Page 140 - 3D Fibre Reinforced Polymer Composites

30 Woven Composites 129

toughening of the composite (Gutnon et al., 1989), although it is likely to be small.
Most of the toughening occurs by the formation of a z-binder bridging zone that can
extend up to -30 mm behind the crack tip. These binders that bridge the delamination
are able to carry a large amount of the applied force, which reduces the stress acting on
the crack tip and thereby increases the delamination resistance.

Fracture & Pull-Out

Figure 5.19 Schematic of mode I delamination cracking in a 3D woven composite.

Figure 5.20 shows a z-binder bridging a delamination in a 3D woven composite. In 3D
woven composites the z-binder yarns may be distorted during manufacture (as shown in
Figure 53, and this reduces the effective stiffness and strength of the bridging z-
binders. Therefore, the interlaminar toughness provided by the z-binders is often not as
high as when they are aligned normal to the crack plane. In addition, the damage
incurred by the z-binder yarns during weaving reduces their tensile strength, which will
also affect the interlaminar toughness. The high toughness from the z-binders often
causes extensive crack branching in 3D woven composites that promotes further
toughening. The applied stress acting on the binders within the bridging zone is not
equal; but rather a low stress is exerted on the binders close to the crack tip and a larger
stress on binders at the rear of the bridging zone where the crack opening displacement
is the greatest. The binders at the rear of the bridging zone eventually break along the
crack plane (as shown in Figure 5.21) or near the outer surface of the composite where
the binder has been weakened during weaving in order to form a tight bend (see Figure
5.4). When the binders break near the outer surface they are gradually pulled-out of the
composite, and the work done during the binder pull-out also adds to the toughness.
Figure 5.22 shows a binder yarn standing proud of the fracture surface of a 3D woven
composite after being pulled-out. In summary, the superior mode I delamination
toughness of 3D woven composites is due to the toughening processes of debonding,
bridging and pull-out of the z-binders and crack branching.