Page 160 - 3D Fibre Reinforced Polymer Composites

Knitted Composite Materials 149

Ncedlc loop Sinker loop Sides or legs

Figure 7.2 Illustration of the main segments in a knit loop

7.2 IN-PLANE MECHANICAL PROPERTIES

7.2.1 Tensile Properties
The tensile performance of knitted composites has been a primary area of investigation
within the published literature. Most of the investigation has focussed upon the tensile
properties of weft knitted E-glass fabrics, generally with a plain knit architecture, that
have been consolidated with epoxy resins. Often there is little information in the
literature on the knitting parameters, such as loop lengths, shapes and densities, that will
allow a more detailed analysis, however the available results allow some general
behaviour to be established.
Experimental data on the tensile performance of plain, weft knitted E-glasdepoxy
composites has been collated from Ramakrishna et al. (1997), Huang et al. (2001) and
Hohfeld et al. (1994) and the variation of tensile strength and modulus with fibre
volume fraction is illustrated in Figures 7.3 and 7.4 respectively. It can be clearly seen
that the tensile performance of the plain knit E-glass/epoxy composites increases with
an increase in the volume fraction of glass fibres. It can also be seen that the tensile
properties are similar to that expected from randomly orientated E-glass mat
reinforcements of the same fibre volume fraction. This is to be expected as the
architecture of the plain knit has very few straight sections of yarn and certainly none
that are of any significant length. This aspect of the plain knit architecture, as well as
many other standard knit styles, will generally limited the mechanical performance of
knitted composites to values much lower than that expected from conventional 2D
woven fabrics (strengths of E-glass fabrics - 350 - 400 MPa).
Leong et al. (2000) and Anwar et al. (1997), presented tensile results of composites
manufactured from other weft knit architectures (fibre volume fractions of - 53%). Rib
(1x1) specimens had a tensile strength of 96 MPa and a modulus of 14.7 GPa, whilst
full milano architectures gave strength and modulus values of 122 MPa and 14.9 GPa
respectively. Clearly the style of knit architecture is influencing the tensile performance
of the knitted composite. This effect of the knit architecture upon the mechanical
properties of the composite is also observed in the results of Wu et al. (1993) and