Question

The stress necessary to initiate yielding, is considerably

• A. more than that necessary to continue it
• B. less than that necessary to continue it
• C. more than that necessary to stop it
• D. less than that necessary to stop it

Hint:

Think of it like static friction vs kinetic friction:
It always takes more force to start the motion (upper yield point) than to keep it sliding (lower yield point).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question refers to the comparative stress levels required to start (initiate) plastic deformation versus the stress level required to propagate or sustain (continue) plastic deformation in materials that display a yield drop.

Step 2: Detailed Explanation:

• Yielding Initiation vs Propagation:
In many crystalline materials (notably annealed low-carbon steel, certain aluminum alloys, and polymers), the onset of plastic deformation requires overcoming a high initial energy barrier.
The stress corresponding to the initiation of yielding is represented by the Upper Yield Point.
Once yielding is initiated at a local region, the stress drops sharply to a lower level, represented by the Lower Yield Point.

• Dislocation Dynamics:
The high initial stress is required to pull the pinned dislocations out of their interstitial solute traps (Cottrell atmospheres).
Once unlocked, the dislocations multiply rapidly, and their movement requires a lower stress level to sustain the deformation.
Thus, the stress required to initiate yielding (upper yield stress) is considerably more than that necessary to continue it (lower yield/flow stress).

Step 3: Final Answer:
The stress necessary to initiate yielding is considerably more than that necessary to continue it.