This set of Engineering Materials and Metallurgy Question Bank focuses on “Fracture Toughness, Protection against Fracture”.
1. The fracture resistance of a material is defined as its ________
a) Fracture strength
b) Fracture toughness
c) Fracture hardness
d) Fracture resilience
Explanation: When cracks are present, the ability of a material to resist fracture is an important characteristic. The fracture resistance of a material in the presence of cracks and discontinuities is defined as its fracture toughness.
2. What is Gc with reference to Griffith Theory?
a) Stress energy released
b) Strain energy released
c) Energy density
d) Griffith constant
Explanation: Fracture toughness is regarded as the rate of strain energy released per unit area of the crack. Its SI unit is joule per meter square (J m-2) and is denoted as Gc.
3. What is the fracture toughness of aluminum alloy?
a) 2 J m-2
b) 3 J m-2
c) 4 J m-2
d) 6 J m-2
Explanation: Although aluminum has a range of 20-100 J m-2, the true surface energy lays at less than 2 J m-2. This value for polystyrene is about 2 J m-2 even though it is relatively brittle.
4. Polymer materials become brittle below ________
a) Critical temperature
b) Glass transition temperature
c) Melting point temperature
d) Room temperature
Explanation: It is known that polymeric materials become brittle below the glass transition temperature Tg. Molecular activity is stationary below this temperature.
5. Ductile fracture occurs due to _______ stress.
Explanation: Ductile fractures occur with large plastic deformation. This failure is caused due to the effect of shear stress. Brittle fracture, on the other hand, is caused due to direct stress.
6. The rate of crack propagation in brittle fracture is _______
Explanation: Brittle fracture occurs at a point where microcracks are more in number. Here, the rate of crack propagation is rapid. On the other hand, the rate of crack propagation in ductile materials is slow.
7. Which of the following can be applied to prevent ductile fracture?
a) Fine grains
b) Low hardness
c) Low Young’s modulus
d) High-temperature treatment
Explanation: To prevent ductile fracture, the material must have fine grains, a high hardness value, and must not show any defects or dislocations. Additionally, it is preferred that the material has high Young’s modulus and cohesive energy.
8. Which of the following is not applicable for prevention of fatigue fracture?
a) Reduction of corrosive environment
b) Coarse grain structure
c) No residual stresses
d) Good surface finish
Explanation: To prevent fatigue fracture, a good design must be used to avoid stresses, and must have no residual stresses or dislocations. It must have a fine grain structure and a control over the surface finish. Special care must be taken to effect of corrosion by polishing and coating the material.
9. Pyroceram has excellent thermal resistance due to its ________
a) Surface finish
b) Melting point
c) Ion exchange
d) Grain size
Explanation: Pyroceram is otherwise known as crystallized glass. It has excellent thermal and mechanical shock resistance due to its fine grain size. The grain size found in pyroceram is about 0.1 μm.
10. __________ can be added to steels to prevent intergranular failure.
Explanation: Intergranular failure occurs due to the continuous brittle phase at the boundary. In steels, the presence of sulfur creates a brittle iron sulfide film. This can be prevented by the addition of manganese in steel.
11. Which of the following must not be used to prevent fatigue fracture?
d) Shot peening
Explanation: Polishing of the surface removes surface irregularities, while shot peening introduces compressive stress, both of which increases the fatigue strength. Carburizing and nitriding increase shock resistance of creation of cracks. On the other hand, decarburizing lowers the fatigue resistance by producing a soft surface layer.
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