This set of Mechanical Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Metallographic Aspect of Fracture”.
1. The necking begins when the ___________
a) true stress and engineering stress becomes equal
b) true strain and engineering strain becomes equal
c) the true strain becomes equal to the strain hardening coefficient
d) strain hardening coefficient becomes equal to an engineering strain
View Answer
Explanation: When the maximum load or the true strain becomes equal to the strain hardening coefficient, the necking begins.
2. The formation of necking produces the state of _________________
a) triaxle stress
b) biaxial stress
c) uniaxial stress
d) pure shear stress
View Answer
Explanation: The formation of necking produces the triaxle state of stress. The hydrostatic component of tension act along the axis of the specimen at the center of the necked region.
3. The crack growth in ductile material is by method of ______________
a) shearing
b) void coalescence
c) dislocation movement
d) impurity segregation
View Answer
Explanation: In ductile fracture, the necked region forms the range of narrow microvoids. The void merges and coalescence together to break apart the material finally. It occurs by elongation of void and bridging the gap between the voids.
4. The main cause of presence of dimple in ductile fracture surface is ___________
a) slip bands
b) impurity atoms
c) micro voids
d) corrosion
View Answer
Explanation: The micro void is formed during necking, which then merges and coalescences. After breaking apart, it leaves the material with a rough surface of the void cavity, which is known as dimples.
5. The preferred sites for void formation are inclusions, second-phase particles, or the fine oxide particles, while in pure metals it is grain-boundary triple points.
a) True
b) False
View Answer
Explanation: The void formation is much easier on these second phase particles compared to metal matrix regular position. This is because it requires less energy for the structure of void in these heterogeneous sites.
6. The ductility of true strain values _____________ with increase in second phase particles.
a) increases
b) decrease
c) remains constant
d) no relationship
View Answer
Explanation: The presence of a second phase particle increases the strength of material, but they provide a site for void formation, which leads to ductile failure earlier than the pure metal.
7. The presence of second phase particle reduces the ductility of the material. The type of particle which has the least effect in lowering the ductility is __________
a) spherical precipitates
b) platelike precipitates
c) rodlike precipitates
d) acicular precipitates
View Answer
Explanation: The shearing of the spherical particle is most difficult, and they do not produce any region of high-stress concentration. Also, cross-slip is more straightforward in spherical particles.
8. In the flow curve equation given below, if the value of strain hardening exponent is increased, what will be the effect on the ductility of material?
σ=Kεn; Where n is strain hardening exponent.
a) It increases
b) It decreases
c) It remains constant
d) Not dependent on n
View Answer
Explanation: The strain hardening exponent has a relationship with the ductility that is necking begins when true strain is equal to strain hardening exponent. So if the value of n is increased, the true strain at necking is also increased. So ultimately the ductility is improved.
9. The presence of notch in material will _______________ the ductile to brittle transition temperature of the material.
a) increase
b) decrease
c) remain constant
d) not dependent on notch
View Answer
Explanation: The notch creates a region of high stress and condition of triaxial stress. The plastic deformation starts to begin at the root of the notch and leads to brittle failure in the material if applied stress is sufficiently high.
10. The Ductile fracture depends strongly on strain rate compared with brittle fracture, which is relatively insensitive to strain rate.
a) True
b) False
View Answer
Explanation: The brittle fracture is strongly dependent on the strain rate because of its nature of failure, which is sudden without any warning. Whereas ductile fracture is insensitive to strain rate.
Sanfoundry Global Education & Learning Series – Mechanical Metallurgy.
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