what changes in the structure of an object that allows something to permanently bend (i.e folding paper)
In: Physics
Paper is just many many tiny wood slivers. Now what happens when you bend a big sliver? It breaks but usually stays intact. That’s what’s happening when you bend paper. Just lots and lots of broken slivers.
Every material has what’s called an elastic limit. When you stretch/strain a material past this limit the deformation stops being reversible, a *plastic deformation*. If you look up a stress vs strain graph the linear part at the beginning is the elastic part.
When you stretch something into the plastic zone and let go of it, before a certain point of stress it will shrink back and recover the same amount it would if you held it at the elastic limit, it just wont go all the way back to normal
For paper this limit is probably really low and the cellular thing that happens with folding is explained better in here, you don’t have to actually fold paper to get it to have a permanent deformation though
Every object that you see around you is actually made of teeny tiny little pieces held together in various ways. Sometimes if you apply heat, water, a combination of the two or sometimes just brute strength you can bend something in just the right way that the little pieces change their formation and the thing you want to bend, stays bent.
Sorry, I just want to make sure you’re clear on the use of “i.e.”
i.e. = “id est” (Latin), meaning “that is” or “in other words”. So your question reads “what… allows something to permanently bend, specifically paper”
e.g. = “exempli gratia”, meaning “for example”. So your question using “e.g.” instead would read “what… allows something to permanently bend, such as (but not limited to) paper”
It’s a subtle difference, but it changes how specific your question is, which might change how specific the answers you get are.
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!
Probably late to the game, but gonna try a proper maybe… ELI7?
If you zoom in smaller and smaller things are made of billions of tiny atoms that are basically little balls that are stuck together. When 2 atoms are stuck together we say they’re bonded. The sticking is a bit like how magnets stick together – they’re attracted to each other, but you can still pull them apart, breaking the bond. Atoms stick themselves together into large structures, and sometimes these structures make even bigger structures – like how a chocolate bar is made of collections of chocolate that’s bonded to rice crispies etc.. For solid objects in order to stay the same shape, the atoms can’t move around – the bonds stay the same… Unless…
If you push hard enough, just like pulling magnets hard enough, you can break the bonds and start to move the atoms around. If after you stop pushing the atoms, they can’t move back to where they were before, then the material will permanently change shape.
In the specific case of paper, atoms make molecules called proteins, that form weak bonds to other proteins and these form fibres that in turn bond together with weak bonds and that makes paper! Folding paper in half, some fibres will slide over eachother in order to change the shape, but they can’t slide back, so the shape change is permanent.
Elastic deformation i.e. when it springs back is a little more complicated as things like rubber achieve it in a different way to metal for example.
Feel free to ask any questions / query stuff.
E: just to add some credibility to my answer, I have a Masters in Materials Science.
You are breaking the structure. You create a compound on the one side and s tear on the other. Making that state it’s new state. And it will now remain folded or be able to reverse.
Plastic vs elastic deformation. Pushing something past it’s yield point (a mechanical property of the material).
The lattice structure of the material changes and the individual molecules slide past on another. Let me expand a little on what a lattice structure is the orientation of the molecules think of like a rubix cube of molecules. When these molecules slip past each other they creat plastic deformation of the material cause the permanent deformation.
Plastic deformation of material. where external forces cause permanent deformation.
Elastic deformation is when the external forces aren’t above the yeild strength of the material. So the material goes back to it’s original shape without deformation.
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