Mechanical Metallurgy Questions and Answers – Fatigue of Metals – Machine Design Approach – 1

This set of Mechanical Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Fatigue of Metals – Machine Design Approach – 1”.

1. The material with higher stacking fault energy has higher fatigue strength whereas the material with lower stacking fault will have a lower fatigue limit.
a) True
b) False

Explanation: The material with high stacking fault energy will easily cross-slip the dislocation, so the slip band formation and gliding are natural. This results in the large plastic zone at the crack tip and the fatigue strength reduces. The material with lower stacking fault energy has higher fatigue strength.

2. The effect of grain size on the fatigue strength is predominant in material with high stacking fault energy while the effect of grain size is negligible in low stacking fault energy material.
a) True
b) False

Explanation: The low stacking fault energy material does not form cell structure on deformation, so the crack boundary acts as the source to impede the crack growth, hence the fatigue strength depends upon the grain size. The inverse of this is that the material with high stacking fault energy does not significantly change with grain size.

3. What is the relationship between the grain size and the fatigue strength for low stacking fault energy material?
a) Fatigue strength is directly proportional to grain size
b) Fatigue strength is inversely proportional to grain size
c) Fatigue strength is directly proportional to the square of grain size
d) Fatigue strength is inversely proportional to the square root of grain size

Explanation: The fatigue strength of the low stacking fault energy material is inversely proportional to the square root of the grain size of the material.

4. Which of the following structure has the best fatigue property?
a) Pearlite
b) Martensite
c) Tempered martensite
d) Bainite

Explanation: The tempered and quenched structure form thin carbide film which results in stress concentration and reduces the fatigue life.

5. The ratio of fatigue strength of the material in the transverse direction to longitudinal direction (of their extrusion) will be highest in ___________
a) induction furnace melted sample
b) arc melting furnace melted sample
c) vacuum melting sample
d) cupola furnace meted sample

Explanation: The variation in the fatigue strength along the longitudinal and the transverse direction happens because of the presence of the inclusion in the sample. If the inclusion is high, it will create a region of stress concentration on the tip and reduces the fatigue strength. So, the material with minimum inclusion will have most upper strength along transverse direction, and the ratio will be high. In case of vacuum melting, the presence of the inclusion in the sample will be minimum, so ratio will be highest.
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6. The fatigue strength of the pure metal is shown by which of the following curve?

a) A
b) B
c) C
d) D

Explanation: The fatigue strength of the pure metal keeps on decreasing with increase in the number of stress cycles. It follows nonlinear curve.

7. The fatigue strength of the metal strain aging due to interstitial atoms is shown by which of the following curve?

a) A
b) B
c) C
d) D

Explanation: The Fatigue strength of alloys with interstitial solute atoms will decrease rapidly with the stress cycle initially, but becomes stagnant after a certain number of stress cycles. This is called the endurance limit of the material.

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