Field of the Invention

The present invention relates to a method for manufacturing of a fibre-reinforced 1am- inate, which method comprises the steps of:

- placing a number of laterally extending layers of fibre mats on top of each other to form the laminate and

- building up a first part of the laminate by placing first fibre mats which in a first lat eral direction has a greater stiffness than in a second lateral direction, which first fibre mats comprises a fibre material which is partly impregnated with resin and pre-cured such that the resin forms spaced resin stripes along the first lateral direction.

Background of the Invention

It is well known to use fibre mats comprising glass fibres, carbon fibres, aramid fibres, organic fibres and combinations thereof when moulding composite parts.

Such fibre mats are typically used for forming a laminate which is impregnated with a resin and cured into a hardened state, thus forming a composite article. Such article can be a blade for a wind turbine.

It is common to apply a resin along some of the fibres, especially in mats with unidi- rectionally aligned fibres. Such mats are known under the term unidirectional mats or UD mats. The length of the fibre mats may be very long. It is common to provide the mats in roll containing mats in form of a web having a length of several hundreds of meters.

Fibre mats are in general provided in the form of webs having a long length compared to the width. Composite laminates usually comprise several layers of fibre mats. Normally, it is troublesome to handle multiple layers in a mould, since wrinkles can have a detri mental effect on the finished product.

The structural characteristics of a fibre-reinforced laminate are usually governed by the amount, type and orientation of the reinforcement fibres. Typically, the stiffness and strength of fibres can only be taken into account to the extent that loading occurs in the longitudinal fibre direction. Therefore, a traditionally designed laminate as sumes that the fibres of the finished laminate will be oriented in the same direction as the direction of the fibres when placed in the mould. However, in some cases wrinkles in the fibre layers may occur as a result of the manufacturing process. In such cases, the wrinkled fibres no longer have the desired orientation, and severe overload of the laminate may be the result. Wrinkles may occur for a number of reasons.

Wrinkles in fibre-reinforced laminates are typically prevented by a combination of arrangements. A method for avoiding wrinkles in the fibre layers consists of including layers of wrinkle-preventing material between the fibre layers. Wrinkle-preventing material is manufactured in such a way that it is stiff er than normal fibre material. When positioned between layers of, for example, fibreglass mats, the fibres in the mats are largely prevented from wrinkling, because the mats are kept flat by the wrin kle-preventing material.

Such wrinkle- preventing material is used in order to be able to lay up the fibre mats in a mould without the risk of one or more layers being wrinkled.

If wrinkles occur in fibre-reinforced laminates despite preventive action, then repair or rejection of the laminate will usually be required, as the loss of stiffness and/or strength in wrinkles will often exceed any acceptable safety margins.

There is a need in the technical field of composite moulding of overcoming the afore mentioned drawbacks of the prior art solutions. Object of the Invention

The object of the invention is to overcome the drawbacks of the prior art solutions and to provide a method for manufacturing a fibre-reinforced laminate where wrinkles are obviated.

Description of the Invention

The aforementioned object of the invention is achieved by a method as described in the preamble of claim 1; said method further comprises the step of:

- building up a second part of the laminate by placing further fibre mats which in a lateral direction being different from said first lateral direction has a greater stiffness than in the first lateral direction, which further fibre mats each comprises a fibre mate rial which is partly impregnated with resin and pre-cured such that the resin forms spaced resin stripes along said different lateral direction,

- repeating placing of said first and said further layers until the desired thickness of the complete laminate is obtained.

In context of the present invention, a laterally extending material is a material with a lateral dimension which is considerably larger than the thickness of the material. Moreover, the laterally extended layer and/or the laterally extended material may comprise different portions with a greater stiffness in a first lateral direction than in a second lateral direction, wherein the first lateral directions of the different portions may be orientated differently relative to each other.

The greater stiffness of a laterally extended layer in a second lateral direction com- pared to a first lateral direction effectively prevents an occurrence of wrinkles. Be cause the laterally extended layer retains its flexibility in the first direction, it is easier to drape compared to a completely stiff layer.

As all layers comprise wrinkle-preventing material, there is no risk that wrinkles will be established when laying up the laminate. Here it shall be remembered that when laying up laminates for large items such as wind turbine blades, it is often seen that persons are walking on the laminate. This will incur footprints which may establish pressure damages and wrinkles if they are not eliminated when manufacturing the laminate according to the present invention.

It is advantageous to use wrinkle-preventing fibre mats to build up the laminates, as less fibre mats than normally may be necessary. A reduction of the weight of the product formed is possible. This is especially important in connection with wind tur bine blades.

The control of the layers established with wrinkle-preventing material is especially important in connection with vacuum forming. In a mould where the laminate is formed under vacuum using a vacuum cloth, it is possible to obviate the deformation of the material which may occur due to the forces the vacuum cloth exerts on the lam inate. Advantageously, a partly impregnated and pre-cured layer of the same material as the material of the other layers may be used as the layer of a material which has in a first lateral direction a greater stiffness than in a second lateral direction.

The layer used to building up the laminate comprises a fibre material which is partly impregnated with resin and pre-cured such that the resin forms spaced resin stripes. This means that the layer is only partly impregnated by resin. Therefore, the un impregnated parts of the layer will become integrated in the laminate in the same way as the rest of the fibre material when applying resin to the laminate when it is placed in a mould.

The material has in a lateral direction a greater stiffness. This is obtained by only part ly impregnating and only partly pre-curing the resin. Preferably, most of the material can be left dry. By this means, a maximal flexibility of the material in the second lat eral direction can be maintained.

The un-impregnated parts of the laterally extended material can be impregnated by resin at the same time as the rest of the fibre material in a mould when a resin infusion process is started. Hereby the laterally extended, wrinkle-preventing material becomes an integrated part of the finished laminate. If a laterally extended, wrinkle-preventing material is pro duced by completely impregnating the fibre material, there is a risk that a weak layer only comprising resin not being reinforced is formed between the laterally extended layers containing the wrinkle-preventing material. This could lead to delamination. This risk is eliminated by the present invention. The laterally extended, wrinkle preventing material can be placed in such a way that the stripes of resin run substan tially in the direction where the strength of the laminate is most critical. If strength is required in more than one direction, the different layers of the laterally extended, wrinkle-preventing material is used and placed in such a way that the stripes in two subsequent layers run mutually perpendicular to each other or under any mutual angle for establishing the desired strength.

The laterally extended, wrinkle-preventing material can be made of several layers of fibre material. Each layer can have a different length or width such that a laterally extended, wrinkle-preventing material of varying thickness results. This can be im portant if the laterally extended, wrinkle-preventing material is used in a laminate, the cross-section of which is not constant. If, for example, the thickness of a laminate ta pers off towards one end, the number of layers of laterally extended, wrinkle- preventing material will gradually be reduced over some length of the laminate. If a layer of laterally extended, wrinkle-preventing material is thick, the layers adjacent to the laterally extended, wrinkle-preventing material will tend to fold around the end of the laterally extended, wrinkle-preventing material. This can have the same effect on the strength of the laminate as the wrinkles of the laterally extended, wrinkle- preventing material is supposed to prevent. Advantageously, the used laterally extend ed material can also have a tapered shape.

In prior art, pre-cured, solid, perforated or mesh-like laminate is described. The lami nate provided according to the present invention will have several advantages over this prior art laminates. It retains its flexibility in one direction and is therefore easier to drape. Moreover, it is only partly impregnated by resin. Therefore, the un impregnated parts of the material will become integrated in the laminate in the same way as the rest of the fibre material in a method for manufacturing of a fibre- reinforced laminate. Furthermore, the laterally extended fibre material can be pro- duced with a varying thickness. This makes it suitable for use in, for example, lami nates with a tapered shape.

The present invention provides for a controlled forming of the layers during a vacuum forming process. In prior art laminates having a substantial thickness, a risk of de forming the material may occur. This is for example the situation at the blade root in wind turbine blades or other parts where a transition in thickness occurs in the lami nate used for forming the wind turbine blade. This risk is eliminated with the con trolled forming of the layers.

In an embodiment of the invention, said method is peculiar in that said step of build ing up the second part of the laminate comprises the step of:

- using second fibre mats which in the second lateral direction has a greater stiffness than in the first lateral direction, which second fibre mats each comprises a fibre mate rial which is partly impregnated with resin and pre-cured such that the resin forms spaced resin stripes along the second lateral direction.

When forming laminates for use in a mould being curved in a direction perpendicular to a longitudinal direction, there is a risk that during the laying of the layers of the laminate, folds/wrinkles extending in the longitudinal direction are formed. These folds will be hidden when laying a following layer in the laminate. However, having stiffness in the direction perpendicular to the longitudinal direction increases the stiff ness and eliminates the formation of folds.

The first and second direction may be perpendicular to each other. However; also oth er mutual angles may be established between the first and second lateral direction.

Hereby it is possible to provide a laminate having different desired strength properties in lateral direction and with different orientation of the actual direction in which the stiffness is established.

In the fibre mat, a side edge area along said side edges will not be provided with resin stripes. When the resin is cured, the side edges would be stiff if cured resin extends to the side edges. This have some drawbacks and accordingly it is preferred that the fi- bres in the side edge areas are soft and pliable. When the side edges are provided in a layer inside the laminate, there is a risk that stiff, reinforced fibres may interfere with layers lying next to an actual layer having the stiff, reinforced fibres at the side edges. Accordingly, the side edge areas along each side edge of a fibre mat are left without resin applied. Hereby soft and pliable fibres are arranged in the side edge areas, even after the curing of the resin. This is especially important for fibre mats in which the reinforced fibres extend to a side edge being provided inside the mould. This will gen erally be the case for fibre mats having the stiffness extending under an angle com- pared to the longitudinal direction of a mould in a mould where the laminate is laid up.

The need of soft and pliable side edge areas is less in fibre mats having a length sub stantially corresponding to the length of a mould and having the stiffness orientated in the longitudinal direction of a mould where the laminate is laid up.

I the final laminate laid up in the mould, there will always be some kind of covering over the outermost layers in the laminate. Typically, a gelcoat is applied over the outermost layer facing a vacuum cloth in the mould, and a coating or surface layer is applied in the mould before building up the laminate.

If the resin-treated layer is outermost in the final laminate, there might occur a visual ly undesired appearance, as a crenelated surface is established due to the cured resin stripes. Such crenelated surface may also influence on the surface properties in the final product in an undesired way.

In a further embodiment of the invention, said method is peculiar in that said step of building up the second part of the laminate comprises the step of:

- using third fibre mats which in a lateral direction being different from said first lat- eral direction and said second lateral direction has a greater stiffness than in the first lateral direction and in the second lateral direction, which third fibre mats each com prises a fibre material which is partly impregnated with resin and pre-cured such that the resin forms spaced resin stripes along said lateral direction being different from said first lateral direction and said second lateral direction. The laminate may be manufactured from more than two fibre mats. A third fibre mat can have a lateral direction in which the stiffness is established in a direction which is different from the first lateral direction and the second lateral direction.

Also, a fourth fibre mat which can have a stiffness in a direction which is different from the first three lateral directions can be used.

Accordingly, it is possible to manufacture the laminate with a number of layers which each has a strengthening in a lateral direction being different from the direction of the strengthening in other layers of the laminate.

In a further embodiment of the invention, said method is peculiar in that said fibre mats are unidirectional fibre mats, wherein said resin has been applied on a plurality of unidirectionally aligned fibres on said unidirectional fibre mat for generating a res in-treated unidirectional fibre mats.

As already explained, it is common to apply resin along some of the fibres in mats with unidirectionally aligned fibres. E.g. resin is applied to each 5 ^th fibre, or resin is applied with a predetermined mutual distance, e.g. 50 mm. The resin can also be ap plied to bundles of fibres in a resin-treated unidirectional fibre mat.

With the present invention it is preferred that the different fibre mats used for manu facturing of the laminate are unidirectional mats. In each of the mats, the resin is pro vided in order to form spaced resin stripes along the direction of the unidirectionally aligned fibres.

Accordingly, there is a strengthening of each mat with a greater stiffness in a direction parallel to the unidirectionally aligned fibres.

Hereby it is possible to have a secure anti-wrinkling effect and simultaneously obtain ing a strength in any desired direction corresponding to the direction of the unidirec tionally aligned fibres. In a further embodiment of the invention, said method is peculiar in that said method further comprises the step of:

- building the laminate, with at least one layer of the laminate being a layer of fibre mats provided with different stiffness in more different lateral directions.

It is possible to manufacture a fibre mat being impregnated with resin in more direc tions. Accordingly, the direction of the stiffness which is obtained in one fibre mat after curing of the resin stripes may have different orientation compared to the direc tion of the stiffness in other fibre mats in the laminate.

The direction of such resin stripes will preferably correspond to the direction of unidi- rectionally aligned fibres.

In a further embodiment of the invention, said method is peculiar in that said method further comprises the step of:

- building the laminate by using fibre mats in second part of the laminate, where such fibre mats has the lateral direction with greater stiffness orientated under an angle of +/- 30 to 60° in relation to said first direction.

In fibre mats used for building up the second part of the laminate, the orientation of the fibres has typically a direction being perpendicular to the first lateral direction. However, the direction may also have an angle which is in most cases +/- 30° or +/- 60° in relation to the first lateral direction. Also, a direction +/- 45° in relation to the first lateral direction is used.

However, it is possible to establish the fibre mats in the second part of the laminate having any stiffness which are orientated under any angle in relation to the first lateral direction.

The first lateral direction in the fibre mats in the first part of the laminate may be a direction perpendicular to the length of the fibre mat. The fibre mat will normally be produced in the form of a web having a long length. The web is provided in the form of a roll of untreated fibre material which is led through a resin application step and a curing step. In such resin application step, it is possible to apply resin stripes having any orientation in relation to the transport direction of the web through such resin ap plication step.

In a further embodiment of the invention, said method is peculiar in that said at least two different layers, which have a greater stiffness in a lateral direction than in anoth er lateral direction, are placed in such a way that the lateral directions of the at least two different layers having the greater stiffness include an angle between 0° and 180°.

Hereby it is possible to manufacture a laminate having stiffness with an orientation in substantial any lateral direction. However, in general it will be preferred that the angle between the orientation of the greater stiffness in the different layers will have an an gle being perpendicular +/- 45° in relation to each other.

In a further embodiment of the invention, said method is peculiar in that said two lat eral directions having the greater stiffness are perpendicular to each other.

The situation will typically be that the lateral directions where the greater stiffness occur will be obtained by a reinforcement direction in a transport direction of the web used for forming the fibre mat for the first part of the laminate and in a direction per pendicular to the transport direction for a web used for forming a fibre mat for the second part of the laminate.

In a further embodiment of the invention, said method is peculiar in that the fibre rein forced laminate is part of a wind turbine blade.

A method according to the present invention is suitable for products having a long length compared to the width of the product, e.g. like the form of a wind turbine blade.

Moreover, seeing that wind turbine blade has a curved form, which may be a double curved form, it is essential to establish stiffness in different directions in the layers of the laminate in order to obviate wrinkling during the manufacture of the wind turbine blade. The laterally extended, wrinkle-preventing material according to the invention is suit able for the manufacture of wind turbine blades because of its stiffness in one direc tion combined with its flexibility in the perpendicular direction. A wind turbine blade must have a high strength in the direction of its longitudinal axis. In the direction per- pendicular to the longitudinal axis, a wind turbine blade is strongly curved. Therefore, a fibre material which is flexible in this direction is easier to drape. Also, the thickness of a wind turbine blade tapers off towards the tip of the blade. Therefore, it is advan tageous to use a laterally extended, wrinkle-preventing material with varying thick ness.

Hereafter, the invention will be described in connection with drawings illustrating non-limiting examples of a method for manufacturing a fibre-reinforced laminate.

Description of the Drawing

Fig. 1 schematically illustrates a section through a first embodiment of a laminate, Fig. 2 schematically illustrates a section through a second embodiment of a lami nate,

Fig. 3 schematically illustrates a section through a third embodiment of a laminate,

Fig. 4 schematically illustrates a section through a shell for a wind turbine blade,

Fig. 5 schematically illustrates laminate in a mould for a wind turbine blade,

Fig. 6 schematically illustrates a cross-section through a curved mould containing a layer for a fibre mat with a wrinkle, and

Fig. 7 schematically illustrates a partial cross-sectional view through a curved mould in the longitudinal direction of a wind turbine blade.

Detailed Description of the Invention

In Figs. 1, 2 and 3 different embodiments of laminates 1 are illustrated.

Each of the laminates 1 is illustrated in a mould 8.

It is illustrated that the laminates 1 are formed of more layers, each of which is built up of different fibre mats. The fibre mats comprise first fibre mats 2, second fibre mats 3, third fibre mats 4 and fourth fibre mats 5. It is possible to use even more dif- ferent fibre layers and the number of fibre layers illustrated are only for illustrative purposes, as a laminate will be manufactured from a higher number of fibre mats.

The laminate is built of a first part and a second part.

The first part of the laminate may be the layer made from the first fibre mats 2. These first fibre mats has in a first lateral direction 7 (see Fig 5) a greater stiffness than in a second lateral direction. The second lateral direction is not indicated but can be any other lateral direction under an angle to said first lateral direction 7. The greater stiff ness is obtained as the fibre mats 2 are provided with a number of parallel resin stripes 9 being orientated along said first lateral direction 7.

The second part of the laminate is made from the second fibre mats 3, the third fibre mats 4 and the fourth fibre mats 5. The greater stiffness is obtained in the fibre mats 3,4,5 as they are provided with a number of parallel resin stripes 9 being orientated along any other lateral direction than said first lateral direction 7. This occurs more clearly from Fig. 5 which illustrates a direction perpendicular to said first lateral direc tion and a direction being 45° in relation to said first lateral direction.

Figs. 1-3 illustrate that the fibre mats 2, 3, 4 and 5 can be used in different combina tions. Each of the laminates 1 will be provided with increased stiffening in a lateral direction being different from layer to layer.

Figs. 1-3 also illustrate that more layers may be used having the greater stiffness in same direction than in other fibre mats in other layers of the laminate 1.

Fig. 1-3 illustrate that it is possible to repeat placing of a first layer 2 and further lay ers 3,4,5 until the desired thickness of the complete laminate 1 is obtained.

It is also possible to have even more different fibre mats than illustrated, having a fur ther different lateral direction in which the greater stiffness occurs.

Fig. 4 is a schematic section through a shell 6 for a wind turbine blade. A wind turbine blade is normally made of an upper and lower shell. Each shell is manufactured from a laminate comprising a number of fibre-reinforced layers.

The wind turbine blade will normally, in some parts of the shell, have a thickened section which is illustrated in Fig. 4. In the thickened section, the number of fibre- reinforced layers with increased stiffness could be increased in respect to the outer sections illustrated in Fig. 4

In Fig. 4, different fibre mats 2, 3, 4, 5 are illustrated in the thickened section of the shell 6. Also in this Figure, the number of fibre mats illustrated is only illustrative as the shell 6 will comprise a higher number of layers.

Fig. 5 illustrates a mould 8 with different layers comprising different fibre mats 2, 3, 5 to form the laminate 1 which shall constitute a shell 6 for a wind turbine blade.

In Fig. 5, parts are cut away in order to illustrate the resin stripes 9 having different orientations in the different fibre mats used for forming the layers of the laminate 1.

The fibre mat 2 is illustrated with resin stripes 9 in the first lateral direction 7 which corresponds to a longitudinal direction for the mould 8. The fibre mat 3 has resin stripes 9 being perpendicular to the first lateral direction 7. The fibre mat 5 has resin stripes 9 being 45° in relation to the first lateral direction 7.

As it occurs from the above, all fibre mats used for the layers in the laminate 1 are wrinkle-preventing fibre mats which have different orientations of the lateral direction in which the increased stiffness is established.

Fig. 6 illustrates a mould 8 for a wind turbine blade in which a first layer 2 having the resin stripes 9 extending in the first lateral direction 7 is arranged. This first lateral direction will in the laminate 1 illustrated correspond to the longitudinal direction for the laminate 1. In a shell 6 for a wind turbine blade as illustrated in Fig. 5, the longitu dinal direction will be in the longitudinal direction of the wind turbine blade. Accord ingly, Fig. 6 represents a cross-sectional view through a mould 8 and a first layer for a shell 6 for a wind turbine blade. The laminate 1 is intended for a shell 6 of a wind turbine blade. When such layer is laid in a curved mould 8 together with other layers, a laminate is established. The cur vature can cause forces in the perpendicular direction to the web laid up in the curved mould 8. Hereby a wrinkle 10 may occur. Such wrinkle 10 may in normal procedures be hidden when the following layer in a laminate is laid up. Hereby wrinkles or folds may be established in the laminate.

When the fibre mat 2 is used for building up a first part of the laminate 1, there is an advantage by building up a second part of the laminate by using the fibre mats 3, 4 and 5. Hereby the increased stiffness is obtained in the second part of the laminate, in the second lateral direction 11 under a different angle, e.g. perpendicular. Hereby the wrinkles or folds 10 are obviated.

A wind turbine blade will have a double curvature. Fig. 7 illustrates a partial cross- sectional view in the longitudinal direction of a wind turbine blade. Here the mould 8 for the wind turbine blade is provided, in which the first layer 2 has the resin stripes (not visible) extending in the first lateral direction 7. This first lateral direction will in the laminate illustrated correspond to the longitudinal direction for the laminate. The shell 6 has a curvature in the longitudinal direction of the wind turbine blade, which corresponds to the first lateral direction 7.

In this mould, a further layer 3 is laid. The layer 3 will provide increased stiffness in the second lateral direction 11 (see Fig. 5). When such layer is laid in a curved mould 8 together with other layers, a laminate is established. The curvature can cause forces in the direction perpendicular to the longitudinal direction of the curved mould 8. Hereby a wrinkle 10 may occur. Such wrinkle 10 may in normal procedures be hidden when the following layer in a laminate is laid up. Hereby wrinkles or folds may be established in the laminate. When the fibre mat 3 is used for building up a part of the laminate there is an ad vantage by building up a further part of the laminate by using the fibre mats 2, 4 and 5. Hereby the increased stiffness is obtained in the further part of the laminate, in the second lateral direction 11 under a different angle, e.g. perpendicular. Hereby the wrinkles or folds 10 are obviated. Reference numbers

1. laminate

2. first fibre mat

3. second fibre mat

4. third fibre mat

5. fourth fibre mat

6. shell for wind turbine blade

7. first lateral direction

8. mould

9. resin stripe

10. wrinkle

11. second lateral direction