Page 139 - 3D Fibre Reinforced Polymer Composites

128 30 Fibre Reinforced Polymer Composites

5.4 INTERLAMINAR FRACTURE PROPERTIES OF 3D WOVEN COMPOSITES

An important advantage of 3D woven composites over conventional 2D laminates is a
high resistance to delamination cracking. 2D laminates are prone to delamination
cracking when subject to impact or out-of-plane loads due to their low interlaminar
fracture toughness properties. 2D laminates made of thermoset prepreg material, such
as carbodepoxy tape, are particularly susceptible to delamination damage due mainly to
the poor fracture toughness of the resin matrix. The low delamination resistance of
carbodepoxy is a significant factor impeding the more widespread use of these
laminates in aircraft structures prone to impact from stone and bird strikes, such as the
leading edges of wings and tail-sections. The superior delamination toughness of 3D
woven composites has been a strong incentive for the use of these materials in highly
loaded or impact prone aircraft structures such as wing panel joints (Wong, 1992),
flanges, and turbine rotors (Mouritz et al., 1999).
The delamination resistance of 3D woven composites has been characterised for the
mode I and I1 load conditions. The mode I condition is also known as tensile crack
opening and mode I1 as shear crack sliding. Most delamination studies on 3D woven
composites have been for mode I loading (Byun et a1.,1989; Guenon et al., 1989;
Arendts et al., 1993; Tanzawa et al., 1997; Mouritz et al., 1999). Little work has been
performed on the mode I1 delamination properties, and this is an area requiring further
research because delaminations caused by impact can propagate as shear cracks. The
delamination properties of 3D woven composites subject to mode I11 (tearing) loading
have not yet been determined, due possibly to the difficultly in performing mode I11
fracture tests on 3D materials.
The mode I interlaminar fracture properties of 3D composites with an orthogonal or
interlocked woven structure have been thoroughly investigated (Byun et a1.,1989;
Guenon et al., 1989; Arendts et al., 1993; Tanzawa et al., 1997; Mouritz et al., 1999). It
is found that the mode I delamination resistance of 3D woven composites is superior to
2D laminates. The delamination toughness increases with the volume content, elastic
modulus, tensile strength and pull-out resistance of the z-binders. However, even
relatively modest amounts of z-binder reinforcement can provide a large improvement
to the delamination resistance. For example, GuCnon et al. (1989) found that the
delamination toughness for a 3D carbodepoxy composite with a z-binder content of
only 1% was about 14 times higher than a 2D carbodepoxy prepreg laminate.
Increasing the z-binder content can promote even larger improvements to the mode I
interlaminar fracture toughness. The largest reported increase is for a 3D woven
composite with an 8% binder content that has a mode I delamination resistance more
than 20 times higher than for a 2D laminate (Arendts et al., 1993). Such large
improvements to delamination resistance are comparable to that found with other types
of 3D composites, such as knitted, stitched and z-pinned materials which will be
described later.
The high mode I interlaminar fracture toughness of 3D woven composites is due to a
number of toughening processes caused by the z-binders, and these are shown
schematically in Figure 5.19. When a delamination starts to grow between the plies in a
3D woven composite, the crack tip passes around the z-binders without causing them
any damage. In some materials the z-binders may debond from the surrounding
composite when the interfacial adhesion strength is poor, although the binders
themselves remain undamaged. The energy needed to debond the binders induces some