Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 448

rate equals the fracture energy of the interface. To illustrate this, consider delami-
nation of a thin layer of a thick beam. An approximate value for the energy release
rate can be calculated by an Euler model, assuming clamped ends. For a long
delamination, the energy release rate approaches a value given by [9]:

22
11

(1 − ns ) t

G =

2 E

( 13)
where G is the energy release rate, s 1 is the critical stress in the layer that under-
goes buckling-driven delamination (material #1), E 1 and v 1 are the Young's modu-
lus and Poisson's ratio of material #1 and t is the thickness of the layer. Recall,
that the criterion for delamination is G = G c , where G c is the fracture energy of the
interface (at the appropriate mode mixity).
Note the effect of the thickness of the thin laminate, t . A thin laminate (small t ) -
corresponding to a delamination lying close to the top of the original laminate -
gives a low buckling stress. However, from (13) it is apparent that a small value of
t gives a low energy release rate. Thus, if the delamination is positioned close to
the surface (small t ), the delaminated laminate can easily buckle, but the delamina-
tion crack cannot propagate since the energy release rate is lower than the fracture
energy. Conversely, if the original delamination is positioned deep in the laminate,
then, from (13), the energy release rate is high and the delamination can propagate
if the laminate can buckle. However, the stress to cause buckling increases with
increasing t . It follows that a delamination positioned deep inside the laminate will
not cause local buckling - the delamination cannot propagate. Thus, neither
delaminations very close to the surface or very far away from the surface will
grow. However, a certain range may exist where both the buckling criterion and the
crack propagation criterion are fulfi lled. This simple argument illustrates that the
delamination position through the thickness or a component such as a blade is of
great importance when determining the impact that a delamination will have on
blade failure. Additional details can be found in Karihaloo and Stang [69].
Occasionally, delamination occurs with signifi cant crack bridging, viz., a zone
behind the crack tip where fi bers connect the crack faces; this is denoted cross-
over bridging. Since this zone can be large, the use of linear-elastic fracture
mechanics is invalid. Instead, the fracture resistance can be described in terms of
the J integral and cohesive laws, representing the mechanical response of the
bridging fi bers. An approach for determination of mixed mode cohesive laws by
the use of DCB-UBM specimens and the J integral [33] was demonstrated by
Sørensen and Jacobsen [ 30 ].

1

7.2.3 Adhesive joints
Crack growth inside an adhesive layer can occur when the energy release rate
equals the fracture energy of the adhesive. However, often cracking of adhesive
joints occurs as interface fracture, since the crack is subjected to both normal
and shear stresses at the crack tip (mixed mode cracking) so that the crack kinks
to the adhesive/laminate interface. Often, the adhesive/laminate interface is the
weakest plane. Therefore, the crack tends to remain at the interface for a wide