Page 178 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 167
The damage to preforms caused by stitching is one of the major drawbacks of this
technique. Stitching can cause many different types of damage, and these are shown in
Figure 8.4. The different damage types are:

Fibre breakage: Breakage occurs from abrasion generated by the needle and stitch
yarn sliding against the fibres during the stitching process. Breakage in prepreg tape can
also occur by the needle tip crushing the fibres, which cannot be easily pushed aside by
the needle because of the resin matrix. Fibre breakage in a dry fabric preform in shown
in Figure 8.4a.
Fibre misalignment: Significant misalignment of the in-plane fibres occurs because
they are distorted around the needle and stitch thread. The amount of distortion to fibres
depends on the fibre density, stitch yarn thickness and, in some cases, stitching density,
and maximum misalignment angles of between 5" and 20' have been measured (Mouritz
and Cox, 2000; Mouritz et al., 1996; Reeder, 1995). A schematic diagram and
photograph of fibre misalignment is shown in Figures 8.4b and 8.4~.
Fibre crimping: Crimping occurs by the stitches drawing the fibres at the surface
into the preform. The effect of fibre crimping is illustrated in Figure 8.4d, and the
severity of crimping increases with the line-tension on the stitching yarn.
Resin-rich regions: The crimping and misalignment of fibres in preforms leads to
the formation of small regions with a low fibre content around the stitches(Figure 8.4e).
This leads to the formation of resin-rich regions when the preform is consolidated into a
composite using liquid moulding processes.
Stitch distortions: The stitches can be distorted by heavy compaction of the preform
using liquid moulding, hot pressing or autoclaving techniques (Rossi, 1989). This type
of damage is shown in Figure 8.4f.
Microcracking: Cracking of the resin near the stitches can occur due to thermally-
induced strains arising from the mismatch in the coefficients of thermal expansion of
the stitches and surrounding composite material (Farrow et al., 1996; Hyer et al., 1994).
In some stitched materials, this can cause debonding between the stitches and
composite.
Compaction: Applying a high tensile load to the thread to ensure it is taut during
stitching can compact the preform plies. As a result, consolidated stitched composites
can have fibre volume fractions that are several percent higher than expected.

Not all stitched composites contain all the different types of damage listed above. The
most common types of damage to stitched composites are fibre breakage, misalignment
and crimping.
In addition to damage to the composite, the stitch thread itself can be damaged.
Damage to fibres in the threads occurs by twisting, bending, sliding and looping actions
as the thread passes through the sewing machine and formed into a stitch. The damage
can be significant and cause a large loss in strength (Dransfield, 1995; Jain and Mai,
1997; Morales, 1990). For example, Morales (1990) found that the tensile strength of
Kevlar thread fell from 4790 MPa to 3706 MPa after stitching. The situation can be
even worse when stitching with carbon thread, when a reduction in strength from 3500
MPa to only about 1550 MPa can occur (Morales, 1990).