Short Answer:

When a mild steel specimen is subjected to a tensile load, it undergoes several stages of deformation such as elastic deformation, yielding, strain hardening, necking, and finally fracture. At first, it behaves elastically, meaning it returns to its original shape when the load is removed. After reaching the yield point, it deforms permanently and finally breaks after significant elongation.

In simple terms, the behavior of mild steel under tension shows how the material stretches and changes shape as the load increases. It helps to understand important properties like yield strength, ductility, and ultimate tensile strength.

Detailed Explanation :

Behavior of Mild Steel under Tension

Mild steel is a ductile material that exhibits both elastic and plastic behavior when subjected to a tensile load. When a tensile test is performed on a mild steel specimen, the stress-strain curve obtained provides valuable information about the mechanical properties of the material. This curve helps to understand how mild steel reacts to an increasing tensile force until it finally breaks.

The test is usually conducted using a universal testing machine (UTM). The specimen is gradually pulled apart, and the corresponding stress and strain are recorded. The complete behavior of mild steel under tension can be explained through different stages, each showing a distinct type of deformation.

1. Elastic Region

When the tensile load is applied gradually to the mild steel specimen, it first behaves elastically. In this region, the deformation is temporary and reversible. The relationship between stress and strain follows Hooke’s Law, which means stress is directly proportional to strain.

If the load is removed at this stage, the specimen will regain its original length and shape. The slope of the stress-strain curve in this region represents the modulus of elasticity (E). This stage continues until the proportional limit is reached. Beyond this limit, Hooke’s law is no longer valid.

2. Yielding Stage

As the load increases beyond the elastic limit, the material enters the yielding stage. At this point, the material starts deforming plastically — meaning the deformation becomes permanent. In mild steel, this stage is very distinct and noticeable.

• Upper Yield Point: It is the point where the material starts to deform plastically for the first time.
• Lower Yield Point: After the upper yield point, the stress slightly drops and remains nearly constant over a large strain range. This is the lower yield point.

During yielding, a large amount of plastic deformation occurs without any noticeable increase in load. The appearance of Lüders bands (visible lines on the specimen surface) can be observed, indicating the spread of plastic deformation along the specimen.

3. Strain Hardening Stage

After the yielding stage, the material starts to resist deformation due to internal molecular rearrangement. This stage is known as the strain hardening or work hardening stage. Here, the stress again increases with strain.

During strain hardening, the internal structure of the metal gets strengthened as dislocations within the atomic lattice interact and lock each other. As a result, the specimen requires a higher load to cause further deformation. This continues until the stress reaches its maximum value, called the ultimate tensile stress (UTS).

The UTS represents the maximum load per unit area that the material can withstand before failure begins. For mild steel, the UTS is quite high, which shows that the material can bear large loads before breaking.

4. Necking Stage

After reaching the ultimate tensile stress, the load required to continue stretching the specimen starts to decrease. However, the true stress (actual stress based on the reduced cross-sectional area) continues to rise.

At this stage, the deformation becomes localized, and a narrow region called a neck forms at the weakest section of the specimen. The material in this region undergoes large plastic deformation while the rest of the specimen remains nearly unchanged. This phenomenon is called necking. The reduction in area in this localized region eventually leads to fracture.

5. Fracture Stage

As the neck continues to thin down, it can no longer bear the applied stress, and the specimen finally breaks. The fracture surface of mild steel typically has a cup-and-cone shape, which is a characteristic feature of ductile fracture.

The central portion of the fracture appears fibrous (indicating plastic deformation), while the outer edges appear granular (indicating shear failure). The fracture occurs due to a combination of tensile and shear stresses.

Stress-Strain Curve Explanation

The behavior of mild steel under tension can be represented on a stress-strain curve with the following key points:

• O–A: Proportional limit (elastic and follows Hooke’s Law).
• A–B: Elastic limit (end of reversible deformation).
• B–C: Yielding region (plastic deformation begins).
• C–D: Strain hardening region (stress increases again).
• D: Ultimate tensile strength (maximum stress).
• D–E: Necking region (reduction in area, leading to fracture).
• E: Breaking point (failure of specimen).

This curve clearly shows the different stages of mild steel behavior and is widely used to study its mechanical properties.

Mechanical Properties from Tension Test

From the tensile test of mild steel, the following properties can be determined:

• Elastic limit
• Yield strength
• Ultimate tensile strength
• Breaking stress
• Percentage elongation
• Ductility

These properties help engineers select suitable materials for different mechanical and structural applications.

Conclusion

In conclusion, the behavior of mild steel under tension demonstrates how the material transitions from elastic to plastic deformation and finally to fracture. It shows clear stages — elastic deformation, yielding, strain hardening, necking, and fracture. Mild steel’s ability to undergo large plastic deformation before breaking makes it highly ductile and safe for engineering use. Understanding this behavior is essential for designing safe and reliable mechanical components like shafts, beams, and structural frames.