Page 181 - 3D Fibre Reinforced Polymer Composites

170 30 Fibre Reinforced Polymer Composites

8.3.2 Tension, Compression and Flexure Properties of Stitched Composites
The tension, compression and bending modulus and strength are material properties of
great engineering importance in load-bearing structures, and therefore these properties
have been measured for many types of stitched composites, including carbon-, glass-
and Kevlar fibre laminates. Large databases for the tension, compression and flexural
properties are now available for most of the main engineering composites, including
carbodepoxy laminates used in aircraft. However, reliable models for predicting the in-
plane mechanical properties of stitched composites are not available.
A review of the published mechanical property data for stitched composites shows
apparent contradictions between materials (Mouritz and Cox, 2000; Mouritz et al.,
1999). The data indicates that most stitched composites have slightly lower tension,
compression and flexural properties than their equivalent unstitched laminate, although
some stitched materials exhibit no change or a modest improvement to their mechanical
properties. For a few materials the properties are dramatically improved or severely
degraded by stitching. The apparent contradictions are shown in Figure 8.5, which
compares the tensile modulus and strength for two composites stitched under identical
conditions (Kan and Lee, 1994). The Young’s modulus for the glasdpolyester
decreases with increasing stitch density whereas the modulus for the KevlarPVB-
phenol increases erratically with stitch density. The tensile strength for the
glass/polyester also drops rapidly with increasing stitch density while the strength for
the KevlarPVB-phenol increases slightly with stitch density before decreasing. Similar
contradictions occur for the compression and flexure properties of stitched composites.
Mouritz and Cox (2000) analysed tension, compression and flexural property data
from the literature for a variety of carbon-, glass-, Kevlar- and Spectra-fibre reinforced
polymer composites stitched over a range of area densities (from 0.2 to 25 stitches/cm2).
The composites were stitched with different thread materials using lock, modified lock
and chain stitches. The mechanical property data collected by Mouritz and Cox (2000)
is plotted in Figures 8.6 to 8.8. The data is plotted as normalised Young’s modulus
(E&) and normalised strength (do,) against stitch density for tension, compression
and flexure. The subscripts t, c and f to (EE,) and (doo) represent tension,
compression and flexure, respectively. The normalised Young’s modulus is the
modulus of the stitched composite (E) normalised to the modulus of the equivalent
unstitched material (E,) subject to the same load condition. Similarly, the normalised
strength is the strength of the stitched composite (a) divided by the strength of the
unstitched laminate (ao) for the same load condition. In the figures, CFRP represents
carbon fibre reinforced polymer, GFRP is glass reinforced polymer, KFRP is Kevlar
reinforced polymer, and SFRP is Spectra reinforced polymer laminate.
With the exception of a few outlying data points, it is shown in Figures 8.6 to 8.8
that stitching improves or degrades the modulus and strength by no more than -20%.
Within this variance, there is no clear correlation between the change to the mechanical
properties and stitch density. This implies that tension, compression and flexural failure
is not determined by the collective action of many stitches but rather that a single stitch
or a small number of stitches and the damage arising from them (eg. distortion and
breakage of fibres) can determine strength.
This data trend is of practical significance because it shows that the tension,
compression and flexure properties for most composites will be changed by less than
20% regardless of the amount of stitching. However, the impact damage resistance and