Chapter 8 - Failure of Materials Flashcards
1
Q
In this course we will focus solely on failure of?
A
Metals.
2
Q
Define: Toughness.
A
The amount of energy absorbed by plastic deformation before failure.
3
Q
What are the two major types of fracture?
A
- Ductile.
2. Brittle.
4
Q
What is the key feature of a ductile fracture?
A
Lots of plastic deformation.
5
Q
What is the key feature of a brittle fracture?
A
Flat fracture surface. No plastic deformation.
6
Q
In a ductile fracture what must be continuously supplied for the crack to propagate?
A
Energy. Because the material plastically deforms all the way up to failure, energy must be continually supplied or the deformation stops. This is referred to as stable crack growth.
7
Q
What are the different kinds of loading that materials can be put under?
A
Tension, Torsion, Shear, Bending, Compression. In real life situations multiple can occur simultaneously and through multiple directions.
8
Q
What is the first stage of tensile fracture of a ductile material?
A
In the necked region the complex nature of the stress field results in microvoid formation around microscopic particles like oxide inclusions or at grain boundaries.
9
Q
What is the second stage of tensile fracture of a ductile material?
A
The microvoids formed in stage one begin to coalesce to form a large central void which then begins to propagate outwards.
10
Q
What is the third stage of tensile fracture of a ductile material?
A
When the advancing crack nears the surface final failure occurs along ~45* shear planes to give the typical “cup-and-cone” fracture.
11
Q
At what point on a stress strain graph does the first stage of tensile fracture of a ductile material occur?
A
At the UTS (Ultimate Tensile Strength).
12
Q
At what point on a stress strain graph does the third stage of tensile fracture of a ductile material occur?
A
At the point of fracture (the end).
13
Q
What shape does the microvoids in the second stage of tensile fracture of a ductile material form?
A
An elipse.
14
Q
Why is the surface of a ductile fracture rough?
A
In ductile failure each grain undergoes its own microscopic form of failure by extreme plastic deformation and micro-necking (Many small “cups-and-cones”.
15
Q
How much energy is required to fracture a completely brittle material?
A
The amount of energy required is however much is needed to create the two new fracture surfaces by breaking bonds.
16
Q
Why is brittle fracture so dangerous?
A
Because there is no plastic deformation at all there is no warning of failure.
17
Q
What is brittle fracture often associated with?
A
Pre-existing cracks and flaws.
18
Q
Why does brittle fracture sometimes occur below the material’s yield point?
A
Local stresses at stress concentrating cracks or flaws may exceed the engineering fracture stress of the material.
19
Q
Why are brittle fracture surfaces planar/flat/featureless?
A
This may be due to the crack propagating by the separation of atoms along crystal planes (called cleavage) or along grain boundaries.
20
Q
True/False: Materials that have undergone brittle fracture can fit together as a matched pair.
A
True. There is no plastic deformation so the surfaces will fit together.
21
Q
What situations favor brittle fracture?
A
- Low temperatures.
- Sudden loading.
- The complex stress state at the tip of cracks, sharp notches, and flaws.
- It may occur in a ductile metal where there is a brittle phase near the grain boundaries of the ductile phase.
22
Q
How does temperature affect whether a fracture will be brittle or ductile?
A
At low temperatures, low atomic movement, makes it hard for dislocations to move. No dislocations moving means no plastic deformation, therefore brittle fracture.
Low temps - Brittle.
High temps - Ductile.
23
Q
How does the speed of loading affect whether a fracture will be brittle or ductile?
A
If a load is applied too quickly the material will not get the chance to plastically deform. It is therefore more likely to undergo brittle fracture.
Slow loading - Ductile.
Fast loading - Brittle.
24
Q
True/False: Materials can only fail through either brittle or ductile failure.
A
False. There is a spectrum of failure between ductile and brittle. Failure can occur in ways that include both some brittle and some ductile failure.
25
Why is the area under a stress-strain graph only able to give us an approximate indication of toughness and not a reliable indicator of toughness?
Because the information gained through a test is only able to provide information on how a material will react under the same conditions as the test. If the test conditions change at all you can get different outcomes.
26
What is an impact test?
A hammer attached to a pendulum is allowed to swing down and impact with a sample. The difference in height between the hammers starting position and the position of the hammer at the top of the swing through will be the energy absorbed by the fracture.
27
In order to compare the toughness of two different materials using an impact test the two specimens must have...?
Identical dimensions and Identical notch geometry.
28
Why does the specimen in an impact test have a notch built into it?
The purpose of the notch is to create a site on the specimen of high stress concentration. This increases the likelihood of brittle fracture to occur. Therefore if a material shows high toughness/ductility during this test it is likely that it will fare well in real world scenarios.
29
Why does a notch create a stress concentration?
The stress being transferred from one end of the specimen to the other must move around the notch and begins to "bunch-up" at the edge of the notch.
30
What is triaxial stress?
The increases stress at the point of the notch makes the metal there want to plastically deform. But the material surrounding it won't allow it to. This causes transverse tensile stresses perpendicular to the axial stress. This means there is stress in three directions.
31
Why does triaxial stress increase the likelihood of brittle fracture occuring?
The transverse stresses means that more stress must be applied before the material at the edge of the notch will deform, (Increases the yield stress). Increased yield stress makes the material less ductile and more brittle.
32
BCC Metals are ____ at low temperatures and _____ at high temperatures.
Brittle at low temperatures, Ductile at higher temperatures.
33
What is the temperature above which a metal starts to exhibit high ductility?
The ductile-brittle transition temperature Tt.
34
True/False: FCC metals have a well-defined ductile-brittle transition temperature (Tt).
False. The close-packed slip planes and directions means that dislocation movement is not as reliant on thermal vibration of the atoms. Therefore there is no well-defined Tt.
35
Polymers also have a ductile-brittle transition temperature (Tt), what is it called?'
The glass transition temperature, (Tg) performs the same function.
36
How is the ductile-brittle transition temperature (Tt) determined?
Using an impact test. If we plot a graph of the impact energy for fracture against the temperature of the material, for BCC metals there is usually a clear change in ductility over a small range of temperature.
37
Why is knowledge of the ductile-brittle transition temperature (Tt) so important?
To avoid sudden (brittle) fracture during use a material must have a Tt above it's minimum operating temperature.
38
How does the inclusion of carbon change the Tt of steel?
With increasing carbon % the Tt increases, and the maximum ductility will reduce.
39
How does the inclusion of Manganese change the Tt of steel?
With increasing manganese % the Tt decreases, and the maximum ductility will increase.
40
What effect does the presence of impurities have on the Tt of steel?
Impurities act as stress raisers and therefore decrease ductility and increases Tt.
41
What effect does grain size have on the Tt of steel?
Smaller grains mean that cracks travel a smaller distance before encountering a change in the material slowing the crack. This means smaller grains increase ductility and reduce the Tt. Smaller grains also increases strength!!! SMALL GRAINS GOOD!!!
42
What effect does the rate of application of stress have on the Tt?
Dislocations need time to move and generate plastic deformation. Faster loading increases Tt and decreases ductility.
43
What effect will the geometry of a crack or flaw have on the Tt?
A sharp crack will have a high stress concentration, making the material more brittle and increase Tt. A blunt crack will have a lower stress concentration, making it less brittle and lowering the Tt. No crack/flaw is best though.
44
What is Fracture mechanics?
Fracture mechanics is the science of predicting the safety of an engineering component when we know that is is under stress and contains defects that can act as stress concentrators or notches.
45
What are the two major questions that fracture mechanics can answer?
1. What is the maximum stress a component should be exposed to during service if it contains flaws of certain size and shape?
2. What is the maximum defect size that a component can contain if the design engineer knows the maximum stress that it will be subjected to.
46
What are the two types of cracks?
1. Edge through crack.
| 2. Internal through crack.
47
How is the length of a crack measured?
For an edge through crack "a" = the length from the surface to the maximum depth of the crack.
For an internal through crack "a" = HALF of the length from one side of the crack to the other.
48
What is the stress intensity factor (K1)?
K1 is a measure of the degree to which an external stress is amplified at the tip of a crack.
49
What is the fracture mechanics equation?
K1 = Y x Sigma x root( pi x a)
50
In the fracture mechanics equation what does Y refer to?
Y = Dimensionless geometric factor. This is looked up in a text book. (no units)
51
In the fracture mechanics equation what does Sigma refer to?
Nominal applied stress on the part. Not the stress in the region of the crack/notch. (Pa)
52
In the fracture mechanics equation what does "a" refer to?
The length of an edge crack or half the length of an internal crack. (m^1/2)
53
What units is K1 measured in?
Pa.m^1/2
| Pascals times square root metres.
54
What is K1c?
When K1 reaches the K1c the condition for fast fracture is reached. This is a material property and is looked up in a test.
55
As ductility increases the K1c of a material will tend to _____.
Increase.
56
What are the three main variables to consider for fracture mechanics?
1. K1c ( material property)
2. Sigma (stress determined by design)
3. "a" (defects in material from processing)
57
Why is fracture mechanics important to the engineer?
If two of the variables are specified then the last one is fixed, and can be calculated. i.e. if you know the K1c, and the stress a designed object will be under, you can calculate the maximum allowable defect size.
58
True/False: We want K1 to be greater than K1c?
False. When K1 is over K1c fast fracture may occur. We want a low K1 and a high K1c.
59
True/False: Cracks can be tolerated in a loaded component.
True. As long as K1 does not reach the critical value K1c for that material. This allows engineers to design with imperfect materials.
60
When a material's strength (UTS) increases it's fracture toughness (K1c) will tend to _____.
Decrease. Typically materials that are high in one will be low in the other. Finding the balance between these two factors is the key.
61
Why is knowledge of fracture mechanics so important if you can just choose a material with a high yield stress?
Because fracture can occur before the material's yield strength is reached due to the presence of a stress concentrating flaw or crack of a certain critical size.
62
How can a brittle fracture occur if K1 is below K1c?
Under some conditions cracks can grow slowly even when the stress is below that required to cause sudden fracture. Cracks can also form and advance slowly in a component that contains no initial crack.
63
What are the two most important conditions where failure can occur below the yield stress?
1. When a component is exposed to a corrosive environment.
| 2. When there is an oscillating or cyclic pattern of stress referred to as fatigue loading.
64
How does corrosion cause cracks to grow?
The component is likely to corrode at the grain boundaries as they are the higher energy positions these boundaries can then be followed deeper into the material with the crack growing as a result of both the stress on the material and continued corrosion.
65
What are the three most important considerations for fatigue failure?
1. It is responsible for >90% of metallic failures.
2. Causes failure at stresses below the yield stress.
3. Causes brittle failure. Even in ductile metals.
66
What are the common types of stress cycles that can produce fatigue failure?
1. Reverse stress cycle - Stress cycles between tension and compression.
2. Repeating stress cycle - Stress cycles between tension and no stress.
3. Fluctuating stress cycle - Stress cycles between two different amounts of tension.
4. Intermittent stress cycle - Stress cycles in a non-smooth/random/irregular way.
67
What are the important variables to consider for fatigue failure?
1. Sigma(mean)
2. Sigma(range)
3. Sigma(amplitude)
68
How is Sigma(mean) calculated?
( Max stress + Min stress ) /2
Tension is positive.
Compression is negative.
69
How is Sigma(range) calculated?
( Max stress - Min stress )
Tension is positive.
Compression is negative.
70
How is Sigma(amplitude) calculated?
( Max stress - Min stress ) /2
Tension is positive.
Compression is negative.
71
Where is fatigue failure most likely to occur?
It is most typical for fatigue cracks to initiate at some form of pre-existing crack or defect.
72
What is the most common sources of defects and cracks in a component?
Oxide inclusions, surface grooves, surface scratches. It is extremely difficult (impossible) to remove all defects from a manufacturing process.
73
What must happen for fatigue failure to occur?
1. Cyclic stress with a tensile component.
2. Maximum cycle stress must be lower than yield stress.
3. A crack must either pre-exist or form in the component and grow sufficiently so that the cross-sectional area of the component is reduced to the point that the remaining area of unfatigued material is insufficient to bear the applied load.
4. Fatigue can occur after many millions of cycles at a lower peak stress, or after a lesser number of cycles at a higher peak stress.
74
True/False: A cycle of varying compressive forces will never produce fatigue failure.
True. There must be a tension component in order for a crack to appear/grow. Compression will force the crack to close.
75
How is the fatigue failure of a material tested?
Using a Rotating Bending Test.
76
True/False: Fatigue failure begins on the surface of a component.
True.
77
What two kinds of fatigue curves are there?
1. Ferrous Metals
| 2. Non-Ferrous Metals
78
What is the graph of the fatigue curve graphed against.
Max stress vs Number of cycles.
| The number of cycles is measured on a log scale.
79
What is a fatigue limit?
The fatigue limit (endurance limit) is the stress level below which fatigue will not occur. This is usually 1/4 to 1/2 it's tensile strength.
80
True/False: Non-Ferrous metals will always suffer from fatigue failure eventually.
True. There is no fatigue limit for non-ferrous metals.
81
What is the fatigue strength?
The stress that will allow a minimum number of cycles N to be achieved before failure occurs.
82
What are Beachmarks?
Beachmarks are lines in the surface of a material that has undergone fatigue failure. They represent periods of major crack advance, and are able to be seen by the naked eye.
83
What are Fatigue Striations?
Fatigue Striations are submicroscopic lines in the surface of a material that has undergone fatigue failure. They are only visible through a microscope and represent how the crack changed with each cycle.
84
Why does a fatigue fracture from something with tension and compression not have beachmarks?
Because the compression forces the crack back together, which "polishes" the surface removing all evidence of the fatigue fracture.
85
How can the fatigue properties of a metal component be improved?
1. Improve the surface finish.
2. Put the surface layer into residual compression.
3. Good design and manufacturing.
86
Why does improving the surface finish improve a metals fatigue properties?
Because it removes scratches etc. which nucleate fatigue cracks.
87
Why does putting the surface layer into residual compression improve a metals fatigue properties?
By adding a consistent compression this will lower the effective tension the component will be put under.
88
How do good design and manufacturing processes improve a metals fatigue properties?
By minimising stress raisers (e.g. sharp corners) which provide places for cracks to start.
89
True/False: Creep only occurs at high temperatures.
True. Though what is considered high can change depending on the material.
90
What is creep?
Creep is a situation where an applied stress causes a material to deform slowly. The deformation is dependent on time, temperature, and stress.
91
At what temperature (relative to its melting temperature) does creep begin to occur?
Tservice > 0.3 - 0.4 Tmelt (Pure Metals)
| Tservice > 0.4 - 0.5 Tmelt (Ceramics)
92
Why does Creep occur at high temperatures?
Because diffusion is very important for creep, which increases with temperature, and takes a long time.
93
True/False: Creep only occurs in metals.
False. Creep is quite a general deformation process, and is observed in metals, polymers, ice (e.g. glaciers), concrete, wood (especially when moist) etc...
94
What is the creep rate?
```
Creep rate = Creep Strain / time.
Creep rate = C x e ^ -( Q / R x T)
C = Constant
Q = Activation Temp for Creep
R = Universal Gas Constant (8.314 J/mol.K
T = Temp in Kelvin
```
95
What are the sections of a creep graph?
1. Initial instantaneous elastic deformation.
2. Primary non-linear creep (usually only a small amount of strain).
3. Secondary or Steady-State creep (largest amount of strain, most important, linear).
4. Tertiary creep (ends in creep fracture).
96
Why do engineers usually focus on designing for steady state or secondary creep?
Because it can be reliably predicted.
97
What are the three major mechanisms of creep?
1. Dislocation climb.
2. Stress-induced migration of atoms and vacancies within each grain.
3. Creep by grain boundary sliding.
98
How does dislocation climb occur?
Vacancies diffuse into the dislocations, allowing them to move around obstacles that would usually halt their movement.
99
True/False: Because creep relies on diffusion, it can be modelled by an Arrenhius type equation.
True.
100
How does stress-induced migration of atoms and vacancies within each grain occur?
Atoms that are diffusing will tend to diffuse in the direction of an applied tensile stress. While the vacancies tend to move in a direction perpendicular to the applied stress. Material becomes thinner and longer.
101
How does grain boundary sliding occur?
With high temperature the shear stresses formed by the tensile stress along with the greater energy at the grain boundaries causes the grains to slip relative to each other.
102
What is usually created as a result of grain boundary sliding?
Microvoids, the grain boundaries are not completely smooth, so as they move past each other they create a large number of vacancies. These vacancies join together to create microvoids.
103
How does grain boundary sliding result in failure during tertiary creep?
The microvoids created by grain boundary sliding will join together and will reduce the materials ability to resist stress.
104
How do we make materials more resistant to creep?
1. Increase the melting point.
2. Alloy should have a structure that creates maximum obstacles to dislocation movement and dislocation climb.
3. If possible, alloys that have strong covalent bonding.
4. Should have a large grain size increase distances over which atoms have to diffuse before they reach grain boundaries where diffusion is much faster.
5. Single crystals may be best.
6. Arrange for precipitates at grain boundaries.
105
How does a high melting point affect a materials creep resistance?
As creep can only occur above 0.3 of Tmelt a higher melting temperature will increase the temperature the component can operate in without creep occuring.
106
How does minimizing dislocation movement affect a materials creep resistance?
Solute atoms, heat stable precipitates, secondary phases help to stop dislocations from moving, and climbing.
107
How do covalent bonds affect a materials creep resistance?
Covalent bonds are stronger than metallic bonds, which makes it harder for dislocations to move.
108
How does grain size affect a materials creep resistance?
If the grains are larger the total area over which grain sliding can occur is reduced. Ultimately having a material made of a single crystal would stop grain boundary sliding completely.
109
How does having precipitates at the grain boundaries affect creep resistance?
Hard precipitates at the grain boundaries will slow/stop grains from being able to slide against one
another.
