What is the stress-strain diagram for mild steel?

Short Answer:

The stress-strain diagram for mild steel is a graphical representation that shows how mild steel behaves under gradually increasing tensile stress. It explains the relationship between the applied stress and the resulting strain (deformation) in the material.

In simple words, when a mild steel specimen is subjected to a tensile test, the diagram obtained shows different stages like the proportional limit, elastic limit, yield point, ultimate strength, and breaking point. This diagram helps to understand the mechanical properties such as elasticity, yield strength, ductility, and toughness of mild steel.

Detailed Explanation :

Stress-Strain Diagram for Mild Steel

The stress-strain diagram is one of the most important tools in the study of material behavior. It shows how a material deforms (strain) when subjected to an external force (stress). For mild steel, which is a ductile material, the stress-strain curve is obtained from a tensile test conducted on a standard specimen using a universal testing machine (UTM).

This curve helps to determine important mechanical properties of mild steel such as modulus of elasticity, yield stress, ultimate tensile stress, elongation, and percentage reduction in area. Mild steel is chosen for testing because of its predictable and ductile behavior under load.

Procedure to Obtain the Stress-Strain Diagram

  1. A standard cylindrical or flat specimen of mild steel is prepared with known dimensions.
  2. The specimen is placed in a universal testing machine and is gradually subjected to tensile load.
  3. The applied load and corresponding elongation are recorded until the specimen fractures.
  4. From these readings, stress and strain values are calculated using formulas:

 

  1. A graph is plotted between stress (vertical axis) and strain (horizontal axis). The shape of this graph gives the stress-strain diagram for mild steel.

Description of the Stress-Strain Diagram for Mild Steel

The stress-strain diagram for mild steel consists of several distinct regions, each representing a different stage of material behavior.

  1. Proportional Limit (Point A)

This is the point up to which stress is directly proportional to strain, meaning the material obeys Hooke’s Law.

The curve in this region is a straight line, and the slope of this line represents the modulus of elasticity (E).
Beyond this point, the relationship between stress and strain is no longer perfectly linear.

  1. Elastic Limit (Point B)

The elastic limit is the maximum stress that the material can withstand without any permanent deformation.
If the load is removed within this region, the material completely returns to its original shape.
Beyond this point, the material begins to deform permanently and loses its ability to fully recover.

  1. Yield Point (Points C and D)

After the elastic limit, a sudden increase in strain occurs with little or no increase in stress.
This region is called the yielding region, and the corresponding stress is called the yield stress.
There are generally two yield points:

  • Upper yield point (C): The stress at which yielding begins.
  • Lower yield point (D): The stress at which yielding continues with uniform strain.

This stage represents the beginning of plastic deformation, where the material changes shape permanently.

  1. Strain Hardening Region (D to E)

After yielding, the material starts to resist deformation more strongly.
This stage is called strain hardening or work hardening.
The stress required to continue deformation increases again until it reaches a maximum value.
The point E corresponds to the ultimate tensile strength (UTS) of the material, which is the highest stress the material can withstand before necking begins.

  1. Necking and Fracture (E to F)

After the ultimate point (E), a localized reduction in cross-sectional area occurs in the specimen — this is known as necking.
As necking progresses, the stress starts decreasing because the area reduces significantly until the specimen finally fractures at point F, called the breaking point.

At this point, the total strain is the sum of elastic strain and plastic strain.

Key Points on Stress-Strain Curve for Mild Steel

Region Behavior Nature of Deformation
O–A Linear region Elastic, obeys Hooke’s law
A–B Elastic limit Reversible deformation
B–D Yielding Plastic deformation starts
D–E Strain hardening Strength increases
E–F Necking Fracture occurs

Important Mechanical Properties from the Diagram

  1. Modulus of Elasticity (E):
    The slope of the linear portion (O–A) gives the stiffness of the material.
  2. Yield Strength (σy):
    The stress at which plastic deformation begins.
  3. Ultimate Tensile Strength (σu):
    The maximum stress the material can sustain before necking.
  4. Breaking Stress (σb):
    The stress at the point of fracture.
  5. Ductility:
    Measured by the percentage elongation or reduction in area between original and final dimensions.
  6. Toughness:
    The total area under the stress-strain curve, representing the energy absorbed before failure.

Typical Values for Mild Steel

  • Modulus of Elasticity,
  • Yield Stress,  (approx.)
  • Ultimate Tensile Strength,
  • Percentage Elongation ≈ 20–25%

These values show that mild steel is both strong and ductile, making it suitable for structural and mechanical applications.

Significance of Stress-Strain Diagram

  • It helps in understanding material behavior under different loading conditions.
  • It determines safe working stress for design.
  • It identifies elastic and plastic limits of materials.
  • It provides the basis for mechanical property evaluation such as stiffness, strength, and ductility.
  • It helps in selecting suitable materials for engineering structures and components.
Conclusion

The stress-strain diagram for mild steel clearly shows how the material responds under tensile loading, starting from elastic behavior to final fracture. The curve includes key points such as the proportional limit, elastic limit, yield point, ultimate strength, and breaking point. Mild steel exhibits both strength and ductility, making it ideal for use in machinery, buildings, and bridges. This diagram is fundamental in mechanical engineering because it helps determine safe stress levels and understand how materials behave under real working conditions.