Page 328 - Fiber Fracture
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310 C. Viney
the domains is crossed twice by the protein backbone, forming a hinge that enables
actin fibre to exhibit torsional flexibility. Thus, one mechanical property of the fibre
is controlled at the molecular length scale, by primary (covalent) bonds.
The ability of actin fibre to maintain rigidity and strength under tension (necessary in
its load-transmitting roles) depends on the forces that bind G-actin into aggregates.
Thus, some mechanical properties of the fibre are controlled at a supramolecular
length scale, by secondary (non-covalent) bonds.
Because the intermolecular secondary bonds are weaker than the intramolecular
primary bonds, the fibre can fail without destroying the integrity of the constituent
molecules. The molecules are therefore immediately available for repairing the fibre.
A hierarchical structure can therefore enable different mechanical properties to be
selectively and independently tailored by different aspects of that structure. While it
is possible in the case of actin to identify specific structural features and bonding
types with specific mechanical properties, there are many hierarchical biological fibres
for which the corresponding associations are more complex and have yet to be
determined fully. As an example, let us consider spider dragline silk (strictly: silk
from the major ampullate glands of spiders). The unique combination of mechanical
properties exhibited by this fibre can be described qualitatively in terms of a multi-
phase microstructure (Viney, 2000). Progress has also been made towards developing
quantitative links between microstructure and some individual mechanical properties of
this material (Termonia, 2000). However, several microstructure-property relationships
for silk - including the nature of the flaws that appear to be ultimately responsible for
fracture (PCrez-Rigueiro et al., 1998) - remain to be resolved.
If we know how the hierarchical microstructure of a material is assembled, we are
in a good position to understand how that microstructure will be deconstructed as the
material fails. Which bonds are most susceptible to being disrupted will depend on how
the sample is loaded (in tension, compression, bending or torsion); we have noted in
the case of actin how different microstructural features confer resistance to failure in
different loading geometries.
A Hierarchical Structure Optimises Toughness
In courses on materials engineering, we learn almost from day one that toughness
requires afine microstructure, with no mention of hierarchy. Here we consider whether
a hierarchical microstructure confers any toughening benefits additional to those
associated with a fine microstructure.
The need for a fine microstructure is usually encountered and justified in the context
of the Griffith formula, which quantifies the stress CT needed to propagate a pre-existing
2 (y’
crack through a metal or ceramic material (Cottrell, 1975):
where c is the length of a surface crack (or half the length of an internal crack), E is
Young’s modulus, and y is the energy per unit area of new surface created by the crack.
According to Eq. 1, the breaking strength remains high if crack lengths can be kept