Failure of materials

Question 1

What in 4 things in the oral environment can affect dental materials?

Answer 1

1. Mechanical forces

Maximum occlusal forces may range from 200-3500N.
First and second molars 400-800N
Incisal forces 150-300N

2. Variation pH - Acidic and alkaline drinks.
3. Variation in temperature- Consumption of hot and cold food and drinks
4. Moist conditions Saliva and oral fluids.

Question 2

What is the composition of enamel?

Answer 2

1. 95-98% inorganic - mainly hydroxyapatite.
2. 1-2% organic.

Question 3

What makes enamel hard and brittle?

Answer 3

Enamel rods

These run from the DEJ to the external surface of the tooth.

Question 4

What is the composition of dentine?

Answer 4

1. 70% inorganic mineral and acellular, hydroxyapatite present but smaller and softer than in enamel.
2. 30% organic: water, collagen, mucopolysaccharides.

Question 5

What is the main structure in dentine?

Answer 5

Dentinal tubule

Extends from external surface to pulp.

Can transmit pain to pulp if exposed.

Question 6

What are 6 desired features of restorative materials?

Answer 6

1. Restore function
2. Comfortable for patient
3. restore aesthetics and maintain overtime
4. restore occlusion and withstands masticatory forces.
5. restoration should be integral and stay ints place
6. Prevent formation of caries and recurrence.

Question 7

What are the 2 main overarching reasons why dental materials fail?

Answer 7

1. Oral environment

Materials are exposed to chemical, thermal and mechanical challenges.

1. Occlusal forces

Ranges from 200 to 3500N.

Highest in posterior region.

Question 8

what are the 5 types of forces?

Answer 8

1. Tensile force- Elongation- where the material is pulled from both sides.
2. Compression- the material is pushed from both sides
3. Shear- material experiences forces from opposite directions:
4. torsion force - Twisting
5. bending force - Bending

Question 9

What 2 force zones does compressive force result in?

Answer 9

all force applications cause complex stress states.

compressio results in:
1. Tensile zone
2. Shear zone

Question 10

What is critical stress?

Answer 10

The point at which the material is about to undergo physical failure.

Question 11

• What does the magnitude of critical stress depend on?

Answer 11

The loading conditions.

E.g. in general a material loaded in shear has a lower critical stress than one loaded in tension.

Question 12

Define stress in dental materials.

Answer 12

When an external force is applied on a test specimen, an internal force, equal in magnitude but OPPOSITE IN DIRECTION is set up in the body.

Question 13

What is the formula for stress and what are the units for stress?

Answer 13

1. Stress = Force / Area
2. Pascal (Pa)

Question 14

Define strain in dental materials.

Answer 14

When an external force is applied on a test specimen it results in a CHANGE IN THE DIMENSION.

Question 15

What is the formula for strain and what are the units for strain?

Answer 15

1. Strain = change in length / original length.
2. No units.

Question 16

What is the elastic region?

Answer 16

1. Linear
2. Region where dimensions remain intact
3. No deformation occurs in this region.

Question 17

What is the yield point/elastic limit?

Answer 17

The point at where if exceeded, the material will experience permanent deformation and changes in dimension.

Question 18

What is the plastic region?

Answer 18

The region where materials undergo deformation and dimension changes

Question 19

What is the failure point?

Answer 19

The point at which fracture of the material occurs.

Question 20

What is Hooke’s law?

Answer 20

Stress is proportional to strain:

Question 21

What is Young’s Modulus?

Answer 21

The pressure below the proportional limit where no deformation occurs:

Question 22

What is the principle of elastic deformation?

Answer 22

The material goes back to its original shape after the load has been placed on it.

Question 23

the elastic modulus and what does it measure?

Answer 23

1. The gradient in the elastic zone.
2. The stiffness of the material in the elastic zone

Question 24

What is proof stress?

Answer 24

The amount of stress a material can endure until it undergoes 0.1-0.2% of deformation.

Question 25

What is the elastic modulus of a material determined by

Answer 25

Interatomic and intermolecules forces of material: 1. Stronger force- stiffer, more rigid material. 2. Same in compression and tension. 3. Independent of heat treatment.

Question 26

How is the elastic modulus calculated and what are its units?

Answer 26

1. Stress/strain in the elastic zone. 2. MPa or GPa

Question 27

If a material has a steeper gradient in the elastic zone what does that say about its stiffness?

Answer 27

Very stiff - indicated by high elastic modulus.

Question 28

If a material has a no/a small plastic zone what does this indicate?

Answer 28

Very stiff- has a lack of ductile features.

Question 29

Why is it important to know the yield point?

Answer 29

As we need to use materials in their elastic mode such as wires for braces.

Question 30

What are the elastic modulus’ of enamel, dentine, resin based composite, porcelain, Pd-Ag alloy, zirconia and alumina?

Answer 30

Enamel - 80-90 GPa Dentine - 15-18 GPa Resin Based Composite - 10-18 GPa Porcelain - 70-80 GPa Pd-Ag alloy - 180-200 GPa Zirconia - 210 GPa Alumina - 340 GPa

Question 31

What does a tensile testing machine do?

Answer 31

1. Pulls the sample from both ends, 2. And measures the force required to pull the specimen apart, 3. And how much the sample stretches before breaking.

Question 32

What is good and bad about tensile strength testing?

Answer 32

Easy to analyse but It is difficult to prepare the specimen.

Question 33

What is true stress?

Answer 33

True stress is the applied load divided by the actual cross-sectional area (the changing area with respect to time) of the specimen at that load

Question 34

Why may engineering stress not yield accurate results?

Answer 34

Stress (engineering stress) is **the applied force divided by the undeformed area over which the force is applied** . May not be accurate because: 1. The cross sectional area changes and the sample decreases in height as testing is conduced. 2. Frictional forces due to clamping have to be overcome. 3. The barrelling effect occurs as the frictional force is not uniform across the specimen:

Question 35

In ceramics what tends to be higher: the tensile strength or the compressive strength? In composite materials what tends to be higher: the tensile strength or the compressive strength

Answer 35

ceramics: compressive composite: tensile

Question 36

What test is used to characterise dental composite restorations and why?

Answer 36

Diametral compressive strength (indirect tensile strength) As you can prep small specimens to test.

Question 37

What is flexural strength and what does high flexural strength indicate about a material?

Answer 37

The ability to resist deformation under load. 1. They do not break 2. They have a 5% deformation of the outer surface 3. They can resist flexing, bending and twisting making them desirable.

Question 38

What’s the difference between a flexure test and a bend test?

Answer 38

1. Flexure test - measure bend strength of brittle material. 2. Bend test - measures crack resistance of ductile material.

Question 39

Where does the fracture occur in a bending strength test?

Answer 39

Side of specimen which is in tension.

Question 40

Define hardness in dental materials.

Answer 40

The resistance a material has to permanent surface indentation.

Question 41

What does Brinell or Rockwell hardness use to test for hardness?

Answer 41

A ball for ductile materials. 1. Size of indent. 2. The smaller then indent for a given load the higher the hardness of material.

Question 42

How is plastic deformation measured in Brinell or Rockwell hardness?

Answer 42

The indent is made to exceed the local yield strength of the material to reach plastic deformation.

Question 43

What is the difference between Knoop and Vickers hardness tests and why are they both useful?

Answer 43

1. Knoop uses a asymmetric diamond tip (A) 2. Vickers uses a symmetric diamond tip. (B) 3. Both make microscopic indentations which are useful for studies on teeth:

Question 44

What are the results for Vickers indentations on enamel and explain these results.

Answer 44

1. Small indent- shows enamel is hard and has a higher elastic region. 2. Indent has cracks- enamel is brittle and has a smaller plastic region.

Question 45

What are the results for Vickers indentations on dentine and explain these results.

Answer 45

1. Large indent- dentine is soft and has a smaller elastic region 2. No cracking - dentine not as brittle as a enamel and therefore has a larger plastic region.