Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 450

where g is a non-dimensional function that depends on the stiffness mismatch.
From eqn (15) it is clear that the critical stress in the gelcoat, 1c s , increases with
decreasing t . In other words, channeling cracking can be suppressed by decreasing
the thickness of the gelcoat layer.

7.3 Examples of sub-components with damage

It is of great importance to perform additional laboratory tests on larger, generic
samples or on blade sub-components in order to validate the design strategy and
design methods. The approach is fi rst to predict the strength characteristics of the
sub-component using measured fracture properties, next manufacture and test the
sub-component. Finally, the predicted and measured strength behaviors are compared.
Such case studies can validate (or invalidate) the proposed design approach.

7.3.1 Failure of adhesive joints
The fi rst example concerns the strength of adhesively bonded joints for glass fi ber
composites (see Fig. 21). As described in the previous section, the proper parameter
for characterizing cracking of an adhesive joint in a wind turbine blade is the fracture
toughness (or fracture energy) measured by mixed mode fracture mechanics testing.
In the present example [70], mixed mode fracture mechanics testing was conducted
for an adhesive joint comprised of a polymer adhesive joining two beams made

30 cm

J ss

Material #1 Characterisation

M 1
M 2

M+M
1

ψ

2

#2

(b)

(a)

Modelling

2 m

F F

h 2

F

t

F

Δ a h 1

Δ a

F

(d)

(c)

Figure 21: A study of mixed mode cracking of adhesive/glass fi ber composite:
mixed mode values of the fracture resistance, J ss , determined from
DCB specimens loaded with uneven bending moments, were used for
prediction of the load-carrying capacity of medium size specimens,
and compared with experimental results obtained from fl exural loading
of 2 m long “medium size“ beam specimens.