what changes in the structure of an object that allows something to permanently bend (i.e folding paper)
In: Physics
Another user already explained it using your example of paper. I’ll draw from my experience in mechanical engineering.
Take a bar of plain low carbon steel that’s long enough and narrow enough that anyone can bend it. Apply just a small force enough to bend it a little then let go. It bounces back to its original shape. Apply a larger and larger force and eventually it actually *bends and stays bent*. Why?
Well a ductile material like mild steel has a crystalline structure at the microscopic level whose atoms are arranged in a way that they can deform and shift ever so slightly. Their atomic bonds are still strong enough to pull them back to their original configuration. This is called ***elastic deformation*** and all metals and their alloys (and also non-metals) will have a particular ratio (i.e. Young’s Modulus a.k.a. the modulus of elasticity) of force applied to the amount they can deform or elongate.
Why does it stay bent after so much force? There’s a proportional limit for ductile materials beyond which that material begins to “yield” or permanently deform. What happens here is that once a certain stress (force applied/distributed over an area) is reached the interatomic bonds in the crystalline structure begin to break and reform new bonds in new shapes of crystals. This behavior is called ***plastic deformation*** (also permanent deformation).
This is irreversible unless one were to heat the metal above a certain point to “reset” it. If you were to try bending it back and forth the location at which it bent will begin to harden and eventually break.
This is easily demonstrated by unfolding a paperclip and bending it back and forth.
Hope that helped further your understanding!
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