Page 226 - 3D Fibre Reinforced Polymer Composites

Z-Pinned Composites 215

The high interlaminar toughness of Z-fiberTM composites makes these materials highly
resistant to edge delaminations and impact damage (Freitas et al., 1994). Edge
delaminations can be a major problem in composite structures, particularly at free edges
and bolt holes, and it is often necessary to taper the edges and reinforce holes to reduce
the incidence of cracking. Freitas et al. (1994) found that the tensile load needed to
produce edge delaminations in a carbodepoxy tape composite is increased dramatically
with a small amount of Z-fiberTM reinforcement. Figure 9.12 shows the tensile load at
which the onset of edge delamination cracking occurred in the composite with a Z-
fiberTM content of 0%, 0.5% or 1.0%. It is seen that the delamination load increased by
over 70% when the composite was reinforced with a Z-fiberTM content of only 0.5%,
and increased by over 90% with 1.0% Z-fibresm. Z-fibersm are also highly effective
increasing the fatigue life and stabilising the growth of fatigue damage in blade-
stiffened panels (Owsley, 2001).

v 9 "O0[
2 1000 r
6
s 800
.-
c
Ki
S 600
5
-
Ki -
0" 400-
c

0.0 0.5 1 .o
Amount of Z-Fibres (%)

Figure 9.12 Effect of z-pin content on the tensile load needed to induce edge
delaminations in a carbodepoxy composite (adapted from Freitas et al., 1994)

The delamination resistance of composites under impact loading is also improved with
z-pinning, although the impact damage resistance of these materials has not been as
extensively studied as for other types of 3D composites. A preliminary study has shown
that the amount of impact damage experienced by carbodepoxy composites is reduced
by between 30% and 50% with z-pinning (Freitas et al., 1994), and even larger
reductions can be expected with a high amount of reinforcement. The improved impact
damage resistance provides Z-fiberTM composites with higher post-impact mechanical
properties than equivalent materials without through-thickness reinforcement. For