Page 78 - Fiber Fracture

FORMS OF FIBRE FRACTURE 63

C

Fig. 5. Granular fractures. (a) Cellulose fibre. (b) Acrylic fibre. (c) Human hair. For further explanation, see
Fig. 1.

occluded solvent rather like a sponge. The voids subsequently collapse on drying and
are elongated during drawing. However, they remain as weak places in the structure. On
extension, individual fibrillar elements start to break, transferring stress to neighbouring
elements. Sometimes there is evidence of the break spreading out from a surface flaw,
and sometimes the break occurs in separate steps joined by an axial split, Fig. 5c.
In addition to solution-spun textile fibres, granular breaks are also found in some
carbon fibres, which reflect their acrylic fibre origin, and in alumina fibres. Granular
breaks are also shown in the natural fibres, wool and hair, in cotton at zero moisture
content, and in resin-treated, cross-linked cotton at intermediate humidities.

Fibrillar Breaks

In wet cotton the fluidity of the absorbed water between fibrils inhibits stress transfer,
so that fibrils break independently, Fig. 6a.

Axial Split Breaks

The characteristic form of tensile rupture in para-aramid, HMPE and other highly
oriented, chain-extended fibres consists of long axial splits, often multiple splits on one
end, Fig. 6b, and a single split on the other, Fig. 6c,d. This is a result of the axial
molecular strength being much greater than the intermolecular strength. Shear stresses
cause cracks to propagate and eventually cross the fibre and lead to loss of continuity.

Stake-and-Socket

In some degraded polyester and hair fibres, the breaks have the form of a stake
and socket, Fig. 7. An outer ring of degraded material breaks first. Then a circular
crack propagates at an off-axis angle to form a positive cone, which finally pulls out of
the opposing negative cone. The polyester example from UMIST studies is an overall
subject to repeated autoclaving, but this type of break was first reported by Ansell
(1 983) after boiling PVC-coated polyester fabric, and Holmes (1996) has reported
similar breaks after aminolysis of polyester fibres.