Tensile test observation: diameter at the fracture section remains approximately the same as before testing Identify the most likely material behavior.

Introduction / Context:
In a tensile test, the way a specimen fractures reveals whether it is ductile or brittle. Ductile materials exhibit noticeable necking before fracture, while brittle materials fail suddenly with little or no plastic deformation. The observation given is that the diameter at the fracture remains approximately unchanged, which signals brittle behavior.

Given Data / Assumptions:

• Round bar in uniaxial tension to failure.
• Diameter near fracture shows no significant necking.
• Room temperature, quasi-static loading.

Concept / Approach:
Ductile fracture is preceded by plastic flow and localized necking. Brittle fracture occurs with minimal plasticity and no necking, often along cleavage planes. Cast iron is a classic brittle engineering material in tension due to its graphite morphology and microstructure.

Step-by-Step Solution:
Check for necking: None observed.Classify behavior: brittle.Among options, cast iron and glass are brittle, but glass is not typically tested as standard round bar in structural labs; cast iron is the conventional exam answer.

Verification / Alternative check:
Typical fracture surfaces: ductile shows cup-and-cone with reduction of area (mild steel, copper, aluminum); brittle shows granular flat fracture with little contraction (cast iron).

Why Other Options Are Wrong:
Mild steel, copper, aluminum alloy are ductile and neck noticeably. Glass is brittle but not the usual engineering metal specimen in standard tensile testing contexts for this question.

Common Pitfalls:
Misinterpreting a slightly rough surface as necking; assuming all metals behave ductilely; ignoring standardized lab examples used in exam problems.

Final Answer:
Cast iron