JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 36

natural fibre

cases, using simplified conservative load
equations from [6].
For the flap-wise static testing of the blade,
it is the blade root bending moment MyB
(acting to bend the blade tip downwind)
that is of interest. Typically, the blade is
tested against the calculated MyB under
normal operating conditions (Load case
A in [6]) and worst case loading. For this
particular turbine setup, the worst case
loading was found to occur when there is a
yaw error (Load case C in [6]).
As required by [6, 7], target test loads have
been determined inclusive of a safety factor.
The target point load F (F = MyB/l), associated with the target blade root bending
moment MyB, at the normal operation and
worst case loads is 1.48 kN and
3.99 kN, respectively. As is later revealed,
a single-point test method is employed,
where a single concentrated point load is
applied at l m from the blade root.

Experimental set-up
Test equipment
The static flap-bending test was conducted
as per [7]. The blade was fixed horizontal (flap-wise up) and was loaded by an
overhead crane at a single point. Fig. 6
presents the overall loading arrangement.
Test regime
Fig. 7 presents the loading sequence used
to achieve the target test loads F. The test

Fig. 6: Blade flap-wise test loading arrangement

consists of three stages:
i) loading up to the
normal operation load,
ii) loading up to the
worst case load, and iii)
loading to failure.
Results and discussion
A video of the structural
testing of the flax blade
is available at [8]. Fig. 8
presents a graph of tip
displacement versus test
load, for both blades. It
Fig. 7: Blade flap-wise test loading regime
is observed that both
blades satisfy the ultimate strength requireof 743 mm and 2025 mm, respectively,
ments of [7] by surviving the normal operaunder worst case loading. While the large
tion load without any superficial failure and
tip displacement of the flax blade is a
the worst case load without any functional/
concern, in principle, a turbine system can
catastrophic failure. Hence, the flax blade
be designed so that a generous clearance is
can be viewed as a potential replacement
available to avoid tower strike. A modified
for the E-glass blade.
flax blade design incorporating shear/
spar webs/caps will also enable major
Table 2 presents useful information from
reductions in tip deflection by increasing
Fig. 8 which enables direct comparison
the flexural rigidity. The failure load and
of the performance of the E-glass and flax
corresponding tip displacement of the flax
blades at the end of the three test stages.
blade is about 80% and 250% that of the
While the flax
blade survives the
worst case loading
like the E-glass
blade, the former is
significantly more
flexible. The E-glass
and flax blades have
a tip displacement

Fig. 8: Tip deflection vs. test load for the E-glass and flax blade, and images of the
flax blade at i) no load, ii) operational load, iii) worst case load, iv) failure load

36 jec composites magazine / No78 January - February 2013
JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013
