Simulation of Mechanical Behavior and Damage of a Large Composite Wind Turbine Blade under Critical Loads

Received: 11 May 2017 / Accepted: 6 June 2017

#Springer Science+Business Media B.V. 2017

Abstracta Issues such as energy generation/transmission and greenhouse gas emis- sions are the two energy problems we face today. In this context, renewable energy sources are a necessary part of the solution essentially winds power, which is one of the most profitable sources of competition with new fossil energy facilities. This paper present the simulation of mechanical behavior and damage of a 48 m composite wind turbine blade under critical wind loads. The finite element analysis was performed by using ABAQUS code to predict the most critical damage behavior and to apprehend and obtain knowledge of the complex structural behavior of wind turbine blades. The approach developed based on the nonlinear FE analysis using mean values for the material properties and the failure criteria of Tsai-Hill to predict failure modes in large structures and to identify the sensitive zones.

Keywords Composite wind turbine blade . Finite element analysis . Mechanical behavior

1 Introduction

The current global context has highlighted the limitations of fossil resources and the governments are increasingly acknowledging the impacts of greenhouse gas emissions on the global climate, and are seeking to develop sustainable energy and in particular

* M. Tarfaoui

[email protected]

1 Department of Fluid Dynamics, Materials and Structures, ENSTA Bretagne–IRDL/LBMS, 29806 Brest, France

2 Laboratory for Renewable Energy and Dynamic Systems, FSAC - UH2C, Casablanca, Morocco

of the marine energies production systems to aid mitigate the effects [1]. Additionally, faced with growing insecurity over fossil fuel supplies, countries search for investing in clean renewable energy sources because offered a more efficient alternative to generate electricity in order to get increased energy security. Offshore wind power is one of the most mature technologies of renewable marine energy sources and has emerged as a promising alternative source for overcoming the energy crisis and the growing concern about sustainability and emission reduction requirements in the world [2].

We are currently witnessing a substantial growth in the wind energy sector world- wide. This growth is expected to accelerate even more in the foreseeable future [3].

This means that a massive number of wind turbine blades will be produced in the forthcoming years. There is a large potential for economizing material in these blades.

Commercial wind turbines have increased consistently in size during the past thirty years, largely for the economic reasons in an attempt to reduce the cost of electricity generation [4]. This is due to the fact that the wind speed – and hence the wind power captured – increases with altitude and that reducing the number of individual turbine units helps reduce the overall cost of a wind farm, especially in the case of offshore farms. Currently the largest machine; has a rated output of 5 MW and a rotor diameter of 124 m [5].

Large wind turbine blades are typically constructed with thin skins made of composite materials [6]. GFRP are the most commonly employed materials, but CFRP are also discovering their way in recent years [7]. The design of a wind turbine structure requires several considerations such as strength, stability, cost and vibration.

Reduction of vibration is a good measure for a successful, safe design of the blade structure [8]. The increase rate of blade mass with length has been diminishing in the past decades key drivers for decrease [9]:

& Advanced fabrication processes

& Introduction of new materials

& More efficient use of materials and improved structural configurations

The principal loads on the blades are caused by the wind and by gravity. Wind loads principally produce both flapwise and edgewise bending. These loads have both a static and a dynamic component (variations in wind speed and natural wind shear) that influence fatigue on the blade material [10, 11].

In general, Wind turbine blades are complex structures and are designed in this study to deliver the maximum power from the wind at the minimum price. Essentially the design is created by the aerodynamic specifications and structural configuration and materials selec- tion, but economics propose that the blade shape is an arrangement to keep the cost of construction reasonable [12].

The present paper employs the finite element method to simulate the behavior of a

large composite wind turbine blade under critical wind loads. In order to identify the

loads that can lead to the ruin of the structure and to characterize the mechanisms of

intralaminar damage and failure, the TSAI-HILL criterion (TSAIH) this is widely used

in the composite industry. The global model has been compared to the various shapes

of spar and webs. The failure of plies applying different criterion has not been

employed in this research and the only failure criteria used is that of long fibers.

2 Design Summary of the Composite Wind Turbine Blade

The wind turbines blades have a very complex structure as the other structures of renewable marine energies [13]. By using the finite element method to model well in face of their structural complexity taking into account all the structural properties and boundary conditions applied most as close as possible to the actual cases in order to validate the optimal design as well as the dynamic response or because the vibration at the natural frequencies is an important part of the design of the blade [14]. The development of advanced and original composite blades requires a severe synergy between principal designs components to improve the design process and reduce rework and redesign and enhanced functionality and extended lifetime is the interplay between:

The principal purpose in the design of wind turbines is to find a rotor that assembles the basic conditions requested. The design of the aerofoil of a wind turbine blade is a compromise between aerodynamic and structural (stiffness) considerations.

50kW D=15 m

100kW D=20 m

1980 1985 1990 1995 2000 2003

Fig. 2 Growth of blade mass with blade length [9]

Aerodynamic considerations are dominating the design of the outer two-thirds of the blade while structural considerations are more important for the design of the inner one-third of the blade. For this, the first hypothesis of the aerodynamic rotor is its diameter, which can be roughly approximated power coefficient. In addition, it is important to consider into account the influence of the geometry of the rotor, taking into consideration the full important, the aerodynamic performance, strength and stiffness conditions, and costs [14].

It desires to produce a power of 5 MW with a blade of 48 m long, aerodynamic characteristics were collected within a research of marketing match, where they had the distribution curves of chord, the twist, the pre-bend, thickness, and the distance between the pitch axis and the trailing edge of several blades used in the market, and averaged to obtain those that were used to create the numerical model (Figs. 1, 2 and 3).

The blade that we will analyze is joined on a three-bladed of an offshore wind turbine which produces a maximum power of 5 MW. The overall characteristics of the considered blade are presented in Table 1 [5].

A preliminary calculation with BHELICIEL^ software permitted us to discover a satisfactory profile answering the specifications of Table 1. Figure 4 gives a schematic representation of initial profile NACA 4412 selected [15].

•Innovative structural configurations

•Better understanding of size effect

•Aerodynamic shape

•Composite material has excellent long-term durability is essential for wind turbine balde

•Reduction of defects

•Reduction in cycle/production times

•Reduction of labour intensive steps

•Improvement of material properties

•Better understanding of material behaviour

•Damage tolérances approach

•Optimized use of materials

The internal structure of materials

Process technologies for manufacturing

Performances under its operating

conditions Properties

Fig. 3 Key issues for new wind turbine blade

Table 1 Overall characteristics of the blade

Length (mm) 48,000

Maximum cord (mm) 3932

Position twists maximum (mm) R9000

Fluid speed upstream of blade (m/s) 25

Angular velocity (rpm) 15,7

Frequency of solicitation: Fr (Hz) 0,26

Power (MW) 5

3 Finite Element Model Development

The finite element method (FEM) has traditionally been used in the development of wind turbine blades mainly to investigate the global behavior in terms of, for example, eigen frequencies, tip deflections, and global stress/strain levels [16]. This type of FE-simulation usually predicts the global stiffness and stresses with a good accuracy [17]. Local deformations and stresses are often more difficult to predict and little has been published in this area. One reason is that the highly localized deformations and stresses can be non-linear, while the global

Fig. 4 Select profile NACA 4412

Fig. 5 Blades with different shear-web transverse placement

Table 2 Mechanical proprieties of materials used

Material E11(MPa) E22(MPa) G12(MPa) ϑ12 ρ(kg/m³)

Orthotropic Composites UD 38,887 9000 3600 0.249 1869

TRIAX 24,800 11,500 4861 0.416 1826

R4545 11,700 11,700 9770 0.501 1782

Isotropic Composites SKINFOAM 256 256 22 0.3 200

ADHESIVE 3000 3000 1150 0.3 1200

response appears linear for relatively small deflections. Another factor is that a relatively simple shell model can be used for representing the global behavior, while a computationally more expensive 3D – solid model may be necessary to predict this localized behavior. Even with a highly detailed 3D solid model it would rarely be possible to predict deformations or stresses accurately without calibration of the FE-model [14]. This calibration is necessary due to large manufacturing tolerances. Features such as box girder corners and adhesive joints often differ from specifications. Geometric imperfections are often seen and can cause unexpected behavior, especially relating to the strength predictions but also the local defor- mations can be affected. In this paper, box girder corners were not modeled in detail using solids.

The influence of the shear-web placement on the blade mass, bending stiffness and bending torsion coupling is studied in this example. Five different geometries are examined as shown in

Laminates Plies

UD 0

TRIAX [−45/0/45]

R4545 [−45/45]

Fig. 6 Blade sectioned

Table 4 Partition areas

Zone Localization

L Shear webs

ROOT Zone fixing of the blade

A Zone joining of the blade

B&D Zone leading and trailing edge

C Contact blade/Shear webs

E Blade tip

Fig.4. In each one, the shear-webs are moved in opposite directions, from being very close to each other to being near the leading and trailing edges [18]

(a) Zone C

(a) Zone B

Fig. 7 Laminate by zones and example of stratification in zone B and C

(1) Blade with web of form T (39% length of the cord) (2) Blade with one shear web (39% length of the cord)

(3) Blade with two shear webs (between 49% and 29% length of the cord) (4) Blade with three shear webs (between 49% and 29% length of the cord) (5) Blade with web of form H (between 49% and 29% length of the cord)

Figure 5 shows the various models which will be useful for the study of optimization of the geometrical form of the webs.

In the next section of this paper one will consider that [15]:

& Model 1: blade with spar with only one web (in only one part).

& Model 2: blade with spar with two webs (in only one part).

& Model 3: blade with spar forms H (in only one part).

& Model 4: all (4 independent parts): blade, spar with only one web and adhesive.

& Model 5: all (4 independent parts): blade, spar with two webs and adhesive.

3.1 Material Parameters and Lamination Strategy

There is a wide range of materials and manufacturing techniques utilized in the wind turbine industry today. The material combinations used are predominantly composite laminates with embedded threaded steel rods in the root section connecting the blade to the hub in a bolted connection. Polyester, vinylester and epoxy resins are common, matched with reinforcing wood, glass, and carbon fibers. Some designs integrate carbon and glass fiber as well as birch and balsa wood.

Both the materials and the lamination strategy were selected through the UpWind data.

UpWind is a European project funded under the Sixth Framework Program of EU. Its task is the design of powerful wind turbines (8–10 MW) with both onshore and offshore. The materials used, in principle, were 5 different types: UD, Triax, R4545, Foam and Webs, whose properties are shown below in the Table 1.

The mechanical properties of composite materials are given in Table 2.

The laminates used, whose properties are indicated in Table 1 consist of plies with various orientations as one can see it in Table 3.

Where, the UD, the TRIAX and the R4545 are composite materials (orthotropic) and the others are isotropic materials. This was done in order to be able to choose,

(a) S4R (b) C3D8R

Fig. 8 Elements used in FEA for modeling

among all, the best setting which it can obtain a low weight blade, with a low frequency and which is not expensive. The blade was sectioned along into 4 parts, to make combinations of materials depending on their requirement, which are shown in Fig. 6.

The materials and lamination strategy chosen for blade of 48 m long was: in the Spar Cap was add adhesive, a new isotropic material whose properties are shown in the Table 2, and Triax, in order to decrease the natural frequency of the blade. Also, with respect to the UpWind strategy, the Shear Webs will change the plies of Webs by Foam, because this has bigger density and lower frequency. To better understand, Table 4 is presented below, which shows the lamination that we will consider, section by section (Figs. 7 and 8).

Once finished this part, the laminate on the blade by zones is as follows:

(a)

(b)

(c)

(b) Model 4 : S4R+C3D8R (a) Model 1 : S4R

Fig. 9 Mesh of models

3.2 Mesh Sensitivity Analysis

For the 5 developed models, shell elements type S4R was used. Below characteristics of the 5 models with 200 mm a mesh size of elements. Solid elements C3D8R were used to mesh the adhesive (model 4 and 5), Fig. 9. Below, an example of the mesh carried out for model 1 and 4. The difference between the two cases of materials assignment affects the mass of the intrados/extrados face and webs. The overall assets of the blade are almost the same for the two types of modelling.

4 Results and Discussion

The approach developed here based on the nonlinear FE analysis using mean values for the material properties and the failure criteria of Tsai-Hill to predict failure modes in large structures and to identify the sensitive zones. In general, composite materials are less resistant

Fig. 11 Flowchart of BEM method for static energy and calculation of the rotor

to combined stresses than to elemental stresses; the limit of elasticity in transverse tension is generally lower when the ply is also stressed in shear.

In the case of long-fiber composites, the most widely used criterion is the Tsai-Hill criterion, which is written with the above notations as follows:

σ

X

2

þ σ

Y

2

þ τ

S

þ σ

σ

X

2 < 1 Where:

X* the longitudinal elastic limit is equal to X

(tensile limit) if the longitudinal stress σ

is positive, and X

(compression limit) if σ

is negative.

Y* is equal to Y

or Y

according to the sign of the transverse stress σ

τ

The shear stress is equal to S

.

Wind loads have a great influence in the field of civil engineering, especially in the design of structures such as wind turbines which by their nature must be placed in areas where wind is important. Many incidents that have caused partial or total ruin of structures have occurred due to insufficient attention to wind action in the design phase.

In our study we studied three cases:

23,377 0,753 6695 281,141 9056 80,520

18,528 0,414 6209 334,236 7475 112,004

15,068 0,282 5337 362,438 6780 128,369

12,612 0,197 4537 379,194 5930 136,405

6491 0,113 1959 234,180 4083 70,669

4732 0,084 1362 199,189 3548 57,344

3365 0,066 0,923 161,885 3180 44,400

2273 0,049 0,595 122,993 2646 32,184

1287 0,034 0,320 74,301 1981 18,462

93,714 3019 26,840

111,412 2492 37,335

120,813 2260 42,790

126,398 1977 45,468

78,060 1361 23,556

66,396 1183 19,115

53,962 1060 14,800

40,998 0,882 10,728

24,767 0,660 6154

& Case 1: The wind exerts a pressure on the blade which differs according to the surfaces. The pressure side is overpressure: the pressure is +0.95 kPa with respect to the atmospheric pressure and the extrados is in depression, −0.95 kPa. The value of ±0.95 kPa represents the most extreme case to which the blade could be subjected for a wind speed of between 24 m/s and 28 m/s.

& Case 2: The wind exerts a pressure on the blade which differs according to the

surfaces. The pressure side is overpressure: the pressure is +0.95 kPa with respect

Plan of rotation

Fig. 12 2D characteristics of a blade profile

Fig. 13 Force-displacement for the case of loading « Wind-cas1»

the forces and moments acting on the blades as well as the power extracted by the propeller. In order to perform this calculation, it is assumed that the following data is available:

& The radius of the propeller R.

& Specific velocity λ.

& The number of blades B.

& Characteristics of the profile C

(α) and C

(α).

The flowchart of the Fig. 11 summarizes the resolution algorithm.

The calculation is done using software B JAVAFOIL ^ and Excel 2007.

To be more precise, we must take into account the geometric data of the blade.

The calculation consists in cutting the blade in 9 sections according to its length and

Fig. 14 Stress distribution of wind turbine blade (cas1)

Fig. 15 Force-displacement for the case of loading « Wind-cas2»

Fig. 16 Stress distribution of wind turbine blade (cas2)

Fig. 17 Force-displacement for the case of loading « Wind-cas3»

in calculating, for each portion, the forces of lift and drag caused by the wind and the rotation. The following calculation is performed for (Table 5):

& Wind speed: V

= 25 m/s

& Air density: ρ = 1225 kg/m

& Angular velocity: Ω = 1.64 rad/s

4.2 Critical Wind Loads

We chose to apply 3 cases of distribution of loading of the wind and to see the different reactions and displacement according to the three directions (Figs. 12, 13, 14, 15, 16, 17.18 and Table 6).

& Case 1

Fig. 18 Stress distribution of wind turbine blade (cas3)

Table 6 Result of loading case « Wind-cas1 »

Model Von. Mises (MPa) TSAIH (MPa) Damage position in the blade (mm)

1 1,11E + 02 3,71 4992

2 0,883E + 02 3,6 4992

3 0,874E + 02 3,4 5008

4 1,24E + 02 5,18 4975

5 1,21E + 02 4,97 4977

& Case 3

Model Von. Mises (MPa) TSAIH (MPa) Damage position in the blade (mm)

1 1,18E + 02 4,02 4992

2 0,761E + 02 3,89 4991,3

3 0,749E + 02 3,87 5009

4 1,16E + 02 5,05 4972,5

5 1,09E + 02 4,9 4973

5 Conclusion

This article showed a thorough research into the damage of a 48 m composite wind turbine blade under critical loads. Its complex failure characteristics observed at the transition region were satisfactorily predicted in the FE simulation using Tsai-Hill criteria.

The strategy detailed here for modeling the wind turbine blade shows the feasibility of using Tsai-Hill to predict failure modes in large structures and to identify the sensitive zones. The global model has been compared to the different configuration of spar and webs. The failure of plies using different criterion has not been used in this study and the only failure criteria used is that of long-fiber composites. The use of a combination of failure criteria would be necessary to simulate accurately the failure of these structures.

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