Page 330 - Fiber Fracture

312 C. Viney

are of questionable value for characterising such materials, and there is little point in
expecting such materials to retain their optimum functionality in a dry environment.
Attempts to spin fibres from genetically engineered analogues of viral spike protein,
to produce material of similarly high compressive strength, have yielded disappointing
results (Hudson, 1997). The native spikes rely on hydrophobic bonding to maintain their
structure. Measured in air, their mechanical properties, and the properties of correctly
assembled fibres based on analogous proteins, must therefore be inferior compared
to results obtained in water. If a natural material is designed to work in an aqueous
medium, attempts to mimic its properties must take this reality into account.
Of course, it is often possible to resort to covalent cross-linking to stabilise a structure
that has been self-assembled from an aqueous environment. Indeed, nature does this
too in the case of fibrous materials such as hair (keratin) and tendons (collagen) that
must exhibit extracellular stability for long periods of time. This approach will be
acceptable if we want the product properties to reflect the presence of such cross-links,
but otherwise it has to be avoided.
Hydrophobically bonded structures will be sensitive to temperature: the entropy
penalty that has to be paid for immobilising water at the surface of G-actin increases
with increasing temperature, as the driving force for water to disorder increases.
The fracture resistance of such structures will therefore also decline with increasing
temperature, unless post-assembly cross-linking has been able to occur.

The Fracture Characteristics of Natural Fibres Can Be Sensitive to Prior
Deformation

The complexity and hierarchy of natural fibre microstructures can allow a variety of
simultaneous microstructural changes to accompany mechanical deformation.
In microstructures where the majority of molecules already have significant extension
and alignment, there is little scope for molecular order to be affected by deformation.
For this reason, the load-extension curves of cotton and flax (Wagner, 1953) are
essentially linear, and the ability of the material to resist flaw propagation does not
change with strain. If, in contrast, the microstructure contains a significant volume of
material in which the molecules are initially disordered, and/or there are distortable
helical structures, the fracture toughness of the material can be altered significantly by
strain. So, to understand fracture, we must know about the microstructural changes that
occur throughout the deformation process.
As an extreme example, we can profitably consider the case of rubber. Although
not itself a fibrous material, rubber is a good model for the disordered microstructural
component in many natural fibrous polymers, including silk. Most people would agree
that rubber is tough. That is why rubber is used to make tyres and the soles of durable
shoes. However, cracks propagate very readily indeed through the skin of an inflated
rubber balloon, on the basis of which rubber could be regarded as a brittle material. This
apparently dual character can be understood if we note that the microstructure of rubber
is changed substantially during the course of deformation. The initial microstructure
consists of a random array of tangled molecules, through which there is no easy crack
path. On stretching, this microstructure is progressively converted to one in which the