Page 173 - 3D Fibre Reinforced Polymer Composites

162 30 Fibre Reinforced Polymer Composites

technique were generally lower than experimental values and its one-dimensional
approach limits its capability to predict the full set of elastic constants.
More complex methods generally involve partitioning the RVE into a number of
infinitesimal elements (sub-elements), the properties of which are analysed by means of
a unidirectional micromechanics model in the local coordinate systems. A tensor
transformation is then used to transform the sub-elements from local coordinates to a
global one and an averaging scheme, normally either iso-strain (Voight method), iso-
stress (Reuss method) or a variation of these, is used to obtain the overall stiffness
matrix of the RVE.
There are many micromechanical models in the literature that can be used to define
the unidirectional properties of the subelements. Two of these that have been used in the
modelling of knitted composites are the Chamis model, which can only be used to
model the elastic properties, and the Bridging Matrix model, which has the capability to
model the stress-strain behaviour of the composite up to failure.
A comparison of a number of these modelling approaches was made by Huang et a1
(2000) for the prediction of the tensile properties of an E-glass/epoxy composite
reinforced by a single layer of plain weft knitted fabric. The results of that comparison
showed that there is no one combination of micromechanical model and averaging
scheme that currently gives reasonable predictions for the elastic properties and failure
strengths. In general the errors in the predictions ranged between 15% to 29% from the
measured values, and often a modelling scheme whose prediction was close for one
particular property produced a very inaccurate prediction for another property.
More recent modelling work (Huang et al., 2001; Huysmans et al., 2001) is showing
promise for the accurate prediction of the mechanical performance of knitted
composites but a substantial amount of progress is needed before a robust, accurate
modelling approach is available.

7.6 SUMMARY

Knitted fabrics hold a great deal of potential for the manufacture of specific types of
composite components. No other textile reinforcement is as capable as knitted fabric is,
of being formed or directly manufactured into complex shapes. Their excellent impact
performance would appear to make them ideal for service conditions where energy
absorption or damage tolerance was critical. A special sub-group of knitted fabrics,
known as Non-crimp Fabrics, is also capable of manufacturing parts with very high in-
plane mechanical performance at a reduced manufacturing cost and is a prime material
candidate to replace conventional prepreg materials in future aircraft.
As with many of the 3D textile reinforcements described here, the mechanical
performance of knitted fabrics is a very complex and not well understood issue.
Excepting non-crimp materials, knitted composites have in-plane mechanical properties
that lie between that of random mats and traditional 2D weaves, but these properties can
be dramatically changed by the knit architecture and the degree of stretch within the
knit. The generation of a database of knitted composite properties and the development
of models to understand and predict these properties are still in their infancy relative to
the other forms of 3D reinforcement. Further progress in these two areas is required
before knitted fabrics will become a more commonly used reinforcement in composite
structures.