Abstract:
A method of machining a fibrous sheet for a composite structure is described. The sheet comprises a resin matrix having a glass transition temperature, wherein the method comprises cooling the sheet substantially to maintain the temperature of the matrix below its glass transition temperature during machining.

Description:
[0001]    This invention relates to techniques for chamfering layers or plies used in composite structures, such as wind turbine blades. 
         [0002]    Composite structures typically comprise one or more plies, each ply being a fibre-reinforced sheet that may comprise a thermoplastic or thermosetting resin matrix. The fibres may be pre-impregnated with the matrix as a ‘prepreg’ or the matrix may be impregnated into a fibre sheet during fabrication of a composite structure, for example during lay-up or injection-moulding procedures. Alternatively, the fibre-reinforced sheet may be pre-impregnated on just one side by a resin foil, i.e. a ‘semi-preg’. 
         [0003]    Plies are commonly laid atop one another in a layered or laminated arrangement. Single-ply composite structures are also possible, with single-thickness plies abutting in edge-to-edge relationship or overlapping at their edges. The plies are commonly supported by a foam core to define a skin on or around the core. 
         [0004]    In some circumstances, it is desirable to chamfer an edge of a ply. For example, plies may abut edge-to-edge in a composite structure and it is desirable to maximise the surface area of the interface between the abutting plies. This is because the shear strength at the interface is a small fraction—possibly as little as 5%—of the tensile strength of the plies themselves. The alternative of overlapping abutting plies leads to stress concentration and disadvantageously kinks the load path extending from one ply to another. Also, where plies define the external surface of a composite structure, an overlap between the plies makes a smooth finish difficult to achieve. 
         [0005]    It is also well known to taper a composite structure by reducing the number of plies from one location to another across the structure. Such tapering is common in aerofoil members such as wind turbine blades, which taper in both the spanwise direction from blade root to blade tip and in the chordwise direction from leading edge to trailing edge. To achieve this, some plies may be terminated or ‘dropped’ inward of an extremity of the structure, leaving other continuous plies to extend further toward that extremity. 
         [0006]    Plies are preferably dropped in a staggered or interleaved manner to make the transition as gradual as possible. However, each dropped ply introduces a region of weakness in view of discontinuity between the neighbouring plies, with the possibility of resin concentrations or gas pockets in any gaps between plies, especially at the edge of dropped plies. Here, edge chamfering is helpful to minimise gaps, to straighten the load path and to maximise the surface area of the interface between plies. This allows thicker plies to be used, which facilitates the lay-up process because fewer layers are then required in the laminate to achieve a required overall thickness. 
         [0007]    Plies for use in composite structures are difficult to chamfer efficiently, accurately and repeatably, particularly with the shallow taper angle that is desirable to maximise the surface area of the edge interface. The plies are flexible and compressible and so tend to move unpredictably under the forces applied by the chamfering process. Also, the plies may degrade with heat generated by the chamfering process. This is a particular problem with prepregs, if the matrix cures or otherwise transforms with heat. For example, heat generated during chamfering may cause the thermoplastic matrix to soften or melt and clog the chamfering tool. If the matrix softens or melts, it is also possible for the chamfering tool to drag the ply unpredictably, possibly distorting it and so undermining the accuracy of cutting. 
         [0008]    Some examples of ply-tapering tools are disclosed in EP 1786617. These include finger cutters akin to hair trimmers, but finger cutters are not suitable for cutting prepregs in which the fibres are embedded in a matrix because the matrix prevents the fingers from penetrating between the fibres. EP 1786617 also discloses milling cutters with inclined faces, turning about an axis orthogonal to a plane containing the edge being tapered. When configured as shown in EP 1786617, milling cutters impart heat to the ply that may degrade the ply and melt its matrix if the ply is a thermoplastic prepreg; this is also a problem suffered by abrading techniques proposed elsewhere in the art, using a belt sander or the like. Also, when configured as shown in EP 1786617, milling cutters impart a side force to the ply, parallel to the tapered edge, that tends to distort the ply and so undermines the accuracy of cutting. This is also a problem suffered by knife-cutting techniques proposed elsewhere in the art. 
         [0009]    It is against this background that the present invention has been made. 
       SUMMARY OF THE INVENTION 
       [0010]    In accordance with the present invention, there is provided a method of machining a fibrous sheet for a composite structure, the sheet comprising a resin matrix having a glass transition temperature, wherein the method comprises: providing a fibrous sheet at a first temperature; supporting the sheet for machining; and cooling at least part of the sheet to a second temperature below the first temperature, substantially to maintain the temperature of the matrix below its glass transition temperature during machining. 
         [0011]    The resin becomes hard and brittle when cooled, which makes it easier to machine. Generally, the resin becomes harder and more brittle with decreasing temperature. Therefore, it is preferable to cool the material to the lowest temperature possible within realistic practical and economic constraints. 
         [0012]    The glass transition temperature of the uncured resin may also be referred to in the art as the ‘cold T g ’ or the ‘uncured T g ’, and is an intrinsic property of the resin that will vary from material to material. Put simply, the cold/uncured T g  is the glass transition temperature of a matrix that has reacted at ambient temperature, and hence exhibits a relatively low degree of cross-linking. Material suppliers such as Gurit™ can provide details of the cold/uncured T g  of the materials that they supply. However, as a matrix ages, some additional cross linking will occur, causing the cold T g  to increase slightly with time. The T g  of the uncured resin in typical prepreg or semi-preg materials used in the construction of modern wind turbine blades is generally below 0° C., for example around −2° C. In comparison, when a matrix is cured at an elevated temperature, it will exhibit a relatively high degree of cross-linking, resulting in the cured matrix having a much higher T g , typically well in excess of 100° C. 
         [0013]    The inventive concept encompasses a method of making a composite structure, comprising: tapering an edge of a fibrous reinforcement in accordance with the above machining method; and incorporating the sheet into a composite structure with the tapered edge lying against or beside at least one other fibrous reinforcement sheet. 
         [0014]    The present invention also provides an apparatus for machining a fibrous sheet for a composite structure, the apparatus comprising: a support for the sheet; a machining tool movable relative to the support; and a cooling system for cooling the sheet. 
         [0015]    The inventive concept also encompasses a composite structure such as a wind turbine blade produced by the above methods or apparatus. 
         [0016]    Optional features of the present invention are set out in the sub claims appended hereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: 
           [0018]      FIG. 1  is a schematic perspective view of an apparatus for machining a prepreg or semi-preg ply, in which refrigerant is applied to the ply prior to machining; 
           [0019]      FIG. 2  is a schematic perspective view of the apparatus of  FIG. 1 , in which the refrigerant is applied to the machining site during the machining process; 
           [0020]      FIG. 3  is a schematic side view showing a machining tool being movable towards and away from an edge of a ply along an arc-shaped path; and 
           [0021]      FIG. 4  shows the apparatus of  FIG. 1  located within a climate-controlled environment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Referring to  FIG. 1 , a prepreg ply  10  is clamped between refrigerated steel blocks  12   a,    12   b,  and a grinding wheel  14  is arranged to translate across a free edge  16  of the ply  10  to remove material from that edge to create a chamfer. Refrigerant  18  is applied locally to the free edge  16  of the ply  10  during the chamfering operation. The apparatus and chamfering technique are described in further detail below. 
         [0023]    The prepreg ply  10  comprises a sheet of glass fibre fabric, which has been impregnated with a thermoset matrix, which in this example is pre-catalysed epoxy resin. The glass fibre fabric consists of two layers and is commonly referred to as ‘triax’. The first layer includes a set of unidirectional (ud) fibres, whilst the second layer is a layer of ‘biax’, which has a first set of unidirectional fibres oriented at an angle of +45° relative to the fibres in the first layer, and a second set of unidirectional fibres oriented at an angle of −45° relative to the fibres in the first layer. 
         [0024]    The steel blocks  12   a , 12   b  are oblongs and include internal refrigeration channels  20 . A refrigerant is pumped through the channels  20  to cool the blocks  12   a , 12   b  to a temperature of −50° C., and then continuously pumped through the channels  20  to maintain the temperature of the blocks  12   a , 12   b  at −50° C. Alternatively, the blocks  12   a , 12   b  may be placed in a refrigerator at −50° C. for several hours prior to the chamfering operation. In this way, the refrigeration channels  20  may not be required. 
         [0025]    The steel blocks  12   a , 12   b  are placed one on top of the other, with the lower steel block  12   b  being located on an insulating foam block  22  to reduce heat transfer from a work surface  24  to the cold blocks  12   a , 12   b.  An end portion of the prepreg ply  10  is sandwiched between the steel blocks  12   a , 12   b  and the blocks are clamped together by a clamp (not shown) to hold the ply  10  firmly in place. The upper block  12   a  is set back from the lower block  12   b  by approximately 40 mm to define an elongate ledge  26 . The free edge  16  of the ply  10  extends from between the steel blocks  12   a , 12   b  onto this ledge  26 . 
         [0026]    The grinding wheel  14  is arranged to traverse along the ledge  26  in a direction parallel to the exposed free edge  16  of the ply  10  as indicated by the arrow  28  in  FIG. 1 . The grinding wheel  14  has an abrasive cylindrical outer surface  30 , which rotates about an axis parallel to the free edge  16  of the ply  10 , i.e. parallel to its direction of translation  28  across the ply  10 . In use, the grinding wheel  14  is angled slightly with respect to the surface  32  of the ply  10  and traversed across the free edge  16  to create a chamfer of a desired gradient. A shallow chamfer gradient in the range of 1:20 to 1:10 i.e. approximately 2.8° to 6° is particularly desirable. 
         [0027]    Prior to chamfering commencing, the free edge  16  of the ply  10  is sprayed with tetrafluoroethane refrigerant (R134a) from a spray can  34 . It will of course be appreciated that other suitable refrigerants may be used for this purpose, for example liquid nitrogen or liquid carbon dioxide. Spraying the free edge  16  of the ply  10  with refrigerant cools the ply  10  to well below the glass transition temperature (T g ) of the uncured epoxy resin in the prepreg. Typically the T g  of the uncured epoxy is around −2° C. Maintaining the temperature of the resin below its uncured T g  during chamfering ensures that the resin remains hard during the chamfering process. This prevents the resin from becoming tacky and contaminating or clogging the abrasive surface  30  of the grinding wheel  14 , which would otherwise occur if chamfering was conducted at room temperature. The cold steel blocks  12   a , 12   b  ensure that any heat generated during the chamfering operation is channeled away from the ply  10 . 
         [0028]    Experimental tests have shown that a single application of the R134a refrigerant to the free edge  16  of the ply  10  prior to chamfering is sufficient to keep the temperature of the ply  10  below the T g  of the uncured resin. However, if necessary, the refrigerant may be applied repeatedly or continuously during chamfering to keep the temperature of the ply  10  below the T g  of the uncured resin. Applying the refrigerant continuously has the advantage that a flow of refrigerant will carry heat away from the worksite. In the example shown in  FIG. 2 , the refrigerant  18  is applied during machining and is applied locally at the machining site  35 . A nozzle  36  supplying the refrigerant  18  may be arranged to move in tandem with the grinding wheel  14  as represented by the arrows  38  in  FIG. 2 . Applying the refrigerant  18  locally at the machining site  35  is advantageous because it concentrates the refrigerant  18  at the point where heat is generated. 
         [0029]    In order to assist heat dissipation from the free edge  16  of the ply  10 , rather than being translated across the ply  10  in a single motion, the grinding wheel  14  may be pressed against the free edge  16  of the ply  10  in a series of pressing operations across the width of the ply  10 . This is represented schematically in  FIG. 3 , which shows the grinding wheel  14  being movable towards and away from the free edge  16  of the ply  10 , i.e. in and out of contact with the free edge  16 , along an arc-shaped path  40 . Refrigerant is continuously applied to the free edge  16  so that cooling continues between presses, i.e. whilst the grinding wheel  14  is moved out of contact with the free edge  16  of the ply  10 . 
         [0030]    Whilst not shown in the above figures, the humidity of the air surrounding the apparatus is controlled to prevent condensation from forming on the cold ply  10  or elsewhere on the apparatus itself. 
         [0031]    Referring to  FIG. 4 , rather than applying refrigerant directly to the free edge  16  of the prepreg ply  10 , the entire apparatus is located in a climate-controlled environment  42  that is sufficiently cold to maintain the epoxy below its uncured glass transition temperature during the chamfering process. Of course, it is also possible to apply refrigerant directly to the chamfering site  35  if necessary when the apparatus is located in a climate-controlled environment  42  such as this. In this example, refrigerant channels have been removed from the blocks  12   a , 12   b,  however it will be appreciated that such channels may be used in combination with a climate-controlled environment  42 . 
         [0032]    It will be appreciated that many modifications may be made to the techniques described above without departing from the scope of the present invention as defined by the accompanying claims. For example, it will be appreciated that the prepreg ply described by way of example above may be substituted for a semi-preg ply or other fibrous ply comprising a resinous matrix material. Also, whilst triax is described by way of example, it will be appreciated that the invention is not limited to the use of triax. Indeed, the fibres in the ply may have any other orientation, for example the fibres may all be unidirectional (ud). In addition, whilst a grinding wheel has been described above, it will be appreciated that the invention may be used in connection with any other machining tool or technique.