Page 325 - Fiber Fracture

FRACTURE OF NATURAL POLYMERIC FIBRES 307

(2) Polymers synthesised via biotechnological routes can be produced in quantities
that enable the economically viable spinning of continuous fibres (Brown and Viney,
1999). Spinning these under controlled conditions offers the promise of cross-sectional
uniformity and improved strength reproducibility. The benefits of continuous fibres and
artificial spinning have in fact been long established in the context of cellulose fibre
(e.g. rayon, Tencef ) regenerated from solution: both strength and strength reliability are
improved by eliminating the polydispersity of fibre length, by reducing the variability
in fibre cross-section, and by maintaining a reproducible microstructure. In principle it
should be possible to spin silk-like, keratin-like and collagen-like proteins into fibres,
though it may not always be easy or even possible to mimic the microstructurc and
properties of the native material.
(3) Much is now known about the processes of supramolecular self-assembly by
which complex materials are formed in nature. Building on this knowledge, we may look
forward to a future in which molecules can be ‘preprogrammed’ to organise into fibrous
structures, by-passing the need for energy-intensive, dangerous and/or environmentally
undesirable processes. (We must however bear in mind that nature’s thermodynamically
attractive routes to high-performance self-assembled materials are a consequence of life
operating under near-equilibrium conditions. Kinetically, nature’s self-assembly routes
are less successful, producing material at rates that are not economically attractive for
making large objects at present.)
(4) Self-assembly is a promising route for producing small (fine) fibres in nanocom-
posites, where a high fibre-matrix interfacial area confers enhanced toughening and
ensures efficient load transfer to the fibres.

Some Thoughts on the Meaning of ‘Brittle’

For engineering design purposes it is useful to label the fracture behaviour of a
material as either brittle or not. There is no single antonym of ‘brittle’, as ‘tough’
and ‘ductile’ are not always interchangeable. The distinction between brittle and non-
brittle materials is sometimes intuitive, but materials with borderline characteristics (e.g.
limited plasticity) are common. Also, as will be discussed further in the section ‘The
Fracture Characteristics of Natural Fibres Can Be Sensitive to Prior Deformation’, the
characteristics of a material can change from non-brittle to brittle during the course of
deformation. Researchers who specialise in the different classes of material do not use
identical definitions of brittleness (even though their intended meanings are equivalent),
and some differences in usage are evident between materials science and materials
engineering. Such differences are inevitable when a topic is SUN~~IX~ across a wide
interdisciplinary landscape. In this paper, we will encounter four nuances of the term
‘brittle’.
(I) A brittle material can be identified in microstructural terms as one that has no
effective physical features or mechanisms for hindering the growth of cracks.
(2) Alternatively, a phenomenological description is possible by simply noting that
cracks propagate rapidly through a brittle material.
(3) The Griffith formula (Cottrell, 1975, and Eq. 1) relates the breaking strength
of a material to the length of pre-existing cracks, the tensile stiffness (Young’s mod-