Page 170 - 3D Fibre Reinforced Polymer Composites

Knitted Composite Materials 159
7.4 IMPACT PERFORMANCE

7.4.1 Knitted Composites
The superior properties of knitted composites in Mode I fracture toughness is also
reflected in their overall impact performance. Leong et a1 (1998) examined the low- to
medium-energy impact performance of an E-glass/epoxy, weft knitted milano material
under drop-weight conditions. For the range of impact energies tested (up to 7.3 J/mm)
they found that the damage area created within the knitted composite was essentially a
circular region of very dense and complex microcracks. The diameter of this damage
zone increased as you moved from the front face to the back face creating a trapezoidal
shape. The authors found that the compression strength of the impacted composite was
reduced by only 21% for high impact energies, implying that the knitted composite was
very damage tolerant. Also, in comparison with composite specimens manufactured
with uniweave reinforcements, the knitted composite was capable of absorbing a much
higher proportion of the incident impact energy, 64% more than the uniweave
composite at high impact energies.
This energy absorption capability has also been observed by Chou et al. (1992) who
conducted notched Charpy impact tests upon E-glass/epoxy specimens of both weft
knitted 1x1 rib and plain weave composite materials. They found that the absorbed
impact energy of the plain weave composite was 68.3 W/m2 whilst the knitted
composite was at least 2.4 times better at 161.3 kJ/m2. This ability of knitted composites
to absorb substantially greater amounts of impact energy than woven materials would
suggest them as ideal candidates for damage-prone structures or ones requiring a high
energy absorption capability, such as crush members. This concept was investigated by
Ramakrishna et al. (1993) when they examined the energy absorption capabilities of
epoxy composite tubes reinforced with knitted carbon fabrics. The knit architectures
used were weft knitted 1x1 rib structures with and without straight fibres laid in the
course direction. The orientation of the inlay yarns along the tube axis allowed the
specific energy absorption capability of the tube to reach 85 W/kg with only a total fibre
volume fraction of 22.5%. This performance is encouraging when compared to the
highest specific energy of 120 W/kg recorded for carbotdepoxy tubes with a fibre
volume fraction of 45% (reported by Ramakrishna et al 1993).
The impact performance under drop weight conditions of knitted composites with
regard to knit architecture has also been investigated by Khondker et al. (2000). They
examined the impact resistance and tolerance of three different architectural styles of E-
glasdvinyl ester weft knitted composites; milano, 1x1 rib and plain knit. For the three
architectural styles, at similar stitch densities, the authors found that the damage area
created at the same impact energy of 6 J/mm (an indication of the impact resistance)
increased significantly from the lain knit (230 mm’) through the milano (290 mm2) to
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the 1x1 rib structure (350 mm ). In a similar fashion the reduction in compression
strength after impact, which gives an indication of the material’s impact tolerance, also
varied with knit architecture. Again the plain knit demonstrated the best damage
tolerance, losing only 22% of its initial undamaged strength at an impact energy of 6
J/mm whilst the milano and 1x1 rib structures lost 27% and 32% respectively. It is not
clear what aspect of the knit architecture gives the plain knit a superior impact
performance over the milano and 1x1 rib structures.