Re: What causes hysteresis loop during the stretching ot materials?

A hysteresis loop is observed when the stress-strain plot of a material 
being stretched doesn't match that of the same material as it is restored 
to its unstressed state.  Ceramic materials and most metals do not show 
much hysteresis, since they are usually stretched within their elastic 
region.  If you stretch a ceramic beyond its elastic region, it usually 
fractures.  Metals, when stretched beyond their elastic region, often start 
to plastically deform (they bend or elongate, which we call yielding).  
When the metal begins to yield, there is usually a significant change in 
the slope of the stress-strain curve.

After the metal has yielded or plastically deformed, imagine that you then 
want to force it back into its original shape.  When you begin to compress 
the metal (or force it in the opposite direction), again it starts to 
elastically deform.  As the metal is forced beyond its yield strength, it 
again plastically deforms.  You can get the metal back to its original 
shape, but when you put the stress-strain plots in tension and compression 
on top of each other, they don't retrace the same plot.  The area between 
the two plots indicates the work which has been done.  If you measured the 
temperature during this process, you could observe the temperature of the 
metal increasing.

Elastomers, or elastic polymers typically have a hysteresis curve, but for 
different reasons than the metal (and of course, the explanation is more 
complex).  When you stretch a rubber band, the polymer molecules become 
more oriented and less random.  Because of the decrease in randomness, the 
elastomer gets hotter (stretch a rubber band and hold it against your upper 
lip; you can feel the temperature increase).  This temperature increase is 
caused by a decrease in entropy of the rubber band.  As you release the 
tension on the rubber band, it contracts, and gets colder, since the 
molecules must adsorb heat to become more random.  This heat transfer 
between the rubber band and its surroundings accounts for part of the 
reason that elastomers have hysteresis curves associated with them.  

A further explanation can be found for hysteresis if one considers the 
phenomena of "stress relaxation."  Plug that term into a search engine and 
see what you can come up with.  And you might consider why you would want 
to use a metal spring for energy storage, but a rubber band to dampen 
vibrations.