Page 415 - Wind Energy Handbook

BLADES 389

to take account of degradation of the material over time, as well as the material’s
inherent variability.
The GL rules lay down that the material safety factor for calculating the design
strength of GFRP under extreme loads is to be calculated as the product of

• a basic factor, ª MO , of 1.35
• a factor, C 2a ¼ 1:5, to account for the influence of ageing,

• a factor, C 3a ¼ 1:1 to account for strength reduction at higher temperatures,
• a factor, C 4a ¼ 1:2 for hand lay-up laminates or 1.1 where manufacture is partially
automated,

• a factor, C 5a ¼ 1:1 if the laminate is not post cured.
These rules result in a material safety factor in the range 2.45–2.94. In the case of
fatigue loads, the ageing factor of 1.5 is omitted and the factor accounting for lay-up
is replaced by one taking account of the type of fibre reinforcement.

7.1.7 Properties of wood laminates

Although laminated wood/epoxy is classed as a composite, it is markedly different
in form from GFRP. Individual plies are made up of large sheets of wood veneer
(Figure 7.7) instead of a multiplicity of fibres laid up in a matrix, and the epoxy
behaves as an adhesive rather than a matrix, bonding the sheets together at the
longitudinal and transverse joints and bonding each ply to its neighbour. Thus
the fibre volume fraction is close to 100 percent and the anisotropic properties of the
wood laminate derive principally from the anisotropic properties of the wood itself.
Wood strength properties are much greater in the direction parallel to the grain,
so all the veneers are orientated with the grain parallel to the blade axis, in order to
resist blade-bending loads efficiently. However, the veneers cannot be produced in
lengths much greater than 2.5 m, so transverse joints have to be included, which
introduce lines of weakness not normally found in GFRP blades. The effect is
minimized by staggering the joints, and by using scarf joints in preference to butt
joints.
The epoxy adhesive has a secondary function of sealing the veneers against
moisture ingress; additional moisture protection is provided by a layer of glass/
epoxy on both the external and internal surfaces. It is important to maintain
moisture content at a low level, because veneer strength decreases about 6 percent
for every 1 percent rise in moisture content.
A comparison of some of the properties of wood laminates used, or considered
for use, in wind turbine blades is given in Table 7.2. Khaya ivorensis, an African
mahoghany, and Douglas fir used to be the main species used for blade manufac-
ture in the UK and US respectively, but environmental pressures have led to the
phasing out of Khaya in favour of European species such as poplar and birch.
The table gives tensile strengths of unjointed specimens. Bonfield et al. (1992)
report the results of tests on jointed specimens, which showed a significant