Necking

Necking is a localized reduction in cross-sectional area that occurs in ductile materials after they reach their ultimate tensile strength (UTS) during a tensile test. As the material is stretched beyond the UTS, plastic deformation becomes concentrated in one region, causing a visible narrowing, or “neck,” in the specimen. This phenomenon signals the transition from uniform plastic deformation to localized deformation, ultimately leading to fracture.

This specimen experienced significant necking before fracture

Necking on the Stress-Strain Diagram:

On a stress-strain diagram, necking appears after the material reaches its ultimate tensile strength (UTS), which represents the peak stress the material can withstand. The key features are:

  1. Elastic and Plastic Regions: Initially, the material deforms elastically, then plastically, increasing in stress until it reaches the UTS.
  2. Peak Stress (UTS): The maximum point on the stress-strain curve corresponds to the ultimate tensile strength. Up to this point, the entire specimen deforms uniformly.
  3. Onset of Necking: Beyond the UTS, true stress continues increasing, but because engineering stress is based on the original cross-sectional area, the engineering stress decreases due to the reduction in load-bearing area.
  4. Localized Deformation: Instead of the entire specimen stretching uniformly, deformation is concentrated in a small region, which experiences further elongation until fracture occurs.

Engineering Significance:

  • Necking indicates imminent failure in ductile materials. Once necking begins, the material’s load-bearing ability rapidly decreases.
  • In engineering stress-strain diagrams, the stress appears to decline after UTS, but in true stress-strain diagrams, stress continues increasing because the actual cross-sectional area is decreasing.
  • Ductile materials, such as mild steel and aluminum alloys, exhibit significant necking before fracture, allowing for visible warning signs of failure, whereas brittle materials fracture with little to no necking.