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Why does a metal wire breaks if it is continuously bent up and down ?

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when you are bending a piece of wire up and down, you are plastically deforming the metal. this causes crystal imperfections called dislocations in the metal to move about within the crystal grains, and to get piled up and trapped at the edges of the grains. When this occurs, the crystal grains start to come loose from one another and cracks then develop along these grain boundaries. Those cracks link together and grow until one of them is large enough to cause the metal wore to come apart.

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  • $\begingroup$ Not quite. See the link in my comment above. Annealing makes a metal softer and moves atoms apart. Work, such as bending, makes a metal harder and moves atoms closer. Enough bending makes the metal so hard that it doesn't bend anymore, but breaks instead. It starts breaking where it is the hardest, where most work was done and the grains are closer, not where it gets "loose". It gets loose in these areas only after it is already broken there. Your answer has everything in reverse. $\endgroup$
    – safesphere
    Commented Jun 8, 2019 at 6:12
  • $\begingroup$ are you saying dislocation pileup does not disconnect adjacent grains and produce nanocracks? $\endgroup$ Commented Jun 8, 2019 at 6:18
  • $\begingroup$ There is no "pile-up". The metal breaks where it is the hardest. Say, it broke at a particular point. This point no longer has any resistance to bending, so all other points now are under an even stronger work and harden faster. Then the next hardest point breaks. It does not have to be next to the previous one. However, after a while, the only solid points remaining will be those between the broken points. All work will be applied to them, they will harden and break. They don't break, because they are close to the broken points, but because all work is applied to them making them harder. $\endgroup$
    – safesphere
    Commented Jun 8, 2019 at 6:28
  • $\begingroup$ Yep, that's called "work hardening". Hammer a piece of cold annealed metal for a while and it starts to a) get springy, b) get harder, and c) get brittle. $\endgroup$
    – S. McGrew
    Commented Jun 8, 2019 at 13:58
  • $\begingroup$ safesphere, do you know what the mechanism of work hardening is? $\endgroup$ Commented Jun 9, 2019 at 2:38

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