KC revision questions Flashcards
Explain the difference between elastic and plastic behaviour in crystalline materials
- Elastic limit is the critical stress at which elastic behaviour stops and either plastic behaviour or failure occurs
Elastic behaviour:
- like a spring
- not time dependent
- linear stress-strain relationship
- reversible: removal of stress results in the material reverting back to its original length
Plastic behaviour
- occurs once the elastic limit for a material has been passed
- not time dependent
- non-linear stress-strain relationship
- irreversible: removal of stress does not result in the material reverting back to its original length
- occurs due to breaking of bonds between atoms which reform between new neighbouring atoms
Describe the environmental conditions necessary for stress corrosion fatigue to occur and what are the associated consequences of this?
- Fatigue is a form of failure that occurs in structures that subjected to dynamic and fluctuating stresses
- happens at a stress that is below the fracture/failure strength
- Stress corrosion fatigue occurs due to the conjoint action of stress and a corrosive environment
- causes crack formation that would not have formed by the action of stress or corrosion alone
- oral cavity is ideal for corrosion to occur due to factors such as: water, pH, temperature
- stress causes an increase in the driving force for corrosion reactions
- crack initiation is easier at corrosion pits
- presence of moisture at crack tips enable bond rupture
- corrosion reaction does not readily occur
- highly stresses crack tip helps break the covalent bond
In the process of making a metal-porcelain restoration, a sintering process of ~900 degrees with a furnace is necessary. Which material needs to have a higher coefficient of thermal expansion and why?
- the CTE of the alloy must be slightly higher than the porcelain
- this means the metal will contract more during cooling after firing the porcelain
- this puts the ceramic under slight residual compression = less sensitive to applied tensile forces
Brittle materials
- fail in tension due to unstable crack propagation
- in compression the crack are stabilised due to the nature of loading
= porcelains are stronger in compression
Alloys
- able to withstand tensile stresses because they have a higher toughness
- Toughness: ability of a material to absorb energy up to fracture
Explain the difference between high copper and low copper amalgam restorations and the benefits of high copper amalgam
Low copper amalgam
- combines gamma phase and Hg
- issues with expansion during setting so limited to <6wt% Cu
- forms gamma 2 phase which is corrosion prone
- volume decreases with time due to corrosion = weakening strength of amalgam
high copper amalgam
- combination of gamma phase, Ag-Cu, and Hg
- avoids expansion issue but mixing 3 powders
- addition of Ag-Cu further reacts with gamma 2 phase to form gamma 1 phase
= less corrosion prone
= more gamma and gamma 1 phase so better strength
Define viscosity and explain the differences between pseudo-plastic and dilatant flow
- Viscosity is a measure of a fluid’s resistance to gradual deformation by shear or tensile stress
Pseudo-plastic flow
- viscosity decreases with increasing strain
- as stress is increased, the normally disarranged molecules begin to align in the direction of flow
- decreased internal friction (viscosity)
Dilatant flow
- increased viscosity with increasing strain
- at rest: closely packed particles with fluid filling the volume between particles = permits particles to move relative to each other
- at high stress: particles move at high speed so the volume between them decreases so fluid cannot fill it (density of particles increases)
- increased resistance to flow
What is a coupling agent and what function does it provide in dental composite resins?
Coupling agent
- a chemical attached to the filler surface
- forms a covalent bond between the filler and polymer matrix
Functions:
- constrains thermal expansion of the matrix by bonding to low expansion fillers
- limits deformation of the matrix under stress
- improves stress distribution within the composite as unbonded filler causes stress concentration and crack nucleation
- provides wear resistance
How does the fracture stress change with the volume of a brittle material and why?
- fractures stress decreases as the volume of brittle material increases
- this size-dependence is due to the presence of flaws in the bulk material
- a larger volume increases the probability off encountering a larger flaw
- the larger the flaw, the smaller the critical stress required for the crack to spontaneously grow and lead to failure
- the strength of the material is thus determined by the largest flaw/defect
Griffith’s experiment
Briefly describe the three stages of setting in glass ionomer cement
1st stage = dissolution
- hydrolysis of the glass releases silicate and calcium ions
2nd stage = gelation
- silicate forms silica gel
- calcium ions react with poly acid to form the initial set (calcium polyacrylate gel)
3rd stage = hardening
- aluminium reacts with polyacid and replaces calcium ions to form the final set (aluminium polyacrylate gel)
Many polymers are known to exhibit a certain degree of shrinkage after polymerisation. Briefly explain two methods that can be utilised to decrease the amount of shrinkage in polymers
polymer
- a molecule made up of many parts
- polymerisation is the process where monomers are chemically linked together to form a polymer
- dimensional shrinkage occurs during polymerisation because the polymer occupies less space than monomers
to reduce shrinkage:
can bind monomer with an inert filler material
- constrains deformation of the polymer matrix
- also constrains the thermal expansion
- increases mechanical properties
can use larger monomers
- there are less ‘by-products’ removed thus less shrinkage
