Page 295 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FIBRES 217

Fig. 11. (a) Kevlar fibre after a week of cyclic buckling. (b) Fibre broken in tension after cyclic buckling.

Although the disruption apparent in kink-bands after bending looks severe, it does
not appreciably affect fibre strength. Kink-bands formed in a single bending action are
pulled out by tension with little or no loss of break load. However, repeated cycling
leads to increasing damage and eventual failure.
Flex fatigue tests over pins proved difficult with Kevlar due to surface abrasion
(Hearle and Wong, 1977). An alternative was a single-fibre buckling test, in which a
2.5-mm length of a single Kevlar fibre was cycled between 1.5 and 2.1 mm. After a
week at 50 Hz the fibre had not broken, though, as shown in Fig. lla, it was severely
damaged. Fibres broken after cyclic buckling (Fig. 1 lb) show short multiple splits.
These would be the type (c) breaks mentioned by Yang (1993, p. 97).
Probably because of the low coefficient of friction HMPE fibres are less subject to
abrasion. Sengonul and Wilding (1994, 1996) found that flexing of Dyneema fibres over
a pin gave multiple split breaks. This indicates that shear splitting in zones of variable
curvature was the dominant factor.
Yarn buckling tests were carried out in the FIBRE TETHERS 2000 (1994, 1995)
joint industry project. Failure due to axial compression fatigue was also studied in fibres
from fatigued ropes in the study. As discussed below, the constraints on fibres within the
yarns, especially if they were restrained in a shrink-tube in the laboratory test or within
ropes, causes very sharp fibre kinks to form. Kevlar, Vectran and Dyneema all showed
kink-bands within fibres and breaks over short lengths.

THEORETICAL APPROACHES

Failure in Shear

The dominant factor influencing fracture in highly oriented, chain-extended polymer
fibres is the high axial molecular strength, which depends on covalent bonding, and the