Failure of Materials 2

Question 1

What does elastic modulus tell us?

Answer 1

The stiffness of a material.

Question 2

What does a ceramic look like on a stress-strain graph?

Answer 2

High modulus of elasticity leading to a yield point.

Brittle material.

Question 3

What does a metal look like on a stress-strain graph?

Answer 3

High modulus of elasticity and then some elongation. Ductile material.

Question 4

What is a good value to try and match the modulus of dental materials to?

Answer 4

The modulus of enamel and dentine.

Question 5

Define resilience of a material

Answer 5

Ability of a material to absorb energy when is it deformed elastically.
The energy is released on unloading.

Question 6

Define toughness of a material

Answer 6

Amount of energy a material can absorb before rupturing.

Question 7

How do we calculate resilience on a stress-strain graph?

Answer 7

The larger the area under the curve is a more resilient material.

Question 8

What is Poisson’s ratio?

Answer 8

A measure of the affect where a material expands in directions perpendicular to the application of compression.

Question 9

When a cylinder undergoes tensile forces in axial direction, in what direction does it expand?

Answer 9

Elongation in axial direction leading to reduction in cross-section.

Question 10

When a cylinder undergoes compression forces, what happens to the cylinder?

Answer 10

The cylinder increases in cross-section and reduces in length.

Question 11

What is poissons ratio a relationship between?

Answer 11

Ratio between lateral and axial strain within the elastic limit.

It indicates that the change in cross-section is proportional to the deformation in the elastic range.

Question 12

Do brittle or ductile materials have a higher poissons ratio?

Answer 12

Ductile due to showing more permanent reduction in cross section during tensile forces.

Question 13

If there is a difference in value of poissons ratio and elastic modulus between enamel and a sealant used, what can happen?

Answer 13

When load is applied, there will be different deformation.
enamel = little deformation

This can lead to shear the the interface and ultimately failure.

Question 14

What does it mean for a material to be isotopic?

Answer 14

It has uniform physical properties in all directions.

in real life we have anisotropic materials where they have different properties in different directions

Question 15

What is the fracture toughness of a material?

Answer 15

The materials resistance to crack propagation.

Question 16

How is fracture toughness measured?

Answer 16

A specific material of known dimension is taken and a crack of known dimension is created. Then subject to loading.

As stress is increased, the material experiences more stresses that are concentrated and then stress exceeds critical stress leads to fracture breaking the specimen into two pieces.

Question 17

How is stress intensity and fracture toughness related?

Answer 17

Stress intensity = level of stress at tip of crack

Fracture toughness = highest value of stress intensity under specific conditions without fracture

Question 18

What are the 3 modes of failure of a material?

Answer 18

1) Tensile mode (crack moves directly apart)
2) Sliding mode (crack surfaces slide over each other perpendicularly)
3) Tearing motion (crack surfaces move relative to each other and parallel to leading edge of crack)

Question 19

How can we enhance fracture toughness?

How can we do this specifically for ceramic?

Answer 19

By adding particles into a matrix in the material at the site of crack to prevent crack propagation.

Adding zirconia particles into a ceramic matrix to prevent crack propagation.

Question 20

If enamel cracks but there is no enamel-dentine debonding, what happens to the crack?

Answer 20

The dentine will stop the crack from propagating.

Question 21

We can also place a material under dynamic load.

How do we do this and what do we measure?

Answer 21

Dynamic loading = intermittent loading at lower stresses and defined sequences.

Measure = stress number with number of cycles when subjected to dynamic loading.

Question 22

What curve do we use to determine the fatigue of a material?

What is fatigue?

Answer 22

Cycle-stress curve.

Fatigue = progressive localised plastic deformation which occurs in a material subjected to cyclic stresses at high stress concentration locations.

Question 23

What is the endurance limit of a material?

Answer 23

The level os stress at which below there is an indefinite number of cycles without fracturing.

Question 24

THERMAL:

What is the coefficient of thermal expansion?

What do we want these to match?

Answer 24

Increase in length of a body for an increase in one degree.

We want the tooth and the material to match (if different then marginal gaps can form).

Question 25

What is thermal conductivity and why is this property important?

Answer 25

• Quantity of heat transmitted through a unit thickness due to a unit temperature gradient

Higher conductivity can bring heat close to the pulp and lead to pulp death. (insulating material will be needed between tooth and restorative material)

Question 26

What is thermal diffusivity and why is it important?

Answer 26

Thermal diffusivity = measure of the RATE at which a temperature disturbance at once point travels to another point.

We want a low thermal diffusivity otherwise temperature change will be transferred quickly to the pulp.

Question 27

What is the glass transition temperature and why is it important?

Answer 27

The temperature at which a material changes from a hard glassy state to a slow rubbery state.

• When a polymer is heated, the energy of the chains will be increased leading to motion.
  Over a certain temp, the polymer changes from hard to soft (drop in elastic modulus).
• We want Tg to be higher than the highest temperature in the mouth so this change does not occur

Question 28

In terms of optical properties, what properties do we want the dental material to have?

Answer 28

Translucency (to mimic a tooth)