Question

Lüders bands appear during

• A. Fatigue
• B. Fracture
• C. Creep
• D. Yield point elongation

Hint:

To eliminate stretcher strains/Lüders bands in manufacturing:
- Perform temper rolling (a small cold-rolling pass) prior to forming.
- This introduces a small density of unpinned dislocations throughout the sheet, bypassing the sharp yield point phenomenon altogether during subsequent deep drawing.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question asks about the specific stage of mechanical deformation or testing during which Lüders bands become visible on a metal specimen.
Lüders bands (also known as stretcher strains or slip bands) are a well-documented microstructural deformation phenomenon in certain alloys.

Step 2: Key Formula or Approach:
The phenomenon is closely linked to the yield point phenomenon observed during the uniaxial tensile testing of low-carbon steels and some aluminum-magnesium alloys.
The yield point phenomenon involves an upper yield point, a lower yield point, and a subsequent plateaus region known as yield point elongation (YPE).

Step 3: Detailed Explanation:

• Cottrell Atmosphere Pinning: In low-carbon steels, interstitial solute atoms (like carbon and nitrogen) migrate to the strain fields of edge dislocations, pinning them.
- To initiate dislocation motion, a high stress is required to pull the dislocations away from these solute atmospheres (this corresponds to the upper yield point).

• Band Propagation during YPE: Once unpinned, dislocations can move at a lower stress level (the lower yield point).
- Plastic deformation then occurs non-uniformly. Localized bands of intense shear deformation, called Lüders bands, initiate at stress concentrators (like the fillets of the specimen) and propagate along the gauge length.
- This propagation occurs during the yield point elongation stage, where the load remains roughly constant.
- Once the entire gauge length has been traversed by these bands, uniform strain hardening begins, and the bands disappear into general deformation.
- These bands are highly undesirable in sheet metal forming (e.g., deep drawing) because they create a rough, stepped surface finish.


Step 4: Final Answer:
Lüders bands are directly associated with the non-uniform deformation occurring during yield point elongation.
Therefore, the correct choice is option (D).