Received 20 February 2014Received in revised form 7 May 2014Accepted 20 May 2014Available online 11 June 2014

power, wind energy industry has witnessed a number of catastrophic blade failure

bines and people

the risk os are regar

be condential and there is not much information being disclosed. Some researchers managed to provide valuable intion to better understand failure behavior and root causes of large blades through expensive full-scale structuraAmong them, Jensen et al. [24] tested a 34 m wind turbine blade and its load-carrying spar girder to failure and fou

http://dx.doi.org/10.1016/j.engfailanal.2014.05.0241350-6307/ 2014 Elsevier Ltd. All rights reserved.

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: drchenxiao@163.com (X. Chen).

Engineering Failure Analysis 44 (2014) 345350

Contents lists available at ScienceDirect

Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanalin either whole blades or pieces of blade being thrown from the turbine, endangering adjacent wind turliving/working close to the wind farm.

Failure investigation could provide useful information for improving the blade design and minimizingfailure. Due to commercial reasons, technical reports of failure investigation performed on failed bladef bladeded toforma-l tests.nd thatWind power as a type of renewable energy sources has received considerable attention worldwide and its development isgrowing at an unprecedented rate in recent years. In the wind turbine system, the blades of a wind turbine rotor aregenerally regarded as one of the most critical components. Driven by economies-of-scale factors that substantially reducethe cost of wind power, the sizes of wind turbine blades become increasingly large. In recent two years, however, structuralfailure 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 resultsKeywords:Wind turbineBlade failureCompositeDelaminationDebonding

1. Introductionaccidents 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 bladedesigned for multi-megawatt wind turbines. Static loads were applied to simulate extremeload conditions subjected by the blade. After blade failure, visual inspection was carriedout and failure characteristics of the blade were examined. It was found that the bladeexhibited multiple failure modes. Among various failure modes observed, delaminationof unidirectional laminates in the spar cap was identied to be the plausible root causeof the catastrophic failure of the blade. This study emphasized that through-thicknessstresses can signicantly affect the failure of large composite blades and provided somesuggestions to the current design practices.

2014 Elsevier Ltd. All rights reserved.Preliminary failure investigation of a 52.3 m glass/epoxycomposite wind turbine blade

Xiao Chen a,b,c,, Wei Zhao c, Xiao Lu Zhao a,b,c, Jian Zhong Xu a,b,ca Institute of Engineering Thermophysics, Chinese Academy of Sciences, No.11 Beisihuan West Road, Beijing 100190, ChinabNational Laboratory of Wind Turbine Blade Research & Development Center, No.11 Beisihuan West Road, Beijing 100190, ChinacEngineering Research Center on Wind Turbine Blades of Hebei Province, No. 2011 Xiangyang North Street, Baoding 071051, China

a r t i c l e i n f o

Article history:

a b s t r a c t

Despite the enthusiastic pursuing for large wind turbine blades to reduce the cost of wind

346 X. Chen et al. / Engineering Failure Analysis 44 (2014) 345350the Brazier effect induced large deformation in the spar cap and the further delamination buckling were the causes led to theblade collapse. Overgarrd et al. [5,6] tested a 25 m blade to failure and concluded that the ultimate strength of the blade wasgoverned 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 themain reason for the blade to collapse. Chou et al. [8] investigated a typhoon-damaged composite blade with a blade lengthclose 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 wasexpected 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 studycarried out a static failure test on a commercial wind turbine blade with a total length of 52.3 m, which by far, according toauthors knowledge, is the longest length among those reported by public studies. It was expected that through this studythe failure characteristics and failure mechanisms of the state-of-the-art commercial wind turbine blades nowadays com-monly with lengths in a range from 50 to 60 m can be better understood. As a part of early research outcomes, this shortcommunication presented complex failure characteristics of the blade that have not been observed from other blades withshorter lengths and identied the plausible root cause of its failure. Furthermore, this work also provided some suggestionsto 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 MWwindturbines 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 bladewas made of glass fabrics and vacuum infused with epoxy resin, and it had a conventional box-spar construction with twoshear webs. Spar caps contained triaxial laminates at outer and inner surfaces and a large amount of unidirectional laminatesbetween two surfaces. Aft panels, leading edge (LE) panels and two shear webs were sandwich constructions cored with PVCfoams. The composite layup regions of the blade are shown in Fig. 1(b).

2.2. Test procedures

Nomenclature

Blade geometryLE leading edgeTE trailing edgeSS suction sidePS pressure side

Failure modeLF laminate fractureDL delaminationDB sandwich skin-core debondingCF core failureThe blade was rst tested under static loads required by certication bodies [9,10] in order to start a series production ofthis blade type. Two apwise 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 beevaluated in the static test. The blade was cantilever-xed at its root and bending loads were applied in a stepwise formthrough three cranes to achieve target test loads. The blade had sustained all target test loads successfully with no noticeablematerial damage or residual deformation according to the post-test visual inspection. Therefore, it was regarded that thestructural integrity of the blade was not adversely affected by the static tests for the blade certication.

Subsequently, the blade was used for a failure test in the apwise bending with its suction side (SS) under compression. Anew set of test loads designed for 3.0 MW wind turbines was applied to simulate the extreme wind loads the blade wasexpected 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 nocommunication among cranes which applied pulling force upwards simultaneously to obtain each prescribed load leveland 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 fromthe inboard region of the blade were detected. During the loading process towards 100% of the target loads, the acoustic

X. Chen et al. / Engineering Failure Analysis 44 (2014) 345350 347 2.5m section 4.0m section 9.5m section Transition region

52.3m

Root region(a)emissions became signicant 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 failurewas 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 atthe blade root to an airfoil shape at the maximum chord. Major failure covered a blade span ranging approximately from 3.5to 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 combinedform of failure, some typical failure modes, i.e., laminate fracture (LF), composite delamination (DL), sandwich skin-core deb-onding (DB), and core failure (CF) can be identied. It can be observed that the major LF and DL occurred at outer triaxiallaminates in the spar cap, Fig. 4(a). A clear fracture line can be observed at the intersection of spar cap and sandwich panelsas shown in Fig. 4(b). Aft panel and LE panel at the blade transition region were primarily subjected to DB and CF as shown inFig. 4(c).

The blade interior was also inspected and it was found that the rear shear web at the major failure regions was completelyfractured with an approximate failure angle of 45 to the blade longitudinal axis, see Fig. 4(d).

Leading edge panel

Aft panel

Trailing edge

Spar cap

Root

Shear web

(b)

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 apwise bending (before failure).

348 X. Chen et al. / Engineering Failure Analysis 44 (2014) 345350Top: SSp

F LE idFront: LE sideThe blade was then sectioned at 4-m span to facilitate examination on the cross section. It was found that DL occurred notonly 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 atthe transition region was likely caused by DL of unidirectional laminates in the spar cap which was subjected to compressiveforces in the failure test. While other failure modes, such as DL and LF of triaxial laminates at spar cap surfaces, DB and CF ofsandwich panels, were not as detrimental as DL of unidirectional laminates in the spar cap to the overall strength of theblade, and they were regarded to be less responsible for the nal failure of the blade.

Bottom: PS

Fig. 3. Final failure of the blade at the transition region.

DL & LFDL & LF

Intersection lines of spar cap and

d i h lDB sandwich panels

DB

LF

Aft panel

DB LFLE panel

DB LFSpar cap

LF

DL

Aft panel

LF

DB

Spar cap

Spar capSpar cap

CFDB

CF

Aft panelAft panel

TE side300 mm

(a) (b)

(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.

X. Chen et al. / Engineering Failure Analysis 44 (2014) 345350 349DL in triaxial laminate

(a)It is noted that the wind turbine blades are usually regarded as thin-walled composite beams in the current design prac-tices. Only failure stresses and failure strains parallel and transverse to the bers and for shear are necessary to be veried asspecied in GL Guideline for the Certication 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 nite element

DL in unidirectional laminate

30 mm

CFCF

DB

20 mm20 mm

DB

20 mm

(b)

(c)

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.

350 X. Chen et al. / Engineering Failure Analysis 44 (2014) 345350models meshed by two-dimensional shell elements, which are capable to provide all necessary information required by theGuidelines although they implicitly assume that the failure of the blades is only determined by in-plane stresses. Theseassumptions are applicable to analyzing composite blades with small sizes because the through-thickness stresses in smallblades 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 mul-tiple failure modes in the transition region, and more importantly the blade was dominated by interfacial failure. Indeed, DLand 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 alsosignicantly affected by interlaminar stresses due to geometric and material discontinuities at this location. These observedfailure 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 andthe 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 strengthof the weakest link. When blades are small, they exhibit single failure mode which can be analyzed easily according to thecurrent design practices, when blades become large, however, multiple failure modes could occur and the weakest link is notreadily known. Considering the wind energy industry trend of pursuing large blades, it is strongly recommended that thecurrent design practices applicable in analyzing small blades should be used with caution when the failure behavior of largecomposite blades is of concern. Meanwhile, the more sophisticated thick laminate theory and the solid nite element modelsare recommended in the blade analysis in order to accurately capture different failure modes related to both in-plane as wellas 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 identied. From thisstudy, the following conclusions were obtained:

The blade exhibited multiple failure modes of laminate fracture, delamination, sandwich skin-core debonding, sandwichcore failure, and shear web fracture at the transition region.

Among various failure modes, delamination of unidirectional laminates in the spar cap was identied as the plausible rootcause of the catastrophic failure of the blade.

The through-thickness stresses were found to be signicantly 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 thethick laminate theory and three-dimensional solid elements in nite element models should be used.

As continuation of this work, further study is being conducted to establish a numerical model to simulate the multiplefailure modes observed in the blade. Other studies are also planned, with the objective of identifying the process leadingto the catastrophic failure of the blade.

Acknowledgments

The authors would like acknowledge two anonymous reviewers for their constructive comments and helpful suggestionsthat have led to signicant 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;76:5261.[3] Jensen FM, Weaver PM, Cecchini LS, Stang H, Nielsen RF. The Brazier effect in wind turbine blades and its inuence on design. Wind Energy

2012;15:31933.[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;14:23954.[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;41:25770.[6] Overgaard LCT, Lund E. Structural collapse of a wind turbine blade part B: progressive interlaminar failure models. Composite: Part A

2010;41:27183.[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 full-

scale static tests. Compos Struct 2013;97:1529.[8] Chou JS, Chiu CK, Huang IK, Chi KN. Failure analysis of wind turbine blade under critical wind loads. Eng Fail Anal 2013;27:99118.[9] IEC standard 61400-1, third edition, Wind turbines part 1: design requirements, IEC; 2005.[10] Guideline for the Certication 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.

Preliminary failure investigation of a 52.3m glass/epoxy composite wind turbine blade1 Introduction2 Information of blade test2.1 Test specimen2.2 Test procedures

3 Failure observation and results4 Discussion5 Conclusions and future workAcknowledgmentsReferences