Forms of discontinuities in 1 KW and 3 KW Wind Turbine Blades Sabbah Ataya and Mohamed M.Z. Ahmed Department of Metallurgy and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez Canal University. Salah Nasim Str., 43721- Suez, Egypt Abstract Periodical inspection of the wind turbine blades is very important for early detection of any serious damage, treating of which increases the life time of the wind turbines. In the current work a number of 15 blades of 3 KW and 81 blades of 1 KW turbines were inspected using the visual test (VT) and by liquid penetrant test (PT) if needed. The location and size of the discontinuities found were measured. Various types of discontinuities were categorized into three main groups: 1) Cracks in form of transverse cracks, longitudinal cracks and surface cracks, 2) Surface or coating damage in the form of coating pores, surface damages, holes or penetrations and reworked areas and 3) Edge damage in the form of edge cuts or crushing and side separation. The gathered data were analyzed to allocate the different forms of discontinuities. Such analysis presents an indicator for condition of the working rotor blades. It was found that the transverse carks are concentrated at the highly loaded region in trailing edge at the upper third part of the blade. Longitudinal cracks with lengths up to 59 cm were existing at the region of geometric change; i.e. in the root and the cover of the aerodynamic zone. Edge damages such as edge cuts or crushing and partial side separation were found in the sharp trailing edge. Surface and coating damages were observed on the leading edge due to erosion. Key words Discontinuities, damage, inspection, NDT, wind turbine blades Introduction Wind turbine blades are the most critical parts which are difficult and expensive to be replaced. Although the designed life time of the wind turbines is usually 2 years visual inspection has revealed serious damages after only five years (with a total measured number of 7.13 x 1 6 cycles) in inspected 3 KW turbine blades produced by hand lay-up (Martin et al. 28; Ancona et al. 21). The presence of mechanical caused discontinuities makes the blade material subjected to weathering conditions which has a damaging effects on the fiberglass reinforced resin even if this effect is difficult to assess (Pryor et al. 21). Thus the aim of the current work was to inspect wind turbine blades which have a life time ranging between 16 and 19 years. The various discontinuities will be documented, classified, measured and allocated on the blade length. The possible root causes of such damages will be discussed. Inspection Techniques In the current study 15 blades of 3 KW wind turbines and 81 blades of 1 KW wind turbines were inspected by visual inspection test (VT) and some of which were inspected by liquid penetrant test (PT). Before inspection the wind turbine blades have been washed and cleaned by a brush using water and soap. A crane of load capacity of 25 ton and height reaching up to 3 m was used. The VT test is started by giving a name for the blade under investigation followed by attaching a meter scale to the blade side under inspection to determine the position and size of discontinuities. The PT test was used according to the standard technique (ASM 1992; Mix 25) to reveal the fine cracks. Graphical Distribution of the Discontinuities The gathered data from the inspection of the wind turbine blades has been analyzed to show size and location of these cracks. The position of the discontinuity is allocated on the blade (L in m) measured from tip to hub (Figure 1). As two different blade lengths (L o in m) were --------------------------------------------------------------------------------------------------------------- Corresponding Auther: Sabbah Ataya, email: sabah_ataya@s-petrol.suez.edu.eg Telfax: 2() 62336268, Mobile: 2()12 246766
inspected, the relative location (L/L o in m/m) is used to display the inspected blade geometries together. Tip L Trailing edge Leading edge L o Aerodynamic zone cover Root root Aerodynamic zone Figure 1: Schematic of the wind turbine blade and terminology of the main parts cover joints Inspection Results and Discussion The gathered inspection data were processed and classified to determine the different forms of discontinuities their size and location on the rotor blade. The discontinuities were categorized into:- 1) Cracks in form of longitudinal cracks, transverse cracks and surface cracks, 2) Edge cuts or crushing and side separation as deboning of the joint at the trailing edge and 3) Surface or coating damages which were classified according to their size into small and shallow pits or pores (with diameter up to 2 mm) and larger coating damage (for damaged areas with a diameter more than 3 mm), holes or penetration with depth up to 13 mm and reworked or repaired areas. 1. Cracks a. Longitudinal Cracks Longitudinal cracks (LCs) are those cracks growing with the blade length. Figure 2.a shows an example of a LC in a 1KW rotor blade. Long LCs were observed at the root or in the cover of the aerodynamic zone (Figure 1). Most of the LCs were found in the 1KW with a crack length varying from few centimeters to about 59 cm. The depth of the LCs was examined using a thin filler sheet of a.5 mm thick. Some of the cracks were as deep as the thin filler has been inserted up to a depth of 1 mm in the middle of the long LCs, while on the rear thirds of the crack the filler was inserted with difficulty to a depth of just 5 mm. b. Transverse Cracks Transverse cracks (TCs) are growing perpendicular to the blade length. TCs were observed mainly on the trailing edge of the blades of the 1KW and 3KW wind turbines. Figure 2.b shows a clear example of the transverse cracks which observed in the 3KW cutting the trailing edge with a length up to 4 cm. Figure 2.c includes another form of TCs which are mostly shallow and short (from 1.5 up to 2.5 cm long) and found in the root of the 1KW wind turbine blades. This short root TCs are found as individual crack or a group of up to 7 neighboring cracks with inter-crack spacing from 2 to 6 cm. c. Surface Cracks Surface cracks (SCs) are those cracks spreading over areas with different sizes having a random orientation or sometimes with a radial orientation form a certain point as that shown in Figure 2.d. It was observed that the SCs are fine shallow cracks on the coating layer of the blade. For better revealing of the SCs, liquid penetrant test (PT) was used. In Figure 3, the crack length is represented against the relative position (L/L o ) of the cracks. It should be mention that the blade can have an individual LC at the root if any, but the TCs can be found in either individual or group of TCs. Figure 3.a shows that the longer longitudinal cracks are located in the root of the blades near to the hub and in the cover of the aerodynamic zone. Although this section is rigid enough where it has a thickness in similar blades of 27 mm (Martin et al. 28), it can be subjected to high fatigue loads which is the most probable reason to cause failure of the rotating highly loaded parts. Figure 3.a shows that there are many long LCs which have a crack length up to 59 cm which necessitates a structural repair to extend the service life of such wind turbine blades. Other reason for the existence of the
LCs at the root and cover of the aerodynamic zone is the geometric change of the blade at these region which increases the severity of the stresses affecting these areas. a) b) c) d) Figure 2: a) Root Longitudinal cracks, b) Edge transverse crack, c) Root transverse cracks and d) Surface cracks. Crack Size (cm) 6 4 2 a) Longitudenal cracks 1 KW 3 KW b) Transverse cracks 6 4 2 1 8 6 4 2 c) Surface cracks Relative Location L/Lo (m/m) Figure 3: Allocation of (a) longitudinal cracks, (b) transverse cracks and (c) surface cracks in 81 blades of 1 KW and 15 blades of 3 KW wind turbines. 2. Edge damage, Edge cuts, edge crushing and debonding or side separation are different forms of edge damage that observed at various positions along the trailing edge. a. Edge Cuts and crushing Figure 4.a shows an example image of the edge cuts in the trailing edge of the 3KW wind turbine blades that observed at about 3.4 m from the tip with a length of 1 cm. Edge crushing (Figure 4.b) is another form of edge damage which can be found in the trailing edge of both 1 KW and 3 KW rotor blades. The trailing edge in the 3KW wind turbine blade is quite sharp and any mechanical interaction with a hard body can result in such type of damages. This mechanical interaction is also may arise from the impact of the crane lifting basket during transportation, maintenance or washing of the blades. b. Partial Side Separation Partial side separation (SS) is a cracking or debonding in the joining region of the trailing edge. Generally it was observed that the SS is existing in the lower third part of the blade on the trailing edge. Figure 4.c shows an example of the typical SS that was observed in the trailing edge of the 3KW wind turbine blades at about 2.85 m from the tip with an SS
length of 7 cm. This can be due to loosening of the joining material at the interface with fiberglass reinforced resin. a) b) c) Figure 4: a) Edge cuts in a 3 KW rotor blade and b) Crushing of the trailing edge of a 1 KW rotor blade and c) Partial side separation of the trailing edge of a 3 KW rotor blade. Edge Cut length (cm) 16 12 8 4 a) Edge cut (crushing) 1 KW 3 KW Debonding length (cm) b) Side separation (debonding) 15 1 5 Relative Location L/Lo (m/m) Relative Location L/Lo (m/m) Figure 5: a) Edge cuts or crushing and b) debonding or partial side separation for a number of 15 blades of 3KW and 81 blades of 1 KW turbines. Figure 5.a shows that the edge cuts on the trailing edge of the 3 KW blades are concentrated in the lower third of the blade length where the edge is so sharp which makes it easy to be crushed by the impact with any hard body. Partial side separation or debonding occurs also more often in the lower half of the 3 KW blades (Figure 4.b). One of these partial debonding extends over a length of 135 cm. Such separation needs a structural repair to avoid any complete debonding under the applied shear stresses. 3. Surface and Coating Damages: Damages of the surface coating of the blades were found in different forms and sizes most of them were just shallow surface damages and a little were deep in the form of holes or penetration.
a- Coating Pores Coating pores are small pits (with diameter of ~ 1 mm, Figure 6.a.) in the coating layer of the blade. These pits is distributing over a large area of a diameter up to 3 cm with pore interspacing from few millimeters to 3 cm. Other form of this discontinuity was in form of discrete pits with a diameter up to 2 mm as shown in right side of Figure 6.a. This type of discontinuity is expected to be formed in the coating layer during manufacture as fine closed bubbles, then opened to the surface and increased in size by erosion. Comparing the size of these pores with the lives of these rotor blades (16-19 years) it can be found that the increase in size these pores is very slow, but their existence increases the surface roughness of the blades. b- Surface Damage This type of discontinuity is found in different forms: either rounded areas with a diameter from 5 mm (Figure 6.b) to 2 mm or elongated to irregular form on the leading edge which spread to a length up to 1 cm (Figure 6.c). Most of the surface damages are locating on the lower third of the leading edge (Figure 6.b,c) due to the higher linear velocity of this part from the blade and the increased erosion effect by the wind carrying sand and dusts. Damages of the coating layer in this form with such large area subjects the main structural material of the wind turbine blades to the weathering conditions attack. This can affect the fiberglass reinforced resin even if the effect of the climate change and weathering condition is difficult to assess (Pryor et al. 21). Fortunately, the problems arising with icing condition is not faced in the area under investigation. Figure 6: Different sizes of coating damages a) Coating pores, b) Small surface damage and insect contamination, c) Large surface damage and d) Hole or penetration in the leading edge of the rotor blades. c- Holes or Penetrations Holes or penetrations are pits of a clear penetration through the blade material and has a diameter of 3 to 1 mm. Figure 6.d shows an example of the penetration that was found in the wind turbine blade with a depth of 12 mm. This holes is mostly originated from the manufacture because it was still partially covered by the coating layer. Other penetrations were due to mechanical interaction with other hard bodies and subjected to subsequent erosion which makes it wider and deeper. In addition to the previously described surface damages, reworked areas were also documented to be followed up. Some of these repairs still need some cosmetic re-repairs to increase the surface quality, consequently the performance of the rotor blades. Coating and surface damages, improper repairs finishing, dust deposits and insect contamination result in rough surface. Such rough surface affects the aerodynamic performance and reduces the turbine efficiency. The potential for erosion depends on the force at which the particulate matter impacts the airfoil. Wind speed and rotational speed of the blade determine impact velocity (Dalili et al. 29). While, geometric shapes and the relative velocities of both the airfoil and the impacting particle determine the impact force of
the particulates. Wind carrying large amounts of sand can erode the leading edge of the turbine blade and increase surface roughness. Minimal leading edge erosion reduces the blade s performance by about 3-4% (Cripps 211). So that additional effort should be done to reduce the leading edge roughness by cleaning the rotor blades from the accumulated dusts and insects in shorter periods. Conclusions 1-Different discontinuities found in 81 blades of the 1KW and 15 blades of the 3 KW wind turbines which have a life of 16 to 19 years were allocated and categorized into: 1) Cracks in form of transverse cracks, longitudinal cracks and surface cracks, 2) Edge cuts or crushing and side separation and 3) Surface or coating damage in the form of coating pores, surface damages, holes or penetrations and reworked areas. 2-The inspection using PT was more helpful than using VT to reveal the fine surface cracks, otherwise, the white surface of the blade makes it easy to detect other cracks by VT. 3-Longitudinal cracks with lengths up to 59 cm are existing at the region of geometric change; i.e. in the root and the cover of the aerodynamic zone. 4-Transverse carks are concentrated at the highly loaded region of the trailing edge at the upper third part of the blade. 5-Edge damages in the form of edge cuts or crushing and partial side separation and edge are in the sharp trailing edge. 6-Surface and coating damages are mainly on the leading edge due to erosion. Recommendations 1- Cosmetic repairs of most found discontinuities will decrease the surface roughness to minimize the aerodynamic drag which increases the efficiency of the turbine. 2- Structural repairs are essential for both longitudinal root cracks and transverse cracks in the trailing edge at the upper third part of the rotor blade. It is essential also for the partial side separation or debonding of the trailing edge of the 3KW blades. 3- Periodical inspection to follow up the dangerous discontinuities and to explore any new damages is needed and periodical cleaning should be performed to remove the accumulated dust and insects. Acknowledgement The authors express their great thanks and appreciation for the Egyptian New and Renewable Energy Authority (NREA); Eng. Abd El-Rahman Afify (NREA head), Eng. Osama Noman, Eng. Ashour Moussa, Eng. El-Sayed Mansour, and the technicians at the inspected site for their help and support to access and reach the inspected wind turbines. The authors thank also Prof. Dr. Eng. Rashad Ramadan for the helpful discussion. Science and Technology Research Fund (STDF) is deeply acknowledged for the financial support. References Ancona, D., McVeigh, J., Wind Turbine - Materials and Manufacturing Fact Sheet, a report prepared by Princeton Energy Resources International, LLC for the Office of Industrial Technologies, US Department of Energy, 29 August, 21. Dalili, N., Edrisy, A., Carriveau, R., A review of surface engineering issues critical to wind turbine performance, Renewable and Sustainable Energy Reviews 13 (29) 428 438 David Cripps, The future of blade repair, Reinforced plastics, Vol. 55, No. 1(211) 28-32. Martin,J.C., Barroso, A., Parıs, F., Canas, J., Study of damage and repair of blades of a 3kW wind turbine, Energy 33 (28) 168 183 Mix, Paul E., Introduction to Nondestructive Testing, Second Edition, John Wiley & Sons, Inc., New Jersey, 25, p. 221. Nondestructive Evaluation and Quality Control ASM Metals Handbook, 2nd edit., Vol. 17, 1992. Pryor,S.C., Barthelmie, R.J., Climate change impacts on wind energy: A review, Renewable and Sustainable Energy Reviews 14 (21) 43 437