Fundamentals Flashcards
Four important Mechanical Properties
- Strength
- Hardness
- Ductility
- Stiffness
3 basic principles of loading:
- Tension (stretch)
- Compression (shrink)
- Shear (sideways)
The stress at fracture of a three point bend test is called:
- Flexural strength
- Also called fracture strength or bend strength
- Important mechanical parameter for brittle materials
What is torsion
Variation of pure shear where structure is twisted
Load in the mouth is usually a combination of what three basic types of load
- Tension
- Compression
- Shear
What is stress measured in
How is stress calculated?
- Pascals or Newtons per metre squared
- Force over area
What is strain?
Strain is a consequence of a structure under certain stress. It is the fractional change in the dimensions caused by the force.
What is Poisson’s ratio?
It is the ratio of lateral and axial loads. Eg when a strain occurs in one axis, a strain will occur in a perpendicular axis.
What does Hooke’s Law describe?
That stress and strain in a body are strictly proportional so long as the elastic limit is not exceeded
Young Modulus, also known as:
Modulus of Elasticity or Elastic modulus
The slope of the constant of proportionality between stress and stain is the:
Elastic modulus
What is used to measure the stiffness or resistance to deformation of a material?
Modulus of Elasticity
Is the E modulus of dentine higher or lower than enamel?
Lower
The bulk modulus measures the:
Compressibility
It refers to the behaviour of a material under hydrostatic loading - where all three stresses are equal.
In elastic behaviour, is displacement dependent or independent of time?
Independent of time
What is the elastic limit of a material
The point where elastic deformation ceases, and either failure or plastic deformation occurs
How are materials brittle?
They have little ability to deform plastically, so the fracture very close to the elastic limit
In brittle materials, the elastic limit is the same as:
Fracture stress
In ductile materials, the elastic limit is also known as:
Yield point or proportional limit
What is 0.2% proof stress?
An arbitrary definition used to enable uniform positioning of the yield point for ductile materials
What is the concept of resilience and what area of dentistry does it often apply to
The capacity of a material to absorb energy when it is deformed elastically, then upon unloading, have the energy recovered. Used in orthodontics.
What is ‘strength’
The amount of stress necessary to cause certain fracture or a certain amount of plastic deformation.
The PEAK stress on the engineering stress-strain curve is known as the:
Ultimate tensile strength
What is toughness?
Measure of the ability of a material to absorb energy up to fracture. Fracture toughness is indicative of a materials resilience to fracture when a crack is present.
What is referred to in saying ‘crystal structure’
The regular atomic structure with respect to each other, and repeated on an atomic scale
Three types of dislocations or defects in a crystal structure
- Edge dislocation
- Screw disclocation
- Mixed
Yield occurs differently in crystalline and amorphous materials, how?
Crystalline eg metals occurs via dislocation
Amorphous eg plastics, liquids via viscous flow
What is a material hardness?
It’s resistance to localised deformation eg polishability, stretch resistance, stress due to loading from opposing cusps
What is fracture toughness?
Resistance to crack propagation
What is the difference between brittle and ductile fracture?
- Brittle - happens rapidly without warning
- Ductile - occurs with plastic deformation
Why do brittle materials in tension fail?
Cracks are destabilised
In compression they are stabilised
What are three methods of predicting failure of a material
- Ultimate tensile strength
- Fatigue
- Corrosion
What are three impotant factors in Griffiths Criterion
- Crack length
- Youngs modulus
- Surface energy
Basically says that the longer the initial crack, the smaller the critical stress for fracture needed
When do cracks propagate spontaneously?
When the amount of energy stored elastically in the specimen exceeds that required for the creation of new surfaces.
Example of defects helping cracks spontaneously propagate
Jagged edge of food packaging
Why is it important to round line angles in prep?
They are areas of increased stress, so if there is a defect in that area, they facilitate crack propagation.
Why is surface finishing and polishing of ceramics essential
Cracks have a critical depth at which catastrophic propagation will occur, so it’s important to remove surface defects.
Why do brittle materials always show a range of strength values?
Because of the variability of flaw size.
Type of failure that occurs in structures subjected to dynamic and fluctuating stresses.
Fatigue failure
A form of failure which occurs at stresses lower than the fracture strength.
Fatigue failure
What % of metallic failures are due to fatigue?
90%
What is the issue with cyclic loading tests in dental materials?
Loading patterns are more random than cyclical eg chewing, or insertion and removal of dentures
Three factors which affect fatigue life
- Mean stress
- Surface defects
- Environment eg corrosion
The joint action of stress and a corrosive environment is called:
Stress Corrosion Fatigue
Stress corrosion fatigue is formation of a crack due to stress and a corrosive environment, which would not have happened by action of stress or environment alone. It is strongly influenced by:
Surface roughness. Rough surface energy is more prone to corrosion.
What is sub-critical crack propagation?
Propagation which occurs in materials below the critical value of a material. It occurs when there is moisture at the crack tip which allows bonds to rupture. Influenced by water, pH and temp.
Occurs in oral environment for PFM, and all ceramic.
Crack grows until it reaches a critical size and fractures catastrophically
Why is fatigue important to dental biomaterials
Because fatigue is influenced by warm, moist and highly corrosive environmenst
Fatigue can occur in polymers, composites and ceramics
True or false
True
What is visco-elasticity
Exhibiting both viscous and elastic properties when undergoing deformation from an applied stress.
Elastic deformation is reversible whilst viscous deformation is permanent and irreversible.
tRue or false
true
the measure of a fluids resistance to gradual deformation by shear stress or tensile stress.
Viscosity.
Same as ‘thickness’ in liquid
Most dental restoratives, modelling and impression materials start liquid and convert to rigid solids, therefore they are:
Visco-elastic
can composite undergo creep?
Yes. Creep is the small change in shape that results when an object is under continuous constant compression. Can be thought of as slow flow.
What is the spring and dashpot theory
Explain visco-elastic behaviour, where the spring is hookean (elastic) and dashpot is newtonian (viscous)
What is the name of the model of dashpots and springs which is used to describe visco-elastic behavious
Combined Maxwell-Kelvin Voigt Model
