Failure of Materials 2 Flashcards
What does elastic modulus tell us?
The stiffness of a material.
What does a ceramic look like on a stress-strain graph?
High modulus of elasticity leading to a yield point.
Brittle material.
What does a metal look like on a stress-strain graph?
High modulus of elasticity and then some elongation. Ductile material.
What is a good value to try and match the modulus of dental materials to?
The modulus of enamel and dentine.
Define resilience of a material
Ability of a material to absorb energy when is it deformed elastically.
The energy is released on unloading.
Define toughness of a material
Amount of energy a material can absorb before rupturing.
How do we calculate resilience on a stress-strain graph?
The larger the area under the curve is a more resilient material.
What is Poisson’s ratio?
A measure of the affect where a material expands in directions perpendicular to the application of compression.
When a cylinder undergoes tensile forces in axial direction, in what direction does it expand?
Elongation in axial direction leading to reduction in cross-section.
When a cylinder undergoes compression forces, what happens to the cylinder?
The cylinder increases in cross-section and reduces in length.
What is poissons ratio a relationship between?
Ratio between lateral and axial strain within the elastic limit.
It indicates that the change in cross-section is proportional to the deformation in the elastic range.
Do brittle or ductile materials have a higher poissons ratio?
Ductile due to showing more permanent reduction in cross section during tensile forces.
If there is a difference in value of poissons ratio and elastic modulus between enamel and a sealant used, what can happen?
When load is applied, there will be different deformation.
enamel = little deformation
This can lead to shear the the interface and ultimately failure.
What does it mean for a material to be isotopic?
It has uniform physical properties in all directions.
in real life we have anisotropic materials where they have different properties in different directions
What is the fracture toughness of a material?
The materials resistance to crack propagation.
How is fracture toughness measured?
A specific material of known dimension is taken and a crack of known dimension is created. Then subject to loading.
As stress is increased, the material experiences more stresses that are concentrated and then stress exceeds critical stress leads to fracture breaking the specimen into two pieces.
How is stress intensity and fracture toughness related?
Stress intensity = level of stress at tip of crack
Fracture toughness = highest value of stress intensity under specific conditions without fracture
What are the 3 modes of failure of a material?
1) Tensile mode (crack moves directly apart)
2) Sliding mode (crack surfaces slide over each other perpendicularly)
3) Tearing motion (crack surfaces move relative to each other and parallel to leading edge of crack)
How can we enhance fracture toughness?
How can we do this specifically for ceramic?
By adding particles into a matrix in the material at the site of crack to prevent crack propagation.
Adding zirconia particles into a ceramic matrix to prevent crack propagation.
If enamel cracks but there is no enamel-dentine debonding, what happens to the crack?
The dentine will stop the crack from propagating.
We can also place a material under dynamic load.
How do we do this and what do we measure?
Dynamic loading = intermittent loading at lower stresses and defined sequences.
Measure = stress number with number of cycles when subjected to dynamic loading.
What curve do we use to determine the fatigue of a material?
What is fatigue?
Cycle-stress curve.
Fatigue = progressive localised plastic deformation which occurs in a material subjected to cyclic stresses at high stress concentration locations.
What is the endurance limit of a material?
The level os stress at which below there is an indefinite number of cycles without fracturing.
THERMAL:
What is the coefficient of thermal expansion?
What do we want these to match?
Increase in length of a body for an increase in one degree.
We want the tooth and the material to match (if different then marginal gaps can form).
What is thermal conductivity and why is this property important?
- Quantity of heat transmitted through a unit thickness due to a unit temperature gradient
Higher conductivity can bring heat close to the pulp and lead to pulp death. (insulating material will be needed between tooth and restorative material)
What is thermal diffusivity and why is it important?
Thermal diffusivity = measure of the RATE at which a temperature disturbance at once point travels to another point.
We want a low thermal diffusivity otherwise temperature change will be transferred quickly to the pulp.
What is the glass transition temperature and why is it important?
The temperature at which a material changes from a hard glassy state to a slow rubbery state.
- When a polymer is heated, the energy of the chains will be increased leading to motion.
Over a certain temp, the polymer changes from hard to soft (drop in elastic modulus). - We want Tg to be higher than the highest temperature in the mouth so this change does not occur
In terms of optical properties, what properties do we want the dental material to have?
Translucency (to mimic a tooth)
