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Simulation of fatigue failure in a full composite wind turbine blade

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Simulation of fatigue failure in a full composite wind turbine blade
Mahmood M. Shokrieh
*
, Roham Raﬁee
Composites Research Laboratory, Mechanical Engineering Department, Iran University of Science and Technology, Narmak, Tehran 16844, Iran
Available online 13 June 2005
Abstract
Lifetime prediction of a horizontal axis wind turbine composite blade is considered. Load cases are identiﬁed, calculated andevaluated. Static analysis is performed with a full 3-D ﬁnite element method and the critical zone where fatigue failure begins isextracted. Accumulated fatigue damage modeling is employed as a damage estimation rule based on generalized material propertydegradation. Since wind ﬂow (loading) is random, a stochastic approach is employed to develop a computer code in order to sim-ulate wind ﬂow with randomness in its nature on the blade and subsequently each load case is weighted by its rate of occurrenceusing a Weibull wind speed distribution.
2005 Elsevier Ltd. All rights reserved.
Keywords:
Composites; Finite element; Stochastic; Wind turbine blade; Damage; Failure analysis
1. Introduction
Pollution free electricity generation, fast installationand commissioning capability, low operation and main-tenance cost and taking advantage of using free andrenewable energies are all advantages of using wind tur-bines as an electricity generators. Along with theseadvantages, the main disadvantage of this industry isthe temporary nature of wind ﬂow. Therefore, using reli-able and eﬃcient equipment is necessary in order to getas much as energy from wind during the limited periodof time that it ﬂows strongly.The blade is the most important component in a windturbine which nowadays is designed according to a re-ﬁned aerodynamic science in order to capture the maxi-mum energy from the wind ﬂow. Blades of horizontalaxis are now completely made of composite materials.Composite materials satisfy complex design constraintssuch as lower weight and proper stiﬀness, while provid-ing good resistance to the static and fatigue loading.Generally, wind turbines are fatigue critical machinesand the design of many of their components (especiallyblades) are dictated by fatigue considerations. Severalfactors expose wind turbine blades to the fatigue phe-nomena which can be summarized as shown below [1]:1. Long and ﬂexible structures2. Vibrations in its resonant mode3. Randomness in the load spectra due to the nature of the wind4. Continuous operation under diﬀerent conditions5. Low maintenance during lifetimeA wind turbine blade expects to sustain its missionfor about 20–30 years. Fig. 1 shows a comparison be-tween diﬀerent industrial components and their expectedlife cycles.The above mentioned reasons and extensive expectedlifetime cause design constraints for wind turbine struc-tures that fall into either extreme load or fatigue catego-ries. For the case of extreme load design, the loadestimation problem is limited to ﬁnding a single maxi-mum load level that the structure can tolerate. For de-sign against fatigue, however, loads must be deﬁned
0263-8223/$ - see front matter
2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.compstruct.2005.04.027
*
Corresponding author. Tel./fax: +98 21 749 1206.
E-mail address:
shokrieh@iust.ac.ir (M.M. Shokrieh).Composite Structures 74 (2006) 332–342www.elsevier.com/locate/compstruct
for all input conditions and then summed over the distri-bution of input conditions weighted by the relative fre-quency of occurrence.Most ongoing research on fatigue phenomena usesload spectra obtained by digital sampling of a speciﬁcconﬁguration of strain gauges which read the strain ata speciﬁc location near the root of blade. Then, the rep-resentative sample of each load spectrum is weighted byits rate of occurrence that can be obtained from the sta-tistical study of the wind pattern. Finally all weightedload spectra are summed and total load spectrum is de-rived. This load spectrum is utilized to estimate fatiguedamage in the blade using Miner
s rule [2]. One of themain shortcomings of this method is the linear natureof Miner
s rule. Not only is Miner
s rule not a properrule for fatigue consideration in metals, but also it hasbeen proven that this rule is not suitable for composites[3]. Another problem with using Miner
s rule is theweakness of this model to simulate the load sequenceand history of load events. This shortcoming can clearlybe seen in the diﬀerence of predicted lifetimes of bladeswith two orders of magnitudes for two load cases withdiﬀerent load sequences [2]. Furthermore, most investi-gations in fatigue simulation of composite blades arelimited to the deterministic approach. In addition, iden-tiﬁcation of a place to install the strain gauges in orderto extract the load spectrum is also questionable. Usinga massive and high cost material fatigue database [4], isanother problem with these methods. It forces research-ers to characterize each conﬁguration of lay-up sepa-rately and is the main reason for publishing newversions of these databases each year due to introduc-tion of new lay-up conﬁgurations in blade structure byindustry.
2. State of the art
In this paper, a model to study the fatigue phenom-ena for wind turbine composite blades is presented inorder to overcome the aforementioned shortcomingsof current methods. As a case study, a 23 m blade of aV47-660 wind turbine, manufactured by the VestasCompany, was selected. Firstly, loading on the bladeis considered carefully using the ﬁnite element methodand the critical zone where catastrophic fatigue failureinitiates is determined. Then, each load case is weightedand ﬁnally fatigue is studied using a developed stiﬀnessdegradation method. The main advantages of this meth-od can be expressed in its ability to simulate the load se-quence and load history. Due to randomness of windﬂow; stochastic analysis will be employed instead of deterministic analysis. Based on the capability of themethod for fatigue modeling, performing a large quan-tity of experiments in order to characterize complete fa-tigue behavior of materials is avoided.
3. Modeling
The investigated blade consists of three main partscalled shell, spar and root-joint (see Fig. 2).The shell is responsible to help create the requiredpressure distribution on the blade. The cross sectionsof the blade are diﬀerent airfoils based on aerodynamicconsiderations. The shell also twists about 15
due toaerodynamic reasons and also has a tapered shape fromroot to tip.A brief summary of pertinent data related to theinvestigated blade is shown in Table 1.The spar, which is also called the main beam, has tosupport loads on the blade that arise from diﬀerentsources. The shell structure carries only 20% of totalloads while the rest has to be carried by the spar. Thecross section of the spar has a box shape.The root joint is the only metallic part in the currentblade that connects whole blade structure to the hub byscrews. This metallic joint is covered by composite lam-inates internally and externally.
Fig. 2. Depiction of root-joint, shell and spar.
Number of Fatigue Cycles to Failure
A l l o w a b l e C y c l i c S t r e s s
Commercial AircraftBridges Hydrofoils Helicopter Wind Turbine Blade 20-30 year Life
10
5
10
6
10
7
10
8
10
9
Fig. 1. Schematic
S
–
N
curves for diﬀerent industrial components [1].
M.M. Shokrieh, R. Raﬁee / Composite Structures 74 (2006) 332–342
333
In order to provide data for the ﬁnite element model,a geometrical model was created based on cross sectionproﬁles of shell and spar using Auto-CAD [5] software.Then the wire frame model, which is the fundamentalgeometrical model, was transferred to ANSYS ﬁnite ele-ment software [6]. After that, geometrical modeling wascompleted by creating surfaces using the loft method. Inthe meshing process, second order shell elements wereemployed to increase accuracy of the modeling. In addi-tion, the selected element type is compatible with com-posites and, in order to not having any triangularelements, a manual meshing method was employed.Therefore all elements have quadratic shape with 8nodes and acceptable aspect ratio. Elements of theroot-joint segment are second order cubic solidelements.Convergence criteria should be considered to evalu-ate the results. Convergence analysis is performed on ametallic model of the blade. By improving mesh densitystep-by-step a suitable number of elements is obtained.The stabilization of tip deﬂection and Von-Misses equiv-alent stress at a location far from the applied loadsare the criteria of convergence. Also in order to examinethe case with both bending and torsion in the structure,the loads were applied on the trailing edge and bound-ary conditions consisted of ﬁxing all 6 degrees of free-dom of nodes that are placed at the root. Thedepiction of the FE model is shown in Fig. 3.Fig. 4 shows the results of convergence analysis.From Fig. 4, it is clear that convergence is obtained withthe use of about 10,000 or more elements.
4. Material characterization
The investigated blade consists of three main types of pre-preg glass/epoxy composites: Unidirectional plies,bi-axial and tri-axial materials.Bi-axial and tri-axial plies contain two and threesame unidirectional fabrics, respectively, which arestitched together (thus are not woven). The main reasonfor not using woven form of composites is due to disad-vantages of using them based on their fatigue properties.In the woven form, due to out-of-plane curvature of fabrics, stress concentrations happen and consequentlyfatigue performance of these materials decreases dra-matically [3].Tri-axial and bi-axial fabrics are used in the shellstructure and unidirectional and bi-axial are used inthe spar structure. The conﬁguration of the bi-axial lam-ina is [0/90]
T
and the conﬁguration of the tri-axial lam-inate is [0/+45/
45]
T
. Two kinds of foam (PMI andPVC) are used respectively at the spar and shell loca-tions, respectively, in order to construct the sandwichpanel.
Table 1General speciﬁcations of investigated bladeLength 22,900 mmMaximum chord 2087 mmStation of maximum chord R4500Minimum chord 282.5 mmTwist 15.17
Station of CG R8100Weight of blade 1250 kgTip to tower distance 4.5 mSurface area 28 m
2
Airfoil cross-section types FFA-W3, NACA-63-xxx, MIXFig. 3. Finite element model of turbine blade showing loads andboundary conditions for convergence consideration.
Convergency Based on Stress
600109266042371221312170161541813232102848508766457284358263434281876
Number of Elements
V o n M i s s e s S t r e s s
Convergency Based on Deflection
6001092187626346604572843583428850810284132321541817016766421312 23712
Number of Elements
T i p D e f l e c t i o n
Fig. 4. Convergence graphs of FE model.334
M.M. Shokrieh, R. Raﬁee / Composite Structures 74 (2006) 332–342
The required properties for analyzing the structureare mechanical and strength properties where the ﬁrstgroup is used for stress analysis and the second one isused for failure analysis [7]. Material characterizationis used to extract the aforementioned properties. Fullmaterial properties of the U-D fabric are availableexperimentally [4] but for the bi-axial and tri-axial com-posites, the mechanical properties are not complete. Theavailable data of these materials are limited to the elasticmodulus in the 0
and 45
directions [8]. The bi-axialand tri-axial laminates and the direction where theexperimental results are available are shown in Fig. 5.The available experimental data are summarized inTable 2.As it can be seen from Table 2, elastic modulus
E
1
and
E
2
are equal in both [+45/
45]
T
and [0/90]
T
and[0/90/
45]
T
due to symmetry.
4.1. Extracting non-available mechanical properties
First, stiﬀness matrices of tri-axial and bi-axial mate-rials are calculated by considering the proportion of fab-ric in each direction and then the matrices were inversed.In parallel, compliance matrices based on available datawere made and ﬁnally the recently obtained compliancematrices were compared to the previously extractedcompliance matrices from the stiﬀness matrices. Finally,after omitting redundant equations, the remaining onesform 14 sets of equations with 14 unknowns enablingthe solution for the mechanical properties of constructedunidirectional plies.Those equations are not reported here due to spacelimitations and complete list of the governing equationscan be found in Ref. [9]. Due to this fact that those 14equations are highly dependent and nonlinear, solvingthem by conventional method is impossible and it needsa start point. The best suggestion for a starting point isthe Poisson
s ratio for the bi-axial material in conﬁgura-tion of [0/90]
T
. Experiments show that the Poisson
sratio of [0/90]
T
conﬁguration is somewhere between0.05 and 0.1 and these values also are veriﬁed by theory[10]. In Table 3, the results obtained by using diﬀerent
amount of Poisson
s ratio are summarized.As can be seen, the best results are achieved whenPoisson
s ratio is set to 0.06 because of its
E
Y
. Therefore,mechanical properties of unidirectional plies are avail-able from experiments [4] and mechanical properties of bi-axial and tri-axial laminates can be calculated basedon the technique in this research and using mechanicalproperties of their constructed U-D ply which is calcu-lated and inserted in Table 3. The mechanical propertiesare summarized in Table 4.If we calculate the ﬂexural modulus using the ob-tained data, there is very good agreement to the experi-mentally available value for this parameter as shown inTable 5.One of the main advantages of using the inversemethod can be realized in the fatigue modeling methodthat is employed in this paper and will be discussed laterin detail.
45˚ 0˚ 0˚ 45˚ Biax LaminateTriax Laminate
Fig. 5. Experiment directions of bi-axial and tri-axial laminates.Table 2Available and non-available mechanical properties [4,8]Composites Conﬁguration
E
1
[GPa]
E
2
[GPa]
m
12
E
6
[GPa]Unidirectional – 43 9.77 0.32 3.31Bi-axial [±45]
T
6.8 6.8 N/A
a
N/ABi-axial [0/90]
T
16.7 16.7 N/A N/ATri-axial [0/±45]
T
20.7 ± 3.1 N/A N/A N/ATri-axial [0/90/
45]
T
15.1 ± 2.3 15.1 ± 2.3 N/A N/A
a
N/A: Not available.Table 3Mechanical properties of constructed U-D ply varied by change inmajor Poisson
s ratio of bi-axial laminateMPB
a
E
X
[GPa]
E
Y
[GPa]
m
G
[GPa]0.05 28.549 4.5643 .21 2.1070.06 28.892 4.3624 .26 2.0960.07 29.010 4.2215 .32 2.0900.08 29.127 4.100 .37 2.0850.09 29.197 3.8240 .40 2.0800.10 29.234 3.6942 .43 2.077
a
MPB: Major Poisson
s ratio of bi-axial [0/90]
T
.Table 4Mechanical properties of involved materials in the blade
E
1
[GPa]
E
2
[GPa]MajorPoisson
s ratio
G
XY
[GPa]Unidirectional ply 43 9.77 0.32 3.31Bi-axial [0/90]
T
16.7 16.7 0.06 2.01Tri-axial [0/+45/
45]
T
17.6 7.01 0.52 5.07Table 5Experimental and theoretical comparison of ﬂexural modulusTri-axial [0/90/
45] Tri-axial [0/45/
45]FlexuralmodulusExperimental[8]Theoretical Experimental[8]Theoretical15.1 ± 2.3 15.23 16.7 ± 2.5 17.85
M.M. Shokrieh, R. Raﬁee / Composite Structures 74 (2006) 332–342
335
The last step of this stage is devoted to extracting theweight of the ﬁnite element model and comparing it withrealistic data. These data are shown in Table 6 and de-scribe the health of model from lay-up conﬁgurationand prove that the model is in a good accordance withactual structures.
5. Loading
The load cases are determined by the combination of speciﬁc operating and external conditions. Operatingand external conditions are assumed to be statisticallyindependent [11]. Operating conditions divided into fourgroups known as normal, fault, after occurrence of thefault and transportation, erection and maintenance(see Fig. 6). External conditions are divided into twogroups called normal and extreme. The load cases tobe used for fatigue analysis generally arise only fromthe combination of normal external and operating con-ditions. It is assumed in this context that because theyoccur so rarely, the other combinations will not haveany signiﬁcant eﬀect on fatigue strength [11].The involved subgroups of normal operating andnormal external conditions are shown in Fig. 6.All of the load cases that occur during the mentionedevents can be categorized as follows:1. Aerodynamic loads on the blade2. Weight of the blade3. Annual gust4. Changes in the wind direction5. Centrifugal force6. Force that arise from start/stop angular acceleration7. Gyroscopic forces due to yaw movements8. Activation of mechanical brake9. Thermal eﬀectAll of the above load cases were calculated and it wasevident that gyroscopic forces and forces from mechan-ical brake activation can be ignored in comparison withthe other loads [12]. A temperature range was consid-ered between
10
and +40
according to the surround-ing ambient environment.
6. Static analysis
At the ﬁrst step of static analysis, in order to insurethat the model is compatible with real structures, a freevibration analysis of model is performed [12]. The result-ing data are in a good agreement with experimental data[12]. The results of free vibration analysis show thatmodel is in accordance with real structures from diﬀer-ent aspects such as dimension, material properties andlay-up sequence.In the second step, calculated forces from the previ-ous section are applied and the response of the structureis studied. In some cases, performing non-linear analysiswas necessary due to large rotation eﬀects as a source of non-linearity. The results of these analyses are summa-rized in Table 7.
Table 6Mass comparison of the bladeMass [kg] CG location [mm]FE model 1268.45 7575.23 (from the root)Realistic 1250.00 7500.00 (from the root)
Stand-byStart-upPower ProductionNormal Shut-down
Condition
Operating ConditionsExternal Conditions
Normal OperatingFault
After the Occurrence of the Fault
Transport, Erection & Maintenance
ExtremeNormalNormal Operating GustTemperature RangesChanges in the Wind Direction
Fig. 6. General view of operating and external conditions of a wind turbine.336
M.M. Shokrieh, R. Raﬁee / Composite Structures 74 (2006) 332–342

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