Page 81 - Fiber Fracture

66 J.W.S. Hearle

yield stress. Deformation is easier on the compression side than on the tension side. The
neutral plane moves out from the central position towards the tension side and maintains
the tensile strain below the break extension.

Lateral Pressure

Breaks due to lateral pressure reflect the features of the object employed. Sharp
knives give a clean cut; scissors have a characteristic form; and blunt instruments give a
squashed appearance. These differences are important in forensic studies, as described
by Hearle et al. (1998; chaptcrs 44 to 46).

‘FATIGUE’ BREAKS

Complex Forms of Loading
The term ‘fatigue’ is rather loosely used in the fibre literature to cover all circumstances
other than those in which break occurs as a result of a monotonically increasing force.
‘Static fatigue’ or creep rupture, which occurs after a long time under load, gives forms
similar to more rapid tensile breaks. Cyclic loading gives rise to a variety of forms.

Tensile Fatigue

Bunsell and Hearle (1971) showed that cyclic loading of nylon commonly gave
tensile breaks after the same time as for creep rupture at the peak cyclic load. The
exception was when the fibres were cycled from zero load to about 50% of break load.
In these circumstances, the break showed a tail on one end, which is typically about
five fibre diameters long and had stripped off the other end, Fig. 9a,b. The sequence
of failure was first the development of a small transverse crack, which then turned and
ran along the fibre at an angle of about 5” to the fibre axis, Fig. 9c. When the crack
had crossed half the fibre, the stress on the residual cross-section caused final ductile
rupture, Fig. 9d.
A similar loading sequence caused failure in polyester fibres, but on some polyester
fibres studied later by Oudet and Bunsell (1987) a low critical minimum load gave the
same form of break. An important, and unexplained difference from nylon is that the
axial cracks are closely parallel to the fibre axis. Consequently, the tails are extremely
long, Fig. 9e,f. In one example, the crack had propagated beyond the final break zone,
which was effectively a creep rupture failure from a central flaw, Fig. 9g.

Flex Fatigue

The commonest way of studying flex fatigue in fibres is to pull a fibre backwards
and forwards over a pin. Typically the pin diameter is about ten times the fibre diameter,
giving a nominal bending strain of lo%, and the tension is about 1/10 of the fibre break
load, but the mode of failure depends on the exact conditions. In Kevlar and wool fibres