Page 329 - Fiber Fracture
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FRACTURE OF NATURAL POLYMERIC FIBRES 31 1
small. Provided that cracks are initially smaller than the grain size, high toughness is
ensured: grain boundaries are effective as obstacles to crack growth, and they contribute
to the factor y in Eq. 1. In the case of a fibrous polymer we have to interpret ‘grain
size’ as the linear dimension of a structural subunit in the direction of initial crack
growth, and ‘grain boundaries’ become the interfaces between such subunits. A fine
microstructure, which is able to deflect cracks along complex paths, is synonymous with
high toughness. A coarse microstructure can accommodate large crack lengths within
a grain (or crystal, or other subunit), and so is associated with a low stress to trigger
catastrophic failure.
However, sometimes a crack will not initiate within a microstructure but will be
imposed on the material from outside, for example by impact or cutting. In such
circumstances, it is useful if the material contains interfaces that can impede the growth
of an initially large crack. Such interfaces must (a) be separated by large distances
(to accommodate a large crack between them, while not significantly diminishing the
load-carrying capacity of the material as a whole), and (b) have geometrical and failure
characteristics that interact optimally with the stress field of a large crack.
These requirements can be met by a microstructure that is hierarchical, where
different scales of structure can stop different sizes of crack. Although microstructural
hierarchy of natural fibres is a fortuitous consequence of fibre self-assembly, it is
also a fortunate consequence. It allows independent optimisation of several mechanical
properties, and it confers damage tolerance as well as toughness. Fibrous materials that
have a hierarchical microstructure are able to fail gracefully.
Water Plays Multiple Roles in the Assembly and Stabilisation of Natural Fibres
Most of the steps involved in the synthesis and assembly of biological fibrous
materials take place in the presence of water. The water acts as a solvent and transport
medium for reactants. It also can play a significant role in promoting adhesion between
biological macromolecules, for example the G-actin monomers in F-actin. The driving
force is entropic. G-actin molecules that have become aggregated will immobilise
significantly fewer water molecules compared to the same number of independent G-
actins, so the entropy of the water increases. Although aggregation necessarily decreases
the G-actin entropy, the accompanying increase in the disorder of water is more than
enough to compensate (Steinmetz et al., 1997; Tuszynski et al., 1997). For every G-actin
molecule that is added to an aggregate, several water molecules can be liberated. Thus,
the water does not act as a ‘glue’ linking G-actin molecules, but rather serves to promote
association of G-actins by virtue of being excluded from the space between them. An
analogy is provided by ‘non-stick’ hydrophobic Teflona surfaces, which can develop a
strong affinity for other hydrophobic materials when immersed in water.
Many natural fibrous materials are stabilised by this type of hydrophobic bonding
between structural subunits at one or more length scales. Examples (Renuart and
Viney, 2000) include keratins, collagen, silk, viral spikes, actin and tubulin. Materials
such as the latter three are optimised for continuous use in an aqueous environment,
in which case hydrophobic bonds may provide a particularly significant source of
stability. Property measurements, including tensile testing to failure, performed in air