Page 221 - 3D Fibre Reinforced Polymer Composites

210 30 Fibre Reinforced Polymer Composites
failure mechanisms. The discussion here is confined to describing changes to the tensile
and compressive strengths of z-pinned composites that is based on a small number of
studies. Freitas et al. (1991, 1994, 1996) have shown that the tensile properties of a tape
carbodepoxy laminate are unaffected when the amount of Z-fibersm is low (under
1.5%). However, the tensile properties can be degraded with high amounts of pinning.
For example, Figure 9.6 shows the effect of Z-fiberTM content on the tensile strength of
a unidirectional tape laminate. The strength decreases rapidly as the Z-fiberm content is
increased up to an areal density of 10%. It is seen that the loss in tensile strength can be
significant, with the strength of the composite with a Z-fiberm content of 10% being
only 60% of the original strength. The failure mechanism of z-pinned composites under
tensile loading has not been studied. It is speculated, however, that the reduction to
tensile strength is due to the misalignment and fracture of fibres. However, more
experimental work is needed to identify the tensile failure mechanism and micro-
mechanical models are needed for predicting the elastic modulus and tensile strength of
z-pinned composites.

700r

% 300-
c
200-

100-

I I I I L

Figure 9.6 Effect of Z-fiberTM content on the tensile strength of a carbodepoxy
composite. Adapted from Freitas et al. (1996)

The effect of z-pinning on the compressive properties and failure mechanisms of
composites is currently under investigation, and data on the compressive strength of z-
pinned composites is limited. Stevens and Fleck (1999a, 1999b) have shown that the
compressive properties of composites can be degraded by Zfibersm, with reductions in
compressive strength of up to 33% being measured for unidirectional tape laminates.
The reduction to compression strength is due to the misalignment of in-plane fibres
around the z-pins (as shown in Figure 9.5) that causes the tows to fail by kinking at
lower compressive loads. As described earlier in Chapter 5. the failure mechanism of