Page 366 - Fiber Fracture

348 J.W.S. Hearle

in Fig. 12d,e are found by fitting stress-strain curves for one type of fibre and are then
used to predict the response for the other type. Although this model provides some
insights into the structural mechanics, it neglects many features, notably the influence of
orientation.

Fracture

Fracture in these solution-spun fibres is probably triggered by the amorphous material
reaching its limiting extension and polymer molecules breaking. Fig. 13a shows how
two crystalline regions in viscose rayon will be linked together by tie-molecules,
which break when they reach a critical load. In the wet state, the free chain ends
will not contribute to the tension. Consequently, as shown in Fig. 13b, the degree of
polymerisation (DP) has a major effect on the fibre strength. At low DP, there will be a
large number of free chain ends, but as chain length increases there will be fewer and
the strength approaches an asymptotic maximum. Fig. 13b also shows the considerable
influence on strength of degree of orientation, as given by the birefringence.
Although one can identify the cause of fracture with chain breakage, its manifestation
is determined by larger-scale structural discontinuities in solution-spun fibres. The fibres
coagulate in a sponge-like form with solid material separated by voids of residual
solvent. On drying, the solvent is removed and stretching elongates the voids. What
remains is a coarse structure with regions of integral material separated by weaknesses
in the structure corresponding to the original void surfaces. The rupture of one integral
zone does not lead to continuous crack propagation, because of the region of weakness,
but does transfer sufficient stress to neighbouring zones to cause them to break in
contiguous positions. This leads to granular breaks as shown in fig. 5 of the 3rd
paper (this volume). These SEM pictures are similar to low-magnification views of the

f
L

Fig. 12. A model for the stress-strain properties of viscose rayon, Hearle (1967). (a-c) Composite models
of crystalline C and disordered D material in (a) lamellar L, (b) micellar M, and (c) fibrillar F forms.