Necking (engineering)

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In materials or mechanical engineering, necking is a mode of ductile flow of a material in tension. This is visible when applied stress passes a material’s ultimate strength. The material's cross-sectional area decreases, becoming thinner, and increases in length before it fails completely.

When a ductile material is deformed in tension, it first behaves nearly linearly. Upon further applied stress, past a material’s ultimate strength, the material’s cross-sectional area decreases. While the cross-sectional area decreases, the amount of stress within that certain area actually will increase, since stress is given by:

$\sigma = {F \over A_{instant}}$ .

where σ is the stress, F is the force being applied, and A_instant is the instantaneous cross-sectional area. The decreasing cross-sectional area causes the stress to increase although the force being applied remains the same.

Necking usually occurs due to either pre-existing flaws or flaws that are caused by stress. The usual order for failure by necking is:

Neck formation
Cavity void formation.
Void coalescence to form cracks caused by dislocation movement
Crack propagation through neck
Final shear failure.

The two sides of the fracture point can be matched up fairly well after the material has broken. This type of fracture is known as a cup-and-cone type fracture and is fairly common in failures due to necking. The cup-and-cone appearance of the fracture surface is caused by the ductility of the material.