Design of the Fiber Lay‐up for the Composite Wind Turbine Blade inDesign of the Fiber Lay up for the Composite Wind Turbine Blade in VARTM

Tzai-Shiung Li and Wen-Bin YoungDepartment of Aeronautics and Astronautics, National Cheng Kung University , Tainan, Taiwan, R.O.C.

IntroductionThe wind turbine blade sustains various kinds of loadings during the operating and parking state. Due to the increasing size of the

wind turbine blade, it is important to arrange the composite materials in a sufficient way to reach the optimal utilization of the materialstrength. In the fabrication process of the vacuum assisted resin transfer molding, the fiber content of the turbine blade depends on thevacuum pressure.

In this study, a design of the fiber layup for the vacuum assisted resin transfer molding (VARTM) is conducted to achieve theefficient utilization the material strength. This design is for the wind turbine blade consisting of shell skins with or without the sparstructure. Based on the numerical stress analysis of the turbine blade, a simple iterative method was proposed to determine thethickness distribution of the composite skin shell of the blade structure. The thickness distribution is selected based on the concept oft c ess d st but o o t e co pos te s s e o t e b ade st uctu e e t c ess d st but o s se ected based o t e co cept ouniform loading. In other word, with the applied wind loading during the parking state, the turbine blade will be designed to have thesame safety factor in each station by applying the Tsai‐Wu failure criterion. In the fabrication process of the VARTM, the fiber content ofthe turbine blade skin shell depends on the vacuum pressure and the number of layup fiber mats. A methodology is constructed todetermine the number of fiber layers used in VARTM that will have a uniform safety factor in each station.

Preliminary designBlade element moment (BEM) method is able to provide If the blade element is designed to be subjected to the stress of

the necessary means to predict the aerodynamic forces andmoments acting on a turbine blade. The theory divides theentire blade to several elements and calculates the wind loadat each element as shown in Figure 1.

In the initial design of the thickness distribution, the bladecan be simplified into two kinds of cross sections as shown inFigure 2. The root part can be simplified as the cylindricalshape and the airfoil of the blade can be simplified as parallel

the material strength, the corresponding blade thickness can bedetermined by

where the thicknesses of the root and blade cross section are tsH andtsA

3

( )8

HsH

H

It

c

2

( )( )

0.162sA

I rt r

ct

( ) ( ) / 2( )

( )p

M r t rI r

r

plates.sA

Equal strength designTo determine the thickness distribution of the composite blade

skin with efficient material utilization, a simple way is to have about thesame Tsai‐Wu values for all the blade elements. A finite element stressanalysis of the turbine blade using the initial skin shell design will resultin Tsai‐Wu values that vary dramatically for each element. Therefore,modification of the thickness distribution according to the Tsai‐Wu

Fig. 2 The simplified cross sections for the blade (a) cylindrical shape for the root (b) 

Design of fiber layupIn the design of the blade shin

shell thickness, the effect of fabricationprocess is not considered. During theVARTM process the composite

under atmospheric pressure. Theresulting design is shown in Fig. 5.

modification of the thickness distribution according to the Tsai Wuvalue of each element can be constructed. The designed thickness isshown in Fig. 3.

Fig. 1 The cross section of a blade element under wind loading

parallel plates for the blade

VARTM process, the compositethickness is related to the layup numberof fiber mats under the compression ofthe atmospheric pressure. Therefore, inthe design of blade skin shell thickness,the layup number of fiber is the directparameter that must be determinedbefore the process.

Stress analysis for the blade with

FIG. 3  (a) the thickness distribution of the skin shell (b) Tsai‐Wu values at each element

ConclusionsThe Tsai‐Wu criterion is used as the failure criterion

for the composite By considering the VARTM process

Fig. 4  The Tsai‐Wu values for (a) the design thickness without considering the fiber compression (b) modified thickness

Stress analysis for the blade withthe modified skin shell thickness isperformed and the result is shown inFig. 4. The Tsai‐Wu values at eachelement are all near the design goalafter modifying the skin shell thicknessby considering the fiber compression

Fig. 5  The setup of the fiber layer for the VARTM process

for the composite. By considering the VARTM process,the fiber volume fraction in each element depends onthe thickness. A procedure for the thickness design isproposed to include the effect of the compression inthe fabrication. The fiber arrangement and layers forthe VARTM can be determined by this designprocedure.