Page 299 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED. CHAIN-EXTENDED POLYMER FIBRES 28 1

compatible with explanation of breakage that comes from the morphology.” The
statistical mechanics, which involves thermally activated jumps over the energy barriers
as in Fig. 3, is valid, and can be expected to apply to predictions of modulus, which
relates to the central value of structural variability. In the absence of explicit structural
detail, it seems questionable to apply the model to failure modes, which depend on
extreme structural situations.
The agreement with experiment shown in Pig. 14 is fascinating, and with the vast
increase in computer power since 1986, it would be valuable to follow up the approach
pioneered by Termonia and Smith for models which included the possible defects in the
structure. In HMPE fibres, it seems right to attribute creep to the movement of whole
molecules past one another, which eventually leads to separation. However, the most
likely mechanism would be the movement of defects such as those described by Reneker
and Mazur (1 983). A kink in a polyethylene chain due to an extra -CH2- group could
move like a ripple in a carpet.

Axial Compression Fatigue

As listed in Table 3, sharp kinks can develop in oriented systems at many scales. Fibre
fracture of Yang’s type (c) is the result of breaks after cyclic axial compression acting at
the smallest scale, usually on thc insidc of bends. Studies by Hobbs et (11. (2000) at the
yam level provide valuable insights. Although the details will be different, the essentials
of the mechanics will be similar inside fibres at the molecular level. It is therefore useful
to review the analysis here. These studies were carried out because of the problem of
axial compression fatigue in mooring ropes. In 1983, Kevlar ropes, which had been
deployed in the Gulf of Mexico in order to moor an oil-rig construction vessel, broke
when they were picked up for connection a few weeks later (Riewald, 1986; Riewald
et al., 1986). The ropes were tested and found to have lost 80% of their strength.
Microscopic examination showed kinks in yarns at intervals along their length. All the
fibres in a yarn kinked cooperatively at these locations, and dyeing gave evidence of
kink-bands within the fibres. Tension-tension rope fatigue testing in FIBRE TETHERS
2000 confirmed that axial compression fatigue occurred when one component in a rope
goes into axial compression even though the rope as a whole remains in tension. Typical
examples showed zig-zag sections, or, where damage was more severe, broken pieces
a few millimetres long separated by undamaged lengths of a few centimetres. As a
result, it has been recommended that aramid ropes in deep-water moorings should not
be allowed to go below 10% of rope break load for more than 2000 cycles, HMPE ropes

Tuhle 3. Examples of axial compression kinks
System Scale
Mountain ranges kilometres
Cliffs metres
Yams in ropes rnillimetres
Molecular fine structure in polymers nanometres