Properties of materials Flashcards by Rachel Sladden

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

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Define mechanical properties

Indicates how material or component will respond in use i.e. how it will perform in response to applied forces

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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

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What does ISO standard for mechanical properties indicate?

Minimum standard it must achieve in MPa

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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)

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Are ceramics better under tension or compression?

Good in compression, not in tension

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What are the units of stress and strain?

Stress is in MPa

Strain is unitless (distance / distance)

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What is stress?

Stress is the force per unit cross-sectional area acting on a material
Stress = F/A

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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

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What happens to stress as item area dereases?

Stress increases

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How do you work out force?

mass x acceleration

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What is resilience?

The amount of energy a material can absorb without undergoing any plastic deformation (Elastic Area)

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What is toughness?

The amount of energy a material can absorb to the point of fracture

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What is ductility?

The amount of plastic strain at fracture

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On a stress-strain curve, which is on the Y-axis / X-axis?

Strain on X axis

Stress on Y axis

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What is the elastic limit?

End of straight line on graph
At yield stress
After that it’s plastic flow region (permanent deformation)

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What is elastic (Young’s) modulus?

A measurement of the stiffness of material
Worked out by gradient of graph
Units - Pa or MPa

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What is stiffness?

Resistance to elastic deformation

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What is the fracture strength?

Force at fracture

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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