Bent Metal Returning to Original Shape

name         Simon
status       other
grade        other
location     Australia

Question -   When a metal paper clip is straightened out why is
it difficult to bend back to its original shape?
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Because you have gone over the materials yield strength.  Materials have a
property known as yield strength.  It is the point in which a material will
be permanently deformed.  As long as you stay well below the yield strength,
you should be able to return to the original shape.

Thanks for using NEWTON.

Christopher Murphy, P.E.
Air Force Research Laboratory
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When a metal paper clip is straightened out, it does not return to 
its original state, like a rubber band does because the metal atoms 
have been permanently displaced from their original location and 
that information has been permanently lost to the paper clip. It 
cannot "remember" where it was, to use everyday terminology. There 
is a more accurate, but subtle, reason if you compare the behavior 
of the metal and a rubber band. When the rubber band is stretched 
its entropy (the number of molecular configurations available to it 
in the stretched state) actually decreases. This is reflected in the 
observation that if you place a rubber band on your lips and stretch 
it you can feel it cool slightly. So when you let go the rubber band 
molecules seek the state that maximizes the number of molecular 
configurations available to it -- that is its original 'unstretched' 
state -- and so it returns to that state spontaneously. In contrast, 
if you place a paper clip on your lips when you distort it, you 
increase the number of atomic states available to it. This is 
reflected in the observation that you can feel the paper clip warm 
slightly. When you let go the paper clip seeks the state than 
maximizes the number of atomic configurations available to it -- its 
entropy increases.
The second law of thermodynamics states that (approximately isolated 
objects) spontaneously go to the state that maximizes the number of 
atomic/molecular configurations available to it. In the case of the 
paper clip it is the distorted shape. In the case of a rubber band 
it is the original unstretched state.

Vince Calder
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