Page 84 - Fiber Fracture

FORMS OF FIBRE FRACTURE 69

Torsional Fatigue

Cyclic twisting fatigue has been less studied than flex fatigue. Repetitive action at the
high twists needed can wear out the tester as well as the fibres. At high twist amplitudes,
Goswami et al. (1980) found breaks in polyester fibres at about 500 cycles, which
were similar to those in monotonically increasing twist. At lower twist levels, breaks at
around 100,000 cycles were due to multiple splitting.

Combined Twisting and Beding

Biaxial rotation is a useful method of laboratory fatigue testing of fibres. For thick
monofilaments a short length is bent into an arc of a circle and then rotated from both
ends. For fine fibres the bend is concentrated over a pin. Clearly, as the bent fibre rotates,
material away from the centre of the fibre alternates between tension and compression,
and in the variable curvature off the pin, there will be shear stresses. However, the
energy loss in each cycle means that torque must be applied through the rotating grips
in order to drive the rotation. This combination of bending and twisting leads to failure
by multiple splitting along helical lines, Fig. 11.

Surface Wear

Surface wear can be studied in yam-on-yam abrasion by cyclic displacement of a
twisted loop or in abrasion against other surfaces by hanging a fibre over a rotating pin.
It also occurs in axial cycling over a pin, to a limited extent in nylon and polyester fibres
and to a greater extent in Kevlar and wool. Severe wear reduces the fibre cross-section
until break occurs under the tensile load, Fig. 12a,b. In Kevlar, the splitting then destroys
the worn surface, but in wool both surfaces can be seen. The worn surfaces are usually
fairly smooth with some fibrillation visible at the edges.
Studies of nylon and polyester show up more detail, Fig. 12c,d. The local shear
stresses causcd by surface friction commonly lead to wear by surface peeling. Some of
the most interesting results are from inter-fibre abrasion in ropes subject to tension-
tension cycling. Wet nylon abrades rapidly with cracks running at an angle across the
fibre and converting continuous filaments into short fibres with a consequent loss of
rope strength, Fig. 12e-g. Polyester ropes have a much slower loss of strength because,
as in tensile fatigue, the cracks run parallel to the fibre axis.
Studies of nylon and polyester show up more detail, Fig. 12c,d. The local shear
stresses caused by surface friction commonly lead to wear by surface peeling. Some of
the most interesting results are from inter-fibre abrasion in ropes subject to tension-
tension cycling. Wet nylon abrades rapidly with cracks running at an angle across the
fibre and converting continuous filaments into short fibres with a consequent loss of
rope strength, Fig. 12e-g. Polyester ropes have a much slower loss of strength because,
as in tensile fatigue, the cracks run parallel to the fibre axis.