Failure of material in the oral environment Flashcards
Define resilence.
The capacity of a material to absorb energy when it is deformed elastically.
How is resilence measured on a stress strain graph?
The area under the curve up to the elastic limit:
What does the resilience zone indicate about the stiffness of a material?
- Bigger zone = more flexible
- Therefore the resilence zone is the same as the modulus of elasticity:
What 2 strains occur under axial loading?
- Axial strain
- Lateral strain
What does a tensile force to do the area of cross-section and original length?
- Reduction of cross-section.
- Increase in original length
What does a compressive force to do the area of cross-section and original length?
- Increase in cross-section
- Reduction of original length
What is Poisson’s ratio and what does it indicate?
- The ratio between the lateral and axial strain in the elastic limit.
- That the change in cross-section is proportional to the deformation in the elastic range.
Why must elastic modulus’ and poisson’s ratio of the material replacing the tissue be similar to the tissues around it?
So deformation rates are similar with both the materials and tissues to prevent shear forces e.g. sealant and enamel:
What is the unit for fracture toughness and when does a fracture occur?
- Kc
- When Kc = K (critical value)
- Then catastrophic crack propagation occurs.
What is the crack growth-stress intensity factor?
The resistance of a material towards crack propagation
What specimens are used to determine fracture toughness?
Notched specimens
What value does fracture toughness give?
A value of the work in creating 2 new surfaces (when cracking occurs)
What is the relationship between stress intensity and fracture toughness?
- The stress intensity, KI represents the level of “stress” at the tip of the crack
- The fracture toughness, KIC, is the highest level of stress intensity that a material under very specific (plane-strain) conditions that a material can withstand without fracture.
What are the 3 modes of failure in dental materials?
Mode I - tensile crack opening KIc.
Mode II - sliding crack opening (in plane shear) KIIc.
Mode III - tearing crack opening (anti-plane shear) KIIIc
How can the critical value of
K be stopped to avoid the crack?
Both Y (shape) and stress are constant as the crack propagates.
Some methods to limit crack propagation include:
- Addition of rubber particles in PMMA dentures act as a barrier to cracks.
- Same with alumina into ceramic crowns.
- What part of the tooth stops cracks from propagating in the enamel?
dentine
What is dynamic loading?
Where materials are subjected to fluctuating or intermittent stresses.
How can a material fail with stresses less than the maximum stress being exerted?
With repeated cyclic loading.
What does determination of the relationship of stress level and number of cycles to failure allow an estimation of with the material?
The reliability of material.
Define fatigue in material testing.
Accumulation of small amounts of intermittent stress.
Explain the clinical significance of determining fatigue behaviour of direct restorative materials.
It allows for an estimation of the reliability of the material.
The stress level and number of cycles to failure can be used to determine the material’s endurance limit.
How can fatigue testing be done?
- By subjecting the specimen to cyclic loading over a range of loads.
- The number of cycles required to cause failure is counted.
How is stress plotted when calculating fatigue and what is the curve called?
- Plotted as a function of the log of the cycles required to cause failure against stress amplitude.
- Known as S-N curves:
Give 4 thermal properties of materials in the oral cavity.
- Coefficient of Thermal Expansion
- Glass transition temperature
- Thermal Conductivity and Thermal Diffusivity
- Polymerisation exotherm
What 2 things can thermal changes cause to a material?
- Expansion
- Contraction.
- What is the coefficient of thermal expansion defined as?
The fractional increase in length of a body for each degree rise in temperature
whats the importance of cofficent of thermal expansion
If two materials (like a filling and tooth) expand at different rates, it can cause gaps, cracks, or debonding.
Define thermal conductivity.
How well a material transfers heat through it.
What is the equation / formula for thermal conductivity?
thermal conductivity =
heat x distance / area x temp gradient
If a material has a high thermal conductivity what does this mean?
- Transfers a lot of heat.
- Therefore requires adequate insulation as it itself is a poor insulator.
Define thermal diffusivity.
A measure of the rate (HOW RAPIDLY) at which a temperature disturbance at one point in a body travels to another point.
What is the equation / formula for thermal diffusivity?
look at notes
For a deep restoration what should the thermal diffusivity be for the material used?
- Must have low thermal diffusivity.
- As it stops heat damaging the pulp.
What is glass transiton temperature and its units?
- The temperarure at which the polymer is heated to a point where the segmental motions of the chains increase and finally overcome the interactions.
- Thus the glassy brittle stage progresses to a rubbery and less rigid form whilst the modulus drops rapidly over a narrow range of temperature.
- Tg
What Tg must restorations and denture based materials have?
- One higher than the maximum temperature in the oral cavity.
- In order to preserve the material’s physical and mechanical properties.
How can the determination of the Tg of a composite filling indicate if there is inadequate curing?
Composites undergo polymerisation to set. Tg is a measure of when a polymer is heated to a point of collapse.
If the Tg is low, that suggests the polymer network is weak → meaning poor or incomplete curing.
Therefore, measuring Tg helps assess how well the composite has cured.
Lower-than-expected Tg = inadequate polymerisation.
What is polymerisation exotherm?
The temperature when a monomer undergoes polymerisation.
What ideally should the polymerisation exotherm of dental materials in the mouth be?
Low to not cause damage to the pulp.
define hue
Describes basic colour.
Define chroma.
The strength or dominance of the hue or intensity of colour.
Define value in material optics.
The overall intensity of how light or dark the colour is.
What are the features of a transparent material?
- Allows light to be transmitted without distoration or change in the colour of an object
- E.g. glass.
What are the features of a translucent material?
Allows light to be transmitted but with scattering and loss of definition and true colour of the object.
What are the features of an opaque material?
Allows only scattering of light and NO TRANMISSION OF LIGHT
