zachtheperson t1_j9906oo wrote
Reply to comment by Glasnerven in When something is bent (a metal ruler for example) and returns to its original shape, what is happening on the molecular level? Where is the information of the original shape stored and what forces do the unbending? by JewNugget2525
What's the difference then with something like a metal bar, where if you bend it far enough it does actually stay bent, but if you bend it less it goes back?
auraseer t1_j991ord wrote
If you bend it too far, you have passed the elastic limit. It is no longer undergoing elastic deformation, but instead, plastic deformation.
In that mode, you are applying enough force to overcome the atoms' tendency to stay put. Some of the atoms get moved out of place, and rearrange into new places in the crystal structure. They settle into places that are lower energy in the object's bent shape.
Once the crystal structure is deformed, and new atomic bonds form, those new bonds replace the old ones. The object's new shape is the lowest energy configuration, and that is now the shape it wants to stay in.
picklesTommyPickles t1_j9adfvt wrote
This is super interesting. Hopefully you don't mind if I continue one more! When you bend the bar and it undergoes plastic deformation, why is it that if you bend the bar enough it will eventually begin to degrade and eventually "fail"? Failure in this context is weaken at the bend or break completely in half.
Based on what I've learned from your posts, it would appear that something happens at the atomic level to the bonds if you overwork the metal.
CrazySheepherder1339 t1_j9ajeu5 wrote
Think of a paperclip as a metal bar.
As kids we would keep flipping the inner part of a paperclip. And kept folding it till it breaks.
So, when you bend the paperclip into plastic deformation. It is difficult/impossible to bend it back. You would have to melt it completly. without melting it. It bends at the point of lease resistance. So after 1 bend, there is a new structure and new point of least resistance is somewhere else near the original bend. So even if you try to bend it back, it won't be the same structure. In this case some of the bonds might have completly broken, but there is still enough to hold it together.
When you keep bend a paper clip, some areas will have elastic defirmation, some will have plastic, and some will break. So if you keep bending it back and forth around a certain area, eventually enough break around the point that you are bending it, that it just splits into 2 pieces.
picklesTommyPickles t1_j9ak8iq wrote
So basically when you bend metal, you're losing some information in the form of broken bonds? Bending enough times in the same location results in enough lost information to result in a fracture?
CrazySheepherder1339 t1_j9am3mx wrote
Yes! Metal is ductile and malleable, so it can last a little longer than things that are more brittle, like a pen clip.
Suppose there are 5 bonds. when bend a metal clip, maybe 1 breaks, 2 reconnect to different pairs, and 2 stay the same.
In the example the "_" means a bond is broken
So if the connection is aa,bb,cc,dd,ee
it becomes a_,bc,cb,dd,ee. Notice how atom b and c switched their bonds. And the second time A_ , b_, c_, dc,ee
But with plastic it would be Aa,bb,cc,dd,ee then just break without switching A_,b_c_,dd,ee Then the second time, it breaks
Fo0ker t1_j9at67e wrote
Yes, that's literaly the cause of metal fatigue, if you bent a spoon back and forth enough times, it'll look ok but the slighest heating of shift and it breaks because you're breaking what's left of the oringinal form. This is the trick behind Yuri Geller rubbing spoons til they fall apart.
It's also why the british plane Comet crashed, the big windows let the metal flex enough just past the elastic limit to "fatigue" the metal and make little cracks in the structure. Over time they built up and you get planes falling from the sky.
kdeff t1_j9bfg98 wrote
There is a difference though, between bending a paperclip back and forth a few times so it plastically deforms and breaks - and fatigue.
The former is exceeding it's ultimate strength and breaking the paperclip. Fatigue is a different phenomenon caused by cyclic loading and not related to the ultimate yield strength - slip bands form and eventually cause a crack.
Seems like a technicality but they are two different phenomenon, and material health is asses completely differently when looking at overstress vs. fatigue!
[deleted] t1_j9c8qg8 wrote
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CrazySheepherder1339 t1_ja9qxrw wrote
So essentially for fatigue, the repetitive localized micro-plastic deformations, will keep shifting/form the slip bands until the straw breaks the camels back?
Could repetitive elastic deformation cause fatigue? My gut thought is that by defenition it can't? Basically if it does, it is actually micro plastic deformation and not elastic deformation.
kdeff t1_jabnfg1 wrote
Fatigue is sort of a mix of plastic and elastic deformation. It can happen when a material is only being elastically stressed - but the mechanism of operation is still dislocation motion (like plastic deformation).
The dislocations that move in this case require much lower stress to move - ie. not all dislocations move at exactly the yield stress of the material (that's sort of an average). But in this case, dislocations move back and forth along the same path (the path of low resistance), and eventually form a slip band which can eventually lead to failure of the material.
This is referred to generally as high-cycle fatigue, ie. it takes a lt of cycles to cause failure, because the stresses are low, and SN (stress vs #cycles) curves are used to assess damage and predict time to failure (compared with a stress/strain curve used to predict failure from overstress).
Coomb t1_j9avvbs wrote
The Comet failures were driven far more by the fact that the windows had sharp corners, which concentrate stress, than by the absolute size of the windows.
_AlreadyTaken_ t1_j9b5pv7 wrote
Things like metal fatigue are due to creating cracks between metal crystals as they dislocate until they connect and form fractures.
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