# Mechanical Behaviour Questions and Answers – Creep at Lower and High Temperatures

This set of Mechanical Behaviour MCQs focuses on “Creep at Lower and High Temperatures”.

1. At lower temperature (below 0.3 Tm), what causes creep phenomenon?
a) Diffusion process
b) Diffusion less process
c) Recrystallization
d) Softening

Explanation: At lower temperature (below 0.3 Tm), diffusion less process causes creep phenomenon. Diffusion requires high temperature. Hence the temperature range is 0.05 to 0.3 Tm.

2. What is the predominant process below 0.5 Tm?
a) Viscous creep
b) Tertiary stage
c) Secondary stage
d) Transient creep

Explanation: Transient creep is the predominate process below 0.5 Tm. Above 0.5 Tm, diffusion starts taking place and recovery determines work hardening.

3. The stress that a material can withstand for a specified period of time without excessive deformation at higher temperature is ______
a) Fatigue strength
b) Creep strength
c) Tensile strength
d) Yield strength

Explanation: The stress that a material can withstand for a specified period of time without excessive deformation is creep strength. Temperature is taken into consideration.

4. When material under stress is heated, creep rate ______ with time.
a) Increases
b) Decreases
c) Remains constant
d) First increases, then decrease

Explanation: Thermal energy causes dislocation movement. When material under stress is heated, creep rate increases with time.

5. Creep rate according to Cottrell equation depends on the activation energy to move ______
a) Dislocations
b) Jogs
c) Kinks
d) Solute atoms

Explanation: Creep rate according to Cottrell equation, depends on the activation energy to move dislocations. Dislocations with lowest activation energy move fast.
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6. What are the predominant recovery processes?
a) Cross slip of screw dislocations
b) Cross slip of edge dislocations
c) Climbing of screw dislocations
d) Climbing of edge dislocations

Explanation: Two processes are predominant in recovery processes. These are the cross slip of screw dislocations and dislocation annihilation.

7. The transient creep described mathematically by _________ is expressed logarithmically by _________
c) Coffin, Manson
d) W.A. Wood, Orowan

Explanation: The transient creep described mathematically by Andrade is expressed logarithmically by Cottrell. Cottrell equation is ε = At-1. Here, ε is strain rate, t is time and A is constant.

8. At lower temperatures creep rate _________ with time.
a) Increases
b) Decreases
c) Remains constant
d) First increases, then decrease

Explanation: At lower temperatures creep rate decreases with time. It is because activation energy is not available.

9. Why does creep rate decrease with time at a low temperature?
a) Vacancy movement
b) Activation energy unavailable
c) Dislocation movement
d) Kinetic energy low

Explanation: At low temperatures, activation energy is not available. So, creep rate decreases with time at a low temperature.

10. Decreasing creep rate during __________ arises from increasing dislocation density.
a) Secondary creep
b) Viscous creep
c) Transient creep
d) Tertiary creep

Explanation: Decreasing creep rate during transient creep arises from increasing dislocation density. Formation of low angle grain boundaries takes place.

11. Decreasing creep rate in transient creep is because of _________ dislocation density.
a) Increased
b) Decreased
c) Zero
d) Locked

Explanation: Decreasing creep rate in transient creep is because of increased dislocation density. It is because of low angle grain boundaries.

12. Climb of edge dislocation is a chief recovery in FCC metals.
a) True
b) False

Explanation: Climb of edge dislocation is not a chief recovery in FCC metals. Cross slip of screw dislocations is a chief recovery in FCC metals.

13. A vacancy is a must for self-diffusion.
a) True
b) False

Explanation: In order to self-diffusion of an atom, it must be surrounded by a vacant site. For this, some activation energy is needed.

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