POWER December 2010 - 62

PLANT DESIGN
New Design Tool Improves
Manufacture of Composite Wind
Turbine Blades
Composite materials are ideal for producing wind turbine blades because of their
strength, light weight, and ability to be tailored to provide the precise mechanical
properties needed for any blade design. Now, best practices originally
developed for rotorcraft blade manufacturing can be applied to designing and
manufacturing wind turbine blades that are constructed from composites.
By Rani Richardson, Dassault Systèmes
C
omposite materials are well suited for
use in the manufacture of wind turbine
blades, yet the structural complexity
of these materials also presents difficult
challenges in optimizing design and manufacturing
methods. From a design and manufacturing
standpoint, wind turbine blades
have many similarities to helicopter rotorcraft
blades and other aerospace structures.
Wind turbine manufacturers, therefore, have
the unique opportunity to adapt and apply
the best practices developed and refined
over several decades in the aerospace industry
and help their businesses succeed in a
tough competitive environment by reducing
design lead time, improving blade durability,
and optimizing design and manufacturing
processes.
Composites dominate the wind turbine
blade market due to their superior fatigue
characteristics and stiffness-to-weight ratio,
their unique ability to be fabricated into complex
geometries, and their potential for aero
elastic tailoring. The ability to tailor composites
to specific loads and other requirements
is among these materials' greatest strengths,
but it also increases the complexity of the design
process.
Challenges of Traditional Wind
Turbine Blade Design
Optimizing the properties of the blades is
critical to delivering the needed bending
strength and fatigue performance while
minimizing cost and weight. The properties
of composite blades can be tightly controlled
and varied over their span through
the appropriate selection of ply orientation,
thickness, and lay-up. Because high hopes
are riding on this alternative energy source,
the wind turbine blade industry is working
hard to improve manufacturing efficiency
62
and address blade failure issues, but challenges
remain great, and failure rates are as
high as 20% within three years.
Most causes of failure can be attributed
to substandard design and manufacturing
practices that could be eliminated by
adopting many of the best practices for
composites design and manufacturing in
the aerospace industry. A significant part of
the challenge may be that nearly all composite
blades today are designed on basic
computer-aided design (CAD) systems intended
for making solid metal and plastic
parts of far less complexity than today's
sophisticated, multi-layered composites.
The limited capabilities of the traditional
CAD systems means that analysis and
manufacturing engineering are carried out
in separate and disconnected environments,
creating inefficiencies and introducing the
potential for error.
In the typical process, the aerodynamicist
first defines the outer shape of the blade
using specialized fluid flow software packages
and wind tunnel testing. This design
is then provided to mechanical engineers,
who define the detailed design and basic
ply guidelines for the blade. The CAD software
has no facility for keeping track of the
laminate information involved in composites
design, so the engineer typically tracks
this information in a spreadsheet.
The engineer might start by defining areas
of the part with similar thicknesses or
structural zones. The zone information is
usually maintained manually in a spreadsheet.
Then the engineer will define a ply
stack that delivers the mechanical properties
required for each zone, as indicated
by previous experience. Most companies
involved in composite design have design
rules that are used to guide this process. For
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example, the smaller plies may be located
on the inside of the stack and the thicker
plies on the outside.
An analyst typically creates a finite element
model from scratch to evaluate the
bending strength and other characteristics of
the blade. Various loading conditions are used
to simulate different wind conditions. The
proposed blade design may be challenged
by load-time histories to predict its fatigue
strength. The analyst recommends changes
to the plies based on the analysis results. The
design typically goes back and forth between
the engineer and analyst multiple times-all
the while being manually tracked via spreadsheet,
which has room for error.
The Wind Turbine Blade
Manufacturing Process
Defining the manufacturing process comes
next. In the traditional approach, the final
shapes of the flat patterns are determined
on the shop floor by expensive and timeconsuming
trial and error: cutting plies, laying
them onto the molding tool using manual
measurement, checking for distortion, recutting,
and trying again. The complex aerodynamic
shape of wind turbine blades makes
it hard to predict how composite materials
will conform to the mold's complex surface.
Sometimes distortion causes the material to
draw away from the mold. In other cases, a
ply sequence imbalance across a three-dimensional
(3D) shape generates stresses that
cause wrinkling or warping. A major difficulty
is developing flat patterns that will meet
the ply guidelines without fabric distortion,
such as bunching up on the mold. Using conventional
methods, this process cannot even
begin until the molding tool is built.
At this point, the common procedure is to
cut fabric plies by hand and try to fit them
POWER | December 2010
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POWER December 2010

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