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Static test until structural collapse after fatigue testing of a full-scale wind turbine blade
Accepted Manuscript Static test until structural collapse after fatigue testing of a full-scale wind turbine blade Hak Gu Lee, Jisang Park PII: DOI: Reference:
S0263-8223(15)00938-1 http://dx.doi.org/10.1016/j.compstruct.2015.10.007 COST 6915
To appear in:
Composite Structures
Please cite this article as: Lee, H.G., Park, J., Static test until structural collapse after fatigue testing of a full-scale wind turbine blade, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.10.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Static test until structural collapse after fatigue testing of a full-scale wind turbine blade Hak Gu Lee* and Jisang Park Wind-turbine Technology Research Center, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam, 641-831, Republic of Korea
Abstract The effect of fatigue damage on the residual strength of a large wind turbine blade is a very important issue in the wind industry. To test the residual strength after fatigue testing, this study prepared a test specimen, a 48.3 m wind turbine blade that had experienced initial static tests and then fatigue tests in accordance with the technical specification IEC TS 61400-23. The prepared specimen was loaded sequentially along the positive flapwise, the positive edgewise, and the negative flapwise direction, while video-recording the test situations. The wind turbine blade was able to sustain the loads in the first two tests, but the blade collapsed in the third test when the negative flapwise load reached 70% of the maximum target value, a value representing 50% of the most severe load in the first test. Based on the recorded information and the fracture patterns at the blade’s broken section, the collapse processes were analyzed. From this analysis we suggest a modified laminate pattern that can enhance the residual strength of a fatigue damaged wind turbine blade.
Keywords: Wind turbine blade; Fatigue damage; Fracture pattern; Structural collapse
*
Corresponding author. XVX^G Tel.: +82-55-280-3261; fax: +82-55-280-3498. E-mail address: [email protected] (H.G. Lee).G
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1. Introduction The durability of wind turbine blades is becoming more critical as blade lengths increase. With the recent trend toward large slender wind turbine blades, the number of blade failures has doubled in the last ten years. About 30 blade failures are now occurring per year in commercially operated wind farms throughout the world [1]. In order to evaluate the ultimate strengths and fatigue lives of these recently designed larger blades, international standards and equivalent guidelines [2-6] require blade manufactures to carry out static and fatigue tests of full-scale prototype blades. There have been several previous studies analyzing the ultimate strengths of full-scale wind turbine blades. Jensen et al. [7] tested a 34 m wind turbine blade until its structural collapse. The failure mechanism was debonding of the outer skin followed by delamination buckling. Jensen et al. considered the root cause the Braizer effect of the shell structure due to bending. Overgaard et al. [8-9] conducted a flapwise static test of a 25 m composite wind turbine blade to collapse. The influence of the Brazier effect on the longitudinal strain in the primary load-carrying laminate was insignificant. Thus, Overgaard et al. asserted it is the delamination and buckling phenomena that govern the structural stability of a wind turbine blade. Yang et al. [10] carried out a flapwise static test of a 40 m wind turbine blade to collapse. The dominant failure mechanism was debonding between the pressure-side and the suction-side aerodynamic shells followed by the instable propagation of the debonding cracks. From these previous studies, it can be concluded that the ultimate strengths of full-scale wind turbine blades are mainly affected by delamination, bucking, and debonding phenomena. There have been few studies pertaining to fatigue lives of full-scale wind turbine blades due to the extremely high cost of fatigue testing. Leeuwen et al. [11] conducted fatigue
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tests of 37 wind turbine blades 3.4 m in length as well as 35 coupons. Flapwise fatigue failures occurred at the tensile side, but edgewise fatigue failures were initiated in the trailing edge bonding lines followed by further crack propagation in the laminate. Blade fatigue data, when compared with coupon data, fitted reasonably with flapwise tests, but did not compare well with edgewise tests. Marín et al. [12,13] analyzed fatigue damage of a 300 kW wind turbine blade. A crack caused by fatigue damage had been initiated at the abrupt geometric-transient region between the root zone and the aerodynamic zone, and then propagated into the laminate. Lee et al. [14] experienced blade root failure during dual-axis resonance fatigue testing of a 3MW full-scale composite wind turbine blade. The bumping motion of the blade shell altered load distribution at the end of the blade root, resulting in the alleviation of load in some locations and the increase of load in other locations. The severe increase of load incurred the partial separation of the Tbolt joints followed by the delamination at the end of the root. Besides the contributions mentioned above, more studies are needed to better understand and prevent blade failures in wind farms in the future. In this study, the residual strength of a fatigue damaged wind turbine blade was tested. After inspecting the fatigue damage on the blade, we carried out additional static tests, while video-recording the test situations. In the positive flapwise direction (from the pressure-side shell to the suction-side shell), the blade was able to sustain the most severe load, but when a static load was applied in the opposite direction the blade collapsed when subjected to 70% of the maximum target load, i.e. equivalent to 50% of the most severe load in the positive flapwise direction. Based on the recorded information and the fracture patterns at the blade’s broken section, the root cause was analyzed. As a result we suggested a modified laminate pattern able to improve the
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residual strength of a wind turbine blade.
2. Test specimen The test blade is a 48.3 m glass/epoxy composite blade with a mass of 11,630 kg and a 1st flapwise natural frequency of 0.72 Hz. It had been developed as a result of a R&D project funded by the Korean government, and had been certified after passing static and fatigue certification tests in accordance with IEC TS 61400-23. The static certification tests were conducted using a test setup and loading conditions as follows. After mounting the test blade on a test rig like a horizontal cantilever beam, five wooden saddle blocks with steel frames were attached at 12.5 m, 19.0 m, 28.0 m, 38.0 m, and 44.5 m from the blade root in order to apply the static test loads on the blade using five electric winches. The static certification tests consisted of four different static tests having different loading directions: the positive and the negative flapwise directions and the positive and the negative edgewise directions. The positive direction describes loading from the pressure-side shell to the suction-side shell for the flapwise test and from the trailing edge to the leading edge for the edgewise test. Among the four directional tests, the positive flapwise test as shown in Fig. 1(a) applies the largest load to the blade. The target test load at each directional test was calculated by multiplying the partial safety factors described in IEC TS 61400-23 and the extreme design load in each test direction. The fatigue certification tests were conducted using a test setup and loading conditions as follows. To oscillate such a huge composite structure, a fatigue test of a full-scale wind turbine blade exploits the blade resonance phenomenon. Fatigue loading conditions are controlled by attaching several additional masses on the test blade in
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order to modify the blade mode shape [15]. The test setup used in the flapwise fatigue test was equipped with one additional mass of 1365 kg at 41.4 m from the root, one exciter of 4450 kg at 28.0 m, and the aerodynamic fairing of 65 kg from 41.4 m to 48.0 m as shown in Fig. 1(b). The test setup used in the edgewise fatigue test was equipped with two additional masses of 4395 kg at 17.0 m and 1380 kg at 28.0 m and one exciter of 3505 kg at 21.0 m as shown in Fig. 1(c). The target test load of each directional test was calculated by multiplying the partial safety factors described in IEC TS 61400-23 and the equivalent fatigue design load in each test direction. The number of testing cycles was one million for the flapwise fatigue test and two million for the edgewise fatigue test. After finishing all certification tests, the blade was inspected as shown in Fig. 2. Several kinds of damage were observed near the blade section at 20.0 m from the root such as delamination between the sparcap and the skin layer, fiber/matrix debonding, and defects in the trailing edge bonding line. This study does not include damage not related to the structural collapse of the blade.
3. Static test to structural collapse Fig. 3 shows the additional static tests until structural collapse of the blade. The blade successfully sustained the positive flapwise static loading as shown in Fig. 3(a). Next, although there were a few cracking sounds on the blade, it sustained the positive edgewise static loading as shown in Fig. 3(b). Lastly, during the negative flapwise loading as shown in Fig. 3(c), the blade collapsed when the test load reached 70% of the maximum negative flapwise load, i.e. equivalent to 50% of the most severe load in the positive flapwise direction. The motion of the blade when it collapsed was video-
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recorded by cameras in the test facility. Fig. 4 represents the fracture patterns in the blade’s broken section located at 20.0 m from the root. The spar cap in the pressure-side shell was completely broken, and the two shear webs cracked obliquely and longitudinally. Also the trailing edge split open along the bonding line. As shown in Fig. 5, the debonding failure was composed of two parts, the interfacial failure between the adhesive and the [±45] layer and the delamination in the trailing edge [0] layers.
4. Root cause analysis The initial motion as the blade collapsed gives a clue on the root cause. By analyzing the recorded video frame-by-frame, it was found that a bending motion and a torsional motion occurred simultaneously on the first frame, but unfortunately it was impossible to distinguish which action precipitated the other. Generally bending and torsion of a wind turbine blade leave different fracture patterns in the two shear webs. The main failure mechanism of the shear webs is compressive buckling failure. Thus the longitudinal cracks in the shear webs mean the existence of large transverse forces, and the oblique cracks in the shear webs mean the existence of large shear forces. The ovalization of the cross section, the Brazier effect [16], can generate the large transverse forces, but this is not the root cause because during the positive flapwise loading the cross section already sustained the test load two times larger than the fracture load. Bending and torsion of the blade induce shear stresses in the shear webs. If bending is the root cause, the oblique cracks in the two shear webs must be parallel as shown in Fig. 6(a). In the test to structural collapse, the two oblique cracks in Fig. 4 are not parallel but have a crossed pattern. Torsion by itself cannot
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create the observed oblique cracks because the shear flows in the two adjacent cells mitigate the shear stresses in the shear webs as shown in the left diagram in Fig. 6(b). However, if debonding occurs first in the trailing edge bonding line, the shear flow almost disappears in the third cell between the trailing edge and the second shear web. As a result, the shear stresses in the second shear web soar as shown in the right diagram in Fig. 6(b), and then incur the oblique cracks we observed in the blade’s broken section. Consequently, it can be ascertained that the main failure mechanism is the trailing edge debonding followed by the torsional failure of the blade section. The trailing edge debonding experienced a two-step failure. It is certain that the interfacial failure between the adhesive and the [±45] layer occurred during the negative flapwise loading because there was no damage on the trailing edge after finishing the positive edgewise loading. However, the delamination in the [0] layers might occur during the positive edgewise loading, creating the cracking sounds we heard. To estimate the strain state in the [0] layers during the positive edgewise loading, we performed an FE analysis using a commercial FE solver, ABAUS 6.13, and its 3 and 4 node shell elements, S3R and S4R. The number of the elements used in the blade FE shell model was 23,090. The boundary condition of the model was the clamped condition at the end of the blade root, and the loading condition was the same as the maximum edgewise load applied by the five electric winches. The FE shell model did not include the adhesive bonding layers of the blade, so the result of the strain analysis in Fig. 7 shows us only the deformation trend of the blade section. During the positive edgewise loading, the pressure side-shell and the suction-side shell are moving away from each other, creating the peel strains in the trailing edge bonding line. During edgewise fatigue testing of the wind turbine blade, high longitudinal stresses in the
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trailing edge [0] layers cause fatigue damage such as fiber/matrix debonding. Hence when the fatigue damaged blade is subjected to the large edgewise static load, delamination can occur in the trailing edge [0] layers due to the peel strains in the fatigue damaged matrix. Fig. 8 represents the collapse processes at the blade’s broken section. The edgewise fatigue test induced the fiber/matrix debonding in the trailing edge [0] layers. Next the peel strains during the edgewise static test caused the delamination in the fatigue damaged trailing edge [0] layers, resulting in the reduction of the trailing edge bonding width. In the positive flapwise static test, the interfacial failure between the adhesive and the [±45] layer occurred when the reduced bonding width was not able to sustain the shear flow any more. The transition from the closed cell to the open cell diminished the torsional rigidity of the blade section, bringing about the torsional motion of the blade. The torsional motion broke the shear webs obliquely with a crossed pattern. Then the ovalization of the blade section incurred the longitudinal cracks in the shear webs as well as the breakage of the pressure-side spar cap. Consequently, it can be concluded that the main root cause of the structural collapse is the reduction of the trailing edge bonding width caused by the delamination in the fatigue damaged trailing edge [0] layers. The laminate pattern of the wind turbine blade was modified to enhance its residual strength as shown in Fig. 9. The existence of the [±45] inner layer that has contact with the adhesive can transfer shear stresses even if there is delamination in the fatigue damaged trailing edge [0] layers. This means that the delamination does not lead to the reduction of the trailing edge bonding width. Consequently, it can be asserted that the contact of [0] layers with bonding adhesive be avoided in order to enhance the residual
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strength of a fatigue damaged wind turbine blade.
5. Conclusion This study analyzed the structural collapse of a fatigue damaged wind turbine blade. During the positive edgewise loading, the peel strains in the trailing edge bonding line induced the delamination in the fatigue damaged [0] layers, creating cracking sounds. During the negative flapwise loading, the interfacial failure in the bonding line occurred when the remaining bonding width, after the delamination, was not able to sustain the shear flow any more. The sudden drop in the torsional rigidity incurred the oblique cracks in the two shear webs. Then the ovalization of the damaged cross section brought about the longitudinal cracks in the shear webs and the breakage of the pressure-side spar cap. Therefore, it was ascertained that the root cause is the reduction of the trailing edge bonding width caused by the delamination in the fatigue damaged trailing edge [0] layers. We suggested the [±45] inner layer have contact with the adhesive instead of the [0] inner layer in order to improve the blade’s residual strength. Furthermore, the contact of a [0] layer with bonding adhesive should be avoided to enhance the residual strength of a fatigue damaged wind turbine blade.
ACKNOWLEDGEMENTS This work was supported by the New & Renewable Energy of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded Korea government Ministry of Trade, Industry and Energy (No.2012T100201707).
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REFERENCES [1]
http://www.caithnesswindfarms.co.uk/AccidentStatistics.htm , 2015.
[2]
International Electrotechnical Commission. International Standard IEC 61400-23 Wind turbines – Part23: Full-scale structural testing of rotor blades, 2014.
[3]
International Electrotechnical Commission. Technical Specification IEC TS 61400-23 Wind turbine generator systems – Part23: Full-scale structural testing of rotor blades, 2001.
[4]
Det Norske Vertias. Standard DNV-DS-J102 Design and manufacture of wind turbine blades, offshore and onshore wind turbines, 2010.
[5]
Germanischer Lloyd. Rules and Guidelines IV Industrial Services 1 Guideline for the Certification of Wind Turbines, 2010.
[6]
Germanischer Lloyd. Rules and Guidelines IV Industrial Services 2 Guideline for the Certification of Offshore Wind Turbines, 2012.
[7]
Jensen FM, Falzon BG, Ankersen J, Stang H. Structural testing and numerical simulation of 34 m composite wind turbine blade, Composite Structures 2006;76:52-61.
[8]
Overgaard LCT, Lund E, Thomsen OT. Structural collapse of a wind turbine blade. Part A: Static test and equivalent single layered models. Composites: Part A 2010;41:257-270.
[9]
Overgaard LCT, Lund E. Structural collapse of a wind turbine blade. Part B: Progressive interlaminar failure models. Composites: Part A 2010;41:271-283.
[10] Yang J, Peng C, Xiao J, Zeng J, Xing S, Jin J, Deng H. Structural investigation of composite wind turbine blade considering structural collapse in full-scale static tests. Composite Structures 2013;97:15-29.
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[11] van Leeuwen H, van Delft D, Heijdra J, Braam H, Jørgensen ER, Lekou D, Vionis P. Comparing fatigue strength from full scale blade tests with coupon-based predictions. Transactions of the ASME 2002;124:404-411. [12] Marín JC, Barroso A, París F, Cañas J. Study of damage and repair of blades a 300 kW wind turbine. Energy 2008;33:1068-1083. [13] Marín JC, Barroso A, París F, Cañas J. Study of fatigue damage in wind turbine blades. Engineering Failure Analysis 2009;16:656-668. [14] Lee HG, Kang MG, Park J. Fatigue failure of a composite wind turbine blade at its
root
end.
Composite
Structures
2015;
http://dx.doi.org/10.1016/
j.compstruct.2015.08.010. [15] Lee HG, Park J. Optimization of resonance-type fatigue testing of a full-scale wind turbine blade. Wind Energy 2015; DOI:10.1002/we.1837. [16] Tatting BF, Gürdal Z, Vasiliev VV. The Brazier effect for finite length composite cylinders under bending. International Journal of Solids Structures 1997; 34:14191440.
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(a)
(b)
(c) Fig. 1. Certification tests of the wind turbine blade: (a) the static certification test along the positive flapwise direction, (b) the fatigue certification test along the flapwise direction, and (c) the fatigue certification test along the edgewise direction.
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Fig. 2. Damage at 20.0 m from the root after fatigue testing of the wind turbine blade.
GGG
(a)
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(b)
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(c) Fig. 3. Tests subjected to static bending loads along: (a) the positive flapwise direction, (b) the positive edgewise direction, and (c) the negative flapwise direction.
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Fig. 4. Fracture patterns in the blade’s broken section located at 20.0 m from the root.
Fig. 5. Interfacial failure of the bonding line and delamination in the trailing edge [0] layers.
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(a)
(b) Fig. 6. Fracture patterns in the shear webs according to failure mechanism: (a) failure caused by bending loads and (b) failure caused by trailing edge debonding followed by torsional loads.
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Fig. 7. Strain result of the blade’s FE shell model near the trailing edge at the blade’s broken section when the maximum positive edgewise bending moment is applied.
Fig. 8. Collapse processes at the blades’ broken section.
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Fig. 9. Modified laminate pattern able to enhance the residual strength of the wind turbine blade.
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S0263-8223(15)00938-1 http://dx.doi.org/10.1016/j.compstruct.2015.10.007 COST 6915
To appear in:
Composite Structures
Please cite this article as: Lee, H.G., Park, J., Static test until structural collapse after fatigue testing of a full-scale wind turbine blade, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.10.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Static test until structural collapse after fatigue testing of a full-scale wind turbine blade Hak Gu Lee* and Jisang Park Wind-turbine Technology Research Center, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam, 641-831, Republic of Korea
Abstract The effect of fatigue damage on the residual strength of a large wind turbine blade is a very important issue in the wind industry. To test the residual strength after fatigue testing, this study prepared a test specimen, a 48.3 m wind turbine blade that had experienced initial static tests and then fatigue tests in accordance with the technical specification IEC TS 61400-23. The prepared specimen was loaded sequentially along the positive flapwise, the positive edgewise, and the negative flapwise direction, while video-recording the test situations. The wind turbine blade was able to sustain the loads in the first two tests, but the blade collapsed in the third test when the negative flapwise load reached 70% of the maximum target value, a value representing 50% of the most severe load in the first test. Based on the recorded information and the fracture patterns at the blade’s broken section, the collapse processes were analyzed. From this analysis we suggest a modified laminate pattern that can enhance the residual strength of a fatigue damaged wind turbine blade.
Keywords: Wind turbine blade; Fatigue damage; Fracture pattern; Structural collapse
*
Corresponding author. XVX^G Tel.: +82-55-280-3261; fax: +82-55-280-3498. E-mail address: [email protected] (H.G. Lee).G
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1. Introduction The durability of wind turbine blades is becoming more critical as blade lengths increase. With the recent trend toward large slender wind turbine blades, the number of blade failures has doubled in the last ten years. About 30 blade failures are now occurring per year in commercially operated wind farms throughout the world [1]. In order to evaluate the ultimate strengths and fatigue lives of these recently designed larger blades, international standards and equivalent guidelines [2-6] require blade manufactures to carry out static and fatigue tests of full-scale prototype blades. There have been several previous studies analyzing the ultimate strengths of full-scale wind turbine blades. Jensen et al. [7] tested a 34 m wind turbine blade until its structural collapse. The failure mechanism was debonding of the outer skin followed by delamination buckling. Jensen et al. considered the root cause the Braizer effect of the shell structure due to bending. Overgaard et al. [8-9] conducted a flapwise static test of a 25 m composite wind turbine blade to collapse. The influence of the Brazier effect on the longitudinal strain in the primary load-carrying laminate was insignificant. Thus, Overgaard et al. asserted it is the delamination and buckling phenomena that govern the structural stability of a wind turbine blade. Yang et al. [10] carried out a flapwise static test of a 40 m wind turbine blade to collapse. The dominant failure mechanism was debonding between the pressure-side and the suction-side aerodynamic shells followed by the instable propagation of the debonding cracks. From these previous studies, it can be concluded that the ultimate strengths of full-scale wind turbine blades are mainly affected by delamination, bucking, and debonding phenomena. There have been few studies pertaining to fatigue lives of full-scale wind turbine blades due to the extremely high cost of fatigue testing. Leeuwen et al. [11] conducted fatigue
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tests of 37 wind turbine blades 3.4 m in length as well as 35 coupons. Flapwise fatigue failures occurred at the tensile side, but edgewise fatigue failures were initiated in the trailing edge bonding lines followed by further crack propagation in the laminate. Blade fatigue data, when compared with coupon data, fitted reasonably with flapwise tests, but did not compare well with edgewise tests. Marín et al. [12,13] analyzed fatigue damage of a 300 kW wind turbine blade. A crack caused by fatigue damage had been initiated at the abrupt geometric-transient region between the root zone and the aerodynamic zone, and then propagated into the laminate. Lee et al. [14] experienced blade root failure during dual-axis resonance fatigue testing of a 3MW full-scale composite wind turbine blade. The bumping motion of the blade shell altered load distribution at the end of the blade root, resulting in the alleviation of load in some locations and the increase of load in other locations. The severe increase of load incurred the partial separation of the Tbolt joints followed by the delamination at the end of the root. Besides the contributions mentioned above, more studies are needed to better understand and prevent blade failures in wind farms in the future. In this study, the residual strength of a fatigue damaged wind turbine blade was tested. After inspecting the fatigue damage on the blade, we carried out additional static tests, while video-recording the test situations. In the positive flapwise direction (from the pressure-side shell to the suction-side shell), the blade was able to sustain the most severe load, but when a static load was applied in the opposite direction the blade collapsed when subjected to 70% of the maximum target load, i.e. equivalent to 50% of the most severe load in the positive flapwise direction. Based on the recorded information and the fracture patterns at the blade’s broken section, the root cause was analyzed. As a result we suggested a modified laminate pattern able to improve the
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residual strength of a wind turbine blade.
2. Test specimen The test blade is a 48.3 m glass/epoxy composite blade with a mass of 11,630 kg and a 1st flapwise natural frequency of 0.72 Hz. It had been developed as a result of a R&D project funded by the Korean government, and had been certified after passing static and fatigue certification tests in accordance with IEC TS 61400-23. The static certification tests were conducted using a test setup and loading conditions as follows. After mounting the test blade on a test rig like a horizontal cantilever beam, five wooden saddle blocks with steel frames were attached at 12.5 m, 19.0 m, 28.0 m, 38.0 m, and 44.5 m from the blade root in order to apply the static test loads on the blade using five electric winches. The static certification tests consisted of four different static tests having different loading directions: the positive and the negative flapwise directions and the positive and the negative edgewise directions. The positive direction describes loading from the pressure-side shell to the suction-side shell for the flapwise test and from the trailing edge to the leading edge for the edgewise test. Among the four directional tests, the positive flapwise test as shown in Fig. 1(a) applies the largest load to the blade. The target test load at each directional test was calculated by multiplying the partial safety factors described in IEC TS 61400-23 and the extreme design load in each test direction. The fatigue certification tests were conducted using a test setup and loading conditions as follows. To oscillate such a huge composite structure, a fatigue test of a full-scale wind turbine blade exploits the blade resonance phenomenon. Fatigue loading conditions are controlled by attaching several additional masses on the test blade in
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order to modify the blade mode shape [15]. The test setup used in the flapwise fatigue test was equipped with one additional mass of 1365 kg at 41.4 m from the root, one exciter of 4450 kg at 28.0 m, and the aerodynamic fairing of 65 kg from 41.4 m to 48.0 m as shown in Fig. 1(b). The test setup used in the edgewise fatigue test was equipped with two additional masses of 4395 kg at 17.0 m and 1380 kg at 28.0 m and one exciter of 3505 kg at 21.0 m as shown in Fig. 1(c). The target test load of each directional test was calculated by multiplying the partial safety factors described in IEC TS 61400-23 and the equivalent fatigue design load in each test direction. The number of testing cycles was one million for the flapwise fatigue test and two million for the edgewise fatigue test. After finishing all certification tests, the blade was inspected as shown in Fig. 2. Several kinds of damage were observed near the blade section at 20.0 m from the root such as delamination between the sparcap and the skin layer, fiber/matrix debonding, and defects in the trailing edge bonding line. This study does not include damage not related to the structural collapse of the blade.
3. Static test to structural collapse Fig. 3 shows the additional static tests until structural collapse of the blade. The blade successfully sustained the positive flapwise static loading as shown in Fig. 3(a). Next, although there were a few cracking sounds on the blade, it sustained the positive edgewise static loading as shown in Fig. 3(b). Lastly, during the negative flapwise loading as shown in Fig. 3(c), the blade collapsed when the test load reached 70% of the maximum negative flapwise load, i.e. equivalent to 50% of the most severe load in the positive flapwise direction. The motion of the blade when it collapsed was video-
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recorded by cameras in the test facility. Fig. 4 represents the fracture patterns in the blade’s broken section located at 20.0 m from the root. The spar cap in the pressure-side shell was completely broken, and the two shear webs cracked obliquely and longitudinally. Also the trailing edge split open along the bonding line. As shown in Fig. 5, the debonding failure was composed of two parts, the interfacial failure between the adhesive and the [±45] layer and the delamination in the trailing edge [0] layers.
4. Root cause analysis The initial motion as the blade collapsed gives a clue on the root cause. By analyzing the recorded video frame-by-frame, it was found that a bending motion and a torsional motion occurred simultaneously on the first frame, but unfortunately it was impossible to distinguish which action precipitated the other. Generally bending and torsion of a wind turbine blade leave different fracture patterns in the two shear webs. The main failure mechanism of the shear webs is compressive buckling failure. Thus the longitudinal cracks in the shear webs mean the existence of large transverse forces, and the oblique cracks in the shear webs mean the existence of large shear forces. The ovalization of the cross section, the Brazier effect [16], can generate the large transverse forces, but this is not the root cause because during the positive flapwise loading the cross section already sustained the test load two times larger than the fracture load. Bending and torsion of the blade induce shear stresses in the shear webs. If bending is the root cause, the oblique cracks in the two shear webs must be parallel as shown in Fig. 6(a). In the test to structural collapse, the two oblique cracks in Fig. 4 are not parallel but have a crossed pattern. Torsion by itself cannot
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create the observed oblique cracks because the shear flows in the two adjacent cells mitigate the shear stresses in the shear webs as shown in the left diagram in Fig. 6(b). However, if debonding occurs first in the trailing edge bonding line, the shear flow almost disappears in the third cell between the trailing edge and the second shear web. As a result, the shear stresses in the second shear web soar as shown in the right diagram in Fig. 6(b), and then incur the oblique cracks we observed in the blade’s broken section. Consequently, it can be ascertained that the main failure mechanism is the trailing edge debonding followed by the torsional failure of the blade section. The trailing edge debonding experienced a two-step failure. It is certain that the interfacial failure between the adhesive and the [±45] layer occurred during the negative flapwise loading because there was no damage on the trailing edge after finishing the positive edgewise loading. However, the delamination in the [0] layers might occur during the positive edgewise loading, creating the cracking sounds we heard. To estimate the strain state in the [0] layers during the positive edgewise loading, we performed an FE analysis using a commercial FE solver, ABAUS 6.13, and its 3 and 4 node shell elements, S3R and S4R. The number of the elements used in the blade FE shell model was 23,090. The boundary condition of the model was the clamped condition at the end of the blade root, and the loading condition was the same as the maximum edgewise load applied by the five electric winches. The FE shell model did not include the adhesive bonding layers of the blade, so the result of the strain analysis in Fig. 7 shows us only the deformation trend of the blade section. During the positive edgewise loading, the pressure side-shell and the suction-side shell are moving away from each other, creating the peel strains in the trailing edge bonding line. During edgewise fatigue testing of the wind turbine blade, high longitudinal stresses in the
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trailing edge [0] layers cause fatigue damage such as fiber/matrix debonding. Hence when the fatigue damaged blade is subjected to the large edgewise static load, delamination can occur in the trailing edge [0] layers due to the peel strains in the fatigue damaged matrix. Fig. 8 represents the collapse processes at the blade’s broken section. The edgewise fatigue test induced the fiber/matrix debonding in the trailing edge [0] layers. Next the peel strains during the edgewise static test caused the delamination in the fatigue damaged trailing edge [0] layers, resulting in the reduction of the trailing edge bonding width. In the positive flapwise static test, the interfacial failure between the adhesive and the [±45] layer occurred when the reduced bonding width was not able to sustain the shear flow any more. The transition from the closed cell to the open cell diminished the torsional rigidity of the blade section, bringing about the torsional motion of the blade. The torsional motion broke the shear webs obliquely with a crossed pattern. Then the ovalization of the blade section incurred the longitudinal cracks in the shear webs as well as the breakage of the pressure-side spar cap. Consequently, it can be concluded that the main root cause of the structural collapse is the reduction of the trailing edge bonding width caused by the delamination in the fatigue damaged trailing edge [0] layers. The laminate pattern of the wind turbine blade was modified to enhance its residual strength as shown in Fig. 9. The existence of the [±45] inner layer that has contact with the adhesive can transfer shear stresses even if there is delamination in the fatigue damaged trailing edge [0] layers. This means that the delamination does not lead to the reduction of the trailing edge bonding width. Consequently, it can be asserted that the contact of [0] layers with bonding adhesive be avoided in order to enhance the residual
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strength of a fatigue damaged wind turbine blade.
5. Conclusion This study analyzed the structural collapse of a fatigue damaged wind turbine blade. During the positive edgewise loading, the peel strains in the trailing edge bonding line induced the delamination in the fatigue damaged [0] layers, creating cracking sounds. During the negative flapwise loading, the interfacial failure in the bonding line occurred when the remaining bonding width, after the delamination, was not able to sustain the shear flow any more. The sudden drop in the torsional rigidity incurred the oblique cracks in the two shear webs. Then the ovalization of the damaged cross section brought about the longitudinal cracks in the shear webs and the breakage of the pressure-side spar cap. Therefore, it was ascertained that the root cause is the reduction of the trailing edge bonding width caused by the delamination in the fatigue damaged trailing edge [0] layers. We suggested the [±45] inner layer have contact with the adhesive instead of the [0] inner layer in order to improve the blade’s residual strength. Furthermore, the contact of a [0] layer with bonding adhesive should be avoided to enhance the residual strength of a fatigue damaged wind turbine blade.
ACKNOWLEDGEMENTS This work was supported by the New & Renewable Energy of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded Korea government Ministry of Trade, Industry and Energy (No.2012T100201707).
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(a)
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(c) Fig. 1. Certification tests of the wind turbine blade: (a) the static certification test along the positive flapwise direction, (b) the fatigue certification test along the flapwise direction, and (c) the fatigue certification test along the edgewise direction.
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Fig. 2. Damage at 20.0 m from the root after fatigue testing of the wind turbine blade.
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(c) Fig. 3. Tests subjected to static bending loads along: (a) the positive flapwise direction, (b) the positive edgewise direction, and (c) the negative flapwise direction.
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Fig. 4. Fracture patterns in the blade’s broken section located at 20.0 m from the root.
Fig. 5. Interfacial failure of the bonding line and delamination in the trailing edge [0] layers.
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(a)
(b) Fig. 6. Fracture patterns in the shear webs according to failure mechanism: (a) failure caused by bending loads and (b) failure caused by trailing edge debonding followed by torsional loads.
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Fig. 7. Strain result of the blade’s FE shell model near the trailing edge at the blade’s broken section when the maximum positive edgewise bending moment is applied.
Fig. 8. Collapse processes at the blades’ broken section.
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Fig. 9. Modified laminate pattern able to enhance the residual strength of the wind turbine blade.
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