What is necking?

Short Answer:

Necking is the process that occurs in a material during a tensile test when, after reaching the ultimate tensile strength (UTS), the material’s cross-sectional area begins to reduce locally, forming a narrow region called a neck. This is the final stage before fracture.

In simple words, necking is the localized reduction in diameter of a specimen under tension. It happens because the material can no longer uniformly resist the applied load. Necking indicates that the material has entered the stage of plastic instability and will soon fail.

Detailed Explanation:

Necking

Definition and Meaning:
When a ductile material, such as steel or copper, is subjected to a tensile load, it elongates and its cross-sectional area decreases uniformly in the beginning. As the load increases, the material reaches a maximum point on the stress-strain curve, known as the ultimate tensile strength (UTS). Beyond this point, the material cannot distribute the strain evenly, and the deformation becomes localized at one region — this is called necking.

The necking region appears as a narrow section along the length of the specimen, and once it starts, further deformation concentrates at that point until the specimen finally fractures. Therefore, necking is considered the final stage of plastic deformation before failure.

Mathematically, the reduction in cross-sectional area due to necking can be expressed as:

Where:

  • A₀ = Original cross-sectional area
  • Aₙ = Cross-sectional area at the neck

This reduction in area is a measure of ductility, and materials that exhibit large necking are considered ductile.

Process of Necking Formation

When a material is pulled under tension, it goes through several stages of deformation before necking occurs. These stages can be described as follows:

  1. Elastic Deformation Stage:
    • At first, the deformation is elastic, meaning the material returns to its original shape when the load is removed.
    • The stress-strain curve in this stage is a straight line, following Hooke’s Law (stress ∝ strain).
  2. Plastic Deformation Stage:
    • Once the material crosses its yield point, it starts to deform plastically, meaning permanent deformation occurs even after removing the load.
    • The material continues to elongate, and the cross-sectional area reduces uniformly along the length.
  3. Ultimate Tensile Strength Stage:
    • As the load increases further, the material resists more deformation until it reaches its maximum load-bearing capacity, known as the ultimate tensile strength (UTS).
    • At this stage, internal atomic structures begin to rearrange, and the material starts to lose its ability to carry additional load.
  4. Necking Stage:
    • Beyond the UTS point, further deformation becomes non-uniform.
    • A small region of the specimen experiences localized plastic deformation, and the cross-section reduces sharply in that area.
    • This local reduction is called necking.
    • As the neck forms, the actual stress at that section increases rapidly due to the reduced area, even though the applied load decreases.
  5. Fracture Stage:
    • As necking progresses, the localized strain continues to increase until the material fractures at the necked section.
    • The fracture typically occurs at an angle (for ductile materials) due to combined tensile and shear stresses.

Cause of Necking

Necking occurs because of plastic instability. When a material is stretched beyond its capacity to distribute the strain uniformly, one particular section starts to deform faster than the rest.

This happens due to the following reasons:

  1. Reduction in Cross-sectional Area:
    As the material elongates, its area decreases, leading to an increase in true stress at that region.
  2. Localized Plastic Flow:
    Once plastic deformation starts in a small region, it causes more strain in that area, and the surrounding material cannot catch up. This leads to localized deformation (neck formation).
  3. Load Redistribution:
    After reaching UTS, the load required to continue deformation decreases. However, deformation concentrates in the neck region, causing further thinning until fracture occurs.

Necking on Stress-Strain Diagram

In a typical stress-strain curve for a ductile material such as mild steel:

  • The linear portion up to the elastic limit represents the elastic region.
  • The curve rises non-linearly through the plastic region until it reaches the ultimate tensile point (UTS).
  • Beyond the UTS, the curve starts to drop, indicating that the load-carrying capacity decreases even though the strain continues to increase.
  • The downward curve represents the necking region, ending with fracture.

In this stage, engineering stress appears to decrease because the area reduces, but true stress (which accounts for the actual area) continues to increase until fracture.

Characteristics of Necking

  1. Localized Deformation:
    Necking occurs only in one localized region of the specimen.
  2. Reduction in Cross-sectional Area:
    A significant decrease in the diameter or thickness occurs at the necked region.
  3. Visible Narrowing:
    The specimen becomes visibly narrower at the neck compared to the rest of its length.
  4. Precursor to Failure:
    Necking always occurs just before fracture in ductile materials.
  5. True Stress Increases:
    Even though the load decreases, true stress increases in the necked region because the area reduces rapidly.

Factors Affecting Necking

  1. Material Ductility:
    • Ductile materials (like steel, copper, aluminum) show significant necking before fracture.
    • Brittle materials (like cast iron or glass) fail suddenly without necking.
  2. Temperature:
    • At higher temperatures, materials become more ductile and exhibit more necking.
    • At low temperatures, necking is minimal as materials tend to fracture suddenly.
  3. Strain Rate:
    • Rapid loading (high strain rate) can reduce necking because the material has less time to deform uniformly.
  4. Material Composition:
    • The presence of impurities or alloying elements affects ductility and the extent of necking.
  5. Heat Treatment:
    • Processes like annealing increase ductility and promote more necking before failure.

Importance of Necking

  1. Indicator of Ductility:
    The amount of necking in a specimen is a direct measure of how ductile a material is. The more necking, the more ductile the material.
  2. Prediction of Failure:
    Necking indicates that the material has reached its limit and will soon fail.
  3. Material Testing:
    Necking helps in determining the ultimate tensile strength (UTS) and fracture strength of materials.
  4. Design Safety:
    In engineering design, necking must be avoided in actual components. Designers ensure stresses stay well below the UTS to prevent neck formation.
  5. Manufacturing Uses:
    Controlled necking is used in metal forming processes like wire drawing, where localized deformation is beneficial.

Example of Necking

When a mild steel rod is stretched under tension, it elongates uniformly up to a certain limit. Beyond the ultimate stress, one section becomes narrower than the rest — this narrow part is the neck. The load required to stretch the material further decreases, and finally, the rod breaks at the necked section.

If the original diameter was 10 mm and the diameter at the neck before fracture is 8 mm, then:

This 36% reduction indicates significant necking and high ductility.

Conclusion:

Necking is the localized reduction in cross-sectional area that occurs in a material under tensile loading, after it reaches its ultimate tensile strength (UTS). It represents the stage of plastic instability, where deformation becomes concentrated in one region until fracture occurs. Necking is a key indicator of ductility, and its occurrence helps in understanding the final failure behavior of materials. In engineering, components are designed to work below this stage to prevent failure and ensure safety and reliability.