Abstract:
The present invention relates to a method of manufacturing a fibre-reinforced laminate ( 101 ), which laminate comprises at least one area on the surface of the laminate configured with projecting fibres for being joined to one or more other elements. This is accomplished by arrangement of a hybrid mat ( 505 ) most distally in the laminate which comprises at least two layers with fibres ( 105 ) transversally across the joining face ( 402 ) of the layers. The outermost layer ( 301 ) of the hybrid mat is removed prior to joining, whereas the innermost layer ( 506 ) of the hybrid mat remains a constituent of the laminate ( 101 ). The invention relates to different methods of causing fibres to project from the surface of the laminate, said fibres thus having a reinforcing effect on the joints. The invention also relates to a laminate manufactured by the disclosed methods and a blade for a wind power plant comprising such laminate.

Description:
[0001]    The present invention relates to a fibre-reinforced laminate and a method of manufacturing a laminate, said laminate comprising at least one area on the surface of the laminate configured for being interconnected with one or more other bodies. The invention also relates to a blade for a wind power plant comprising such laminate. 
       BACKGROUND 
       [0002]    When several parts are assembled it is often the strength of the joint as such which is decisive to the strength of the finished element. In the context of fibre-reinforced laminates purely mechanical joints such as bolting, riveting, etc., are most often not particularly suitable. Instead gluing is most often applied or, if two fibre-reinforced elements are concerned, optionally joining by impregnation of the one element onto the other. In order to achieve a joint with sufficient strength, it is usually necessary to treat the surface of the areas on the one or both of the elements to be joined with a view to achieving larger surface area and hence increased adhesion. 
         [0003]    Several such surface treatments are available, the most simple and probable most common one is to grind the surface in those places where the joining is to take place in order to subsequently glue or in any other way obtain joining. Hereby a rough surface is accomplished with a number of exposed fibres. However, the grinding is very time-consuming and a very work-intensive process, and simultaneously it is difficult and expensive to remove the grinding dust resulting from the process, which dust is undesirable for working environment and health considerations. If the finished composite product is subsequently exposed to humidity, remaining grinding dust may also be a great inconvenience, since the dust may collect to form very hard lumps when absorbing humidity. 
         [0004]    Other methods of increasing the surface area and hence the adhesion include sanding, chemical degreasing or use of laser, wherein indentations are formed on the surface of the element due to evaporation of material from the surface. However, sanding of the surface entails the same drawbacks with dust and poor working environment as grinding. The working environment is also an issue in case of chemical degreasing and laser treatment. The methods also share the feature that they all destroy the surface of the laminate to some degree or other where the laminate is treated, which makes it more difficult to ensure the same quality of the laminate outside and within the joining zones. A further drawback of those methods for surface treatment is that they are all relatively work-intensive and hence time-consuming. Most often it is also necessary that the laminate maintains its shape and remains braced during the surface treatment, which is a drawback since it is both space-consuming and prevents the use of the scaffolding for another element, while the finished laminate cures elsewhere. Additionally the joining to the one or the other elements should preferably take place fairly shortly after the surface treatment in order for it to work optimally. When the laser treatment is concerned, it is also an expensive method, both as to acquisition costs of equipment and execution costs. 
         [0005]    U.S. Pat. No. 5,968,639 teaches a method of joining several parts, where Z-pinning is used to cause fibres to project from the surface of the one part. During manufacture of the laminate a foam element filled with Z-pins in the desired amount and orientation is arranged on top of the laminate and underneath the vacuum cloth. During the injection the foam material collapses due to the temperature and/or the pressure, whereby the Z-pins are pressed partially down into the laminate and integrated by moulding into its surface. The remainder of the foam material can be removed just before the joining of the laminate to another part and thus serves to protect the Z-pins in the meantime. Here the Z-pins increase the strength of the joint quite considerably, but the manufacturing method is complicated by the need to fairly accurately control the pressure and the temperature during the injection in order to ensure that the foam material collapses to a suitable degree. Moreover the arranging of the Z-pins considerably increases the production time. 
       OBJECT AND DESCRIPTION OF THE INVENTION 
       [0006]    It is the object to provide joints for fibre-reinforced laminates of great strength and adhesion whereby the above-mentioned problems associated with various surface treatments, such as grinding, are obviated. 
         [0007]    Thus, the present invention relates to a method of manufacturing a laminate comprising the application of resin to one or more fibre-reinforced layers and subsequently curing; said laminate further comprising at least one area on the laminate surface configured for being interconnected with one or more other bodies; on which surface areas a number of fibres are arranged in such a manner that at least some of the fibres project from the surface of that area. A hybrid mat is arranged distally in the laminate, which hybrid mat comprises at least two layers with fibres transversally across the joining face of the layers; and wherein at least the outermost layer of the hybrid mat is removed prior to joining to the other bodies; and wherein at least the innermost layer of the hybrid mat remains a constituent of the laminate. 
         [0008]    When the outermost layer has been removed the remaining layers remain on the surface of the laminate with the fibres projecting from the remaining layers and secured thereon. The fibres, which are thus caused to project from the laminate surface, increase the surface area considerably, whereby the adhesion between the laminate and the object(s) to be joined, eg by gluing, is enhanced. Moreover, the fibres act as a fibre-reinforcement of the joint as such, thereby considerably increasing its strength. A further advantage of this method according to the invention is that a subsequent treatment, chemical as well as mechanical, of the areas to be joined is unnecessary. Hereby the discomforts of dust from the conventional grinding process as described in the introductory part are thus avoided; and likewise it is avoided to have to use particular chemicals for cleaning or other treatment of the surface. The prior art techniques referred to in the introductory part for increasing the surface area and hence for improving the adhesion also share the feature that an amount of material is removed from the surface with ensuing damage to same. Opposed to this, the method according to the invention is advantageous in that the ready-moulded surface of the laminate is not damaged, and it is hence ensured that a uniform quality results between the areas for joining and the remainder of the laminate. 
         [0009]    Yet a considerable advantage of the present method is that the quality of the joints does not depend on when the joining is performed compared to when the laminate was made. This means that, if desired, the joining may take place immediately following manufacture, but the same quality can be obtained even if one waits eg one week or more before joining. This is a major advantage compared to eg gluing of grinded surfaces, where the gluing should preferably be performed fairly quickly following grinding of the relevant surfaces. By such insensitivity to time, a greater flexibility of production is also accomplished, where the individual manufacturing processes are more easily adapted and incorporated which may in turn entail less expensive production from an overall point of view. 
         [0010]    As opposed to other known methods of accomplishing improved glued joints, such as grinding, sanding and laser treatment, where it is most often necessary that the laminate retains its shape and remains braced for the sake of the surface treatment, the method according to the present invention is advantageous in that the joining zones on the laminate are prepared already before the laminate is ultimately cured. The scaffolding of the mould can thus be removed, and manufacture of yet an element can be initiated while the finished laminate cures elsewhere. 
         [0011]    The above-described method according to the invention is further advantageous in that a reinforced joint is accomplished without particular claims as to how and how far the fibres project from the surface. Thus, the fibres may have different lengths, project in different directions and sit in a random pattern while still considerably increasing the strength of the joint. This is advantageous in that the requirements to precision and tolerances during production can hereby be reduced, and likewise the production can be simplified with an ensuing minimization of manufacturing costs. 
         [0012]    Conversely, if those parts of the fibres that project from the surface are of approximately the same length, the fibres can be used to advantage to ensure a uniform distance between the joined elements. Hereby, eg in case of gluing, glue lines of equal heights can be obtained which improves the quality. 
         [0013]    A further advantage of said method is that, provided they are of an electrically conducting material such as eg carbon fibres or metal fibres, such projecting fibres may contribute to equalisation of potential between the combined parts. Such equalisation of potential is advantageous eg when lightning protection of a blade is concerned. 
         [0014]    By this method it is further advantageously accomplished that the hybrid mat can be manufactured in advance as a semi-finished product entirely independently of the laminate manufacturing process as such. From this semi-finished product, sections may optionally be cut that correspond to the shape of the gluing zones or the joints, which are subsequently simply laid as a final layer on top of the remaining laminate layers prior to application of resin material. Thus, the method may in a simple manner be incorporated in the usual laminate manufacturing process, eg a VARTM process. Thereby it is also possible to completely accurately control where the joining areas are to be. The ability to manufacture the hybrid mat in advance also enables that the production may take place in optimum conditions and in the best possible working conditions with an ensuing improved guarantee for quality. It is a further advantage that, by the present method, the fibre-reinforced joints can be made locally in precisely and exclusively the desired locations. Hereby the consumption of material is also minimised. 
         [0015]    Moreover the invention relates to a method of manufacturing a laminate according to the above, where the fibres transversally of the joining face of the layers in the hybrid mat are arranged by Z-pinning, by needle-punching through a fibrous layer in the hybrid mat and into a further layer in the hybrid mat by fibre being pulled from a fibrous layer in the hybrid mat into a further layer in the hybrid mat; or by sewing together at least two layers in the hybrid mat. By all of these methods it is obtained that, following injection and following tearing off of the outermost layer of the hybrid mat, the fibres are firmly secured on the lowermost layer(s) on top of the remaining layers of the laminate; and that these fibres project from the laminate surface. The fibres transversally across the joining face may be arranged by use of Z-pinning. In addition to the ones mentioned previously, it is an advantage that the technique for arranging the fibres is a known and tested technique which is moreover not very cost-intensive. By this method it is also possible to arrange the transversal fibres position-wise and in numbers precisely where they are to be used. It is also possible to vary the number of projecting fibres so as to obtain joints of varying strengths dimensions to the forces that they are to be able to transmit. It is a further advantage that the fibres can be pinned or shifted angled into the laminate, whereby the fibres will project in different directions from the finished surface of the laminate. Hereby an improved and stronger joint is accomplished, where the fibres yield a higher degree of resistance compared to a scenario in which the fibres sit perpendicularly from the laminate surface. Since, in accordance with this method, the fibres may be arranged to extend down through several layers of the laminate, strong anchoring of the fibres is moreover accomplished. Apart from the above-mentioned advantages of such methods, where fibres are caused to project from the laminate surface, the methods are advantageous in describing simple processes with no requirements to special tools or particular materials. 
         [0016]    Yet an embodiment of the present invention relates to a method of manufacturing a laminate as described above, wherein the fibres transversally across the joining face of the layers in the hybrid mat were produced by a fibrous layer being draped into grooves in a further layer in the hybrid mat. 
         [0017]    Yet an embodiment of the present invention relates to a method of manufacturing a laminate as described above, wherein at least one layer in the hybrid mat is manufactured from a rubber-like material. The advantage of such material is that the resin material is unable to soak through this layer, and therefore it can relatively easily be torn off the laminate again and leave the fibres projecting there from. It is further possible to reuse the material. Finally it is a flexible material which can easily be laid across even double-curved surfaces. 
         [0018]    The invention further relates to a method of manufacturing a laminate in accordance with the above, wherein at least one layer of the hybrid mat is manufactured from a foam-rubber-like material, such as expanded polypropylene. A foam-rubber-like material such as expanded polypropylene can advantageously be included in the hybrid mat, as it is easily pinned through and therefore it is easy to pull, pin or sew fibres into it. It is also an inexpensive material. 
         [0019]    The invention moreover relates to a method of manufacturing a laminate according to the above comprising arrangement of at least one film between two layers in the hybrid mat. This is advantageous since the film, which may eg be a vacuum cloth or a peel ply, can facilitate tearing off of the outermost layer(s) of the hybrid mat in such a manner that the fibres remain on the surface and project there from. A film can thus prevent that one unintentionally tears off the entire hybrid mat or that one is unable to tear off the entire outermost layer. Finally a film can ensure that the resin penetrates into the outermost layer of the hybrid mat only in a limited amount via the fibres that sit transversally of the joining face and perforate the film. Thereby the outermost layer is more easily pulled off. 
         [0020]    The invention also relates to a fibre-reinforced laminate manufactured by one or more of the above described methods. Advantages of this are as described above. 
         [0021]    The invention also relates to a laminate according to the above, wherein those parts of the fibres that project from the surface are at least partially impregnated. Hereby it is possible to control how the fibres project from the surface, eg at which angles and how far from the surface the fibres extend. 
         [0022]    Moreover the invention relates to a laminate according to the above, wherein the fibres project from the surface at preferably acute angles relative to the plane of the surface, whereby it is accomplished that the fibres contribute further to the strength of the joint. 
         [0023]    The method further relates to a blade for a wind power plant comprising a laminate as described above. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]    In the following the invention will be described with reference to the figures, wherein: 
           [0025]      FIG. 1  shows a laminate with areas prepared for fibre-reinforced joints; 
           [0026]      FIG. 2  shows a blade for a wind power plant with areas of fibre-reinforced joints; 
           [0027]      FIG. 3  shows the manufacture of fibre-reinforced joints by Z-pinning of an extra layer on top of the laminate; 
           [0028]      FIG. 4  shows a cross-sectional view of a part of a laminate with a hybrid mat prior to application of resin; 
           [0029]      FIG. 5  is a cross-sectional view of a part of the finished laminate prepared for a fibre-reinforced joint; 
           [0030]      FIG. 6  shows the manufacture of a hybrid mat by needle punching; 
           [0031]      FIG. 7  shows the manufacture of a hybrid mat by fibres being pulled from the one layer upwards through a second layer; 
           [0032]      FIG. 8  shows the manufacture of a hybrid mat by layers being joined by sewing; and 
           [0033]      FIG. 9  shows the manufacture of a hybrid mat by a layer being draped upwards into grooves in another layer. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]      FIG. 1  shows a laminate  101  structured from a number of layers or laminas  102 , of which some or all are reinforced by fibres. The kind of fibre-reinforcement is of no actual consequence to the invention and may be configured in many different ways, eg with long uniformly oriented fibres with woven mats, small short fibres in random arrangement, etc. In the shown areas  103  on the surface  104 , the laminate  101  is to be joined to other (not shown) elements, and to improve the strength of the joints, they are reinforced by letting fibres  105  be securely fitted and project from the surface  104 . Such joints can be accomplished eg by gluing or by laying a dry or semi-impregnated element on top of an area  103 , following which the element is covered by a vacuum cloth and finishing injections are performed locally onto the laminate  101 . Some of the laminate surface  104  is shown in an enlarged sectional view in the figure. Moreover, a random section through the laminate  101  is shown, which shows part of an area  103  prepared for joining with fibres  105  projecting from the surface  104 . Fibres  105  need not necessarily sit in a pattern and may also have different lengths. Likewise, the amount of projecting fibres compared to the size of a joint may vary, and an amount of merely 1% of fibres has been found to considerably improve the strength of a joint. The fibres can be un-impregnated, ie dry, as well as completely or partially impregnated. By allowing the fibres to be completely or partially impregnated, it is possible to control the angle  106  formed by the fibres relative to the surface. If the fibres project from the surface in an acute angle  106  a further increased strength of the joint results. This strength is further enhanced if the fibres project in different directions. 
         [0035]    For instance, such fibre-reinforced joints can be used for blades of wind turbines, where each blade is typically assembled from two large blade shells. A blade shell  201  is outlined in  FIG. 2 . Two blade shells  201  are usually glued together along the major part of the rim circumference and to one or more bracing interior beams. The figure shows the areas  103  on a shell, where the laminate  101  from which the blade shell is constructed is to be glued. In these areas  103  the laminate  101  is therefore made with fibres projecting from the surface  104 , said fibres markedly increasing the strength of the joints. Conventionally the surface is grinded where the joints are to be provided to achieve a rough surface and sufficiently good adhesion to the elements glued thereon. However, the grinding is a very time-consuming and work-intensive process, and at the same time it is difficult and expensive to remove the grinding dust deriving from the process and which is undesirable for working environment and health considerations. Also, the grinding dust is undesirable after the blades have been taken into use, since any remaining dust may give rise to problems by absorbing moisture and forming sludge or very hard lumps that clog the drain holes in the blade. By the fibre-reinforced joints according to the invention grinding of the laminate is unnecessary. Additionally the scaffolding for the mould of the blade shell can be removed already while the blade cures. Conventionally this is not possible, since the scaffolding is necessary while the joint zones are being prepared eg by grinding. By the conventional grinding process it is also important to the quality of a glued joint that the gluing takes place relatively quickly following grinding. However, this is not necessary in case of joints with reinforcing fibres according to the invention, and therefore one may more easily adapt the timing of the joining processing to other processes and hence optimise the working procedure. 
         [0036]      FIG. 3  shows another method of manufacturing areas for fibre-reinforcing joints as described previously with fibres  105  projecting from the surface  104  of a laminate. Where it is desired to provide the joints an additional layer  301  is disposed most distally. The layer  301  is attached to the laminate by a number of short fibres  105  that are pressed, pushed or shifted in by means of eg Z-pinning  401 . Here the fibres  105  are shorter than the thickness of the overall laminate, whereby they are caused to sit interiorly of the laminate, but sit transversally across the joining face  402  between the laminate  101  and the extra layer  301 . Unlike conventional Z-pinning, the fibres  105  need not necessarily firmly hold the laminate and need not necessarily be disposed in a pattern; rather they need only be distributed fairly evenly and be provided in such number that, in the finished joint, they will ensure requisite adhesion. Nor is it necessary for the sake of the quality of the joint that all the fibres sit with identical distance into or at the same angle relative to the surface. Conversely, the strength of the joint will increase if the fibres are not sitting perpendicularly to the surface, and the strength will increase further if the fibres project in different directions. Thus, fibres  105  can thus be arranged in varying amounts dimensioned in accordance with the loads the joint is to be able to resist. The laminate assembly, ie the laminate  101  along with the additional outermost layer  301 , is injected with resin, and the outermost layer  301  is torn off either prior to final curing or just prior to joining to other elements, whereby the pinned-in fibres  105  are left to project from the surface  104 . This layer  301  may eg be of glass fibres or a foam material such as eg expanded polypropylene. For the fibres  105 , various known materials can be selected, such as carbon fibres or glass fibres. To facilitate tearing off of the outermost layer  301  and to ensure that the tearing takes place at the joining face  402 , a film can be arranged between the surface  104  of the laminate and the additional outermost layer  301 . Such film could be a peel ply or a vacuum cloth. 
         [0037]      FIG. 4  shows a cross-sectional view of a laminate  101  just prior to application of the resin material. The figure illustrates a manufacturing method for a fibre-reinforced joint according to the invention. Here the laminate  101  is manufactured by the VARTM process (Vacuum Assisted Resin Transfer Moulding), but in other embodiments it is an option that it is accomplished in other manufacturing processes, such as eg by simple wet laying (optionally hand laying), by RTM or on an element moulded by pultrusion or injection moulding. As is the case with ordinary moulding, the material layers  102  are laid into the mould part  501 . On top of this, an injection layer or injection netting  502  is optionally laid to improve distribution of the resin material and yet on top of this the injection passages  503  in a pattern and with a mutual distance that are suitable for providing optimal injection of the resin. All of this is covered in a vacuum cloth  504  which is sealed at the edge of the mould part, and hereby the laminate is ready for injection. In those areas where it is desired to prepare a fibre-reinforced joint, most distally on top of the laminate and below the injection layer  502 , a so-called hybrid mat  505  is provided. If the joint is to be arranged on the other side of the laminate the hybrid mat  505  is arranged between the mould part  501  and the laminate  101 . It is a characteristic feature of this hybrid mat  505  that it consists of at least two layers with fibres  105  sitting transversally of the joining face  402  of the layers. In different embodiments the transversal fibres  105  may sit in different manners, either only interiorly of the hybrid mat or extending all the way through its outermost or lowermost layer or combinations thereof. The joining of the layers and the transversal fibres  105  in the hybrid mat  505  can thus take place in advance independently of the production of the laminate  101 , and it is furthermore an option to produce the hybrid mat as yard goods from which desired pieces can be cut and unrolled to the specific production. The ability to produce the hybrid mat  505  in advance also provides the option that the production may take place in optimum conditions and in the best possible working conditions with ensuing improved guarantee of the quality of the mat. During injection the resin material is conveyed via the injection passages  503  throughout the entire injection layer  502  and from there down and out through all the layers in both the laminate  101  and the hybrid mat  505 . When the injection has ended, the vacuum cloth  504 , the injection passages  503  and the injection layer  502  are removed as usual. Besides, at least the outermost layer  301  of the hybrid mat  505  is removed. 
         [0038]    Now remains the lowermost layer(s)  506  of the hybrid mat  505 , combined and secured onto the laminate surface  104 , with fibres  105  projecting out of the surface  104  as outlined in  FIG. 5 . In order to make it easier to tear off the outermost layer(s)  301  of the hybrid mat  505  and to ensure that one does not tear off the entire mat and to ensure that the outermost layer  301  is not too securely attached, a further layer can be disposed between the outermost layer  301  and the lowermost layer  506 , thereby ensuring that the separation takes place at this layer. Examples of such layer include a thin vacuum film or a thin peel ply. 
         [0039]      FIG. 6  shows a method of manufacturing a hybrid mat  505  consisting of an outermost layer  301  and a lowermost fibre-reinforced layer  506 , between which two layers some of the fibres from the lowermost layer  506  are needle-punched, ie pushed or pressed, into the outermost layer  301 . The method is quite simple and may be performed eg by pressing a spear  701  or the like upwards through the fibre layer  506  and up into the second layer  301  as illustrated in the figure. According to yet an embodiment a plate or a roller is applied onto which a number of spears are mounted for pressing up the fibres  105 . 
         [0040]    A hybrid mat  505  with fibres arranged transversally of a joining face  402  between two layers can also be manufactured by pulling of fibres from a fibrours layer  506  up into a second layer  301  as outlined in  FIG. 7 . Fibres are pulled up by means of a tool  801  in the shape of eg a relatively fine crochet needle, which tool  801  is inserted down through the one layer  301 , seizes one or more fibres from the fibrous layer  506  and pulls them along. The holes  802  in the outermost layer  301  which is torn off prior to joining of the laminate  101  to other elements can either be made in advance or be made by the tool  801  during the process. According to yet an embodiment a plate or a roller is used on which a number of “crochet needles” are applied to grate open or tear the fibres  105  along. 
         [0041]    Yet another method of manufacturing a hybrid mat  505  consisting of several layers with fibres  105  transversally of the joining face  402  of the layers is shown in  FIG. 8 . Here the transversal fibres are arranged quite simply by two or more layers being joined by sewing with ordinary stitches  901 , wherein the top thread  902  and bobbin thread  903  interlock in each stitch. Which particular type of sewing stitch  901  is used is immaterial to the invention and so is the pattern sown. Here neither of the layers in the hybrid mat need be fibre-reinforced, but they may be. In order to facilitate tearing of the one layer of the hybrid mat off later on in the process, it may be advantageous to add a film between the layers in the hybrid mat as mentioned and described earlier. When the outermost layer(s) of the hybrid mat is to be removed, it can be accomplished merely by it being torn off, while the top thread in the sewing  902  is torn apart. Alternatively the top thread  902  can be cut or shorn to pieces in a number of locations prior to tearing, or the top thread  902  can be pulled out of the sewing  901 . The latter can be accomplished in a simple manner provided the strain on the top thread  902  during sewing exceeded the strain on the bobbin thread  903 . Thus, in the above it will primarily be the bobbin thread  903  that is left on the laminate surface  104  and projects. The method as described above is advantageous in being inexpensive, quick and simple and by using only known techniques. 
         [0042]    An alternative embodiment for the manufacture of the hybrid mat  505  is shown in  FIG. 9 . Hereby longitudinally extending grooves  1001  are cut or otherwise provided in the one side of the layer  301  which is to be torn off the laminate at a later stage. A thin drapeable fibre-reinforced layer  506  is then draped into these grooves  1001  as outlined in the figure, whereby the fibre layer is folded up into the grooves  1001  and forms a series of pleats  1002 . As described above, the hybrid mat  505  is laid onto the laminate where it is later to be glued or in any other way jointed to other elements. The pleated layer  506  is to face towards the laminate. The resin material is applied, and the outermost layer of the hybrid mat  301  is torn off, following which the layer  301  is left sitting firmly on the laminate and with the pleats projecting from the surface and forming the fibre-reinforcement of the joint. 
         [0043]    It will be understood that the invention as disclosed in the present description with figures can be modified or amended while continuing to be comprised by the protective scope of the below claims.