# Mechanical Metallurgy Questions and Answers – Surface Effect & Fatigue – 2

This set of Mechanical Metallurgy Assessment Questions and Answers focuses on “Surface Effect & Fatigue – 2”.

1. The number of slip band in the fatigue is directly proportional to the number of cycles.
a) True
b) False

Explanation: The statement is false because there is no direct relationship between these two. Generally, the number of slip bands reaches a stage of saturation after few cycles.

2. The fatigue crack propagation is _____________ in nature.
a) intergranular
b) transgranular
c) along the high atomic density planes
d) along the low-density atomic planes

Explanation: The fatigue crack propagation is generally transgranular. So it flows across the grain cutting the grain boundary perpendicularly.

3. The fatigue strain does not cause considerable strain hardening of successive cycle with growth of cracks.
a) True
b) False

Explanation: The work of W.A.Woods shows that the fatigue crack growth is not associated with extensive movement of slip bands but only a few Nanometer growths in each cycle. So, this intrusion and extrusion caused by the cycles do not cause strain hardening, but the crack growth occurs.

4. The plastic blunting process is the growth mechanism of crack, followed by the ___________
a) first stage of crack growth
b) second stage of the crack growth
c) third stage of the crack growth
d) termination of the crack

Explanation: The stage-II crack propagation occurs by the plastic blunting process. In this, the crack tip expands and contract on the application of the tensile load causing the progression the crack.

5. With decrease in the temperature, the fatigue strength of the material ___________
a) decreases
b) increases
c) remains constant
d) no relationship

Explanation: The fatigue of the material is associated with the production and movement of vacancies. At lower temperature, the movement of the vacancy freezes so the fatigue strength of the material increases.

6. The crack propagation rate with stress is given as ______
Where;
-> da/dN is crack growth with each cycle
-> C is constant
-> σm is mean stress
-> σa is alternating stress
-> a is the crack length
-> m and n are constant
a) da/dN = C σam an
b) da/dN = C σmm an
c) da/dN = C σa a
d) da/dN = σa a

Explanation: The crack growth with each cycle is given by the alternating stress:
-> da/dN = C σam an
The value of the m varies from 2 to 4, and the value of n varies from 1 to 2.
So, if the alternating stress is high, the crack growth with each cycle will be more and vice versa.

7. The relationship between the stress intensity factor and crack growth is given as _______
a) da/dN = A (ΔK)
b) da/dN = A (ΔK)p
c) da/dN = (ΔK)
d) da/dN = A (1/ΔK)p

Explanation: To use the concept of the fatigue crack growth in the engineering design, the relationship between the stress intensity factor and crack growth was established. It is as follow:
-> da/dN = A (ΔK)p
Where A & p are constants. ΔK is the range of the stress intensity factor.

8. Increasing the mean stress ___________ the crack growth rate.
a) increases
b) decreases
c) does not change
d) no relationship

Explanation: Increasing the mean stress increases the stress ratio for both stress and stress intensity, so the crack growth rate will increase.

9. The correct graph to show the relationship between the crack growth rate (da/dN) vs stress intensity factor range (ΔK) is shown in the figure ________
a)

b)

c)

d)

Explanation: The crack growth rate with the stress intensity factor is given by the below-shown curve:

10. In the given curve, the slope of the middle linear portion is equal to _________

a) da/dN
b) ΔK
c) A
d) p

Explanation: The equation of the line is given as:
-> da/dN=A(ΔK)p
On the log-log scale, the slope of the line will be given by p.

Sanfoundry Global Education & Learning Series – Mechanical Metallurgy.

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