It is not that the fiber itself
became stronger; it is that the knot slows the take up of tension providing
increased toughness. Since that describes
almost all situations including the stopping of a bullet, the discovery is important
and will certainly be applied even to cheap fibers.
How about nylon stockings? You get the idea.
I continue to look forward to a
real space elevator cable using graphene and this work advances feasibility.
Simple Trick Turns Commercial Polymer Into World’s Toughest Fiber
A materials scientist has created the world’s toughest fiber using a
mechanism based on a slip knot.
April 29, 2013
In material science, toughness is a measure of the amount of energy a material can absorb before breaking. Kevlar, for example, can absorb some 80 Joules per gram before breaking but this is dwarfed by certain natural materials which are much tougher. The silk produced by the giant riverine orb spider, for instance, can absorb around 390 Joules per gram before breaking.
So there is great interest in finding new materials that can match or
beat the performance of natural materials for applications that require high
levels of energy absorption.
Today, Nicola Pugno at the University
of Trento in Italy
The new idea is deceptively simple–it involves no more than tying a
slip knot in the fibre, creating a loop of extra fibre that can passes through
the knot as it comes under tension.
The mechanism is straightforward. When the fibre is placed in tension,
the slip knot begins to tighten and the extra material passes through the knot,
dissipating energy through friction.
Of course, the fibre eventually breaks but only after all the material
in the loop has passed through the slip knot.
Clearly this doesn’t make the material any stronger (toughness and
strength are different properties that are generally uncorrelated). However,
it’s not hard to see how the energy dissipation would dramatically increase the
amount of energy the fibre absorbs before it breaks, thereby increasing its
toughness.
Pugno says he has tested the idea on a number of materials using
different numbers of loops and slip knots. The best results come from using
three slip knots, he says.
By applying this simple trick to a commercial polymer fibre called
Endumax, he has increased its toughness from 44 Joules per gram to a remarkable
1070 Joules per gram. That’s the highest value ever recorded. “The proof of
concept is experimentally realized making the world’s toughest fibre,” he
says.
That’s better even than fibres made from nanotubes which materials
scientists are just beginning to make. The strongest of these, made from carbon
nanotubes, has a toughness of 970 Joules per gram.
Pugno says his work is just the beginning and that it ought to be
possible to use his slip-knot technique to make graphene fibres with a toughness
of 100,000 Joules per gram.
There are challenges ahead, of course. Ideally, the force required to pass the fibre through the knot should be just below the material’s breaking point and this depends on factors such as the knot topology. The choice of knot has an important influence on the behaviour of the material and further work here could lead to novel designs. Pugno says he is currently patenting his slip-knot design and so has not yet published it.
The potential applications of this idea are many. Tougher materials could obviously be used in areas where energy absorption is important, such as the manufacture of body armour, for example.
That’s an effective and cheap idea that has significant potential.
It’s also extraordinarily simple, which might just be the reason it has been
overlooked until now.
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