Page 167 - 3D Fibre Reinforced Polymer Composites

156 30 Fibre Reinforced Polymer Composites

properties, the compressive performance of the E-glass weft knit milano material is not
significantly effected by any stretching of the knitted fabric prior to consolidation. Both
the isotropic nature and insensitivity to stretch and knit architectural changes of the
compressive properties are believed to be due to the mode of failure that occurs under
compressive loading. Khondker et al. (2001a) identified that failure involved the
formation of yarn kinks, which were a direct result of the buckling of the most highly
curved sections of the loaded yarns (see Figure 7.8). This type of failure is very
dependent upon the properties of the matrix thus, although the fibre volume fraction and
knit architecture will have some effect on the compression performance, the matrix
properties will tend to be the dominating factor.

Figure 7.8 Compressive failure in knitted composites through kink formation (courtesy
of the Cooperative Research Centre for Advanced Composite Structures Ltd)

7.2.3 In-Plane Properties of Non-Crimp Fabrics

In Chapter 2 the production of a specialised sub-group of knitted fabrics, known as
Non-crimp fabrics, was described. Non-crimp fabrics were designed primarily as an
alternative reinforcement material to tape and woven prepregs. The use of these fabrics
will result in reduced costs in the lay up of composite structures due to their multilayer
structure. Although produced using warp knitting techniques this family of fabrics
contains substantial proportions of relatively straight, in plane yarns which will
dominate the mechanical performance of the composite materials for which it is a
reinforcement. Thus the proportion and orientation of these in-plane yarns will be the
controlling factor in much of the mechanical performance rather than the structure of
the warp knit yarns. The improvement that is observed in the mechanical performance