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COMPOSITES

The objective of this study was to adapt ultrasonic and radiographic techniques for the inspection of wind turbine blades and to compare the obtained results. The measurements performed show that radiographic techniques are capable of reliably detecting a number of structural defects within the blade. The adapted air-coupled ultrasonic technique, using Lamb waves, proved to be the most promising in terms of implementation as it only requires access to one side. However, the novel combination of contact pulse-echo and immersion techniques using a moving container of water identified the shape and size of defects better. Through comparisons of images obtained using both radiographic and ultrasonic techniques, different defect properties can be identified. Hence, the best results are achieved when both techniques are combined together.

Keywords: Wind turbine blade, composite materials, ultrasonic, guided waves, radiography.

1. IntroductionWind turbines have been in use for many years(1) and are becoming increasingly popular because they are a renewable source of energy with a reduced environmental impact. In order to maximise the efficiency of the energy exploited from the wind, the various components of a wind turbine need to be inspected regularly. One of the most critical components of a wind turbine is the blades. Blade failure can be very costly as it can damage the turbine itself and other surrounding structures. Flaws can appear both during manufacture and operation, and typically consist of delaminations in the spar laminate, a change in thickness of the laminate and loss of cohesion between the skin, epoxy layer and/or main spar. Ideally, non-destructive testing (NDT) techniques should be used to reduce the possibility of failure and extend overall component life(1-3).

Due to variable wind conditions, wind turbine blades experience very complex loading sequences(4). The application of fibre-reinforced composites in the construction of wind turbine blades is fatigue critical and, due to the heterogeneous and anisotropic nature, their fatigue behaviour is very complex and can result in several internal structure damage mechanisms developing during the component life(5,6). Several techniques are available for monitoring

the performance of wind turbines as well as early fault detection to minimise breakdowns. Inspection methods based on ultrasound, thermography and acoustic emission have been applied that enable manufacturing quality control and in-service inspection(3).

The propagation characteristics of bulk ultrasonic waves in pulse-echo or transmission modes have been used to determine material properties of multi-layered blade samples. This technique is especially useful for the detection and characterisation of internal defects in non-homogeneous blades(7-9). Ultrasonic C-scan imaging has been used for area mapping of the composite delamination and interface disbonds. However, the interference of overlapped reflections, scattering and attenuation of the reflected ultrasonic waves from the multi-layered structure often affects results. The scattering also has a negative impact on the propagation of ultrasonic waves in general and requires the use of lower frequencies in ultrasonic testing(7). Another ultrasonic technique suitable for the testing of wind turbine blades is based on guided waves, allowing an estimation of the position and type of internal defect. In this case, the guided waves are scattered in all directions through interaction with a structural discontinuity, and mode conversion occurs. As a result, the received signals are weaker and covered by noise(10-12).

Radiography was used to identify density differences within the blade structure. The basic principle of radiography is to transmit ionising radiation through a material and measure its attenuation. The amount of attenuation is dependent on the density and thickness of the material. Therefore, pixel readout (grey level) corresponds to the total attenuation along the photon path. This allows very small density changes to be detected, particularly volumetric flaws parallel to the X-ray beam. This includes missing glue between laminates, cracks and voids in the laminates, non-intended orientation of fibres or kink band(13). Furthermore, microfocus X-ray sources are available that ensure a highly focused beam with focal spot sizes in the 10 microns range. The focal spot size is the limiting factor in a microfocus X-ray system, meaning spatial resolutions approaching 10 microns are achievable, resulting in increased detail(3,6).

The objective of this study is to compare the adapted air-coupled ultrasonic technique; the combination of contact and immersion ultrasonic with a moving water container technique; and the radiographic technique, suitable for in-service NDT of wind turbine blades, taking into account the complex material and component structure as well as the harsh environmental conditions often found at wind turbine sites.

2. The sample The study was carried out on a 1005 x 870 mm wind turbine blade sample constructed from a composite glass fibre reinforced plastic (GFRP) material. A drawing of the sample giving dimensions and identifying the defects is shown in Figure 1. These artificial defects are circular flat-bottom holes with various diameters which simulated lack of bonding/glue, structural defects and delaminations. Three defects on the main spar (with diameters 19, 49 and 81 mm) and one on the trailing edge (diameter 15 mm) were investigated.

NDT of wind turbine blades using adapted ultrasonic and radiographic techniques

E Jasiūnienė, R Raišutis, R Šliteris, A Voleišis, A Vladišauskas, D Mitchard and M AmosSubmitted 05.05.09 Accepted 03.08.09

Elena Jasiūnienė, Renaldas Raišutis, Reimondas Šliteris, Algirdas Voleišis and Alfonsas Vladišauskas are with Ultrasound Institute, Kaunas University of Technology, Studentu 50, LT-51368 Kaunas, Lithuania. Tel: +370-37-351162; Fax: +370-37-451489.

Daniel Mitchard and Mathew Amos are with TWI Technology Centre (Wales) Ltd, ECM2, Heol Cefn Gwrgan, Margam, Port Talbot SA13 2EZ. Tel: +44 (0)1639 873100; Fax: +44 (0)1639 864679.

Corresponding author: E-mail: elena.jasiuniene@ktu.lt

DOI: 10.1784/insi.2009.51.9.477

The multi-layered structure of the wind turbine blade consists of a skin layer (dye coating with GFRP), glue/foam layer and a GFRP foundation layer, as shown in Figure 2(a). The sample not only possessed a number of artificial defects but also a number of manufacturing non-homogeneities, such as delaminations between adjacent layers of GFRP (Figure 2(b)), lack of glue Figure 2(c), etc.

3. NDT techniquesSeveral techniques, such as radiography and a number of different ultrasonic techniques, were adapted and used for the NDT of the wind turbine blade sample due to the multi-layered, variable thickness and arbitrary curved surface (Figure 1) and a large variety of defects (Figure 2). The purpose of the study was to evaluate the imaging capabilities of these different techniques through attempts to identify and characterise the various defects. The techniques used were as follows:q Adapted air-coupled ultrasonics at 290 kHz using guided Lamb

waves;q Novel combination of pulse-echo immersion ultrasonics using a

moving water container with two different transducers (focused f = 2.2 MHz and planar f = 400 kHz);

q A specially developed microfocus radiography system capable of up to 450 kV with a 50 μm focal spot size, using an image intensifier CCD camera and an a-Si flat-panel digital detector.

3.1 Air-coupled ultrasonicsThe air-coupled ultrasonic measurement system was developed at the Ultrasound Institute of Kaunas University of Technology, Lithuania. A pair of air-coupled transducers was used for non-contact scanning of the wind turbine sample, as illustrated in Figure 3(a). The transducers were accurately positioned by the mechanical scanning unit as shown in Figure 3(b). The specially developed pair of air-coupled ultrasonic transducers (transmitter and receiver) had a central frequency of 290 kHz, transduction losses -66 dB (at 100 mm distance in air in through-transmission mode), bandwidth 150 kHz at -6 dB level and was mounted into a pitch-catch configuration for the generation and reception of guided Lamb

Figure 1. A drawing giving dimensions and identifying artificially-made defects in the test sample

Figure 2. Main spar cross-sections showing: (a) a part without defects, (b) delamination inside the multi-layered wall of the main spar and (c) lack of glue between the skin and main spar

Figure 3. (a) A structural diagram and (b) the experimental set-up of the air-coupled technique using guided Lamb waves

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waves. The central frequency of ultrasonic transducers was selected according to the numerical simulation results of guided waves propagation in a complicated structure wind turbine blade(12). The transmitter was driven by the burst of eight periods and 750 V amplitude and the A0 Lamb wave mode was used for defect detection. An advantage of the technique being used is that only access to one side of the sample was required(11,12).

3.2 Pulse-echo immersion ultrasonics with a moving water container

The novel pulse-echo immersion ultrasonic measurement system with a moving water container was developed at the Ultrasound Institute in Kaunas University of Technology, Lithuania. The novel combination of the contact pulse-echo and the immersion ultrasonic technique was performed using two different transducers, one focused f = 2.2 MHz and one planar f = 400 kHz(11,14).

A structural diagram of the ultrasonic technique with a moving container of water used as a water delay line is shown in Figure 4(a). In order to obtain the uniform acoustic contact the polythene film was used as a bottom of the water container and oil between the film and surface of the sample serves as a coupling medium. The experimental set-ups of the ultrasonic techniques with the focused and planar transducers are shown in Figures 4(b) and (c) accordingly.

The 22 mm-diameter focused transducer (Panametrics V305) had a central frequency of 2.2 MHz and a focal distance of 50 mm; the focal point was estimated to be at the front surface of the sample. The transducer was excited by a 0.22 μs pulse with an amplitude of 200 V.

The well-damped planar 400 kHz ultrasonic transducer, transduction losses -30 dB (at 48 mm distance in water in pulse-echo mode, reflection from planar surface), bandwidth 250 kHz at -6 dB level. The transducer has λ/4 acoustic matching layers and composite structure in order to avoid reverberations. It was specially developed for deep penetration (more than 20 mm) of ultrasonic waves into the highly attenuating-scattering structure of the wind turbine blade. The transducer with a diameter of 26 mm was excited by a 1.25 μs pulse with amplitude of 17 V. The mechanical positioning of the probe was performed using a mobile single-axis scanning unit.

3.3 Radiographic techniquesThe radiographic inspection system was developed at TWI Technology Centre (Wales) Ltd, UK, specifically for investigating wind turbine blades (Figure 5). X-ray images were taken using a 450 kV X-ray tube at energies from 80 kV to 140 kV at 100 μA. The focus spot size was measured to be approximately 50 μm. The sample of the blade was mounted on a manipulator capable of 360º of rotation. Two detectors were used to acquire the data: an amorphous silicone digital flat panel with 2024 x 2024 pixels, and an image intensifier CCD camera with 1012 x 1012 pixels. The source-to-object distance (SOD) was approximately 1 m and source-to-detector distance (SDD) was approximately 1.8 m.

4. Experimental resultsThree artificial defects on the main spar, of diameter 19, 49 and 81 mm, and one artificial defect on the trailing edge of the sample, of diameter 15 mm, were investigated. Section 4.1 will describe results from the first two defects, section 4.2 will describe results from the third defect on the spar, and the defect on the trailing edge of the sample will be described in section 4.3.

4.1 Investigation of the 19 and 49 mm spar defectsThe first two defects are shown in Figure 6. They were inspected using the adapted technique of air-coupled guided Lamb waves at f = 290 kHz, the novel combination of contact and pulse-

Figure 4. (a) The general set-up for pulse-echo immersion testing of the sample, (b) experimental set-up using a focused transducer, (c) experimental set-up using a planar transducer

Figure 5. The experimental set-up for radiographic testing of the wind turbine blade

echo immersion techniques using focused f = 2.2 MHz and planar f = 400 kHz transducers with a moving water container. Also, 140 kV and 100 μA X-rays technique with the 2024 x 2024 pixel a-Si flat panel detector was used.

Figure 7 (a) shows the C-scan image obtained using the adapted air-coupled guided Lamb wave technique (shown in Figure 3). The measurements were performed using a scanning step of 2 mm, and both defects can be easily recognised and detected using the conventional amplitude detection technique and image correction. The image shows good contrast, which enables an estimation of the geometry of the defects and their approximate dimensions. Additionally, the darker zone at y = 110 mm indicates that besides the artificial defects there are natural defects and variations in the material properties and structure.

Figures 7(b) and (c) show corrected C-scan images using the focused 2.2 MHz ultrasonic transducer and the planar 400 kHz transducer of the same defects as above. The measurements were performed with a scanning step of 1 mm.

In the case of the focused transducer, the reflections from the outer surface of the sample were removed using filtering in the time and frequency domains. The shape and diameter of the bigger defect can be easily reconstructed. However, the smaller defect can be easily lost in other artefacts, demonstrating the natural internal non-homogeneities in the structure.

In the case of the planar transducer, the reflections from the surface were removed by subtracting the reference signal from the raw C-scan image. The averaged response of the multi-layered medium was taken as the reference signal. Both defects can be easily recognised and detected using the conventional amplitude detection technique.

Figure 8 shows a radiographic image acquired at 140 kV and 100 μA with the 2024 x 2024 a-Si flat panel detector. The density difference caused by the defects was easily detected using the X-ray system. The image shows very good contrast, detecting fibre orientation and allowing accurate measurements to be taken of the geometry of the defects. However, small variations in surface structure and planar material properties are not detectable. This is because these indications have caused no significant density change in the direction of the beam orientation.

4.2 Investigations of the 81 mm spar defectThe 81 mm defect has areas containing lack of glue, and is shown in Figure 9. This defect was inspected using the same ultrasonic techniques as previous defects. Also, 80 kV and 100 μA X-rays technique with the 1012 x 1012 image intensifier CCD camera was used.

Figure 10(a) shows the C-scan image obtained using the adapted air-coupled guided Lamb wave technique (shown in Figure 3(a)). The measurements were performed using a scanning step of 2 mm. However, this defect was not easily recognised as other areas of natural delamination dominated the results.

Figure 6. The 19 and 49 mm artificial defects in the main spar

Figure 7. A C-scan image of 19 and 49 mm defects: (a) using the air-coupled guided Lamb waves technique, (b) using the focused ultrasonic transducer, (c) using the planar ultrasonic transducer

Figure 8. A 2D digital radiograph of the 19 and 49 mm defects using the 2024 x 2024 a-Si digital flat panel

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Figure 9. The 81 mm artificial defect in the main spar

Figures 10(b) and (c) show C-scan images of the same defect using the focused 2.2 MHz and the planar 400 kHz ultrasonic transducers. The measurements were performed with a scanning step of 1 mm. In the case of the focused transducer, the shape and size of the defect cannot be fully reconstructed because it is shadowed by natural delamination. In the case of the planar transducer, the reflections from the surface of the sample were removed by subtracting the reference signal from the raw C-scan image. Here, the approximate dimensions of the defect can be estimated, and non-homogeneities elsewhere in the sample were detected.

Figures 11(a) and (b) show radiographic images acquired at 80 kV and 100 μA with the 1012 x 1012 image intensifier CCD detector. This was used as it is capable of real-time radiography, which was beneficial for optimising the beam angle for any defects present in a sample of the wind turbine blade. The images acquired show very good contrast allowing accurate measurements to be taken of the geometry of the defects. The lack of glue regions within the drilled flat-bottom hole is clearly visible and the non-uniformities in the internal structure detected using ultrasonics (Figure 10(c)) have been validated using X-rays (Figure 11 (a) and (b)). Judging from the radiographic inspection, it appears that these indications are again lack of glue within the structure.

4.3 Investigation of the trailing edge defect One artificial defect of 15 mm diameter on the trailing edge of the sample was inspected using the adapted air-coupled ultrasonic guided waves technique and using X-ray settings of 80 kV and 100 μA with the 1012 x 1012 image intensifier CCD detector. The defect is shown in Figure 12.

Figure 13 shows the C-scan image obtained using the air-coupled guided Lamb wave technique. The measurements were performed using a scanning step of 1 mm, and the position of the defect was easily detected.

Figure 14 shows a radiographic image acquired at 80 kV and

Figure 10. A C-scan image of the 81 mm defect: (a) using the air-coupled guided Lamb wave technique, (b) using the focused ultrasonic transducer, (c) using the planar ultrasonic transducer

Figure 11. (a) A 2D digital radiograph of the 81 mm defect and (b) the outer region using the 1012 x 1012 image intensifier CCD detector

100 μA with the 1012 x 1012 image intensifier detector. The image again shows very good contrast allowing accurate measurements to be taken of the geometry of the defects. This defect was positioned on the trailing edge (thinnest section) of the blade meaning only a low energy was required for penetration resulting in increased image contrast. This is highlighted by the increased visibility of the fibre orientation within the structure. The defect was easily detected and only appears elongated as the sample was not positioned perpendicular to the X-ray beam.

5. ConclusionsWind turbine blades are multi-layered structures with variable thickness of each individual layer. They have arbitrary curved surfaces and are made from anisotropic materials with manufacturing non-homogeneities. The purpose of this comparative investigation was to evaluate the imaging capability of adapted ultrasonic and radiographic techniques for the detection of artificial defects in a GFRP wind turbine blade sample.

Two ultrasonic techniques adapted specifically for investigating wind turbine blades were used: air-coupled testing using guided Lamb waves with f = 290 kHz, and a novel combination of contact and pulse-echo immersion testing using a moving water container with a focused transducer at f = 2.2 MHz and planar transducer at f = 400 kHz. The measurements show that the simplest to implement on-site, and consequently the most promising, is the air-coupled technique. It was selected due to one-side access and the non-contact experimental set-up. However, the pulse-echo immersion testing with focused transducer can identify the position and approximate size with better accuracy and can find defects near the surface, such as lack of glue just below the skin layer. The same technique using a planar transducer shows internal defects and variations in the material properties at different depths of inspected volume.

The radiographic images proved that the adapted X-ray technique is capable of detecting volumetric defects such as lack of glue and internal structural irregularities. The X-ray source and detector provided suitable resolution and contrast sensitivity with the combination of real-time imaging and 360º of rotation increasing the probability of detection.

The large aspect ratio and sandwich structure associated with a wind turbine blade make X-ray beam orientation a very important inspection parameter. Imaging through the thinnest sections (perpendicular to the sandwich structure) requires low energy resulting in better image contrast. This increased contrast sensitivity enables differences in structures of low absorption to be displayed, such as the fibre and resin. Studies have also shown that on-site radiographic inspection of wind turbines is a viable technique if the energy is kept relatively low; this allows safety requirements such as shielding to be easily met(15).

The downside of X-ray imaging in this direction is that planar defects cannot be detected and the different layers of the sandwich structure are superimposed over one another, resulting in difficult interpretation. To combat these issues would require the X-ray beam to be parallel with the longest length and increasing the energy to penetrate the thicker material. This would increase the radiation hazard and reduce contrast sensitivity.

The comparative study presented in this work has demonstrated a possibility to show the variability of the internal structure of wind turbine blades. The work has shown that the artificial defects are easily detected alongside natural defects in the material structure. Comparisons of images obtained using the ultrasonic and radiographic techniques show that each can identify different defect characteristics, and that the best results can be achieved by combining the techniques.

Acknowledgements The part of this work was sponsored by the European Union under the Framework-6 ‘Development of a Portable, High Energy, Nanofocus Computer Tomography system for Glass Reinforced Plastic Wind Turbine Blades’ CONCEPT (Computerised Open Environment Portable Tomography) project. CONCEPT is collaboration between the following organisations: TWI (UK), X-Tek (UK), Detection Technology (Finland), General High Voltage (UK), Innospexion (Denmark), Eon (UK), RWE nPower (UK), Kaunas University of Technology (Lithuania), London South Bank University (UK) and Germanischer Lloyd (Germany). The project is coordinated and managed by TWI (UK) and is partly funded by the EC under the programme ref: COOP-CT-2006-032949.

Figure 12. The 15 mm artificial defect in the trailing edge of the sample

Figure 13. A C-scan image of the trailing edge defect using the air-coupled guided Lamb waves technique

Figure 14. A 2D digital radiograph of the 15 mm defect using the 1012 x 1012 image intensifier CCD detector

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