Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 439

7 Modeling of wind turbine blades
7.1 Modeling of structural behavior of wind turbine blades
7.1.1 Modeling of entire wind turbine blade
This section outlines basic rules for structural design of wind turbine blades.
A more thorough description of the overall design of wind turbines for various
onshore and offshore applications can be found in the DNV/Risø guidelines [48]
and in other chapters of this topic.
Some designs are constructed with a load-carrying box girder (main spar) that
supports the outer aeroshell as shown in Fig. 2, which illustrates a typical struc-
tural layout for a wind turbine blade with a load-carrying girder. The purpose of
the box girder is to give the blade suffi cient strength and stiffness, both globally
and locally. Globally, the blade should be suffi ciently stiff in order not to collide
with the tower under all types of loading. Locally, the webs, together with the stiffness
of the outer shell, ensure that the shape of the aerodynamic profi le is maintained.
The box girder or the webs usually extend from the root of the blade to a position
close to the tip. The load-carrying fl ange of the box girder, sometimes called the cap,
is usually a single skin construction (i.e. consisting of a single thick laminate, with
most of the fi bers aligned along the blade length, i.e. the z -direction, see Fig. 2). The
webs are usually quite thin sandwich structures; the main purpose of these is to take
the shear loads of the blade. The proper design of the blade requires careful analysis.
For example, geometrical non-linear effects can result in higher than expected loading
of the webs which may result in blade failure at a stress level that is much lower than
predicted when the design is based on linear calculations. The design should ensure
that the failure criteria discussed in the previous section are not exceeded anywhere in
the blade during regular and extreme load situations. This section presents an
overview of modeling tools used to predict the static and dynamic behavior of wind
turbine blades and to determine the stress and strain distribution within a blade.
7.1.2 Beam models
The global defl ection of wind turbine blades, Eigen frequencies and other global
behavior can in general be analyzed with good accuracy by use of beam models.
However, if greater accuracy is needed or more locally structural phenomena need
to be analyzed, more detailed shell and/or solid FE models must be used.
The idea of a beam model is to describe the cross section properties in terms of
suitable coeffi cients, such as area, moment of inertia, torsional stiffness, etc. The
behavior of the beam is then described entirely in terms of one-dimensional func-
tions, such as axial and transverse displacement and torsion. In order for such a
theory to give an accurate representation of the actual behavior of a wind turbine
blade, it is important that the description of the cross section parameters contain all
the relevant information. This includes stiffness parameters, the center of mass, the
elastic center and the center of torsion. For a typical wind turbine blade these three
centers will be located fairly close to each other, but are not coincident.
While the basic properties of cylindrical beams date back to the late 19th century,
consistent theories accounting for cross section variation, pre-twist of the blade