Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 449

range of mode mixities. Assuming that the fracture process zone remains small, an
appropriate criterion for crack propagation is

Gyy () = ()

( 14)

c

where G is the energy release rate, y is the mode mixity and G c ( y ) is the fracture
energy for the specifi c mode mixity. The criterion should be understood as follows:
an analysis of a structure with a crack along an adhesive joint gives an energy
release rate and a mode mixity value. Then, the appropriate fracture energy, cor-
responding to the determined mode mixity should be used. This is required since
the fracture energy depends strongly on the mode mixity, as shown in Fig. 9.
In case of a large scale fracture process zone, e.g. in the form of fi ber bridging,
the appropriate material laws are the J integral and cohesive laws, as described
above. An example of this will be given in Section 7.3.

7.2.4 Sandwich failure
Delamination crack growth in sandwich structures is another example of interface
fracture. As described above, the appropriate cracking criterion for interface crack-
ing is given by eqn (14). However, some core materials (e.g. low-density polymer
foams) possess low fracture energy and the crack can kink into the core material.

7.2.5 Gelcoat/skin delamination
Delamination of the gelcoat is also interface cracking. Delamination of the gelcoat
can be caused by residual stresses originating from the hardening of the gelcoat.
As mentioned, the peel test (Fig. 8e) is frequently used for characterizing the inter-
facial fracture energy. Another useful test method is the DCB-UBM test confi gura-
tion. A sandwich DCB specimen can be made by gluing an addition beam onto
the gelcoat layer. By changing the ratio between the applied moments, the mode
mixity can be adjusted so that the crack does not kink out of the interface between
the gelcoat and the skin (face sheet) layers, but remains at the interface.

7.2.6 Channeling cracking in the gelcoat
Typically, the cracking in the gelcoat takes place in the form of cracks that extends
widely across the gelcoat layer, but the crack extends only to the depth of the
gelcoat layer - it does not penetrate into the underlying laminate. This mode of
cracking is denoted channeling cracking . Channeling cracking has been modeled
by Nakamura and Kamath [36] who found that the energy release rate of a crack
in a coating attains a steady-state value once the crack has extended a distance
of a few times the coating thickness. The crack then propagates under a constant
stress - therefore channeling cracks can spread widely across the gelcoat layer. We
denote the Mode I fracture energy of the gelcoat with G 1c . The critical stress level
below which no channeling cracking can occur is given by [35]:

2 GE

gt

c 1c 1

s =

( 15 )

1ch

2
1

p (1 − n )