When a stretching force, also known as a tensile force, is applied to metals, the metal undergoes a process known as plastic deformation. This occurs beyond the elastic limit of the metal, where it can no longer return to its original shape after the force is removed.

Here's what happens when a stretching force is applied to metals:

1. Elastic deformation: Initially, when a small stretching force is applied, the metal undergoes elastic deformation. This means that the metal stretches or deforms reversibly. When the force is removed, the metal returns to its original shape and size.

2. Plastic deformation: As the stretching force increases beyond the elastic limit, the metal enters the plastic deformation phase. At this point, the material undergoes permanent deformation. The metal's atoms shift and rearrange, causing the metal to elongate and change shape.

3. Necking: As the stretching force continues to be applied, a localized region of the metal known as the "neck" starts to form. This region experiences intense thinning and becomes the weakest point in the metal.

4. Fracture: Eventually, the stress concentration in the neck reaches a critical point, causing the metal to break or fracture. The metal ruptures at the weakest point, typically at the neck.

The behavior of metals under stretching force can be influenced by several factors such as the metal's composition, grain structure, temperature, strain rate, and environmental conditions. Some metals are more ductile and can withstand significant plastic deformation before breaking, while others are more brittle and undergo minimal deformation before fracturing.