Page 155 - 3D Fibre Reinforced Polymer Composites

144 30 Fibre Reinforced Polymer Composites
compares the fracture toughness of 3D braided composites with other forms of
composite reinforcement, therefore it is not possible at this time to make any strict
comparisons as to any potential improvements. However, the mode I fracture behaviour
of a 4-step braided carbodepoxy material was examined by Filatovs et al. (1994) in a
compact tension arrangement and the effect of the notch orientation relative to the
direction of the braiding axis was investigated. It was found that the fyce required to
initiate and grow a crack through the 3D braid increased by a factor of 4 as the braid
axis orientation varied from in line with the notch to transverse to it. The lowest value
for crack propagation force was observed when the notch axis was at the same angle to
the braid axis as the braiding yarns themselves, thus allowing crack propagation to
occur partially along sections of the braiding yarns. Unfortunately, the authors did not
translate these results into measurements of fracture toughness and did not compare
them with measurements on conventional 2D laminates.
There is more published work that examines the general damage tolerance of 3D
braided composites. In their work on the general mechanical properties of 4-step
braided, carbodepoxy composites Gause et al. (1987) also compared the OHT and
OHC strength of 1x1 and lxlx* braids with a 2D laminate (Table 6.3). The 3D braids
were observed to retain a very high proportion of their undamaged gross tensile strength
(99% and 86% for the 1x1 and lxlx4iF respectively) compared to the 49% retained by
the 2D laminate. In compression their retained strengths were of a similar order (42-
47%). Under drop weight impact tests the 3D braids were found to perform far better at
limiting the extent of damage, having less than half of the damage area created at the
higher test energies than the 2D laminate.
KO et al. (1991) also examined the strength retention of carbon/PEEK 3D braids
compared to 2D laminates under OHT conditions. Although the 2D laminate had far
superior undamaged tensile properties (1081 MPa versus 586 ma), it was found that
with similar proportions of fibres in the 0" and k20" directions the 3D braided
specimens had a far greater retention of tensile strength than the 2D laminates (79% and
58% respectively). Impact tests were also conducted upon the specimens and it was
found that the 3D braided materials had higher compression after impact strength and an
order of magnitude lower damage area than the 2D laminates.
Brookstein et al. (1993) compared the CAI performance of 2D and 3D braided
composites (Table 6.2) and found that at the two impact energy levels tested (3 and 7
J/mm) the 3D braided composites had approximately the same or better compression
strength compared to the 2D braided samples. This less significant difference between
the impact performance of 3D and 2D braids compared to 3D braids and 2D tape
laminates can be understood through the general architecture of braids. Even with an
absence of through-thickness braiding yams, the architecture of a 2D braided laminate
is very undulated with the layers of braided fabric nesting significantly with each-other.
This makes it very difficult for impact damage to propagate extensively within the
composite as compared to the relatively straight crack paths available in tape laminates.
Overall, the damage resistance and tolerance of 3D braided composites is seen to be
significantly greater than that of 2D tape laminates and at least the same as, or greater
than, that of 2D braided composites. However, no data exists for 3D braided polymer
matrix composites that examines the effect that the braid architecture or the braiding
process has upon their fracture or damage performance. Much of this investigation has
been conducted in ceramic and metal matrix 3D braided composites.