Why does a metal wire breaks if it is continuously bent up and down ?
[Physics] Why does a metal wire breaks if it is continuously bent up and down
material-science
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They do actually, but in normal conditions you can not observe this. The sublimation rate will depend on temperature and energy of sublimation, which is different for different metals. Also in normal conditions you will have some kind of equilibrium - some of the particles that leave the surface will condensate back. But in vacuum techniques you need to take this to account - there are metals with low sublimation energy - such as Zinc, Magnesium, Cadmium e.t.c. You can see them here, here, for example, in the list of non-suitable for vacuum equipment. As an extreme case you can imagine a piece of metal, which is heated from inside and put into deep space, if its mass is small and gravitation attraction can be neglected, it will evaporate.
I would guess you're thinking about the phenomenon called creep.
Your piece of metal is made up from crystals in which each atom has a well defined position with respect to the atoms around it, and it takes a lot of energy to make an atom jump completely out of its position. When you elastically deform the metal you displace atoms very slightly, but when you release the metal the atoms return to their equilibrium positions and the metal returns to its original shape. Permanent deformation happens when you apply so much force that it makes some of the atoms jump completely out of their original locations and into new positions.
However even under small deformations it is possible for atoms to move causing the deformation to become permanent. Metals generally contain defects in their crystal structure called dislocations and the atoms near a dislocation are generally more mobile than atoms in the rest of the metal. Under even small applied forces movement of atoms at a dislocation can cause the dislocation to move and the metal to permanently deform.
Alternatively, at any temperature above absolute zero the atoms in a metal have some thermal energy so they are vibrating about their equilibrium positions. As you increase the temperature the atoms vibrate more and this makes it easier for them to move. The obvious extreme example of this is melting, where the atoms vibrate so much it breaks down the regular crystal lattice and becomes a liquid. However even below the melting point increased temperature can allow atoms to move and this can cause a small deformation to become permanent.
To what extent creep happens, and how fast, depends on the type of metal and how the metal has been treated. Obviously your tweezers are designed to be flexible and will have been made from a metal that does not creep readily.
Best Answer
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.