Page 154 - 3D Fibre Reinforced Polymer Composites

Braided Composite Materials 143
overall composite properties. They postulated that the extent of crowding may be more
pronounced as the yam size increases, therefore improving the tensile properties for
larger yarn size specimens, however no supporting evidence was presented for this
theory.
The flexural and shear properties do not show any clear trend of improvement with
decreased braid angle, again indicating that the effect of yam size, although significant,
is not a clearly understood phenomenon.

6.2.4 Comparison with 2D Laminates

Gause et al. (1987) compared the properties of their 1x1 and IxlxY' 3D braided
specimens with a 24 ply laminate of AS1/3501 prepreg with a lay-up orientation
designed to mimic the proportions of fibres contained in the lxlx%F 3D braided
material (Table 6.3). The authors found that there was no clear trend in the comparison
of undamaged in-plane properties between 2D and 3D materials. The tensile strengths in
both directions as well as transverse tensile modulus was found to be worse for the 3D
braid but the longitudinal compressive properties and tensile modulus were found to be
better. In the case of open hole properties the 3D braided materials retained a far greater
proportion of their tensile strength than the 2D laminate, at least 86% gross strength
compared to approximately 50% for the 2D laminate. However the comparison of open
hole compressive strength did not follow a similar trend, although this may be due to a
lower than expected value of compression strength for the 2D laminate.
It should be expected that 3D braided composites will not have undamaged, in-plane
properties that match, or are superior to, 2D prepreg tape laminates of similar fibre
orientation and volume fractions. This is due to the fact that the yarns within the braid
will suffer from a certain level of crimping as a result of the braiding process and this
will reduce their performance relative to the uncrimped fibres in the prepreg tape. A
better comparison to make is of 2D and 3D braided composites and this was done
within the work published by Brookstein et al. (1993). The results that are summarised
in Table 6.2 also give a comparison between the properties of 2D triaxial braids and 3D
Multilayer Interlock braids manufactured from the same 12K AS4 carbon tow and
epoxy resin (results normalised to 50% fibre volume fraction). Except for the case of the
compressive strength the results show that for both braid patterns the 2D braids have
better performance in the longitudinal direction than the 3D braids but lower in the
transverse direction. The authors suggested that it was possible that the 0" fibres in the
3D braids were pushed away from the axis by the geometrical configuration of the
interlocking braiding yarns and therefore were improving the transverse performance of
the specimens at the detriment of the longitudinal.
It is clear from the published literature that more data is needed before a strict
comparison can be made between the in-plane properties of 3D braided composites and
the standard 2D laminates.

6.3 FRACTURE TOUGHNESS AND DAMAGE PERFORMANCE
As with all 3D textile composites, the addition of the third dimension of reinforcement
is expected to invest composites made from 3D braided material with improved
toughness and damage characteristics. There has been very little published that