This set of Mechanical Metallurgy Questions and Answers for Aptitude test focuses on “Creep Curve – 2”.
Explanation: The idealized curve consists of three regions, the primary state, secondary or the steady-state, and the final stage.
Explanation: In the constant stress state of the creep curve, the tertiary state of creep curve is absent, or the second state elongates to the third stage itself. So the generalized curve looks like given above.
3. The primary state of the creep curve represents the ___________ creep rate in the strain vs. time curve.
Explanation: On the application of load, the material instantaneously strains rapidly and then becomes constant, so in the primary state, the creep rate is decreasing.
4. Which state of the creep curve state represents the constant creep rate?
d) No stage
Explanation: The secondary region of the creep curve is linear and represents the constant creep rate of the curve. Time to failure by creep is mainly determined by this stage only.
5. The steady-state creep (secondary state) has constant creep due to the competing result of the strain hardening and recovery.
Explanation: In the initial state, the creep rate is decreasing because the creep resistance of the material increases. After this stage, the strain hardening and recovery process starts counteracting each other, and the creep rate becomes steady, hence constant creep rate.
6. The tertiary state of the creep rate increases with time. It occurs in every case and has increasing creep rate.
Explanation: The tertiary creep rate is only observed in the constant load case. Because as the necking is processed, the cross-sectional area reduces with steady load which results in increasing strain rate. Generally, it needs high stress and high temperature.
7. Which state of the creep curve state is associated with the metallurgical changes in the material?
d) No stage
Explanation: The third stage of creep is often associated with metallurgical changes such as coarsening of precipitates particles, recrystallization or diffusional changes in the phases that are present.
8. If the temperature of the material is higher, the creep rate will also be _____________
c) independent of temperature
d) sometimes higher sometimes lower
Explanation: At higher temperatures, the diffusion will be higher, and the grain boundary phenomena will also take place rapidly, So the creep rate will also become higher.
9. The following curves represent the constant stress creep curve. Sample A, B, C, D are tested at different stress values. The correct relationship of the magnitude of stress for the following stress values will be _________
Explanation: As the stress is increasing, the curve shifts towards the more and more strain values with lesser and lesser time. So Sample A is tested at the highest stress and the sample D at the lowest stress.
10. The creep rate determination will be most critical for which component?
a) Train wheel
b) Steam turbine
c) Mega steel structure
d) Car body
Explanation: The creep is the phenomena associated with the high-temperature application. The steam turbine operates at very high temperatures with thermal cycles, so understanding of creep behaviour of the material becomes very important in such cases.
Sanfoundry Global Education & Learning Series – Mechanical Metallurgy.
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