JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 33

permeability

Optimizing fabrics for wind blade infusion

When optimizing the infusion process for the manufacture of wind
turbine blades, a key factor to consider is the permeability of the reinforcement fabrics. Multiaxial reinforcements specialist Formax works
with blade manufacturers to develop fabrics with the right amount of
permeability to enable the best balance between processability and
performance.

By
Tom James
DirecTor of innovaTion
formax

he wind energy market has seen
strong growth over the last 20 years.
To meet this demand, wind turbine
manufacturers have been developing models
with longer rotor blades and higher power
outputs: 7 MW turbines with 75 m blades
are now available for offshore wind farms. As
blade length increases, the choice of materials and manufacturing process becomes ever
more important to ensure the blades are
fully optimized to achieve the highest mechanical performance, at the lowest possible
weight and at acceptable cost.

Permeability testing
To enhance the technical support it offers to
customers, the company continues to invest
in its testing and simulation capabilities. As
there is not yet an official test standard for
permeability testing of structural fabrics for
use in composites, the company recently

Fig. 1: The bespoke permeability testing rig at
Formax

commissioned a bespoke permeability
measurement rig in a collaboration with the
University of Nottingham, UK.
The new rig (Figure 1) calculates the permeability of the preform stacks using calculations based on Darcy's Law, an equation describing the flow of a fluid through a porous
medium. It is a development of an existing
fixture at the University of Nottingham and
is similar in size and appearance to a flatplate resin transfer moulding (RTM) tool.
Two 25-mm-thick aluminium plates form
the upper and lower surfaces of a 500 mm x
500 mm cavity with a thickness that can be
varied using shims. The top plate is attached
to a floating hinge which allows for efficient
analysis of a variety of cavity thicknesses and
volume fractions. The scope of this cavity
thickness adjustment makes it possible
to test fabrics to the customer's targeted
volume fraction. The rig's lower plate houses
an array of pressure sensors which measure
the hydrostatic pressure at fixed, known distances from the injection point in the centre
of the lower plate.
A low-viscosity silicone oil is used as test
fluid, because it has a very well defined
temperature/viscosity relationship at typical
testing temperatures and allows for tests to
be conducted in a fraction of the time taken
for impregnation with a polymer. Injected
in the centre of the lower plate at a pressure of 2 bar (and with the perimeter of the

No99 September 2015 / jec

Feature Wind Energy

The two composite technologies used to
manufacture large wind turbine blades are
prepreg and infusion. Turbine manufacturers select a process and associated
materials based on a business case for each
individual project. A specialist in developing custom multiaxial reinforcements for
demanding applications, Formax has been
engineering unidirectional, biaxial and
triaxial reinforcements (in both glass and
carbon fibre) for the wind energy industry
for more than 15 years, and understands
that the permeability of the fabric reinforcement is a critical factor in the infusion
process. The higher the permeability of
the fabric, the faster the infusion, which is
desirable from the process point of view.
However, faster infusion can lead to lower

mechanical properties resulting from
incomplete wet-out of fibres, creating dry
spots (potential defects) in the final laminate. In addition, carbon fibre is generally
more difficult to infuse successfully than
glass. This is because it has a smaller fibre
diameter, which means carbon fibres can
pack together more tightly, reducing the
permeability of the fibre bundles.

composites magazine 33

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