# Mechanical Behaviour Questions and Answers – Forces Between Dislocations

This set of Tricky Mechanical Behaviour Questions and Answers focuses on “Forces Between Dislocations”.

1. Dislocations of opposite sign on the _____ slip plane will _____ each other.
a) same, attract
b) same, repel
c) different, attract
d) different, repel
View Answer

Answer: a
Explanation: Dislocations of opposite sign on the same slip plane will attract each other. These will run together. Finally, these will annihilate each other.

2. The force of attraction or repulsion _____ with decreasing distance of separation.
a) increases
b) decreases
c) remains constant
d) first increases and then decreases
View Answer

Answer: a
Explanation: F reputation = Gb2/2π (1-v) r
F attraction = – Gb2/2π (1-v) r
Here G is shear modulus, b is Burgers vector, r is a distance of separation and v is Poisson’s ratio. Here in both cases, force is inversely proportional to separation distance.

3. Attraction force between dislocations of opposite sign is _____
a) ∝ b
b) ∝ b1/2
c) ∝ b2
d) ∝ b3
View Answer

Answer: c
Explanation: Attraction force is given by F (a) = – Gb2/2π (1-v) r Here G is shear modulus, b is Burgers vector, r is distance of separation and v is Poisson’s ratio. Here force is proportional to square of Burgers vector.
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4. Edge dislocations have _____ stress field therefore _____ interactions can occur between parallel edge dislocations.
a) symmetrical, a number of
b) symmetrical, less number of
c) unsymmetrical, a number of
d) unsymmetrical, less number of
View Answer

Answer: c
Explanation: Edge dislocations show unsymmetrical stress field. Because above slip plane atoms are in compression state and below in tension state. This stress gives a number of interactions between parallel edge dislocations.

5. Which statement is false?
a) Two edge dislocations of the same sign on parallel slip planes repel each other if ϴ<45°
b) Two edge dislocations of the same sign on parallel slip planes attract each other if ϴ<45°
c) Dislocations of opposite sign on the same slip plane attract each other
d) Dislocations of same sign on the same slip plane repel each other
View Answer

Answer: b
Explanation: Edge dislocations of same sign on parallel slip planes attract each other if ϴ>45°. It is due to the interaction of their strain fields. It will repel if ϴ<45°.

6. The nature of the interaction of dislocations doesn’t depend on _____
a) relative position of dislocations
b) burgers vector
c) character of dislocations
d) source of dislocation
View Answer

Answer: d
Explanation: The nature of interaction means whether attraction or repulsion. It depends on a relative position of dislocations, Burgers vector and character of dislocation. It doesn’t depend on a source of dislocation.

7. Annihilation due to the attraction between dislocations lying on parallel slip plane gives either ____ or _____
a) void, interstitialcy
b) pore, interstitialcy
c) interstitialcy, vacancy row
d) crack, vacancy row
View Answer

Answer: c
Explanation: Edge dislocations of opposite sign attract each other. Due to the action of force, these tend to annihilate. These leave either row of vacancies or interstitialcy.
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8. What is the ratio of elastic strain energies, when two parallel edge dislocations of same sign on the same plane are separated farther apart to when they are very close together?
a) 1:1
b) 1:2
c) 2:3
d) 2:1
View Answer

Answer: b
Explanation: When dislocations are very close together, they can be approximated as a single dislocation of strength 2b. The elastic energy thus becomes twice that of the dislocations when they are large distance separated.

9. Screw dislocations always attract each other.
a) True
b) False
View Answer

Answer: b
Explanation: Screw dislocations may repel or attract each other based on stress fields. Screw dislocations of opposite sign having the same Burgers vector attract. While dislocations of the same sign repel each other.
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10. From annihilation of dislocations, total energy of system _____
a) increases
b) decreases
c) remains constant
d) first increases and then decreases
View Answer

Answer: b
Explanation: Dislocations of opposite sign attract each other and annihilate. This reduces the elastic strain energy associated to pair. Hence the total energy of the system reduces.

11. Screw dislocations of opposite sign having same Burgers vector ____ each other and two parallel edge dislocations of same sign on same plane ____ each other.
a) attract, attract
b) attract, repel
c) repel, attract
d) repel, repel
View Answer

Answer: b
Explanation: Screw dislocations of opposite sign having same Burgers vector will attract. It will attract with a force per unit length of Gb2/2πr. Two parallel edge dislocations having same sign rest on the same plane repel each other.

12. The nature of force between dislocations depends on the relative position of dislocations.
a) True
b) False
View Answer

Answer: a
Explanation: Two edge dislocations of the same sign resting on parallel slip planes ϴ<45°, repel each other. If ϴ>45°, these attract each other.

Sanfoundry Global Education & Learning Series – Mechanical Behaviour & Testing of Materials.

To practice tricky questions and answers on all areas of Mechanical Behaviour, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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