20 Questions In Our Quiz On Creep Failure In Metals

Welcome to your Quiz on Creep Failure in Metals

1. Which stage of creep is primarily characterized by a balance between strain hardening and recovery processes?

a. Primary Creep $ \; \; $

b. Secondary Creep $ \; \; $

c. Tertiary Creep $ \; \; $

d. Elastic Creep $ \; \; $

2. Which of the following best defines creep in metals?

a. A temporary deformation of a material that occurs when it is subjected to a stress below its elastic limit for a short period. $ \; \; $

b. A permanent deformation of a material that occurs over time under a constant load or stress, typically at elevated temperatures. $ \; \; $

c. The sudden failure of a material due to repetitive cyclic loading below its ultimate tensile strength. $ \; \; $

d. A deformation process in metals caused by rapid cooling, resulting in changes to the crystal structure. $ \; \; $

3. Which of the following statements about the stages of creep in metals is correct?

a. In the primary stage, the creep rate is constant due to the balance between work hardening and recovery. $ \; \; $

b. The secondary stage is characterized by a steadily decreasing creep rate due to strain hardening. $ \; \; $

c. The tertiary stage is marked by an accelerating creep rate leading to necking and eventual failure of the material. $ \; \; $

d. The primary stage of creep is irrelevant in most engineering applications due to its negligible contribution to deformation. $ \; \; $

4. How does increasing the grain size of a material affect its creep resistance?

a. Increases creep resistance by reducing grain boundary sliding $ \; \; $

b. Decreases creep resistance by reducing dislocation density $ \; \; $

c. Has no effect on creep resistance $ \; \; $

d. Increases creep rate due to higher diffusivity $ \; \; $

5. What is the primary reason for tertiary creep leading to failure?

a. Strain hardening increases excessively $ \; \; $

b. Recovery mechanisms dominate completely $ \; \; $

c. Microstructural damage and necking occur $ \; \; $

d. Thermal expansion causes fracture $ \; \; $

6. In which scenario is diffusional creep the dominant mechanism?

a. Low temperatures and high stress $ \; \; $

b. High temperatures and low stress $ \; \; $

c. High stress and low temperatures $ \; \; $

d. Moderate stress and grain size refinement $ \; \; $

7. Which factor most significantly increases the rate of steady-state creep?

a. Increasing applied stress $ \; \; $

b. Decreasing temperature $ \; \; $

c. Decreasing grain size $ \; \; $

d. Decreasing stress exponent $ \; \; $

8. What is the role of alloying elements like molybdenum in creep-resistant alloys?

a. Increase grain size for creep resistance $ \; \; $

b. Improves oxidation resistance $ \; \; $

c. Improves creep strength by forming stable carbides at grain boundary and within the grain $ \; \; $

d. Both b & c $ \; \; $

9. What is the defining characteristic of materials that experience creep rupture under constant load?

a. Fracture occurs with zero plastic strain $ \; \; $

b. Fracture occurs after a predictable time at a given temperature $ \; \; $

c. Fracture occurs only under cyclic loading $ \; \; $

d. Fracture is independent of stress or temperature $ \; \; $

10. A cylindrical bar made of a material with the following properties is subjected to a constant tensile load of 20 kN at a temperature of 550°C for 1000 hours. Determine the total creep strain after 1,000 hours. The bar has a diameter of 20 mm and the following creep constants:

Primary Creep Strain: $ \epsilon _{p}= A. t^{n} $, where $ A= 1.5 \times 10^{-5} $, t in hours, n=0.3

Secondary Creep Rate: $ \dot{\epsilon}_{s} = B. \sigma^{m} $ where $ B= 2 \times 10^{-10}$, m=3.

Neglect Tertiary Creep

a. 0.052 $ \; \; $

b. 0.3 $ \; \; $

c. 0.4 $ \; \; $

d. 0.2 $ \; \; $

11. What is the onset temperature of creep for metals?

a. $ \geq 0.4 \times Melting \, Point \, Temperature(Kelvin) $. $ \; \; $

b. $ \geq 0.2 \times Melting \, Point \, Temperature(Kelvin) $. $ \; \; $

c. $ \geq 0.1 \times Melting \, Point \, Temperature(Kelvin) $. $ \; \; $

d. None of the above $ \; \; $

12. Whether creep occurs at high temperatures only?

a. Yes $ \; \; $

b. No $ \; \; $

13. A 2-inch × 2-inch iron bar should operate for 5 years at $ \, 600^\circ C \, $. What is the maximum allowable load that can be applied to avoid rupture of iron bar due to creep? (Refer enclosed graph)

Larson miller parameter graph

a. 3000 Pounds $ \; \; $

b. 5200 Pounds $ \; \; $

c. 1000 Pounds $ \; \; $

d. 500 Pounds $ \; \; $

14. A turbine blade operates at a high temperature where creep is significant. Considering the Larson-Miller parameter (LMP) for a material is given by $LMP= T(C+log(t_{r})$ where T is the absolute temperature in Kelvin, $t_{r} \, $ is the time to rupture in hours, and C is a constant. If the temperature increases by 100 K and C remains constant, how will $t_{r}$ change? Assume LMP remains constant.

a. $ t_{r} \, $ decreases significantly $ \; \; $

b. $ t_{r} \, $ increases significantly $ \; \; $

c. $ t_{r} \,$ remains constant $ \; \; $

d. The effect depends on the value of C $ \; \; $

15. Which of the following mechanisms is primarily responsible for the failure of metals due to creep at high temperatures?

a. Grain boundary sliding and void formation $ \; \; $

b. Brittle fracture at low stress levels $ \; \; $

c. Rapid work hardening $ \; \; $

d. Corrosion-assisted cracking $ \; \; $

16. Find the total elongation of turbine blade which is subjected to a tensile stress of 80 MPa at $450 ^\circ C \, $ for 10,000hrs. The initial length of turbine blade is 1000mm. The instantaneous and primary creep elongations are 2mm. Assume the steady state creep strain rate is $5 \times 10 ^{-7} h^{-1} \, $. Tertiary creep elongation is negligible as compared to primary and secondary.

a. 4mm $ \; \; $

b. 6mm $ \; \; $

c. 7mm $ \; \; $

d. 10mm $ \; \; $

17. Find the activation energy for creep for Nickel alloy rod having the steady state creep behaviour as shown in the following figure. The stress exponent ‘n’ and constant ‘K’ is independent of temperature.

a. 78787 J/mol $ \; \; $

b. 857680 J/mol $ \; \; $

c. 898898 J/mol $ \; \; $

d. 676656J/mol $ \; \; $

18. For the above problem (Question no.17) find the steady state creep rate at $ 850 ^\circ C \, $ 400 MPa stress.

a. $1.99 \times 10^{-8} h^{-1} $ $ \; \; $

b. $3.2 \times 10^{-8} h^{-1} $ $ \; \; $

c. $0.9 \times 10^{-8} h^{-1} $ $ \; \; $

d. $5 \times 10^{-8} h^{-1} $ $ \; \; $

19. Above what temperature, creep behavior is important for Stainless steel of grade 304L?

a. $ 417 ^\circ C \, $ $ \; \; $

b. $ 300 ^\circ C \, $ $ \; \; $

c. $ 350 ^\circ C \, $ $ \; \; $

d. $ 290 ^\circ C \, $ $ \; \; $

20. Whether creep occurs only in metals?

a. Yes $ \; \; $

b. No $ \; \; $

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Understanding Creep Failure: A Quiz on Creep Failure in Metals