JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 34

permeability
Focus
Formax, headquartered in Leicester, UK, is
a leader in the development and manufacture of tailored reinforcement material
solutions, offering an unrivalled range of
reinforcements, including carbon and glass
multiaxials, UDs, stitched core and combi
fabrics. The company is supplier to a global
customer base in the target markets of
wind energy, automotive, marine, sports
and industrial, as well as other selected
application areas.

tool vented to atmospheric pressure), the
oil infiltrates the preform stack while the
pressure sensors in the bottom plate detect
the pressure gradient across the stack. Data
acquisition and processing software then
takes these measurements and generates
permeability values and the principal and
secondary flow directions for the fabric
sample.
The results from the fabric permeability
measurement tool have been validated
against the fixture at the University of
Nottingham and it has already been used to
assist customers in the automotive, marine
and tidal energy industries.
To provide more insight into the effect of
fabric architecture on the permeability,
a fabric can be tested at a range of fibre
volume fractions to produce a graph
providing estimates for permeabilities
at a given volume fraction. As Figure 2
shows, permeability drops sharply with an
increase in volume fraction, before levelling out as the volume fraction gets closer
to its theoretical maximum.
These values can be used with analytical
hand-calculation methods and, with the required experience, flow simulation software
packages such as PAM-RTM to provide
data for fill time and locations of dry spots.
A good understanding of the limitations of
such simulation codes is necessary, especially for wind energy customers looking to
analyse their vacuum infusion process. The
modelling of flow through a fixed cavity
volume of an RTM tool is significantly more
straightforward than for the flow under

Fig. 2: The permeability (K) vs. volume fraction (Vf) curve for a carbon fibre biaxial fabric with (red) and without (blue) flow enhancement media. The flow enhancement is a low g/m2 layer of nonwoven thermoplastic
filament which is stitched between the fibre layers to create an inter-ply passage for polymer flow

vacuum bag, due to the variation in cavity
thickness, fabric compression and fabric permeability associated with infusion. In order
to further the advancement of simulation
solutions for vacuum infusion, the company
is funding a four-year engineering doctorate
at the University of Nottingham.
Combined with mechanical testing and process simulation, permeability data enables
the team to engineer fabric architectures
which allow higher and more consistent
infusion rates and result in the optimal laminate quality. A range of different infusion
cores based on polypropylene, polyester,
natural fibre and glass fabric can be included
within the fabric structure to further enhance infusion.

The optimal carbon UD
One focus of Formax's wind energy research
is the development of carbon fibre unidirectional (UD) fabrics designed for use
in the main load-carrying elements of the
blade, which are the spar caps that run from
the root to the tip of the blade. These are
critical structures within the blade, with the
highest mechanical performance (stiffness
and compression strength). This requires
exact fibre alignment and wet-out in order
to produce high-quality laminates with low
void content.
There are a number of options for the
manufacture of spar caps. Glass or carbon
fibre can be used, and the caps can be

34 jec composites magazine / No99 September 2015

manufactured using prepreg or infusion
technology. Typically, blade manufacturers
start to move from glass to more expensive
carbon fibre for blades above 35 m in length
in order to achieve the higher mechanical
properties required. When the business
case prevents the use of high-performance
(but expensive) prepregs, the composite
engineer may look to infusion of dry carbon
UD fabrics for the required performance
at lower cost. With its new permeability
characterization capability, the company
is now developing a range of carbon UDs
with fabric architectures that are bespoke
to spar cap laminates. Permeability can be
optimized in the X, Y and Z directions to
suit the specific spar geometry and infusion
lay-up, whilst maximizing utilization of the
mechanical properties of carbon fibre.
Permeability can be enhanced through
intra-layer or inter-layer fabric architecture
features. Intra-layer features are conventional
elements of a structural fabric (e.g. those created due to a given stitch style) which exist
within each fibre layer, increasing flow rate
during infusion and creating a permanent
feature within the composite. Inter-layer features sit between the fibre layers and act as
polymer flow paths during the infusion process. Formax is developing novel inter-layer
features which are temporary and increase
permeability during the infusion process,
but do not remain in the final composite. n
More information:
www.formax.co.uk

http://www.formax.co.uk

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015