Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 435

Testing of thin surface layers, such as gelcoats on wind turbine blades, offers
special challenges, since standard fracture mechanics test methods cannot be
applied. The determination of the fracture energy of a gelcoat can be determined
by tensile [34] or bending experiments based on the concept of steady-state crack-
ing of a channeling crack [35, 36]. The peel test can be used for measuring the
fracture energy of the interfaced between a gelcoat and a substrate [37]; however,
large-scale plasticity in the gelcoat may lead to erroneous results if this is not
accounted for [38]. Alternatively, a DCB sandwich specimen can be made by
bonding an additional beam onto the gelcoat attached to a substrate.

6.3 Failure under cyclic loads

Test methods used under static loads can in many cases also be used for the study of
fatigue damage evolution. In some cases, however, special requirements mean that spe-
cial concerns have to be accounted for in the selection of test specimens. Moreover, the
data collection, the data analysis and materials properties used for describing fatigue
are different from those used to describe strength properties under static loading.
Under cyclic loading, it is useful to distinguish between failure due to a damage
zone and failure due to crack growth. For materials that fail by a damage zone,
such as a unidirectional fi ber reinforced composite loaded in the fi ber direction,
the life under cyclic loading can be described by a so-called S-N curve, which is
the relationship between the maximum applied stress, s max , and the number of
cycles to failure, N f . A schematics of an S-N curve is given in Fig. 10. The applica-
tion of S-N data in design is straightforward. For example, assume that a compo-
nent should be designed such that it safely survives a given number of load cycles.
Then, from the S-N curve one reads off the maximum applied stress, s max , corre-
sponding to that number of cycles. The S-N curve depends on the minimum applied
stress, s min ; usually expressed in terms of the R -ratio, which is R = s min / s max . The
approach can refi ned to predict for a small fraction, say 1/1000 of failed specimens
(instead of the average fatigue life) and to account for different maximum load
(load spectrums), e.g. by the use of the Palmer-Miners rule [39].
S-N curves for tension-tension and compression-compression testing of
glass-epoxy unidirectional laminates for the spar beam for wind turbine blades are
shown in Figs 11 and 12. The data are from the Optidat database [40]. The mea-
sured fatigue data are evaluated according to ASTM E739, and the 50% median
line and the lower 95% confi dence limit are shown in these diagrams. The statisti-
cal 95% lower confi dence limits based on the 95% survival line for the Siemens
Wind Power shell materials is also shown in the fi gure. This analysis is based on
the work by DNV [41]. The fatigue lifetime depends on both the applied amplitude
and the applied mean value and can be presented in other useful graphs [42].
Of particular importance is whether or not an endurance fatigue limit exists for a
material. A fatigue limit implies that a stress limit exists, such that if the material is
never loaded beyond this value, f s , then the material will never fail due to fatigue.
In addition to buckling and failure due to static overload, perhaps the most
important mode of damage that needs to be addressed in design is the cyclic growth