The assumption that a plane section before twisting remains plane after twisting is valid for:
Try it first: pick an option, then read the explanation.
Correct answer: D. only hollow or solid circular cross-sections
Asked in SSC JE 03/03/2017 (Evening)
Detailed Explanation
The elementary torsion theory assumes plane cross-sections remain plane and radii remain straight. This is exactly true only because of the axial symmetry of a circle.
Why “only hollow or solid circular cross-sections” Is Correct
It holds only for solid or hollow circular sections. Rectangular, elliptical, I and other sections warp, so their analysis needs Saint-Venant's theory.
Why the Other Options Are Wrong
- A. all types of cross-section: Non-circular sections warp.
- B. all cross-sections with curved boundaries: An ellipse has a curved boundary but still warps.
- C. only solid cross-sections: Solid rectangles and triangles warp.
Key Point
No warping → circular (solid or hollow) sections only.
Key Concepts for Students
- Warping of open thin sections can be large.
- Saint-Venant's theory handles non-circular shapes.
- Shafts are made circular partly for this reason.
Practice more: this is question 21 in Torsion MCQs – Part 2. Try the whole set and check every answer instantly.