Technical field

The present disclosure relates to a root connection of a wind turbine blade. In particular, the disclosure relates to an insert for a wind turbine blade root connection and to a method of manufacturing an element for forming such an insert. The disclosure also relates to a method of inspecting the integrity of an element for a wind turbine blade root connection.

Background

Wind turbine blades for large horizontal axis wind turbines have significant mass, perhaps in the region of 10 tonnes or more, up to 30 tonnes or more. Wind turbine blades are fastened to a hub to make up a rotor which rotates on a main shaft to drive a generator. The blades are attached at their root end to a hub flange.

A blade is typically connected to the hub flange using a group of bolts. The stresses on a blade hub connection are considerable, owing chiefly to blade mass and wind force, as well as the effect of perpetual rotation of the rotor, which tends to vary the degree and direction of the forces on the blade with every rotation of the rotor. Vibrations in the system can also be considerable.

With an expected lifetime of 20 years or more, the fatigue performance of the blade hub connection is critical. A group of bolts, often known as stud bolts, may be connected to the blade root using threaded bushings embedded into the fibre composite material of the root end of the blade. Such bushings are usually cylindrical, often made of steel. The bushings transfer the loads from the blade to the stud bolts. The stud bolts transfer those loads to the hub, which is a rigid, often cast, component.

Considering the fatigue requirements placed on a blade hub connection, the manner of embedding the bushings into the blade root is critical, as is the ability to ascertain the integrity of the connection between the bushing and the surrounding fibre composite material. Because of the configuration of bushings embedded in a blade root, it is difficult or not possible to use ultrasonic non-destructive test methods to interrogate the integrity of the connection between the bushing and the surrounding fibre composite material of the insert. This may also be the case when bushings are themselves embedded in an insert body of fibre-reinforced resin material, prior to arranging the inserts at a blade root. It is against this background that the present invention has been developed.

Summary of the invention

There is proposed an insert for a wind turbine blade root end connection, the insert comprising a bushing in a fibrous composite material body, wherein the body comprises: a plurality of elongate fibrous battens arranged around the bushing and a transition layer located between the bushing and the elongate fibrous battens; wherein one or more of the elongate fibrous battens comprises a translucent portion positioned to allow inspection of the transition layer. Preferably the transition layer is a fibrous layer. The transition layer may in particular comprise fibrous or filamentary material wrapped or wound around the busing. Preferably tightly wrapped around the bushing. Optionally, the transition layer may comprise fibrous material sheets. Optionally, the transition layer may comprise filamentary material wound around the bushing. Optionally, the transition layer may comprise one or more layers of fibrous material sheets wrapped around the bushing and bound in place with filamentary material windings. The transition layer is preferably a resin infused fibrous layer, in which the fibrous and/or filamentary material is embedded in a resin matrix.

Each of the elongate fibrous battens in the insert body, arranged around the bushing may comprise a translucent portion positioned to allow inspection of the transition layer. In this way it may be permitted to inspect the transition layer under each one of the fibrous battens. Preferably, each said translucent portion may extend along substantially the entire length of an associated elongate fibrous batten. This may allow inspection of the entire extent of a portion of a transition layer beneath a fibrous batten. Translucency of a fibrous batten may be understood in the present context as allowing the passage of enough light through the batten to enable a determination of defects underneath it. Still preferably, a translucent portion of a batten may be substantially transparent. Still preferably, a translucent batten may be substantially transparent.

A fibrous batten configured for the present invention may in particular comprise glass fibre filamentary or fibrous material in a resin matrix. In particular, an elongate fibrous batten may comprise glass fibre composite material. The glass filaments or fibres of the glass fibre composite material of said battens may be substantially uniaxial. In one optional embodiment, the glass fibres of the glass fibre composite may comprise glass fibre rope or glassfibre tow. Preferably, the elongate fibrous battens may comprise resin transfer moulded glass fibre composite. Accordingly, the elongate fibrous battens may be made by resin transfer moulding prior to being provided in a said insert.

In preferred embodiments an elongate fibrous batten may have a substantially triangular cross-section. In this way, four such battens arranged evenly about a periphery of a bushing may generate an insert having an approximately rectangular cross section. Still preferably, a fibrous batten may be prismoid, with two planar longitudinally extending faces subtending a third, longitudinally extending, concave face.

The invention may also comprise a wind turbine blade comprising a plurality of inserts according to the invention. Still further, the invention may comprise a moulded blank capable of being subdivided into one or more said inserts. Still further, the invention may comprise a moulded blank capable of being subdivided into one or more identical inserts.

The invention may comprise a method of making an element for a wind turbine blade root end connection, the method comprising placing into a mould a bushing and a plurality of elongate fibrous battens around the bushing, wherein one or more of the elongate fibrous battens comprises a translucent portion positioned in the vicinity of the of the bushing; subsequently forming the element in said mould using a resin infusion process including curing, wherein the formed element comprises a transition layer located between the bushing and the elongate fibrous battens; subsequently extracting said element from said mould and inspecting said element for blemishes by passing light through the translucent portion of the one or more fibrous battens. An element made by the method defined herein may in particular comprise a blank from which one or more root inserts may be hewn, or it may comprise a root insert.

Preferably, the bushing may be wrapped with fibrous or filamentary material before placing the bushing in the mould. The bushing may be wrapped with fibrous and/or filamentary material before placing the bushing in the mould. The bushing may be wrapped with layers of fibrous sheet material before placing the bushing in the mould. Filamentary material may be wound around the bushing before placing the bushing in the mould. Filamentary material may be wound around layers of fibrous sheet material wrapped around the bushing, before placing the bushing in the mould. Fibrous and/or filamentary material wrapped around the bushing may become infused with resin during the resin transfer moulding process for the element being formed. The region immediately around the bushing thereby constituting a fibrous composite material transition layer. This layer, between the bushing and the body of the insert, may be known as a transition layer.

Inspecting the element may comprises determining the presence and/or size of blemishes in the vicinity of the transition layer. Blemishes in this context may comprise any irregularity or defect. More specifically, it is intended to detect irregularities or defects which may compromise the performance of the connection between a wind turbine blade and a hub. For inspecting the element, it may be preferred to pass light through a translucent portion an elongate fibrous batten and then to observe light reflected back through the translucent portion. Light in the visible spectrum may be preferred. Inspection using visible light may in particular be easier to implement than using electronic detectors of other e.g. non-visible wavelengths. Inspecting the element may thereby in particular comprise making a visual inspection through the one or more elongate fibrous battens.

Preferably, each of the elongate fibrous battens may comprise a translucent material portion or may be fully comprised of translucent material. Inspecting the element may comprise observing the integrity of the connection through each elongate fibrous batten. In this context, the term “each” may signify “all”.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Brief description of the drawings

The present invention will now be described by way of non-limiting examples with reference to the following figures, in which: Figure 1 shows a horizontal axis wind turbine;

Figure 2 shows a typical wind turbine blade;

Figure 3 shows a schematic view of the root end of the blade of Figure 2;

Figure 4 shows an enlarged view of part of the root of Figure 3; Figure 5 shows a schematic view of a wind turbine blade root insert;

Figure 6 shows a schematic longitudinal cross-sectional view of a bushing;

Figure 7 shows a schematic cross-sectional view of the bushing of Figure 5 embedded in a composite body;

Figure 8 shows a schematic view of an insert blank in a mould;

Figure 9 shows a schematic view of an alternative arrangement of components for an insert; and

Figure 10 shows a schematic view of an insert with a blemish located in the vicinity of the bushing; and Figure 11 shows a schematic view of a method of inspecting an insert or an insert blank.

Detailed description

Figure 1 shows a horizontal axis wind turbine 10. The wind turbine 10 comprises a tower 12 supporting a nacelle 14 to which a rotor 16 is mounted. The rotor 16 comprises a plurality of wind turbine blades 18 that extend radially from a central hub 19. In this example, the rotor 16 comprises 3 blades 18.

Figure 2 is a schematic view of one of the blades 18. The blade 18 extends from a generally circular root 20 to a tip 22 in a longitudinal or “spanwise” direction, and between a leading edge 24 and a trailing edge 26 in a transverse or “chordwise” direction. The blade 18 comprises a shell 27 which may be formed primarily of fibre- reinforced plastic (FRP). The blade 18 comprises a suction surface 28 and a pressure surface 29. The suction 28 and pressure 29 surfaces define a thickness dimension of the blade.

The blade 18 transitions from a circular profile at the root 20 end to an airfoil profile moving from the root 20 of the blade 18 towards a shoulder 25 of the blade 18 which is the widest part of the blade 18 where the blade has its maximum chord. The blade 18 has an airfoil profile of progressively decreasing thickness in an outboard portion of the blade which extends from the shoulder 25 to the tip 22 of the blade 18.

Figure 3 shows the root end 20 of the blade 18, which root end 20 terminates at a root face 21. Figure 4 is an end view of a sector of the root 20. The root 20 is preferably attached to the hub 19 by stud-bolts (not shown) which may extend from or through a hub flange (not shown) into bushings 40, typically made of metal, such as steel, four of which are shown in Figure 4. Each bushing 40 extends in a longitudinal direction and has an internal axial bore 109. Each bushing 40 is embedded in a body 108 as shown in Figure 5.

Figure 5 shows an insert 105 comprising a body 108 with a bushing 40 embedded in it. Each insert 105 has a generally quadrilateral, preferably slightly trapezoidal, cross- section. In use, the inserts 105 are laid side by side in a ring around the circumference of the root 20 between inner and outer blade shell walls 41 and 42. The walls 41 , 42 preferably comprise a glass fibre reinforced composite material. In this example, wall 41 forms an outer layer of the shell 27 at the blade root 20 while wall 42 forms an inner layer of the shell 27 at the root 20. During blade manufacture, the inserts 105 may be placed in a blade shell mould and then integrated with the materials which will make up blade shell 27 through a resin infusion moulding process such as vacuum infusion. To achieve this, the inserts 105 are preferably pre-manufactured and then laid into a blade mould by positioning them on lay-up material for the shell 27. Additional lay-up material may be applied over the inserts 105 in the mould prior to infusion.

Figure 5 shows a root insert 105 having a root face end 107 and a distal end 106, and whose shape tapers down away from its root face end 107. An insert 105 may for example be made from a moulded, longitudinal blank 104 (Figure 8). A moulded blank 104 may be subdivided into two or more inserts 105. Optionally, a moulded blank 104 may comprise a fibre-reinforced body including two or more bushings 40. The blank 104, once extracted from a mould, may then be subdivided into two or more inserts 105, each insert 105 preferably including a single bushing 40. In embodiments, an insert 105 may comprise two or more bushings 40, e.g. arranged side by side in the insert body 108. In the example of Fig. 5 the insert 105 comprises a single bushing 40. In the example of Fig. 5, the insert 105 may have been obtained by bisecting a longitudinal blank 104, which blank 104 includes a bushing 40 at each end. The blank 104 may be bisected by making a diagonally extending cut through it, to leave two tapered inserts 105 as shown by way of example in Fig. 5.

Note that the side faces 143 (Figure 4) of the inserts 105 are preferably planar so that the inserts 105 can be arranged in a continuous ring with the planar side faces 143 of adjacent inserts 105 abutting each other as shown in Figure 4. If the inserts 105 have a slightly trapezoidal shape as shown, then the respective side faces 143 may be arranged side-by-side to form a circular arrangement as illustrated in Figures 3 and 4 without requiring wedge-shaped spacers between them. Optionally, adjacent inserts 105 may contact each other. Optionally, a boundary layer of material may be interposed between adjacent inserts 105. Optionally, such a boundary layer may be comprised of a fibrous material layer such as a glassfibre or polymer fibre layer. Preferably, such a fibrous boundary layer may be a thin sheet material layer.

The bushing 40 serves to both transfer loads between a hub connection element (not shown), and to transfer loads between the bushing 40 and the body 108 of the insert 105. As shown in Figure 6, the axial bore 109 of the bushing 40 extends from the outermost, or “rootmost”, end 60a of the bushing 40 axially along the bushing in a direction towards the innermost, or “tipmost”, end 60b of the bushing 40. The bore 109 comprises a female threaded portion 66 towards its rootmost end for transferring loads between a hub connection element (not shown) and the bushing 40 via stud bolts (not shown). Optionally, a frustoconical recess 65 may be located at the tipmost end of the bushing 40. This recess may receive an end of a core 62. The core 62 may be made from a variety of materials, such as polyethylene terephthalate (PET) foam, pultruded glass, glass-fibre reinforced composite material, or wood.

In embodiments, the outermost surface 59 of the bushing 40 may comprise a substantially smooth portion 61 located towards the root end 60a of the bushing 40, and a textured portion 63 located towards the tip end 60b of the bushing 40. In the illustrated embodiment, the textured portion 63 of the outermost surface 59 comprises a series of grooves 68 and peaks 69. However, any suitable texture or profile may be used.

In the embodiment of Fig. 6, the said threaded portion 66, for threadably engaging a threaded stud bolt, is provided within an axial region of said bushing 40 whose outer surface is more smooth than the outer surface of a textured portion 63 of the bushing 40. Preferably, the threaded portion 66 of said bushing 40 may be provided in an axial region of the bushing 40 which is axially offset from the bushing’s textured portion 63. As can be seen in Fig. 6, the internal bore 109 and the outer surface of the bushing 40 define a wall thickness of the bushing 40. In embodiments, the axial region of said bushing 40 which surrounds the threaded region 66 has a minimum wall thickness which is greater than a minimum wall thickness of the bushing 40 in that part of its textured region 63 which surrounds and is axially coextensive with the bore 109. Optionally, the axial region of said bushing 40 which surrounds the threaded region 66 has a minimum wall thickness which is greater than an average wall thickness of the bushing 40 in that part of its textured region 63 which surrounds and is axially coextensive with the bore 109. In this embodiment, the bushing 40 is partially surrounded by a wrap material 101 which, when cured in a resin matrix (as described below), forms a transition layer 102 (Figure 7, Figure 8) to transfer loads between the bushing 40 and the body 108 of the insert 105. Preferably, the wrap material 101 comprises layers of sheet material. Optionally, the layers of sheet material comprise fibrous material layers. Advantageously, the wrap material 101 is configured to ensure a robust connection between the bushing 40 and the insert body 108 when cured in a resin matrix.

The wrap material 101 is preferably located mainly or completely around the textured portion 63 of the outermost surface 59 of the bushing 40. The wrap material 101 may typically comprise a plurality of fibrous layers positioned one over another. A fibrous layer may in particular primarily include glass fibre material such as a glass fibre fabric layer. Each fibrous layer may be held in place by a filamentary tow (not shown) wound tightly around the fibrous layer to guide it into the grooves 68, and anchor it in position, until the transition layer 102 is formed. The fibrous material of the wrap material 101 may be dry during lay-up, or may comprise pre-impregnated fibrous material as is well known in the art. The layers of the wrap material 101 may be impregnated with resin and/or cured along with the remainder of the insert body 108. The transition layer 102 forms a part of the body 108 of the insert 105. During manufacture of an insert 105, a bushing 40 may be wrapped in layers of sheet material to form the wrap material 101 . In particular, a textured region 63 of a bushing 40 may be wrapped with sheet material. As discussed above, these layers of sheet material will later make up a so-called transition layer 102 around the bushing 40. The transition layer 102 may thereby be built up in layers on the textured surface 63 of the bushing 40. In the illustrated example according to Fig. 6, the outer surface of a bushing 40 at a rootmost region thereof may be more smooth than the outer surface of an axially offset textured region 63 thereof. Moreover, the wall thickness of a rootmost region of the bushing 40 may be greater than the wall thickness of the bushing in an axially offset textured region 63 thereof. In embodiments, the outer surface of a bushing may present a circumferential shoulder between a smooth rootmost region of the bushing and a textured region 63 thereof. In embodiments, the wrap material 101 may thereby be built up in layers on the textured surface 63 of the bushing 40 until it is substantially flush with the substantially smooth outer surface 61 of the bushing 40. In alternative embodiments, the wrap material 101 may be further built up, and extended, so that it also covers the smooth portion 61. In a further alternative, the entire outermost surface 59 of the bushing 40 may be textured such that there is no smooth portion 61. In a still further alternative, the entire outer surface 59 of the bushing 40 may be substantially smooth.

One advantage of axially offsetting a smooth outer surface region of a bushing 40 from a textured outer surface region 63 thereof may be to reduce the transmission of tensile forces between the bushing 40 and the insert body 108 in a root region thereof. This may be especially beneficial when an internal thread 66 in a bore 109 of the bushing 40 is arranged within the axial boundaries of a smooth outer surface region thereof. Such an arrangement may ensure in particular that a zone in which tensile forces are transferred between a stud bolt and a bushing 40 is axially offset from a zone in which forces are transferred between a bushing and an insert body 108. This may lower overall peak tensile forces in the bushing 40, giving it greater resilience as a fatigue component.

Figure 7 shows a schematic cross-sectional view across a blank 104 or insert 105 e.g. along plane A-A of Figure 6. As shown, the bushing 40 with its surrounding transition layer 102 is located between four elongate fibrous battens 150. The elongate fibrous battens 150 extend from the root end of the insert 105 towards the tip end of the insert 105. These extend battens 150 in particular may extend beyond the tipmost end 60b of the bushing 40. Preferably, for example, the battens 150 may also surround a core 62. Additional fibre-reinforced composite material 180 may surround the elongate fibrous battens 150 to form the finished blank 104 or insert 105. The body 108 of the insert 105 thereby comprises the transition layer 102, the elongate fibrous battens 150, and optionally an outer wrap of fibrous composite material 180.

The elongate fibrous battens 150 may comprise any suitable cross-sectional shape. Examples include circular or rectangular or oval or other polyhedral cross section shapes. In one embodiment, the elongate fibrous battens 150 may have a substantially triangular cross-section. As shown for example in Fig. 7, two sides 111 of the elongate fibrous battens 150 are substantially planar while a third side 112 is concave curved so as to substantially conform to a circular outer surface 61 of the bushing 40, and to a generally circular outer surface of the wrap material 101 . In addition, the surface area of the cross-sections need not be the same for all elongate fibrous battens 150, and any suitable number of elongate fibrous battens 150 may be used. A batten 150 having a cross sectional shape as illustrated in Fig. 7 may be known as a deltoid.

Preferably, the elongate fibrous battens 150 are pre-formed before being positioned in a mould (see Figure 8) along with other elements of an insert to form a moulded blank 104 from which an insert 105 may be derived or hewn 105. The illustrated elongate fibrous battens 150 may comprise fibrous or filamentary material in a resin matrix. These may be made by pultruding a fibrous material covered in with uncured resin through a shaping die and then curing the resin. The thereby obtained batten 150 may be used as shaped by the die, or it may be further finished e.g. machined or abraded to the required cross- sectional shape or dimensions. Alternatively, or preferably, the battens 150 may be made from fibrous or filamentary material in a resin matrix using a resin-transfer moulding (RTM) process. In particular, dry fibrous material may be placed in a mould which is then closed on all sides and infused with resin by injection which may include vacuum assisted injection. Once the dry fibrous material has become permeated with resin, a curing process is applied to the mould to harden the resin. The mould can then be opened and the hardened, shaped batten 150 can be removed. The batten 150 may be further finished, e.g. shaped, before use. The RTM process may in particular allow the generated battens 150 to have better transparency or translucency properties. Optionally, the battens 150 may comprise glass fibre composite comprising substantially uniaxial glass fibres embedded in a resin matrix. In an alternative example, the fibres of the elongate fibrous battens 150 may comprise fibre rope and/or fibre fabric e.g. glass fibre rope or fabric. The elongate fibrous battens 150 are translucent such that light can pass through them. Preferably, the elongate fibrous battens 150 are transparent such that it is possible to see through them with the naked eye. Note that throughout this description, “translucent” is used to mean a material through which light of any frequency may pass, and “transparent” is used to mean a material through which a human can see with the naked eye. Therefore, in this description, transparent material is a sub-set of translucent material.

Figure 8 shows a mould 250 suitable for forming an insert blank 104. In the example of Fig. 8, an insert blank 104 may be moulded, from which two or more inserts 150 may be generated. The mould comprises a shaped cavity 188 and shaped lid 181 and is suitable for forming the blanks 104 by a resin transfer moulding process. To form the blanks 104, a wrapped bushing 40 is placed in the mould 250 together with the pre-formed elongate fibrous battens 150. Optionally, an additional outer wrap of fabric 180 may substantially surround the wrapped bushing 40 and elongate fibrous battens 150. In embodiments, the body of the blank 104 may further include a core 60. The lid 181 is positioned over the mould cavity 188 to close the mould 250. The blank 104 can thereby be made by a resin transfer moulding process (RTM) known per se. A RTM process may include use of vacuum in a so-called vacuum assisted resin transfer moulding process. In some RTM processes, resin may be injected into a mould.

As discussed above, the fibrous material of the transition layer 102 may be pre-cured before the bushing 40 is placed into the mould 250, or - more preferably - the fibrous wrap material 101 which forms the transition layer 102 may be formed of pre impregnated uncured, or dry glass fibre material which is infused with resin during the RTM process of formation of the blank 104 for an insert 105.

Figure 9 shows a perspective view of an alternative configuration of a lay-up for a blank 104 for an insert 105 during manufacture. In this example, the bushing 40 comprises a textured outer surface 59 which extends from the tipmost end of the bushing 40 substantially to the rootmost end 60a of the bushing 40. Four elongate fibrous battens 150 are positioned around the bushing 40. In this embodiment, there is no wrap material 101 shown surrounding the bushing 40. In another alternative embodiment, the outer surface 59 of the bushing 40 may be substantially smooth along its entire length. If desired, wrap material 101 may be provided which extends partially or fully along the entire length of the textured or smooth bushing 40.

As discussed above, the integrity of the root end connection of a wind turbine blade is critical. Because the elongate fibrous battens 150 of the inserts 105 are translucent, it is possible to observe the integrity of the connection between the elongate fibrous battens 150 and the underlying layers, and thereby inspect the integrity of the blanks 104 or insert 105 emerging from a mould. Typically and preferably, the underlying layer to be inspected will be a transition layer 102 comprising the wrap material 101 in a cured resin matrix. However, in cases where there is no fibrous or filamentary wrap material 101 present in the transition layer 102, then the resin around the outermost surface 59 of the bushing 40, between said bushing 40 and the battens 150 may itself be considered to be a transition layer.

Figure 10 shows a schematic perspective view of a blank 104 or an insert 105 comprising a bushing 40 and a body 108. The body 108 includes a transition layer 102, elongate fibrous battens 150 and additionally includes an optional outer wrap of fibrous composite material 180. A visible blemish 190 is illustrated. This may e.g. correspond to a badly infused patch of fibrous material forming the wrap material 101 , sometimes known as a void. Such voids have poorer physical properties than fully infused fibrous composite regions of a fibrous composite element. In the example shown the blemish 190 is located in the transition layer 102. In this example, the blemish 190 is a void area of dry fibre where the fibres of the transition layer 102 have not been fully wetted (or infused) by the resin during the resin transfer moulding process. Such voids undermine the integrity of the finished inset 105 as they can be the source of cracks in use. Blemishes may comprise other imperfections such as cracks in the resin, air bubbles, or contaminants. It is vital that such blemishes are identified before an insert 105 goes into service in a wind turbine blade.

One method of inspecting the integrity of a blank 104 or an insert 105 may comprise a technician using his or her naked eye to inspect the blank 104 or insert 105 for blemishes by looking at the transition layer 102 through the translucent material of the elongate fibrous battens 150. If this inspection method is to be used, the process used to form the elongate fibrous battens 150 is preferably controlled so that the elongate fibrous battens 150 are substantially transparent to make visual identification of any blemishes easier. A resin transfer moulding process is particularly suitable for this purpose.

Figure 11 depicts a method of inspecting the integrity of the blank 104 or insert 105. The method comprises using a light source 200 to pass light through the elongate fibrous battens 150, and a light detector 201 to detect light reflected back through the elongate fibrous battens 150. The light source 200 may be ambient light or light derived from a bulb, LED or other suitable visible light source which is directed at the blank 104 or insert 105 to be inspected. The light detector 201 may be the human eye, or a mechanical, or electronic, light detector or any other suitable light detecting equipment. The light source 200 may comprise light outside of the range of frequencies visible to the human eye and the light detector 201 may be selected accordingly. It will be clear to the skilled person that the translucent portions of the elongate fibrous battens 150 must be translucent enough to allow the light being used for inspection to pass through the translucent portion and to be reflected back through again for detection by the detector 201.

The light detected by the detector 201 may be interpreted by a technician, or by a computer algorithm or the like, to ascertain the presence and/or size of any blemishes 190. As mentioned above, the light used to inspect the blank 104 or insert 105 need not be in the range of frequencies visible to the human eye. For example, infra-red or ultra violet light may be used.

As discussed above, a wrap material 101 may not be present. In such a case, the resin located between the elongate fibrous battens 150 and the outermost surface 59 of the bushing 40 may be considered to be a transition layer. Dry patches, cracks, air bubbles, or impurities in the resin transition layer may be detected visually or by light transmitting/detecting apparatus as described above.

It may be desirable to cover the blank 104 or insert 105 in another material such as carbon-fibre composite, or another opaque material before lay-up in the root end 20 of the blade 18. In such cases it is necessary to inspect the blank 104 or insert 105 for defects 190 in the vicinity of the transition layer and/or bushing before the opaque layer is applied.

It may be desirable to form the transition layer 102 from a material other than glass fibre composite. For example, carbon fire composite. Formation of the transition layer 102 from other materials does not affect the efficacy of the described void detection methods.

It is not necessary for the whole length of the elongate fibrous battens 150 to be translucent and/or transparent since it is only the integrity of the transition layer 102 that is of interest. In view of this, if desired, the elongate fibrous battens 150 may only comprise a translucent portion in the vicinity of the bushing 40 and/or transition layer 102. Similarly, it is not necessary that all of the elongate fibrous battens 150 comprise a translucent portion. It may be that only a portion of the bushing 40 and/or transition layer 102 needs to be inspected. In this case, only the elongate fibrous battens 150 in the vicinity of the bushing 40 and/or transition layer 102 of interest need be translucent.