Environmental Engineering Reference
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The twist angle predicted by the outer surface shell model deviates from the
experimental values by as much as 32% near the loaded end of the blade section
(see Fig. 19). The major reason for this disagreement is the offset confi guration of
the shell elements. As also found by Laird et al . [ 57 ], this confi guration has serious
problems modeling correct torsional behavior. In contrast, it was found that the
outer surface shell FE model can accurately predict fl apwise bending response
and, to a reasonable degree, accurate edgewise bending response [58].
Mid-thickness FE models are generally not capable of modeling accurate fl ap-
wise and edgewise bending when the model includes details like ply-drops in the
spar cap [58]. Using rigid elements to connect regions with different material
thickness can therefore not in general be recommended.
Finally, by comparing results from experiments with the global displacements
and rotations for the combined shell/solid FE models, it can be concluded that the
shell/solid model provides good accuracy for predicting fl apwise, edgewise and
torsional behavior. Also, by comparing results from the shell/solid model of the
modifi ed blade section with experiments, it has been shown that this FE model
type can also be used to model bend-twist coupling [58]. These studies will con-
tinue in order to develop more understanding regarding why shell models with
material offsets have problems with modeling the torsional behavior.
The combined shell/solid model is more detailed and accurate than the other
two shell models but degrees of freedom needed for the combined shell/solid
model is also considerable larger and therefore more time consuming to analyze.
7.2 Models of specifi c failure modes
7.2.1 Criteria for laminate failure
Stress- or strain-based criteria are widely used for prediction damage and fracture
of individual laminas in laminated structures [59]. Such criteria are easy to use in
connection with numerical modeling of wind turbine blades, since the stress (or
strain) is usually calculated as a part of the analysis. Denote the in-plane stresses
as follows: s L is the normal stress acting in the fi ber direction (the L direction
defi ned in Section 5.1), s T is the normal stress acting perpendicular to the fi ber
direction (the T direction) and t LT is the in-plane shear stress.
The simplest type of stress-based criteria assumes that failure is governed by
one stress component. Thus the criterion for tensile failure in the fi ber direction
(the L direction) is ( s L > 0):
s
s +
L
1
( 2 )
Lu
s + is the tensile strength when the material is loaded in uniaxial tension
along the fi bers (the longitudinal direction), as described in Section 5.2. The criterion
for compression failure in the fi ber direction ( s < 0) is:
where
Lu
s
s
L
1
( 3 )
Lu
 
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