Page 186 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 175
Simple rule-of-mixtures theory can be used to explain the small improvements (< 10%)
to the Young’s modulus and strength of stitched composites. It is well known that the
modulus and strength of composites are dependent on the volume fraction of in-plane
fibres aligned in the load direction. Stitching with a high tensile force on the yam so the
thread is taut can raise the fibre volume content by several percent by compacting the
preform. The fibre volume fraction can be increased further by compaction of the
stitched preform during consolidation inside a closed mould. It has been suggested by
Mouritz and Cox (2000) that any modest increase to the Young’s modulus and strength
of stitched composites is probably due to a small increase in the fibre volume content
due to compaction of the in-plane fibres by stitching. Unfortunately, however, most
researchers who find an improvement to the mechanical properties of composites after
stitching do not report the fibre volume contents for the stitched and unstitched
materials. Therefore, while it is likely that compaction is the cause for the small
improvements to modulus and strength, this has not been confirmed by experiment.
A few stitched composites display remarkably high mechanical properties,
particularly under flexural loading. It is seen in Figure 8.8 that the flexural modulus and
strength values for some stitched composites are up to 3.5 and 1.75 times higher than
the unstitched laminate, respectively. Such large improvements to the flexural
properties cannot be due solely to fibre compaction from stitching, which usually
increases the fibre volume content by a few percent. The mechanism responsible for the
large increase to the flexural properties of stitched composites has not been determined.
However, a study by Chang et al. (1989) found that some unstitched composites fail in
bending by delamination cracking, but when these materials are stitched the
delamination cracks are suppressed and this increases the flexural properties.
The reduction to the tensile modulus and strength of stitched composites is attributed
mostly to three types of damage caused by stitching. These are fibre breakage (see
Figure 8.4a), distortion and crimping of the in-plane fibres causing misalignment from
the load direction (see Figures 8.4b-8.4d). These types of damage are usually localised
to a small region surrounding the stitches, and therefore their effect on the modulus and
strength is expected to be modest. Because the number and size of the regions
containing misaligned fibres is small, preliminary modelling by Mouritz and Cox
(2000) indicate that the reduction to the Young’s modulus of stitched composites is
modest (usually less than about 5%). This prediction shows reasonable agreement with
the observed reduction of up to 20% in many stitched composites.
The mechanism of compression failure of stitched composites is a complex process,
and is believed to be a competition between different failure modes. Many types of
unstitched composites, and in particular prepreg laminates, fail under uniaxial
compression loading by delamination cracking between the plies. Delamination is the
most common compression failure mechanism in unstitched materials, and it is often
initiated by edge stresses or a pre-existing defect (such as a void or crack).
Delamination failure can be suppressed in stitched composites by the stitches stopping
Euler buckling of the delaminated plies. Farley et al. (1992a; 1992b; 1993c) observed
that with delamination suppressed in stitched composites, the compressive failure
mechanism changes to kinking of the most severely crimped load-bearing fibres. The
damage mechanism leading to a kink failure is believed to start at the surface of stitched
materials where the crimping of the fibres is the most severe (see Figure 8.4d). Under
compression loading, axial shear stresses rapidly rise in a heavily crimped fibre bundle,
which cause failure of the polymer matrix within the bundle via microcracking and