eli5: Why do hard things not bend back into shape?

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One of those things I never really pondered and now kinda hard to wrap my head around.

 

**What happens on an atomic level when e.g. a metal bar bends? It’d seem that the atoms in the bend would get squished while those on the outside would get stretched apart (since the number of atoms doesn’t change), so why doesn’t the natural repulsion/attraction of atoms strife towards an even distribution of them and slowly bend it back into shape?**

Or do the inner atoms push outwards when bending so there is an even distribution? But if so, then why does the material become weakened and eventually tear/break when you bend it back and forth?

 

The longer I think about it the more I start to question why things stay in shape in the first place. Why does a sheet of paper remain a somewhat thin layer of atoms rather than clumping up to a ball? If I laid out a bunch of electromagnetic balls next to each other like a sheet of paper and then turned them on they probably wouldn’t just be cool with clicking onto the balls next to them and being a flat plane, but rather tumble all over each other and build up bumps and probably try to end up in a ball (balls just seem like a perfect distribution for thousands/millions of little things being magnetically attracted).
So what makes the atoms in solid materials just be cool with staying in whatever weird shape we put them in?

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*”What happens on an atomic level when e.g. a metal bar bends?”*

The crystal structure of metal is rarely perfect. The imperfections allow the atoms to pop to a new location one atom at a time. These imperfections are called dislocations. With the movement of each atom, there is an infinitesimal change in shape of the metal object.

This video demonstrates it much better than I can describe it:

[https://www.youtube.com/watch?v=sn1Y6zIS91g](https://www.youtube.com/watch?v=sn1Y6zIS91g)

*”It’d seem that the atoms in the bend would get squished while those on the outside would get stretched apart (since the number of atoms doesn’t change), so why doesn’t the natural repulsion/attraction of atoms strife towards an even distribution of them and slowly bend it back into shape?”*

What you are describing is why metal springs back if flexed a little. Below a certain threshold, there isn’t enough force to make the atoms move at a dislocation, so the metal returns to its original shape when the force is removed. Above the threshold, atoms start moving and permanent deformation occurs.

Note that any individual atom doesn’t need to move far when metal bends. To see this, consider bending a metal tube. The wall of the tube on the inside of the bend is being compressed. At 6:55 in the above video you can see that as the bubble raft is compressed it gets wider. This is exactly what happens to the wall of the tube on the inside of the bend. The wall thickness will increase slightly as the tube is bent. And at 7:05 you can see what happens to the metal tube on the outside of the bend. As the metal stretches, the wall thickness of the tube decreases. All of this happens without atoms needing to move very far. The movement all happens within the wall thickness of the tube — none of the atoms need to move around the tube from the inside of the bend to the outside of the bend.

*”…why does the material become weakened and eventually tear/break when you bend it back and forth?”*

I don’t have a complete answer for this. I’m not even sure it is completely understood by the experts. The less than complete answer is something like this:

Metals can never be manufactured perfectly. There will always have some defects. When a piece of metal is loaded in tension, these defects cause a large increase in the internal forces (called stress) in the material right at the defect (they create a stress concentration or stress raiser). This high stress can exceed the strength of the metal locally and a crack can form. The crack makes a bigger stress raiser, and continues to grow until complete failure.

Why in some places do dislocation move through a metal without leaving a crack, but in others cracks form at a defect? Sorry, I don’t know.

*”So what makes the atoms in solid materials just be cool with staying in whatever weird shape we put them in?”*

Someone else might have a better answer to this, but I think it’s lack of energy. That is, for a atom to move requires energy, We supply a lot of energy to flatten a lump of metal into a sheet. When the metal is cool, there isn’t enough energy to push an atom from a place where it is relatively happy to one where it would be just a tiny bit happier.

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