Page 51 - 3D Fibre Reinforced Polymer Composites

40 3D Fibre Reinforced Polymer Composites
In spite of the restrictions, non-crimp fabric is being used extensively for the
manufacture of high performance yachts and in the manufacture of wind turbine blades.
Its use is also increasing within the aerospace industry and it is considered to be the
prime material candidate for use in future aircraft programs (Hinrichsen, 2000). This
fabric has the advantages that fewer numbers of layers need be used to build up the
required structure, therefore reducing the cost of labour. Due to the relatively
uncrimped nature of the yarns, laminates produced using NCF have been found to
exhibit superior in-plane properties for a given volume fraction of reinforcement than do
laminates produced using woven fabric in which the yarns can be more highly crimped
(Hogg et al., 1993). However, unlike the true 3D structures described in earlier sections
(weaving, braiding, etc.) the polyester knitting thread does not improve the impact
performance of the composite. Non-crimp fabric has also shown a much greater ability
to conform to relatively complex shapes without the wrinkling that is normally
produced in standard woven fabric. This is due to the ability of the fabric layers to shear
a certain amount relative to each other without the knit loops restricting this movement.

2.5 STITCHING

2.5.1 Traditional Stitching
Although the use of stitching in the production of composite components has only been
reported since the 1980’s, it is arguably the simplest of the four main textile
manufacturing techniques that have been described here and one that can be performed
with the smallest investment in specialised machinery. Basically the stitching process
consists of inserting a needle, carrying the stitch thread, through a stack of fabric layers
to form a 3D structure (see Figure 2.31). Standard textile industry stitching equipment is
capable of stitching preforms of glass and carbon fabrics and there are many high
performance yarns that can be used as stitching threads. Aramid yarns have been the
most commonly used for stitched composites as they are relatively easy to use in
stitching machines and are more resistant to rough handling than glass and carbon.
However the use of aramid stitching threads can cause difficulties in the final composite
component due to their propensity to absorb moisture and the difficulty in bonding the
aramid yarn to many standard polymer resins. The manufacturer must therefore be
aware that these problems may lead to a reduction in the mechanical performance of the
component in certain situations. Glass and carbon yarn do not have the problems of
moisture absorption and weak interfaces that aramid yam does, but they are
significantly more difficult to use in stitching machines. This is due to their inherent
brittleness, which can lead to yarn breakage when stitch knots are being formed and
fraying of the yarn in its passage through the stitching machine. Apart from trying to
minimise the potential fraying on the stitch thread the main requirement for a suitable
stitching machine is that the needle be capable of penetrating through the number of
fabric layers to be stitched together in a precise and controlled manner.
Although common, industrial stitching equipment can be used, there has been some
development of more complex machines specifically designed for the production of
stitched composite components. To date the most ambitious program has been that
undertaken by NASA in association with Boeing (Beckworth and Hyland, 1998). This