Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 452

advances. Therefore, such specimen is well suited for crack growth studies involving
large-scale bridging. It was found that single-cycle overloads of 20 and 30% did
not signifi cantly affect the subsequent crack growth rate. Cycling under different
values of J gave, after a

series of lower amplitude load cycles, a lower crack growth rate than in the earlier
stages of the experiment. This behavior is attributed to crack defl ection away from
the adhesive/laminate interface into the laminate. A possible mechanism for the
crack defl ection is the additional fi ber bridging which would increase the crack
growth resistance along the crack path, resulting in crack defl ection away from the
adhesive interface into the laminate. Investigations using cyclically loaded DCB-
UBM specimens have also shown a decreasing crack growth rate during constant

Δ J gave difference crack growth rates. However, a given Δ

Δ J amplitude in glass fi ber laminates [ 72 ].

7.3.3 Buckling-driven delamination of panels
In order to understand the effect of delamination on the compressive behavior
of laminated composite materials used in wind turbine blades, it is instructive
to examine recent results from compression tests performed on composite and
sandwich panels. Short et al . [68] tested glass fi ber reinforced polymer specimens
containing artifi cial delaminations of various size and depth. Good agreement
between FE predictions and experimental measurements were found for the entire
range of delamination geometries that were tested. FE and simple closed-form
models were also developed for delaminated panels with isotropic properties. This
enabled a study of the effect of delamination geometry on compressive failure
without the complicating effects of orthotropic material properties. A buckling
mode map, allowing the buckling mode to be predicted for any combination of
delamination size and through-thickness position was developed and is shown in
Fig. 23. The results of this study can be used to derive a graph of non-dimensional
failure load versus non-dimensional failure stress as shown in Fig. 24.
Nilsson et al . [73] made a numerical and experimental investigation of buckling-
driven delamination and growth in carbon fi ber/epoxy panels with an implanted arti-
fi cial delamination. The average maximum load for the delaminated panels was
approximately 10% lower than the maximum load for the undelaminated panels.
The maximum load was found to be almost unaffected by delamination depth. The
experimental results as well as the numerical analyses showed a strong interaction
between delamination growth and global buckling load for all delamination depths.
Delamination growth initiated at or slightly below the global buckling load in all
specimens. In all tests, the delaminations grew more or less symmetrically and per-
pendicular to the direction of load. At the maximum load, the crack growth rate
increased sharply. It was concluded [73] that structures where delaminations are
likely to occur should be designed against the possibility of global buckling.
For wind turbine blades, delaminations will likely be present along curved surfaces.
The effect of curvature on the failure load of laminates containing delaminations
was studied experimentally [74]. Tests on fl at and curved GFRP specimens with
delaminations between plies showed that the failure load for fl at specimens was
the same as or higher than that for curved specimens. As shown in Fig. 25, the