Necking (engineering)

Necking, in engineering or materials science, is a mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material.[1] The resulting prominent decrease in local cross-sectional area provides the basis for the name "neck". Because the local strains in the neck are large, necking is often closely associated with yielding, a form of plastic deformation associated with ductile materials, often metals or polymers.[2] Once necking has begun, the neck becomes the exclusive location of yielding in the material, as the reduced area gives the neck the largest local stress.

A polyethylene sample with a stable neck.
Schematic depiction of necking. Prior to necking, the entire material evenly undergoes plastic deformation. Once an unstable neck forms, the cross-sectional area of the neck will continue to decrease under load, while the un-necked material no longer plastically deforms.
A stress–strain curve typical of structural steel.

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