Fatigue and Fracture Mechanics MCQs Aeronautical Engineering)

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What is fatigue in materials science?
A) The gradual weakening of a material due to high temperatures
B) The process of material failure due to cyclic loading
C) The failure of a material due to impact loading
D) The corrosion of material due to environmental exposure
Answer: B) The process of material failure due to cyclic loading
2. Which of the following is a key characteristic of fatigue failure?
A) Occurs after a single load application
B) Results from prolonged exposure to high temperatures
C) Occurs suddenly without prior warning
D) Results from low cycle but high stress amplitude
Answer: C) Occurs suddenly without prior warning
3. What is the most common method used to represent the fatigue life of a material?
A) S-N Curve (Stress-Number of cycles curve)
B) T-T Curve (Time-Temperature curve)
C) P-H Curve (Pressure-Hardness curve)
D) L-D Curve (Load-Displacement curve)
Answer: A) S-N Curve (Stress-Number of cycles curve)
4. What does the endurance limit represent in a fatigue test?
A) The maximum stress the material can withstand before plastic deformation
B) The stress level below which a material can endure an infinite number of cycles without failure
C) The minimum stress required to cause failure in a single cycle
D) The maximum stress a material can withstand at high temperatures
Answer: B) The stress level below which a material can endure an infinite number of cycles without failure
5. Which factor does NOT significantly affect fatigue life?
A) Surface finish
B) Grain size
C) Load frequency
D) Ambient humidity
Answer: D) Ambient humidity
6. Which of the following describes the Paris-Erdogan law in fracture mechanics?
A) It relates the stress intensity factor to the rate of crack growth.
B) It describes the relationship between stress and strain in plastic deformation.
C) It is used to predict the endurance limit of materials.
D) It relates the creep rate to temperature and stress.
Answer: A) It relates the stress intensity factor to the rate of crack growth.
7. What is the main purpose of shot peening in improving fatigue life?
A) To increase the hardness of the material
B) To reduce the weight of the material
C) To introduce compressive residual stresses on the surface
D) To improve the thermal conductivity of the material
Answer: C) To introduce compressive residual stresses on the surface
8. What type of fracture is most commonly associated with fatigue failure?
A) Brittle fracture
B) Ductile fracture
C) Intergranular fracture
D) Transgranular fracture
Answer: A) Brittle fracture
9. What is the primary mode of crack propagation in fatigue failure?
A) Mode I (Opening mode)
B) Mode II (Sliding mode)
C) Mode III (Tearing mode)
D) Mode IV (Combined mode)
Answer: A) Mode I (Opening mode)
10. In fracture mechanics, what does the stress intensity factor (K) signify?
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– A) The toughness of the material
– B) The intensity of the stress field near the crack tip
– C) The maximum load the material can withstand
– D) The rate of crack growth
– **Answer:** B) The intensity of the stress field near the crack tip
11. What does the term “fracture toughness” refer to?
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– A) The ability of a material to resist deformation under high loads
– B) The energy required to propagate a crack
– C) The resistance of a material to crack propagation
– D) The ability of a material to absorb energy before fracture
– **Answer:** C) The resistance of a material to crack propagation
12. Which test is commonly used to determine fracture toughness?
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– A) Charpy impact test
– B) Vickers hardness test
– C) Compact tension (CT) test
– D) Brinell hardness test
– **Answer:** C) Compact tension (CT) test
13. In the context of fracture mechanics, what is a “stress concentration”?
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– A) A point in the material where stress is uniformly distributed
– B) A localized increase in stress due to geometric discontinuities
– C) The average stress applied to a material
– D) The stress at which a material yields
– **Answer:** B) A localized increase in stress due to geometric discontinuities
14. What is the significance of the critical crack length in fracture mechanics?
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– A) It represents the length at which a crack becomes visible.
– B) It is the length at which a crack will propagate without additional loading.
– C) It is the maximum length a crack can reach before the material yields.
– D) It is the length beyond which a material can no longer withstand fatigue.
– **Answer:** B) It is the length at which a crack will propagate without additional loading.
15. What is the role of crack closure in fatigue crack growth?
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– A) It accelerates crack propagation.
– B) It retards crack propagation.
– C) It has no effect on crack growth.
– D) It changes the direction of crack growth.
– **Answer:** B) It retards crack propagation.
16. What is a “Paris Law” in the context of fatigue crack growth?
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– A) A law describing the yield strength of materials.
– B) A law relating crack growth rate to the range of stress intensity factor.
– C) A law describing the plastic deformation of materials.
– D) A law relating crack closure effects to the fatigue life.
– **Answer:** B) A law relating crack growth rate to the range of stress intensity factor.
17. Which of the following materials typically has the highest fracture toughness?
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– A) Glass
– B) High-strength steel
– C) Ceramic
– D) Aluminum alloy
– **Answer:** B) High-strength steel
18. In which stage of fatigue life does crack initiation occur?
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– A) Early life stage
– B) Crack propagation stage
– C) Final fracture stage
– D) None of the above
– **Answer:** A) Early life stage
19. Which of the following is NOT a typical method for detecting fatigue cracks?
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– A) Ultrasonic testing
– B) X-ray diffraction
– C) Magnetic particle inspection
– D) Visual inspection
– **Answer:** B) X-ray diffraction
20. What does the term “fatigue strength” refer to?
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– A) The maximum stress a material can endure for a specified number of cycles without failure.
– B) The stress level at which the material fails after a single load application.
– C) The load at which a material undergoes plastic deformation.
– D) The stress level required to propagate a pre-existing crack.
– **Answer:** A) The maximum stress a material can endure for a specified number of cycles without failure.
21. What is the significance of the Griffith criterion in fracture mechanics?
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– A) It determines the stress at which yielding occurs.
– B) It relates the crack length to the critical stress required for crack propagation.
– C) It describes the rate of crack growth under cyclic loading.
– D) It predicts the onset of fatigue in materials.
– **Answer:** B) It relates the crack length to the critical stress required for crack propagation.
22. Which of the following factors does NOT contribute to fatigue crack initiation?
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– A) Surface roughness
– B) Residual stresses
– C) Load frequency
– D) Environmental corrosion
– **Answer:** C) Load frequency
23. What is the main effect of surface roughness on fatigue life?
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– A) It increases the endurance limit.
– B) It decreases the fatigue strength.
– C) It has no effect on fatigue life.
– D) It improves the fracture toughness.
– **Answer:** B) It decreases the fatigue strength.
24. Which of the following is a common method for improving the fatigue life of a material?
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– A) Reducing the number of load cycles
– B) Increasing the operating temperature
– C) Surface hardening
– D) Using a lower strength material
– **Answer:** C) Surface hardening
25. What does the term “low cycle fatigue” refer to?
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– A) Fatigue occurring at high stress levels and low number of cycles
– B) Fatigue occurring at low stress levels and high number of cycles
– C) Fatigue occurring due to environmental factors
– D) Fatigue occurring only in high-strength materials
– **Answer:** A) Fatigue occurring at high stress levels and low number of cycles
26. Which of the following is true about high cycle fatigue?
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– A) It occurs at high stress levels and low number of cycles.
– B) It occurs at low stress levels and high number of cycles.
– C) It is independent of stress levels.
– D) It only occurs in brittle materials.
– **Answer:** B) It occurs at low stress levels and high number of cycles.
27. Which term is used to describe the failure of materials due to repeated stress below the yield strength?
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– A) Creep
– B) Fracture
– C) Fatigue
– D) Plastic deformation
– **Answer:** C) Fatigue
28. What is the primary cause of crack growth during fatigue?
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– A) Static loading
– B) Cyclic loading
– C) Environmental exposure
– D) High temperature
– **Answer:** B) Cyclic loading
29. Which of the following can significantly reduce the fatigue life of a material?
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– A) Smooth surface finish
– B) High tensile residual stress
– C) Low stress amplitude
– D) Homogeneous material structure
– **Answer:** B) High tensile residual stress
30. What is the role of “notches” in fatigue failure?
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– A) They increase the fatigue strength.
– B) They act as stress concentrators, reducing fatigue life.
– C) They reduce the overall stress in the material.
– D) They have no effect on fatigue life.
– **Answer:** B) They act as stress concentrators, reducing fatigue life.
31. Which of the following is NOT a stage in fatigue failure?
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– A) Crack initiation
– B) Crack growth
– C) Stable crack propagation
– D) Final fracture
– **Answer:** C) Stable crack propagation
32. What does the term “fracture mechanics” refer to?
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– A) The study of the mechanisms of material deformation
– B) The study of the mechanisms and effects of crack propagation in materials
– C) The analysis of material behavior under high temperatures
– D) The study of surface interactions in materials
– **Answer:** B) The study of the mechanisms and effects of crack propagation in materials
33. In which part of an S-N curve does the fatigue life typically become infinite?
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– A) Near the fatigue limit
– B) At the high-stress region
– C) At the low-cycle region
– D) At the high-cycle region
– **Answer:** A) Near the fatigue limit
34. Which factor is typically responsible for initiating cracks in materials under cyclic loading?
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– A) Plastic deformation
– B) Surface defects
– C) High temperatures
– D) Environmental conditions
– **Answer:** B) Surface defects
35. What is the main purpose of using a “Wöhler curve” (S-N curve)?
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– A) To determine the toughness of materials
– B) To relate the number of cycles to failure to the applied stress amplitude
– C) To find the critical crack length
– D) To predict the yield strength of materials
– **Answer:** B) To relate the number of cycles to failure to the applied stress amplitude
36. What effect does increasing the load frequency have on fatigue life, assuming all other factors remain constant?
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– A) Increases fatigue life
– B) Decreases fatigue life
– C) Has no effect on fatigue life
– D) Improves fracture toughness
– **Answer:** B) Decreases fatigue life
37. Which of the following is NOT a common characteristic of fatigue failure?
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– A) Occurs at stress levels below the material’s yield strength
– B) Often begins at surface imperfections
– C) Is influenced by the environmental conditions
– D) Always occurs in a single cycle
– **Answer:** D) Always occurs in a single cycle
38. What does “KIC” represent in fracture mechanics?
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– A) Fracture toughness
– B) Yield strength
– C) Endurance limit
– D) Fatigue limit
– **Answer:** A) Fracture toughness
39. Which of the following factors is most likely to increase the fatigue life of a material?
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– A) Increasing the stress amplitude
– B) Introducing compressive residual stresses
– C) Increasing the load frequency
– D) Roughening the surface
– **Answer:** B) Introducing compressive residual stresses
40. What is the main mechanism of crack growth in high-cycle fatigue?
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– A) Plastic deformation
– B) Brittle fracture
– C) Cyclic slip
– D) Thermal expansion
– **Answer:** C) Cyclic slip
41. In fracture mechanics, what is the role of “crack tip plasticity”?
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– A) It increases the crack growth rate.
– B) It reduces the stress intensity factor.
– C) It shields the crack tip from the applied stress.
– D) It accelerates the final fracture.
– **Answer:** C) It shields the crack tip from the applied stress.
42. Which of the following materials would most likely exhibit brittle fracture?
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– A) Mild steel
– B) High-strength aluminum alloy
– C) Polymers
– D) Ceramics
– **Answer:** D) Ceramics
43. Which of the following describes the “stress ratio” (R) in fatigue testing?
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– A) Ratio of minimum stress to maximum stress in a cycle
– B) Ratio of maximum stress to minimum stress in a cycle
– C) Ratio of applied load to material yield strength
– D) Ratio of crack length to material thickness
– **Answer:** A) Ratio of minimum stress to maximum stress in a cycle
44. Which of the following is NOT a method for detecting fatigue cracks?
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– A) Ultrasonic testing
– B) Radiography
– C) Magnetic particle inspection
– D) Thermal conductivity measurement
– **Answer:** D) Thermal conductivity measurement
45. Which factor is most important in determining the fatigue strength of a material?
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– A) Grain size
– B) Surface finish
– C) Load frequency
– D) Material density
– **Answer:** B) Surface finish
46. What is “crack initiation” in the context of fatigue?
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– A) The stage where a crack begins to propagate rapidly
– B) The first stage of fatigue failure where micro-cracks form
– C) The final stage of fatigue failure
– D) The stage where a crack becomes detectable by non-destructive testing
– **Answer:** B) The first stage of fatigue failure where micro-cracks form
47. What does the term “fracture toughness” measure?
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– A) The ability of a material to resist crack initiation
– B) The ability of a material to absorb energy during fracture
– C) The resistance of a material to crack propagation
– D) The hardness of a material
– **Answer:** C) The resistance of a material to crack propagation
48. Which of the following techniques is used to retard fatigue crack growth?
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– A) Increasing the load amplitude
– B) Inducing compressive residual stresses
– C) Reducing the material’s hardness
– D) Increasing the operating temperature
– **Answer:** B) Inducing compressive residual stresses
49. Which of the following is a primary advantage of using fracture mechanics in engineering?
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– A) It predicts the exact time of failure.
– B) It provides a detailed understanding of elastic deformation.
– C) It allows for the safe design of structures with pre-existing flaws.
– D) It improves the thermal properties of materials.
– **Answer:** C) It allows for the safe design of structures with pre-existing flaws.
50. Which of the following is the most effective method to improve fatigue resistance in metals?
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– A) Annealing
– B) Polishing the surface
– C) Cold working
– D) Applying thermal insulation
– **Answer:** B) Polishing the surface

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