Question

For a material with a pronounced yield point in its stress-strain diagram, such as mild steel, the yield point phenomenon is attributed to:

• A. Sudden orientation of dislocations in the direction of the applied stress
• B. The release and redistribution of internal stresses
• C. The abrupt movement of dislocations after overcoming the Peierls-Nabarro barrier
• D. An increase in the rate of strain hardening due to temperature rise

Hint:

Yield point = dislocations start to move fast! It’s all about breaking through the Peierls-Nabarro barrier.

Answer 1

The Correct Option is C

In materials like mild steel, the stress-strain curve shows a distinct yield point — where the material begins to deform plastically with minimal increase in stress. This unique behavior is attributed to the dislocation mechanics within the crystal lattice.
Peierls-Nabarro Barrier and Yield Point:

• The Peierls-Nabarro barrier represents the energy needed to move dislocations through a crystal lattice.
• Initially, dislocations are pinned due to obstacles like interstitial atoms (e.g., carbon in steel).
• Once the applied stress is sufficient to overcome this barrier, dislocations move abruptly, leading to the sharp yield point behavior.
• This sudden movement initiates plastic flow and is followed by a yield plateau.

Why Other Options Are Incorrect:

• (A) Dislocation orientation isn’t the primary factor — it’s their mobility that matters.
• (B) Redistribution of internal stresses may occur, but it does not explain the abrupt yield phenomenon.
• (D) Strain hardening occurs later in the plastic region, not at the yield point.

Therefore, the yield point phenomenon in mild steel is due to the abrupt movement of dislocations after overcoming the Peierls-Nabarro barrier.