Page 349 - Fiber Fracture

FRACTURE OF COMMON TEXTILE FIBRES 33 1

INTRODUCTION

Most of the presentations at the Fibre Fracture conference, which appear as papers in
this book, were primarily concerned with fracture in high-performance fibres. Among
polymeric fibres these include the synthetic HM-HT fibres in the papers by Hearle
(paper 11) and by Termonia and natural fibres in the context of living organisms in
the paper by Viney. However, there is another group of fibres that are by far the
most important economically, namely the fibres that are used by most of the textile
industry. About half are used in clothing, a quarter in various household uses, and a
quarter in technical and enginecring uses. Cotton (cellulose) and polyester (polyethylene
terephthalate) each account for 30 to 40% of total usage, with smaller amounts of
polyamides (nylon), acrylics (polyacrylonitrile), other synthetics, including copolymers,
rayon (regenerated cellulose), cellulose acetate, other natural cellulose fibres, wool,
other hair fibres, and silk. In order to put this in context, it can be noted that global
polyester production is in the tens of millions of tonnes per year, whereas the various
high-performance fibres are in hundreds or thousands of tonnes.
For most of these uses, fracture in the narrow sense of failure under a peak load is
not directly relevant. Durability under the sequence of complex loading experienced in
use is of greater importance, although some products are discarded when they are no
longer fashionable or for other reasons, rather than when they are worn out. However,
the tensile break load is commonly used as a scaling factor for the intensity of applied
loads, whether in use or in fatigue testing.
After thousands of years of practical experience, the textile industry generally takes
an empirical approach to the choice of fibres and the design of fabrics. Even for
the manufactured fibres, which were introduced in the 20th century, the mechanics
of the internal structure is at a much weaker theoretical level than the chemistry.
Perhaps the first case where a full engineering approach has involved fibre producers,
manufacturers and users, is in the use of fibre ropes for deep-water moorings (Tension
Technology International and Noble Denton Europe, 1999). There are now about 15
oil-rigs deployed by Petrobras off the coast of Brazil, and, for 20 years, the US Navy
has been interested in the possibility of deep-water mooring of mobile bases. When
these problems were first examined in a Joint Industry Study (Noble Denton Europe and
National Engineering Laboratory, 1995), the general view was that high-performance
fibres, such as aramids, Vectran, and HMPE or carbon fibres in pultruded rods, would
be the materials to use. In reality, although the strength of high-tenacity polyester of the
type used in tyres and ropes is only 1.1 GPa, which is 1/3 of aramid strength, this has
proved to be the fibre to use. A typical rig would have 16 lines of 700 tonnes break load
ropes, each about 1.4 km long, which will use 400,000 kg of polyester yam.
The principal criteria for deep-water moorings, which engineering design has to
satisfy, are that the peak loads should be safely below the break load, that the offset of
the rig, which depends on fibre rope modulus, should be limited, and that the fatigue
life should typically be at least 20 years. Marine engineers have mooring analysis
programs, which input data on sea and weather states and are used to compute the
response of the rig. The mechanics is partly strain-driven by wave heights and partly
stress-driven by wind and current forces. The problem for fibre moorings is different