- Email: [email protected]
epoxy composite wind turbine blade
Engineering Failure Analysis 44 (2014) 345–350
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Preliminary failure investigation of a 52.3 m glass/epoxy composite wind turbine blade Xiao Chen a,b,c,⇑, Wei Zhao c, Xiao Lu Zhao a,b,c, Jian Zhong Xu a,b,c a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, No.11 Beisihuan West Road, Beijing 100190, China National Laboratory of Wind Turbine Blade Research & Development Center, No.11 Beisihuan West Road, Beijing 100190, China c Engineering Research Center on Wind Turbine Blades of Hebei Province, No. 2011 Xiangyang North Street, Baoding 071051, China b
a r t i c l e
i n f o
Article history: Received 20 February 2014 Received in revised form 7 May 2014 Accepted 20 May 2014 Available online 11 June 2014 Keywords: Wind turbine Blade failure Composite Delamination Debonding
a b s t r a c t Despite the enthusiastic pursuing for large wind turbine blades to reduce the cost of wind power, wind energy industry has witnessed a number of catastrophic blade failure accidents in recent years. In order to provide more insights into the failure of large blades, this short communication presents preliminary investigation on a 52.3 m composite blade designed for multi-megawatt wind turbines. Static loads were applied to simulate extreme load conditions subjected by the blade. After blade failure, visual inspection was carried out and failure characteristics of the blade were examined. It was found that the blade exhibited multiple failure modes. Among various failure modes observed, delamination of unidirectional laminates in the spar cap was identified to be the plausible root cause of the catastrophic failure of the blade. This study emphasized that through-thickness stresses can significantly affect the failure of large composite blades and provided some suggestions to the current design practices. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Wind power as a type of renewable energy sources has received considerable attention worldwide and its development is growing at an unprecedented rate in recent years. In the wind turbine system, the blades of a wind turbine rotor are generally regarded as one of the most critical components. Driven by economies-of-scale factors that substantially reduce the cost of wind power, the sizes of wind turbine blades become increasingly large. In recent two years, however, structural failure of large composite blades with lengths around 50 m has attracted negative attention to the wind energy sector [1]. The catastrophic blade failure caused by extreme loading conditions such as typhoon and blade tower impact usually results in either whole blades or pieces of blade being thrown from the turbine, endangering adjacent wind turbines and people living/working close to the wind farm. Failure investigation could provide useful information for improving the blade design and minimizing the risk of blade failure. Due to commercial reasons, technical reports of failure investigation performed on failed blades are regarded to be confidential and there is not much information being disclosed. Some researchers managed to provide valuable information to better understand failure behavior and root causes of large blades through expensive full-scale structural tests. Among them, Jensen et al. [2–4] tested a 34 m wind turbine blade and its load-carrying spar girder to failure and found that ⇑ Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, No.11 Beisihuan West Road, Beijing 100190, China. Tel.: +86 135 5239 6959; fax: +86 010 8254 3037. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.engfailanal.2014.05.024 1350-6307/Ó 2014 Elsevier Ltd. All rights reserved.
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X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
Nomenclature Blade geometry LE leading edge TE trailing edge SS suction side PS pressure side Failure mode LF laminate fracture DL delamination DB sandwich skin-core debonding CF core failure
the Brazier effect induced large deformation in the spar cap and the further delamination buckling were the causes led to the blade collapse. Overgarrd et al. [5,6] tested a 25 m blade to failure and concluded that the ultimate strength of the blade was governed by instability phenomena in the form of delamination and buckling instead of the Brazier effect. Yang et al. [7] studied structural collapse of a 40 m blade and found that debonding of aerodynamic shells from adhesive joints was the main reason for the blade to collapse. Chou et al. [8] investigated a typhoon-damaged composite blade with a blade length close to 39.5 m and showed that the blade failed at a wind-speed of 53.4 m/s by delamination and cracking, although it was expected to resist forces at a wind speed of 80 m/s. In order to provide more insights into failure behavior and mechanisms of large composite blades, authors of this study carried out a static failure test on a commercial wind turbine blade with a total length of 52.3 m, which by far, according to authors’ knowledge, is the longest length among those reported by public studies. It was expected that through this study the failure characteristics and failure mechanisms of the state-of-the-art commercial wind turbine blades nowadays commonly with lengths in a range from 50 to 60 m can be better understood. As a part of early research outcomes, this short communication presented complex failure characteristics of the blade that have not been observed from other blades with shorter lengths and identified the plausible root cause of its failure. Furthermore, this work also provided some suggestions to the current blade design practices based on the failure investigation of this large blade. 2. Information of blade test 2.1. Test specimen According to manufacturing information, the blade under investigation was a prototype blade designed for 2.5 MW wind turbines in a Class III b wind site and had a total length of 52.3 m. The geometry of the blade is shown in Fig. 1(a). The blade was made of glass fabrics and vacuum infused with epoxy resin, and it had a conventional box-spar construction with two shear webs. Spar caps contained triaxial laminates at outer and inner surfaces and a large amount of unidirectional laminates between two surfaces. Aft panels, leading edge (LE) panels and two shear webs were sandwich constructions cored with PVC foams. The composite layup regions of the blade are shown in Fig. 1(b). 2.2. Test procedures The blade was first tested under static loads required by certification bodies [9,10] in order to start a series production of this blade type. Two flapwise directions and two edgewise directions of bending were used based on IEC standard 61400-23: Full-scale structural testing of rotor blades [11], which notes these directions being the most important load conditions to be evaluated in the static test. The blade was cantilever-fixed at its root and bending loads were applied in a stepwise form through three cranes to achieve target test loads. The blade had sustained all target test loads successfully with no noticeable material damage or residual deformation according to the post-test visual inspection. Therefore, it was regarded that the structural integrity of the blade was not adversely affected by the static tests for the blade certification. Subsequently, the blade was used for a failure test in the flapwise bending with its suction side (SS) under compression. A new set of test loads designed for 3.0 MW wind turbines was applied to simulate the extreme wind loads the blade was expected to subjected to. The maximum root moment was 12,213 kN m and the maximum root shear force was 431 kN. During the failure test, test loads were applied quasi-statically by using four cranes following a loading procedure of 0%, 40%, 60%, 80%, 100% of the target loads. Load cells were mounted at each crane to record the applied loads. There was no communication among cranes which applied pulling force upwards simultaneously to obtain each prescribed load level and then held for around ten seconds before the next load level was applied, see Fig. 2. Applied loads were continued to increase after the blade survived 80% of the target loads at which acoustic emissions from the inboard region of the blade were detected. During the loading process towards 100% of the target loads, the acoustic
347
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
Root (a) region
Transition region
52.3m
2.5m section
(b)
4.0m section
9.5m section
Spar cap
Leading edge panel
Aft panel Root
Trailing edge
Shear web Fig. 1. The geometry and composite layup regions of the blade. (a) Blade geometry. (b) Composite layup regions of the blade.
Fig. 2. The blade under flapwise bending (before failure).
emissions became significant and the catastrophic failure of the blade occurred drastically at the inboard region of the blade. From the load cell recordings, the load level at the blade failure was estimated to be approximately 90% of the target test loads. 3. Failure observation and results The blade failed at the transition region where the cross-sectional geometry of the blade transits from a circular shape at the blade root to an airfoil shape at the maximum chord. Major failure covered a blade span ranging approximately from 3.5 to 5.5 m, see Fig. 3. Visual observation of failure features are shown in Fig. 4. It was found that although failed regions exhibited a combined form of failure, some typical failure modes, i.e., laminate fracture (LF), composite delamination (DL), sandwich skin-core debonding (DB), and core failure (CF) can be identified. It can be observed that the major LF and DL occurred at outer triaxial laminates in the spar cap, Fig. 4(a). A clear fracture line can be observed at the intersection of spar cap and sandwich panels as shown in Fig. 4(b). Aft panel and LE panel at the blade transition region were primarily subjected to DB and CF as shown in Fig. 4(c). The blade interior was also inspected and it was found that the rear shear web at the major failure regions was completely fractured with an approximate failure angle of 45° to the blade longitudinal axis, see Fig. 4(d).
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X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
Top: p SS
F Front: LE side id
Bottom: PS
Fig. 3. Final failure of the blade at the transition region.
DL & LF Intersection lines of spar cap and sandwich d i h panels l
DB
LF DL
DB
Aft panel LF
LF Aft panel
DB LE panel
LF
DB
Spar cap
Spar cap
(a)
(b)
Spar cap
DB CF Aft panel
300 mm
TE side
(c)
(d)
Fig. 4. Failure observed at the blade transition region. (a) Typical failure modes found at the suction side. (b) Close-up of failure around 4-m blade span. (c) Failure modes found at aft panel. (d) Failure of the rear shear web.
The blade was then sectioned at 4-m span to facilitate examination on the cross section. It was found that DL occurred not only at triaxial laminates constituting the outer surface of the blade but also at unidirectional laminates as shown in Fig. 5(a). Furthermore, sandwich panels exhibited DB and CF at this cross section, see Fig. 5(b) and (c). 4. Discussion Considering that spar caps were designed to carry the primary bending moments, the catastrophic failure of the blade at the transition region was likely caused by DL of unidirectional laminates in the spar cap which was subjected to compressive forces in the failure test. While other failure modes, such as DL and LF of triaxial laminates at spar cap surfaces, DB and CF of sandwich panels, were not as detrimental as DL of unidirectional laminates in the spar cap to the overall strength of the blade, and they were regarded to be less responsible for the final failure of the blade.
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
349
(a) DL in triaxial laminate
DL in unidirectional laminate 30 mm
(b) CF
DB
20 mm
(c)
DB
20 mm Fig. 5. Failure observed at the 4-m cross section. (a) Spar cap at the suction side of the blade. (b) LE panel. (c) Aft panel.
It is noted that the wind turbine blades are usually regarded as thin-walled composite beams in the current design practices. Only failure stresses and failure strains parallel and transverse to the fibers and for shear are necessary to be verified as specified in GL Guideline for the Certification of Wind Turbines [10], which is widely used by wind energy industry worldwide. Consequently, the strength of the blades is commonly analyzed by the classic thin laminate theory and the finite element
350
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
models meshed by two-dimensional shell elements, which are capable to provide all necessary information required by the Guidelines although they implicitly assume that the failure of the blades is only determined by in-plane stresses. These assumptions are applicable to analyzing composite blades with small sizes because the through-thickness stresses in small blades with thin laminates are negligible and only in-plane failure mode need to be considered. However, from the investigation presented in this study, it was evident that the large blade under concern exhibited multiple failure modes in the transition region, and more importantly the blade was dominated by interfacial failure. Indeed, DL and DB were intimately governed by the properties and stress (or strain) states of the interfaces between constituent layers. Furthermore, in-plane failure mode, i.e., LF, which was found at the intersection of spar cap and sandwich panels, was also significantly affected by interlaminar stresses due to geometric and material discontinuities at this location. These observed failure modes are essentially related to through-thickness stresses in composite laminates and sandwich constructions. Therefore, it is impossible to predict the dominating failure modes of the blade using the classic thin laminate theory and the shell element models neglecting the effect of through-thickness stresses. Furthermore, in the complex structural systems like composite blades, structural strength is determined by the strength of the weakest link. When blades are small, they exhibit single failure mode which can be analyzed easily according to the current design practices, when blades become large, however, multiple failure modes could occur and the weakest link is not readily known. Considering the wind energy industry trend of pursuing large blades, it is strongly recommended that the current design practices applicable in analyzing small blades should be used with caution when the failure behavior of large composite blades is of concern. Meanwhile, the more sophisticated thick laminate theory and the solid finite element models are recommended in the blade analysis in order to accurately capture different failure modes related to both in-plane as well as through-thickness stresses. 5. Conclusions and future work Preliminary failure investigation based on visual inspection was performed on a 52.3 m glass/epoxy composite blade, which has been loaded under static bending. Failure characteristics and the plausible root cause were identified. From this study, the following conclusions were obtained: – The blade exhibited multiple failure modes of laminate fracture, delamination, sandwich skin-core debonding, sandwich core failure, and shear web fracture at the transition region. – Among various failure modes, delamination of unidirectional laminates in the spar cap was identified as the plausible root cause of the catastrophic failure of the blade. – The through-thickness stresses were found to be significantly affect the failure behavior of this large composite blade. – The current design practices are not applicable to the strength analysis of the large blades. It is recommended that the thick laminate theory and three-dimensional solid elements in finite element models should be used. As continuation of this work, further study is being conducted to establish a numerical model to simulate the multiple failure modes observed in the blade. Other studies are also planned, with the objective of identifying the process leading to the catastrophic failure of the blade. Acknowledgments The authors would like acknowledge two anonymous reviewers for their constructive comments and helpful suggestions that have led to significant improvement of the paper. References [1] http://www.windaction.org, retrieved on 6 May, 2014. [2] Jensen FM, Falzon BG, Ankerson J, Stang H. Structural testing and numerical simulation of a 34 m composite wind turbine blade. Compos Struct 2006;76:52–61. [3] Jensen FM, Weaver PM, Cecchini LS, Stang H, Nielsen RF. The Brazier effect in wind turbine blades and its influence on design. Wind Energy 2012;15:319–33. [4] Jensen FM, Puri AS, Dear JP, Branner K, Morris A. Investigating the impact of non-linear geometrical effects on wind turbine blades – part 1: current status of design and test methods and future challenges in design optimization. Wind Energy 2011;14:239–54. [5] Overgaard LCT, Lund E, Thomsen OT. Structural collapse of a wind turbine blade – part A: static test and equivalent single layered models. Composite: Part A 2010;41:257–70. [6] Overgaard LCT, Lund E. Structural collapse of a wind turbine blade – part B: progressive interlaminar failure models. Composite: Part A 2010;41:271–83. [7] Yang JS, Peng CY, Xiao JY, Zeng JC, Xing SL, Jin JT, et al. Structural investigation of composite wind turbine blade considering structural collapse in fullscale static tests. Compos Struct 2013;97:15–29. [8] Chou JS, Chiu CK, Huang IK, Chi KN. Failure analysis of wind turbine blade under critical wind loads. Eng Fail Anal 2013;27:99–118. [9] IEC standard 61400-1, third edition, Wind turbines – part 1: design requirements, IEC; 2005. [10] Guideline for the Certification of Wind Turbines, Edition 2010, Germanischer Lloyd; 2010. [11] IEC standard 61400-23, 1st ed., Wind turbines – part 23: full-scale structural testing of rotor blades, IEC; 2011.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Preliminary failure investigation of a 52.3 m glass/epoxy composite wind turbine blade Xiao Chen a,b,c,⇑, Wei Zhao c, Xiao Lu Zhao a,b,c, Jian Zhong Xu a,b,c a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, No.11 Beisihuan West Road, Beijing 100190, China National Laboratory of Wind Turbine Blade Research & Development Center, No.11 Beisihuan West Road, Beijing 100190, China c Engineering Research Center on Wind Turbine Blades of Hebei Province, No. 2011 Xiangyang North Street, Baoding 071051, China b
a r t i c l e
i n f o
Article history: Received 20 February 2014 Received in revised form 7 May 2014 Accepted 20 May 2014 Available online 11 June 2014 Keywords: Wind turbine Blade failure Composite Delamination Debonding
a b s t r a c t Despite the enthusiastic pursuing for large wind turbine blades to reduce the cost of wind power, wind energy industry has witnessed a number of catastrophic blade failure accidents in recent years. In order to provide more insights into the failure of large blades, this short communication presents preliminary investigation on a 52.3 m composite blade designed for multi-megawatt wind turbines. Static loads were applied to simulate extreme load conditions subjected by the blade. After blade failure, visual inspection was carried out and failure characteristics of the blade were examined. It was found that the blade exhibited multiple failure modes. Among various failure modes observed, delamination of unidirectional laminates in the spar cap was identified to be the plausible root cause of the catastrophic failure of the blade. This study emphasized that through-thickness stresses can significantly affect the failure of large composite blades and provided some suggestions to the current design practices. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Wind power as a type of renewable energy sources has received considerable attention worldwide and its development is growing at an unprecedented rate in recent years. In the wind turbine system, the blades of a wind turbine rotor are generally regarded as one of the most critical components. Driven by economies-of-scale factors that substantially reduce the cost of wind power, the sizes of wind turbine blades become increasingly large. In recent two years, however, structural failure of large composite blades with lengths around 50 m has attracted negative attention to the wind energy sector [1]. The catastrophic blade failure caused by extreme loading conditions such as typhoon and blade tower impact usually results in either whole blades or pieces of blade being thrown from the turbine, endangering adjacent wind turbines and people living/working close to the wind farm. Failure investigation could provide useful information for improving the blade design and minimizing the risk of blade failure. Due to commercial reasons, technical reports of failure investigation performed on failed blades are regarded to be confidential and there is not much information being disclosed. Some researchers managed to provide valuable information to better understand failure behavior and root causes of large blades through expensive full-scale structural tests. Among them, Jensen et al. [2–4] tested a 34 m wind turbine blade and its load-carrying spar girder to failure and found that ⇑ Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, No.11 Beisihuan West Road, Beijing 100190, China. Tel.: +86 135 5239 6959; fax: +86 010 8254 3037. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.engfailanal.2014.05.024 1350-6307/Ó 2014 Elsevier Ltd. All rights reserved.
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X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
Nomenclature Blade geometry LE leading edge TE trailing edge SS suction side PS pressure side Failure mode LF laminate fracture DL delamination DB sandwich skin-core debonding CF core failure
the Brazier effect induced large deformation in the spar cap and the further delamination buckling were the causes led to the blade collapse. Overgarrd et al. [5,6] tested a 25 m blade to failure and concluded that the ultimate strength of the blade was governed by instability phenomena in the form of delamination and buckling instead of the Brazier effect. Yang et al. [7] studied structural collapse of a 40 m blade and found that debonding of aerodynamic shells from adhesive joints was the main reason for the blade to collapse. Chou et al. [8] investigated a typhoon-damaged composite blade with a blade length close to 39.5 m and showed that the blade failed at a wind-speed of 53.4 m/s by delamination and cracking, although it was expected to resist forces at a wind speed of 80 m/s. In order to provide more insights into failure behavior and mechanisms of large composite blades, authors of this study carried out a static failure test on a commercial wind turbine blade with a total length of 52.3 m, which by far, according to authors’ knowledge, is the longest length among those reported by public studies. It was expected that through this study the failure characteristics and failure mechanisms of the state-of-the-art commercial wind turbine blades nowadays commonly with lengths in a range from 50 to 60 m can be better understood. As a part of early research outcomes, this short communication presented complex failure characteristics of the blade that have not been observed from other blades with shorter lengths and identified the plausible root cause of its failure. Furthermore, this work also provided some suggestions to the current blade design practices based on the failure investigation of this large blade. 2. Information of blade test 2.1. Test specimen According to manufacturing information, the blade under investigation was a prototype blade designed for 2.5 MW wind turbines in a Class III b wind site and had a total length of 52.3 m. The geometry of the blade is shown in Fig. 1(a). The blade was made of glass fabrics and vacuum infused with epoxy resin, and it had a conventional box-spar construction with two shear webs. Spar caps contained triaxial laminates at outer and inner surfaces and a large amount of unidirectional laminates between two surfaces. Aft panels, leading edge (LE) panels and two shear webs were sandwich constructions cored with PVC foams. The composite layup regions of the blade are shown in Fig. 1(b). 2.2. Test procedures The blade was first tested under static loads required by certification bodies [9,10] in order to start a series production of this blade type. Two flapwise directions and two edgewise directions of bending were used based on IEC standard 61400-23: Full-scale structural testing of rotor blades [11], which notes these directions being the most important load conditions to be evaluated in the static test. The blade was cantilever-fixed at its root and bending loads were applied in a stepwise form through three cranes to achieve target test loads. The blade had sustained all target test loads successfully with no noticeable material damage or residual deformation according to the post-test visual inspection. Therefore, it was regarded that the structural integrity of the blade was not adversely affected by the static tests for the blade certification. Subsequently, the blade was used for a failure test in the flapwise bending with its suction side (SS) under compression. A new set of test loads designed for 3.0 MW wind turbines was applied to simulate the extreme wind loads the blade was expected to subjected to. The maximum root moment was 12,213 kN m and the maximum root shear force was 431 kN. During the failure test, test loads were applied quasi-statically by using four cranes following a loading procedure of 0%, 40%, 60%, 80%, 100% of the target loads. Load cells were mounted at each crane to record the applied loads. There was no communication among cranes which applied pulling force upwards simultaneously to obtain each prescribed load level and then held for around ten seconds before the next load level was applied, see Fig. 2. Applied loads were continued to increase after the blade survived 80% of the target loads at which acoustic emissions from the inboard region of the blade were detected. During the loading process towards 100% of the target loads, the acoustic
347
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
Root (a) region
Transition region
52.3m
2.5m section
(b)
4.0m section
9.5m section
Spar cap
Leading edge panel
Aft panel Root
Trailing edge
Shear web Fig. 1. The geometry and composite layup regions of the blade. (a) Blade geometry. (b) Composite layup regions of the blade.
Fig. 2. The blade under flapwise bending (before failure).
emissions became significant and the catastrophic failure of the blade occurred drastically at the inboard region of the blade. From the load cell recordings, the load level at the blade failure was estimated to be approximately 90% of the target test loads. 3. Failure observation and results The blade failed at the transition region where the cross-sectional geometry of the blade transits from a circular shape at the blade root to an airfoil shape at the maximum chord. Major failure covered a blade span ranging approximately from 3.5 to 5.5 m, see Fig. 3. Visual observation of failure features are shown in Fig. 4. It was found that although failed regions exhibited a combined form of failure, some typical failure modes, i.e., laminate fracture (LF), composite delamination (DL), sandwich skin-core debonding (DB), and core failure (CF) can be identified. It can be observed that the major LF and DL occurred at outer triaxial laminates in the spar cap, Fig. 4(a). A clear fracture line can be observed at the intersection of spar cap and sandwich panels as shown in Fig. 4(b). Aft panel and LE panel at the blade transition region were primarily subjected to DB and CF as shown in Fig. 4(c). The blade interior was also inspected and it was found that the rear shear web at the major failure regions was completely fractured with an approximate failure angle of 45° to the blade longitudinal axis, see Fig. 4(d).
348
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
Top: p SS
F Front: LE side id
Bottom: PS
Fig. 3. Final failure of the blade at the transition region.
DL & LF Intersection lines of spar cap and sandwich d i h panels l
DB
LF DL
DB
Aft panel LF
LF Aft panel
DB LE panel
LF
DB
Spar cap
Spar cap
(a)
(b)
Spar cap
DB CF Aft panel
300 mm
TE side
(c)
(d)
Fig. 4. Failure observed at the blade transition region. (a) Typical failure modes found at the suction side. (b) Close-up of failure around 4-m blade span. (c) Failure modes found at aft panel. (d) Failure of the rear shear web.
The blade was then sectioned at 4-m span to facilitate examination on the cross section. It was found that DL occurred not only at triaxial laminates constituting the outer surface of the blade but also at unidirectional laminates as shown in Fig. 5(a). Furthermore, sandwich panels exhibited DB and CF at this cross section, see Fig. 5(b) and (c). 4. Discussion Considering that spar caps were designed to carry the primary bending moments, the catastrophic failure of the blade at the transition region was likely caused by DL of unidirectional laminates in the spar cap which was subjected to compressive forces in the failure test. While other failure modes, such as DL and LF of triaxial laminates at spar cap surfaces, DB and CF of sandwich panels, were not as detrimental as DL of unidirectional laminates in the spar cap to the overall strength of the blade, and they were regarded to be less responsible for the final failure of the blade.
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
349
(a) DL in triaxial laminate
DL in unidirectional laminate 30 mm
(b) CF
DB
20 mm
(c)
DB
20 mm Fig. 5. Failure observed at the 4-m cross section. (a) Spar cap at the suction side of the blade. (b) LE panel. (c) Aft panel.
It is noted that the wind turbine blades are usually regarded as thin-walled composite beams in the current design practices. Only failure stresses and failure strains parallel and transverse to the fibers and for shear are necessary to be verified as specified in GL Guideline for the Certification of Wind Turbines [10], which is widely used by wind energy industry worldwide. Consequently, the strength of the blades is commonly analyzed by the classic thin laminate theory and the finite element
350
X. Chen et al. / Engineering Failure Analysis 44 (2014) 345–350
models meshed by two-dimensional shell elements, which are capable to provide all necessary information required by the Guidelines although they implicitly assume that the failure of the blades is only determined by in-plane stresses. These assumptions are applicable to analyzing composite blades with small sizes because the through-thickness stresses in small blades with thin laminates are negligible and only in-plane failure mode need to be considered. However, from the investigation presented in this study, it was evident that the large blade under concern exhibited multiple failure modes in the transition region, and more importantly the blade was dominated by interfacial failure. Indeed, DL and DB were intimately governed by the properties and stress (or strain) states of the interfaces between constituent layers. Furthermore, in-plane failure mode, i.e., LF, which was found at the intersection of spar cap and sandwich panels, was also significantly affected by interlaminar stresses due to geometric and material discontinuities at this location. These observed failure modes are essentially related to through-thickness stresses in composite laminates and sandwich constructions. Therefore, it is impossible to predict the dominating failure modes of the blade using the classic thin laminate theory and the shell element models neglecting the effect of through-thickness stresses. Furthermore, in the complex structural systems like composite blades, structural strength is determined by the strength of the weakest link. When blades are small, they exhibit single failure mode which can be analyzed easily according to the current design practices, when blades become large, however, multiple failure modes could occur and the weakest link is not readily known. Considering the wind energy industry trend of pursuing large blades, it is strongly recommended that the current design practices applicable in analyzing small blades should be used with caution when the failure behavior of large composite blades is of concern. Meanwhile, the more sophisticated thick laminate theory and the solid finite element models are recommended in the blade analysis in order to accurately capture different failure modes related to both in-plane as well as through-thickness stresses. 5. Conclusions and future work Preliminary failure investigation based on visual inspection was performed on a 52.3 m glass/epoxy composite blade, which has been loaded under static bending. Failure characteristics and the plausible root cause were identified. From this study, the following conclusions were obtained: – The blade exhibited multiple failure modes of laminate fracture, delamination, sandwich skin-core debonding, sandwich core failure, and shear web fracture at the transition region. – Among various failure modes, delamination of unidirectional laminates in the spar cap was identified as the plausible root cause of the catastrophic failure of the blade. – The through-thickness stresses were found to be significantly affect the failure behavior of this large composite blade. – The current design practices are not applicable to the strength analysis of the large blades. It is recommended that the thick laminate theory and three-dimensional solid elements in finite element models should be used. As continuation of this work, further study is being conducted to establish a numerical model to simulate the multiple failure modes observed in the blade. Other studies are also planned, with the objective of identifying the process leading to the catastrophic failure of the blade. Acknowledgments The authors would like acknowledge two anonymous reviewers for their constructive comments and helpful suggestions that have led to significant improvement of the paper. References [1] http://www.windaction.org, retrieved on 6 May, 2014. [2] Jensen FM, Falzon BG, Ankerson J, Stang H. Structural testing and numerical simulation of a 34 m composite wind turbine blade. Compos Struct 2006;76:52–61. [3] Jensen FM, Weaver PM, Cecchini LS, Stang H, Nielsen RF. The Brazier effect in wind turbine blades and its influence on design. Wind Energy 2012;15:319–33. [4] Jensen FM, Puri AS, Dear JP, Branner K, Morris A. Investigating the impact of non-linear geometrical effects on wind turbine blades – part 1: current status of design and test methods and future challenges in design optimization. Wind Energy 2011;14:239–54. [5] Overgaard LCT, Lund E, Thomsen OT. Structural collapse of a wind turbine blade – part A: static test and equivalent single layered models. Composite: Part A 2010;41:257–70. [6] Overgaard LCT, Lund E. Structural collapse of a wind turbine blade – part B: progressive interlaminar failure models. Composite: Part A 2010;41:271–83. [7] Yang JS, Peng CY, Xiao JY, Zeng JC, Xing SL, Jin JT, et al. Structural investigation of composite wind turbine blade considering structural collapse in fullscale static tests. Compos Struct 2013;97:15–29. [8] Chou JS, Chiu CK, Huang IK, Chi KN. Failure analysis of wind turbine blade under critical wind loads. Eng Fail Anal 2013;27:99–118. [9] IEC standard 61400-1, third edition, Wind turbines – part 1: design requirements, IEC; 2005. [10] Guideline for the Certification of Wind Turbines, Edition 2010, Germanischer Lloyd; 2010. [11] IEC standard 61400-23, 1st ed., Wind turbines – part 23: full-scale structural testing of rotor blades, IEC; 2011.