Page 33 - 3D Fibre Reinforced Polymer Composites

22 30 Fibre Reinforced Polymer Composites
2.2.4 Multiaxial Weaving
One of the main problems facing the use of multilayer woven fabrics is the difficulty in
producing a fabric that contains fibres orientated at k45" in the plane of the preform.
Standard industry looms, which are capable of producing multilayer fabric, cannot
manufacture this fabric with fibres at angles other than 0" and 90". It is possible to
orient the through-thickness binder yarns at angles such as +45" but this will not
significantly affect the in-plane, off-axis properties of the composite. Although some
orthogonal non-woven preforms can be produced with yarn architectures of this type,
the equipment and processes used in their production are generally not suited for large
volume production. This restricts the potential components that can be made using
multilayer fabric as the necessity to add +45" fabric will often negate the advantages
that can be gained in using a single, integrally woven preform that contains fibres in the
thickness direction. The more recent machinery developments have therefore tended to
concentrate upon the formation of preforms with multiaxial yams.
Curiskis et a1 (1997) have reviewed and described the techniques that are being
employed to produce multiaxial preforms. Process such as Triaxial Weaving, Lappet
Weaving and Split Reed Systems have been used by a number of researchers to develop
equipment capable of producing multiaxial, multilayer preforms and a number of
patents have been filed relating to the development of this equipment (Ruzand and
Guenot, 1994; Farley, 1993; Anahara et al., 1991; Addis, 1996; Mohamed and Bilisik,
1995). Although promising results have been demonstrated, the current reported
technology still appears to be in the development stage and preforms seem limited to
having the +45" yarns only towards the outer surfaces and not at other levels within the
thickness of the preform (see Figure 2.10).

2.2.5 Distance Fabrics
A final subset of the weaving technologies relates to the production of a preform style
known generally as Distance Fabric. This family of preforms is produced by the use of
the traditional textile technique known as Velvet Weaving. In this multilayer weaving
process two sets of warp yarns, spaced by a fixed distance, are woven as separate
fabrics but are also interlinked by the transfer of specific warp yarns from one fabric
layer to the other. These warp yarns, known as pile yarns, are woven into each face
fabric thus forming a strong linkage between the two faces and creating a sandwich
structure as shown in Figure 2.11. The spacing between the face fabrics can be adjusted
by controlling the separation of the warp yams in the weaving loom and the angle of the
pile yarns can be varied from vertical (90") to bias angles (e.g. k45") although currently
these bias angles can be only produced in the warp direction. Distance Fabric material
is commercially available and comes in a range of heights up to - 23 mm. Due to the
strong linkage between the face fabrics it is highly suited for the production of peel-
resistant and delamination resistant sandwich structures (Bannister et al., 1999).

2.3 BRAIDING

The braiding process is familiar to many fields of engineering as standard two-
dimensionally braided carbon and glass fabric has been used for a number of years in a