Environmental Engineering Reference
In-Depth Information
Table 2: Classifi cation of various materials used in wind turbine blades and the
anisotropy level used to characterize their elastic constants.
Isotropic materials
Orthotropic materials
Adhesive
Glass fi ber/polyester composites
Steel
Carbon fi ber/epoxy composites
Polymer foam
Wood (e.g. birch or balsa)
Gelcoat
Bamboo
with aligned continuous fi bers, have different elastic properties in different directions.
Therefore, the elastic properties must be related to a coordinate system. It is con-
venient to use a global x - y - z coordinate system (see Fig. 2) and a local coordinate
system that follows the direction of the fi bers. The longitudinal direction (the fi ber
direction) is assigned the subscript L, the (in-plane) direction perpendicular to
the longitudinal direction is called the transverse direction and given subscript
T, and the out-of-plane direction, orthogonal to the L and T directions is denoted
TT, i.e. T with a prime. Then, the elastic properties are specifi ed in terms of E L ,
E T , E T
where E i denotes the Young's modulus
in the i -direction and n ij is the Poisson's ratio in direction j due to a normal stress
in the i -direction, and G ij is the shear modulus in the i - j -plane. Table 2 lists some
common materials used in wind turbine blades and their anisotropy classifi cation.
, n LT , n LT ′
, n TT′
, G LT , G LT ′
and G TT
5.2 Strength and fracture toughness properties
Damage and failure modes are described by various parameters that may be stress-
based, energy-based or length-based (e.g. critical defect length). A damage mode
that involves a distributed damage zone is usually described in terms of a critical
stress value, i.e. by a maximum stress criterion (tensile or compressive strength).
Crack growth along a fracture plane is a localized phenomenon. The onset of crack
growth can be described in terms of a maximum stress intensity factor (fracture
toughness) or a maximum energy release rate (fracture energy). A crack experi-
encing fi ber bridging requires modeling of the bridging fi bers. This can be done by
a cohesive law (a traction-separation law). The area under the traction-separation
law is the work of separation. Table 1 lists parameters that are typically used to
characterize common damage and failure modes. These concepts are applicable
to static failure. A similar distinction can be made for cyclic damage evolution.
A complete analysis that would involve the design against the failure modes listed
in Table 1 will require the knowledge of all the relevant materials parameters.
A maximum stress criterion (or maximum strain criterion) can be used for mate-
rials that develop a damage zone (using appropriate safety factors, typically around
1.5-1.8). As an example, unidirectional fi ber composites, loaded in uniaxial ten-
sion in the fi ber direction, usually display a distributed damage zone during fail-
ure. Consequently, an appropriate strength measure is the tensile strength,
s +
(here, subscript L indicates the longitudinal direction, subscript u indicates ulti-
mate strength and superscript + indicates tension). Other failure modes that are
usually characterized in terms of stress criteria are the compressive strength of
Lu
Search WWH ::




Custom Search