Material Testing — Tensile Test on Mild Steel After fracture in a standard tensile test on a round bar of mild steel, how does the local diameter at the fracture section change?

Introduction:
A tensile test reveals elastic properties, yielding, strain hardening, necking, and ductile fracture of metals such as mild steel. The visible change in cross-section at the fracture provides direct evidence of ductility.

Given Data / Assumptions:

• Specimen: round bar of mild steel (ductile material).
• Standard uniaxial tensile test to fracture.
• Uniform temperature, quasi-static loading.

Concept / Approach:
After uniform elongation, a ductile metal enters diffuse necking, then localized necking. True stress rises even as engineering stress may drop. Cross-section reduces markedly at the neck just before ductile fracture (cup-and-cone profile).

Step-by-Step Solution:
Elastic region: diameter change negligible and recoverable.Plastic region: volume constancy approximately holds, so as length increases, area must reduce.Necking: localized reduction in area causes a sharp decrease in diameter at the neck.Fracture: the broken ends show smaller diameter where necking developed.

Verification / Alternative check:
Fracture surface and gauge length measurements show reduction of area = ((A0 - Af) / A0) * 100%.

Why Other Options Are Wrong:
remain same: contradicts measured reduction of area in ductile fracture.increase: impossible under tensile necking.depend upon rate of loading: normal quasi-static tests still show necking; rate effects do not reverse the trend.

Common Pitfalls:
Confusing engineering stress–strain with true stress–strain and misinterpreting the post-UTS behavior.

Final Answer:
decrease