# Mechanical Behaviour Questions and Answers – Elastic Properties of Dislocations

This set of Basic Mechanical Behaviour Questions and Answers focuses on “Elastic Properties of Dislocations”.

1. What is the order of core energy?
a) 0.5 – 1.5 eV
b) 0.1 – 1.0 eV
c) 2.0 – 4.0 eV
d) 3.0 – 10 eV

Explanation: Core energy contributes 10% to the total energy due to dislocation. It is of the order of 0.5 to 1.5 eV. In the core region, distortion is maximum.

2. Which statement is true regarding screw and edge dislocation?
a) Screw and edge dislocation both show non-symmetry
b) Screw and edge dislocation both are equally symmetric
c) Symmetry of edge dislocation is greater than screw dislocation
d) Symmetry of screw dislocation is greater than edge dislocation

Explanation: In edge dislocation atoms above slip plane, are in compression state and below in tension state. Therefore, it shows less symmetry than screw dislocation.

3. What is the ratio of elastic strain energy per unit length of an edge dislocation and screw dislocation?
a) 1:1
b) 1:2
c) 2:3
d) 3:2

Explanation: Elastic strain energy of edge dislocation and screw dislocation is 3/2 IGb2 and IGb2. Here G is shear modulus, l is length and b is Burgers vector.

4. Elastic strain energy of screw dislocation is ____ of that of edge dislocation for unit length?
a) Higher
b) Lesser
c) Equal
d) Half

Explanation: Elastic strain energy of screw dislocation is lesser than that of edge dislocation for unit length. It is 2/3 times than that of edge dislocation. That’s why screw dislocations are more stable.

5. Elastic energy of a dislocation is _____ the Burgers vector.
a) Directly proportional to
b) Proportional to the square of
c) Not dependent on
d) Proportional to the square root of

Explanation: Elastic energy of a dislocation is proportional to the square of Burgers vector. It is represented by Frank’s rule. Which is Eelastic ∝ b2.
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6. The stability of dislocations ____ with an increase in the Burgers vector.
a) Increases
b) Decreases
c) Remains constant
d) Increases then decrease

Explanation: From Frank’s rule, Eelastic ∝ b2. Thus, with an increase in Burger’s vector, the energy of dislocation increases. Hence, the stability of dislocation decreases.

7. What is the expression for the force required to bend the dislocation to radius R?
a) Gb/2R
b) Gb/R
c) GbR
d) Gb2/R

Explanation: The force required to bend the dislocation is τ = Gb/2R. Here G is shear modulus, b is Burgers vector and R is the radius.

8. What is the percentage contribution of elastic strain in the total energy of dislocation?
a) 10%
b) 30%
c) 70%
d) 90%

Explanation: The region outside the core gives rise in elastic strain energy. Its contribution to the total energy is 90%. While core energy contributes only 10%.

9. Elastic strain energy _____ with an increase in length.
a) Increases
b) Decreases
c) Remains constant
d) Increases then decrease

Explanation: Eelastic = lGb2. Where l is length, G is shear modulus and b is Burgers vector. So elastic energy is proportional to length. Strain energy is generally expressed for unit length.

10. Line tension (T) is _____
a) ∝ b
b) ∝ b1/2
c) ∝ b2
d) ∝ b3

Explanation: Line tension is given as T = Gb2/2
Here, G is shear modulus and b is Burgers vector. So line tension associated with dislocation is proportional to the square of Burgers vector.

11. A dislocation can avoid obstacles by a glide.
a) True
b) False

Explanation: A dislocation can’t glide further when encounters obstacles. It can cross slip or climb. Thus it can change the slip plane.

12. As a distance between precipitate particles increases, shear stress to bend dislocation decreases.
a) True
b) False

Explanation: Stress required to bend dislocation is given as τ = Gb sinϴ/l. Here b is Burgers vector and G is the shear modulus of the material. As a distance between particles (l) increases, the shear stress required to bend dislocation decreases.

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