JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 37

failed due to the top surface buckling under
compressive loads.

Tab. 2: Flax is costlier than E-glass at every stage

E-glass blade
Stage of
test loading

Flax blade

% of normal
% of normal
Tip displacement
Tip displacement
operation
operation
mm % of blade length
mm % of blade length
load
load

Normal operation

100

270

100

388

Worst case

270

743

270

2025

Failure

N/A

280

2300

more studies are required
to understand the behaviour of PFCs employed
in specific applications,
as opposed to analysing
their performance through
coupon tests.

The potential of flax as a structural replacement for E-glass in SWT blade manufacture has been investigated. It is found that
although the flax/polyester blade is 10%
lighter than the E-glass/polyester blade,
the cost of materials of the former is found
to be around 3 times that of the latter.
Flap-bending tests demonstrate that both
blades satisfy structural integrity requirements. It is interesting to see that while a
flax blade can compete against an E-glass
blade in terms of strength but not stiffness, flax fibres and their composites are
generally recognized for the opposite. The
study concludes that under current market
conditions, flax can be considered as a
sustainable and structural, but not low-cost,
alternative to E-glass. n
More information:
eaxds1@nottingham.ac.uk
References
1. Wambua P, Ivens J, Verpoest I. Composites Science and
Technology 2003;63:1259-1264.
2. Zini E, Scandola M. Polymer Composites 2011;32(12):1905-1915.
3. Shah D, Schubel PJ, Licence P, Clifford MJ. Journal of Materials
Science 2012;47:2700-2711.
4. Carus M. «Bio-Composites: Technologies, Applications and
Markets», in 4th International Conference on Sustainable Materials, Polymers and Composites. Birmingham, UK, 6-7 July 2011.
5. Shah D, Schubel PJ, Clifford MJ, Licence P. JEC Composites
Magazine #73:p. 51-54 (2012).
6. BS EN 61400-2:2006. Wind turbines - Part 2: Design requirements for small wind turbines. London: British Standards
Institution, 2006.

The flexural rigidity of the blade EI in the
flap-wise direction has also been estimated. Fig. 8 clearly shows that the flax blade
deflects more than the E-glass blade, and
the latter exhibits a higher flexural rigidity
at almost all points along the blade length
(Fig. 9). The mean flexural stiffness EImean
is calculated as 43.4 kNm2 for the E-glass
blade and 24.6 kNm2 for the flax blade.
It is interesting to see that while a flax blade
can compete against an E-glass blade in
terms of strength but not stiffness, flax
fibres and their composites are recognized
for the opposite [1, 10]. This shows that

7. BS-EN 61400-23:2002. Wind turbine generator systems - Part
23: Full-scale structural testing of rotor blades. London: British
Standards Institution, 2002.
8. Schubel P, Shah DU, Parnham E. Turbine blade destruction
(available at http://www.youtube.com/watch?v=75P8Qe1opPE).
In: Haran B, Slater S, editor, 2012.
9. Shah D, Schubel PJ, Clifford MJ, Licence P. Polymer Composites
2012;33(9):1494-1504.
10. Shah D, Schubel PJ, Clifford MJ, Licence P. «Mechanical
characterization of vacuum infused thermoset matrix composites
reinforced with aligned hydroxyethylcellulose sized plant bast
fibre yarns», in 4th International Conference on Sustainable

Fig. 10: Images of the fractured flax blade
showing a) the region of failure, and b) wrinkle
formation due to buckling

Materials, Polymers and Composites. Birmingham, UK., 6-7 July

No78 January - February 2013 /

2011.

Feature Wind energy

The E-glass and flax blades
failed under different
modes. The E-glass blade
failed at the root-hub
Fig. 9: Variation in flexural rigidity along the blade length for the Ejunction showing extensive
glass and flax blades when subjected to normal operating test loads
delamination and eventual
E-glass blade, respectively.
splitting of the trailing edge. The flax blade
From Fig. 8, it can be observed that
failed about 1 m from the blade root (Fig.
while the tip displacement increases at a
10a), possibly due to a step-change in the
constant rate (linear growth; R2 = 0.996)
stacking sequence which may have resulted
with load for the E-glass blade, the tip
in stress concentration. The flax blade
displacement increases at a growing rate
(quadratic growth; R2 = 0.989) with load
for the flax blade. This is consistent with
the observation that E-glass fibres and their
composites have a linear elastic load-displacement curve, while plant fibres and their
composites have a non-linear visco-elastic
load-displacement curve [9].

Conclusions

jec composites magazine 37

http://www.youtube.com/watch?v=75P8Qe1opPE

JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

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