The term "necking" is used in engineering and materials sciences to describe the localized reduction of cross-sectional area of a specimen under tensile load. Necking occurs when an instability in the material causes its cross-section to decrease by a greater proportion than the strain hardens when undergoing tensile deformation. If the material begins to harden by a smaller proportion than the decrease in cross-sectional area, strain concentrates at the location of highest stress or lowest hardness. The greater the local strain, the greater the local decrease in cross-sectional area, which in turn causes even more concentration of strain, leading to an instability that causes the formation of a neck. Necking behavior is disregarded in calculating engineering stress but is taken into account in determining true stress.

specimen necking