Abstract:
A fiber reinforced plastic-structure and to a method to produce the fiber reinforced plastic-structure are disclosed. At least two elements are used to build up the shape of the fiber reinforced plastic-structure. Two adjacent elements are connected via its contact-surfaces by an applied glue or resin. A mat is located between the contact-surfaces before the glue or resin is used to connect the elements. The mat comprises chopped fibers, which are oriented in a random manner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 09010467.0 EP filed Aug. 20, 2009, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a fiber reinforced plastic-structure and to a method to produce the fiber reinforced plastic-structure, while at least two elements are used to build up the shape of the fiber reinforced plastic-structure. 
       BACKGROUND OF INVENTION 
       [0003]    It is known to build up a wind-turbine-blade for example by the use of fiber-reinforced laminates. The fiber-reinforced laminates may consist of chopped strand mats (CSM) or of woven fabric mats (like multi-axis interlaid scrims), of warp-thread reinforced unidirectional performs, of single or joined roving bundles and of any known fiber-material like glass, Kevlar, carbon or hemp. 
         [0004]    The fiber-reinforcement may be supplemented with pre-fabri-cated components. For example fiberglass inserts, pultruded rods, . . . , etc. 
         [0005]    The fiber-reinforcement may even be combined with sandwich core materials like balsa wood, foam or honeycomb. 
         [0006]    A wind-turbine-blade is built up by a number of layers in a so called laminate stack. The structure comprises piled up plastic-laminates, pre-casted components or elements or other fiber reinforced plastic-structures. 
         [0007]    A lower mould is used to carry the main blade-structure, while an upper mould is used to enclose the three-dimensional structure of the blade, together with the lower mould. The connected moulds are evacuated by air while a liquid matrix material (like resin) is subsequently infused into the mould. 
         [0008]    The resin cures out, while this process is achieved by applying pressure and temperature to the enclosed structure. This kind of process is called “Vacuum Assisted Resin Transfer Method, VARTM”. 
         [0009]    A fiber reinforced plastic-structure comprises single elements. These single elements may comprise fiber-reinforced laminates, pieces of balsa-wood and/or other pre-casted elements. The single elements need to be connected. 
         [0010]    For the connection the single elements are arranged into a desired shape and are connected by help of glue, which is applied to contact-surfaces of adjacent elements. 
         [0011]    It is possible to connect the single elements by using the VARTM, while resin is used as matrix-material. 
         [0012]    It is also possible to use glue to connect the single elements. 
         [0013]    If the elements are connected by resin or glue a resulting bond-connection shows only a low so called “Inter Laminar Shear Strength Value, ILSS-value”. 
         [0014]    This is especially the case if the contact-surfaces of the elements are smooth. 
         [0015]    Often a so called “Shear Fracture” occurs between the smooth surfaces, so a fracture propagates along a resulting glue-line. This is weakening the resulting structure. 
         [0016]    To reduce this effect it is known to use a glue, which contains a filler. For example a so called “Mineral Filler” or a “Needle Shape Filler” is used. 
         [0017]    This kind of glue is based on a two component epoxy or is based on a polyurethane system. It is also possible to base it on unsaturated polyester, to which a curing agent is added. 
         [0018]    One disadvantage of this filler-based glue is, that its application is done in the shape of a glue-paste. This often creates voids and air bubbles along the glue line, leading to a crack formation. 
         [0019]    Often a glue-paste will result in a brittle glue-line and in a glue line, which shows cracks. 
       SUMMARY OF INVENTION 
       [0020]    It is therefore the aim of the invention, to provide an improved fiber reinforced plastic-structure and a method to produce it. 
         [0021]    This aim is solved by the features of the independent claims. 
         [0022]    Other embodiments of the invention are object of the dependent claims. 
         [0023]    According to the invention the fiber reinforced plastic-structure comprises at least two single elements. The elements are used to build up the shape of the structure. The two adjacent elements are connected via its contact-surfaces by an applied glue or resin. A mat is located between the contact-surfaces before the glue or resin is used to connect the elements. The mat comprises chopped fibers, which are oriented in a random manner. 
         [0024]    So the used mat is a so called “Chopped Strand Mat, CSM”. 
         [0025]    The random orientation of the fibers in the mat prevents formation and propagation of cracks in an unbroken path in the connection zone. So a strong and robust connection of the elements is achieved—which is a great advantage compared to prior art, where two surfaces are connected by the use of glue-paste or the like and where cracks are likely to form and evolve in an unbroken path. 
         [0026]    In a preferred embodiment the fiber reinforced plastic-structure is used to build up a blade of a wind-turbine preferably. 
         [0027]    The glue or resin is applied by help of the known VART-method in a preferred embodiment. 
         [0028]    It is possible to apply the VART-method to the elements of the fiber reinforced plastic-structure to create a single reinforced fiber reinforced plastic-structure. 
         [0029]    It is also possible to arrange the elements of the fiber reinforced plastic-structure together with other components of a wind-turbine-blade for example into moulds and to apply the VART-method to the whole blade-structure. So the elements of the fiber reinforced plastic-structure are integrated into the blade-sandwich-structure. The elements are connected together and are also connected with the other used components of the blade by applying a single VARTM-process to the whole blade. 
         [0030]    In a preferred embodiment the mat comprises fibers, which are made of a pre-impregnated laminate, a so called “pre-preg”. For this purpose glass fibers, carbon fibers or other possible fibers are impregnated with an epoxy resin, while the resin is destined to cure at a predetermined temperature. 
         [0031]    Because of the invention a glue-bond with a high quality is obtained. The glue-bond shows a very high “Inter Laminar Shear Strength, ILSS”. 
         [0032]    According to the invention the used mat comprises cut fibers with a random orientation. The mat is impregnated with epoxy and is placed in a bond-zone between two elements or parts. So a good bond-zone with an improved ILSS-value is achieved. 
         [0033]    In a preferred embodiment the random oriented fibers show a length from 5 mm up to 50 mm, while they are impregnated with a heat curing epoxy resin. 
         [0034]    Due to the invention it is also possible, to control the thickness and the quality of the bond-zone very easily. As the pre-impregnated CSM material is used as a mat, air-bubbles and voids along the bond-line or within the bond-zone are reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The invention will be described in more detail by help of some drawings. 
           [0036]      FIG. 1  shows different types of fiber-layouts being used to build up a fiber reinforced plastic-structure, 
           [0037]      FIG. 2  shows a possibility to produce a mat being used according to the invention, 
           [0038]      FIG. 3  shows a cross-section of a blade, comprising a number of elements, which are connected according to the invention, 
           [0039]      FIG. 4  shows a method to use the mat according to the invention during a blade-production-process. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0040]      FIG. 1A  shows a unidirectional laminate  1 , comprising a number of fibers, which are aligned in a parallel direction. The laminate shows therefore a high specific stiffness along its length. 
         [0041]    The laminate  1  shows a very smooth surface, which might lead to an impaired inter laminar shear strength value for a glue connection to the final laminate  1 . 
         [0042]      FIG. 1B  shows a multidirectional laminate  2 , comprising a first number of fibers, which are aligned in a 0° direction. A second number of fibers are controlled aligned in a +45° direction while a third number of fibers are controlled aligned in a −45° direction. 
         [0043]    The resulting laminate  2  shows an improved specific stiffness in the relevant directions 0°, +45° and −45°. 
         [0044]    The laminate  2  shows a very smooth surface, which might lead to an impaired inter laminar shear strength value for a glue connection to the final laminate  2 . 
         [0045]      FIG. 1C  shows a laminate  3 , comprising random-oriented, cut fibers. These fibers are forming a mat. 
         [0046]    According to the invention the mat will be located especially between smooth surfaces of two adjacent and in some cases pre-fabricated elements. 
         [0047]    This mat is named “chopped strand mat, CSM”. 
         [0048]      FIG. 2  shows a possibility to produce a mat being used according to the invention. 
         [0049]    Short-cut fibers  4  are brought onto a carrier, while the fibers  4  show a random-orientation. 
         [0050]    The fibers  4  are combined with a heat-curing resin  5 . 
         [0051]    The fibers  4  and the resin are guided between two rotating elements  6 , which are used to create the mat, being used for the invention. 
         [0052]    For example pressure is applied to the combined fibers and resin. 
         [0053]    In a preferred embodiment a plastic protective liner is also applied on each side of the mat (not shown in detail). This product is known as “pre-preg”. 
         [0054]    The plastic liner is used to protect the mat, as long as it is on stock. The liners are removed later, when the mat is destined to be used. 
         [0055]    Due to this a pre-impregnated mat or laminate L is created, comprising short-cut and random oriented fibers  4  and resin  5 , while the mat L is sealed by protection plastic liners. 
         [0056]      FIG. 3  shows a cross-section of a blade BL, comprising a number of elements, which are connected according to the invention. 
         [0057]    For example a pre-casted beam  7  is located in the middle of the blade BL, while two pre-casted blade-shells  8   a ,  8   b  are forming an outer shape of the blade BL. 
         [0058]    A lower blade-shell  8   a  needs to be connected with an upper blade-shell  8   b . According to the invention pre-impregnated mats  9  are located between the two shells  8   a ,  8   b.    
         [0059]    Accordingly the pre-casted beam  7  needs to be connected with the lower blade-shell  8   a  and the upper blade-shell  8   b . According to the invention pre-impregnated mats  9  are located between the two shells  8   a ,  8   b  and the pre-casted beam  7 . 
         [0060]    In a preferred embodiment the used CSM-prepreg-mats are placed by a robot-device or by hand in the dedicated positions. 
         [0061]    All the parts of the blade BL are pressed together and vacuum may be applied to enforce the connection. 
         [0062]    Next heat is applied to the structure, so the applied resin of the mat is allowed to cure. So the applied CSM-prepreg-mats connect the described parts of the blade BL, as shown as completed blade  10  on the right side of  FIG. 3 . 
         [0063]      FIG. 4  shows a method to use the mat according to the invention during a blade-production-process. The blade is shown in a cross-sectional-view. 
         [0064]    A number of dry fiber-laminates are placed into a lower mould  12 , forming a dry main structure of the blade. 
         [0065]    Additionally other components may be put onto the lower mould  12  to form a three-dimensional-shape of the blade. These components may comprise for example dry laminates or mats, pre-fabricated components or layers of balsa-wood, etc. 
         [0066]    The cross-sectional view of the blade in  FIG. 4  shows exemplary a web as an additional component, while the web is located in a middle section of the blade in a vertical position. 
         [0067]    According to the invention CSM-mats are located between relevant surfaces of adjacent components. 
         [0068]    Another number of dry fiber-laminates  13 , holding the rest of the dry blade laminate, needs to be placed on top of the main blade structure. 
         [0069]    For this an upper mould  11  is used. While the upper mould  11  is placed on the floor with its concavity in upward direction, a vacuum liner  14  comprising a layer of CSM-prepreg is placed to cover the dry fiber-laminates  13 . 
         [0070]    Vacuum is applied under the liner  14  and therefore it is possible to lift the upper mould  11  with the stack of reinforcement laminate  13  and rotate it around its length axis, enabling it to be placed accurately over the lower mould  12 . 
         [0071]    The upper mould  11  and the lower mould  12  are connected. 
         [0072]    All the parts or components within the enclosed moulds are pressed together—vacuum may be applied to enforce the structure. 
         [0073]    Vacuum for the VARTM-process is applied and resin is infused into the blade-structure. Subsequently heat is applied to the moulds to cure the resin and to cure the CSM-pre-preg-mat to finish the blade.