Page 77 - Fiber Fracture

62 J.W.S. Hearle

Fig. 4. (a) Light-degraded nylon. (b,c) High-speed break of nylon from pendulum impact. For further
explanation, see Fig. 1.

increase in stress on the unbroken side causes additional plastic yielding and the crack
opens into a V-notch, Fig. 3b. When this has grown to a critical size, catastrophic failure
occurs over the remaining cross-section, Fig. 3c. Tests on polyester film marked with a
grid show that the additional elongation represented by the open end of the V-notch is
accommodated in the unbroken material by a band of shear, Fig. 3d, which would extend
back by many fibre diameters. Plastic deformation over a large volume of the whole
circular specimen is a challenge to the theoreticians of fracture mechanics.
There are variant forms. The initial flaw may be a small point, a line perpendicular
to the fibre axis, or an angled line, and this changes the detail of the break, Fig. 3e.
Occasionally, breaks start from opposite sides of the fibre, giving two V-notch zones
and a central catastrophic region. Breaks normally start from the fibre surface, but
occasionally from an internal flaw, when the V-notch becomes a double cone, Fig. 3f.
A variant on this form occurs in light-degraded nylon. Voids form round the delustrant,
titanium dioxide particles, and multiple breaks start from these voids to give a turreted
appearance, Fig. 4a.

High-speed Breaks

As the rate of extension is increased, the V-notch region gets smaller, Fig. 4b, and
the catastrophic region gets larger. At ballistic rates, the change is complete and the
break appears as a mushroom, Fig. 4c. This is explained as being due to a change from
isothermal to adiabatic conditions. Heat generated by the rapid plastic flow causes the
material to melt, or at least soften. The elastic energy stored in the fibre remote from the
break zone causes snap-back when break occurs. When snap-back stops, the softened
material collapses into the mushroom cap.

Granular Breaks

Cellulosic and acrylic fibres, which are spun from solution, show granular breaks,
which are similar to lower-magnification views of the structure of a fibre-reinforced
composite, Fig. 5a,b. The reasons are similar. The fibres coagulate from solution with