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picklesTommyPickles t1_j9ak8iq wrote

Reply to comment by CrazySheepherder1339 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

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?

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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

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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.

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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!

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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.

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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).

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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.

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