Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 446

The cracking of a 90° ply due to a tensile stress in the layer deserves more
comments. For thin layers, where the fl aw size is comparable to the layer thick-
ness, a 90° layer can form transverse cracks (sometimes also called tunneling
cracks). That is a cracking mode where the 90° layer develops a crack that spans
the layer thickness and runs across the laminate parallel to the fi ber direction. This
is a steady-state problem: when the crack length is a few times the layer thickness,
the energy release rate takes a constant value independent of the crack length. A
stress criterion for the stress level in the 90° layer at which a tunneling crack can
propagate in the 90° layer is [62-64]:

EG
fh

22 Ic

s =

(11 )

T

where E 22 is the Young's modulus in the directions perpendicular to the fi ber direc-
tion, G Ic is the Mode I fracture energy of the 90° layer and h is the thickness of
the 90° layer undergoing tunneling cracking, and f is a dimensionless parameter
(of the order of unity) that depends on the elastic properties of the lamina and the
surrounding layers. From (11) it follows that, for fi xed material properties, the
stress level at which tunneling cracking can occur decreases with increasing h .
Conversely, tunneling cracking can be suppressed by decreasing the layer thick-
ness. The development of a tunneling crack unloads the 90° ply only in the vicin-
ity of the crack. Remote from the crack, the stress fi eld is unaffected; the stress
fulfi ls the criterion (11), so that anther tunneling crack can propagate. This leads
to a characteristic damage state called multiple cracking, consisting of tunneling
cracks developing with fairly regularly even crack spacing.
For cyclic loading, similar criteria can be used; the only modifi cation is that the
stress value corresponding to the failure life or the fatigue limit is used instead of
the monotonic strength values.

7.2.2 Delamination of composites
Delamination is fracture along the interface between different layers. As mention,
delamination can be analyzed by linear-elastic fracture mechanics concepts. The
criterion for crack propagation is

GG

( 12)

c

where G is the energy release rate and G c is the fracture energy. The fracture
energy is a material property that can be measured by fracture mechanics testing as
described in Section 6.2. The energy release rate must be calculated for the given
structure, accounting for the elastic properties, the geometry (including crack size)
and applied loads.
For cyclic crack growth, the aim is to determine how fast a crack can propagate
during cyclic loading and to estimate the number of load cycles that remains before
the blade fractures rapidly. The following procedure can be used for predicting the
crack size as a function load cycles: First, the structure (a sub-structure or an entire
blade) containing a crack is analyzed, e.g. by the use of a FE model. The model