The present invention is directed towards a shearography inspection apparatus. The disclosed apparatus is particularly well adapted for complex applications, such as the inspection of non-horizontal or non-stationary surfaces. For instance, preferred implementations of the apparatus can be attached onto the surface of a large engineering component such as a wind turbine blade via a robotic deployment mechanism to conduct non-destructive testing of the engineering component in a remotely controlled manner. The apparatus can be used for testing of any material but is particularly useful in the inspection of composite materials.

Shearography, also known as speckle pattern shearing interferometry, is an optical inspection method widely used for inspection of engineering components on the ground. Digital shearography uses impingement of laser light on a material surface to generate interference (‘speckle’) patterns which are captured by image sensors (e.g. charge-coupled device [CCD] or complementary metal- oxide [CMOS] semiconductor sensors). Speckle patterns of the material surface are compared in different load states (i.e. with and without load) using image analysis algorithms, permitting detection of surface and subsurface defects such as debonds, delamination, cracks, voids, wrinkles, impact damage, fluid ingress and so forth. Advantages of digital shearography include the fact that it is non- contact, not of particularly high apparatus or operational cost, provides immediate signal acquisition, safe (no ionising radiation, chemicals or gaseous emissions), and capable of inspecting relatively large areas. Further background relating to digital shearography can be found in the papers ‘Zhao, Q.; Dan, X.; Sun, R; Wang, Y; Wu, S.; Yang, L. Digital Shearography for NDT: Phase Measurement Technique and Recent Developments. Appl. Sci. 2018, 8, 2662’, and ‘Yang LX, Hung YY; Digital shearography for nondestructive evaluation and application in automotive and aerospace industries. Proceedings of the 16th WCNDT, Montreal (2004). Digital shearography is a particularly useful technique for inspection of composite materials, such as honeycomb composites and polymer matrix composites reinforced by fibres. In the wind power industry, shearography has been used to inspect wind turbine blades (WTBs) during the manufacturing stage and after transportation. As the WTBs are on the ground at these times, the requirement of a stable working condition for shearography can be readily met, hence shearography can work properly. However, after the WTBs are installed on a wind tower, inspection of the WTBs on-site becomes very challenging due to the perpetual vibration of the WTB in the sky even when the wind speed is low. This is because shearography as an interferometric technique, requiring a stable working condition where the relative vibration between the shearography system and the target object under inspection is preferably less than a few tens of the wavelength of the laser light used. In other words, the relative vibration should preferably be less than 10-20 micrometres. However the vibration of the WTBs are usually in the range of a few centimetres to meters, depending on the size and location of the WTB, e.g. the tip of the WTB vibrates significantly more than the root area of the WTB. As a result, inspection of WTB using existing shearography systems available on the market has never been reported. As the wind energy sector has been growing rapidly, more and more WTBs need regular inspection and maintenance.

Similar issues arise in the inspection of other large engineering components, such as the wings of aircraft, which are typically designed to allow a degree of flexing relative to the aircraft body and hence exhibit vibrational movement during testing.

Known shearography systems typically comprise a source of coherent light, optical shearing means, image acquisition means and loading means, as well as the concomitant power supply, optics, electronics and mounting hardware to support these. Coherent light is usually in the form of laser light and will be referred to below as such. The optical shearing means can comprise any device which is able to bring light scattered from two different points on the surface being tested to the same single point on the image sensor or other acquisition means. For instance, the optical shearing means may be implemented via an optical wedge, prism or interferometer (such as a modified Michelson Interferometer). Image acquisition means can be via CCD, CMOS or other (two- dimensional) imaging sensor. The loading means (which may be provided as an integral part of the shearography apparatus, or separately) are used to put the component being inspected in a state of stress, using thermal, mechanical or other excitation. Thermal excitation is via some form of heating, such as a hot air gun, whilst mechanical excitation can be provided by vacuum, bending or vibration.

For industrial deployment, the laser source, optical shearing device and image sensor are typically packaged in such a way that the laser light is emitted directly from an exit window of the apparatus housing, onto the surface of material to be inspected, and then captured via an imaging camera also within the apparatus housing. Due to the nature of laser optics, the area of material inspected in each inspection cycle is dependent upon the size of the area that can be illuminated by the laser source, as well as the imaging area of the camera. Generally the angle of view of the camera is limited (usually less than 30°) and/or the angular spread of the laser light is small, with the result that it is necessary to maintain a relatively large stand-off distance between the shearography apparatus and the target surface, ideally 1 to 4m. When the target component is positioned horizontally on the ground, this stand-off distance can be maintained relatively easily by an apparatus with sufficient stability placed on the target surface. However, when the shearography apparatus is deployed onto a wind tower, the WTB can only be practically positioned vertically parallel to the wind tower. This means that the shearography system should be also positioned approximately vertically, at a relatively large distance (1 to 4m) from the WTB. Due the constant vibration of the WTB as mentioned above, in order to minimise the relative motion between the shearography and the target surface the shearography apparatus would desirably be attached to the WTB surface. However, to attach a relatively heavy (3 to 5kg) and sensitive shearography package of laser/optical/camera components at a relatively large distance from the WTB is highly challenging. Such a deployment is also likely to have a high tendency of flip over, to be unstable and have high strength requirements in whatever arm or construction is used to hold the shearography components package the required distance from the inspected surface. This is not an optimal arrangement and is likely to lead to overly specified and heavy apparatus which will be susceptible to vibration and likely poor inspection performance.

It is an object of the aspects of the present invention to provide a shearography inspection apparatus that meets the needs of the applications and industries mentioned above and overcomes or at least reduces one or more of the problems mentioned. Desirably, the apparatus should be robust, flexible, lightweight and capable of capturing high resolution correlated speckle images during inspection over relatively large inspection areas.

Aspects of the present invention provide a (portable) shearography inspection apparatus particularly suitable for inspecting a large engineering component positioned at disadvantageous orientations (vertical, upside-down etc.). The shearography inspection apparatus comprises: a laser source, optical shearing means, image acquisition means, wherein said laser indirectly illuminates the material surface.

In accordance with the present invention, a shearography inspection apparatus for inspection of a target surface, comprises: a coherent light source configured to illuminate an inspection area of the target surface via a first optical path; an optical shearing device configured to receive light reflected from the inspection area of the target surface via a second optical path; and an image acquisition device; wherein the optical shearing device is configured to direct light received from two different points on the target surface to the same point on the image acquisition device; and further comprising a light redirection assembly located in the first and second optical paths.

By providing a light redirection assembly in this way, the optical path length is not limited to the physical distance between the body of the apparatus (which includes the light source, shearing unit and image acquisition device). This enables the body to be fitted close to the target surface, doing away with the need for any special arm or construction for holding the apparatus at a sufficient distance from the object. Rather the apparatus can be securely affixed to the target surface (by gravity if the surface is horizontal, or via suction pads or the like if not) with much reduced risk of tipping over or other movement relative to the object. This allows the shearography system to be deployed to a wind tower or other object by a robotic system to conduct inspection remotely by inspectors working on the ground (for an on-shore wind turbine, for example) or on a vessel (for an offshore wind turbine, for example). The attachment can be achieved by a gripping or climbing robot secured directly onto the surface to be tested, by means of e.g. vacuum suckers, caterpillar tracks etc. Apart from use on wind turbines, such robotic systems are useful when inspecting other large engineering components, such as the wings of aircraft.

Preferably the light redirection assembly is configured such that the lengths of the first and second optical paths are greater than the distances between the coherent light source and the target surface and between the optical shearing device, respectively. This allows the inspection area to made larger than would otherwise be possible with the close positioning of the apparatus to the target surface.

Advantageously, the coherent light source is configured to emit light in a direction away from the target surface in use, and the light redirection assembly is adapted to receive light from the coherent light source and to redirect the light towards the target surface in use. It should be noted that “a direction away” from the target surface means any direction including a component parallel to the surface normal, proceeding from the start point away from the target surface (not towards it). In some embodiments this direction may be parallel to the surface normal but this is not essential. Preferably, the light redirection assembly is adapted to receive light reflected from the inspection area of the target surface in use and to redirect the light towards the optical shearing device.

The light redirection assembly could take any form, including for instance a waveguide structure or fibre optics (means being provided to maintain stability thereof), provided the dimensions of the assembly allow for the expansion/contraction of the beam diameter along the optical paths.

In particularly advantageous embodiments, the light redirection assembly comprises one or more reflective elements, configured to reflect light from the coherent light source to the target surface and from the target surface to the optical shearing means in use., wherein preferably the same reflective element is configured to reflect light from the coherent light source to the target surface and from the target surface to the optical shearing means in use. The use of the same reflective element simplifies manufacture and control of the apparatus. Advantageously, the or each reflective element is: a mirror, a prism, or any other reflective surface.

Preferably, the laser source is indirectly applied to (e.g. focused on) the material surface via an extra (i.e. located in the first and second optical paths rather than as part of the individual laser/camera optics packages in a housing of the apparatus) reflective mirror (forming the light redirection assembly). The laser source may be initially directed away from the target material surface toward the mirror, which then reflects the light on to the surface. The laser illuminated surface of the material is then imaged by the shearography camera (the image acquisition device) via the same extra mirror and where the image shearing means is positioned in front of the camera. The use of an extra mirror allows a sufficient imaging distance (e.g. 1 to 4m) when the main shearography body is positioned in proximate to the material surface under inspection; hence facilitating the attachment of the shearography system to the material surface such that the tendency of flip over is minimal.

The light redirection assembly could be fixed such that the position and size of the inspection area on the target surface is also fixed. However, in preferred embodiments, the assembly further comprises a light redirection control device for adjusting the light redirection assembly so as to move the inspection area on the target surface relative to the apparatus. In this way the size and/or position of the inspection area can be changed without needing to move the apparatus as a whole. This can be implemented, for instance, by mounting reflective element(s) making up the light redirection assembly so that they can pivot or rotate about one or more axes, or translate in one or more directions.

Preferably the lengths of the first and second optical paths are in the range 1 to 4 metres for the reasons discussed above.

In advantageous implementations, the coherent light source, optical shearing device and image acquisition device are located in a housing of the apparatus, and the light redirection assembly is located partially or entirely outside the housing. For instance, where the light redirection assembly comprises a reflective element, the reflective element may be held by an extension arm at a suitable distance away from the housing. Since such an arm and reflective element (e.g. mirror) structure will be relatively light, this does not cause significant unbalance to the apparatus.

Preferably, the housing is affixed to a rig configured to attach the apparatus to the target surface in use, the rig preferably further comprising a transport device configured to move the apparatus along the target surface in use. For instance, the rig may include an attachment device such as suction pads for fixing the apparatus to the target surface. The transport device may include caterpillar tracks, wheels, rollers or similar.

Advantageously, the optical redirection assembly is configured such that the first and second optical paths each include a portion extending from the housing in the direction away from the rig to the optical redirection assembly and a portion extending from the optical redirection assembly in the direction towards the rig to the target surface.

Most preferably, the coherent light source is a laser. Any of the laser sources mentioned in the papers cited above can be utilised, for instance.

Advantageously, the optical shearing device comprises any of: an optical wedge, a prism or an interferometer, preferably a Michelson interferometer or a Mach Zehnder interferometer. The papers cited above provide details of suitable preferred implementations.

Preferably, the image acquisition means comprises a camera, a charged coupled device (CCD) sensor array or a complementary metal-oxide semiconductor (CMOS) sensor array. Again, particular examples can be found in the papers cited above.

Advantageously, the apparatus further comprises a data memory configured to receive and store images acquired by the image acquisition device and/or a processor configured to receive images acquired by the image acquisition device and carry out comparison thereof to output shearography data. Hence, the data processing can either be carried out on board the apparatus or, as may be preferable due to the amount of processing power required, the images can be stored and retrieved later for processing. Preferably the apparatus further comprises a communications module configured to communicate data to and/or from an external device; the communications module preferably being wireless. If the communications device is wireless (or a sufficiently long wired connection is practicable), the data can be retrieved in real-time (or near to) for analysis.

The apparatus may optionally further comprise a loading device (for applying a load to the object), preferably a thermal loading device such as a hot air gun, an infrared lamp, a heated plate, an induction heater, or a laser heater. Alternatively the necessary loading could be performed by a separate entity.

As already mentioned, preferably the apparatus further comprises a transport device configured for movement of the apparatus across the target surface, the transport device preferably comprising a track-driven climbing robot with vacuum suction cups. Any system capable of carrying the shearography apparatus safely and securely to every position of the surface to be inspected and manoeuvring through remote or autonomous control could be used to implement the transport device.

Advantageously, the apparatus further comprises a remote control unit configured to control the shearography inspection apparatus in response to commands received from an external device.

Thus, preferably, the shearography apparatus is robotically deployed. Being robotically deployed over a large structure/object, that is a structure which has a surface area many times larger than the inspection area which can be inspected by a single shearography image acquisition cycle, the shearography module is required to move over the surface to be inspected and carry out multiple image acquisition cycles in different locations on the surface. A robotically deployed shearography system is particularly beneficial when inspecting large structures which are positioned in disadvantageous orientation such as vertical and up- side-down, dangerous to inspect or otherwise overly burdensome for manual inspection. Wind turbine blades have been shown to be ideal for inspection using shearography but are typically hard to reach on a wind tower due to their usual height from the ground and dangerous to inspect due to conditions local to their placement (e.g. offshore).

The invention therefore further provides a shearography inspection system comprising a shearography inspection apparatus as described above and a remote control module for communicating commands to the apparatus.

Also provided is a shearography inspection assembly comprising a shearography inspection apparatus as described above and an object having a target surface for inspection, the object being any of a wind turbine blade, an aircraft fuselage or wing, a turbine blade, a ship hull or superstructure, a vehicle, a roof, a dome, a wall or any other mechanical or architectural structure.

The invention also provides a method of shearographic inspection of an object, comprising:

(a) placing a shearography inspection apparatus as described above on a target surface of the object;

(b1) using the shearography inspection apparatus to obtain a first interferometric image of a first inspection area of the target surface;

(b2) changing the load conditions on the object;

(b3) under the changed load conditions, using the shearography inspection apparatus to obtain a second interferometric image of the first inspection area of the target surface; and

(b4) comparing the first and second interferometric images of the first inspection area.

In step (a), the apparatus (e.g. a robot) can be deployed on the object (e.g. wind turbine blade) by several methods such as by a climbing mechanism, crane (nacelle, platform, self-hoisting, tower climbing and such like), aerial winch or drone. Of course, if the object is at ground level and is horizontal, the apparatus can simply be manually lifted and put on top of the surface. It should be noted that the term “on” in step (a) above does not imply any particular relative orientation of the target surface and the apparatus: for instance the apparatus can be attached on a vertical surface of the object or indeed on an underneath surface, using appropriate attachment mechanisms.

If the object is a WTB, preferably, the robot is placed on the blade whilst the blade is in the vertical position (parallel to the turbine tower) and the robot fixes onto the blade (e.g. by suction cups or clamps). The inspection can be carried out by then moving the robot to various positions on the blade.

Thus preferably the method further comprises, after step (b3):

(c) moving the shearography inspection apparatus or adjusting the light redirection assembly;

(d1) using the shearography inspection apparatus to obtain a first interferometric image of a second inspection area of the target surface;

(d2) changing the load conditions on the object;

(d3) under the changed load conditions, using the shearography inspection apparatus to obtain a second interferometric image of the second inspection area of the target surface; and

(d4) comparing the first and second interferometric images of the second inspection area.

As already mentioned, step (a) and, if performed, step (c) advantageously comprises affixing the shearography inspection apparatus to the target surface, preferably by clamping and/or suction. This is especially the case if the surface is non-horizontal or an underneath surface.

Preferably, the object is any of: a wind turbine blade, an aircraft fuselage or wing, a turbine blade, a ship hull or superstructure, a vehicle, a roof, a dome, a wall or any other mechanical or architectural structure. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures in which:

Figure 1 shows in perspective view a schematic illustration of a shearography inspection apparatus according to an embodiment of the invention, on a surface of an object to be inspected;

Figure 2 shows a side view of the shearography inspection apparatus of Figure 1 ;

Figure 3 schematically depicts selected components of the shearography inspection apparatus of Figure 1 ;

Figure 4 illustrates a first optical path of the shearography inspection apparatus of Figure 1 ; and

Figure 5 illustrates a second optical path of the shearography inspection apparatus of Figure 1.

Figure 1 shows, in perspective view, an embodiment of a shearography inspection apparatus 10 on a surface S to be inspected, Figure 2 showing the same arrangement in side view. It should be noted that the plane x-y in which the surface S lies could have any orientation and in many implementations will not be horizontal. The surface S could also be an underneath surface of an object. The surface S may be flat, curved or any other shape. The surface S could be a surface of any object but typically is of a large engineering component such as a wind turbine blade (WTB), aircraft wing or similar.

In Figure 1 , the path of laser light emitted from the apparatus to the inspected material surface (the “first optical path”, I) and the imaging path from the material surface to the camera inside the shearography box (the “second optical path”, R) are highlighted. The inspection area A is also shown with its periphery in dashed lines. In practice, of course, neither the optical paths I, R, nor the periphery of the inspection area A will necessarily be visible.

Figure 2 shows the shearography apparatus 10 by a robotic mechanism (described below) on to a large component, such as a WTB, on a vertical orientation (i.e. in this example, the x axis is vertical).

As shown in Figures 1 and 2, in this embodiment the main body (or housing) 1 of the shearography apparatus 10 contains the laser source, optical components (image shearing means), image sensor and associated electronics. This will be described in more detail below. An extra mirror 5, forming the light redirection assembly in this embodiment, enables the laser source and imaging camera to be directed to the inspection area A. Laser exit window 2 emits a laser beam which is reflected by mirror 5 to fall on an inspected material surface S as laser illuminated region 8. The camera in the shearography main body 1 captures an image of the inspection area A through the window 3 via the mirror 5. The mirror 5 is mounted on a mounting arm 5a.

In this embodiment, a robotic mechanism (comprising rig 6, tracks 7 and suction pads 8) enables the shearography main body 1 and the extra mirror 5 to be attached to the target surface S. A heating source 11 is also mounted on the mounting arm 6, such as an infrared radiation source. The radiation from heating source 11 may also be directed onto surface S by mirror 5. Alternatively, the heating source could comprise a hot air gun, such as a Steinel HG2220E which operates at 2200W, 80 - 630 °C, airflow rate 500 l/min.

In more detail, the apparatus 10 comprises a main body or housing 1 , inside which the key optical and control components are located, which is mounted to a rig 6. The rig 6 is provided with means for attaching the assembly to a surface S to be inspected, which in this embodiment take the form of suction cups or suction pads 8 which are disposed on retractable actuating arms and can be moved towards and away from the surface S under control of a motor (not shown). In other embodiments, the attachment means could instead comprise any of: clamps, chains/tie wires, gripping arms, pre-installed fixation points (e.g. hook and eye; magnetic fixing points; tracks. The rig 6 in this example is also provided with a transportation device, here in the form of caterpillar tracks 7, for moving the apparatus 10 along surface S. The tracks 7 are powered by an on board motor (not shown).

The interior of housing 1 is shown schematically in Figure 3. Housed therein are a coherent light source 20, an optical shearing device 30, an image acquisition device 40 and (optionally) control means 50 which may include any of: a data storage unit (memory), a data processor, a power source, and/or a communications module.

The coherent light source 20 comprises a laser source 21 and, optionally suitable optics for emitting the laser beam. The coherent laser light is output along a first optical path I through window 2. The first optical path I will be described further below but ultimately it directs the laser light onto an inspection area A on the surface S under test. Light reflected from the inspection area A travels along second optical path R and is received by optical shearing device 30 through window 3 (which again may or may not include an optical component). In this example, the optical shearing device 30 comprises a modified Michelson interferometer. However, any other type of optical shearing device, which is capable of superimposing light from two spaced points on the area A onto a single point, can be used instead.

The interferometer comprises a beam splitter 31 (such as a half silvered mirror) which divides the incoming light into two parts. The reflected part is directed onto mirror 32, which is arranged perpendicular to the optical axis and hence reflects the light through 180 degrees, such that it form an image on the image acquisition device 40. The image acquisition device 40 may comprise a CCD array or similar. Simultaneously, the transmitted part of the incoming light is passed by beam splitter 31 to another mirror 33, which is arranged at a small non-perpendicular angle to the optical axis of the light striking it. Mirror 33 therefore reflects the light to a slightly shifted position on the beam splitter 31 (shown by dashed lines), which reflects it onto image acquisition device 40, forming another image of inspection area A which is slightly shifted relative to the first. In effect, light from two different points on the surface S has been directed onto the same point on image acquisition means 40. The two simultaneously obtained images interfere with one another such that image acquisition device 40 receives an interference pattern (or speckle pattern), rather than an actual direct image of area A. The received image (or pattern) can then be stored in memory provided by control means 50.

Figures 4 and 5 show respectively the first and second optical paths I, R, in more detail. In each case, only the components of the shearography inspection apparatus which form part of the paths are shown, for clarity. The first optical path I, is that followed by the light from source 20 (inside housing 1), through window 2, to the inspection area A on surface S. Unlike conventional systems, here an optical redirection assembly (comprising in this example mirror 5) is located between the source 20 and the area A in the first optical path. Thus, light is emitted from housing 1 through window 2 in a direction away from the surface S (i.e. in a direction having a component parallel to the z axis), along a first portion of the first optical path with edges denoted as h, l ₂ , in Figure 4. The light is incident on mirror 5, which is held by support arm 5a further from the surface S than the housing 1. The mirror 5 reflects the light back towards the surface S, thereby illuminating inspection area A on the surface S. This second portion of the first optical path has its edges denoted by l ₃ and U in Figure 4. The overall length of the first optical path (i.e. adding first and second portions together) is preferably in the range 1 to 4 meters so as to form a sufficiently large inspection area A.

The second optical path R, is that followed by the light reflected by inspection area A, through window 3, to the optical shearing device 30 (inside housing 1). Again, the optical redirection assembly (comprising in this example mirror 5) is located between the area A and the optical shearing device 30 in the second optical path. Thus, light is reflected from area A towards mirror 5 (i.e. in a direction having a component parallel to the z axis), along a first portion of the second optical path with edges denoted as Ri, R ₂ , in Figure 5. The light is incident on mirror 5, which reflects the light back towards the housing 1, where it is received by optical shearing device 30 through window 3. This second portion of the second optical path has its edges denoted by R ₃ and R in Figure 5. The overall length of the second optical path (i.e. adding first and second portions together) is also preferably in the range 1 to 4 meters (and is preferably substantially the same length as the first optical path).

It should be appreciated that the optical redirection assembly 5 could be implemented in different ways and with any appropriate optical components able to guide the light in the direction(s) required. For instance, the mirror 5 could be replaced by other reflective component(s) such as prisms, or optical fibres could be used provided the beam is able to expand. The optical redirection assembly is preferably configured so as to make the length of the first and second optical paths greater than the distance between the shearography apparatus and the surface to which it is attached, as can be seen from Figures 1 and 2 (where the apparatus is only spaced from the surface by the height of caterpillar tracks 7).

As shown in Figure 1 , the mirror 5 may be movably mounted so that it can be pivoted and/or translated. This allows adjustment of the first and second optical paths enabling the size and position of inspection area A to be changed. Appropriate actuators for moving the mirror 5 will be provided in this case.

Having captured a first image (interference pattern) using the process described above, the load conditions on the object are changed. For example, this could be done using a loading device such as thermal source 11 mentioned above which is provided on apparatus 10. Alternatively, the loading means could be provided separately. In the present example, the load conditions are changed by switching on or off the thermal source 11 , causing the temperature of the inspection area A of the surface S to change and thereby changing the stress experienced by the material. A second image (interference pattern) is now obtained by repeating the process described above. The first and second images can either be stored in memory for later processing or, if an on-board processor is provided, compared to one another to identify any defects existing in the inspection area A of the material.

A particularly preferred example of a method for shearographic inspection of a WTB will now be described. In this example, a shearography inspection apparatus 10 as described with reference to Figures 1 to 5 above is used, and equipped with a loading device in the form of a heat gun (such as a Steinel HG2220E, operated at 2200W, 80 - 630 °C, airflow rate 500 l/min).

Step-1 : Position the apparatus 10 at the required surface S of a wind turbine blade which is positioned vertically and is approximately parallel to the wind tower.

Step-2: Start the vacuum suction cups 8 of the apparatus 10 and switch the motor or other vibratory components of the apparatus off to avoid vibration from the apparatus. This means the whole apparatus 10 including shearography unit 1 is attached to the wind turbine blade surface by the vacuum suction cups 8.

Step-3: Switch on the laser source 20 of the shearography system, and switch on the heating source (e.g. a heat gun) to heat the inspection area A of the wind turbine blade for a period of time (e.g. 10 to 15 seconds). In an example using the heat gun mentioned above, heating was carried out from around 0.5m from the test sample surface S, raising the temperature of the inspection area A by around 5°C with 10s heating. At the same time start recording video clips of laser speckle images (i.e. store the interference images received by image acquisition device 40).

Step-4: Switch off the heating source, and continue recording video clips of laser speckle images for a period of time (e.g. 30-60 seconds) where the wind turbine blade deforms due to natural cooling. Step-5: Speckle pattern recording complete. Switch on the motor of the robotic mechanism and then switch off the vacuum suction cups 8.

Step-6: Use the transportation device 7 to move the apparatus 10 to a new location on the blade surface to carry out a new inspection by the shearography system.

Step-7: Repeat steps 2 to 6, until all the areas of the WTB are inspected.

Step-8: Retract the whole apparatus 10 and start post-processing of the recorded video clips to produce shearography fringe patterns where potential subsurface defects within the wind turbine blade can be identified. The apparatus 10 is preferably provided with a communications module 9 enabling high communications bandwidth between an external device (base station) and the apparatus, so that all operations of the apparatus can be remotely controlled. It is also preferably that sufficiently large storage (memory) is provided onboard, for data collection. Ideally, data analysis and image processing could be carried out during inspection. However, this is usually not possible due to huge computational time required; hence the data analysis and image processing could be conducted off-line. Defect detection results will typically be provided afterward.