Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 423

humidity and temperature and should be suffi ciently wear resistant to last for
20 years.
2.4 Processing methods
Wind turbine blades can be manufactured using procedures similar to that used for
composite aircraft structures and composite boat hulls; namely lamination by the
use of pre-pregs, hand lay-up and vacuum assisted resin transfer molding.
Both the manufacturing approaches shown in Fig. 3 involve the manufacturing
of the aeroshells as the fi rst step. Since the aeroshells do not carry much load
themselves, they are primarily made of lightweight sandwich structures. The
fi bers (or pre-pregs) and sandwich materials are usually placed by hand in an
open mould. The mould is closed before curing, so that fumes can be removed
during the processing. Next, in the second step, the moulds are opened and the
webs or box girder is placed in between the aeroshells. In the third step, the
moulds with the two half-shells are closed and the two half-shells and webs (or
box girder) are bonded together. Such processing methods have several advan-
tages. First, the quality of the aeroshells and webs (or box girder) can be con-
trolled before the fi nal assemblage. Secondly, the adhesive layers allow a relatively
large dimensional tolerance, as the adhesive layer thickness can range from a
millimeter to several centimeters. This is particularly useful with increasing sizes
of the blade components.
3 Testing of wind turbine blades
3.1 Purpose
Full-scale testing (Fig. 4) is mandatory for certifi cation of large wind turbine
blades. The basic purpose of these blade tests is to demonstrate that the blade type
has the prescribed reliability with reference to specifi c limit states with a reason-
able level of certainty. According to Det Norske Veritas (DNV) [7], a limit state is
a defi ned as a state beyond which the structure no longer satisfi es the requirements.
The following categories of limit states are of relevance for structures: ultimate
limit state (ULS), fatigue limit state (FLS), and serviceability limit state (SLS).
The blade should be manufactured according to a certain set of specifi cations in
order to ensure that the test blade is representative of the whole series of blades.
In other words, the purpose of the blade tests is to verify that the specifi ed limit
states are not reached and that the type of blade possesses the projected strength
and lifetime.
Normally, the full-scale tests used for certifi cation are performed on a very lim-
ited number of samples; only one or two blades of a given design are tested so that
no statistical distribution of production blade strength can be obtained. Therefore,
although the tests do give information valid for the blade type, they cannot replace
either a rigorous design process or the use of a quality control system for blade
production.