Abstract:
Method of bonding a shear web ( 50 ) to a wind turbine blade shell ( 75 ) and the obtained blade, wherein the shear web ( 50 ) comprises a web and a mounting flange ( 56 ) oriented transverse to the web ( 50 ). The method involves: providing a seal ( 66, 68 ) on the mounting flange ( 56 ) of the shear web ( 50 ) such that when the mounting flange ( 56 ) is positioned against the blade shell ( 75 ), a cavity ( 76 ) is defined by the seal between the mounting flange ( 56 ) and the blade shell ( 75 ). The air of the cavity ( 76 ) is then evacuated and adhesive is injected into the cavity ( 76 ). The use of pieces ( 80 ) to keep the distance between the mounting flange ( 56 ) and the blade shell ( 75 ) is preferred.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to the manufacture of wind turbine blades, and more specifically to an improved method of bonding a shear web to a wind turbine blade shell. 
       BACKGROUND 
       [0002]    Wind turbine blades, such as the long blades employed on modern utility-scale wind turbines, generally comprise a substantially hollow blade shell made primarily of composite materials, such as glass-fibre reinforced plastic. Referring to  FIG. 1 , this shows a cross-sectional view of a known wind turbine blade  10 . The blade shell is made up of two half shells, a windward shell  12  and a leeward shell  14 , which are bonded together along the leading edge  16  and trailing edge  18  of the respective half shells  12 ,  14 . Each half shell  12 ,  14  is made up of a plurality of glass-fibre fabric layers and other structural components such as foam core material  20  and carbon fibre reinforcements  22 . 
         [0003]    A pair of shear webs  24  are bonded between the respective half shells  12 ,  14 . The shear webs  24  are longitudinally-extending structures that bridge the two half shells  12 ,  14  of the blade  10  and serve to transfer shear loads from the blade  10  to a wind turbine hub in use. The shear webs  24  are I-beams, i.e. each shear web  24  is substantially I-shaped in cross section, and comprises a generally vertical web  26  disposed between upper and lower mounting flanges  28 ,  30 . The upper and lower mounting flanges  28 ,  30  are arranged transversely to the web  26  and define substantially flat surfaces for mounting the shear web  24  to the leeward and windward blade shells  14 ,  12  respectively. More specifically, the lower mounting flange  30  of each shear web  24  is bonded to a respective shear web mounting region  32  defined on an inner surface  34  of the windward half shell  12 , whilst the upper mounting flange  28  of each shear web  24  is bonded to a respective shear web mounting region  36  defined on an inner surface  38  of the leeward half shell  14 . 
         [0004]    A method of manufacturing the wind turbine blade of  FIG. 1  will now be described briefly with reference to  FIGS. 2 a   - 2   c.    
         [0005]    Referring initially to  FIG. 2 a   , this shows a mould  40  for the wind turbine blade  10  divided into two female half moulds, a windward mould  42  and a leeward mould  44 , which are arranged side by side in an open configuration of the mould  40 . The windward blade shell  12  and the leeward blade shell  14  are moulded separately in their respective mould halves  42 ,  44 . As shown in  FIG. 2 a   , the windward blade shell  12  is supported on a mould surface  46  of the windward mould  42  and the leeward blade shell  14  is supported on a mould surface  48  of the leeward mould  44 . 
         [0006]    After forming the blade shells  12 ,  14  in the respective mould halves  42 ,  44 , adhesive is applied along the leading edge  16  and trailing edge  18  of the windward half shell  12 , and/or along the leading edge  16  and trailing edge  18  of the leeward half shell  14 . Adhesive is also applied along the shear web mounting regions  32  defined on the inner surface  34  of the windward half shell  12 . Further adhesive is applied along the upper mounting flanges  28  of the shear webs  24  (see  FIG. 2 b   ). 
         [0007]    Referring to  FIG. 2 b   , once the adhesive has been applied to the various surfaces  16 ,  18 ,  28 , the shear webs  24  are then lifted into the windward half mould  42  and the lower mounting flanges  30  of the shear webs  24  are positioned against the inner surface  34  of the windward half shell  12  in the mounting regions  32 . 
         [0008]    Referring now to  FIG. 2 c   , once the shear webs  24  have been positioned against the windward blade shell  12 , the leeward mould  44 , including the leeward blade shell  14 , is lifted, turned and placed on top of the windward blade mould  42 . This process is referred to as ‘closing the mould’. Under the weight of the leeward half shell  14 , the adhesive between the respective half shells  12 ,  14  and the adhesive between the shear webs  24  and the half shells  12 ,  14  is squeezed at these respective interfaces. With the mould  40  remaining closed, the adhesive is left to cure, i.e. harden. Once the adhesive has cured, the mould  40  is then opened and the completed blade  10  is removed. The cured adhesive firmly bonds the half shells  12 ,  14  together and firmly bonds the shear webs  24  to the half shells  12 ,  14 . 
         [0009]    In the process described above, once the adhesive has been applied to the various components, it is important to load the shear webs  24  into the windward mould  42  as quickly as possible to avoid the adhesive curing partially before the shear webs  24  are correctly positioned. However, it can be difficult and time consuming to position the shear webs  24  correctly, and if the adhesive cures partially in the meantime its viscosity will increase and this may adversely affect the resulting bondlines between the shear webs  24  and the blade shells  12 ,  14 . 
         [0010]    Adhesives such as epoxy, which is typically used in the above process, can adversely react with moisture and carbon dioxide in the air in a process known as ‘carbamation’. If this happens, a greasy residue may develop on the adhesive, which may compromise the strength of the bonded joints. For this reason, it is desirable to limit the exposure of the adhesive to air, or otherwise equipment such as de-humidifiers and hot-air blowers can be used to mitigate the risk of carbamation. 
         [0011]    Epoxy and many other adhesives may also present a handling hazard to workers. It is therefore desirable to avoid or at least minimise contact with, or exposure to, the adhesive where possible. 
         [0012]    Typically several tonnes of adhesive are required to join the blades together and to bond the shear webs in place. In the process described above, a significant proportion of the adhesive is squeezed out of the bonding interfaces when the shells are brought together. This adhesive is effectively wasted, yet it still contributes to the overall weight of the completed wind turbine blade, and represents a substantial material cost. 
         [0013]    It is an object of the present invention to provide an improved method of bonding components together which avoids or otherwise overcomes one or all of the above problems. 
       SUMMARY OF THE INVENTION 
       [0014]    According to the present invention there is provided a method of bonding a shear web to a wind turbine blade shell, the shear web comprising a web and a mounting flange oriented transverse to the web, and the method comprising:
       a. positioning the shear web relative to the blade shell such that the mounting flange is in mutually opposed relation with an inner surface of the blade shell;   b. providing a primary seal between the mounting flange and the inner surface of the blade shell, the seal defining a substantially enclosed primary cavity between the mounting flange and the inner surface of the blade shell;   c. removing air from the primary cavity to create a vacuum in the primary cavity;   d. admitting adhesive into the primary cavity; and   e. curing the adhesive.       
 
         [0020]    The method of the present invention is a repeatable process that creates consistently well-defined bondlines. The bondline is defined by the dimensions of the primary cavity, which is filled with adhesive. Wastage of adhesive is eliminated because the adhesive is contained within the primary cavity and there is no ‘squeeze out’ of adhesive. Accordingly, the material cost of the adhesive and the overall weight of the blade is reduced in comparison to the prior art bonding process. 
         [0021]    As the shear web is in position before injection of the adhesive, the risk of the adhesive curing before the web is loaded into the blade and positioned correctly is eliminated. This allows more time to position the web correctly before the bonding process takes place. Also, this allows different adhesives to be used, for example a more reactive adhesive having a faster curing time can be used in the present method. 
         [0022]    As the adhesive in the present method does not make contact with the air, the risk of carbamation is avoided. Contact between operators and the adhesive is also advantageously eliminated in the present method because adhesive is injected into the sealed cavity. Accordingly, this reduces any risks to personnel associated with handling adhesive. 
         [0023]    The method may comprise providing one or more spacer elements in the primary cavity between the mounting flange and the inner surface of the blade shell. The spacer elements are preferably configured to maintain the mounting flange and the inner surface of the blade in spaced apart relation when a vacuum is created in the primary cavity. This advantageously keeps the primary cavity open and prevents the mounting flange being pulled too close to the inner surface of the blade shell. The spacers also conveniently set the bond height between the shear web and the blade shell. The spacers are preferably made from substantially incompressible material such as wood, plastic or metal. 
         [0024]    The method may involve admitting adhesive into the primary cavity via one or more adhesive ports arranged in fluid communication with the primary cavity. The method may further comprise removing air from the primary cavity through one or more vacuum ports provided in fluid communication with the primary cavity. In preferred embodiments of the invention, the adhesive ports and vacuum ports are provided in the mounting flange of the shear web. However, in other examples the ports may be provided elsewhere, for example in the primary seal or in the blade shell. 
         [0025]    The method preferably comprises monitoring a pressure in the primary cavity and determining when the cavity is full of adhesive from a sudden change in the monitored pressure. The pressure can be monitored remotely from the shear web, for example at a remotely-located vacuum pump. This advantageously allows the process to be controlled and monitored remotely. 
         [0026]    In particularly preferred embodiments of the invention, the method involves providing a secondary seal between the mounting flange and the inner surface of the blade shell. Preferably the secondary seal at least partially surrounds the primary seal, and more preferably the secondary seal completely surrounds the primary seal. The secondary seal is preferably spaced apart from the primary seal to define a peripheral cavity between the primary and secondary seals. The peripheral cavity preferably at least partially surrounds the primary cavity, and more preferably completely surrounds the primary cavity. The method preferably involves removing air from the peripheral cavity to create a vacuum in the peripheral cavity. 
         [0027]    Preferably the primary seal and/or the secondary seal are integrated with the mounting flange of the shear web prior to arranging the shear web against the blade shell. 
         [0028]    The method preferably comprises removing air from the peripheral cavity through one or more vacuum ports in fluid communication with the peripheral cavity. The vacuum ports are preferably provided in the mounting flange of the shear web, but may alternatively be provided in any other suitable position such as in the secondary seal or in the blade shell. 
         [0029]    Preferably the method comprises reducing the pressure in the peripheral cavity to a lower level than in the primary cavity so as to create a stronger vacuum in the peripheral cavity than in the primary cavity. Advantageously the vacuum in the peripheral cavity serves as a clamp around the primary seal and reacts the pressure of the adhesive injected into the primary cavity. This reduces the likelihood of the primary seal being broken by the adhesive pressure. Any air ingress in the process will also be removed via the peripheral cavity and hence away from the bondline. 
         [0030]    The method may advantageously comprise monitoring the pressure in the peripheral cavity and detecting the presence of a leak in the primary seal in the event of a sudden change in the monitored pressure. The pressure can advantageously be monitored at a location remote from the shear web, for example at a remotely-located vacuum pump, thus allowing remote monitoring and control of the process. 
         [0031]    The method preferably comprises admitting adhesive into the primary cavity once a vacuum has been created in both the primary and peripheral cavities. 
         [0032]    The method preferably comprises causing the adhesive to flow in a longitudinal direction within the primary cavity. This can be achieved by spacing the adhesive ports apart from the vacuum ports in a longitudinal direction. In other embodiments the ports may be configured to cause a chordwise flow of adhesive. 
         [0033]    The invention thus provides a method of making a wind turbine blade comprising bonding a shear web to an inner surface of a wind turbine blade shell according to the method described above. The blade shell may be a first half shell of the wind turbine blade and the method may further comprise bonding a second half shell to the first half shell. The invention also provides a wind turbine blade made according to the above method. 
         [0034]    The invention further provides a wind turbine blade comprising a blade shell and a shear web located inside the blade shell, wherein the shear web comprises a web and a mounting flange oriented transverse to the web, the mounting flange being bonded to an inner surface of the blade shell by cured adhesive within an adhesive-filled cavity bounded by a primary seal between the mounting flange and the inner surface of the blade shell. 
         [0035]    The wind turbine blade may further comprise one or more spacer elements within the adhesive-filled cavity between the mounting flange and the inner surface of the blade shell. The spacer elements are preferably made of substantially incompressible material. The mounting flange of the shear web may include one or more adhesive inlet ports and/or one or more vacuum ports. 
         [0036]    The wind turbine blade preferably comprises a secondary seal between the mounting flange and the inner surface of the blade shell. The secondary seal preferably at least partially surrounds the primary seal and more preferably completely surrounds the primary seal. The secondary seal is preferably spaced apart from the primary seal to define a peripheral cavity between the first and secondary seals. 
         [0037]    As the adhesive is contained within the primary cavity, the peripheral cavity is preferably substantially free from adhesive. 
         [0038]    The shear web preferably comprises a pair of mounting flanges that extend respectively along opposite longitudinal edges of the shear web. One of the mounting flanges is preferably bonded to the inner surface of the blade shell on a windward side of the shell, and the other mounting flange is bonded to the inner surface of the blade shell on a leeward side of the shell. 
         [0039]    The invention also provides a wind turbine having a wind turbine blade as described above. 
         [0040]    The invention also provides a shear web comprising a web and a mounting flange oriented transverse to the web, wherein one or more seals are integrated with the mounting flange. The mounting flange preferably further comprises one or more adhesive ports and/or one or more vacuum ports. 
         [0041]    Optional features described in relation to the invention when expressed in terms of a method also apply to the invention when expressed in terms of a wind turbine blade or in terms of a shear web, and vice versa. Repetition of such features has been avoided where possible purely for reasons of conciseness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIGS. 1 and 2   a - 2   c  have already been described above by way of background to the present invention. In order that the present invention may be more readily understood, examples of the invention will now be described with reference to  FIGS. 3-9 , in which: 
           [0043]      FIG. 3  is a perspective view of part of a shear web comprising a web disposed between upper and lower mounting flanges according to an embodiment of the present invention; 
           [0044]      FIG. 4  is a transparent perspective view of part of the lower mounting flange of the shear web; 
           [0045]      FIG. 5  is a plan view of an outer surface of the lower mounting flange of the shear web; 
           [0046]      FIG. 6  is a plan view of an inner surface of the lower mounting flange of the shear web; 
           [0047]      FIG. 7  is a schematic cross-sectional view of the lower mounting flange of the shear web taken along the line A-A in  FIG. 4 , with the shear web positioned against an inner surface of a blade shell; 
           [0048]      FIG. 8  is a cross-sectional view of the lower mounting flange of the shear web when sealed against the inner surface of the blade shell; and 
           [0049]      FIG. 9  shows a vacuum system and an adhesive system connected to vacuum ports and adhesive inlet ports located on the lower mounting flange of the shear web. 
       
    
    
     DETAILED DESCRIPTION 
       [0050]    Referring to  FIG. 3 , this shows a first end of a longitudinally-extending shear web  50  according to an embodiment of the present invention. The shear web  50  is approximately 80 metres in length (L) and varies in height (H) along its length, with a maximum height at one end of about 4.5 metres. The shear web  50  is substantially I-shaped in cross section and comprises a substantially vertical web  52  disposed between upper and lower mounting flanges  54 ,  56 . The mounting flanges  54 ,  56  are arranged substantially perpendicular to the web  52  in this example, i.e. substantially horizontal. 
         [0051]    The lower mounting flange  56  comprises an inner surface  58  and an outer surface  60 . The terms ‘inner’ and ‘outer’ are relative to a central longitudinal axis of a wind turbine blade in which the shear web  50  is affixed in use (e.g. the central longitudinal axis (C) of the blade  10  in  FIG. 1 , which extends perpendicular to the page in  FIG. 1 ). The outer surface  60  of the lower mounting flange  56  is shown in plan view in  FIG. 5 , whilst the inner surface  58  of the lower mounting flange  56  is shown in plan view in  FIG. 6 . The dashed lines in  FIG. 6  indicate the position of the web  52 . 
         [0052]    Referring now also to  FIG. 4 , which is a transparent perspective view of the lower mounting flange  56  in which the web  52  has been omitted for clarity, and also to the plan views of  FIGS. 5 and 6 , a plurality of vacuum ports  62   a,    62   b  and a plurality of adhesive inlet ports  64  are located on the inner surface  58  of the lower mounting flange  56 . A primary seal  66  and a secondary seal  68  are provided on the outer surface  60  of the lower mounting flange  56 . 
         [0053]    The primary and secondary seals  66 ,  68  are vacuum-tight seals and are made from butyl-rubber. The primary seal  66  is arranged in a rectangle such that a rectangular primary region  70  is defined within the primary seal  66 . The secondary seal  68  is also arranged in a rectangle and surrounds the primary seal  66 . The secondary seal  68  is spaced slightly apart from the primary seal  66 , by approximately 2 cm in this example, such that a peripheral region  72  is defined between the two seals  66 ,  68 . 
         [0054]    When the lower mounting flange  56  of the shear web  50  is positioned against a surface (see  FIG. 7 ), such as the inner surface  74  of a blade shell  75 , the primary seal  66  defines a substantially rectangular primary cavity  76  between the lower mounting flange  56  and the surface  74 , and the secondary seal  68  defines a peripheral cavity  78  surrounding the primary cavity  76 . In other words, the primary region  70  and the peripheral region  72  described above become substantially enclosed cavities  76 ,  78 . The primary cavity  76  is located directly below the web portion  52  of the shear web  50 . 
         [0055]    Referring still to  FIGS. 4-6  in addition to  FIG. 7 , the plurality of vacuum ports  62   a,    62   b  referred to above includes a first set of vacuum ports  62   a  and a second set of vacuum ports  62   b.  The first set of vacuum ports  62   a  are in fluid communication with the primary cavity  76  in use, i.e. they extend into the primary region  70  bound by the primary seal  66 . The second set of vacuum ports  62   b  are in fluid communication with the peripheral cavity  78 , i.e. they extend into the peripheral region  72  defined between the seals  66 ,  68 . The adhesive inlet ports  64  referred to above are in fluid communication with the primary cavity  76 . Notably, the first set of vacuum ports  62   a  are spaced apart from the adhesive inlet ports  64  in a longitudinal direction L (see  FIG. 1 ) of the shear web  50 . The effect of this arrangement will be described in further detail later. 
         [0056]    As shown most clearly in  FIG. 3 , the vacuum ports  62   a,    62   b  and the adhesive inlet ports  64  project upwardly from the inner surface  58  of the lower mounting flange  56  for easy access. Referring to the plan view of  FIG. 6 , the various ports  62   a,    62   b,    64  are distributed around the web  52 , which also projects upwardly from the lower mounting flange  56 . 
         [0057]    Referring now specifically to  FIG. 7 , which is a partial cross-section through the shear web  50  taken along the line A-A in  FIG. 4 , a plurality of spacers  80  are also mounted to the outer surface  60  of the lower mounting flange  56 . The spacers  80  are located inside the primary region  70  bounded by the primary seal  66 , i.e. inside the primary cavity  76  when the shear web  50  is against the surface  74 . The spacers  80  are made from substantially non-compressible material such as wood. 
         [0058]    During the manufacture of a wind turbine blade, the shear web  50  is bonded to an inner surface  74  of a blade shell as will now be described. 
         [0059]    Referring to  FIG. 9 , a first set of vacuum lines  82  (one of which is shown in  FIG. 9 ) are connected respectively to the first set of vacuum ports  62   a  and a second set of vacuum lines  84  (one of which is shown in  FIG. 9 ) are connected to the second set of vacuum ports  62   b.  The first set of vacuum lines  82  are connected to a first vacuum pump  86 , and the second set of vacuum lines  84  are connected to a second vacuum pump  88 . The first vacuum pump  86  includes a first pressure gauge  90  and the second vacuum pump  88  includes a second pressure gauge  92 . A plurality of adhesive inlet hoses  94  are each connected at one end to a respective adhesive inlet port  64  and at the other end to a source of adhesive  96 . One or more adhesive inlet valves  98  are provided in the adhesive lines  94  for controlling the flow of adhesive. One or more adhesive pumps may also be provided in the adhesive inlet lines  94  if required. The vacuum pumps  86 ,  88  and the adhesive source  96  are conveniently located remotely from the shear web  50 , for example in a process room. 
         [0060]    Referring again to  FIG. 7 , the shear web  50  is initially positioned against the inner surface  74  of the blade shell  75  and arranged such that the web  52  is located over a predefined bonding region  100  defined on the blade shell  75 , i.e. the region where a bondline between the shear web  50  and the blade shell  75  is to be created. When positioned against the inner surface  74  of the blade shell  75 , the shear web  50  is supported against the surface  74  by the primary and secondary seals  66 ,  68 . In this position the seals  66 ,  68  are compressed slightly under the weight of the shear web  50 . Notably, the thickness of the spacer blocks  80  is selected such that there is clearance  102  between the spacers  80  and the surface  74  of the shell  75  when the seals  66 ,  68  are compressed under the weight of the shear web  50 . 
         [0061]    Referring now additionally to  FIG. 8 , with the shear web  50  in position against the surface  74  of the blade shell  75 , the vacuum pumps  86 ,  88  are turned on. The first vacuum pump  86  draws air out of the primary cavity  76  through the first set of vacuum lines  82  and the second vacuum pump  88  draws air out of the peripheral cavity  78  through the second set of vacuum lines  84 . The removal of air from the primary and peripheral cavities  76 ,  78  creates an effective vacuum within each of the cavities  76 ,  78 . The creation of a vacuum within the respective cavities  76 ,  78  causes the lower mounting flange  56  to be pulled closer to the inner surface  74  of the blade shell  75 , which compresses the primary and secondary seals  66 ,  68  further and causes them to seal tightly against the surface  74 . 
         [0062]    The lower mounting flange  56  is pulled towards the surface  74  until the spacer blocks  80  make contact with the surface  74  of the shell  75 . As the spacers  80  are incompressible, the lower mounting flange  56  is prevented from being pulled any closer towards the inner surface  74  of the blade shell  75 . The spacers  80  thereby ensure that the primary cavity  76  remains open, i.e. that a clearance is maintained between the lower mounting flange  56  and the inner surface  74  of the blade shell  75  when a vacuum is established in the respective cavities  76 ,  78 . 
         [0063]    As the primary cavity  76  and the peripheral cavity  78  are sealed independently of one another, and are connected to independent vacuum systems, two distinct vacuum zones are created in the primary and peripheral cavities  76 ,  78  respectively. The vacuum pumps  86 ,  88  are configured to maintain a slightly stronger vacuum in the peripheral cavity  78  than in the primary cavity  76 , or in other words a slightly lower pressure in the peripheral cavity  78  than in the primary cavity  76 . In this example, a vacuum pressure of approximately −1 bar is maintained in the peripheral cavity  78  and a slightly higher pressure is maintained in the primary cavity  76 . Accordingly, the vacuum in the peripheral cavity  78  pulls the lower mounting flange  56  of the shear web  50  at a higher level than the primary cavity  76  and functions as a clamp around the primary cavity  76 . 
         [0064]    Once vacuums have been established in the respective cavities  76 ,  78 , adhesive is admitted into the primary cavity  76  via the adhesive inlet ports  64 . As mentioned previously, the adhesive inlet ports  64  are spaced apart from the first set of vacuum ports  62   a  in the longitudinal direction L of the shear web  50 . Accordingly, the adhesive flows or ‘infuses’ in a longitudinal direction L inside the primary cavity  76  from the adhesive inlet ports  64  towards the first set of vacuum outlet ports  62   a.  As the adhesive is drawn into the primary cavity  76  under vacuum, the injection back pressure is minimised and the primary cavity  76  fills with adhesive. 
         [0065]    The vacuum pressures in the primary and peripheral cavities  76 ,  78  are monitored throughout the adhesive infusion process using the vacuum gauges  90 ,  92  associated with the vacuum pumps  86 ,  88 . Once the primary cavity  76  is full of adhesive, the pressure inside the primary cavity  76  will suddenly change (i.e. the vacuum gauge  90  will suddenly register a very high negative pressure of, for example, around −200 bar). This sudden change in pressure indicates that the primary cavity  76  is full of adhesive, i.e. the adhesive injection process is complete. At this stage, the adhesive supply is turned off by closing the adhesive inlet valve(s)  98  to prevent further adhesive entering the primary cavity  76 . 
         [0066]    The adhesive is then left to cure, i.e. harden, which results in the lower mounting flange  56  forming a strong bond to the inner surface  74  of the blade shell  75 . 
         [0067]    As the peripheral cavity  78  pulls at a higher level than the primary cavity  76 , the peripheral cavity  78  advantageously reacts the pressure from the injected adhesive at the adhesive inlet ports  64  and over the bond area and thus prevents the injected adhesive from forcing the two surfaces  60 ,  74  apart and breaking the primary seal  66 . In other words, the evacuated peripheral cavity  78  serves to prevent leaks developing at the primary seal  66 . In the unlikely event that the primary seal  66  does develop a leak, adhesive will flow into the peripheral cavity  78 . This will cause a sudden change in pressure in the peripheral cavity  78 , which can be detected by the pressure gauge  92  of the vacuum pump  88  associated with the peripheral cavity  78 . If a leak is detected then it may be necessary to suspend the infusion process until the leak has been repaired. 
         [0068]    A further advantage of the stronger vacuum in the peripheral cavity  78  is that any air ingress in the process will be removed from the system away from the bondline because the bondline is created within the primary cavity  76 . 
         [0069]    As the adhesive is contained within the primary cavity  76 , wastage of adhesive caused by squeeze out is eliminated. This reduces the overall amount of adhesive required in the bondline and hence reduces the overall weight of the completed blade and the materials cost of the adhesive. The dimensions of the resulting bondline between the shear web  50  and the blade shell  75  are advantageously well defined, i.e. they are defined by the dimensions of the primary cavity  76 . The dimensions of the primary cavity  76  are predefined by the shape of the region  70  bound by the primary seal  66  and the height of the spacers  80 . Accordingly, the invention provides a repeatable process for creating consistently well-defined bondlines. 
         [0070]    As the process can be controlled and monitored by the remotely-located vacuum gauges  90 ,  92 , the entire bonding process can be controlled and monitored remotely from the blade. This is particularly advantageous and facilitates a single-stage bonding process, i.e. in which the shear web  50  is bonded to both the windward half shell and the leeward half shell simultaneously using simultaneous infusion at each bondline. Such a process is made possible with the bonding method of the present invention because there is no requirement to have direct access to the bondline during the bonding process. 
         [0071]    Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. 
         [0072]    For example, whilst the above examples relate to the creation of a bond between the lower mounting flange  56  of the shear web  50  and the blade shell, the bond could alternatively or additionally be created between the upper mounting flange  54  and the blade shell. As mentioned above, the process could be used simultaneously along the upper and lower mounting flanges  54 ,  56  to bond the shear web  50  to both the windward shell and the leeward shell simultaneously. The method would therefore involve closing the mould prior to bonding the shear web  50  to the respective blade shells. Accordingly, the method lends itself to a single-stage bonding process. 
         [0073]    Whilst a shear web  50  having an I-shaped cross section is described in the above examples, the shear web  50  may have a different shape in other examples. For example, the shear web  50  may be substantially C-shaped in cross-section or the web  52  may have L-shaped flanges at each end. The mounting flanges  54 ,  56  need not be perpendicular to the web  52 , and in other examples the mounting flanges  54 ,  56  may be arranged at other transverse angles to the web  52 , for example more or less than ninety degrees. The angle of the mounting flanges  54 ,  56  relative to the web  52  will depend on the local contour of the blade shell at which the shear web  50  is to be fixed, i.e. in a chordwise direction of the blade shell. 
         [0074]    Whilst in the above examples the adhesive inlet ports  64  and the vacuum ports  62   a  are arranged so as to create a longitudinal, i.e. spanwise flow of adhesive in the primary cavity  76 , the ports  62   a,    64  may alternatively be arranged to create a chordwise flow for example. In this case, the resin inlet ports  64  may be spaced apart from the vacuum ports  62   a  in a widthwise direction of the primary cavity  76 . 
         [0075]    Whilst the various ports  62   a,    62   b,    64  in the above examples are provided in the mounting flange  56  of the shear web  50 , the ports  62   a,    62   b,    64  may alternatively be provided in other suitable positions. For example, the ports  62   a,    62   b,    64  may be provided in the seals  66 ,  68  or in the blade shell or other such surface to which the shear web  50  is bonded. 
         [0076]    Whilst the above examples relate to the creation of a bond between a shear web  50  and a wind turbine blade shell  75 , various features of the invention (for example the double seal arrangement) may be utilised for other bonds, such as the bonds between respective half shells of a wind turbine blade. 
         [0077]    For the avoidance of doubt, relative terms such as ‘upper’ and ‘lower’ as used in the preceding description are used for convenience and refer to the orientation of features as shown in the figures. These terms are not intended to limit the scope of the invention.