Experimental study on the evolution of necking zones of metallic materials

Localization of plastic strain, also known as necking, is an important failure mechanism of metals. Although metals are widely used in engineering, it remains unclear how the necking zones evolve during loading process. In this work, strain localization is investigated experimentally in three types of metallic materials, including mild steel Q235, high strength low alloy steel HSLA350, and aluminum alloy AL6061. Displacements, strain distributions, and instantaneous cross-sectional areas of the specimens were measured using a three-dimensional digital image correlation (3D-DIC) measurement system. True stress–strain relations of the specimens were obtained. A new method is proposed to determine the necking zone length from both the longitudinal and transverse strain profiles. At different locations within the necking zone, true stress is distributed unevenly. It is found that the necking zone length increases with the increasing axial elongation of the specimens. This study provides an in-depth understanding of the evolution of necking in metals, which is of significant importance to study the post-necking and failure behavior of metallic materials as well as for calculation of ductility of metal structures.