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| Various plastic items. Created by Bing Image Creator. |
Materials can be elastic, plastic or both. Plastic
materials such as soft toffee, chewing gum or plasticine deform when
stretched or compressed, but don't return to their original shape when
the external force is removed. Elasticity
is a property of a material that allows it to regain its original shape
when subject to a force. So for instance rubber is elastic and
similarly if you flex a steel ruler or stretch a spring, they regain
their shape when the force is removed. When a force is applied to a
material, it undergoes a strain,
which in mechanics (a branch of physics that deals with energy and
forces and their effect on bodies) is defined as the ratio of the
extension of the material to its original length. The stress
in the material is the tensile reaction due to the external force
causing the stretching (effectively the material "pulls back" because of
nuclear forces in the atoms). For many materials there's a proportional
relationship between stress and strain up to a certain point.
Eventually a point is reached however when a material no longer behaves
elastically, called the elastic limit,
beyond which it will deform plastically. If you've ever bent a piece of
metal, you'll know that it'll spring back, but if you bend it too much,
it'll stay bent. That's because the elastic limit was exceeded. Even
glass is elastic and if you push on a glass window or shower door, you
can see the glass flexing. However glass has very little plasticity and
usually fractures suddenly when stresses are not much greater than the
elastic limit.
This article on Phys.org discusses the issues
with polymers and making them stiff and elastic. Currently they can be
made elastic or stiff, but not both at the same time. Stiff
technically means that there's very little strain for a given stress.
So steel is stiff but rubber isn't. However a stiff and strong spring
such as used in the suspension of a truck or train is still elastic, and
can flex and return to its original shape if subject to large forces.
That's not the case with plastic and if it's made stiff, it can't
stretch elastically. So it behaves more like concrete when stretched,
and fractures. A new technique for making polymers will hopefully
improve the properties of polymers so they behave better when used for
such things as prosthetics and medical implants.