Why does necking occur during tensile testing of ductile materials?

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SUMMARY

Necking occurs during tensile testing of ductile materials when the limiting uniform elongation is reached, coinciding with the ultimate tensile strength. This phenomenon is attributed to the conservation of mass and volume, as described by the Poisson effect. The necking region experiences significant shearing and is characterized by local instability due to the formation of micro-voids during plastic flow. These micro-voids coalesce into micro-cracks, acting as stress raisers that ultimately lead to fracture.

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Why does necking occur during tensile testing of ductile materials?
 
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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.
 
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 !
 

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