Failure of Materials 1 Flashcards
What do materials of the mouth need to withstand?
1) Mechanical forces
2) pH variation
3) Temp variation
4) Moist conditions
What will the best dental materials be able to replicate?
Matching properties of the dental materials themselves
Give some brief details on the structure of enamel
- Inorganic materials (95-98%)
- CaP ions making up hydoxyapatite crystals
- Crystals are arranged into rods
Give some brief details on dentine structure
- 70% inorganic and 30% organic
- Contains water
- HA crystals are much smaller
- Dentine has a tubular structure containing dentinal fluid
What are the properties of an ideal restorative material?
- Biocompatible
- Similar properties to enamel and dentine
- Perform in oral environment (pH, temp, moist)
- Assist in tissue regeneration (provide fluoride)
- Remain in place and integral
- Withstand masticatory forces
- Restore aesthetics
- Prevent caries formation
What are the likely reasons for a material failing?
- Oral environment (temp, pH, moisture)
- Masticatory forces
- Bacteria build up
- Secondary caries
Explain what each of these forces means:
- Tensile
- Compression
- Shear
- Torsion
- Bending
Tensile = pulling out in opposite directions
Compression = forces acting from both sides towards the same point
Shear = forces acting on opposite directions
Torsion = twisting motion around axis
Bending = two twisting motions around axis in opposite directions
What two forces is a compressive force formed of?
Shear and tensile forces
Why does a material fail mechanically?
- Physical failure
- Critical stress is exceeded
What are the definitions of stress ans strain?
Stress = a force applied to a material in any given direction - N
Strain = the resultant change in dimension due to the force applied - no units
In a stress-strain graph, what does the gradient represent?
The elastic modulus
What is the definition of the elastic limit (yield point)?
The point beyond which a material will experience permanent deformation (graph is no longer linear).
What type of deformation occurs after the yield point?
Plastic deformation
What difference can we seen between ductile and brittle materials on the S-S graph?
Ductile - deformation occurs for a large extent before failure, large curved part of graph
Brittle - deformation does not occur, the material breaks at the yield point
Before the yield point, what happens when stress is taken off the material?
Material will return to its original shape
What is proof stress?
The point at which 0.1-0.2% deformation (strain) is the proof stress.
What does the elastic modulus measure?
The stiffness of a solid material.
- defines the relationship between stress and strain in the linear elastic region
- higher number = more stiff
What is the stiffness of a material determined by?
The interatomic and intermolecular forces of a material.
Stronger force = more rigid material.
What material in dentistry do we need to use in elastic mode?
Orthodontic wires
What is tensile strength and how do we measure it?
The ability of a material to resist a force that tries to pull it apart.
Une a universal testing machine to pull apart a material of set dimension.
What is compressive strength and how do we test it?
Maximum compressive load a material can withstand without undergoing failure.
Used cylindrical specimens of known dimensions and compressive force appplied.
What can be a problem when doing compressive force tests?
Clamping forces need to be overcome.
Do ceramics have a higher or lower tensile force than composites?
Lower
What is indirect tensile strength?
The diametral compressive strength, also known as the indirect tensile strength, is a property for characterising dental composite restorations.
What is flexural strength and how do we test it?
- Ability to resist deformation under load
For materials that deform a lot but do not break, the load yield is typically measured at 5% deformation of the outer surface and is reported as the flexural strength of the material.
Green specimen supported by 2 supports. Specimen is loaded in the middle and the bending (deformation) is measured.
Define hardness of a material
The resistance to permanent surface indentation.
Sufficient hardness ensures that restorations placed are resistant to in service scratching, due to mastication or abrasion.
How is hardness measured?
The Brinell and the Rockwell hardness tests uses a ball.
- Usually used for a ductile material.
- The larger the indentation, the more ductile the material is.
- The harder the material, the sammler the indent.
- The indent must exceed the local yield strength to give the plastic deformation.
Is enamel or dentine harder?
Enamel
