Structural Ceramics: Mechanical Properties Flashcards
1
Q
She likes this as a test question
A
2
Q
she likes to test on this
A
3
Q
she likes this on test
A
Stress intensity factor is greater than fracture toughness
4
Q
What are the Weibull parameters? What is desirable?
A
m and sigma 0
we want them as high as possible
5
Q
What would a more reliable material look like on a Weibull analysis?
A
higher slope
6
Q
What are structural ceramics?
A
ceramics that demonstrate enhanced mechanical properties under demanding mechanical loading conditions
7
Q
What are the demanding mechanical loading conditions of structural ceramics ALSO caused by?
A
- large thermal gradients
- erosive and corrosive environments
8
Q
What are functional ceramics?
A
Ceramic materials that are used in applications where electronic, magnetic and/or optical properties are key for their performance.
9
Q
What is the most common application for ceramics?
A
compression
10
Q
When do ceramics need reinforcement?
A
When bending and tension are involved
11
Q
What kind of ceramics are used for applications where strength is key?
A
structural ceramics
12
Q
What are advanced structural ceramics?
A
Ceramic materials that demonstrate enhanced mechanical properties under demanding mechanical loading conditions
13
Q
What is the drawback of advanced structural ceramics?
A
Expensive to replace in erosive, corrosive or high T environments
14
Q
What are the three types of bonding that are important in ceramics?
A
- ionic - strong
- covalent - strong
- van der waals - strong (~1/10 strength of ionic/covalent)
15
Q
What kind of bond energy do ionic and covalent bonds have? What characteristics does this mean?
A
large bond energy
Large Tm
large E
small a
16
Q
What does a strongly vs. weakly bonded Force vs separation r plot look like? what does the slope indicate? What is the slope proportional to? What does the slope depend on?
A
large slope = strong
small slope = weak
slope proportional to the modulus of elasticity, E
depends on the bond strength of material
17
Q
What is the unit for the elastic modulus?
A
GPa or psi
18
Q
What does the graph of PE vs. interatomic distance look like and what property for bonding does it relate to?
A
Elastic modulus
19
Q
How does the modulus of elasticity compare for metal /ceramics/polymers/ composites?
A
ranges higher than other materials
20
Q
How does the TS of metals/ceramics/polymers/composites compare?
A
large range - lower than metals higher than polymers
21
Q
What are the three different types of tests to measure Tensile strength?
A
- 3-point bending
- 4-point bending
- tensile test
22
Q
How many forces/supporting pins are in the 3-point bending test? 4-point?
A
3-point
- 1 force, 2 supporting pins
4-point
- 2 forces, 2 supporting pins
23
Q
How does the tensile test work? What does it work best for and why?
A
loads are applied longitudinally
works well for metals but not ceramic bc of their brittleness.
the grips can introduce microcracks that affect ceramics
24
Q
Where does maximum tensile stress occur in 3 and 4-point bending?
A
3->on the bottom of the sample directly under the applied load
4-> equally for a
25
What are important characteristics and equations that go with the 3 point bending test?
1. microcracks are distributed on the bottom side of the sample in the L-span
When the applied load exceeds the fracture load, the equation applies (if it breaks in the middle)
26
What does the stress distribution of a 3-point bending test look like?
27
In a 3 point bending test where is the max tensile stress?
occurs on the bottom of the sample directly under the applied load
28
Does the 3-point or 4-point provide more reliable results?
4-point
29
What kinds of values do 3-point vs. 4-point bend tests give?
3-point give higher values of σf
30
What is the formula used with 4 point bending? when is it accepted?
more accepted than 3-point equation
accepted when fracture is in the span of a
31
For the 4-point bending test, how is the tensile stress at the bottom of the sample distributed?
In the length of the sample represented by "a"-- so between the 2 forces-- the tensile stress on the bottom is uniform (the same)
32
What does the stress distribution for 4 point bending look like?
33
What is dislocation motion like in metals/covalent ceramics/ionic ceramics? Why?
Metals
- easy dislocation
- non-directional bonding and close-packed directions
Covalent ceramics
- motion difficult
- directional (angular) bonding
Ionic ceramics
- motion difficult
- need to avoid nearest neighbors of like sign (+ and -)
34
What are the two different types of fracture?
Ductile and brittle fracture
35
What is ductile fracture?
fracture accompanied by significant plastic deformation
36
What is brittle fracture?
- little or no plastic deformation
- catastrophic
37
What does ductile, moderately ductile, and brittle fracture look like?
38
What kind of fracture is the most desirable? Why?
ductile
there's warning before fracture
39
What do cracks look like for impact/point loading, bending, torsion, and internal pressure?
40
What do intergranular (between grain) brittle fracture surfaces look like?
41
What do transgranular (through grains) fracture surfaces look like?
42
Is plastic or elastic deformation permanent?
plastic
43
What are the steps of plastic deformation?
1. initial
2. small load- bonds stretch and planes shear (elastic+plastic)
3. unload-- planes still sheared (plastic) but the elastic bounces back
44
What does a F vs. deformation graph look like for linear elastic/plastic?
** basically look at where the plastic +elastic (bright red) ends on the deformation axis. then it goes backwards once the load is released bc there was still some elastic (so it's less deformed than it was when the f was still acting on it)
45
What does elastic/plastic deformation look like on a stress vs. strain graph?
**so basically the load was removed and then the line going down is all the elasticity (same slope) coming out. the difference on the x-axis on where it starts and ends is plastic strain, or the plastic def /og basically
46
What does the yield strength comparison look like between metals/ceramics/polymers/composites?
effectively no yield strength for ceramics
47
What is toughness? How is it measured on a graph?
Energy to break a unit volume of a material
The area under a stress-strain curve
48
What do the stress strain fracture toughnesses look like on a graph?
ceramic - small blue
metals - large green
polymers - very small - pink
49
What kinds of energy are in brittle/ductile fracture?
brittle - elastic energy
ductile fracture - elastic + plastic
50
What is fracture toughness?
A measure of the ability of the ceramic to resist crack propogation
51
How does the fracture toughness of ceramics compare to the other materials?
52
What do you use to measure fracture toughness?
single-edge notched beam
53
How does the tensile strength of engineering materials compare to the tensile strength of perfect materials?
less than perfect
54
What is the ratio of compressive to tensile strength of ceramics?
compressive strength is 10x higher than tensile
55
How does the stress strain behavior of perfect materials, typical ceramics, strengthened metals, and typical polymers behave @ room T?
56
What did DaVinci observe? What is the explanation?
longer the wire-- smaller the load for failure
flaws cause premature failure & larger samples have longer flaws
57
How does the strength of metals and ceramics relate to flaws? How is this expressed on a graph?
Metal strength is largely independent of flaws
Ceramic strength is controlled by flaws
58
What are flaws?
stress concentrators
59
What kind of crack propagates easier?
cracks with sharp tips, more than blunt tips
60
What is the Griffith crack criteria?
longer crack = higher stress at crack tip
sharper crack = higher stress conc.
61
How does plastic deformation show up in cracks?
They have blunt tips
62
What cracks propagate the easiest?
cracks w sharp tips, no bluntness
63
What is this showing?
the max stress at crack tip is at the crack tip and as you move outwards from the crack into the solid material the stress just becomes the stress applied
64
What kind of energy is in the crack? How does energy behave in a crack?
Elastic strain energy
- energy is stored in the material as it elastically deforms
- energy is released when the crack propagates
- creation of new surfaces requires energy
65
When does a crack propagate?
If crack-tip stress (σm) exceeds a critical stress (σc)
66
What do you do in this formula for ductile materials?
replace
ys with (ys +yp ), where yp is plastic deformation energy
67
What are three toughening mechanisms?
1. crack deflection
2. crack bridging
3. transformation toughening
68
What happens in crack deflection?
Grain boundaries make the crack have to avoid all the boundaries to propagate so it’s harder to crack
69
What happens in crack bridging? With what?
Materials bridge the crack and decrease the stress at the crack tip
- elongated grains
- continuous fibers
- whiskers
- particles
70
What happens in transformation toughening?
A material undergoes a phase transformation at the crack tip to improve toughness
71
Explain how transformation toughening works more detailed
As a crack goes through the initial grains, a stress field is created acting against the crack propagation by the transforming grains
72
How would transformation toughening work with monoclinic and tetragonal particles?
The tetragonal phase particles change to monoclinic in the stressed region around the crack
73
What is one ceramic that undergoes transformation toughening?
zirconia
74
75
- deflection and bridging
- deflection
- branching
- pullout
- pullout
76
What's another extra fancy toughening mechanism?
bio-inspired advanced ceramics
mimicking mother-pearl structures
77
What is the condition for crack growth?
Stress intensity factor > fracture toughness
78
What is a variable for fracture toughness?
Kc
79
What are the three modes of loading for fracture toughness?
mode 1 is the most common and its KIC
80
What cracks grow first?
largest, most highly stressed cracks
81
What are the two ways that fracture is graphed?
- max flaw size dictates design stress
- design stress dictates max flaw size
82
What is the ratio of the compressive to tensile strength of ceramics?
compressive strength is 10X higher than tensile
83
What does it mean when a material exhibits r-curve behavior? What does it look like on a graph?
higher reliability and enhanced thermal shock resistance
84
At low and intermediate T, What happens when K > KIC?
85
At low and intermediate T, when K< KIC, what happens?
86
What happens as ceramics sustain cycling loads? What happens if a crack is stable K
cycling loads = fatigue = subcritical crack growth
steady stress and corrosive environment = subcritical crack growth
87
At high T (T>0.5Tm) , what happens under high stress?
slow deformation under high stress at high T = creep = subcritical crack growth
88
What are three different paths that ceramics get subcritical crack growth?
cyclic loading = fatigue = SCG
steady stress & corrosive environment = SCG
slow def under high stress at high T = creep = SCG
89
What is subcritical crack growth SCG?
slow growth of a subcritical flaw (under fracture toughness) as a result of its exposure to combined effect of stress and corrosive environment
90
What are two kinds of cracks that can experience subcritical crack growth?
- preexisting crack
- nucleation of a new crack during service
91
At low-mid and high temperatures, what are the main ways that the subcritical crack grows?
Low-mid T -> the stress increases the chemical reactivity of the atom bonds at the crack tip (weakens them)
high T -> high heat and stress form cavities (little holes) ahead of the crack tip, which lets the crack grow
92
What is fatigue?
Fatigue is when a material slowly gets damaged and eventually fails after being exposed to repeated or cyclic loading, even if the stress is well below its normal strength
93
Do ceramics fatigue?
yes
94
What formula is used to test fatigue? What kind of experiments and specimens can you use?
Experiments
-- tension-tension
-- compression-compression
-- tension-compression
Specimens
- smooth "crack free"
- containing "long cracks"
95
What are some observations related to ceramic fatigue?
1. Cyclic loading diminishes the effect of reinforcement shielding effect
2. Ceramics with R-curve behavior appear to be more susceptible to fatigue
3. At high T, the cyclic fatigue is not as damaging
*4) no micromechanical model can successfully explain all fatigue data in ceramics
*5) fatigue micromechanics are fundamentally different than in metals
96
What is creep?
slow continuous deformation of a solid with time that only occurs at high temperatures T>0.5Tm
97
What is the driving force of creep?
the process:
1. stress gradient at high T (so not uniform -> tensile at some parts and compressive in other areas)
2. the vacancies can move easier at high T and they go to the tensile regions bc it's energetically easier
3. the amount of vacancies in the tensile region > vacancy concentration in compressed regions
4. this causes the atoms to move into these vacancies in the tensile regions
5. causes permanent deformation bc changes the shape
97
What is the formula for steady state creep rate? Is it temp dependent?
Yes
98
What are three different methods of design of structural applications?
1. empirical
2. statistical
3. probabilistic
99
Why do we need toughening methods?
Increase the energy needed to extend a crack
100
What is the empirical design method for structural applications based on?
based on prior experience
101
What does the statistical design method for structural applications work for?
Works well for metals because their σf values fit a Gaussian curve
102
What is the probabilistic design method for structural applications used for?
widely used for ceramics due to their non Gaussian behavior
103
Why is there a wider range of fracture stress for ceramics?
The influence of cracks (microcracks)
104
How do the graphs of frequency of failure vs. fractures stress look like for metals vs. ceramics? Why?
Wider range of fracture stress values for ceramics due to microcracks
105
What is the Weibull analysis used for (4)?
1. To assess the statistical variations of when failure will occur
2. Extreme value distribution: analyzes values that are far away from the mean
3. Effect of volume and type of material on results
4. Predict what will occur at larger or smaller volumes
106
What are the Weibull parameters? What do we want?
m: slope of curve. It is a measure of reliability
σ0: stress at which 63.2% of the samples fail
we want both to be high
107
What are trends of the Weibull analysis graphs?
Pf vs σf
Pf- probability of failure
108
What does a Weibull graph of large vs. small samples look like
same slope (reliability)
small is futher right = at higher strength (less microcracks)
109
What does a higher slope on a Weibull Analysis graph mean? What about further to the right?
samples are more reliable
stronger
110
How do you know if there's a smaller probability of failure? What do we care about with the failures and reliability?
smaller probability of failure at any given stress for samples w SiC (higher slope)
we want more narrow distribution of failures and a higher reliability
111
(wasn't on adv slides) What are the steps to the Weibull analysis?
1. rank in order from least to highest
2. calculate Pf using the equation
- n = rank of current sample
- N -= total # of samples
3. make a table
- ln(sigma) is x-coordinate
- ln(ln(1/(1-Pf)) is y-coordinate
4. make graph
5. find slope
112
What is the benefit of a proof test? Cons?
A proof test can truncate a distribution and reduce probability of failure under service conditions
- it must exactly simulate the service condition
- costly and can introduce new flaws
113
What are 2 examples of structural ceramics and their applications?
ZrO2
- furnaces
Si3N4
- automotive engines
114
What are three examples of applications of structural ceramics?
1. heat engines
2. space shuttle
3. tissue scaffold
115
Why are structural ceramics used in heat engines? Which ceramic?
1. good thermal shock resistance
2. low thermal expansion
3. high specific strength
4. low thermal conductivity
Si3N4
116
What kinds of ceramics are used for space shuttles?
UHTC
117
How are ceramics used for tissue scaffold?
growing tendon on scaffold
- strong fibre to sustain the tension
118
What does creep depend on?
1. stress
2. time
3. temp
4. grain size and shape
5. microstructure
6. volume fraction and viscosity
7. dislocation mobility
119
What are three mechanisms of creep?
1. diffusion creep -> flux of atoms
2. viscous creep -> glassy phases cavitation and stress rupture
3. dislocation creep -> movement of blocks of atoms
120
What dictates the strength and brittle behavior of ceramics?
bonding
121
What are the differences between theoretical and actual values of strength due to?
role of microstructure and presence of flaws and cracks (microcracks)
122
What do ceramics need if they're going to be used in tension?
reinforcement :
- deflection
- crack bridging
- transformation toughening
123
What determines the ability of a material to avoid crack propagation?
Fracture toughness
