JEC COMPOSITES MAGAZINE - Issue #132 - January/February 2020 - 40

FEATURE design
allows the design of longer and stiffer
blades.
Now that the availability of affordable
carbon fibre more or less solved the
commercial issue of its high-volume
use, a question that still needs to be
answered is which carbon fibre-based
material and process should be used to
manufacture the spar cap. There are currently 3 "main" options on the market to
choose from: prepregs, pultrusion and
infusion.
Prepregs
Carbon fibre prepregs have been
around for more than 50 years and are
still widely used. As the fibre is already
pre-impregnated with the resin, the
user does not need to worry about how
to impregnate the material or how to
adjust the desired fibre-volume fraction
(FVF). Additionally, the fibres are
well aligned, leading to good results in
mechanical tests - at least in the lab.
But there are also a few downsides.
During the lay-up process, air is trapped
between the individual layers, resulting
in an increased porosity of the spar cap
(usually a single-digit percentage value),
reducing the built quality accordingly.
This is inherent to the material and
technology and cannot be avoided
(Figure 2).

Fig.2: Air inclusions in a prepreg laminate

Additionally, the material's stiffness
makes it more difficult - or sometimes
even impossible - to manufacture spar
caps with a twist, pre-bend or both. The
limited shelf life of prepregs is another
inconvenience.
Pultrusion
The latest trend, which has been
attracting a lot of attention recently, is
pultruded rods, namely carbon fibres
pulled through a die, impregnated with
a vinyl ester or epoxy resin system and
then cured immediately. The resulting
standard rods vary from 100 mm to 300
mm in width and from 3 mm to 5 mm in

40

thickness and are wound up on coils before they are sent to the manufacturers.
The technical advantages are that the
rods are already cured, offer a well-defined FVF and good fibre alignment,
leading to high mechanical properties.
However, there are also some disadvantages. The rods need to be unwound, cut
and chamfered with expensive machinery. The usually square cross-section of
the rods can lead to resin-rich zones in
the part, especially when the spar cap is
not flat but twisted prebuilt. Also, a glass
fibre fabric layer is usually introduced
between the overlapping rod layers.
This reduces the spar cap's mechanical
properties compared with the pure
pultruded rods obtained during the lab
test. Last but not least, the stiffness of
the rods, which accounts for their good
mechanical properties, can also lead to
problems. It is more difficult to lay up
the material in a mould with a noticeable pre-bend and make sure it does not
shift when vacuum is drawn. Therefore,
the possibility to position the spar cap
freely within the blade is limited.
Infusion
The third option is to produce carbon
fibre spar caps by infusing dry carbon
unidirectional fabrics with fibres running in the 0° direction.
Infused laminates made of CF UD
fabrics and epoxy resin systems reveal
lower mechanical properties in lab
tests than prepregs and pultruded rods.
This definitely needs to be taken into
account when designing the blade.
However, what you see is what you get:
The lab values reflect the mechanical
performance that can be expected from
the real spar cap. To achieve a good
built quality with these mechanical
properties, a good fibre alignment in
the mould is crucial. As UD fabrics are
softer than prepregs and pultruded rods,
the lay-up process has to be performed
carefully with the material coming from
a cart running above the mould.
The softness, or so-called drapability, of
the UD fabric tapes does not only cause
problems though. It allows the fabric to
adapt to the mould shape more easily

jec composites magazine / N°132 January - February 2020

than the other material options, which
is important for spar caps with a prebend, twist or both. It is also exactly this
drapability that allows the designer to
move the spar cap in the position that is
best for the blade structure. This offers
more design freedom for the spar cap
concept, including the use of CF UD
tapes for trailing edges.

Specimen No.:

B232/13-L546 (B461/12-L546)

Specimen

Measured
area

26,393mm

No. of
measured
field

19

Void
number

40

2

Void area
based on
measured
area
Void area
fraction of
measured
area

0,458
mm2

1,737%

Statistic

Average
value

88,1933
um

Number

40

Minimum

20,1534
um

Standard
deviation

82,54 um

Maximum

337,9640
um

Absolut
deviation

317,8106
um

Evaluation

Manually

Microscope analyse

Device

Neophot 32

Description whold area = 300 mm2
Void area = 0,458 mm2
Void fraction = 0,15 %

Fig. 3: Void analysis on a thick carbon fibre
laminate infused in VAP® technology (IMA)
JEC COMPOSITES MAGAZINE - Issue #132 - January/February 2020

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