Page 354 - Fiber Fracture

336 J.W.S. Hearle
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Fig. 4. Fracture of resin-treated cotton: (a) at 65% rh, showing a granular form cutting across the fibre; (b)
in wet state, showing a crack following the helical line between fibrils. From Hearle et al. (1998).

As with the effect of the matrix in fibre-reinforced composites, the bonding between the
fibrils is not strong enough to cause direct crack propagation across the fibre from a
break in a fibril, but is strong enough to transfer some stress and trigger the next break in
a neighbouring fibril. The result is a granular break as described in the 3rd paper in this
volume. Fig. 4 shows that a granular fracture occurs at 65% rh in cotton that has been
treated with a resin cross-linking agent, which increases the bonding between fibrils.
At an intermediate humidity (65% rh), absorbed water weakens the bonding of fibrils.
In untreated cotton, the shear stress associated with untwisting then becomes the source
of failure. Starting at the stress concentration adjacent to a reversal and a weakness at
point N in Fig. 2, a shear crack follows the helical line around the fibre, as shown in fig.
8 of the 3rd paper in this volume. Eventually the crack reaches the position N further
along the fibre, which leads to rupture by tearing back along the NN line. A similar form
is shown by a resin-treated cotton in the wet state.
In the wet state, the inter-fibrillar bonding is weaker still, and an untreated fibre acts
as a bundle of separate fibrils, which break independently. This is the fibrillar form
of fracture shown in fig. 6 of the 3rd paper in this volume. The progressive increase
of water absorption leads to a continuous increase of tenacity from dry to wet cotton.
This is an interesting example of where a reduction of intermolecular cohesion leads to
greater strength, as a result of relieving stress concentrations and allowing other modes
of deformation.