Page 290 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FIBRES 273

Fig. 6. (a) Crack propagation with bifurcation. (b) Broken fibre showing an end of type (a) on the left and
of type (b) on the right. (c) If an inner bifurcation grows faster than an outer one, the multiple splitting on
one end would point away from the break and not towards the break.

Fig. 8 shows that long axial splits also occur in the tensile fracture of a high-modulus
polyethylene fibre.

Creep Rupture

Time is always a factor in determining the effective strength of polymer fibres.
Higher strengths occur in ballistic impact resistance and lower strengths in long-term
loading situations. Even when another fatigue mechanism is the main cause of failure,
the final stage leading to breakage is creep rupture.
The time-dependent behaviour is different in the two types of highly oriented, chain-
extended polymer fibres. Table 1 gives the results of studies in FTBRE TETHERS 2000
(1995), which were made because creep rupture is a concern in deep-water mooring
of oil-rigs. The low-load creep in aramid fibres is due to a straightening of the initial
structure. It reduces in rate, even on a logarithmic scale, with time and is not a source of
creep rupture. In Vectran, the creep is less and is absent after 10 days under load.
HMPE fibres show high creep and in the worst cases break after a few days under
moderate loads. Table 1 shows that there are big differences in creep response in

Table 1. Creep properties from Fibre Tethers 2000
o/c creep 1 min to 100 days Days to rupture
~~ ~
8 of break load 15 30 15 30
Kevlar 29 0.13 0.2 I >357 2213
Kevlar 49 0.09 0.08 >217 >217
1
Twaron 1000 0.2 0.25 >357 2214
Vectran 0.06 0.09 > 13 >218
Spectra 900 15.8 break 182 4
Spectra 1000 7.96 break 33 I 28
Dyneema SK60 I .05 6.0 2354 I23