Module 3.3 - Mechanical Behavior Flashcards
Brittleness of material is dictated by what?
Ease of disolcation motion
Why is slip difficult for ceramics?
The energy barrier for dislocation motion is MUCH LARGER than the thermal energy available to the system.
Why is dislocation motion easier for metals?
Their bonding is non-directional and directions are close-packed for slip
For covalent ceramics why is dislocation motion more difficult?
It has directional or angular bonding, so slip needs to take place at a very specific orientation.
For ionic ceramics, why is the dislocation motion difficult?
Because of the repulsion between the positive and negative neighbors.
Flaws increase the what in their vicinity?
Stress in their vicinity
Where is maximum stressed experienced for cracks?
Crack tips
What is fracture toughness?
A material’s resistance to crack propagation
What happens if the stress experienced at the tip of the crack is less than critical stress?
The crack tip is stable and the crack doesn’t propagate
What happens if the stress experienced at the tip of the crack is greater than the critical stress?
Crack tip will be unstable and rapid and sudden fracture could occur
For the same crack length and radius, which will cause faster fracture, an internal or external crack?
External
What is the Griffith criterion?
That a crack will propagate when the decrease in elastic strain energy is at least equal to create the new crack
In crack propagation, in relation to the Griffith criterion, what energies are equal?
elastic energy released = energy consumed in forming new surface
What is the critical condition for fracture?
Rate of E release > Rate of E consumption
True or false, when a crack propagates, it also ensures that fracture will occur?
False.
What is surface energy in terms of the Griffith criterion?
Energy absorbed in crack growth
What is the energy released as the crack extends?
Strain energy
A crack will propagate under what conditions related to elastic strain energy and surface energy?
When the decrease in elastic strain energy is equal to the energy needed to create new crack surface.
What are the three modes of fracture?
- Opening
- Sliding
- Tearing
In tensile loading, what is the orientation of crack propagation?
Perpendicular to the applied stress
In tensile loading, what is the orientation of crack propagation?
Parallel to the compression axis
In tensile loading, a fracture is caused by what in terms of propagation?
Unstable propagation of a single crack
In compression loading, a fracture is caused by what in terms of propagation?
slow extension and linking up of many cracks to form the crack zone
What are the flaws introduced to ceramics during processing, machining or service? Give the 4.
- Pores
- Inclusions
- Agglomerates and Large Grains
- Surface Flaws
Pores are detrimental to the strength of ceramics. Why?
Because they act as stress concentrators and reduce the overall cross-sectional area.
Can pores alone can cause failure. Why?
No they cannot, and fracture is dictated by the presence of other defects in the vicinity
What happens if the pore is much larger than the surrounding grains?
Sharp cusps around the surface of the pore may form, and these cusps act as stress concentrators
Where do inclusions come from, and what do they cause?
Impurities in the starting powders can react with the matrix. These impurities form inclusions with different thermal and mechanical properties form the original matrix.
Why are inclusions detrimental to strength of the material?
They may have a CTE mismatch, and therefore cause large residual stresses
What happens if the CTE of the inclusion < CTE of the matrix?
Cracks will be formed
What happens if the CTE of the matrix < CTE of the inclusion?
The inclusions will detach from the matrix and create pore-like openings
What are agglomerates? and how are they formed?
In regions in a ceramic material with more fine particles, they undergo rapid densification. From here, stress is induced within the surrounding compact.
What does the presence of agglomerates cause?
Formation of voids and cracks around agglomerates
What cause large grains?
Exaggerated grain growth during sintering
For large grains, what happens if the ceramic is non-cubic?
Large grains are anisotropic, it acts as an inclusion in a fine-grained matrix. There will be CTE mismatch.
What are the possible ways that surface flaws can be introduced into the material?
- High temperature GB grooving
- Post-fabrication machining
- Accidental Damage
How does grain size affect strength?
Larger grains = larger flaws
When grains sizes are very fine, where does fracture usually occur from?
Process or Surface flaws
When the grain size is very fine, what happens to the strength?
It becomes grain size insensitive
Why are compressive surface residual stresses introduced onto the surface?
It inhibits failure from surface flaws
How do compressive surface residual stresses affect thermal shock and contact damage resistance?
It enhances thermal shock and contact damage resistance
When compressive surface residual stress is introduced onto the surface what happens inside the material?
Tensile stress develops inside
Due to compressive residual stresses on the material surface, what happens if a crack propagates?
If a crack propagates, the material is now weaker than before the compressive strength is introduced.
When residual compressive stresses are introduced onto the surface of the material, what happens on the surface?
You’re generating a surface layer with a higher volume than the original matrix
If an outer layer with lower CTE is incorporated to a material, what does this do?
It introduces compressive residual stresses onto the surface
Glass tempering introduces compressive residual stresses onto the surface. True or false?
True.
Physical saturation of the outer layer with atoms (i.e. ion implantation) and also exchanging smaller ions for larger ions will cause what on the surface of the material?
Compressive residual stresses on the surface of the material
Transformation stresses in zirconia ceramics can be used to induce compressive residual stresses on the material’s surface. True or false?
True.
At high temperatures, the surrounding atmosphere may react with the surface of the ceramic. What reactions may be caused that either strengthen or weaken the ceramic?
Formation of protective passivation (blunts and partially heals surface flaws to increase strength), and atmosphere may attack the surface causing it to corrode, pit, or etch off the surfaces.
What are the three main toughening mechanisms for ceramics?
- Crack deflection
- Crack Bridging
- Crack Toughening
What is the mechanism of crack deflection?
The plane of the crack propagation is deflected away from normal to the stress, hence stress intensity at the crack tip is lower. This can be done through grain boundaries or reinforcements.
What is the mechanism of crack bridging?
Crack surfaces are behind the crack tip are bridge through adding reinforcing phases (i.e. whiskers, fibers, elongated grains). These reinforcing phases bear the applied stress to lower stress at the crack tip.
What is transformation toughening?
The geometry of the reinforcing phase changes once the material undergoes stress because the matrix can no longer constrain it. This induces residual compressive stresses on the crack surface and tip.
Ceramics exhibit little plastic deformation and are brittle. Ture or false?
True
What is the specific loading conditions where dislocation motion can be observed in ceramics?
Slip
What are the two considerations for slip?
Lattice resistance
Orientation of crystal with respect to applied stress
According to the von Mises criterial, for plastic deformation to occur, how many independent slip systems are needed?
5 independent slip systems
What does the Hall-Petch relation specify?
We could achieve strength in materials that are as high as their own theoretical strength by reducing the grain size.
What is dislocation pinning in ceramics?
DIslocations in ceramics can be pinned by solute atoms
What is creep?
An increase in the strain with constant applied stress
When does creep begin to occur?
When T>0.5 Tm
What are the stages of creep and how can you differentiate them?
- Primary Creep - strain rate decreases with time
- Secondary Creep - strain rate is minimized and has a steady state value
- Tertiary Creep - strain rate is increasing with time
What are the three mechanisms of creep?
- Dislocation Glide
- Dislocation Creep
- Diffusion Creep
What is a dislocation glide?
When dislocations move along slip planes (overcomes barriers by thermal activation)
What is dislocation creep?
Similar to dislocation glide aided by vacancy diffusion moving through climb or glide. Overcomes barriers through thermally assisted mechanisms involving diffusion of vacancies/interstitials.
What happens during diffusion creep?
Vacancies are diffused.
What are the two types of diffusion creep? Differentiate.
Nabarro-Herring - Vacancy diffusion happens THROUGH crystal lattice
Cobble Creep - Vacancy diffusion happens along grain boundaries
What is grain boundary sliding?
Creep resulting in an intergranular film (IGF) at high temperature.
In grain boundary sliding, what mechanisms result in a permanent change in shape?
- Glass is squeezed out of boundaries during compression, flowing to those under tensions
- DIssolution and precipitation
