GB2601126A

GB2601126A - A precured fibrous strip for a load-carrying structure for a wind turbine blade

Info

Publication number: GB2601126A
Application number: GB2018113.7A
Authority: GB
Inventors: Razeghi Rama; Bjørn Jørgensen Jeppe
Original assignee: LM Wind Power AS; Blade Dynamics Ltd
Current assignee: LM Wind Power AS; LM Wind Power UK Ltd
Priority date: 2020-11-18
Filing date: 2020-11-18
Publication date: 2022-05-25
Also published as: GB202018113D0

Abstract

A precured fibrous composite strip 50, 50’ for a load-carrying structure, such as a spar cap 40, for a wind turbine blade; the strip has first and a second longitudinal ends 51, 52; a first and second sides 53, 54 defining a width; and upper and lower surfaces 55, 56 defining a thickness. The strip comprises a chamfer region 60 at an end of the strip comprising: a primary chamfer 60a formed in the upper surface and a secondary chamfer 60b in the lower surface; wherein the chamfers are shaped so that the chamfer region has an asymmetric profile shape. Preferably, the chamfer lengths are different, with the primary chamfer length being at least 10% larger than the secondary chamfer; the double chamfer surfaces may have changing taper angles or may be shaped to be substantially plane, parabolic, spline-shaped or s-shaped chamfers. Preferably, the chamfer region has a blunt face with an end step thickness. The precured strip is preferably a pultruded strip with unidirectional fibres oriented in the longitudinal direction. A spar cap comprising a strip is further provided preferably wherein a plurality of strips are stacked in an array to form the spar cap. A wind turbine blade comprising a strip and a method of manufacturing a spar cap comprising stacking the strips are further provided.

Description

Title: A precured fibrous strip for a load-carrying structure for a wind turbine blade

Field of the Invention

The present invention relates to a precured fibrous strip for a load-carrying structure, such as a spar cap for a wind turbine blade, a spar cap comprising such a fibrous strip, a method of manufacturing a spar cap for a wind turbine blade, and a wind turbine blade comprising a spar cap including such a fibrous strip or manufactured according to said method.

Background of the Invention

Wind turbine blades are often manufactured according to one of two constructional designs, namely a design where a thin aerodynamic shell is glued onto a spar beam, or a design where spar caps, also called main laminates, are integrated into the aerodynamic shell.

In the first design, the spar beam constitutes the load-bearing structure of the blade. The spar beam as well as the aerodynamic shell or shell parts are manufactured separately. The aerodynamic shell is often manufactured as two shell parts, typically as a pressure side shell part and a suction side shell part. The two shell parts are glued or otherwise connected to the spar beam and are further glued to each other along a leading edge and a trailing edge of the shell parts. This design has the advantage that the critical load-carrying structure may be manufactured separately and therefore easier to control. Further, this design allows for various different manufacturing methods for producing the beam, such as moulding and filament winding.

In the second design, the spar caps or main laminates are integrated into the shell and are moulded together with the aerodynamic shell. The main laminates typically comprise a high number of fibre layers compared to the remainder of the blade and may form a local thickening of the wind turbine shell, at least with respect to the number of fibre layers. Thus, the main laminate may form a fibre insertion in the blade. In this design, the main laminates constitute the load-carrying structure. The blade shells are typically designed with a first main laminate integrated in the pressure side shell part and a second main laminate integrated in the suction side shell part. The first main laminate and the second main laminate are typically connected via one or more shear webs, which for instance may be C-shaped or I-shaped. For very long blades, the blade shells further along at least a part of the longitudinal extent comprise an additional first main laminate in the pressure side shell, and an additional second main laminate in the suction side shell. These additional main laminates may also be connected via one or more shear webs. This design has the advantage that it is easier to control the aerodynamic shape of the blade via the moulding of the blade shell part.

Vacuum infusion or VARTM (vacuum assisted resin transfer moulding) is one method, which is typically employed for manufacturing composite structures, such as wind turbine blades comprising a fibre-reinforced matrix material.

During the process of filling the mould, a vacuum, said vacuum in this connection being understood as an under-pressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels, the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus, it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus, dry spots are areas where the fibre material is not impregnated, and where there can be air pockets, which are difficult or impossible to remove by controlling the vacuum pressure and a possible overpressure at the inlet side. In vacuum infusion techniques employing a rigid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by puncturing the bag in the respective location and by drawing out air for example by means of a syringe needle. Liquid polymer can optionally be injected in the respective location, and this can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mould parts, staff have to stand on the vacuum bag. This is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus in a local weakening of the structure, which can cause for instance buckling effects In most cases, the polymer or resin applied is polyester, vinyl ester or epoxy, but may also be PUR or pDCPD, and the fibre reinforcement is most often based on glass fibres or carbon fibres or even hybrids thereof. Epoxies have advantages with respect to various properties, such as shrinkage during curing (which in some circumstances may lead to less wrinkles in the laminate), electrical properties and mechanical and fatigue strengths. Polyester and vinyl esters have the advantage that they provide better bonding properties to gelcoats. Thereby, a gelcoat may be applied to the outer surface of the shell during the manufacturing of the shell by applying a gelcoat to the mould before fibre-reinforcement material is arranged in the mould. Thus, various post-moulding operations, such as painting the blade, may be avoided. Further, polyesters and vinyl esters are cheaper than epoxies.

Consequently, the manufacturing process may be simplified, and costs may be lowered.

Often the composite structures comprise a core material covered with a fibre reinforced material, such as one or more fibre reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid, lightweight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, for instance by providing channels or grooves in the surface of the core material.

Resin transfer moulding (RTM) is a manufacturing method, which is similar to VARTM. In RTM, the liquid resin is not drawn into the mould cavity due to a vacuum generated in the mould cavity.

Instead, the liquid resin is forced into the mould cavity via an overpressure at the inlet side.

Prepreg moulding is a method in which reinforcement fibres are pre-impregnated with a pre-catalysed resin. The resin is typically solid or near-solid at room temperature. The prepregs are arranged by hand or machine onto a mould surface, vacuum bagged and then heated to a temperature, where the resin is allowed to reflow and eventually cured. This method has the main advantage that the resin content in the fibre material is accurately set beforehand. The prepregs are easy and clean to work with and make automation and labour saving feasible. The disadvantage with prepregs is that the material cost is higher than for non-impregnated fibres. Further, the core material needs to be made of a material which is able to withstand the process temperatures needed for bringing the resin to reflow. Prepreg moulding may be used both in connection with an RTM and a VARTM process.

Further, it is possible to manufacture hollow mouldings in one piece by use of outer mould parts and a mould core. Such a method is for instance described in EP 1 310 351 and may readily be combined with RTM, VARTM and prepreg moulding.

As for instance blades for wind turbines have become longer and larger in the course of time and may now be more than 100 meters long, the impregnation time in connection with manufacturing such blades has increased, because more fibre material has to be impregnated with polymer.

Furthermore, the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots, said control may e.g. include a time-related control of inlet channels and vacuum channels. This increases the time required for drawing in or injecting polymer. As a result, the polymer has to stay liquid for a longer time, normally also resulting in an increase in the curing time.

Further, as the blades are becoming longer, it has become increasingly customary to manufacture blades from premanufactured parts. In particular, the spar cap of the structure may be manufactured as precured parts, such as pultruded elements. This improves the alignment of fibres for the spar cap and prevents wrinkles in the layup. Further, it improves the infusion time and decreases the time for in-mould preparation of the wind turbine blade. The precured parts may for instance be pultruded fibre-reinforced planks and may be stacked, e.g. in an array.

The spar cap typically has a varying thickness along the span of the spar cap, e.g. having a tapered thickness towards the root end of the blade and/or the tip end of the blade. In order to accommodate the varying thickness, an end region of the precured elements may be tapered. This provides a gradual stiffness transition.

However, even with the taper sections at the ends of the precured elements, resin rich areas may still be formed at the taper sections. Further, if fibre layers are draped on top of the stacked precured parts, wrinkles may be formed at the tapered end regions. In addition, the tip of the tapered end regions is prone to breaking or cracking.

Summary of the Invention

It is an object of the invention to obtain a new precured fibrous strip, a new spar cap, a new method of manufacturing a spar cap for a wind turbine blade, and a new wind turbine blade, which overcome or ameliorate at least one of the disadvantages of the prior art or which provide a useful alternative.

According to a first aspect, this is obtained by a precured fibrous composite strip for a load-carrying structure, such as a spar cap, for a wind turbine blade, wherein the strip has a first longitudinal end and a second longitudinal end; a first side and a second side with a width defined as the distance between the first side and the second side; and an upper surface and a lower surface with a thickness defined as the distance between the upper surface and the lower surface; and wherein the strip comprises a first chamfer region at the first end of the strip comprising: a first primary chamfer formed in the upper surface at the first end of the strip, the first primary chamfer having a first primary chamfer length; and a first secondary chamfer formed in the lower surface at the first end of the strip, the first secondary chamfer having a first secondary chamfer length; wherein the first primary chamfer and the first secondary chamfer are shaped so that the chamfer region has an asymmetric profile shape.

S

By letting the precured fibrous composite strip having a chamfer at both the upper and lower surface at the first end of the strip, the tapering of the strips can better be tailored to required needs. In particular, the asymmetric design allows more freedom to tailor the chamfer thickness to mitigate possible failure modes. The length of the chamfer region can be reduced compared to e.g. conventional pultruded strips with a tapered end section, or in the alternative, the overall tapering angle for the chamfers can be smaller, which has the benefit of an improved shear load transfer and reduction of the peel loading. This has the benefit that the overall strength of a joint can be improved, in particular if the chamfer region has a transition to fibre layers in the finished spar cap or wind turbine blade comprising such a spar cap. It is also possible to obtain a trade-off that allows the length of the chamfer region to be reduced while also making the taper angle smaller.

It is clear that the terms "upper" and "lower" may define the precured fibrous composite strip in accordance with the strip being positioned on a horizontal surface. However, it is recognised that in use, the upper and lower surfaces of the precured fibrous strips need not necessarily be oriented upwardly and downwardly, respectively. It is also clear that the precured fibrous strips may be arranged so that the lower surface faces upwards and the upper surface faces downwards.

It is also clear that the precured fibrous strip may comprise a second chamfer region at the second longitudinal end of the strip. Similar to the first chamfer region, the second chamfer region may comprise a second primary chamfer formed in the upper surface at the second end of the strip, the second primary chamfer having a second primary chamfer length; and a second secondary chamfer formed in the lower surface at the second end of the strip, the second secondary chamfer having a second secondary chamfer length; wherein the second primary chamfer and the first secondary chamfer are shaped so that the chamfer region has an asymmetric profile shape. Further, the strip may also comprise a similar chamfer region at the first side and/or at the second side.

The term asymmetric profile means that the first primary chamfer is not a mirror image of the first secondary chamfer.

According to a second aspect, the object is obtained by a spar cap for a wind turbine blade comprising a precured fibrous strip according to the first aspect.

According to a third aspect, the object is obtained by a wind turbine blade comprising a spar cap according to the second aspect.

According to a fourth aspect, the object is obtained by a method of manufacturing a spar cap for a wind turbine blade or a wind turbine blade shell comprising a spar cap, the method comprising: providing a plurality of precured fibrous strips including at least one precured fibrous strip according to the first aspect, stacking the plurality of precured fibrous strips such that interface regions are formed between adjacent precured fibrous strips, supplying resin to the plurality of precured fibrous strips and causing the resin to fill the interlace regions between adjacent strips, and curing the resin in order to form the spar cap.

Finally, the object is obtained by a spar cap manufactured according to the above method and a wind turbine blade comprising such a spar cap.

In the following, preferred embodiments according to the above aspects are described. The various embodiments may be combined in any conceived combination.

In a preferred embodiment, the first primary chamfer length is different from the first secondary chamfer length. In other words, the start of the first primary chamfer and the second primary chamfer are mutually offset in the longitudinal direction of the precured fibrous strip. This embodiment has the advantage that thickness transitions can be made in steps, even if the chamfers are planar or straight. The first primary chamfer length may be at least 10% larger than the first secondary chamfer length, or vice versa. The first primary chamfer length may also be at least 20%, 300/0 or 40°/0 larger than the first secondary chamfer length, or vice versa. In one embodiment the first primary chamfer length is 25%-90% of the first secondary chamfer length, or vice versa.

In one advantageous embodiment, the first primary chamfer and/or the second primary chamfer have a substantially plane or straight surface. The first primary chamfer and the second primary chamfer may have different angles and/or different lengths. With two plane or straight surfaces, the thickness transition of the first end may be varied in two steps.

However, it is also possible to have a variable chamfer angle along the first primary chamfer and/or the first secondary chamfer. In a first preferred embodiment, the first primary chamfer and/or the second primary chamfer have a decreasing taper angle towards the first longitudinal end. The first primary chamfer and/or the second primary chamfer may for instance have a shape chosen from the group of: a substantially parabolic chamfer, a substantially spline-shaped chamfer or a substantially s-shaped chamfer.

According to a preferred embodiment, the chamfer region has a blunt face at the first longitudinal end, wherein the blunt face has an end step thickness. By leaving the end face slightly blunt, the end of the precured fibrous strip is even less likely to break off or form cracks. The end step thickness may be between 0.01 mm and 0.3 mm, preferably between 0.05 mm and 0.2 mm, e.g. around 0.1 In yet another preferred embodiment, the chamfer region has a thickness to length ratio in the range 1:10 to 1:250, preferably in the range 1:20 to 1:200, and more preferably in the range 1:50 to 1:150, e.g. around 1:100.

In a preferred embodiment, the strip comprises unidirectionally oriented reinforcement fibres, such as glass fibres or carbon fibres, oriented in the longitudinal direction. The strip is preferably a pultruded element. However, the strip may also be premanufactured in other ways, such as by extrusion or moulding.

In another preferred embodiment, the thickness of the strip is between 1 mm and 10 mm, preferably between 3 mm and 8 mm, and more preferably between 4 mm and 7 mm, e.g. around 5 mm.

In another preferred embodiment, the width of the strip is between 30 mm and 300 mm, preferably between 50 mm and 200 mm, e.g. around 100 mm.

The length of the strip may advantageously be at least 100 mm and be up to the length of the part that it is used for, e.g. up to the length of a spar cap or wind turbine blade. The length may e.g. be up to 100 m. In the future, even longer strips may be used.

The plurality of precured fibrous strips are preferably stacked in an array. The strips may be stacked on top of each other so as to be aligned or mutually displaced in the transverse direction.

The above-mentioned method may comprise arranging at least a first fibre layer so that at least a first fibre layer is draped to cover at least the first primary chamfer of the at least one precured fibrous strip, and arranging a second fibre layer to cover at least the first secondary chamfer of the at least one precured fibrous strip. As previously mentioned, the solution provides a benefit to the overall strength of a joint between the fibre layers and the precured fibrous strips.

The above-mentioned method may comprise the step of arranging flow-promoting material between at least some of the stacked precured fibrous strips. This may promote flow between densely packed strips.

The method may also comprise the step of draping one or more fibre layers over the plurality of stacked precured fibrous strips before the step of supplying resin. The one or more fibre layers may form an inner skin, e.g. of the blade shell. Similarly, one or more fibre layers may be arranged outermost to form an outer skin, e.g. of the blade shell.

Brief Description of the Figures

The invention is explained in detail below with reference to embodiments shown in the drawings, in which Fig. 1 shows a wind turbine, Fig. 2 shows a schematic view of a wind turbine blade, Fig. 3 shows a schematic view of a wind turbine blade shell, Fig. 4 shows a side view of a spar cap, Figs. 5a and 5b show embodiments for a cross-section of a spar cap, Fig. 6 shows a chamfer region of a precured fibrous strip, Figs. 7a-7g show various profiles for the chamfer region of a precured fibrous strip, and Fig. 8 shows steps in a manufacturing method.

Detailed Description of the Invention

In the following, a number of exemplary embodiments are described in order to understand the invention.

Fig. 1 illustrates a conventional modern upwind wind turbine according to the so-called "Danish concept" with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 nearest the hub and a blade tip 14 farthest from the hub 8.

Fig. 2 shows a schematic view of a first embodiment of a wind turbine blade 10 disclosure. The wind turbine blade 10 has the shape of a conventional wind turbine blade and comprises a root region 30 closest to the hub, a profiled or an airfoil region 34 farthest away from the hub and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 comprises a leading edge 18 facing the direction of rotation of the blade 10 when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region 30 due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade 10 to the hub. The diameter (or the chord) of the root region 30 may be constant along the entire root area 30. The transition region 32 has a transitional profile gradually changing from the circular or elliptical shape of the root region 30 to the airfoil profile of the airfoil region 34. The chord length of the transition region 32 typically increases with increasing distance r from the hub. The airfoil region 34 has an airfoil profile with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10. The width of the chord decreases with increasing distance r from the hub.

A shoulder 39 of the blade 10 is defined as the position where the blade 10 has its largest chord length. The shoulder 39 is typically provided at the boundary between the transition region 32 and the airfoil region 34.

It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub.

The blade is typically made from a pressure side shell part 36 and a suction side shell part 38 that are glued to each other along bond lines at the leading edge 18 and the trailing edge 20 of the blade.

In the following, the invention is explained with respect to the manufacture of the pressure side shell part 36 or suction side shell part 38.

Fig. 3 shows a perspective view of a blade shell part, here illustrated with the suction side shell part 38, which is provided with a load-carrying structure, which forms a spar cap 40 or main laminate. The spar cap 40 can be integrated into the blade shell or it can be a separate spar cap that is attached, e.g. by adhesion, to the blade shell 38. The spar cap 40 may be part of a separate spar structure. However, it is also possible to provide a blade with spar caps provided at both the pressure side shell part 36 and the suction side shell part 38, with one or more shear webs attached between the spar caps.

Fig. 4 shows a side view of a manufactured spar cap 40. The spar cap is made from a plurality of precured fibrous strips 50, 50' that extends in the longitudinal direction of the spar cap 40. The precured fibrous strips 50, 50' may be stacked on top of each other as shown in Fig. 4. In a cross-sectional view, the strips 50, 50' may be stacked right on top of each other as shown in Fig. 5a, or they may be displaced between layers, e.g. as shown in Fig. 5b with partially overlapping strips.

The precured fibrous strips 50, 50' preferably comprise unidirectionally oriented reinforcement fibres, such as glass fibres or carbon fibres, oriented in the longitudinal direction. Further, the strips are preferably pultruded elements.

The pultruded strips 50 each have a first longitudinal end 51 and a second longitudinal end 52, a first side 53 and a second side 54 with a width defined as the distance between the first side 53 and the second side 54, and an upper surface 55 and a lower surface 56 with a thickness defined as the distance between the upper surface 55 and the lower surface 56. The upper surface 55 and lower surface 56 are defined in relation to how the pultruded strips are laid up, and the upper surface 55 will typically be arranged towards the inner surface of the spar cap 40, whereas the lower surface 56 will typically face towards the outer surface of the spar cap 40, as seen in relation to the blade shell 38. The strips each comprise a chamfer region 60 with a taper length at the first longitudinal end 51 and may further comprise a second chamfer region 70 at the second longitudinal end 52.

In addition, flow-promoting material 80, such as a fibre layer or fibre veil, may be arranged between precured fibrous strips 50 in order to promote resin flow during the infusion process. In the shown embodiment, the flow promoting material 80 is arranged in layers between layers of precured fibrous strips 50, 50'. However, flow-promoting material may also be arranged between neighbouring strips 50, 50'. Further, the precured fibrous strips may be arranged between an inner skin layer 82 comprising one or more fibre layers, and an outer skin layer 84 comprising one or more fibre layers, as shown in Fig. 4.

In the shown embodiment, the upper precured fibrous strips 50' are formed with a conventional single chamfer at the first longitudinal end 51 and the second longitudinal end 52. The lower-most precured fibrous strips 50 are formed with a first primary chamfer 60a formed in the upper surface 55 at the first end 51 of the strip 50, the first primary chamfer 60a having a first primary chamfer length, and a first secondary chamfer 60b formed in the lower surface 56 at the first end 51 of the strip 50, the first secondary chamfer 60b having a first secondary chamfer length.

The first primary chamfer 60a and the first secondary chamfer 60b are shaped so that the chamfer region has an asymmetric profile shape. By letting the precured fibrous composite strip having a chamfer 60a, 60b at both the upper surface 55 and the lower surface 56 at the first end 51 of the strip 50, the tapering of the strips can better be tailored to required needs. In particular, the asymmetric design allows more freedom to tailor the chamfer thickness to mitigate possible failure modes. The length of the chamfer region can be reduced compared to e.g. conventional pultruded strips with a tapered end section (e.g. the ones shown with reference numeral 50), or in the alternative, the overall tapering angle for the chamfers can be smaller, which has the benefit of an improved shear load transfer and reduction of the peel loading. This has the benefit that the overall strength of a joint can be improved, in particular if the chamfer region has a transition to fibre layers in the finished spar cap or wind turbine blade comprising such a spar cap, e.g. a transition from the inner skin layer(s) 82 and the outer skin layer(s) 84.

The first primary chamfer 60a and the first secondary chamfer 60b advantageously have a thickness to length ratio in the range 1:10 to 1:250, preferably in the range 1:20 to 1:200, and more preferably in the range 1:50 to 1:150, e.g. around 1:100. The thickness of the strip 50 may advantageously be between 1 mm and 10 mm, preferably between 3 mm and 8 mm, and more preferably between 4 mm and 7 mm, e.g. around 5 mm. The width of the strip is between 30 mm and 300 mm, preferably between 50 mm and 200 mm, e.g. around 100 mm. The length of the strips 50 may advantageously be at least 100 mm and be up to the length of the blade, e.g. up to 100 m. In the future, even longer strips may be used.

A typical pultruded strip may typically have a thickness of 5 mm, and the chamfer region may have a length of 500 mm, thus providing a thickness to length ratio of 1:100 for the chamfer region. As examples, with the new configuration with two chamfers, the length of the chamfer region may be approximately halved without having to alter the tapering angle of the chamfers, or alternatively the thickness to length ration of the individual chamfers may be changed to approximately 1:200 without having to increase the overall length of the chamfer region. A trade-off between the two is also possible. This leaves various possibilities for better tailoring and improving the strength of an end joint for the precured fibrous strip.

Fig. 6 shows the chamfer region 60 of the precured fibrous strip 50 in greater detail and illustrates that the fibre layers forming the outer skin 84 and the fibre layers forming the inner skin 82 may be draped to cover the first primary chamfer 60a and the first secondary chamfer 60b.

In a preferred embodiment, the first primary chamfer length is different from the first secondary chamfer length. In other words, the start of the first primary chamfer and the start of the second primary chamfer are mutually offset in the longitudinal direction of the precured fibrous strip. The thickness of the two chamfers may also be different.

In one advantageous embodiment, the first primary chamfer and/or the second primary chamfer have a substantially plane or straight surface. The first primary chamfer and the second primary chamfer may have different angles and/or different lengths. With two plane or straight surfaces, the thickness transition of the first end may be varied in two steps. However, it is also possible to have a variable chamfer angle along the first primary chamfer and/or the first secondary chamfer. In a first preferred embodiment, the first primary chamfer and/or the second primary chamfer have a decreasing taper angle towards the first longitudinal end. The first primary chamfer and/or the second primary chamfer may for instance have a shape chosen from the group of: a substantially parabolic chamfer, a substantially spline-shaped chamfer or a substantially s-shaped chamfer.

Fig. 7 shows various examples of precured fibrous strips having a chamfer region with a first primary chamfer formed in the upper surface at the first end of the strip, and a first secondary chamfer formed in the lower surface at the first end of the strip and having an asymmetric profile. The profiles shown in Figs. 7a, 7b, and 7c are provided with plane chamfers for both the first primary chamfer and the first secondary chamfer. In the profile shown in Fig. 7a, the heights of the first primary chamfer and the first secondary chamfer are the same, but the lengths are different, with the effect that the tapering angles of the two chamfers are different. In the profile shown in Fig. 7b, the tapering angle of the first primary chamfer and the first secondary chamfer are the same, but the lengths are different, with the effect that the height of the two chamfers are also different. In the profile shown in Fig. 7c, the lengths of the first primary chamfer and the first secondary chamfer are the same, but the tapering angles are different, with the effect that the height of the two chamfers are also different.

The profiles shown in Figs. 7d and 7e are provided with substantially parabolic-shaped chamfers for both the first primary chamfer and the first secondary chamfer. In the profile shown in Fig. 7d, the heights of the first primary chamfer and the first secondary chamfer are the same, but the lengths are different. In the profile shown in Fig. 7e, the length of the first primary chamfer and the first secondary chamfer are the same, but the heights are different.

The profiles shown in Figs. 7f and 7g are provided with substantially s-shaped or spline-shaped chamfers for both the first primary chamfer and the first secondary chamfer. In the profile shown in Fig. 7f, the heights of the first primary chamfer and the first secondary chamfer are the same, but the lengths are different. In the profile shown in Fig. 7g, the length of the first primary chamfer and the first secondary chamfer are the same, but the heights are different.

It is recognised that it is possible to combine characteristics of the various profiles shown in Figs. 7a-7g. It is for instance possible to provide a profile having a plane primary chamfer and a secondary chamfer being substantially parabolic-shaped, s-shaped or spline-shaped. It is also possible to provide a profile having a substantially parabolic-shaped primary chamfer and a secondary chamfer being substantially s-shaped or spline-shaped. It is also recognised that each of the profiles may be provided mirrored or upside-down.

While the profiles shown in Figs. 7a-7c are shown without a blunt face at the first longitudinal end, the chamfer region preferably has a blunt face at the first longitudinal end similar to the ones shown in Figs. 7d-7g, wherein the blunt face has an end step thickness By leaving the end face slightly blunt, the end of the precured fibrous strip is even less likely to break off or form cracks. The end step thickness may be between 0.01 mm and 0.3 mm, preferably between 0.05 mm and 0.2 mm, e.g. around 0.1 mm.

In the following, a method of manufacturing a spar cap for a wind turbine blade according to the present disclosure is described. The method comprises the steps shown in Fig. 8.

In a first step 90, a plurality of precured fibrous strips including at least one precured fibrous strip 50 as described above is provided. In a second step 91, the plurality of precured fibrous strips are stacked such that interface regions are formed between adjacent precured fibrous strips. In an optional third step 92, flow-promoting material is arranged between at least some of the stacked precured fibrous strips. In a fourth step 93, resin is supplied to the plurality of precured fibrous strips and causing the resin to fill the interface regions between adjacent strips. In a fifth step 94, the resin is cured in order to form the spar cap.

The plurality of precured strips may be stacked in an array. Further, the manufacturing method may additionally comprise the step of draping fibre layers over the plurality of stacked precured fibrous strips before the step of supplying resin, e.g. such that at least a first fibre layer is draped to cover at least the first primary chamfer of the at least one precured fibrous strip, and a second fibre layer is draped to cover at least the first secondary chamfer of the at least one precured fibrous strip.

List of reference numerals 2 wind turbine 4 tower 6 nacelle 8 hub blade 14 blade tip 16 blade root 18 leading edge 20 trailing edge 22 pitch axis root region 32 transition region 34 airfoil region 36 pressure side shell 38 suction side shell 39 shoulder spar cap SO precured fibrous strips! pultruded elements 51 first longitudinal end 52 second longitudinal end 53 first side 54 second side upper surface 56 lower surface chamfer region / first chamfer region 60a first primary chamfer 60b first secondary chamfer chamfer region / second chamfer region 70a second primary chamfer 70b second secondary chamfer flow promoting material 82 inner skin layer(s) 84 outer skin layer(s) 90-94 method steps

Claims

Claims 1. A precured fibrous composite strip for a load-carrying structure, such as a spar cap, for a wind turbine blade, wherein the strip has a first longitudinal end and a second longitudinal end; a first side and a second side with a width defined as the distance between the first side and the second side; and an upper surface and a lower surface with a thickness defined as the distance between the upper surface and the lower surface; and wherein the strip comprises a first chamfer region at the first end of the strip comprising: a first primary chamfer formed in the upper surface at the first end of the strip, the first primary chamfer having a first primary chamfer length; and a first secondary chamfer formed in the lower surface at the first end of the strip, the first secondary chamfer having a first secondary chamfer length; wherein the first primary chamfer and the first secondary chamfer are shaped so that the chamfer region has an asymmetric profile shape.
2. The precured fibrous strip of claim 1, wherein the first primary chamfer length is different from the first secondary chamfer length.
3. The precured fibrous strip according to claim 1 or 2, wherein the first primary chamfer length is at least 10% larger than the first secondary chamfer length, or vice versa.
4. The precured fibrous strip of claim 3, wherein the first primary chamfer length is 40%-90% of the first secondary chamfer length, or vice versa.
5. The precured fibrous strip of any of claims 1-4, wherein the first primary chamfer and/or the second primary chamfer have a substantially plane surface.
6. The precured fibrous strip of any of claims 1-5, wherein the first primary chamfer and/or the second primary chamfer have a decreasing taper angle towards the first longitudinal end.
7. The precured fibrous strip of any of claims 1-6, wherein the first primary chamfer and/or the second primary chamfer have a shape chosen from the group of: a substantially parabolic chamfer, a substantially spline-shaped chamfer or a substantially s-shaped chamfer.
8. The precured fibrous strip according to any of claims 1-7, wherein the chamfer region has a blunt face at the first longitudinal end, wherein the blunt face has an end step thickness.
9. The precured fibrous strip according to any of claims 1-8, wherein the end step thickness is between 0.01 mm and 0.3 mm, preferably between 0.05 mm and 0.2 mm, e.g. around 0.1 mm.
10. The precured fibrous strip according to any of claims 1-9, wherein the first primary chamfer and/or the first secondary chamfer have a thickness to length ratio in the range 1:10 to 1:250, preferably in the range 1:20 to 1:200, and more preferably in the range 1:50 to 1:150, e.g. around 1:100.
11. The precured fibrous strip according to any of claims 1-10, wherein the strip comprises unidirectionally oriented reinforcement fibres, such as glass fibres or carbon fibres, oriented in the longitudinal direction.
12. The precured fibrous strip according to claim 11, wherein the strip is a pultruded element.
13. The precured fibrous strip according to any of claims 1-12, wherein the thickness of the strip is between 1 mm and 10 mm, preferably between 3 mm and 8 mm, and more preferably between 4 mm and 7 mm, e.g. around 5 mm.
14. The precured fibrous strip according to any of claims 1-13, wherein the width of the strip is between 30 mm and 300 mm, preferably between 50 mm and 200 mm, e.g. around 100 mm.
15. The precured fibrous strip according to any of claims 1-14, wherein the length of the strips is between 100 mm and 100 m.
16. A spar cap for a wind turbine blade comprising a precured fibrous strip according to any of claims 1-15.
17. The spar cap of claim 16, wherein the spar cap comprises a plurality of stacked precured fibrous strips and comprising at least one precured fibrous strip according to any of claims 1-15.
18. The spar cap of claim 17, wherein the plurality of precured fibrous strips are stacked in an array.
19. A wind turbine blade comprising a spar cap according to any of claims 16-18.
20. A method of manufacturing a spar cap for a wind turbine blade or a wind turbine blade shell comprising a spar cap, the method comprising: providing a plurality of precured fibrous strips including at least one precured fibrous strip according to any of claims 1-15, stacking the plurality of precured fibrous strips such that interface regions are formed between adjacent precured fibrous strips, supplying resin to the plurality of precured fibrous strips and causing the resin to fill the interface regions between adjacent strips, and curing the resin in order to form the spar cap.
21. The method according to claim 20, wherein the method comprises arranging at least a first fibre layer so that at least the first fibre layer is draped to cover at least the first primary chamfer of the at least one precured fibrous strip, and arranging a second fibre layer so that the second fibre layer is draped to cover at least the first secondary chamfer of the at least one precured fibrous strip.
22. The method according to claim 20 or 21, wherein the plurality of precured strips are stacked in an array.
23. The method according to any of claims 20-22, wherein the method comprises the step of arranging flow-promoting material between at least some of the stacked precured fibrous strips.
24. The method according to any of claims 20-23, wherein the method comprises the step of draping a fibre layer over the plurality of stacked precured fibrous strips before the step of supplying resin.
25. A spar cap manufactured according to any of claims 20-25.
26. A wind turbine blade comprising a spar cap according to claim 25.