Page 214 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 203
In the experiments performed by Tong et a1 (1998), the specimens were prepared by (a)
overlaying two [0/k45/90Is fabric stacks followed by debulking under vacuum and heat
to produce a preform of single-lap panel; (b) applying transverse stitches following the
designed pattern; and (c) injecting resin and consolidating the panel under clamping
pressure and a curing temperature of 80°C for 4 hours.
All specimens were manufactured from Ciba Composites Injectex@ uniweave
carbon fabric GU230-EO1 and GY260 epoxy resin/HY9 17 hardenerDY070 accelerator.
The uniweave material has 90% of its fibers oriented in the warp direction and the
remaining fibers in the weft direction to hold the warp fibres in place for ease of
handling. The Injectex8 has been developed for precise fabric placement at preform
stage prior to resin infusion. The stitch material used was a twisted 4Otex (2x20) Kevlar
thread, and zigzag stitching pattern was employed with the overstitch limited to 1 mm
as schematically shown in Figure 8.38.
The measured axial loads increased almost linearly with the applied axial
displacement for all specimens up to the final failure. For all specimens catastrophic
failure occurs upon attaining the ultimate load. The average failure loads are tabulated
in Table 8.1. The results show that the stitched single-lap joints are stronger than the
unstitched ones. For the long specimens with an unsupported length of 90 mm,
through-thickness stitching leads to an average increase in joint strength by about 25%.
For the short specimens with an unsupported length of 70 mm, there is an average
increase in joint strength of about 22%.

Stitch pitch (4.3 mm) yx 4 Fwidth(4mm)

4k-
Overstitch (less Gap (4 mm)
than 1 mm)

Figure 8.38 Top view of the four-row zigzag stitch used in the overlap of the single-lap
joint (Tong et al, 1998a)

Table 8.1 Effect of stitching on static failure strength of single lap joints fabricated by
stitching preform and using RTM
Specimen Unsupported Average failure
group length (mm) load (kN)
Unstitched 90 mm 11.33
Unstitched 70 mm 12.37
Stitched 90 mm 14.11
Stitched 70 mm 15.06