Patent Publication Number: US-2023160797-A1

Title: Specimen test method

Description:
The present disclosure relates to testing of specimens, more specifically, the present disclosure relates to a test rig and a method for mechanical load testing of specimens, such as specimen comprising interfaces between composite material and other elements, such as metal elements, e.g. bolt connections. 
     BACKGROUND 
     Wind turbines blades are typically fastened to a hub of the wind turbine by bolted joints. Similarly, other connections of a wind turbine blade, e.g. modular blade joints, may rely on bolted joints. Some of these connections are subject to quite extensive stress, and therefore such assemblies must be thoroughly tested to make sure the joints are able to handle the stress. 
     Bolt connections are usually tested mechanically for quasi-static and fatigue cases using specimens with one or more bolted joints. The mechanical tests are conventionally performed by subjecting the specimen to axial loadings, while measuring response to increasing loads as well as repeated cyclic loads. 
     SUMMARY 
     It is an object of the present disclosure to provide a method and a test setup, such as a test rig, to facilitate more precise and sufficient load testing of test specimens of structural components. Particular structural components of a wind turbine blade, such as bolt connections, e.g. root bolt connections. 
     Thus, the present disclosure relates to a test rig and a method for mechanical load testing of a specimen. Particularly of a specimen extending along a longitudinal axis from a first specimen end to a second specimen end and comprising a composite material extending along the longitudinal axis from a first composite end to a second composite end and a primary elongate component extending along the longitudinal axis from a first primary component end to a second primary component end. The first primary component end forms the first specimen end. The composite material encapsulates the primary elongate component along a first interface region extending along the longitudinal axis from the second primary component end to the first composite end. 
     Accordingly, a method for mechanical load testing of the specimen is disclosed, wherein the method comprises: securing the first specimen end to a first connection part of a test rig; securing the second specimen end to a second connection part of the test rig; and applying a load to the specimen by applying compression and/or tension forces between the first connection part and the second connection part of the test rig. The applied load is provided such that it results in an axial load component and a bending moment being imposed to the specimen. 
     Also disclosed is a test rig for mechanical load testing of the specimen, wherein the test rig comprises a first connection part adapted to secure the first specimen end, and a second connection part adapted to secure the second specimen end. The test rig is operable to apply a load to the specimen by applying compression and/or tension forces between the first connection part and the second connection part of the test rig. The applied load is provided such that it results in an axial load component and a bending moment being imposed to the specimen. 
     By testing the specimens by applying a load comprising both an axial load and a bending moment, a load is applied being more in line with the actual load conditions of the component during intended use. Thereby, the test loads are closer to real use-conditions, meaning that potential failures may be prevented, and error margins may potentially be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the disclosure will be described in more detail in the following with regard to the accompanying figures. The figures show one way of implementing the present disclosure and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set. 
         FIG.  1    schematically illustrates a conventional modern upwind wind turbine, 
         FIG.  2    schematically illustrates an exemplary wind turbine blade, 
         FIG.  3    schematically illustrates a root of an exemplary wind turbine blade, 
         FIG.  4    schematically illustrates a specimen of a root connector, 
         FIG.  5    schematically illustrates a specimen of a root connector, 
         FIGS.  6 A- 6 C  schematically illustrates examples of applying a load to a specimen, 
         FIGS.  7 A- 7 B  schematically illustrates examples of applying a load to a specimen, 
         FIGS.  8 A- 8 C  schematically illustrates exemplary test rigs, 
         FIG.  9    schematically illustrates an exemplary specimen, and 
         FIG.  10    schematically illustrates an exemplary connection frame. 
     
    
    
     DETAILED DESCRIPTION 
     A method and a test rig for mechanical load testing of a specimen are disclosed. Particularly of a specimen extending along a longitudinal axis from a first specimen end to a second specimen end. The specimen may further comprise a composite material extending along the longitudinal axis from a first composite end to a second composite end and a primary elongate component extending along the longitudinal axis from a first primary component end to a second primary component end. The composite material may encapsulate, and may bond to, the primary elongate component along a first interface region extending along the longitudinal axis from the second primary component end to the first composite end. The first primary component end may form the first specimen end. 
     The second composite end may be the second specimen end. Alternatively, the specimen may comprise a secondary elongate component extending along the longitudinal axis from a first secondary component end to a second secondary component end. The second secondary component end may be the second specimen end. The composite material may encapsulate the secondary elongate component along a second interface region extending along the longitudinal axis. The secondary elongate component may be coaxial with the primary elongate component. 
     The composite material may be substantially symmetrical about the longitudinal axis. The primary elongate component may be substantially symmetrical about the longitudinal axis. The secondary elongate component may be substantially symmetrical about the longitudinal axis. The composite material, the primary elongate component and optionally the secondary elongate component may be coaxial about the longitudinal axis. The primary elongate component may be a substantially cylindrical element, such as a bolt and/or a bolt insert. The secondary elongate component may be a substantially cylindrical element, such as a bolt and/or a bolt insert. The composite material may form a substantially cylindrical element. The composite material may comprise reinforcing fibres, such as glass fibre and/or carbon fibre, suspended in a matrix of resin. The resin may be epoxy resin, polyester resin or vinyl ester resin (or other). 
     The test rig may comprise a first connection part. The first specimen end may be secured to the first connection part of the test rig. The first connection part may be adapted to secure the first specimen end. The first specimen end may be secured to the first connection part to allow rotational movement of the first specimen end, e.g. relative to the first connection part. Securing the first specimen end to the first connection part may comprise securing the first specimen end to the first connection part to allow rotational movement of the first specimen end, e.g. relative to the first connection part. The first connection part may be adapted to secure the first specimen end to allow rotational movement of the first specimen end relative to the first connection part. The test rig may be adapted to allow translational movement of the first specimen end, e.g. relative to the second connection part. 
     The test rig may comprise a second connection part. The second connection part may be formed by a floor or a common platform of the test rig. The second specimen end may be secured to the second connection part of the test rig. The second connection part may be adapted to secure the second specimen end. 
     Securing the second specimen end to the second connection part may comprise fastening the second specimen end to the second connection part, e.g. to prevent rotational and/or translational movement of the second specimen end relative to the second connection part. The second connection part may be adapted to secure the second specimen end to prevent rotational movement of the second specimen end, e.g. relative to the second connection part. Alternatively, securing the second specimen end to the second connection part of the test rig may comprise securing the second specimen end to the second connection part, e.g. to allow rotational movement of the second specimen end, e.g. relative to the second connection part. The second connection part may be adapted to secure the second specimen end to allow rotational movement of the second specimen end relative to the second connection part. The test rig may be adapted to allow translational movement of the second specimen end, e.g. relative to the first connection part. 
     A load may be applied to the specimen by applying compression and/or tension forces between the first connection part and the second connection part of the test rig. The test rig may be operable to apply a load to the specimen by applying the compression and/or tension forces between the first connection part and the second connection part. The applied load may be provided such that it results in an axial load component and a bending moment being imposed to the specimen. 
     Applying the load to the specimen may comprise applying a first force between the first connection part and the second connection part in a first direction at an angle relative to the longitudinal axis of the specimen. For example, the test rig may be operable to apply the load to the specimen by applying the first force between the first connection part and the second connection part in the first direction at the angle relative to the longitudinal axis of the specimen. 
     Applying the load to the specimen may comprise applying a second force between the first connection part and the second connection part in a second direction parallel to the longitudinal axis of the specimen. The second direction may be offset from the longitudinal axis by an offset distance. The offset distance may be between 0.1-100 mm, such as between 5 and 30 mm, such as between 10 and 20 mm. The first connection part may be adapted to secure the first specimen with an offset distance between the longitudinal axis of the specimen and the first connection part. For example, such that the test rig may be operable to apply the load to the specimen by applying the second force between the first connection part and the second connection part in the second direction parallel to the longitudinal axis of the specimen and offset by the offset distance. 
     Applying the load to the specimen may comprise applying a force distribution across the width of the first specimen end. The force distribution may be asymmetric about the longitudinal axis. For example, the test rig may be operable to apply the load to the specimen by applying the force distribution across the width of the first specimen end. 
     The test rig may comprise a first actuator and optionally a second actuator operable to apply the load to the specimen. The first connection part may form part of the first actuator. The second connection part may form part of the second actuator. The first actuator and the second actuator may be coaxial. Alternatively, the first actuator and the second actuator may be connected to a beam forming part of the first connection part. 
     The test rig may comprise a beam. For example, the first connection part of the test rig may comprise the beam. The beam may be extending between a first beam end and a second beam end. The first actuator may be attached to the first beam end. The second actuator may be attached to the second beam end. Securing the first specimen end to the first connection part may comprise securing the first specimen end to an intermediate beam position between the first beam end and the second beam end. The first connection part may be adapted to secure the first specimen by securing the first specimen end to the intermediate beam position. The first actuator and the second actuator may be operated to apply different forces, thereby imposing a bending moment in the specimen. 
     Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
       FIG.  1    schematically illustrates a conventional modern upwind wind turbine  2  according to the so-called “Danish concept” with a tower  4 , a nacelle  6  and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub  8  and three blades  10  extending radially from the hub  8 , each having a blade root  16  nearest the hub and a blade tip  14  furthest from the hub  8 . 
       FIG.  2    schematically illustrates an exemplary wind turbine blade  10 . The wind turbine blade  10  has the shape of a conventional wind turbine blade with a root end  17  and a tip end  15  and comprises a root region  30  closest to the hub, a profiled or an airfoil region  34  furthest away from the hub and a transition region  32  between the root region  30  and the airfoil region  34 . The blade  10  comprises a leading edge  18  facing the direction of rotation of the blade  10 , when the blade is mounted on the hub, and a trailing edge  20  facing the opposite direction of the leading edge  18 . 
     The airfoil region  34  (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region  30  due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade  10  to the hub. The diameter (or the chord) of the root region  30  may be constant along the entire root area  30 . The transition region  32  has a transitional profile gradually changing from the circular or elliptical shape of the root region  30  to the airfoil profile of the airfoil region  34 . The chord length of the transition region  32  typically increases with increasing distance r from the hub. The airfoil region  34  has an airfoil profile with a chord extending between the leading edge  18  and the trailing edge  20  of the blade  10 . The width of the chord decreases with increasing distance from the hub. 
     A shoulder  40  of the blade  10  is defined as the position, where the blade  10  has its largest chord length. The shoulder  40  is typically provided at the boundary between the transition region  32  and the airfoil region  34 . 
     It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub. 
     The wind turbine blade  10  comprises a blade shell comprising two blade shell parts or half shells, a first blade shell part  24  and a second blade shell part  26 , typically made of fibre-reinforced polymer. The wind turbine blade  10  may comprise additional shell parts, such as a third shell part and/or a fourth shell part. The first blade shell part  24  is typically a pressure side or upwind blade shell part. The second blade shell part  26  is typically a suction side or downwind blade shell part. The first blade shell part  24  and the second blade shell part  26  are fastened together with adhesive, such as glue, along bond lines or glue joints  28  extending along the trailing edge  20  and the leading edge  18  of the blade  10 . Typically, the root ends of the blade shell parts  24 ,  26  has a semi-circular or semi-oval outer cross-sectional shape. 
       FIG.  3    schematically illustrates a root  16  of an exemplary wind turbine blade, such as the wind turbine blade  10  as illustrated in the previous figures. 
     The root  16  comprises, at the root end  17 , a plurality of bolt inserts  42 . The plurality of bolt receivers  42  may be fitted with a plurality of bolts  44 . For simplicity, only three bolts  44  are illustrated. However, it will be understood that a bolt  44  may be provided for each bolt insert  42 . The bolt  44  may be attached to the bolt insert  42  by being threadedly connected with an internal thread inside the bolt insert  42 , e.g. a barrel nut, which may be encapsulated in the material of the root end  17  during manufacture of the root  16 . Alternatively, the bolts  44  may be embedded directly in the root  16 , e.g. by embedding bolts  44  in the root end  17  during manufacture of the root  16 . 
     The bolt inserts  42  and/or the bolts  44  are responsible for attachment of the wind turbine blade to the hub of the wind turbine. Thus, the bolt inserts  42  and/or the bolts  44  are critical components to avoid extreme failure of the wind turbine. Under dimensioned bolt inserts  42  and/or connector bolts  44  and/or weak coupling between the material of the root  16  and the bolt insert  42  and/or connector bolts  44 , could result in catastrophic failure, such as disconnection of an entire blade from the hub. 
       FIG.  4    schematically illustrates a specimen  50  of a root connector, e.g. comprising a bolt insert  42  and a bolt  44  as illustrated in relation to  FIG.  3   . The specimen  50  extends along a longitudinal axis L from a first end  50 A to a second end  50 B. The specimen  50  comprises composite material  52  extending along the longitudinal axis L from a first composite end  52 A to a second composite end  52 B. The specimen  50  comprises a primary elongate component  54 , e.g. comprising a bolt insert  42  and/or a bolt  44  as illustrated in  FIG.  3   . The primary elongate component  54  extends along the longitudinal axis L, from a first primary component end  54 A to a second primary component end  54 B. 
     As seen in the illustrated example, the first primary component end  54 A corresponds to the first specimen end  50 A. In the illustrated example, the second composite end  52 B is the second specimen end  50 B. 
     The composite material  52  encapsulates the primary elongate component  54  along a first interface region  56  extending along the longitudinal axis L from the second primary component end  54 B to the first composite end  52 A. 
     The composite material  52  and the primary elongate component  54  is substantially symmetrical about the longitudinal axis L. 
       FIG.  5    schematically illustrates an alternative specimen  60  of a root connector, e.g. comprising a bolt insert  42  and a bolt  44  as illustrated in relation to  FIG.  3   . The specimen  60  of  FIG.  5    is a symmetric specimen comprising identical ends, which may simplify some tests. Furthermore, strength tests of the specimen  60  as compared to the specimen  50  of  FIG.  4    has the advantage that two interfaces are included in the test, resulting in a more reliable test. 
     The specimen  60  extends along a longitudinal axis L from a first end  60 A to a second end  60 B. The specimen  60  comprises composite material  62  extending along the longitudinal axis L from a first composite end  62 A to a second composite end  62 B. The specimen  60  comprises a primary elongate component  64 , e.g. comprising a bolt insert  42  and/or a connector bolt  44  as illustrated in  FIG.  3   . The primary elongate component  64  extends along the longitudinal axis L, from a first primary component end  64 A to a second primary component end  64 B. The specimen  60  comprises a secondary elongate component  68 , e.g. comprising another bolt insert  42  and/or another bolt  44  as illustrated in  FIG.  3   . The secondary elongate component  68  extends along the longitudinal axis L from a first secondary component end  68 A to a second secondary component end  68 B. The secondary elongate component  68  is coaxial with the primary elongate component  64 . 
     As seen in the illustrated example, the first primary component end  64 A corresponds to the first specimen end  60 A. In the illustrated example, the second secondary component end  68 B is the second specimen end  60 B. 
     The composite material  62  encapsulates the primary elongate component  64  along a first interface region  66  extending along the longitudinal axis L from the second primary component end  64 B to the first composite end  62 A. The composite material  62  encapsulates the secondary elongate component  68  along a second interface region  69  extending along the longitudinal axis L from the first secondary component end  68 A to the second composite end  62 B. 
     The composite material  62 , the primary elongate component  64 , the secondary elongate component  68  is substantially symmetrical about the longitudinal axis L. 
       FIGS.  6 A- 6 C  schematically illustrates examples of applying a load to a specimen  50 , such as the specimen  50  as described in relation to  FIG.  4   , wherein the applied load results in an axial load component and a bending moment being imposed to the specimen  50 . 
     In the illustrated examples, the second specimen end  50 B is secured, e.g. to a second connection part of a test rig, by fastening the second specimen end  50 B to prevent rotational movement and translational movement of the second specimen end  50 B, e.g. relative to the element to which the second specimen end  50 B is attached, e.g. the second connection part of the test rig. For example, the second specimen end  50 B may be attached to a floor. In some alternative examples, the second specimen end  50 B may be secured to allow rotational and/or translational movement of the second specimen end  50 B. 
     The first specimen end  50 A is secured to a first connection part of a test rig (not shown), and the test rig is configured to apply a load between the first connection part and the second specimen end  50 B, which may be secured to a second connection part, such as a floor, of the test rig. The first specimen end  50 A may be secured to allow rotational and/or translational movement of the first specimen end  50 A. 
     The applied load is provided such that it results in an axial load component and a bending moment being imposed to the specimen  50 . For example, as illustrated in  FIG.  6 A , applying the load may comprise applying a first force F 1  in a first direction at an angle A relative to the longitudinal axis L of the specimen  50 . Alternatively, as illustrated in  FIG.  6 B , applying the load may comprise applying a second force F 2  in a second direction, e.g. parallel to the longitudinal axis L of the specimen, wherein the second direction is offset from the longitudinal axis L by an offset distance D. Alternatively, as illustrated in  FIG.  6 C , applying the load may comprise applying a force distribution F 3  across the width of the first specimen end  50 A, wherein the force distribution is asymmetric about the longitudinal axis L. 
       FIGS.  7 A- 7 B  schematically illustrates examples of applying a load to a specimen  60 , such as the specimen  60  as described in relation to  FIG.  5   , wherein the applied load results in an axial load component and a bending moment being imposed to the specimen  60 . 
     The first specimen end  60 A is secured to a first connection part of a test rig (not shown) and the second specimen end  60 B is secured to a second connection part of the test rig. The test rig is configured to apply a load between the first specimen end  60 A and the second specimen end  60 B. The first specimen end  60 A and/or the second specimen end  60 B may be secured to allow rotational and/or translational movement thereof. 
     The applied load is provided such that it results in an axial load component and a bending moment being imposed to the specimen  60 . For example, as illustrated in  FIG.  7 A , applying the load may comprise applying a second force F 2  in a second direction, e.g. parallel to the longitudinal axis L of the specimen, wherein the second direction is offset from the longitudinal axis L by an offset distance D. Alternatively, as illustrated in  FIG.  7 B , applying the load may comprise applying a force distribution F 3  across the width of the first specimen end  60 A, wherein the force distribution is asymmetric about the longitudinal axis L. 
       FIGS.  8 A- 8 C  schematically illustrates exemplary test rigs  70 ,  80 ,  90  for applying a load to a specimen, such as the specimen  50  as described in relation to  FIG.  4    and/or the specimen  60  as described in relation to  FIG.  5   , such that the applied load results in an axial load component and a bending moment being imposed to the specimen  50 ,  60 . 
     In  FIG.  8 A  the test rig  70  comprises a first connection part  72 , secured to the first specimen end  50 A of the specimen  50 , e.g. the first connection part  72  is secured to the primary elongate component of the specimen  50 . The test rig  70  comprises a second connection part  74  being secured to the second specimen end  50 B. The first connection part  72  is secured to the first specimen end  50 A by a first connection frame  100 . 
     The first specimen end  50 A is secured to the first connection part  72  of the test rig  70  to allow rotational movement of the first specimen end  50 A relative to the first connection part  72 . The second specimen end  50 B is fastened to the second connection part  74  of the test rig  70  to prevent rotational movement and translational movement of the second specimen end  50 B relative to the second connection part  74 . 
     The test rig  70  comprises an actuator  76 , configured to apply a force, e.g. a linear force. The actuator  76  is operable to apply compression and/or tension forces between the first connection part  72  and the second connection part  72  of the test rig  70 . The first connection part  72  forms part of the actuator  76 . The actuator  76  applies a force between the first connection part and the second connection part in a direction X parallel to the longitudinal axis L of the specimen  50 , wherein the direction X is offset from the longitudinal axis L by an offset distance D. Because of the offset distance D, a moment is imposed to the specimen by applying the force along the direction X. The moment experienced by the specimen  50  in the test rig  70  of  FIG.  8 A , is greatest at the second specimen end  50 B of the specimen  50  and decreasing towards the first specimen end  50 A. 
     The offset distance D is, for the purpose of illustration, exaggerated. The offset distance may be in the range of 5-30 mm, while the length of the first specimen  50  may have a length of about 0.5 m. 
     In  FIG.  8 B  the test rig  80  comprises a first connection part  82 , secured to the first specimen end  50 A of the specimen  50 , e.g. the first connection part  82  is secured to the primary elongate component of the specimen  50 . The test rig  80  comprises a second connection part  84  being secured to the second specimen end  50 B. The second connection part  84  may be provided by a floor. 
     The second specimen end  50 B is fastened to the second connection part  84  of the test rig  80  to prevent rotational movement and translational movement of the second specimen end  50 B relative to the second connection part  84 . 
     The test rig  80  comprises a first actuator  86 A and a second actuator  86 B, configured to jointly apply a force and/or a force distribution to the specimen  50 . The actuators  86 A,  86 B are operable to apply compression and/or tension forces. The first connection part  82  of the test rig  80  comprises a beam  88  extending between a first beam end  88 A and a second beam end  88 B. The first actuator  86 A is attached to the first beam end  88 A and the second actuator  86 B is attached to the second beam end  88 B. The first specimen end  50 A is secured to an intermediate beam position between the first beam end  88 A and the second beam end  88 B. 
     The first actuator  86 A and the second actuator  86 B are configured to, via the beam  88 , jointly apply a force and/or a force distribution to the specimen  50  between the first connection part  82  and the second connection part  84  of the test rig  80 . The actuators  86 A,  86 B via the beam  88  apply a load between the first connection part  82  and the second connection part  84 , and by varying the contribution of the first actuator  86 A and the second actuator  86 B to the applied load, a bending moment may be imposed to the specimen  50 . The bending moment experienced by the specimen  50  in the test rig  80  of  FIG.  8 B , is greatest at the second specimen end  50 B of the specimen  50  and decreasing towards the first specimen end  50 A. 
     In  FIG.  8 C  the test rig  90  comprises a first connection part  92 , secured to the first specimen end  60 A of the specimen  60 , e.g. the first connection part  92  is secured to the primary elongate component  64  of the specimen  60 . The test rig  90  comprises a second connection part  94  being secured to the second specimen end  60 B, e.g. the second connection part  94  is secured to the secondary elongate component  68  of the specimen  60 . The first connection part  92  is secured to the first specimen end  60 A by a first connection frame  100 . The second connection part  94  is secured to the second specimen end  60 B by a second connection frame  102 . 
     The first specimen end  60 A is secured to the first connection part  92  of the test rig  90  to allow rotational movement of the first specimen end  60 A relative to the first connection part  92 . The second specimen end  60 B is secured to the second connection part  94  of the test rig  90  to allow rotational movement of the second specimen end  60 B relative to the second connection part  94 . 
     The test rig  90  comprises a first actuator  96 A and a second actuator  96 B, configured to jointly apply a force and/or a force distribution to the specimen  60 . The actuators  96 A,  96 B are operable to apply compression and/or tension forces between the first connection part  92  and the second connection part  94  of the test rig  90 . The first connection part  92  forms part of the first actuator  96 A. The second connection part  94  forms part of the second actuator  96 B. The actuators  96 A,  96 B apply a force between the first connection part  92  and the second connection part  94  in a direction X parallel to the longitudinal axis L of the specimen  60 , wherein the direction X is offset from the longitudinal axis L by an offset distance D. Because of the offset distance D, a moment is imposed to the specimen by applying the force along the direction X. The moment experienced by the specimen  60  in the test rig  90  of  FIG.  8 C , is, because of the symmetry of the specimen  60 , greatest in the middle of the specimen  60  between the first specimen end  60 A and the second specimen end  60 B, and decreasing towards the first specimen end  60 A and the second specimen end  60 B. 
     The offset distance D is, for the purpose of illustration, exaggerated. The offset distance may be in the range of 5-30 mm, while the length of the first specimen  50  may have a length of about 0.5 m. 
       FIG.  9    schematically illustrates an exemplary specimen  60  being a symmetrical specimen, wherein the first specimen end  60 A is connected to a first connection part  92  of a test rig by a first connection frame  100 . Similarly, the second specimen end  60 B is connected to a second connection part  94  of the test rig by a second connection frame  102 . 
       FIG.  10    schematically illustrates an exemplary connection frame  100 ,  102 , such as the connection frames  100 ,  102  of  FIG.  9   , also the connection part  92 ,  94  of the test rig is illustrated. As illustrated, the connection frame  100 ,  102  comprises a specimen connection point  104 . The specimen connection point  104  is provided at a position with a distance D 1  from one side of the connection frame  100 ,  102  and a distance D 2  from the opposite side of the connection frame  100 ,  102 . The distance D 1  may be bigger than the distance D 2 . Thus, the specimen connection point  104  is offset from the centre of the frame  100 ,  102 , i.e. not coaxial with the axis of the connection part  92 ,  94 . 
     Thereby, a linear compression or tension applied via the connection part  92 ,  94  impose a bending moment in the specimen. 
     As also illustrated, the connection part  92 ,  94  is connected with the connection frame  100 ,  102  by a hinge-coupling, allowing rotation between the connection part  92 ,  94  and the connection frame  100 ,  102 , e.g. in the plane of the offset of the specimen connection point  104 . Thereby, avoiding bending moments being transferred to the test rig. 
     In the illustrated examples, the loads are illustrated as a tension force. However, it should be understood that also a compression force may be applied using the same principles. 
     The disclosure has been described with reference to a preferred embodiment. However, the scope of the invention is not limited to the illustrated embodiment, and alterations and modifications can be carried out without deviating from the scope of the invention. 
     Throughout the description, the use of the terms “first”, “second”, “third”, “fourth”, “primary”, “secondary”, “tertiary” etc. does not imply any particular order or importance, but are included to identify individual elements. Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa. 
     LIST OF REFERENCES 
     
         
           2  wind turbine 
           4  tower 
           6  nacelle 
           8  hub 
           10  blade 
           14  blade tip 
           15  tip end 
           16  blade root 
           17  root end 
           18  leading edge 
           20  trailing edge 
           24  first blade shell part (pressure side) 
           26  second blade shell part (suction side) 
           28  bond lines/glue joints 
           30  root region 
           32  transition region 
           34  airfoil region 
           40  shoulder 
           42  bolt insert 
           44  bolt 
           50 ,  60  specimen 
           50 A,  60 A first specimen end 
           50 B,  60 B second specimen end 
           52 ,  62  composite material 
           52 A,  62 A first composite end 
           52 B,  62 B second composite end 
           54 ,  64  primary elongate component 
           54 A,  64 A first primary component end 
           54 B,  64 B second primary component end 
           56 ,  66  first interface region 
           68  secondary elongate component 
           68 A first secondary component end 
           68 B second secondary component end 
           69  second interface region 
           70 ,  80 ,  90  test rig 
           72 ,  82 ,  92  first connection part 
           74 ,  84 ,  94  second connection part 
           76 ,  86 A,  96 A first actuator 
           86 B,  96 B second actuator 
           88  beam 
           88 A first beam end 
           88 B second beam end 
           100  first connection frame 
           102  second connection frame 
         L longitudinal axis 
         X force direction 
         A angle 
         D offset distance 
         F 1  first force 
         F 2  second force 
         F 3  force distribution