Strength of Ceramics Flashcards
She likes this as a test question
she likes to test on this
she likes this on test
Stress intensity factor is greater than fracture toughness
What are the Weibull parameters? What is desirable?
m and sigma 0
we want them as high as possible
What would a more reliable material look like on a Weibull analysis?
higher slope
What are structural ceramics?
ceramics that demonstrate enhanced mechanical properties under demanding mechanical loading conditions
What are the demanding mechanical loading conditions of structural ceramics caused by?
- large thermal gradients
- erosive and corrosive environments
What are functional ceramics?
Ceramic materials that are used in applications where electronic, magnetic and/or optical properties are key for their performance.
What is the most common application for ceramics?
compression
When do ceramics need reinforcement?
When bending and tension are involved
What kind of ceramics are used for applications where strength is key?
structural ceramics
What are advanced structural ceramics?
Ceramic materials that demonstrate enhanced mechanical properties under demanding mechanical loading conditions
When are advanced structural ceramics the material of choice? What’s the drawback?
In erosive, corrosive or high temperature environments.
They are expensive.
What are the three types of bonding that are important in ceramics?
- ionic
- covalent
- van der waals
What kind of bond energy do ionic and covalent bonds have? What characteristics does this mean?
large bond energy
Large Tm
large E
small a
What does a strongly vs. weakly bonded stress strain plot look like? what does the slope indicate? What is the slope proportional to?
large slope = strong
small slope = weak
slope proportional to the modulus of elasticity, E
What is the unit for the elastic modulus?
GPa or psi
What is the elastic modulus?
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
How do atoms behave when a stress is released?
The atoms reduce their energy by returning to their equilibrium separation distance
What does the graph of PE vs. interatomic distance look like and what property for bonding does it relate to?
Elastic modulus
What is the force between atoms analogous to?
The restoring force of a spring in Hooke’s Law
How does the modulus of elasticity compare for metal /ceramics/polymers/ composites?
How does the TS of metals/ceramics/polymers/composites compare?
What are the three different types of tests to measure Tensile strength?
- 3-point bending
- 4-point bending
- tensile test
How many forces/supporting pins are in the 3-point bending test? 4-point?
3-point
- 1 force, 2 supporting pins
4-point
- 2 forces, 2 supporting pins
How does the tensile test work? What does it work best for and why?
loads are applied longitudinally
works well for metals but not ceramic bc of their brittleness.
the grips can introduce microcracks that affect ceramics
Where does maximum tensile stress occur in 3 and 4-point bending?
on the bottom of the sample directly under the applied load
Does the 3-point or 4-point provide more reliable results?
4-point
What kinds of values do 3-point vs. 4-point bend tests give?
3-point give higher values of σf
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 -)
What are the two different types of fracture?
Ductile and brittle fracture
What is ductile fracture?
fracture accompanied by significant plastic deformation
What is brittle fracture?
- little or no plastic deformation
- catastrophic
What does ductile, moderately ductile, and brittle fracture look like?
What kind of fracture is the most desirable? Why?
ductile
there’s warning before fracture
What do cracks look like for impact/point loading, bending, torsion, and internal pressure?
What do intergranular (between grain) brittle fracture surfaces look like?
What do transgranular (through grains) fracture surfaces look like?
Is plastic or elastic deformation permanent?
plastic
What are the steps of plastic deformation?
- initial
- small load- bonds stretch and planes shear (elastic+plastic)
- unload– planes still sheared (plastic)
What does a F vs. deformation graph look like for linear elastic/plastic?
What does elastic/plastic deformation look like on a stress vs. strain graph?
What does the yield strength comparison look like between metals/ceramics/polymers/composites?
effectively no yield strength for ceramics
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
What kinds of energy are in brittle/ductile fracture?
brittle - elastic energy
ductile fracture - elastic + plastic
What is fracture toughness?
A measure of the ability of the ceramic to resist crack propogation
How does the fracture toughness of ceramics compare to the other materials?
What do you use to measure fracture toughness?
single-edge notched beam
How does the tensile strength of engineering materials compare to the tensile strength of perfect materials?
less than perfect
How does the stress strain behavior of perfect materials, typical ceramics, strengthened materials, and typical polymers behave @ room T?
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
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
What are flaws?
stress concentrators
What kind of crack propagates easier?
cracks with sharp tips, more than blunt tips
How does plastic deformation show up in cracks?
They have blunt tips
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
When does a crack propagate?
If crack-tip stress (σm) exceeds a critical stress (σc)
What do you do in this formula for ductile materials?
replace
ys with (ys +yp ), where yp is plastic deformation energy
What are three toughening mechanisms?
- crack deflection
- crack bridging
- transformation toughening
What happens in crack deflection?
Grain boundaries make the crack have to avoid all the boundaries to propagate so it’s harder to crack
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
What happens in transformation toughening?
A material undergoes a phase transformation at the crack tip to improve toughness
What is the condition for crack growth?
Stress intensity factor > fracture toughness
What cracks grow first?
largest, most highly stressed cracks
What are the two ways that fracture is graphed?
- max flaw size dictates design stress
- design stress dictates max flaw size
What is the ratio of the compressive to tensile strength of ceramics?
compressive strength is 10X higher than tensile
What are three different methods of design of structural applications?
- empirical
- statistical
- probabilistic
Why do we need toughening methods?
Increase the energy needed to extend a crack
What is the empirical design method for structural applications based on?
based on prior experience
What does the statistical design method for structural applications work for?
Works well for metals because their σf values fit a Gaussian curve
What is the probabilistic design method for structural applications used for?
widely used for ceramics due to their non Gaussian behavior
Why is there a wider range of fracture stress for ceramics?
The influence of cracks (microcracks)
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
What is the Weibull analysis used for (4)?
- To assess the statistical variations of when failure will occur
- Extreme value distribution: analyzes values that are far away from the mean
- Effect of volume and type of material on results
- Predict what will occur at larger or smaller volumes
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
What are trends of the Weibull analysis graphs?
Pf vs σf
Pf- probability of failure
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
we want more narrow distribution of failures and a higher reliability
What are the steps to the Weibull analysis?
- rank in order from least to highest
- calculate Pf using the equation
- n = rank of current sample
- N -= total # of samples
- make a table
- ln(sigma) is x-coordinate
- ln(ln(1/(1-Pf)) is y-coordinate
- make graph
- find slope
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
What are 2 examples of structural ceramics and their applications?
ZrO2
- furnaces
Si3N4
- automotive engines
What are three examples of applications of structural ceramics?
- heat engines
- space shuttle
- tissue scaffold
Why are structural ceramics used in heat engines? Which ceramic?
- good thermal shock resistance
- low thermal expansion
- high specific strength
- low thermal conductivity
Si3N4
What kinds of ceramics are used for space shuttles?
UHTC
How are ceramics used for tissue scaffold?
growing tendon on scaffold
- strong fibre to sustain the tension
