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.

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:
- Elastic and Plastic Regions: Initially, the material deforms elastically, then plastically, increasing in stress until it reaches the UTS.
- 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.
- 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.
- 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.
