When a metal wire is bent repeatedly, it undergoes cyclic stress that weakens its internal structure. Every time it is bent, microscopic cracks develop at the stressed points. These cracks propagate further with each bend, reducing the wire’s strength. Eventually, the accumulated stress surpasses the material’s endurance limit, leading to fracture. This phenomenon is known as metal fatigue. The repeated bending also causes heat generation, which can further weaken the wire by softening the metal at the bending point, accelerating the breaking process.