Page 369 - Fiber Fracture

FRACTURE OF COMMON TEXTILE FIBRES 35 1

Fig. 14. Shear splitting due to internal abrasion in a wet nylon rope under tension-tension cycling, eventually
leading to complete fibre rupture. From Hearle et al. (1998).

fig. 1 l), which is more complicated, is given by Morton and Hearle (1993), chapter 26).
By far the commonest form of damage in worn textiles is multiple splitting, which can
be attributed to bending and/or twisting.
In surface abrasion, the external friction forces are balanced by internal shear stresses,
which lead to peeling damage (3rd paper, fig. 12). Internal abrasion is a major problem
in wet nylon ropes. In one test, where a wet nylon rope was cycled up to 50% of its
break load, it broke after 970 cycles. The shear stresses caused by the rubbing of fibres
against another had caused cracks to run across the fibres converting the continuous
filaments into fibres about 1 cm long, with a consequent reduction of strength, Fig. 14.
In tensile fatigue down to zero or low loads, which gives the breaks shown in fig. 9 of
the 3rd paper (this volume), shear stresses result from an initial transverse crack on the
surface of the fibre. This mechanism was believed to be a source of failure in nylon
brake parachutes for aircraft (Hearle et al., 1998, pl. 40G). An unexplained difference
is that in both abrasion and tensile fatigue, shear stresses cause cracks to run across the
fibre at an angle of about 10" in nylon, which rapidly leads to complete breaks; whereas
in polyester the cracks are parallel to the fibre axis, which is far less damaging.
Axial compression fatigue was discussed in the final section of the 11th paper in
this volume, because of its importance in failure of high-performance fibres. However,
similar breaks occur in the textile fibres covered in this paper. Axial compression occurs
on the inside of bent fibres, and, as shown in fig. loa-e (3rd paper, this volume) is
one cause of failure in flex fatigue tests. A single bend, which leads to the formation
of visible kink bands, is not damaging, but cyclic bending eventually leads to structural
rupture. An example of where this is found to occur in use is in the wear of wool carpets.
Severe damage is found where there is turning walk. This leads to repeated compression
of the pile and the fibres buckle into sharp kinks. Cracks develop on the inside of the
bends and eventually the fibres break, so that the carpet pile is progressively lost.
Some interesting results for nylon and polyester fibres have received no more than
vague explanations. Kurokawa et al. (1973) found that kink-bands were formed in a
single bend in PET fibres up to 80"C, then rose to needing a maximum of 2000 bends at
190°C, before falling to a single bend at 200°C and rising again to 3000 at 240°C. Our