Chapter 2

Question 1

List the 4 broad categories of metal failure

Answer 1

Ductile Fracture
Brittle Fracture
Fatigue
Creep

Question 2

Define the term “ductile-brittle transition temperature”

Answer 2

The ductile-brittle transition temperature is when the object transitions from ductile to brittle. The temperature is at the point where it decreases the material’s ability to absorb force without fracturing, therefore the stress required to cause crack growth will be reduced

Question 3

Identify the characteristics marking of a brittle fracture

Answer 3

Chevron markings
Direction of propagation

Question 4

List the ways to lower the ductile-brittle transition temperature of a steel

Answer 4

Decrease in Carbon content to below 0.15%
Decrease in rate of loading(impact loading)
Decrease of depth notch or increase in radius notch
Increase the nickel content to about 2% to 5%
reduce the grain size by adding grain refining elements

Question 5

Define “fatigue limit”

Answer 5

Fatigue limit is the limiting stress below which there are no fatigue fracture

Question 6

State the methods to introduce compressive stresses on the surface of the component so as to improve fatigue life

Answer 6

Shot-peening
Cold rolling
Case hardening

Question 7

What is meant by “stress concentration” and explain the effect of stress concentration on fatigue life

Answer 7

A point in the object where stress is significantly higher than in the surrounding area. When stress concentration is produced on the surface, it is a starting point for fatigue failures. Any repeated stress may cause a fatigue crack to grow at the stress concentration leading to fatigue failure

Question 8

Explain how the creep rate is affected in each of the 3 typical creep stages

Answer 8

Primary creep - rapid extension with decreasing rate due to strain hardening
Secondary creep - steady state, work hardening is balanced by thermal softening
Tertiary creep - strain rate accelerated, formation of void and structural changes leading to final fracture

Question 9

Discuss the following methods to improve creep resistance
a) High melting point
b) Solid solution strengthening
c) Precipitation or dispersion hardening

Answer 9

a) creep usually occurs when metal operate at temperature above 30% to 40% of their absolute melting point when operating a high melting point metal, it can be used for high-temperature service

b) Solid solution hardening increases the matrix strength by adding different soluble elements which improve the creep resistance of single crystal nickel-base superalloys at high temperatures

c) precipation or dispersion hardening raises the yield strength of the metal and also resist the movement of dislocations that causes creep