Page 334 - Fiber Fracture

316 C. Viney

v)
v)
2
1
Sequenceof - 2 3
load-unload cycles

- -
Strain
Fig. 5. Response of a viscoelastic biomaterial to mechanical preconditioning. Stress-strain curves are
displaced to the right, and hysteresis (the difference between loading and unloading curves in a cycle)
decreases.

standardised by a cyclic conditioning treatment prior to experimental characterisation. A
typical standardisation procedure consists of the following steps: (1) at the temperature
of interest, the maximum planned load is applied for the maximum planned loading
time; (2) this is followed by a recovery period (after unloading) that lasts ten times as
long; (3) this cycle is repeated until reproducible load-extension results are obtained.
A similar (pre)conditioning procedure (Fig. 5) is routinely imposed on natural tissues
and materials before biomechanical characterisation (Fung, 1993). While this again can
provide a useful basis for comparing results from different experiments, such results
may be misleading if we are interested in how the actual natural material behaves,
Le. without Conditioning. To correctly interpret the in-service mechanical properties of
natural fibres in terms of the underlying hierarchical structure, a strong case can be
made for leaving both the properties and the structure as undisturbed as possible.

Cross-Sectional Area Characterisation

Regardless of whether stress is quoted as nominal values (scaled relative to the
initial sample cross-section) or true values (scaled relative to the final cross-section),
representative cross-sectional areas are needed for accurate characterisation of fibre
strength and stiffness. Depending on the type of fibre being tested, the scale on which
the test has to be performed, and the environment in which the fibre strength is being
tested, it may or may not be possible to obtain such a measurement.
For example, most of the tensile strength and stiffness data quoted for natural silks
are inaccurate. Silks typically have a highly non-uniform cross-section, due to the non-
constant linear production rate of the fibre under natural spinning conditions, and the
fact that spinneret orifice sizes can be changed continuously by the spider or larva unless
the animal is anaesthetised. Here, ‘non-uniform’ refers both to the cross-sectional shape,
which does not have a simple outline, and to the fact that this shape and its enclosed
area can vary with position along the fibre (Dunaway, 1994; Dunaway et al., 1995a;