Page 407 - Wind Energy Handbook

BLADES 381

CSM Continuous Strand Mat

CSM/polyester
Adhesive PVC foam
TFT wound spar
CSM/polyester
Gel coat
Figure 7.4 Glass-fibre Blade Construction Using Compact Spar Wound with Transverse
Filament Tape (TFT) on Mandrel. (Reproduced from Corbet (1991), by permission of the DT1
Renewable Energy R&D Programme)
Table 7.1 lists the structural properties of the materials in general use for blade
manufacture and those of some other candidate materials. For comparative pur-
poses, values are also presented of:

• compressive strength-to-weight ratio,
• fatigue strength as a percentage of compressive strength,
• stiffness-to-weight ratio,
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• a panel stability parameter, E/(UCS) .
It is evident that glass- and carbon-fibre composites (GFRP and CFRP) have a
substantially higher compressive strength-to-weight ratio compared with the other
materials. However, this apparent advantage is not as decisive as it appears, for
two reasons. First of all, the fibres of some of the plies making up the laminated
blade shell have to be aligned off-axis (typically at 458) to resist shear loads, giving
reduced strengths in the axial direction. Secondly, the relatively low Young’s
modulus of these composites means that resistance to buckling of the thin skins
governs the design rather than simple compression yielding. The likelihood that
2
buckling will govern is inversely related to the panel stability parameter, E=(UCS) ,
given in the last column of the table, so that materials with high values, such as
wood composites will be least sensitive to buckling. As a result wood composite
blades are generally lighter than equivalent glass-fibre composite blades. Design
against buckling is considered in Section 7.1.10.
It should be noted that the low strength of wood laminate compared with other
materials renders it unsuitable for blades with slender chords operating at high tip
speed, where the flapwise bending moments during operation are inevitably high
in relation to blade thickness. For example, Jamieson and Brown (1992) have shown
that, in the case of a family of stall-regulated machines, the blade stress is highly
sensitive to rotational speed, increasing as the fourth power, if the skin thickness to
chord ratio is maintained constant. Although stresses can be reduced by increasing
the skin thickness, this represents a less and less efficient use of the additional