Page 350 - Fiber Fracture

332 J.W.S. Hearle

to that for steel cable moorings, which depend on catenary forces determined by the
weight of the cable to hold the rig in place, whereas taut fibre moorings depend on rope
tensions determined by their extension. An incidental advantage is that the footprint of
the moorings on the sea-bed is much smaller with taut moorings. In order to input rope
tensions, engineers have to come to terms with the complex visco-elastic properties of
polymer fibres, with moduli that vary with the current state and previous history, in
contrast to the elastic-plastic response of steel. Rope length is a determinant of strain
for given displacements. For the strains in mooring lines resulting from wave motion at
IO00 to 2000 m depths, polyester has the best combination of properties: intermediate
modulus with good, strength and durability. At lesser depths, where strains would be
larger, nylon, with a lower modulus, would be better than polyester, but, at these depths,
catenary steel moorings, which are too heavy in longer lines, have a good record of use.
At greater depths, higher-modulus fibres may be preferred.
For the common textile uses, fibres are characterised by flexibility, fineness and
a high ratio of length to width (McIntyre and Daniels, 1995), but, they must also
have an intermediate extensibility. Most have at least partially recoverable extensions
up to typical break extensions of 7 to 50%, much higher than for brittle solids or
the yield extension of elastic-plastic materials and much lower than for elastomers.
Such properties are achieved by partially oriented, partially crystalline polymers, and
are almost completely satisfied by six chemical types: cellulose, protein, polyamide,
polyester, polyacrylonitrile and polypropylene.
The diverse forms of fibre failure were described in the paper by Hearle (3rd
paper), with some qualitative comments on the mechanics of failure. More details of
failures of textile fibres in both scientific studies and after use are given in Hearle
et al. (1998). However, as Christopher Viney comments (see paper 13), “the entire
stress-strain curve of a fibre is relevant to fracture”. The aim of this paper is to outline
‘the microstructural changes that occur throughout .... deformation’, and lead to ultimate
failure. More information on relevant physical properties of textile fibres is given by
Morton and Hearle (1993). The behaviour differs according to the type of fibre, and
there has only been limited quantitative modelling of the deformation mechanics.
One general comment is that defects are not as strong a controlling feature of
breakage in these extensible textile fibres as in many other materials. Rupture forces
cannot be calculated from modulus and crack depth as in Griffiths brittle fracture, or
even from the later theories of fracture mechanics. As described below, Moseley (1963)
showed that severe damage could be imposed on nylon and polyester fibres with no
effect on strength at room temperature.
By far the greatest share of both experimental studies of mechanical properties of
fibres and theoretical studies of structural mechanics has been on tensile properties. This
paper therefore concentrates on explanations of tensile stress-strain curves and the way
in which they lead to fracture. Some comments on other forces, particularly in cyclic
loading, will be included in a concluding section.