WO2012013192A1

WO2012013192A1 - Chamfering of laminate layers

Info

Publication number: WO2012013192A1
Application number: PCT/DK2011/050286
Authority: WO
Inventors: Anton Bech
Original assignee: Vestas Wind Systems A/S
Priority date: 2010-07-30
Filing date: 2011-07-19
Publication date: 2012-02-02
Also published as: GB201012871D0; EP2598315A1; GB2482345A

Abstract

A method of tapering an edge (25) of a fibrous reinforcement sheet (18) for a composite structure is described. The method includes supporting at least a portion of the sheet (18) and creating a tapered edge by relative translational movement of a rotary tool (20) with respect to the supported portion of the sheet. Rotation of the tool defines a cutting direction transverse to the tapered edge.

Description

CHAMFERING OF LAMINATE LAYERS

This invention relates to techniques for chamfering layers or plies used in composite structures, such as wind turbine blades.

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'.

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.

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.

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.

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.

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.

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.

It is against this background that the present invention has been made. Summary of the invention

In accordance with the present invention, there is provided a method of tapering an edge of a fibrous reinforcement sheet for a composite structure, the method comprising:

supporting at least a portion of the sheet; and creating a tapered edge by relative translational movement of a rotary tool with respect to the supported portion of the sheet, rotation of the tool defining a cutting direction transverse to the tapered edge.

The inventive concept encompasses a method of making a composite structure, comprising: tapering an edge of a fibrous reinforcement sheet in accordance with the above method; and incorporating the sheet into a composite structure with the tapered edge lying against or beside at least one other fibrous reinforcement sheet.

The present invention also provides an apparatus for tapering an edge of a fibrous reinforcement sheet for a composite structure, the apparatus comprising: a support for supporting at least a portion of the sheet; and a rotary tool capable of relative

translational movement with respect to the supported portion of the sheet, rotation of the tool defining a cutting direction transverse to the tapered edge. The inventive concept also encompasses a composite structure such as a wind turbine blade produced by the above methods or apparatus.

Optional features of the present invention are set out in the sub claims appended hereto.

Brief description of the drawings

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:

Figure 1 is a perspective view of a chamfering apparatus in accordance with a first embodiment of the present invention, in which a rotary saw is used to create a chamfered edge in a prepreg ply supported on a curved surface; Figure 2 is a side view of the chamfering apparatus of Figure 1 ;

Figure 3 is a schematic diagram showing the relative radii of the rotary saw and the curved supporting surface of the chamfering apparatus of Figures 1 and 2, illustrating how these dimensions can be selected to create a desired geometry of chamfer;

Figure 4 is a perspective view of a chamfering apparatus in accordance with a second embodiment of the present invention, in which a grinding wheel has been substituted for the rotary saw of Figures 1 and 2;

Figure 5 is a perspective view of a chamfering apparatus in accordance with a third embodiment of the present invention, in which a rotary drum extending across the full width of the prepreg ply has been substituted for the rotary saw of Figures 1 and 2; and

Figure 6 is a schematic side view of the apparatus of Figures 4 or 5 showing how the grinding wheel and rotary drum may be moved towards the ply along an arc- shaped path. Detailed description

Referring to Figures 1 and 2, a chamfering apparatus 10 in accordance with a first embodiment of the present invention includes a support 12 having a curved supporting surface 14. The supporting surface 14 is concave and curves between upper and lower parallel edges 16,17 of the support 12, with an axis of curvature parallel to both edges 16,17. The extent of curvature of the surface 14 varies between the upper and lower edges 16,17 with the radius of curvature of the surface 14 being smaller towards the lower edge 17 than towards the upper edge 16. In fact, the surface 14 straightens towards the upper edge 16 in cross-section. A prepreg ply 18 is supported on the curved surface 14 such that the ply curves in conformity with the surface 14, with a radially-outer surface 19 of the ply 18 being in contact with the curved surface 14, as shown in Figure 2.

A rotary saw 20, comprising a thin circular cutting disc 21 with a series of spaced-apart teeth 22 extending radially from its circumference, is positioned adjacent a radially-inner surface 23 of the supported ply 18. The cutting disc 21 is arranged to rotate about a central axis 24 orthogonal to the plane of the disc 21. The rotary saw 20 is oriented with respect to the support 12 such that the plane of the cutting disc 21 is substantially orthogonal to the curved surface 14, i.e. so that the axis of rotation 24 is substantially parallel to the axis of curvature of the supporting surface 14. The rotary saw 20 is moveable parallel to its rotational axis 24 so that the cutting disc 21 can be translated across the curved surface 14 of the support 12 to create a chamfer in a free edge 25 of the prepreg ply 18 adjacent the lower edge 17 of the support 12. In this example, rotation of the cutting disc 21 defines a cutting direction that is orthogonal to the free edge 25. The apparatus 10 and the chamfering technique are described in further detail below.

In this example, the prepreg ply 18 comprises a sheet of glass fibre fabric, which has been impregnated with a pre-catalysed epoxy resin matrix. 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.

Referring still to Figures 1 and 2, the prepreg ply 18 is clamped in position on the curved surface 14 of the support 12 by an elongate bar 26 that extends across the curved supporting surface 14 adjacent the upper edge 16. The prepreg ply 18 is arranged between the clamping bar 26 and the curved surface 14, and the bar 26 is clamped against the curved surface 14 to hold the prepreg ply 18 firmly in position on the support 12. In addition to being clamped to the support 12, the prepreg ply 18 is maintained in position on the curved surface 14 by means of suction. To this end, the support 12 includes an internal vacuum chamber (not shown), which is connected to a vacuum pump 28 by a pipe 30 (Figure 1 ). A plurality of holes 31 is provided in a region of the curved surface 14 that is overlaid by the prepreg ply 18. The holes 31 communicate with the vacuum chamber, so that when the vacuum pump 28 is activated, air is evacuated from the vacuum chamber and from the region behind the prepreg ply 18. The reduction of pressure in the vacuum chamber causes the prepreg ply 18 to be sucked against the curved surface 14 and maintained in the correct position on the support.

To avoid the epoxy in the prepreg ply 18 from becoming soft and clogging the teeth 22 of the saw 20, the prepreg ply 18 is cooled during the machining process to keep the temperature of the uncured epoxy below its glass transition temperature (T_g), which in this example is approximately -2°C. To this end, refrigerant tubes 32 are provided within the support 12, behind the curved supporting surface 14. A refrigerant or other suitable cooling fluid is pumped through the tubes 32 in use to cool the curved surface 14. If required, cooling fluid such as liquid nitrogen or carbon dioxide may be applied directly to the cutting site, and/or the entire apparatus 10 may be located in a suitably refrigerated environment. Humidity levels around the apparatus 10 are controlled to prevent condensation forming on the cold prepreg ply 18 or elsewhere on the apparatus 10 itself.

Referring specifically to Figure 2, the chamfering apparatus 10 includes a mechanism 34 for moving the support 12 towards the saw 20 in a direction substantially perpendicular to the axis of rotation 24 of the saw 20. The mechanism 34 includes a rod 35 that projects from a rear surface 36 of the support 12. The rod 35 is slidably supported within a guide bearing 37 fixed to a work surface 38. One end 39 of the rod 35 is connected to the support 12, whilst the other end 40 of the rod 35 defines a threaded bore. A threaded shaft 42, extending from an electric stepper motor 44, is received within and mated with the threaded bore of the rod 35. The stepper motor 44 is also fixed to the work surface 38.

Turning the stepper motor 44 anti-clockwise turns the threaded shaft 42 anticlockwise in the bore, which causes the rod 35 to slide within the guide bearing 37 and advance the support 12 towards the saw 20. Conversely, turning the stepper motor 44 clockwise turns the threaded shaft 42 clockwise within the bore 40, which causes the rod 35 to slide within the guide bearing 37 and draw the support 12 away from the saw 20. It will be appreciated that other means of translating the support 12 may be used, for example a jack or sliding arrangement.

Referring now to Figure 3, the radius 46a,46b,46c of the rotary saw 20, and the radius of curvature 48 of the curved surface 14 near the lower edge 17 of the support 12, are selected to provide the requisite geometry of chamfer. The three circular dotted lines 50a, 50b, 50c in Figure 3 represent, schematically, three different sizes of rotary saw 20. It can be seen from Figure 3 that the gradient of chamfer increases as the radius

46a,46b,46c of the rotary saw 20 relative to the radius 48 of the curved surface 14 decreases. A shallow chamfer gradient in the range of between 1 :10 to 1 :20 is particularly desirable when the prepreg ply 18 is to be incorporated in a composite structure such as, for example, a wind turbine blade.

Referring again to Figures 1 and 2, in use, the support 12 is cooled and the prepreg ply 18 is draped on the curved surface 14. The ply 18 is clamped in position on the cooled curved surface 14 using the clamping bar 26. Next, the vacuum pump 28 (Figure 1 ) is activated so that the prepreg ply 18 is sucked against the curved surface 14 and thereby held firmly in position. The rotary saw 20 is positioned at one end of the support 12 and the electric stepper motor 44 is turned anti-clockwise to advance the support 12 towards the axis of rotation 24 of the saw 20 until the prepreg ply 18 is suitably positioned relative to the saw 20 such that the saw 20 will create a chamfer in the free edge 25 of the ply 18 as shown in Figure 3.

It can be seen in Figure 3 that the point of intersection of the saw 20 with the radially- inner surface 23 of the ply 18 is towards the lower edge 17 of the support 12. Also, the axis of rotation 24 of the saw 20 is between the supporting surface 14 and the centre of curvature 54 of the supporting surface 14. Once the saw 20 and the support 12 have been aligned in this way, the saw 20 is translated in a direction parallel to its rotational axis 24, as shown by the arrow 56 in Figure 1 , across the free edge 25 of the prepreg ply 18. As the saw 20 translates across the ply 18, it removes material from the free edge 25 of the ply 18 to create a chamfer or taper in the free edge 25. In this way, the rotary saw 20 acts as a milling cutter.

The chamfered free edge 25 of the prepreg ply 18 may be created in a single pass of the rotary saw 20 across the width of the ply 18. Alternatively, multiple passes may be employed to remove material incrementally from the free edge 25 until the required chamfer is created. If multiple passes are employed, the stepper motor 44 is turned anticlockwise between successive passes, i.e. 'indexed', to advance the support 12 incrementally towards the saw 20. The rotary saw 20 may be moved, i.e. translated, by hand. Alternatively its movement may be automated. It will of course be appreciated that the entire procedure may be, and preferably is, automated under computer control.

The direction of rotation of the rotary saw 20, which is indicated by the arrow 58 on Figure 2, is such that the teeth 22 of the cutting disc 21 move through the thickness of the ply 18, from the radially-inner surface 23 to the radially-outer surface 19 of the ply 18, in a direction towards the free edge 25 of the ply 18. In this way, the rotary saw 20 forces the free edge 25 of the ply 18 against the curved surface 14 as cutting takes place, which ensures that the ply 18 remains firmly in position on the curved surface 14. In this example, rotation of the cutting disc 21 defines a cutting direction orthogonal to the free edge 25 of the ply 18. However, it will be appreciated that in other examples of the invention, the cutting disc 21 may be oriented such that the cutting direction is otherwise transverse to the free edge 25.

The refrigerated conditions are maintained during the machining process to ensure that the temperature of the prepreg ply 18 is kept below the T_g of the uncured resin. As noted above, this ensures that the resin is kept sufficiently hard during the machining process so that it does not clog the teeth 22 of the rotary saw 20. The narrowness of the cutting edge of the rotary saw 20 is particularly advantageous in this respect because it results in a correspondingly narrow cut, which means that heat is only generated in a relatively small region of the free edge 25 of the ply 18 at any one time. In addition, the continuous translation of the rotary saw 20 across the ply 18 ensures that heat is only generated for a short time period at any point along the free edge 25. This allows heat to dissipate quickly and efficiently from the cutting zone.

Once the chamfer has been created in the free edge 25 of the ply 18, the stepper motor 44 is turned clockwise to move the support 12 away from the saw 20. The clamping bar 26 is released and the vacuum pump 28 is turned off to release the prepreg ply 18 from the support 12.

Referring now to Figure 4, this shows a chamfering apparatus 59 in accordance with a second embodiment of the present invention. In this second embodiment, the rotary saw 20 of the first embodiment has been replaced by a grinding wheel 60. The grinding wheel 60 has an abrasive cylindrical outer surface 62, which faces the curved surface 14 of the support 12. The grinding wheel 60 is moveable across the width of the curved surface 14 in a direction parallel to its rotational axis, as represented by the arrow 64 in Figure 4, to create a chamfered edge in the prepreg ply 18. In this example, rotation of the grinding wheel 60 defines a cutting direction orthogonal to the free edge 25 of the ply 18, although it will be appreciated that the grinding wheel 60 may be oriented such that the cutting direction is otherwise transverse to the free edge 25.

As with the first embodiment, the grinding wheel 60 may be moved continuously along the free edge 25 of the ply 18 to create the chamfer. However, as the contact area between the grinding wheel 60 and the ply 18 is larger than the contact area between the saw 20 and the ply 18 in the first embodiment, the grinding wheel 60 tends to generate more heat in use as a result of frictional forces. To assist heat dissipation from the machining site, the chamfer may be created via a series of presses of the grinding wheel 60 against the free edge 25 of the ply 18. For example, the grinding wheel 60 may be pressed against the free edge 25 for a brief time, then moved away from the free edge 25 perpendicular to the rotational axis 64, and then indexed parallel to the rotational axis 64 and pressed again against the free edge 25 to an extent corresponding to the first pressing operation. This process, which is repeated across the full width of the ply 18, provides an opportunity for heat to dissipate from the ply 18 each time the grinding wheel 60 is moved out of contact with the ply 18.

Referring now to Figure 5, this shows a chamfering apparatus 66 in accordance with a third embodiment of the present invention. In this third embodiment, a rotary drum 68 replaces the rotary saw 20 of the first embodiment. The rotary drum 68 has an axis of rotation 70 that is substantially parallel to the axis of curvature of the supporting surface 14. In this example, rotation of the rotary drum 68 defines a cutting direction orthogonal to the free edge 25 of the ply 18, although it will be appreciated that the rotary drum 68 may be oriented such that the cutting direction is otherwise transverse to the free edge 25.

Like the grinding wheel 60 of the second embodiment, the rotary drum 68 has an abrasive cylindrical outer surface 72. However, in contrast to the second embodiment, the rotary drum 68 extends across the full width of the prepreg ply 18. In this way the rotary drum 68 does not need to be translated across the curved surface 14 to chamfer the free edge 25 of the prepreg ply 18. Instead, in use, the stepper motor (not shown) is used to index the support 12 towards the drum 68 in a direction perpendicular to the drum's rotational axis 70 until a chamfer of the required geometry has been created. Alternatively, to assist heat dissipation from the chamfering site, a series of presses of the roller 68 against the free edge 25 of the ply 18 may be employed to create the chamfer in an analogous way to that described above in relation to the grinding wheel 60. It will be appreciated that in other embodiments of the invention, the support 12 may be stationary, and the rotary drum 68 moved towards the support 12 to create the requisite chamfer in the free edge 25 of the prepreg ply 18.

It is preferred that the drum 68 is as long or longer than the width of the prepreg ply 18. However, it will be appreciated that a shorter drum may be used, which can be moved in and out of contact with the free edge 25 in a series of presses as it is indexed parallel to the rotational axis along the full width of the prepreg ply 18, in substantially the same way as described above in relation to the grinding wheel example of Figure 4.

Figure 6 illustrates how the grinding wheel 60 and rotary drum 68 of Figure 4 and 5 respectively may be moved in an arc-shaped path 74 towards and away from the free edge 25 of the supported prepreg ply 18. This type of movement may be used in the pressing operations described above.

It will be appreciated that many modifications may be made to the process described above without departing from the scope of the present invention as defined by the accompanying claims. For example, whilst chamfering of a prepreg ply has been described above, it will be appreciated that the invention is equally applicable to chamfering of other plies, for example semi-pregs or dry plies for use in resin infusion techniques. 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).

Claims

1 . A method of tapering an edge of a fibrous reinforcement sheet for a composite structure, the method comprising: supporting at least a portion of the sheet; and creating a tapered edge by relative translational movement of a rotary tool with respect to the supported portion of the sheet, rotation of the tool defining a cutting direction transverse to the tapered edge.

2. The method of Claim 1 , wherein the cutting direction is substantially orthogonal to the tapered edge.

3. The method of Claim 1 or Claim 2, wherein the cutting direction is toward a free extremity of the tapered edge.

4. The method of any preceding claim, wherein the tool turns about an axis of rotation substantially parallel to the tapered edge.

5. The method of any preceding claim, comprising relative movement between the tool and the sheet to advance the tool across the supported portion of the sheet in a direction substantially parallel to the tapered edge.

6. The method of any preceding claim, comprising relative movement between the tool and the sheet to advance the tool toward the supported portion of the sheet in a direction substantially orthogonal to the tapered edge.

7. The method of any preceding claim, comprising relative movement between the tool and the sheet to retract the tool intermittently from cutting contact with the sheet.

8. The method of Claim 7, comprising advancing the tool into cutting contact with the sheet after each intermittent retraction of the tool from cutting contact with the sheet.

9. The method of Claim 8, comprising advancing the tool into deeper cutting contact with the sheet after each intermittent retraction of the tool from cutting contact with the sheet.

10. The method of any of Claims 7 to 9, comprising advancing the tool in a direction substantially parallel to the tapered edge following each intermittent retraction of the tool from cutting contact with the sheet.

1 1. The method of any preceding claim, comprising forming the tapered edge by continuous relative movement between the tool and the sheet to advance a linear cutting front progressively across the supported portion of the sheet.

12. The method of any preceding claim, comprising forming the tapered edge by stepwise relative movement between the tool and the sheet to develop a tapered region in successive tapered portions extending across the supported portion of the sheet.

13. The method of any preceding claim, comprising forming the tapered edge by a single relative movement between the tool and the sheet that creates a tapered region across a full width of the supported portion of the sheet.

14. The method of any preceding claim, comprising supporting the supported portion of the sheet in a concave-curved configuration with that portion having an axis of curvature substantially parallel to the tapered edge.

15. The method of Claim 14, wherein the tool defines a cutting arc that substantially intersects or substantially forms a tangent with an arc of curvature of the supported portion of the sheet.

16. The method of any preceding claim, comprising cooling the supported portion of the sheet.

17. A method of making a composite structure, comprising: tapering an edge of a fibrous reinforcement sheet in accordance with any preceding claim; and incorporating the sheet into a composite structure with the tapered edge lying against or beside at least one other fibrous reinforcement sheet.

18. The method of Claim 17, wherein the cooperating fibrous reinforcement sheets each have a tapered edge and the tapered edges abut one another.

19. Apparatus for tapering an edge of a fibrous reinforcement sheet for a composite structure, the apparatus comprising: a support for supporting at least a portion of the sheet; and a rotary tool capable of relative translational movement with respect to the supported portion of the sheet, rotation of the tool defining a cutting direction transverse to the tapered edge.

20. The apparatus of Claim 19, wherein the support has a concave-curved supporting surface having an axis of curvature substantially parallel to the tapered edge.

21. The apparatus of Claim 20, and having a clamp and/or a vacuum system for holding at least a portion of the sheet against the support.

22. The apparatus of any of Claims 19 to 21 , wherein the tool defines a cutting arc that substantially intersects or substantially forms a tangent with an arc of curvature of the supporting surface.

23. The apparatus of any of Claims 19 to 22, wherein the tool is a cutting disc that turns in a plane transverse to the tapered edge.

24. The apparatus of any of Claims 19 to 22, wherein the tool is a grinding wheel that presents a cutting surface to the tapered edge, the width of the cutting surface being less than the length of the tapered edge.

25. The apparatus of any of Claims 19 to 22, wherein the tool is a grinding drum that presents a cutting surface to the tapered edge, the width of the cutting surface being equal to or greater than the length of the tapered edge.

26. A composite structure such as a wind turbine blade, produced by the method of any of Claims 1 to 18 or by use of the apparatus of any of Claims 19 to 25.