NeckingPhasen

NeckingPhasen, also written necking phases in English, describe the stages of localized plastic deformation that develop in ductile materials subjected to uniaxial tension. Necking is the concentration of plastic flow into a narrowed region, producing a thinning neck that reduces the cross‑sectional area at a specific location.

In many metals, as yielding transitions to plastic flow, strain can become unstable and localize. On an engineering stress–strain curve, the onset of necking occurs at the maximum engineering stress; beyond this point the load that the specimen can carry decreases even as plastic strain concentrates in the neck. The onset is often associated with the Considère criterion, which roughly states that necking begins when the true stress equals the rate of increase of true stress with respect to true strain.

The necking process is commonly described in several phases. The pre‑necking phase involves uniform elongation where the cross section remains nearly constant and work hardening increases the flow stress. The onset of necking marks the appearance of a localized constriction, after which the neck grows and the deformation becomes highly localized. In the post‑necking phase, the neck propagates along the gauge length, the cross‑section in the neck shrinks rapidly, true stress in the neck rises, and damage accumulates. Finally, fracture occurs when cracks form and coalesce, leading to complete failure of the specimen.

The concept of necking phases is important for understanding material ductility, formability, and failure behavior. Material properties such as strain hardening, strain-rate sensitivity, and temperature influence the timing and evolution of necking phases.