Environmental Engineering Reference
In-Depth Information
Table 1: Failure modes and appropriate strength and fracture concepts used for
the design and analysis of wind turbine blades.
Basic damage modes
Material property (static strength)
Adhesive joint failure
Crack in adhesive layer
G Ic (fracture energy)
Laminate/adhesive interface cracking
G c ( y ) (interface fracture energy)
Interface cracking with fi ber bridging
s ( d ) (cohesive law)
Sandwich failure
Interface cracking
G c ( y ) (interface fracture energy)
Interface cracking with fi ber bridging
s ( d ) (cohesive law)
Laminate failure
Tensile failure (fi ber fracture mode) -
damage zone
s +
(tensile strength in fi ber direction)
Lu
Compressive failure (fi ber fracture
mode) - damage zone
s
(compressive strength in fi ber direction)
Lu
Tensile failure (matrix fracture
mode) - cracking
s + (tensile strength perpendicular to
fi bers)
Tu
Shear failure
t LTu (shear strength)
Splitting crack (crack parallel with
fi ber direction)
G c ( y ) (mixed mode fracture energy)
Delamination crack between plies
G c ( y ) (interface fracture energy)
Gelcoat/skin delamination
Interface cracking
G c ( y ) (interface fracture energy)
Interface cracking with fi ber bridging
s ( d ) (cohesive law), J c (work of separation)
Gelcoat cracking
Thin fi lm cracking
G Ic (fracture energy)
tensile failure (fi ber fracture) in a laminate. When the gelcoat cracks, the laminate
loses its protection against environmental exposure - which can lead to laminate
damage over a long time. In contrast, tensile failure in the form of fi ber fractures
decreases the stiffness and residual strength of the load-carrying laminates - this
can lead to rapid failure within a short period of time. Models and criteria for
assessing the various types of damages will be described in Section 7.
5 Material properties
5.1 Elastic properties
Modern wind turbine blades are three-dimensional structures made by the use
of several different materials and the elastic properties and thermal-physical con-
stants, such as thermal expansion coeffi cient, of materials infl uence the damage
developed in a blade. As a result, the stress fi eld depends on the elastic properties of
the materials used. For isotropic materials, the elastic properties are the Young's modu-
lus, E , and the Poisson's ratio, n . Orthotropic materials, such as composite laminates
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