# Mechanical Behaviour Questions and Answers – Mechanism of Brittle Fracture

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This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Mechanism of Brittle Fracture”.

1. In which type of brittle fracture, crack travels through grain boundaries?
a) Trans granular
b) Intra granular
c) Intergranular
d) Fibrous

Explanation: In intergranular fracture, crack propagates through grain boundary. It is because of weaker grain boundaries. In Trans granular, crack passes through grains.

2. Which theory explains brittle fracture?
a) Griffith
b) Cottrell
c) Taylor
d) Dislocation

Explanation: Griffith extensively studied brittle fracture. He proposed a mechanism to explain it. He did it using theory of microcracks.

3. What is the expression for maximum stress at the tip of crack?
a) σmax = 2 σ (ar)1/2
b) σmax = 2 σ (a/r) 1/2
c) σmax = 2 σar
d) σmax = 2 (σar)1/2

Explanation: Maximum stress at the crack tip is given by σmax = 2 σ(a/r)1/2. Here crack length is 2a. σ is applied stress normal to crack length and r is radius of curvature of the crack tip.

4. Elastic strain energy is proportional to ____________
a) (a)1/2
b) a
c) a2
d) a3

Explanation: The elastic strain energy (Ue) for unit volume is given by following relation:
Ue = πσ2a2/E
Here, E is Young’s modulus, σ is applied tensile stress, 2a is crack length.

5. On tensile loading, the elastic energy stored per unit volume will ___ with an increase in load.
a) Increases
b) Decreases
c) Remains constant
d) Increases or decreases

Explanation: The elastic energy stored increases with increase in load. It is clear from the relation:
Ue = πσ2a2/E
Where E is Young’s modulus, σ is applied tensile stress, 2a is crack length.

6. Surface energy associated with crack is proportional to ______
a) a
b) a2
c) a1/2
d) a-1

Explanation: Us = 4ϒsa. Here Us is surface energy. 2a is crack length and ϒs is surface energy per unit area.

7. Surface energy of crack is given by __________
a) 4ϒsa
b) 2ϒs(a) 1/2
c) 2ϒsE
d) 2ϒsa

Explanation: Surface energy related to a crack is given by Us = 4ϒsa. Here ϒs is surface free energy of crack and 2a is crack length.

8. As length of crack increases, surface energy of crack ___
a) Increases
b) Decreases
c) Remains constant
d) May increase or decrease

Explanation: Us = 4ϒsa. Here ϒs is surface free energy of crack and 2a is crack length. Hence, surface energy increases with crack length.

9. Elastic strain energy ___ with increase in crack length.
a) Increases
b) Decreases
c) Remains constant
d) May increase or decrease

Explanation: Elastic strain energy reduces with increase in crack length. It is due to the stress relieving of material close to the surface of the crack.

10. What does plastic deformation do on crack propagation?
a) Lowers surface energy
b) Sharpens the tip
c) Reduces required work
d) Increases concentration of stress

Explanation: Plastic deformation in metallic materials reduces sharpness at the crack tip. It lowers the surface energy by reducing the concentration of stress. It increases required work to propagate crack.

11. Orowan and Irwin theory is given for cracking in brittle materials.
a) True
b) False

Explanation: Orowan and Irwin modified Griffith theory considering plastic deformation. It was for metallic materials.

12. Compressive stresses don’t make a serious problem when applied on brittle materials.
a) True
b) False

Explanation: Cracks tend to close when compressive stresses are applied. These don’t cause failure. In tensile stress, cracks tend to propagate causing failure.

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