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Knitted Composite Materials 161

7.4.2 Non-Crimp Composites
The impact performance of carbon non-crimp composites was investigated by Bib0 et a1
1998 and compared with a unidirectional prepreg tape composite with the same resin
system and lay up. No significant difference in the impact resistance was observed
between the two material forms with similar damage areas being measured at the same
impact energies, however examination of the damage patterns did reveal a slight
variation. The damage sustained by the non-crimp composite appeared to be affected by
the local presence of the knitting yarns and any undulations in the layered fabric, giving
the damage a more complex appearance than the traditional delaminations and
shearhansverse cracks observed in unidirectional prepreg tape composites. It is
possible that these variations in the local fabric topography are acting as barriers to easy
crack growth, forcing the crack to follow a more convoluted path, although this effect is
not seen globally in the total measured damage area.
The residual compression strength after impact did not show any conclusive
difference between the absolute values measured for the non-crimp and prepreg
specimens. However, given that the undamaged compression strength of non-crimp
composites was generally observed to be significantly lower than that of unidirectional
prepreg tape materials, the authors claimed that the non-crimp composites exhibited a
greater damage tolerance than the prepreg materials.

7.5 MODELLING OF KNITTED COMPOSITES

Given the complex nature of the knit architecture, accurately modelling the strength and
stiffness performance of these materials is a very challenging task. Not withstanding
this, a number of researchers have been developing modelling approaches to varying
degrees of success and comparative studies of these approaches are contained in
worthwhile reviews by Leong et al. (2000) and Huang et a1 (2000).
Historically there have been two general approaches to modelling the performance
of knitted composites; Numerical (using FEM techniques) and Micromechanical.
Although FEM is a very powerful tool for structural analysis the 3D complexity of the
knit architecture and the sensitivity of FEM to boundary conditions make this approach
both time-consuming and the applicability of the results potentially suspect.
Micromechanical approaches have therefore become the more practical means of
modelling the knitted composite.
The mechanical properties of a knitted composite will depend upon three things; the
properties of the constituent materials, the overall fibre volume fraction, and the knit
loop architecture. Of these three areas the most critical in the model development is the
determination of the knit geometry. All of the models that have been developed for
knitted composites start first by describing the Representative Volume Element (RVE)
or unit cell of the knit architecture, ideally by some analytical function as discussed in
Chapter 4. However, currently only the plain knit architecture can be specified by such
a function (Leaf and Glaskin), other knit architectures must have their RVE’s described
through often time-consuming and difficult experimental measurements.
Once the RVE has been described the most simplistic approach reported has been to
use the Krenchel model, which uses a combination of the rule of mixtures and a
reinforcement efficiency factor to describe the elastic modulus. Predictions using this