# Mechanical Behaviour Questions and Answers – Fracture Toughness

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

1. Which of the following mode is the sliding mode?

a) Mode I
b) Mode II
c) Mode III
d) Mode IV

Explanation: Mode I is opening or tensile mode. Mode II is sliding mode. Mode III is tearing mode.

2. Which of the following mode is tearing mode?

a) Mode I
b) Mode II
c) Mode III
d) Mode IV

Explanation: Mode I is opening or tensile mode. Mode II is sliding mode. Mode III is tearing mode. Mode I loading is most common for structural components.

3. What is the fracture toughness of alumina in MPa√m?
a) 1.26
b) 1.76
c) 1.86
d) 1.98

Explanation: Alumina has low fracture toughness. KIC for it is 1.76 MPa√m.

4. Which of the following has the highest fracture toughness?
a) Marageing steel
b) Alumina
c) Silicon nitride
d) PSZ

Explanation: Marageing steel has a maximum fracture toughness of the four options. It has the fracture toughness of 90 MPa√m. Alumina, Silicon nitride and PSZ have a fracture toughness of 1.76, 5 and 11 MPa√m respectively.

5. What is the fracture toughness of 4340 steel in MPa√m?
a) 50
b) 88
c) 46
d) 99

Explanation: 4340 steel has a fracture toughness of 46 MPa√m. 50 and 88 MPa√m is toughness of Ni-Cr steel. Ti-6 Al-5V has a toughness of 99 MPa√m.
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6. ___________ metals fracture toughness is unaffected from temperature increase.
a) SC
b) BCC
c) FCC
d) HCP

Explanation: Increasing temperature usually increases the fracture toughness of BCC and HCP metals. In FCC metals, fracture toughness remains unaffected by temperature.

7. Which type of loading is most common and important for structural components?

a) Mode I
b) Mode II
c) Mode III
d) Mode IV

Explanation: Mode I loading is most common for structural components. Mode II is less frequently used. Mode III is also rare.

8. All the crack tip stresses are __________ to K.
a) Equal
b) Greater
c) Lesser
d) Proportional

Explanation: K is a parameter that measures the magnitude of the stress field. It is called stress intensity parameter. All crack tip stresses are proportional to K.

9. What is the unit of fracture toughness?
a) MPa
b) MPa√m
c) MPam
d) MPam2

Explanation: Fracture toughness has units MPa√m. Its formula is given as KIC = ϒσf √(πa).

10. Largest size flaw can be determined using __________
a) Brinell test
b) NDT
c) Leeb test
d) Shore test

Explanation: Largest size flaw can be determined using NDT. X-ray radiography or ultrasound testing can be used. Other tests above gives hardness.

11. The larger flaws reduce the allowable stress.
a) True
b) False

Explanation: The material’s ability to resist crack growth depends on the factors. The larger flaws reduce the allowable stress.

12. Increase in yield strength increases fracture toughness of a material.
a) True
b) False

Explanation: Increasing yield strength decreases the fracture toughness of a material. It also decreases the allowable flaw size. It is due to lower ductility.

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