Why does necking occur during tensile testing of ductile materials?

[pukb]

Why does necking occur during tensile testing of ductile materials?

 

[Astronuc]

Necking coincides with the limiting uniform elongation, which corresponds with the ultimate tensile strength.

The reduction in (cross-sectional) area is related to conservation of mass (and volume), which is observed in the Poisson effect. One observes a significant amount of shearing in the necking area. Basically it is a region of local instability in the material.

 

[MalliCk]

Formation of necks is mainly due to the formation of micro-voids, that form during the plastic flow of metals/alloys under tension. So, when a micro-void is formed atomic layers slide around them. These micro-voids coalesce to form micro-cracks (Stress Raisers) which causes fracture.
Basically voids are responsible for the necking phenomenon.

Hope this helps you !