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Ceramics > Strength of Ceramics > Flashcards

Strength of Ceramics Flashcards

1
Q

She likes this as a test question

A
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2
Q

she likes to test on this

A
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3
Q

she likes this on test

A

Stress intensity factor is greater than fracture toughness

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4
Q

What are the Weibull parameters? What is desirable?

A

m and sigma 0

we want them as high as possible

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5
Q

What would a more reliable material look like on a Weibull analysis?

A

higher slope

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6
Q

What are structural ceramics?

A

ceramics that demonstrate enhanced mechanical properties under demanding mechanical loading conditions

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7
Q

What are the demanding mechanical loading conditions of structural ceramics caused by?

A
  1. large thermal gradients
  2. erosive and corrosive environments
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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.

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9
Q

What is the most common application for ceramics?

A

compression

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10
Q

When do ceramics need reinforcement?

A

When bending and tension are involved

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11
Q

What kind of ceramics are used for applications where strength is key?

A

structural ceramics

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12
Q

What are advanced structural ceramics?

A

Ceramic materials that demonstrate enhanced mechanical properties under demanding mechanical loading conditions

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13
Q

When are advanced structural ceramics the material of choice? What’s the drawback?

A

In erosive, corrosive or high temperature environments.

They are expensive.

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14
Q

What are the three types of bonding that are important in ceramics?

A
  1. ionic
  2. covalent
  3. van der waals
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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

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16
Q

What does a strongly vs. weakly bonded stress strain plot look like? what does the slope indicate? What is the slope proportional to?

A

large slope = strong
small slope = weak

slope proportional to the modulus of elasticity, E

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17
Q

What is the unit for the elastic modulus?

A

GPa or psi

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18
Q

What is the elastic modulus?

A

A measure of how much deformation will occur for a given applied stress (force). Atoms are pulled apart (or pushed together) slightly when a stress is applied

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19
Q

How do atoms behave when a stress is released?

A

The atoms reduce their energy by returning to their equilibrium separation distance

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20
Q

What does the graph of PE vs. interatomic distance look like and what property for bonding does it relate to?

A

Elastic modulus

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21
Q

What is the force between atoms analogous to?

A

The restoring force of a spring in Hooke’s Law

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22
Q

How does the modulus of elasticity compare for metal /ceramics/polymers/ composites?

A
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23
Q

How does the TS of metals/ceramics/polymers/composites compare?

A
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24
Q

What are the three different types of tests to measure Tensile strength?

A
  1. 3-point bending
  2. 4-point bending
  3. tensile test
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25
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

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26
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

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27
Q

Where does maximum tensile stress occur in 3 and 4-point bending?

A

on the bottom of the sample directly under the applied load

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28
Q

Does the 3-point or 4-point provide more reliable results?

A

4-point

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29
Q

What kinds of values do 3-point vs. 4-point bend tests give?

A

3-point give higher values of σf

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30
Q

What is dislocation motion like in metals/covalent ceramics/ionic ceramics? Why?

A

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 -)

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31
Q

What are the two different types of fracture?

A

Ductile and brittle fracture

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32
Q

What is ductile fracture?

A

fracture accompanied by significant plastic deformation

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33
Q

What is brittle fracture?

A
  • little or no plastic deformation
  • catastrophic
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34
Q

What does ductile, moderately ductile, and brittle fracture look like?

A
35
Q

What kind of fracture is the most desirable? Why?

A

ductile

there’s warning before fracture

36
Q

What do cracks look like for impact/point loading, bending, torsion, and internal pressure?

A
37
Q

What do intergranular (between grain) brittle fracture surfaces look like?

A
38
Q

What do transgranular (through grains) fracture surfaces look like?

A
39
Q

Is plastic or elastic deformation permanent?

A

plastic

40
Q

What are the steps of plastic deformation?

A
  1. initial
  2. small load- bonds stretch and planes shear (elastic+plastic)
  3. unload– planes still sheared (plastic)
41
Q

What does a F vs. deformation graph look like for linear elastic/plastic?

A
42
Q

What does elastic/plastic deformation look like on a stress vs. strain graph?

A
43
Q

What does the yield strength comparison look like between metals/ceramics/polymers/composites?

A

effectively no yield strength for ceramics

44
Q

What is toughness? How is it measured on a graph?

A

Energy to break a unit volume of a material

The area under a stress-strain curve

45
Q

What kinds of energy are in brittle/ductile fracture?

A

brittle - elastic energy
ductile fracture - elastic + plastic

46
Q

What is fracture toughness?

A

A measure of the ability of the ceramic to resist crack propogation

47
Q

How does the fracture toughness of ceramics compare to the other materials?

A
48
Q

What do you use to measure fracture toughness?

A

single-edge notched beam

49
Q

How does the tensile strength of engineering materials compare to the tensile strength of perfect materials?

A

less than perfect

50
Q

How does the stress strain behavior of perfect materials, typical ceramics, strengthened materials, and typical polymers behave @ room T?

A
51
Q

What did DaVinci observe? What is the explanation?

A

longer the wire– smaller the load for failure

flaws cause premature failure & larger samples have longer flaws

52
Q

How does the strength of metals and ceramics relate to flaws? How is this expressed on a graph?

A

Metal strength is largely independent of flaws

Ceramic strength is controlled by flaws

53
Q

What are flaws?

A

stress concentrators

54
Q

What kind of crack propagates easier?

A

cracks with sharp tips, more than blunt tips

55
Q

How does plastic deformation show up in cracks?

A

They have blunt tips

56
Q

How does energy behave in a crack?

A

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

57
Q

When does a crack propagate?

A

If crack-tip stress (σm) exceeds a critical stress (σc)

58
Q

What do you do in this formula for ductile materials?

A

replace
ys with (ys +yp ), where yp is plastic deformation energy

59
Q

What are three toughening mechanisms?

A
  1. crack deflection
  2. crack bridging
  3. transformation toughening
60
Q

What happens in crack deflection?

A

Grain boundaries make the crack have to avoid all the boundaries to propagate so it’s harder to crack

61
Q

What happens in crack bridging? With what?

A

Materials bridge the crack and decrease the stress at the crack tip

  • elongated grains
  • continuous fibers
  • whiskers
  • particles
62
Q

What happens in transformation toughening?

A

A material undergoes a phase transformation at the crack tip to improve toughness

63
Q

What is the condition for crack growth?

A

Stress intensity factor > fracture toughness

64
Q

What cracks grow first?

A

largest, most highly stressed cracks

65
Q

What are the two ways that fracture is graphed?

A
  • max flaw size dictates design stress
  • design stress dictates max flaw size
66
Q

What is the ratio of the compressive to tensile strength of ceramics?

A

compressive strength is 10X higher than tensile

67
Q

What are three different methods of design of structural applications?

A
  1. empirical
  2. statistical
  3. probabilistic
68
Q

Why do we need toughening methods?

A

Increase the energy needed to extend a crack

69
Q

What is the empirical design method for structural applications based on?

A

based on prior experience

70
Q

What does the statistical design method for structural applications work for?

A

Works well for metals because their σf values fit a Gaussian curve

71
Q

What is the probabilistic design method for structural applications used for?

A

widely used for ceramics due to their non Gaussian behavior

72
Q

Why is there a wider range of fracture stress for ceramics?

A

The influence of cracks (microcracks)

73
Q

How do the graphs of frequency of failure vs. fractures stress look like for metals vs. ceramics? Why?

A

Wider range of fracture stress values for ceramics due to microcracks

74
Q

What is the Weibull analysis used for (4)?

A
  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
75
Q

What are the Weibull parameters? What do we want?

A

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

76
Q

What are trends of the Weibull analysis graphs?

A

Pf vs σf

Pf- probability of failure

77
Q

How do you know if there’s a smaller probability of failure? What do we care about with the failures and reliability?

A

smaller probability of failure at any given stress

we want more narrow distribution of failures and a higher reliability

78
Q

What are the steps to the Weibull analysis?

A
  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
79
Q

What is the benefit of a proof test? Cons?

A

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
80
Q

What are 2 examples of structural ceramics and their applications?

A

ZrO2
- furnaces

Si3N4
- automotive engines

81
Q

What are three examples of applications of structural ceramics?

A
  1. heat engines
  2. space shuttle
  3. tissue scaffold
82
Q

Why are structural ceramics used in heat engines? Which ceramic?

A
  1. good thermal shock resistance
  2. low thermal expansion
  3. high specific strength
  4. low thermal conductivity

Si3N4

83
Q

What kinds of ceramics are used for space shuttles?

A

UHTC

84
Q

How are ceramics used for tissue scaffold?

A

growing tendon on scaffold
- strong fibre to sustain the tension

Ceramics (8 decks)

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