Engineering Failure Analysis 35 (2013) 480–488 Contents lists available at SciVerse ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal Damages of wind turbine blade trailing edge: Forms, location, and root causes Sabbah Ataya ⇑, Mohamed M.Z. Ahmed Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Salah Nasim Street, Suez 43721, Egypt a r t i c l e i n f o Article history: Available online 7 June 2013 Keywords: Wind turbine blades Trailing edge Damages Cracks Fiberglass reinforced polymer (FGRP) a b s t r a c t The geometrical form and the manufacturing technique make the trailing edge of the wind turbine blade more susceptible to damage. In this study the trailing edge in a number of 81 blades of 100 kW wind turbines and 18 blades of 300 kW wind turbines of working life ranging between 6.5 107 and 1.1 108 cycles were completely visually scanned. The different damages were classiﬁed and allocated in their exact position relative to the blade length. Cracks in different orientations with the blade length were the frequent types of damages which found on the trailing edge. First, longitudinal cracks (LCs) that found along the blade trailing edge from the blade root to the tip were in lengths that varied from a few centimeters up to around 1.35 m. Second, transverse cracks (TCs) were found in either simple TCs which growing on one shell at the trailing edge or round TCs which growing across the two shells. TCs lengths were ranging from 20 to 50 mm. Third, edge damages were detected in the form of edge cuts or crushing. The possible root causes of the different types of cracks have been discussed. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Trailing edge of the wind turbine blade is the region of the two half shell bonding which include a long line of adhesive joints. The trailing edge is subjected to changeable complex state of stress through the cycle, while the adhesive joining material at this edge can be subjected to shear stresses. Although, the designed fatigue life of the wind turbine blade should be 20 years . Failure of wind turbines due to failure of the blades represents 19.4% of a total of 1028 wind turbine failure cases  and the main failure root cause are mostly due to the operation and environmental conditions. There are no available much data about the various forms of damages in the trailing edge of the wind turbine blades. Thus the aim of the current work was to present the most frequent forms of damages of the wind turbine blade trailing edge and the locations of their occurrence. This has been carried out through the full visual scanning inspection of a considerable number of wind turbine blades, those were working at the boundaries of their proposed life. 2. Inspection procedure In this study 18 blades of 300 KW wind turbines and 81 blades of 100 KW wind turbines were full visually inspected (VT). The blade lengths are 9.5 m and 14.2 m for the 100 KW and 300 KW power, respectively. Before the visual inspection of the wind turbine blades, the blades have been washed and cleaned thoroughly. The VT inspection is started by giving a designation name for the blade under investigation followed by attaching a scale tape to the blade side under inspection ⇑ Corresponding author. Tel.: +20 (0)122 2467 606; fax: +20 (0)623 3602 68. E-mail addresses: email@example.com (S. Ataya), firstname.lastname@example.org (M.M.Z. Ahmed). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.05.011 S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 481 to determine the location of any discontinuity or damage. The inspection was carried out thoroughly from the tip up to the root hub on the trailing edge of the blade. The different discontinuities on the blade were documented using a high resolution digital camera. 3. Results and discussion 3.1. Estimation of the actual working life The inspected wind turbines were installed in different dates, so that their lives were ranging between 17 and 22 years. The actual working life (in cycles) of each wind turbine was calculated based on the operational data using Eq. (1): Actual working life ¼ RPM Age ðMinutesÞ Capacity factor ð1Þ where, RPM is the average number of cycles per minutes of each turbine model, the age is based on the date of start working date after installation of each turbine and the capacity factor describes the net time fraction of working excluding the times when the wind speed is under the cut-in speed. The availability factor of these turbines is ranging between 0.95 and 0.98. The capacity factor is 0.22 and 0.25 and the RPM is 33 and 38 for the 100 kW and 300 kW turbines, respectively. The actual turbines working lives were calculated in number of cycles till the time of inspection and found between 6.5 107 and 1.1 108 cycles which are illustrated against the different turbines in Fig. 1. The calculated working number of cycles was consistent with published information  which conﬁrmed that the working life of 20 years delivered a number of cycles in the order of 108 cycles. 3.2. Trailing edge damage forms Extensive analysis of the documented pictures has been carried out to determine the type, size and location of the different forms of damages. It should be mentioned here that a number of other damage forms exist in the blade shells were not included in this context. The trailing edge damages were categorized as follows:(1) Longitudinal cracks along the trailing edge through the bonding materials. (2) Transverse cracks. (3) Edge cuts or crushing. Table 1 indicates the number of each type of damages in the inspected wind turbine blades. Because the inspected wind turbines were of different types (100 kW and 300 kW) some normalization was made to relate the location of the damage to the blade length. For the wind turbines 100 kW and 300 kW power, the number of inspected blades B was 81 and18 and the blade lengths R0 were 9.5 m and 14.2 m respectively. The number of discontinuities (N) is normalized and expressed in term of number per blade unit length (n) using Eq. (2). n¼ N B R0 ð2Þ Fig. 2 illustrates the number of damage per blade unit length for each type of damages for both 100 kW and 100 kW power wind turbine blades. Clearly, it can be observed that the number of damages per blade length is higher in the 300 KW wind turbine blades than in the 100 kW wind turbine blade in case of the longitudinal crack and edge cuts, while transverse cracks are higher in 100 KW wind turbines than 300 KW wind turbines. Fig. 1. Actual working lives (in cycles) against the turbine number. 482 S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 Table 1 Number of trailing edge damages in the inspected wind turbine rotor blades. Discontinuities 300 kW blades 100 kW blades Total Longitudinal cracks Transverse cracks Edge cuts Total 79 14 43 136 57 59 40 156 136 73 83 292 Fig. 2. Number of damage per blade unit length in the total inspected 100 kWand 300 kW wind turbine blades. The inspected blades were assembled from two half shells manufactured by the hand lay-up technique and then glued together afterwards. The blade is protected from ultraviolet degradation and water penetration by a gelcoat. A carrying spar made from ﬁberglass reinforced resin is supporting the two shells and strengthening the whole structure. Fig. 3 illustrates a cross section of a 100 kW wind turbine blade. Understanding the production method of the wind turbine blades [4–7] and the working conditions will enable the determination of the mutual effect of the different production defects and working conditions on the formation of different damages. Because, it is impossible to have samples of the different defects of these working turbine blades for further laboratory failure analysis, the failure root causes will be determined based on the background of the production techniques, the working conditions, the possible defect formation at certain positions and the comparison with similar published damage forms. During this long lifetime, the rotor blades are exposed to extreme temperatures (from 5 to 50 °C), humidity, rain, sandy storms, solar radiation and salinity. This severe working and aging conditions along the working life (17 to 22 years) have a great contribution in the damage formation. In the following sections, some examples for each type of damages are illustrated and discussed, the damage size as well as their allocation on the rotor radius is presented and the possible root causes of failure are explained. 3.2.1. Longitudinal cracks Longitudinal cracks (LCs) are those cracks oriented with the blade length. Fig. 4a–c shows some examples of LCs in a 100 KW and 300 KW rotor blade. Relatively long LCs were observed at the root or in the cover of the aerodynamic zone. The longest LC with a 60 cm long shown in Fig. 4a that was found in the transition zone where abrupt changes of material thickness and geometric concentrator  takes place. Fig. 4b shows an LC located at 2.85 m away from the 300 KW blade tip Fig. 3. Cross section of a 100 kW rotor blades. S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 483 Fig. 4. Examples of longitudinal cracks that found in the trailing edge of 100 kW (a) and 300 kW (b) wind turbine blades. and growing in the bonding layer of the two shell halves. While the LC in Fig. 4c is accompanied with some damages and removal of the gel coat on both sides of the crack. Fig. 5 illustrates the distribution of the different LCs sizes on the blade length. In this ﬁgure the blade length has been normalized to represent the data obtained from both types of wind turbine blades (100 kW and 300 kW ). The normalized rotor radius is obtained by dividing the location of the crack (R) by the blade length (R0) either 9.5 m or 14.2 m. From Fig. 5 it can be observed that in the 100 kW wind turbine blades the majority of LCs was found near to the hub in the root of the blade and on the cover of the aerodynamic zone. In contrast, in the 300 KW wind turbine blades, the majority of the LCs was found near to the tip of the blade (Fig. 5). The number of LCs per blade unit length was higher in the blades of 300 KW turbines (Fig. 2). The length of the cracks is varied from a few centimeters to about 40 cm. One crack with a length of 135 cm was found in a 300 KW blade. The depth of the LCs was examined using a thin ﬁller sheet of a 0.05 mm thick. Some of the cracks were as deep as the thin ﬁller has been inserted up to a depth of 10 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. Longitudinal cracks LCs which detected at the trailing edge of both 100 KW 300 KW blades were classiﬁed according to their location (as shown in Fig. 5) into edge LCs and root LCs. The edge LCs are the LCs locating on the relative rotor radius from 0.2 to the blade tip. The edge LCs shown in Fig. 5 were separate re-plotted in Fig. 6. Linear ﬁtting of the edge LCs gives an estimation to the position where most of this crack type is occurring. The linear ﬁtting shows that the edge LCs are concentrated at 0.73 of the rotor radius, near to the tip. The root LCs are those cracks which locating on the root and on aerodynamic cover of the blade (i.e. up to 0.19 of the relative rotor lengths, Fig. 5). Fig. 7 includes a focusing on the root LCs which presented in Fig. 5. The line connecting between the points representing the upper and lower end of the crack indicates the crack length. Fig. 5. Longitudinal cracks LCs on the trailing edge of the wind turbine blades. 484 S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 Fig. 6. Estimation of the edge LCs position on the trailing edge of wind turbine blades. Fig. 7. Distribution and length of the root LCs on the trailing edge of wind turbine blades. The blade is made out of two half shells that joined together with a joining material thickness of about 1 cm at the trailing edge. The thickness of the shell is decreasing from the root to the tip. A thickness change from 17 to 27 mm was found near the root of an equivalent 300 KW rotor blade . Also, the shell thickness decreases gradually toward the tip. This leads to a decreased rigidity towards the tip, which is needed, but the drawback of this low rigidity of the rear third of the blade is the increased deﬂection leading to high shear stresses at the interface between the rotor shell and the bonding materials, this can lead to decohesion or cracks at the trailing edge joints. This is mostly the reason of the LCs found near to the tip (Fig. 4b). Also during the joining of the two shell halves two types of defects might occur. One can be called ‘‘overbite’’ which is the existence of excess bonding material and the second can be called ‘‘underbite’’ which is the existence of unﬁlled spaces between the two shell halves. Overbite at the shell edges are treated by grinding and underbite are ﬁlled with bonding materials. The abrupt change of the material thickness at the root and the aerodynamic cover of the blade were led to similar damages in a 300 kW turbine blade as analyzed by Martin et al. , who related the cause to the fatigue loading. So that the reason for the existence of the LCs at the root and cover of the aerodynamic zone (Fig. 4a) mainly due to the interaction of the fatigue loading and the geometric change of the blade at these regions which increases the severity of the stresses affecting these areas . 3.2.2. Transverse cracks Transverse cracks (TCs) are aligned perpendicular to the blade length. TCs were observed on the trailing edge of the 100 KW and 300 KW wind turbine blades with higher number per blade length in the 100 KW turbines. Fig. 8a–c shows some examples of the transverse cracks which observed in the 100 kW and 300 kW penetrates the trailing edge with a length up to 4 cm. TCs are divided into four types each of which will be discussed separately below. 18.104.22.168. Simple TCs. Simple TCs are those cracks existing only in one side of the blade i.e., only in one half shell at the trailing edge and stops at the edge of the blade. An example of the simple TCs that found in the 100 kW wind turbine blades and is shown in Fig. 8a. On the assembly of the two shell halves some misalignment could occur. To correct this misalignment and have a straight trailing edge, some grinding should be done on one side of the trailing edge. This can lead to a reduction in the S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 485 Fig. 8. Examples of the simple and round transverse cracks found in the 300 kW and 100 kW wind turbine blades. (a) Simple TC at 0.64 of the rotor radius of a 100 kW turbine, (b) Round TC at 0.82 of a 300 kW rotor radius and (c) other side of the round TC in (b). reinforcing ﬁberglass which weakens the edge under high loading condition making it subjected to such cracks. The straight end of the simple crack (Fig. 8a) indicates that it could grow further through the thickness of the blade edge. 22.214.171.124. Round TCs. Round TCs are those TCs which extended on both sides of the trailing edge of the blade i.e., growing through the two shell halves of the blade. An example of the round TCs that found on the 300 kW wind turbine blades is shown in (Fig. 8b–c). This round TC has a branched river form due to the obstacles formed from the ﬁber layers to its growth path. The round TCs could be started initially as simple TCs then grow further crossing the trailing edge to the other side of the edge, this is mainly due to the reduction of the of the loading capacity in this region. The possible cause of the round TCs is the weakest reinforcement at this region, as mentioned above in the discussion of the simple TCs, which makes this region incapable of resisting the high load at this region. This round transverse crack is considered an example of the most dangerous cracks due to cutting the whole edge of a highly loaded region of the blade. So that, it was decided to make a follow-up inspection on different time span to monitor the progress of such crack until it is repaired. 126.96.36.199. Short root TCs. Short root TCs are another form of TCs (locating between the black line marks in Fig. 9 which are shallow and short (from 1.5 up to 2.5 cm long). Those cracks are found in the root of the 100 KW wind turbine blades. This short root TCs are found as individual crack or as a group of up to 8 neighboring cracks with inter-crack spacing from 2 to 6 cm. These repeated short TCs is possibly occurring due to an excessive cosmetic bonding material to any misﬁt of the bonded shells. 188.8.131.52. Repeated simple TCs. Repeated simple TCs on one side of the trailing edge are another form of the transverse cracks as shown in Fig. 10a. This type of tax is found in only one blade of the 300 KW wind turbines. The depth of these repeated TCs Fig. 9. Short repeated root TCs at the aerodynamic cover of a 100 kW wind turbine blades. 486 S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 Fig. 10. Different forms of repeated TCs in the 300 kW and 100 kW wind turbine blades. (a) Repeated simple TCs at 0.79 of the rotor radius of a 300 kW turbine and (b) TC’s oriented at 45° to the edge of a 100 kW . was examined using a ﬁne smooth tool and found that they are extended only through the coating layer. This type of repeated cracks is due to the stall of the blade. Wind turbine blade stalling (Fig. 11) is an edgewise vibration  due to increasing the angle at which the wind strikes the blades (angle of attack), and it reduces the induced drag (drag associated with lift). Stalling can happen when the winds speed highly increased. A fully stalled turbine blade when stopped if the ﬂat side of the blade facing direct the wind current. This type of cracking could be due to external force impacting the other side of the trailing edge. 184.108.40.206. Transverse cracks on 45°. Transverse cracks on 45o with the trailing edge (Fig. 10b) This crack type has been found in the turbine where the repeated TCs (Fig. 10b) exist. From the production information, the outer layer (under the gelcoat) is usually randomly oriented ﬁbers, while the next layers are mostly arranged at 45° and 45° with the blade length. Cosmetic ﬁnishing (grinding) led mostly to some removal till reaching the subsequent layers (45° and 45°). With the vibrations (due to turbine stall) or fatigue loading, the ﬁber getting easily loosen from the matrix [1,3,13,14] leading to the appearance of such short cracks which aligned with the ﬁber orientation. The distribution of the different TCs sizes on the blade length in terms of normalized rotor radius is shown in Fig. 12. It is clear that the short root cracks are mainly found in the 100 kW blades. Some TCs with a crack length of 5 cm are detected in 300 kW blades (see Fig. 13). 3.2.3. Edge damages Edge damages in the form of edge cuts or crushing were observed at various positions along the trailing edge of the blades. Fig. 14a shows an example image of the edge cuts in the trailing edge of the 300 kW wind turbine blades that observed at about 3.4 m from the tip with a length of 10 cm. Edge crushing (Fig. 14b) is another form of edge damages which can be found on the trailing edge of both 100 kW and 300 kW rotor blades. The trailing edge of the 300 kW 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. Fig. 11. Position of the wind turbine blade relative to the wind current (a) attached ﬂow and (b) stall [10–12]. S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 487 Fig. 12. Trailing edge transverse cracks TC (100 and 300 kW ). Fig. 13. Trailing edge transverse crack TC length on the rotor radius of 100 and 300 kW wind turbine blades. Fig. 14. (a) Edge cuts in a 300 kW rotor blade and (b) crushing of the trailing edge of a 100 kW rotor blade. Allocation of the edge cuts relative to the normalized rotor radius is presented in Fig. 15. The edge damage sizes are ranging between 1 cm and 13 cm. It can be observed that the edge cuts on the trailing edge of the 300 kW blades are 488 S. Ataya, M.M.Z. Ahmed / Engineering Failure Analysis 35 (2013) 480–488 Fig. 15. Allocation of edge cuts on the normalized rotor radius in 81 and 18 blades of 100 and 300 kW power, respectively. concentrated in the lower third of the blade length where the edge is sharp which makes it easy to be crushed by the impact with any external force. 4. Conclusions For the current study, the following conclusions can be drawn:(1) Various damages found in trailing edge of 81 blades of the 100 kW and 18 blades of the 300 kW wind turbines which have a working life ranging between 17 and 22 years were reported and allocated on the blade length. (2) Longitudinal cracks were detected with lengths up to 0.59 m are extending from the blade root to the region of geometric change (cover of the aerodynamic zone) and debonding of lengths up to 1.35 m on the blade trailing edge. (3) Linear ﬁtting of the location of the edge longitudinal cracks shows that they are mostly occurring at 0.73 of relative rotor length. (4) Transverse cracks (TCs) were classiﬁed into: - simple (one side) TCs, round TCs, repeated short root TCs, simple TCs and trailing edge cracks at 45° to the blade length. (5) Transverse cracks are concentrated in the highly fatigue loaded region on the trailing edge. (6) Edge damages in the form of edge cuts or crushing are concentrated in the rear third of the blade toward the tip. Acknowledgements Science and Technology Development Fund (STDF) of Egypt is deeply acknowledged for the ﬁnancial support of Grant No. 1795 of the call TC/1/Energy/2009/Renewable (Wind Energy). The authors express their great thanks and appreciation for the Egyptian New and Renewable Energy Authority (NREA) 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 Prof. Dr. Eng. Rashad Ramadan, Faculty of Petroleum and Mining Engineering, Egypt, and Mr. Otto Lutz, Authorized Expert on Fibre Composite Materials, Bundorf, Germany, for the helpful discussion. References               Kong C, Kim T, Han D, Sugiyama Y. Investigation of fatigue life for a medium scale composite wind turbine blade. Int J Fatigue 2006;28(10):1382–8. Chou J-S, Chiu C-K, Huang IK, Chi K-N. Failure analysis of wind turbine blade under critical wind loads, Eng Failure Anal 2013;27:99–118. Kensche CW. Fatigue of composites for wind turbines. Int J Fatigue 2006;28(10):1363–74. Manwell JF, MecGowan JG, Rogers AL. Wind energy explained: theory design and application. 2nd ed. John Wiley and Sons Ltd.; 2009. p. 276–304. Piggott H. How to build a wind generator – the axial ﬂux alternator windmill plans. UK: University of Loughborough; May 2003. Ancona, D, McVeigh, J. Wind turbine – materials and manufacturing fact sheet, a report prepared by Princeton energy resources in international, LLC for the ofﬁce of industrial technologies; 2001. US Department of Energy. Jüngert A. Damage detection in wind turbine blades using two different acoustic techniques. The e-J Nondestruct Test 2008. Cripps D. The future of blade repair. Reinf pl 2011;55(1):28–32. Martin JC, Barroso A, Parıs F, Canas J. Study of damage and repair of blades of a 300 kW wind turbine. Energy 2008;33:1068–83. Guidelines for design of wind turbines. 2002, Denmark: Centraltrykkeri. Hu D, Hua O, Du Z. A study on stall-delay for horizontal axis, wind turbine. Renew Energ 2006;31:821–36. Larsen JW, Nielsen SRK, Krenk S. Dynamic stall model for wind turbine airfoils. J Fluid Struct 2007;23:959–82. Costa J, Turon A, Trias D, Blanco N, Mayugo JA. A progressive damage model for unidirectional ﬁbre-reinforced composites based on ﬁbre fragmentation. Part II: stiffness reduction in environment sensitive ﬁbres under fatigue. Compos Sci Technol 2005;65(14):2269–75. Boerstra GK. The multislope model: a new description for the fatigue strength of glass ﬁbre reinforced plastic. Int J Fatigue 2007;29(8):1571–6.