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.

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

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.