A wind turbine blade with reinforcements to prevent peeling

Abstract

The present invention relates to a reinforced blade for a wind turbine, particularly reinforcing the adhesive bond lines to better withstand the forces causing blade failures due to peeling effects in the bond lines. The blade having at least one elongated reinforcing member connected inside the blade shell for increasing the strength of the blade adhesive bond lines. Each of the at least one elongated reinforcing member having a first end and a second end and extending in a longitudinal direction between the first end and the second end and wherein the first end is connected to the blade shell, and the second end is connected to girder or the flat-back, and thereby decreasing peeling stresses in the adhesive bond lines.

Description

[0001] A WIND TURBINE BLADE WITH REINFORCEMENTS TO PREVENT PEELING

[0002] The present invention relates to a reinforced blade for a wind turbine, particularly to a blade having elongated reinforcing members in the blade in order to prevent or reduce deformation and peeling stresses in the bond lines.

[0003] Background

[0004] Typically, a wind turbine blade has an aerodynamic blade shell shape and at least one girder, such as a beam or a spar. The girder can be a single girder, but often two girders are used. The two girders together with the parts of the blade shell extending between the two girders form a so-called box profile. The top and bottom of the box profile are often referred to as the caps or the spar caps. Some types of blades are designed with a spar in the form of a box profile which is manufactured separately and bonded in between prefabricated surface blade shells. The aerodynamic shell is typically made of a laminate of fibre reinforced plastics, fibreglass and / or other materials. Typically, the aerodynamic shell is made from two blade shell parts that are assembled to form the blade shell by adhesive bond lines.

[0005] Under normal operating conditions, the wind turbine blade is subjected to loads at an angle to the flap-wise direction. It is common to resolve this load on the blade into its components in the flap-wise and edge-wise direction. The flap-wise direction is a direction substantially perpendicular to a transverse axis through a cross-section of the blade. The flap-wise direction may thus be construed as the direction, or the opposite / reverse direction, in which the aerodynamic lift acts on the blade. The edge-wise loads occur substantially in a direction perpendicular to the flap-wise direction. The blade is further subject to torsional loads which are mainly aerodynamic, gravity and inertia loads. These loads can subject the blade to harmonic motions or oscillations substantially at the blades’ torsional eigenfrequency.

[0006] When a blade is subjected to edge-wise loading combined with flap-wise loading combined with torsional loading the cross section of the blade is deforming, see Fig. 1. This cross-sectional sheer distortion deformation induces peeling stresses in the adhesive bond lines consequently this can lead to a fatigue failure in the adhesive joint, also called the adhesive bond lines, of the trailing edge, substantially in case the trailing edge is shaped as in a flat-back aerofoil, where the two shell parts are connected, and in the adhesive bond lines between the girders and the blade shells. Furthermore, the deformation of the of the blade shell can lead to large deformations in both the blade shell and the girder and / or the trailing edge area, especially when the blade design comprises flat-back profiles, see Fig. 1 . The occurrence of this cross-sectional shear distortion phenomenon can lead to fatigue failure of the girder to the blade shell adhesive bond lines and / or fatigue failure in the adhesive bond lines between the blade shell at the trailing edge.

[0007] The adhesive bond line fatigue failure in the trailing edge and between the blade shells and the girder(s) may then ultimately cause the blade to break apart. The deformation can also lead to buckling of the blade shell and this reduces the ultimate strength of the blade because the blade shell is often load carrying. Furthermore, the deformations also compromise the aerodynamic efficiency of the blade since the designed shape of the blade profile is no longer maintained. The edge-wise loading combined with flapwise loading combined with torsional loading can further cause the trailing edge of the blade to deform in a stable post buckling pattern. This is caused by bending of the blade from the leading edge towards the trailing edge. The blade material in the leading edge is then subject to tension and the trailing edge to compression. Since the trailing edge may be relative thin, it cannot withstand substantial compression forces before it bends out of its neutral plane. When this happens, some of the load on the trailing edge is transferred to and distributed through part of the blade shell further away from the trailing edge, until equilibrium of the forces is established. Although this deformation may not immediately lead to failure, it decreases the safety margin for the general failure load of the blade and it increases the peeling and shear stresses in the trailing edge.

[0008] Thus, there is a need for a wind turbine blade in which deformations of adhesive bond lines are prevented or reduced and wherein the blade structure is strengthened without significantly increasing the overall weight of the blade. It is yet an object of the present invention to provide a wind turbine blade with increased overall strength.

[0009] It is yet another object of the present invention to provide a wind turbine blade with increased resistance to fatigue failure.

[0010] It is yet another object of the present invention to provide a wind turbine blade with increased torsional stiffness.

[0011] It is yet another object of the present invention to provide a wind turbine blade with increased resistance to buckling of the trailing edge preferably in a flat back trailing edge.

[0012] It is yet another object of the present invention to provide a wind turbine blade with improved resistance against deformations of the blade profile. It is also an object of the present invention to provide a reinforced blade profile for a wind turbine blade.

[0013] It is therefore an object of the present invention to provide a wind turbine blade with improved resistance against deformations of the blade shell. It is another object of the present invention to provide a wind turbine blade with reduced weight and reduced costs.

[0014] It is also an object of the present invention to provide a wind turbine blade with improved reliability of joints between blade shell parts and between the blade shells and the girders.

[0015] It is a further object to provide a wind turbine blade capable of working under severe aerodynamic loads and to optimise the aerodynamic efficiency, e.g. energy output of the blade.

[0016] It is further an object of the present invention to provide alternatives to the prior art.

[0017] Resume

[0018] According to a first aspect of the present invention, the above-mentioned and other objects are fulfilled by a wind turbine blade at more than 40 meters total length comprising a blade shell having a section with an aerodynamic profile, and at least one elongated reinforcing member connected inside the blade shell for increasing the strength of the blade, each of the at least one elongated reinforcing member having a first end and a second end and extending in a longitudinal direction between the first end and the second end and wherein the first end is connected to the upper part of the blade shell or the lower part of the blade shell and the second end is connected to girder or to the flat back. According to a second aspect of the present invention, the above-mentioned and other objects are fulfilled by a method of increasing the strength of a wind turbine blade at more than 40 meters total length having a blade shell with a section having an aerodynamic profile, the method comprising the steps of positioning at least one elongated reinforcing member inside the blade shell, each of the at least one elongated reinforcing member having a first end and a second end and extending in a longitudinal direction between the first end and the second end, and connecting the first end to the upper part of the blade shell or to the lower part of the shell and the second end to the girder or to the flat back. The wind turbine blade may be utilized in a vertical axis wind turbine, such as a Darrieus wind turbine, a wind star turbine, etc., or preferably in a horizontal axis wind turbine, such as common modem wind turbines usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced).

[0019] The blade shell of the wind turbine blade may preferably, but not exclusively, comprise a composite or laminated material. The material may preferably, but not exclusively, comprise fibreglass and / or carbon fibres and / or other durable and flexible or stiff materials typically with a high strength / weight ratio. This may further comprise at least in part light weight metals or alloys. The blade shell may typically be a laminate and / or sandwich- construction. Preferably, at least one of the at least one elongated reinforcing member extends in a direction that is substantially perpendicular to the longitudinal extension of the blade. In case of a curved blade wherein the longitudinal extension of the blade forms a non-linear curve in space, the elongated reinforcing member extends in a direction that is substantially perpendicular to the longitudinal extension of the blade in the vicinity of the elongated reinforcing member in question.

[0020] The elongated reinforcing members may form an angle with the longitudinal extension of the blade in the vicinity of the elongated reinforcing member in question preferably ranging from 70° to 110°, more preferably from 80° to 100°, and even more preferred from 85° to 95°.

[0021] The at least one elongated reinforcing member may form an angle between 10 and 80 degrees, preferably between 30 and 60 degrees and more preferably between 40 and 50 degrees to the profile chord of the blade. The profile chord of the blade is an imaginary surface that contains the leading edge and the trailing edge of the blade and extends therebetween. Thus, the edge-wise direction is a direction in parallel with the profile chord and the flap-wise direction is a direction perpendicular to the profile chord. The wind turbine blade may comprise a plurality of elongated reinforcing members positioned in spaced relationship along the longitudinal extension of the blade.

[0022] The blade according to the invention may also comprise one or more girders. Wind turbine blades with one or more girders are well-known. A conventional girder has a longitudinal extension in the longitudinal direction of the blade and a transverse extension substantially perpendicular to the profile chord of the blade. The one or more conventional girders primarily strengthen the blade along the longitudinal extension of the blade. A girder may also be referred to as a web or a shear web. The conventional girder or web may be constituted by any type of elongate constructional member capable of taking up loads, such as a beam or a spar, e.g., shaped as an l-profile or a C- profile, preferably made from fibre reinforced plastics or other suitable material. Typically, conventional girders extend along substantially the entire length of the blade.

[0023] Preferably, the elongated reinforcing member has a straight shape. If the shape of the elongated reinforcing member is not straight, the shape of the elongated reinforcing member could be straightened when subjected to stretching forces leading to movement of its end points and obviously, this is not desired.

[0024] The elongated reinforcing member may be constituted by any type of elongated constructional member capable of taking up loads. The elongated reinforcing member may comprise one or more elements selected from the group consisting of a rod, a plate, and a tube, capable of resisting both compression forces and tensional forces. Since, the elongated reinforcing member need not necessarily to be capable of resisting compression forces, the elongated reinforcing member may further comprise one or more elements selected from the group consisting of a wire, a rope, a thread, a fibre, and a web of fabric. The elements may have any suitable cross-section, for example a substantially round or polygonal cross-section, such as substantially rectangular, triangular, circular, oval, elliptical, etc, but is preferably circular or oval.

[0025] The elements may be applied individually or may be applied as a number of individual elements together forming a "thicker" element. Particularly, the element may comprise fibres of very high stiffness and strength such as, glass fibres, carbon fibres, aramid fibres, polyethylene fibres, PBO fibres (polypheylene benzobisoxqazole), etc.

[0026] The elongated reinforcing members may be made of any suitable material. Fibre reinforced plastic is presently preferred for rods, plates and tubes. The elongated reinforcing members may also be made of wood, such as bamboo, birch, plywood, etc.

[0027] The elongated reinforcing member may also be made of steel, light metal alloys, etc.

[0028] The elongated reinforcing members may also be made of material based on plant fibres with high cellulose content, such as bast fibres, such as flax, jute, etc. These fibres may be used as reinforcement in a composite material, such as a reinforced plastic, or may be used in the form of wires or rods. The elongated reinforcing member may also be made of a combination of the above-mentioned materials.

[0029] The elongated reinforcing member is required to have a high tensional strength only, i.e. , preferably, the elongated reinforcing member need not necessarily carry other loads so that the elongated reinforcing member may be thin whereby its weight and cost are kept at a minimum. The thickness of the elongated reinforcing member is preferably less than 10 times the maximum thickness of the blade shell, more preferred less than 5 times the maximum thickness of the blade shell, still more preferred less than 2 times the maximum thickness of the blade shell, most preferred less than the maximum thickness of the blade shell.

[0030] The connections on the inner surface of the blade profile may in principle be positioned anywhere on the inner surface but it should be observed that the chosen positioning is suitable for the elongated reinforcing member to be able to provide a reasonable and useful reinforcing effect to the adhesive bond line in question. The connections may comprise any suitable kind of joint such as welded, glued, melted, fused or other simple mechanical connections. The elongated reinforcing member itself may comprise the connections or it may comprise additional connections or connection parts adapted to engage or cooperate with the connections on the inner surface of the blade shell and the girder(s) I flat back.

[0031] The connections may be releasable connections that may comprise any suitable kind of joint, such as a snap-fit, press-fit, groove-and-tongue connection or other simple mechanical connection.

[0032] The elongated reinforcing member secures and keeps the shape of the adhesive bond line immediate surroundings substantially unchanged when the aerodynamic profile is loaded by forces in the edge-wise and flap-wise and torsional direction. This in turn causes the overall strength of the aerodynamic profile to increase significantly since the resistance against buckling is also increased.

[0033] An elongated reinforcing member according to the present invention improves the peeling strength of the adhesive bond lines. One of the at least one elongated reinforcing member may form an angle with another elongated reinforcing member. The angle may range from 10° to 50°. Preferably, the at least one elongated reinforcing member extends substantially in a 45-degree angle to the profile chord of the blade in a cross-section of the blade. Two or more elongated reinforcing members may be positioned in spaced relationship along at least a part of the longitudinal extension of the blade in such a way that neighbouring elongated reinforcing members are mounted with different angles in relation to the profile chord of the blade.

[0034] The maximum distance between two elongated reinforcing members may be based on specific requirements, such as, but not limited to, a need for a particularly strong wind turbine blade design, e.g. when the wind turbine is intended to be subjected to repeatedly severe weather conditions, such as when erected offshore at open sea or onshore in a mountain area.

[0035] The elongated reinforcing members may be positioned in certain sections of the blade only, possibly without any predetermined or calculated maximum distance. Particularly, but not exclusively, the elongated reinforcing members may be located at positions wherein a substantial deformation of the adhesive bond lines is expected or established. This location may be at the inner one third, the middle one third or at the outer one third of the blade length direction.

[0036] At least one of the at least one elongated reinforcing member may comprise a composite material such as glass fibre reinforced plastic or carbon fibre reinforced plastic or any other fibre reinforced material.

[0037] At least one of the at least one elongated reinforcing member may be bonded, screwed, or laminated onto the blade shell or onto the girder or onto the flat back. Brief description of the drawings

[0038] In the following, the invention will be described in greater detail with reference to embodiments shown by the enclosed figures. It should be emphasized that the embodiments shown are used for example purposes only and should not be used to limit the scope of the invention.

[0039] The figures are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the invention, while other details have been left out.

[0040] In addition to the shown embodiments, the invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

[0041] Fig. 1 shows a cross-section through a wind turbine blade,

[0042] Figs. 2 shows a cross-section through a wind turbine blade with elongated reinforcing members according to embodiments of the invention,

[0043] Figs. 3 shows a cross-section through a wind turbine blade with elongated reinforcing members according to other embodiments of the invention,

[0044] Figs. 4 shows a cross-section through a wind turbine blade with elongated reinforcing members according to yet other embodiments of the invention,

[0045] Figs. 5 shows a cross-section through a wind turbine blade with a load carrying rectangular unit or box and with elongated reinforcing members according to yet other embodiments of the invention, Figs. 6 shows a cross-section through a wind turbine blade with elongated reinforcing members according to embodiments of the invention, where the elongate reinforcing members comprise sandwich structures,

[0046] Figs. 7 shows a cross-section through a wind turbine blade with elongated reinforcing members comprising comprise sandwich structures according to yet another embodiments of the invention, and

[0047] Figs. 8 shows a cross-section through a wind turbine blade with spar caps and with elongated reinforcing members according to embodiments of the invention,

[0048] Detailed description of the embodiments

[0049] Fig. 1 shows a schematic cross-section of a wind turbine blade in a nonloaded position (solid lines) and in a loaded position (dashed lines). The cross-section shows an aerofoil of a type, often referred to as a flat-back aerofoil. Throughout the present invention disclosure, a flat-back aerofoil is defined as an aerofoil comprising a trailing edge thickness exceeding 2% of the aerofoil chord length. The word flat-back is used for any shape of the trailing edge regardless of it being flat, round, elliptic, and regardless of the angle that the flat-back area has compared to the internal girder(s) or to the aerofoil chord line. It is evident that the aerofoil section shown in fig. 1 , will experience increased peeling loading in any adhesive bond line between any of the blade shells and any of the girders. It is likewise evident that any adhesive bond line at the flat-back area of the aerofoil section, will experience increased peeling loading when the blade is subject to loading (see the dashed line in fig. 1 ). The present invention disclosure describes different means to reinforce these adhesive bond lines by using elongated reinforcing members implemented substantially to prevent unwanted peeling stresses in the adhesive bond lines and thereby to prevent bonding failures in the adhesive bond lines and ultimately to prolong the lifetime of the blade.

[0050] Fig. 2 shows a schematic cross-section of a wind turbine blade with elongated reinforcing members between the upper part of the blade shell and the girders, between the lower part of the blade shell and the girders, and between the flat-back and the lower blade shell. The elongated reinforcing members are bonded to the blade inner surface and to the girders I flat-back, but they could as well be connected by any other means, like over lamination, bolted, screwed or even by a combination of the elongated reinforcing member penetrating the blade shell, the girders and / or the flat- back and thereafter bonded, over laminated, bolted or screwed. The elongated reinforcing members in fig. 2 are, in a preferred embodiment of the invention, made from a composite material skin like glass fibre or carbon fibre and in this preferred embodiment of the invention, combined with a core material to form a sandwich structure.

[0051] Fig. 3 shows a schematic cross-section of a wind turbine blade with elongated reinforcing members between the upper part of the blade shell and the girder, and between the lower part of the blade shell and the girder. The aerofoil shown in fig. 3 is not a flat-back type of aerofoil, since the trailing edge thickness is less than 2% of the aerofoil chord length, and therefore a reinforcement of the trailing edge bond line is not within the scope of the present invention disclosure. The elongated reinforcing members shown in fig. 3, are, in this preferred embodiment of the invention, made from a solid composite material like glass fibre reinforced plastic or carbon fibre reinforced plastic, but they could as well be made from any other material. The elongated reinforcing members are bonded to the blade inner surface and to the girders I flat-back, but they could as well be connected by any other means like over lamination, bolted, screwed or even by a combination of the elongated reinforcing member penetrating the blade shell, the girders and / or the flat-back and thereafter bonded, over laminated, bolted or screwed.

[0052] Fig. 4 shows a schematic cross-section of a wind turbine blade with elongated reinforcing members between the upper part of the blade shell and the girder, and between the lower part of the blade shell and the girder, and between the flat-back and the lower blade shell. The elongated reinforcing members have a triangular shaped cross-section substantially to improve the stiffness of the reinforcement of the adhesive bond lines. The elongated reinforcing member between a blade shell and a girder is only positioned at one side of the girder, but it could as well also have an elongated reinforcing member at the other side of the girder, and thereby reinforcing the adhesive bond lines even more. The ns are bonded to the blade inner surface and to the girders I flat-back, but they could as well be connected by any other means.

[0053] Fig. 5 shows a schematic cross-section of a wind turbine blade. The crosssection of the blade shows a load carrying rectangular unit or box with both the girders and the spar cap manufactured and bonded into the blade shells in one piece. This kind of internal blade structure design is common in blades from a few blade manufacturers, although it is not the most used design. Fig. 5 shows the elongated reinforcing members between the upper part of the blade shell and the vertical part of the load carrying rectangular unit or box part representing the girders, and between the lower part of the blade shell and the load carrying rectangular unit part likewise representing the girders. The schematic cross-section of the wind turbine blade is not a flat-back type of aerofoil, and therefore no elongated reinforcing members are present at the trailing edge area in fig. 5. The elongated reinforcing members shown in fig. 5, are, as in this preferred embodiment of the invention, made from a solid composite material like glass fibre reinforced plastic or carbon fibre reinforced plastic, but they could as well be made from any other material. The elongated reinforcing members are bonded to the blade inner surface and to the girder part of the load carrying rectangular unit or box, but they could as well be connected by any other means or principles.

[0054] Fig. 6 shows a schematic cross-section of a wind turbine blade with elongated reinforcing members between the upper part of the blade shell and the girder, and between the lower part of the blade shell and the girder, and between the flat-back and the lower part of the blade shell. The elongated reinforcing members comprise, in a preferred embodiment of the invention as shown in fig 6, sandwich structures comprising a core material like foam or balsa wood and a skin covering the core material like glass fibre or carbon fibre. The elongated reinforcing members have a triangular shaped crosssection substantially to improve the stiffness of the reinforcement of the adhesive bond lines. The elongated reinforcing member between a blade shell and a girder is only positioned at one side of the girder, but it could as well also have an elongated reinforcing member at the other side of the girder, and thereby reinforcing the adhesive bond lines even more. The elongated reinforcing members are bonded to the blade inner surface and to the girders I flat-back, but they could as well be connected by any other means like over lamination, bolted, screwed.

[0055] Fig. 7 shows a schematic cross-section of a wind turbine blade with elongated reinforcing members between the upper part of the blade shell and the girders, between the lower part of the blade shell and the girders, and between the flat-back and the lower part of the blade shell. The blade shown in fig. 7 comprise two spar caps with a distance between them, and the girders are, as it is most common in wind turbine blade structures, positioned at the spar caps, since they have a larger strength and stiffness than the sandwich structures of the blade shells. The elongated reinforcing members comprise, in a preferred embodiment of the invention as shown in fig 7, sandwich structures comprising a core material like foam or balsa wood and a skin covering the core material like glass fibre or carbon fibre. The elongated reinforcing members are bonded to the blade inner surface and to the girders I flat-back, but they could as well be connected by any other means like over lamination, bolted, screwed. The elongated reinforcing members shown in fig. 7, are made from fibre reinforced plastics and a core material to form a sandwich structure, but they could as well be made from any other material. The elongated reinforcing members are bonded to the blade inner surface and to the girder part of the load carrying rectangular unit, but they could as well be connected by any other means like over lamination, bolted, screwed.

[0056] Fig. 8 shows a schematic cross-section of a wind turbine blade with elongated reinforcing members between the upper part of the blade shell and the girders, between the lower part of the blade shell and the girders, and between the flat-back and the lower part of the blade shell. The cross-section of the blade shown in fig. 8 shows, as in fig. 2, 4, 6 and 7, two spar caps with a distance between them, and the girders are positioned at the spar caps, since they have a larger strength and stiffness than the sandwich structures of the blade shells. The elongated reinforcing members comprise, in a preferred embodiment of the invention as shown in fig 8, solid composite material as the main load carrying structure. The elongated reinforcing members are screwed (not shown in fig. 8) to the blade inner surface and to the girders I flat-back, but they could as well be connected by any other means like over laminated or bonded. The elongated reinforcing members shown in fig. 8, are made from composite material like glass fibre or carbon fibre reinforced plastics, but they could as well be made from any other material. List of parts

[0057] 1. Girder

[0058] 2. Flat-back 3. Cap, Spar Cap

[0059] 4. Elongated reinforcing member

[0060] 5. Adhesive bond line

[0061] 6. Sandwich

[0062] 7. Upper Part of the Blade Shell 8. Lower Part of the Blade Shell

Claims

Claims1. A wind turbine blade at more than 40 meters total length comprising- a blade shell having a section with an aerodynamic profile, and- at least one elongated reinforcing member connected inside the blade shell for increasing the strength of the blade, wherein each of the at least one elongated reinforcing member has a first end and a second end and extending in a longitudinal direction between the first end and the second end, and wherein the first end is connected to an upper part of the blade shell or to a lower part of the blade shell and the second end is connected to a girder or to a flat back and thereby preventing deformation induced peeling stresses of the adhesive bond lines.

2. A wind turbine blade at more than 40 meters total length according to claim 1 , wherein the at least one elongated reinforcing member comprises a plurality of elongated reinforcing members positioned in spaced relationship along the longitudinal extension of the blade.

3. A wind turbine blade at more than 40 meters total length according to claim 1 or 2, wherein at least one of the at least one elongated reinforcing member extends in a direction that forms an angle with relation to a longitudinal extension of the blade ranging from 70° to 110°, preferably from 80° to 100°, more preferred from 85° to 95°.

4. A wind turbine blade at more than 40 meters total length according to claim 3, wherein at least one of the at least one elongated reinforcing member extends in a direction that is substantially perpendicular to the longitudinal extension of the blade.

5. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member form an angle between 10 and 80 degrees, preferably between 30 and 60 degrees and most preferably between 40 and 50 degrees to the profile chord of the blade.

6. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein the at least one elongated reinforcing member comprises a plurality of elongated reinforcing members positioned in spaced relationship along the longitudinal extension of the blade with a mutual distance that is less than 2xD, wherein D is the distance between the first and second end of one of the plurality of elongated reinforcing members.

7. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein one of the at least one elongated reinforcing member forms an angle with another one of the at least one elongated reinforcing member.

8. A wind turbine blade at more than 40 meters total length according to claim 7, wherein the angle ranges from 10° to 50°.

9. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member is a flexible wire with high tensional strength without a capability of resisting compression forces.

10. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member comprises a composite material such as glass fibre reinforced plastic or carbon fibre reinforced plastic or any other fibre reinforced material.11 . A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member is bonded onto the blades shell or onto the girder or onto the flat back.

12. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member is screwed into the blades shell or into the girder or into the flat back.

13. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member is laminated to the blades shell or to the girder or to the flat back.

14. A wind turbine blade at more than 40 meters total length according to any of the preceding claims, wherein at least one of the at least one elongated reinforcing member is located at the inner one third, the middle one third or at the outer one third of the blade length direction.

15. A method of increasing the strength of a wind turbine blade at more than 40 meters total length having a blade shell with a section having an aerodynamic profile, the method comprising the steps of positioning at least one elongated reinforcing member inside the blade, wherein each of the at least one elongated reinforcing member having a first end and a second end and extending in a longitudinal direction between the first end and the second end, and connecting the first end to the upper part of the blade shell or the lower part of the blade shell and the second end to the girder or to the flat- back, and thereby preventing deformation induced peeling stresses of the adhesive bond lines.

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