Page 165 - 3D Fibre Reinforced Polymer Composites

154 30 Fibre Reinforced Polymer Composites

can be clearly understood in that during fabric stretch the fibres, whose orientations
were rather randomly distributed in all directions, now begin to align more towards the
axis of the fabric stretch. This increase in the proportion of the fibres oriented in the
direction of loading will naturally improve the tensile performance of the material.
The ability of knitted fabrics to be deformed easily and of the knitting process itself
to produce holes integrally formed within the fabric, allows for the possibility of
producing composites with continuous fibres surrounding a notch or bolthole rather than
the broken fibres produced during the drilling of holes in composites. The effect of
formed holes upon the notched tensile strength and bearing performance of knitted
composites was examined by de Haan et al (1997) and Leong et a1 (1998) respectively.
In both investigations the performance of specimens with holes formed into the knit
architecture was significantly improved compared to the specimens with drilled holes
(see Table 7.3). This was due not only to the unbroken yarns surrounding the hole but
also to the increase in the fibre volume fraction around the hole that occurs when the
hole is formed into the knitted fabric.

Table 7.3 Notched (de Haan et al., 1997) and Bearing (Leong et al., 1998) wale
direction tensile properties of weft knitted composites (W/D = 4)
Structural form Notched Bearing

The failure process of a knitted composite is, like its architecture, a complex situation.
A number of researchers (Rudd et al., 1990; Ramakrishna et a]., 1994; Wu et al., 1993;
Ramakrishna et al., 1997; Leong et al., 1999; and Huysmans et al., 2001) have
examined the various stages of tensile failure in warp and weft knitted composites,
ranging from low fibre volume fraction, single layer materials, to high fibre volume
fraction, multilayer specimens. It is generally accepted that the first stage of failure
occurs at reIatively low strain values and is the result of debonding between the resin
and the portions of the knit loops orientated transverse to the loading direction (see
Figure 7.7). Upon increasing load these cracks propagate into the resin-rich regions
between the yarn loops. As these cracks grow and coalesce they are bridged by the
unbroken yarns of the knit loops. The composite behaviour following this is then largely
dependent upon the number and geometry of the yarns crossing the crack plane.
Architectures with highly orientated yams will pick up the load almost immediately
whilst those with significant curvature, or off-axis orientation, may rotate or stretch
before becoming fully loaded. Final failure of the knit loops has been seen to occur in
either one of the two places (and often a mixture of both), at the “legs” of the knit loop
where the local fibre volume fraction is lowest, or at the loop crossover points where the
stress concentrations are highest.

7.2.2 Compressive Properties
Unlike the tensile properties, relatively little has been reported on the compressive
properties of knitted composites. A number of researchers (Wang et al., 1995a; Leong et