Page 331 - Fiber Fracture

FRACTURE OF NATURAL POLYMERIC FIBRES 313

molecules are extended, aligned, and less tangled, and which provides little resistance to
cracks propagating parallel to the length of the molecules.
The amorphous matrix phase in spider dragline silk can be likened to rubber (Gosline
et al., 1984). Such elastomeric behaviour is promoted if water is available to swell
the amorphous regions in the silk microstructure. (Dragline silks undergo a marked
shape change when immersed in water (Work, 1981, 1985; Work and Morosoff, 1982;
Fornes et al., 1983; Gosline et al., 1984, 1995) or salt solutions (Vollrath et al.,
1996.) The radial swelling, to as much as twice the original thickness, is accompanied
by an axial shrinkage of up to 40% of the original length; this dramatic effect is
therefore known as supexontraction.) Many other silks, for example the textile fibre
harvested from the cocoons of Bombyx mori (domesticated) silkworms, do not exhibit
significant supercontraction in water, but they nevertheless can also be regarded as
elastomers (Gosline et al., 1994). This description is relevant when we come to address
the statistical brittleness of silk (the section ‘The Statistical Basis of Fibre Failure
Analysis’). It helps us to interpret the observation (PCrez-Rigueiro et al., 2001; Garrido
et al., 2002) that the breaking stress of silk (recorded at high strain) is much less
reproducible than the yield stress (recorded at low strain).

In a Hierarchical Fibre Microstructure, Molecules That Have ‘Melted’ Can Continue
to Carry Loads Usefully

From everyday experience of conventional materials, we may come to expect that
disordering of a microstructure will always lead to a loss of reinforcement and a
reduction or even failure of load-bearing ability. In fact, this combination of cause
and effect has some notable exceptions, none more significant than the contractile
mechanism of muscle (Pollack, 1990,2001).
We are again dealing with a useful consequence of hierarchical structure in a fibrous
material, and of the attendant anisotropic distribution of primary and secondary bonds.
There are two fibrous materials in muscle: actin (already described in the section
‘Primary and Secondary Bonds Can Have Direct, Distinguishable, Complementary
Effects on Fibre Mechanical Properties’) and myosin. The myosin-containing filaments
consist of bundles of rod-like structures, where each rod is a supramolecular helix
(supercoil, or coiled coil) assembled from two a-helical protein strands (Fig. 3). The
helical structure is able to locally and reversibly transform to a random one, triggered
by one of several environmental signals that can include a change in local packing
constraints, a change in pH, or a change in the concentration of various salts. This
local conformational change leads to a contraction in rod length (Fig. 3). It does
not involve any breaking of primary bonds; it merely requires a local rearrangement
in the number and distribution of protein-protein and protein-environrnent secondary
bonds. Because the myosin in muscle is interconnected (by non-covalent associations),
and is further supported by actin-containing filaments, the molecular-level contraction
leads to a corresponding macroscopic contraction of the muscle, along a structurally
predetermined direction. Although the random coil conformation in myosin is similar
to the conformation of flexible polymer chains in melts and solutions, its localisation
to particular regions within a hierarchical fibre means that the controlled contraction