Definition - What does Necking mean?
Necking is a mode of the tensile deformation in which the large amount of strain is disproportionately localized in a small area of the material. The local cross sectional region of the material offers the base for neck. It is closely related to yielding, a kind of a plastic deformation which is related to ductile materials like polymers or metals as it has large local strains. The strength of the strain which hardens the material increases continually with the increase in the deformation.
Petropedia explains Necking
Necking is a result of instability during the tensile deformation where the cross sectional area of a material decreases by a great proportion. All the real materials consist of heterogeneities-like local variations and flaws in composition or dimensions before the deformation which results in the local fluctuation in strains and stresses. These fluctuations should be infinitesimal in their magnitude so as to determine the location of the embryonic neck. During the tensile deformation the cross sectional area of the material decreases and the strain hardens. The hardening of the material strain depends on the extent of deformation.