Page 80 - Fiber Fracture

FORMS OF FIBRE FRACTURE 65

Fig. 8. Cotton breaks. (a) Raw cotton at 65% rh. (b) Schematic view of break. (c) Wet resin-treated cotton.
For further explanation, see Fig. 1.

along the line of the helix angle. This continues until the crack reaches the other side of
the collapsed zone, when it tears back to the starting point.

OTHER DIRECTIONS OF DEFORMATION

Twist Breaks

Except for glass and other brittle materials, twist angles of 20" to 60" are needed
to break fibres. In these conditions, the torsional shear stresses are out-weighed by the
tensile stresses due to the increased length on the outside of the fibre. Breaks tend to
be geometrically distorted forms of tensile breaks, often with some additional splitting.
Although, I do not know of experimental studies, breaks in brittle fibres would be
variants of the standard forms.

Bending Breaks

Schoppee and Skelton (1974) showed that break occurred in bending of a glass fibre
when the surface strain reached 7.3% and in two carbon fibres at 1.4 and 2.8%. The
rupture is a brittle fracture due to tensile extension, but occurs at values slightly greater
than the breaking strain in tensile tests because the effective length is much lower.
In other high-modulus fibres, such as Kevlar 49, as well as the general textile fibres,
the fibres could be bent back on themselves without breaking, which corresponds to a
nominal surface strain of 100%. This behaviour is explained by the low compressive