Page 351 - Fiber Fracture

FRACTURE OF COMMON TEXTILE FIBRES 333

COTTON AND RELATED FIBRES

Structure and Stress-Strain Curves

Essential features of the structure of cotton and the influence on the stress-strain
curve are shown schematically in Fig. 1. Experimental and theoretical studies of
deformation and fracture have been reported by Hearle and Sparrow (1979a,b).
After a thin primary wall has grown to the external dimensions of the final cotton
fibre, microfibrils are laid down in a helical array as a secondary wall until growth is
completed with a small cylindrical lumen left in the centre of the fibre. Each microfibril
is generated by an enzyme complex as a set of 30 parallel cellulose molecules, which
crystallise in the cellulose I lattice. In this sense, cotton is close to 100% crystalline.
The apparent disorder, which shows up in density measurements and techniques such
as X-ray diffraction, is due to imperfections in the packing of the microfibrils. The
helix angle 8 in the secondary wall varies slightly from outside to inside, but is
typically 21". At intervals along the fibre, the helix alternates from right-handed to

cellulose hollow collapse
molecule tube

C: reversing
assembly

Fig. 1. Cotton fibre structure-property relations: (a) structure; (b) mechanics. The modulus of the crystal
lattice A is reduced in the helical assembly B to a greater extent with free rotation at reversals C.
Straightening of the convoluted ribbon D allows additional extension at low stress to give the stress-strain
curve of a dry fibre. Lower shear resistance in the wet state shifts lines to B', C', D'. From Hearle (1991).