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. 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.