Page 297 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FIBRES 279

Crack propagation path
Skin Core I

Fig. 13. Tensile failure model due to Morgan et al. (1982).

occasionally jump to the right at what presumably represent the ends of molecules. In
the middle of the core, a lining-up of chain ends leads to a longer transverse crack,
and on the right the crack becomes purely axial. I am sceptical about attempts at such
interpretations at the molecular level. The structure is not well enough known. Long
axial breaks, Yang’s type (a) and (b), without transverse cracks except perhaps at the tip
of a split, are more commonly observed. Where there are transverse cracks as part of a
break, they are probably due to kink-band damage. But, although I can raise questions, I
cannot provide detailed answers.

Time and Temperature

The strength of highly oriented, chain-extended polymer fibres is time- and
temperature-dependent, and, in principle, responses can be predicted by statistical
mechanics. Fig. 14 compares experimental results with the theoretical predictions of
Termonia and Smith (1986). The agreement is good. The Monte Carlo methods used in
this and later studies are described in the paper by Termonia (this volume).
The model in Termonia and Smith (1986) is for “ideal fibres made of a perfectly
oriented array of fully extended macromolecules, with no other defects than chain
ends resulting from finite molecular weight”. Parameters defining the primary covalent
bonding and the secondary hydrogen bonding are applied to a system “modelled as
a three-dimensional array of bonds which are viewed as coupled oscillators in a state
of constant thermal vibration”. I have no difficulty in accepting this as a reasonable