Page 225 - 3D Fibre Reinforced Polymer Composites

214 30 Fibre Reinforced Polymer Composites
It is evident that z-pinning is highly effective in raising the delamination resistance of
composites, however further work is required to optimise the pinning conditions to
achieve the maximum improvement in interlaminar fracture toughness. The effects of
the stiffness, strength, diameter and type of the pin as well as the areal density of
pinning on the modes I and I1 interlaminar fracture toughness needs to be thoroughly
investigated. This investigation can be facilitated with the recent development of
micro-mechanical models for predicting the interlaminar fracture toughness of z-pinned
composites. A model has been proposed by Liu and Mai (2001) for mode I toughness
whereas the model by Cox (1999) described in Section 8.4.2 can be used for mode I1
toughness.
The mechanism of interlaminar toughening that occurs with z-pinned composites is
similar to that operating with other types of 3D composites. The pins appear to do little
to prevent the initiation of interlaminar cracks, which might be considered as the onset
of delamination growth up to a length of about 1 to 5 mm. With cracks longer than
about 5 mm, however, the z-pins slow or totally suppress the further growth of
delaminations by a crack bridging action. The toughening processes for modes I and I1
loading are shown schematically in Figure 9.1 1. Interlaminar toughening occurs by the
z-pins bridging the delamination behind the crack front, and through this action is able
to support a significant amount of the applied stress. This greatly reduces the strain
acting on the crack tip and thereby increases the interlaminar fracture toughness and
stabilises the crack growth process. When the separation distance between the crack
faces becomes large the rods are pulled from the composite or break. In the case of
mode I1 loading, the pins also absorb a significant amount of strain energy by shear
deformation until failure occurs by pull-out or rupture.

I+- Crack Bridging Zone +Fracture Pin Pull-Out & 4
zone

Crack Bridging Zone

(b)
Figure 9.11 Schematic of the bridging toughening mechanism in Z-fiberm composites
for (a) mode I and (b) mode I1 loading