Material behavior at the yield point At the yield point of a metallic test specimen under tension, which behavior best describes the material response?

Introduction / Context:
The yield point marks the onset of plasticity in ductile metals during a tensile test. Understanding this transition is fundamental to differentiating elastic design limits from plastic behavior and permanent deformation.

Given Data / Assumptions:

• Conventional engineering stress–strain testing of a ductile metal.
• Elastic behavior precedes yield; beyond yield, plastic strains accumulate.

Concept / Approach:
Hooke's law (stress proportional to strain) applies in the linear elastic range. At yield, the proportional limit is exceeded and plastic (permanent) strain initiates. On unloading after yielding, only the elastic portion of strain is recovered; the plastic portion remains.

Step-by-Step Solution:
Before yield: stress = E * strain; unloading follows the same linear slope E.At yield: nonlinearity appears; plastic mechanisms activate (dislocation motion, slip).After yield: on complete unloading, permanent set (plastic strain) is observed; shape is not fully recovered.

Verification / Alternative check:
Typical mild steel stress–strain curves show a distinct upper and lower yield point; aluminum alloys show a smooth yield transition but still exhibit plastic deformation beyond the 0.2% proof stress.

Why Other Options Are Wrong:
(a) and (b) would imply purely elastic proportional behavior, which ends at or before yield; (c) contradicts the definition of plastic strain; (e) viscoelasticity is a time-dependent phenomenon not specifically tied to the yield point in metals.

Common Pitfalls:
Confusing yield point with ultimate strength; assuming full shape recovery after any unloading; ignoring the difference between proportional limit and yield strength.

Final Answer:
Undergoes plastic deformation