Page 333 - Fiber Fracture

FRACTURE OF NATURAL POLYMERIC FIBRES 315

Concentrated Water
LiBr
Fig. 4. An example of a mechanochemical engine, based on Steinberg et al., 1966 and Pollack, 1990. A
'belt' of collagen is wound around pulleys A, B, C and D. Pulleys C and D are mounted on a common
axis. When concentrated salt solution is added to the left-hand reservoir, the collagen immersed in that
solution contracts, exerting equal forces on the rims of pulleys C and D. Because pulley C has a larger
radius than pulley D, there is a net anticlockwise torque as shown. Rotation continuously immerses new
lengths of collagen in the salt solution, while previously immersed material is able to relax in the right-hand
reservoir. Eventual equalisation of the salt concentration in the two reservoirs prevents this engine from
being a perpetual-motion device.

which reversible loss of molecular order (in the matrix) equates to an enhancement of
mechanical properties.
The Experimental Methods Used for Characterising the Failure Strength and Other
Mechanical Properties of Fibres Must Be Appraised Carefully

Methods that are used for characterising the mechanical properties of artificial fibres
may not be optimal for characterising natural materials.

Conditioning

Mechanical property characterisation of artificial polymers (fibrous and non-fibrous)
is often preceded by a mechanical conditioning treatment (Ward and Hadley, 1993) if
the material is viscoelastic. This treatment is designed to provide a standard, repro-
ducible microstructural state, so that results from different experiments, materials and
laboratories can be compared easily. The conditioning treatment is deemed necessary
because the mechanical properties of viscoelastic materials are affected by their entire
previous mechanical history, as articulated in the Boltzmann superposition principle
(Ward and Hadley, 1993). To predict mechanical behaviour accurately, one ought in
theory to know the entire loading history of specimens since their manufacture! Under
practical conditions, only comparatively recent history is relevant, so specimens can be