# Mechanical Behaviour Questions and Answers – The Shear Strength of Ideal and Real Crystals

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This set of Mechanical Behaviour Questions and Answers for Freshers focuses on “The Shear Strength of Ideal and Real Crystals”.

1. The internal energy of crystal ___________ if atoms are displaced from their mean position.
a) Rises
b) Decreases
c) Remains constant
d) Varies independently

Explanation: In a perfect crystal all atoms occupy equilibrium positions. This arrangement possesses minimum energy. This energy increases with the displacement of atoms from these positions.

2. In a perfect crystal, shear stress is __________
a) Very high
b) Negative
c) Zero
d) Very low

Explanation: Shear stress is the component of stress. In a perfect crystal, not displacement takes place. So shear stress is zero.

3. ____ stress increases from the force vector component which is parallel to the material cross section.
a) Normal
b) Shear
c) Tensile
d) Yield

Explanation: Shear stress is denoted τ. It arises from the force vector component which is parallel to the cross section. Normal stress (σ) arises from the force vector component perpendicular to the cross section.

4. What is SI unit of shear stress?
a) Pa
b) N
c) psi
d) gm.cm-1s-2

Explanation: Shear stress his ratio of the force applied to the cross sectional area. So the unit is N/m2 or Pa. psi is avoirdupois unit.

5. Which symbol is used to denote shear stress?
a) σ
b) τ
c) ε
d) e

Explanation: Shear stress is denoted as τ. Normal stress is denoted by σ. True strain and engineering strain is denoted by ε and e.

6. What is a theoretical shear strength of perfect crystal?
a) G
b) G/2π
c) G/4π
d) G/8π

Explanation: The maximum shear stress required for plastic deformation in a perfect crystal is G/2π. It is known as theoretical shear strength of perfect crystal. Here G is shear modulus.

7. What is the range of theoretical shear strength of ductile metals?
a) 1 to 10 GPa
b) 2 to 20 GPa
c) 10 to 100 GPa
d) 20 to 200 GPa

Explanation: Theoretical shear strength is given by G/2π. Shear modulus (G) of ductile metals is 10 to 100 GPa. It gives the theoretical strength of 2 to 20 GPa.

8. What is a theoretical shear strength of brass perfect crystal?
a) 5 GPa
b) 6 GPa
c) 8 GPa
d) 10 GPa

Explanation: Shear modulus of brass is 40 GPa. By using G/2π, theoretical shear strength comes out to be 6 GPa.

9. Shear stress is a function of the displacement.
a) True
b) False

Explanation: Shear stress changes with increasing displacement of atoms. This displacement is due to external energy. This energy may be in form of thermal energy or mechanical work.

10. Real crystals deform at theoretical shear strength.
a) True
b) False 