why does metal heat up when bent repeatedly???

Answer 1

When you bend metal you are working it, Work is energy, heat is a form of energy. When bending the metal metal in alternating pressure reversal cycles of tensile and compression, this energy input heats up the metal, resuting a fatigue which will lead to the metal breaking (failure).

Answer 2

It's not just when it's bent repeatedly, it's doing it each time it is bent & the heat is increasing each time. A rudimentary explanation is that, on a molecular level you are causing the metal to break & rub against itself. This builds up heat just as two sticks rubbing together cause heat & then fire...

Answer 3

It's all about friction. You're trying to separate the material and because the material is ductile and not brittle, the atoms with stronger bonds are trying to move past the atoms with weaker bonds. Friction force always results in the release of heat through the process of conduction, so the material heats up.