2 - Properties of Dental Materials

Question 1

2.6 What is the mathematical equation for stress?

Answer 1

Stress = ratio of force / area

Units: Pascal or 1 N / m2

Question 2

2.7 What are the five types of stress?

Answer 2

1. Tension (elongation)
2. Compression (shrinkage)
3. Shear (forces directed parallel to each other, but not along the same straight line)
4. Torsion (twisting forces)
5. Bending

Question 3

2. 12 What is strain?

- In what units is it measured by?

Answer 3

Deformation caused by stress

Measured as a unitless value or as a percentage

Question 4

2. 14 Define proportional limit
   - What is the elastic region?
   - What is the plastic region?

Answer 4

The greatest stress sustained without deviation from the linear proportionality of the stress and strain

(The stress-strain graph line is straight just up to this point - then the slope starts to change)

Below the proportional limit point - strain is still reversible (known as the elastic region)

Above this point - strain is irreversible (known as the plastic region)

Question 5

2. 15 Define elastic limit
   - For linearly elastic materials, what is the elastic limit equivalent to?
   - What is the exception to this equivalency?

Answer 5

The greatest stress sustained without permanent deformation

For linearly elastic materials, the proportional limit and elastic limit represent the same stress within the structure (exceptions are super-elastic materials).

Question 6

2.16 Which kind of behavior is typical of permanent deformation as a result of stress - elastic or plastic?

Answer 6

Plastic behavior is typical of permanent deformation

Question 7

2.17 Define yield strength

Answer 7

The amount of stress at which the material begins to function in a plastic manner. Limited permanent strain has occurred.

This is the point in which we can actually detect the material has actually deformed.

Question 8

2.17 Which is higher on the stress-strain graph - proportional limit or yield strength?

Answer 8

Yield strength is slightly higher because it includes a slight amount of the permanent deformation

Question 9

2.18 Give examples of when permanent deformation is bad in the oral cavity

Answer 9

Fillings, crowns, bridges can be deformed - this causes improper occlusion and marginal breakdown.

Partial denture frameworks can be bent out of proper fit

Question 10

2.18 Give examples of when permanent deformation is good in the oral cavity

Answer 10

Orthodontic wires must be bent to retain shape.

Partial clasps must be readjusted

Question 11

2.19 Define ultimate tensile strength (uts)

Answer 11

The max stress that a material can withstand in tension before failing or breaking

Question 12

2.19 Define ultimate compression strength (ucs)

Answer 12

The max stress a material can withstand in compression before fracture or irreversible deformation

Question 13

2.19 Which is typically of greater importance in dentistry and why - yield strength or ultimate tensile strength?

Answer 13

Typically yield strength, because it is an estimate of when a material will start to deform permanently (and possibly begin losing function)

Question 14

2. 20 Define fracture strength (Sf)
   - Is the fracture strength point typically the same as the ultimate strength?
   - For most dental alloys and ceramics subjected to tension, will the ultimate strength and fracture strength be similar?

Answer 14

The stress point at which a brittle material fractures

A material does not always fracture at its point of greatest stress. it may elongate excessively reducing the cross-sectional area (necking) causing a reduction in stress.

For most dental alloys and ceramics, the ultimate strength and fracture strength will be similar.

Question 15

2. 21 Define elongation
   - For alloys, what is elongation an indication of?
   - What units is this expressed in?

Answer 15

A deformation that results from the application of tensile force

An indication of the workability of an alloy. Expressed as a %.

Question 16

2.21 Below the porportional limit - deformation elastic or plastic?

Above the porportional limit - deformation elastic or plastic?

Answer 16

Below: Elastic elongation/deformation

Above; Plastic elongation/deformation

Question 17

2. 23 Define elastic modulus (e modulus)

- How does the slope relate to its elasticity?

Answer 17

The measure of elasticity. Represents the stiffness of a material within the elastic region.

The steeper the slope, the less flexible / the more stiff.

The line is literally the slope of the Stress/strain curve in the elastic portion.

Question 18

2.23 What is responsible for the property of elasticity (e modulus)?

Answer 18

Interatomic/intermolecular forces are responsible for the property of elasticity. The stronger the forces, the more stiff or rigid the material.

Question 19

2.24 Describe the correlation between the value of the Elastic modulus and the elasticity of the material.

Will metals or polymers have a higher E?

Answer 19

The higher the value of E, the more stiff or rigid the material.

Metals will have a higher E.

Question 20

2. 25 Define resilience
   - It is an indication of the amount of energy to do what?
   - Know where it is on the Stress/Strain graph

Answer 20

Resistance of a material to permanent deformation

Indicates the amount of energy needed to deform a material to the proportional limit

It is the area under the elastic portion before the yield strength

Question 21

2. 26 Define toughness
   - It is an indication of the amount of energy to do what?
   - Know where it is on the Stress/Strain graph

Answer 21

Resistance of a material to fracture

Indicates the amount of energy needed to cause fracture

It is the area under the elastic and plastic area

Question 22

2.26 What three factors can be adjusted to affect toughness?

Answer 22

1. Yield strength
2. Ultimate strength
3. Strain

Increasing any of these will increase toughness

Question 23

2.27 What is fracture toughness?

Answer 23

A property which describes the ability of a material containing a crack to resist fracture.

Defects generally weaken a material and may result in fractures at stresses well below the yield stress.

Question 24

2.27 Why do brittle materials tend to have fractures occur at stresses below their yield stress?

Answer 24

Because brittle materials have no ability to deform and redistribute stress. Defects will weaken the material additionally and contribute to early fracturing.

Question 25

2.29 What test is used to measure the tensile properties of brittle materials (amalgam, cements, ceramics, plastic, stone)?

Answer 25

Diametral compression test

• Materials in a cylindrical shape are placed between two surfaces. Load will be increased on the material and load increased until fracture occurs

Question 26

2.30 Brittle materials such as amalgam, resin composities, cements, plaster, and investments have both elastic and plastic properties but the ________ response is small.

Answer 26

The plastic response is small.

Question 27

2. 35 Define fatigue strength

- What two factors does fatigue strength depend on?

Answer 27

The stress at which a material fails under repeated loading
- depends on the magnitude of the load and the number of loadings (the higher the magnitude of the load, the fewer # of loading’s needed to cause failure and vice versa).

Question 28

2.37 Why is the rate of loading important for many materials?

What are some examples of dental materials in which rate of loading is significant?

Answer 28

Some properties of these materials are very dependent on how fast they are stressed.

Increasing the loading rate will produce different stress/strain curves - higher loading rates leads to higher values for E, PL, and US.

Examples: Alginate, elastomeric impression materials, waxes, amalgam, polymers

Question 29

2.38 Viscoelasticity

Compare an elastic material with a viscoelastic material

Answer 29

Elastic material - has mechanical properties independent of loading rate

Viscoelastic - has mechanical properties dependent on loading rate (possesses properties characteristic of both elastic solids and viscous fluids)

Question 30

2.39 Define viscosity

Answer 30

The resistance of a fluid to flow - often time and temperature dependent

Units: poise (p)

Question 31

2. 43 Define stress relaxation

- What’s an example of a dental material that undergoes stress relaxation?

Answer 31

Reduction in stress in a material subjected to constant strain

Ortho bands

Question 32

2. 43 Define creep

- What’s an example of a dental material that undergoes creep?

Answer 32

Increase in strain in material under constant stress

Amalgam

Question 33

2. 50 Define tear strength
   - What dental material is this property important for?
   - What other material property is this dependent on?

Answer 33

Resistance to tearing forces

This property is important for polymers in thin sections (such as impression material in the sulcus or soft liners - critical for maintaining shape when being removed)

Depends on rate of loading (rapid loading = higher values)