What is an elastic material? How is
energy stored in an elastic material? Does a compressed, bent or stretched
elastic material always return to its original shape on releasing the
tension?
When a material is subjected to two equal and
opposite forces they may change the relative positions of the particles i.e. may
change its shape.
This is what happens if you stretch a spring or rubber band,
squash moulding clay or bend a strip of plastic or metal.
When springs or elastic material are
stretched, elastic potential energy is stored in the system.
When the forces causing the stretching is
removed, the spring or elastic returns to its original length (shape)
The more an elastic material is
stretched, the greater the elastic potential energy store.
If the forces of compression/stretching are removed, different
materials behave in different ways.
A spring or rubber band are likely to
return to their original shape and length, displaying their elastic properties.
The potter's clay will stay in its new shape.
The plastic strip like a ruler may return
to its original shape.
A strip of the soft metal like lead will be permanently
deformed but a strip of strong steel, if not bent too far, is likely to return
to its original shape - this happens with a clock spring in a clockwork clock.
Materials which have a tendency to return to
their original shape are called elastic.
Those that do not are referred to as
inelastic
(non-elastic). The more elastic a material, the greater its ability to regain
its original shape.
Engineers designing structures like bridges
need to understand the different ways forces operate. When the applied forces
stretch materials the material is under
tension. If the applied forces squeeze or compress the material,
it is described as being under compression. In a bridge, the supports on either side are under
compression but the main body of the bridge carrying traffic is under tension -
this could be the roadway and/or the cables in a suspension bridge.
If you want to bend, stretch or compress an
elastic object you must apply a force.
As explained above an elastic material
is one that can be deformed in shape by applying a force and returns to its
original shape if the forces are removed - springs and rubber bands clearly
exhibit elasticity.
If an object doesn't return to its original length
and shape it is inelastic.
The
extension of a simple spring by stretching it with an applied force can be used
to illustrate these points.
Initially the extension of a stretched
spring is proportional to stretching force applied (e.g. adding increasing
weight). This simple law is obeyed until you reach the limit of
proportionality (L on
the graph) - in other words the graph is linear up to point L.
After that, between point L and point D, the
stretching is greater than expected - non-linear graph, but the spring will
still return to its original length - the spring is still behaving elastically, but
only for a relatively small further increase in the applied force.
However, if you overstretch an elastic
material it may only partially contract return to its original shape on
removing the applied force, so the spring is permanently deformed. The point
at which this first happens is beyond what is called the
elastic limit (point D on
the graph).
Beyond the elastic limit D, the
greater the stretching force applied the greater the permanent deformation
(from point D onwards) - the less the object returns to its original shape -
this is seen on the graph as it curves over in the extension direction!
When the object doesn't return to its
original shape after removing the forces it is called
plastic deformation.
With some materials, the elastic limit L, is
so low that you see little elastic behaviour and permanent deformation sets in
quickly with a relatively small applied force. In these cases, the force -
extension graph is a curve (non-linear) with virtually no linear portion at the
start.
