Page 410 - Wind Energy Handbook

384 COMPONENT DESIGN

material beyond a skin thickness to chord ratio of 3–4 percent, especially in the
outboard part of the blade, where the blade thickness to chord ratio is low.
Fatigue performance is conveniently measured by mean fatigue strength at 10 7
cycles, as a percentage of ultimate compressive strength. Clearly, carbon-fibre and
khaya/epoxy perform best with a value of about 30 percent. The low value for
welded steel (10 percent), combined with steel’s low strength-to-weight ratio,
renders it uncompetitive for large diameter machines where gravity fatigue loading
becomes important, although it was chosen for some of the early prototype mega-
watt scale machines when the fatigue properties of composite materials were less
well understood.
The stiffness-to-weight ratio determines blade natural frequency. Apart from
CFRP, the values in the table are all in a relatively small range (18–27 GPa),
indicating that material choice will generally only have a marginal effect on
dynamic behaviour.
From the above brief survey, it is apparent that the material with the best all-
round structural properties is carbon-fibre composite. However, it has not found
common use because it is an order of magnitude more costly than other materials.
Instead, the most popular material is glass/polyester, followed by glass/epoxy and
wood/epoxy.
Steel is the cheapest material in the raw state, and can be formed into tapering,
curved panels following the aerofoil profile, except in the sharply curved region
near the leading edge. However, it is much harder to introduce a twist into such
panels, and this consideration, together with the poor fatigue properties, means that
steel is rarely used. By contrast, glass- and carbon-fibre composites lend themselves
to wet lay-up in half-moulds profiled to give the correct aerofoil shape, planform
and twist. Laminated wood composite blades are built up in a similar way, but the
veneer thickness has to be restricted to enable the veneers to flex to the required
curvature during lay-up.
In the following paragraphs, the properties of the materials in most common use
for blade manufacture are considered in more detail.

7.1.6 Properties of glass/polyester and glass/epoxy composites

As noted in Table 7.1, the properties of glass/polyester and glass/epoxy plies with
the same fibre volume fraction and lay-up are generally very similar, i.e., the
influence of the matrix is slight. They will therefore be treated as the same material
in the discussion that follows, except in relation to fatigue, where some differences
have been noted. The glass used in blade construction is E-glass, which has good
structural properties in relation to its cost.
The plate elements forming the spar of a GFRP blade are normally laminates
consisting of several plies, with fibres in different orientations to resist the design
loads. Within a ply (typically 0.5–1.0 mm in thickness), the fibres may all be
arranged in the same direction, i.e. UD or unidirectional or they may run in two
directions at right angles in a wide variety of woven or non-woven fabrics.
Although the strength and stiffness properties of the fibres and matrix are well-
defined, only some of the properties of a ply can be derived from them using simple