Properties of materials Flashcards
What factors have to be considered when choosing a material?
Unmixed compounds: storage conditions, shelf life
During mixing: heat generation, setting time, viscosity
Set material: strength, toughness, solubility
Define mechanical properties
Indicates how material or component will respond in use i.e. how it will perform in response to applied forces
Why are mechanical properties important?
To ensure material can function in mouth e.g. not fracture or deform under biting fores or changes in T
What does ISO standard for mechanical properties indicate?
Minimum standard it must achieve in MPa
Name 10 mechanical properties
Stress and strain Fatigue (repetition) Hardness (resistance to scratching) Abrasion resistance (wear) Tension (stretch) Compression (squash) Torsional (twist) Shear (2 planes moving in different directions) Bending (combination stress - tension outside, compression inside)
Are ceramics better under tension or compression?
Good in compression, not in tension
What are the units of stress and strain?
Stress is in MPa
Strain is unitless (distance / distance)
What is stress?
Stress is the force per unit cross-sectional area acting on a material
Stress = F/A
What is strain?
Strain is the fractional change in dimensions caused by the force.
Strain = (L1-L0)/L0 i.e. change in length / original length
What happens to stress as item area dereases?
Stress increases
How do you work out force?
mass x acceleration
What is resilience?
The amount of energy a material can absorb without undergoing any plastic deformation (Elastic Area)
What is toughness?
The amount of energy a material can absorb to the point of fracture
What is ductility?
The amount of plastic strain at fracture
On a stress-strain curve, which is on the Y-axis / X-axis?
Strain on X axis
Stress on Y axis
What is the elastic limit?
End of straight line on graph
At yield stress
After that it’s plastic flow region (permanent deformation)
What is elastic (Young’s) modulus?
A measurement of the stiffness of material
Worked out by gradient of graph
Units - Pa or MPa
What is stiffness?
Resistance to elastic deformation
What is the fracture strength?
Force at fracture
What is the ultimate tensile strength?
Max force absorbed
Shape of stress-strain curve for ceramic
Brittle so no plastic region
Put strain on it until it breaks
Can take higher loads than plastics before breaking
Shape of stress-strain curve for elastomer
Very small elastic region, very large plastic region
Become more brittle as material gets older
How do we measure stress and strain?
Using a tensile tester
How does fatigue occur?
When materials are subjected to fluctuating, cyclic loads over time (too small to cause failure)
Accumulation of stress can cause crack propagation –> failure
What are fatigue properties given as?
Fatigue life (no. cycles to failure) Fatigue limit/ strength (cyclic stress required to cause failure for set no. cycles)
What needs to be considered to prevent fatigue?
Shape, surface and homogeneity of materials for stress
What is hardness?
The ability to withstand surface indentation by compressive force
Which is the most common indentation technique used for dental materials? What does it use?
Vickers
Diamond
How is hardness determined?
Directly proportional to size of indentation (hardness no. is large for soft materials and small for hard)
Or scratch test
What does hardness give a good indication of?
Materials resistance to wear (abrasion)
Name 3 groups of chemical and physical properties
Elasticity, viscosity and viscoelasticity (how it responds to the force)
Durability and degradation (how it responds to acids)
Thermal properties (how it responds to heat)
Define elasticity, viscosity and viscoelasticity
Elastic: strain and instantaneously return to original state once stress is removed
Viscous: resist flow and strain linearly with time. When load is released, do not immediately return to original state
Viscoelastic: elements of both. Exhibit time-dependent strain
What are non-Newtonian fluids?
e.g. cornflour and water
becomes solid when you put stress on it, otherwise liquid
What is the study of viscosity called?
Reometry
What is durability?
Ability of a material to withstand its environment e.g. if it dissolves, erodes or corrodes
What is the extent to which a material dissolves in fluid measured by?
Its solubility
What does erosion mean in dentistry?
Destruction of enamel and dentine by acid attack
Can be compounded by mechanical forces such as brushing
What is corrosion?
Deterioration of a material
Usually a metal with its environment
Electrochemical process, can be considered as destructive (natural) oxidation
What are the types of degradation?
Corrosion, erosion and abrasion
What can degradation lead to? What is therefore important
Leaching of constituents into oral environment.
Any leached constituents must be safe locally and systemically
When does heat transfer by conduction take place?
When T gradient exists and is directional from hotter (high momlecular energy) to colder (low molecular energy)
What is thermal conductivity?
Rate of heat flow (Watts per meter) per unit of T gradient under steady state conditions
Units: W m-1 C-1
What is thermal diffusivity?
Measure of readily a material conducts thermal energy to how readily it stores it
What is the equation for thermal diffusivity?
thermal diffusivity = thermal conductivity / (density x heat capacity)
Is a high or low thermal diffusivity preferred? Why?
Low. Less heat conducted to the pulp, higher proportion raises T of material
Why does thermal expansion occur?
Most materials expand when heated due to increase in amplitude of atomic/ molecular vibrations due to absorption of heat energy
How is thermal expansion measured?
For solids, often stated as linear coefficient of thermal expansion (fractional length change per degree of T change)
What is important about thermal expansion in dentistry?
Crucial to minimise thermal expansion mismatches between restorative materials and tooth tissue / between neighbouring materials.
e.g. cores/ substructures and crowns
What is important about exothermic reactions in dentistry?
Use materials formed by combi of 2+ substances, often exothermic i.e. heat generated and transferred to surrounding tooth tissue.
Care required with large restorations, esp. those in close proximity to pulp cavity
