Folding a paper I have always amused at the how things are able to retain their shape
For example
A paper that has been rolled up for a long time tries to retain its form when straightened
A paper rolled/folded along one line in a direction resists folding or rolling in the opposite direction
A crumpled paper is difficult to straighten
A bent rod straightens itself
I believe all of it is somehow related to elasticity
And again I fail to imagine why all of them should attain a new shape if sufficient force is applied
I dont understand why elasticity fails to restore their shape after if they are sufficiently stretched even after the stretching force stops acting
Thank you.
 A: If I understood the question correctly it can be rephrased as "Why do materials permanently attain a new shape when an applied force is above the yield strength?". The wording   "yield strength" in material science is the point where a material start to deform plastically instead of elastically. Why does this happen? Let's first think about why do materials elastically deform? When a force is applied and a material starts to deform then the bonds between either atoms or molecules start to expand and if you apply a force small enough so to not break the bonds then they will go back to their original distance due to attraction of electromagnetic force. However if one apply a sufficiently large enough force then some or all of the bonds start to break or it can break and create a new bonds at a different place. If this is the case then the material has plastically deformed.
A: You have a limit of elasticity in every material. Best example is a spring, you can extend it quiet a bit an if you release it goes back to its initial length. but if you extend it more and more the elasticity steps and it just deforms without ever going back.paper has a very, very low limit of elasticity, so it stays in the crumpled state.
