Fig. 3. Elastic hysteresis of an idealized rubber band. The area in the
centre of the hysteresis loop is the energy dissipated due to material
plasticity.[dubious – discuss]
Elastic hysteresis was one of the first types of hysteresis to be examined.
A simple way to understand it is in terms of a rubber band with
weights attached to it. If the top of a rubber band is hung on a hook
and small weights are attached to the bottom of the band one at a time,
it will get longer. As more weights are loaded
onto it, the band
will continue to extend because the force the weights are exerting on
the band is increasing. When each weight is taken off, or unloaded
it will get shorter as the force is reduced. As the weights are taken
off, each weight that produced a specific length as it was loaded onto
the band now produces a slightly longer length as it is unloaded. This
is because the band does not obey Hooke's law perfectly. The hysteresis loop of an idealized rubber band is shown in Fig. 3.
In one sense the rubber band was harder to stretch when it was being
loaded than when it was being unloaded. In another sense, as one unloads
the band, the cause (the force of the weights) lags behind the effect
(the length) because a smaller value of weight produces the same length.
In another sense more energy was required during the loading than the
unloading; that energy must have gone somewhere, it was dissipated or
"lost" as heat.
Elastic hysteresis is more pronounced when the loading and unloading is done quickly than when it is done slowly.
Some materials such as hard metals don't show elastic hysteresis under a
moderate load, whereas other hard materials like granite and marble do.
Materials such as rubber exhibit a high degree of elastic hysteresis.
A word of caution: rubber behaves like a gas. When the rubber band is
stretched it heats up. If it is suddenly released, the rubber cools
down, very easy to perceive just by touching. So, there is a large
hysteresis from the thermal exchange with the environment and a smaller
hysteresis due to internal friction within the rubber. This proper,
intrinsic hysteresis could be measured only if adiabatic isolation of
the rubber band is imposed.