rotor blade as shown in  FIG. 1  in a top view. 
           [0031]      FIG. 7  shows the flow-enhancing layer of  FIG. 6  in a sectional view. 
           [0032]      FIG. 8  shows a second example for a flow-enhancing layer used in the process of producing a rotor blade according to  FIG. 1  in a top view. 
           [0033]      FIG. 9  shows the flow-enhancing layer of  FIG. 8  in a sectional view. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0034]      FIG. 1  is a schematic view of the cross-section of a laminated wind turbine rotor blade  1 . The rotor blade  1  is made of an upper shell  3  and a lower shell  5  each comprising a thickened section  9  and non thickened sections  11 . The upper and lower shells  3 ,  5  comprise a number of fibre reinforcement layers which are not individually shown in the figure. In the thickened section  9  the number of reinforcement layers is increased with respect to the non-thickened sections  11 . 
         [0035]    The thickened section  9  of the upper shell  3  is shown in more detail in  FIG. 2 . In the thickened section  9 , flow-enhancing layers  13  are present between stacks of fibre reinforcement layers  15 . The fibre reinforcement layers  15 , as well as the flow-enhancing layers  13 , are embedded in a resin matrix which has been formed by resin infusion and subsequent curing of the resin. During the infusion process the flow-enhancing layers  13  layered between neighbouring stacks  15  of the fibre reinforcement layers ensure sufficient resin flow between the stacks  15  so that a sufficient wetting of all fibre reinforcement layers in the stacks  15  is achieved. 
         [0036]    The method of forming the wind turbine rotor blade  1  shown in  FIGS. 1 and 2  will now be described with respect to  FIGS. 3 to 5 . In general, the upper and lower shells  3 ,  5  of the rotor blade  1  are produced by placing dry fibre reinforcement layers on top of each other in a mould, wetting the fibre reinforcement layers by means of a resin infusion and subsequently curing the resin. Please note that although described with respect to producing a wind turbine rotor blade  1 , the method which will be described with respect to  FIGS. 3 to 5  can also be used for producing other fibre reinforced laminated structures, e.g. in boat building. 
         [0037]    A first stage of the method for producing the rotor blade  1  shown in  FIG. 1  is shown in  FIG. 3 . The figure schematically shows a cut-out sectional view of the mould  17  and a number of fibre reinforcement layers  19 , e.g. glass fibre layers, carbon fibre layers or aramid fibre layers, which are placed dry in the mould  17  on top of each other so as to form a stack  15  of the fibre reinforcement layers  19 . 
         [0038]    After a stack  15  of fibre reinforcement layers has been placed in the mould  17  a flow-enhancing layer  13  is placed on top of the stack  15  (see  FIG. 4 ). 
         [0039]    After the flow-enhancing layer  13  has been placed on top of the first stack  15  of fibre reinforcement layers  19 , another stack  15  comprising a number of fibre reinforcement layers  19  is placed on top of the flow-enhancing layer  13 , as shown in  FIG. 5 . 
         [0040]    Alternately layering stacks  15  of fibre reinforcement layers  19  and flow-enhancing layers  13  can be continued until the desired thickness of the layering is reached. The number of fibre reinforcement layers  19  can be as high as possible without negatively influencing the wetting of all fibre reinforcement layers  19  within a stack  15 . 
         [0041]    Although not shown in  FIG. 3 to 5 , additional flow-enhancing layers  13  may be present under the lowermost stack  15  of fibre reinforcement layers  19 . In this case, a flow-enhancing layer  13  would be the first layer placed in the mould  17 . The outermost layer of the overall stack consisting of stacks  15  of fibre reinforcement layers  19  alternating with flow-enhancing layers  13  may also be a flow-enhancing layer  13 . 
         [0042]    After the layering of the dry fibre reinforcement layers  19  and the dry flow-enhancing layers  13 , the mould  17  is closed and a vacuum is applied to the mould. Then, a resin, e.g. a polyester resin or an epoxy resin, is infused into the evacuated mould. The resin wets the fibre reinforcement layers thereby using the flow-enhancing layers  13  as flow paths which allow for the distribution of the resin throughout the thick overall stack. After a while all fibre reinforcement layers  19 , and also all flow-enhancing layers  13 , are sufficiently wetted. Then, the resin is cured. After curing the resin, the mould  17  is dismantled. 
         [0043]    Examples of flow-enhancing layers  13  that may be used in the described method are shown in  FIGS. 6 to 9 . 
         [0044]      FIGS. 6 and 7  show a flow-enhancing layer  13  which is implemented as a woven mat. While  FIG. 6  shows a top view onto the mat,  FIG. 7  shows a sectional view through the mat. As can be seen from the figures, threads  21 ,  22  of the woven structure provide space for a resin flow through the flow-enhancing layer  13  above and below the threads  21 ,  22 . Moreover, resin can flow through openings  23  between neighbouring threads from one side of the woven mat to the other. Therefore, the permeability of this woven mat is much higher than that of the fibre reinforcement layers  19 . 
         [0045]    The woven mat may be made from the same material as the fibre reinforcement layers  19 . In addition, the woven mat may be pre-cured so as to be inherently stable. This prevents the reduction of flow space for the resin by preventing compression of the woven mat when the vacuum is applied to the mould. In the present embodiment of the invention the woven mat is made from a glass fibre epoxy laminate. 
         [0046]      FIGS. 8 and 9  show a flow-enhancing layer  13  which is implemented as a corrugated metal plate  25 . While  FIG. 8  shows a top view onto the corrugated metal plate  25 ,  FIG. 9  shows a sectional view through the plate  25 . Although the 
         [0047]    corrugation alone would be sufficient for providing flow space for the resin to flow through the flow enhancing layer  13 , the flow can further be enhanced by providing perforating holes  27  in the corrugated metal plate  25 , as shown in  FIGS. 8 and 9 . Through the holes  27  resin may easily flow from one side of the corrugated metal plate  25  to the other. Although shown as being located at the highest and lowest points of the corrugated metal plate  25 , holes  27  may additionally or alternatively be present between these locations. 
         [0048]    Although not explicitly mentioned, materials other than the material the fibre reinforcement layers  19  are made of or metal can be used as a material for the flow-enhancing layers  13 . By using selected materials, it becomes possible to provide a desired stiffness ratio of the flow-enhancing layers  13  to the fibre reinforcement layers  19  after curing the resin. 
         [0049]    Throughout the description the wetting of the fibre reinforcement layers means a wetting to a desired degree which is sufficient for the desired application of the product produced with the method. The degree of wetting may therefore vary from partially wetting the fibre reinforcement layers up to fully wetting the fibre reinforcement layers, depending on the laminated structure which is to be formed. 
         [0050]    Although a woven mat has been described as an example for the flow enhancing layer, other meshed structures, with or without corrugation, may be used instead. 
         [0051]    In the inventive method, the infusion of resin is facilitated, in particular for thick stacks of fibre reinforcement layers, by using flow-enhancing layers between stacks of fibre reinforcement layers. This allows the manufacture of thicker laminates and thus, enables the manufacture of larger structural integrated laminated structures.