Page 288 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FIBRES 27 1

Fig. 5. Views of structure in highly oriented, chain-extended fibres. (a,b) Panar et al. (1983). (c) Jacobs and
Mencke (1995). (d) Sikkema (2000). (e) Nakagawa (1994). (f,g) Hongu and Phillips (1997). (h) Aramid
pleat structure (Dobb et al., 1977).

are more advanced theories, such as the one of Northolt and van der Hout (1985), which
brings in the shear modulus. In many HM-HT fibres the orientation is so high that this
effect is small. The pleated structure of Kevlar 29 as first wound up, which is shown
in Fig. 5h, does cause an appreciable reduction in modulus and associated creep, but,
since the disorientation is pulled out under high stress, has little effect on strength. The
post-treatments that increase modulus in other types of Kevlar cause little change in
strength.
The above theory and practice show that the modulus, or more generally the tensile
stress-strain curve, of HM-HT fibres can be confidently estimated and can come close
to the theoretical limit. As mentioned above, the ideal strength of a ‘perfect’ structure
is more difficult to estimate, because it depends on the point of inflection in the free
energy diagram. In real fibres, it is also necessary to take account of the fact that break
load is not a central statistic but an extreme value. Strength is therefore dependent on the
weakest and rarest defects or other forms of variability. Fracture will start where there
is a combination of structural weakness and stress concentration, and this will vary with
the mode of deformation. However, a hypothetical strength that is related to rupture of
molecules across a plane perpendicular to the fibre axis (and the molecular orientation)
is not the relevant consideration, though, as suggested below, it may occur as the final
stage of rupture over a reduced cross-section. The dominant feature, which influences
fracture, is the fact that the axial molecular strength is much greater than the transverse
intermolecular strength. This means that axial splitting occurs much more readily than
transverse rupture. Axial cracks manifest themselves in different ways in different
circumstances with different stress distributions and histories. Complete explanations
would require a more certain knowledge of the fine structure than is indicated by Fig. 5
and a comprehensive description of any larger defects in the fibres.