A composite beam is made of two equal strips, one brass and one steel. If the temperature is raised:
Try it first: pick an option, then read the explanation.
Correct answer: D. All of these
Asked in SSC JE 25/01/2018 (Morning)
Detailed Explanation
Brass expands more than steel (α ≈ 19 × 10⁻⁶ vs 12 × 10⁻⁶ per °C). Bonded together, they must share the same length at the interface.
Why “All of these” Is Correct
Brass is restrained → compression; steel is stretched → tension. These equal and opposite forces act along the centres of the two strips, which are a small distance apart, so they form a couple that bends the strip — the bimetallic strip effect. All three statements are true.
| Situation | Stress |
|---|---|
| Bar fully restrained, temperature change ΔT | σ = EαΔT (compressive on heating, tensile on cooling) |
| Composite bar heated (ends held together) | Material with higher α → compression; lower α → tension |
| Composite bar cooled | Higher α → tension; lower α → compression |
Why the Other Options Are Wrong
- A. steel experiences a tensile force: True, but (b) and (c) are also true.
- B. brass experiences a compressive force: True, but (a) and (c) are also true.
- C. the composite beam is subjected to a couple: True, but (a) and (b) are also true.
Key Point
Bimetallic strip: high-α side in compression, low-α side in tension, the strip bends (towards the low-α side).
Key Concepts for Students
- Thermostats use bimetallic strips.
- The strip curves with the higher-α metal on the outside.
- Net axial force is zero; only a couple remains.
Practice more: this is question 28 in Stress and Strain MCQs – Part 2. Try the whole set and check every answer instantly.