Page 45 - 3D Fibre Reinforced Polymer Composites

34 30 Fibre Reinforced Polymer Composites

Standard warp and weft knitted fabric are regarded by many as 2D fabric, however,
machines with two or more needle beds are capable of producing multilayer fabrics with
yams that traverse between the layers. Figure 2.24 shows a schematic of such a fabric
and the range of knit architectures that can be produced with current industrial machines
is quite extensive. These flat fabrics can also be formed with variable widths, splits to
allow multiple, parallel fabrics to be formed, and holes with sealed edges.

Figure 2.24 Schematic of a multilayer knitted fabric

It is clear from the illustrations of knit architectures that the primary difference between
knitted fabric and fabric made by the other textile processes described here is in the high
degree of yarn curvature that results from the knitting process. This architecture results
in a fabric that will provide less structural strength to a composite (compared to woven
and braided fabrics) but is highly conformable and thus ideally suited to manufacture
relatively non-structural components of complex shape. This conformability means that
layers of knitted fabric can be stretched to cover the complete tool surface without the
need to cut and overlap sections. This reduces the amount of material wastage and helps
to decrease the costs of manufacturing complex shape components (Bannister and
Nicolaidis, 1998). Examples of such components are shown in Figure 2.25.
Changing the knit architecture can vary the properties of knitted fabric itself quite
significantly. In this fashion, characteristics such as fabric extensibility, areal weight,
thickness, surface texture, etc, can all be controlled quite closely. This allows knitted
fabric to be tailor-made to suit the particular component being produced. Both warp and
weft knitting also have the ability to produce fabric with relatively straight, oriented
sections of the knitting loop (see Figure 2.26) that can be designed to improve the in-
plane mechanical performance of the fabric. Warp knitting in particular has been used to
produce fabric with additional straight yarns laid into and bound together by the knit
structure, but this will be described more fully in a later section.