Page 353 - Fiber Fracture

FRACTURE OF COMMON TEXTILE FIBRES 335

0 2.0 4.0 6.0 8.0
Extension %
Fig. 3. Load-extension curves for Acala cotton fibres: (a) normal; (b) after stretching wet under tension of 2
g and drying at stretched length. From Hearle and Sparrow (1979a).

The above description applies to dry fibres. In wet fibres, the hydrogen bonding
between microfibrils is replaced by mobile absorbed water, which effectively gives
zero shear modulus. In the schematic diagram of Fig. 1, this is shown as reducing the
modulus from line B to B’, although, as noted above, the effect of the shear modulus is
small. The effect on rotation at reversals and removal of convolutions is much greater
and moves C to C’ and D to D’. Measurements show that the initial modulus of wet
cotton is 1/3 of that at 65% rh, and the strength and break extension are 10% higher
wet that at 65% rh. The changes are greater at lower relative humidities: the values of
strength and break extensions at 25% rh are half those at 100% rh.
Fracture

What eventually leads to rupture? The form of break of cotton depends on the state
of the fibre.
In the dry state, with no absorbed water, the limit is the extension of the cellulose
crystals. This is more difficult to calculate theoretically, because it depends on the
position of the point of inflection in the plot of free energy versus extension, but would
be expected to be at about 2% extension. Due to the helical structure, the extension in
the cotton fibre will be greater and the other resulting stresses may influence the fracture.