Page 150 - 3D Fibre Reinforced Polymer Composites

Braided Composite Materials 139
The flexural behaviour of the materials also showed significantly reduced performance
when specimen edges were machined to produce cut fibres. This experimental data
indicates the high sensitivity that 3D braided composites have to machining damage of
the yarns on the surface. As each braiding yarn within the common 3D braiding
processes will eventually travel to the specimen surface, any machining of this surface
will result in the braiding yarns becoming non-continuous along the specimen length,
with the resultant drop in performance. Due to the fixed nature of the axial fibres they
will run parallel to the specimen surface and thus will not be affected by any machining.
This results in their higher retained properties when compared to composites without
axial fibres.
The data presented in Table 6.1 also illustrates the strong influence that the braiding
pattern has upon the mechanical properties of the composite materials. The presence of
axial fibres within the 1x1 architecture has produced a braid with an apparent braiding
yarn angle (angle between the braiding yarn orientation and the specimen braid axis)
less than that of the 1x1 architecture without axials. The orientation of the braiding
yarns closer to the braid axis, which is the direction along which the testing has been
performed, and the presence of the axial fibres themselves produces a composite with
improved tensile, compressive and flexural properties. This improvement in composite
performance due solely to a reduction in braiding yarn angle is also observed when
comparing the properties of the 1x1 and 3x1 structures, in both cut and uncut edge state.
A decrease of 8O in the braiding yarn angle resulted in an improvement in tensile and
compressive performance of 25 - 50%. Wenning et al. (1993) also observed a similar
improvement in the tensile modulus with a decrease in the fibre angle of 4-step braided
composites.
Other investigations on the influence of braid angle were conducted by Brookstein et
al. (1993), who investigated the properties of carbodepoxy 3D composites that were
braided by the Multilayer Interlock method. Specimens with two braiding patterns (Le.
differing braid angles) were tested, +45"/0"/~45" (Vf = 43%) and ~60"/0"/~60° (Vf =
45%) and the results of these tests were normalised to a nominal 50% fibre volume
fraction for comparative purposes (see Table 6.2). When comparing the properties of
the two 3D braided patterns, Brookstein et al. also found that the tensile and
compressive properties in the longitudinal direction were improved when the braiding
yam angle was reduced, but at the sacrifice of the transverse performance. The design of
3D braided preforms must therefore be a compromise between the required mechanical
performance and the number of axial yarns and the braid angle possible within a certain
braiding technique.
The influence of axial fibres on the composite mechanical performance was also
noted by Gause et al. (1987) who observed significant increases in the longitudinal
tensile and compressive properties of carbodepoxy, 4-step braided specimens when half
of the yarns available for braiding were fixed as axial yarns. Table 6.3 summarises the
results of this work although it should be noted that errors in some of the data contained
in the original publication were corrected in a later publication by KO (1989) and it is
from this publication that the data in Table 6.3 is taken. It is interesting to note that
although the presence of axial yarns has improved the longitudinal properties of the
braided specimens, it comes at the sacrifice of the transverse properties. This is because
there are now fewer yarns available as braiding yarns and thus less reinforcement
oriented towards the transverse direction.