Failure of Materials 1

Question 1

What do materials of the mouth need to withstand?

Answer 1

1) Mechanical forces
2) pH variation
3) Temp variation
4) Moist conditions

Question 2

What will the best dental materials be able to replicate?

Answer 2

Matching properties of the dental materials themselves

Question 3

Give some brief details on the structure of enamel

Answer 3

• Inorganic materials (95-98%)
• CaP ions making up hydoxyapatite crystals
• Crystals are arranged into rods

Question 4

Give some brief details on dentine structure

Answer 4

• 70% inorganic and 30% organic
• Contains water
• HA crystals are much smaller
• Dentine has a tubular structure containing dentinal fluid

Question 5

What are the properties of an ideal restorative material?

Answer 5

• Biocompatible
• Similar properties to enamel and dentine
• Perform in oral environment (pH, temp, moist)
• Assist in tissue regeneration (provide fluoride)
• Remain in place and integral
• Withstand masticatory forces
• Restore aesthetics
• Prevent caries formation

Question 6

What are the likely reasons for a material failing?

Answer 6

• Oral environment (temp, pH, moisture)
• Masticatory forces
• Bacteria build up
• Secondary caries

Question 7

Explain what each of these forces means:

• Tensile
• Compression
• Shear
• Torsion
• Bending

Answer 7

Tensile = pulling out in opposite directions

Compression = forces acting from both sides towards the same point

Shear = forces acting on opposite directions

Torsion = twisting motion around axis

Bending = two twisting motions around axis in opposite directions

Question 8

What two forces is a compressive force formed of?

Answer 8

Shear and tensile forces

Question 9

Why does a material fail mechanically?

Answer 9

• Physical failure

- Critical stress is exceeded

Question 10

What are the definitions of stress ans strain?

Answer 10

Stress = a force applied to a material in any given direction - N

Strain = the resultant change in dimension due to the force applied - no units

Question 11

In a stress-strain graph, what does the gradient represent?

Answer 11

The elastic modulus

Question 12

What is the definition of the elastic limit (yield point)?

Answer 12

The point beyond which a material will experience permanent deformation (graph is no longer linear).

Question 13

What type of deformation occurs after the yield point?

Answer 13

Plastic deformation

Question 14

What difference can we seen between ductile and brittle materials on the S-S graph?

Answer 14

Ductile - deformation occurs for a large extent before failure, large curved part of graph

Brittle - deformation does not occur, the material breaks at the yield point

Question 15

Before the yield point, what happens when stress is taken off the material?

Answer 15

Material will return to its original shape

Question 16

What is proof stress?

Answer 16

The point at which 0.1-0.2% deformation (strain) is the proof stress.

Question 17

What does the elastic modulus measure?

Answer 17

The stiffness of a solid material.

• defines the relationship between stress and strain in the linear elastic region
• higher number = more stiff

Question 18

What is the stiffness of a material determined by?

Answer 18

The interatomic and intermolecular forces of a material.

Stronger force = more rigid material.

Question 19

What material in dentistry do we need to use in elastic mode?

Answer 19

Orthodontic wires

Question 20

What is tensile strength and how do we measure it?

Answer 20

The ability of a material to resist a force that tries to pull it apart.
Une a universal testing machine to pull apart a material of set dimension.

Question 21

What is compressive strength and how do we test it?

Answer 21

Maximum compressive load a material can withstand without undergoing failure.

Used cylindrical specimens of known dimensions and compressive force appplied.

Question 22

What can be a problem when doing compressive force tests?

Answer 22

Clamping forces need to be overcome.

Question 23

Do ceramics have a higher or lower tensile force than composites?

Answer 23

Lower

Question 24

What is indirect tensile strength?

Answer 24

The diametral compressive strength, also known as the indirect tensile strength, is a property for characterising dental composite restorations.

Question 25

What is flexural strength and how do we test it?

Answer 25

• Ability to resist deformation under load

For materials that deform a lot but do not break, the load yield is typically measured at 5% deformation of the outer surface and is reported as the flexural strength of the material.

Green specimen supported by 2 supports. Specimen is loaded in the middle and the bending (deformation) is measured.

Question 26

Define hardness of a material

Answer 26

The resistance to permanent surface indentation.

Sufficient hardness ensures that restorations placed are resistant to in service scratching, due to mastication or abrasion.

Question 27

How is hardness measured?

Answer 27

The Brinell and the Rockwell hardness tests uses a ball.

• Usually used for a ductile material.
• The larger the indentation, the more ductile the material is.
• The harder the material, the sammler the indent.
• The indent must exceed the local yield strength to give the plastic deformation.

Question 28

Is enamel or dentine harder?

Answer 28

Enamel