Kirsty's dental materials

Question 1

What features would create the ideal dental restorative material?

Answer 1

Biocompatible Good bonding ability to tooth or bone Good aesthetics Hold properties similar to those it is reconstructing Capable of initiating tissue repair

Question 2

What are the three categories of dental materials?

Answer 2

preventative restorative auxiliary

Question 3

What features lead to obselence in a dental material?

Answer 3

Failure of the material to satisfy any of the features which render it an ideal restorative Replacement with an equal but cheaper product which performs all the same tasks. replacement with an equal product which is less technically sensitive

Question 4

What is the diffusion coefficient?

Answer 4

The amount of a substance that diffuses across a given area through a unit thickness of the substance in one unit of time.

Question 5

What happens to the diffusion coefficient of metals with a low melting point and why?

Answer 5

The diffusion coefficient increases as melting point decreases because as the temperature increases in a substance the atomic bond energy decreases allowing for more rapid diffusion

Question 6

What is meant by wetting ?

Answer 6

The ability of a liquid to flow easily over a solid surface and adhere to the solid.

Question 7

How does contact angle affect wetting?

Answer 7

The higher the contact angle the less wettable the solid. A contact angle of 0 is a perfectly wettable solid and one of 180 is unwettable

Question 8

What is viscosity?

Answer 8

The resistance of a fluid to flow. It is defined as stress divided by strain. stress being the force divided by area over velocity divided by distance of deformity due to force.

Question 9

What does thixotropic mean?

Answer 9

Repeated applications of pressure cause a fluid to become less viscous and subsequently more flowablee

Question 10

What is creep?

Answer 10

Creep is the the time dependant plastic strain of a material under a static load or a constant stress.

Question 11

What is hue?

Answer 11

the predominant colour of an object

Question 12

What is value relating to colour?

Answer 12

It it is the relative lightness or darkness of a colour on the gray scale.

Question 13

What is chroma?

Answer 13

iI is the degree of saturation of a hue.

Question 14

What is metamerism?

Answer 14

Different lights will trait different spectrums and so sub subsequently a colour matched under one light may differ under another light this concept is known as metamerism.

Question 15

What is thermal diffusivity important?

Answer 15

materials with a high thermal diffusivity will transmit heat rapidly.

Question 16

What is the coefficient of thermal expansion?

Answer 16

The change in length per unit of the original length of the material when its temperature is raised by 1 degree.

Question 17

What is the proportional limit ?

Answer 17

The point at which a material fractures or undergoes permanent deformation having reached the limit of elastic stress.

Question 18

What happens to a material when a stress is applied which exceeds the proportional limit of that material?

Answer 18

It breaks or permanently deforms.

Question 19

What is the difference between elastic and plastic strain?

Answer 19

elastic strain is completely revresible plastic strain results in permanent deformity

Question 20

What are the three types of stress?

Answer 20

Tensile : elongation or stretch stress Compressive: shortening or compression stress Shear : where there is sliding or twisting stress.

Question 21

What are the four reasons why shear failure is unlikely to occur in isolation

Answer 21

As the force to create shear failure must be applied parallel to the material interface the following features will make shear failure less likely 1. many of the brittle materials used in restoratives have rough, curved surfaces 2. The presence of chamfers, bevels etc change the curvature of a bonded surface and make shear failure less likely 3. The force must be applied immediate to the shear interface and this is difficult to achieve. 4.the tensile strength of brittle materials is usually below their shear strength and so this normally fails first

Question 22

What does the straight line region represent in the stress strain curve?

Answer 22

The straight line represents the reversible elastic deformation which occurs before the proportional limit is reached.

Question 23

What does P represent on the stress strain curve and what happens at this point?

Answer 23

P is the point at which the material begins to undergo platic deformation and the stress at this point is the proportional limit.

Question 24

What is stiffness?

Answer 24

Stiffness is determined by the slope of the elastic portion of the stress/strain curve

Question 25

What does UTS represent?

Answer 25

The point at which the material fractures in this case 1625 MPa

it stands for ultimate tensile strength.

Question 26

What does E represent on this graph and how is it calculated?

Answer 26

E = the elastic modulus or youngs modulus and it is calculated as stress/strain from the proportional limt.

ie for dentin 176/.015 =11.7 GPa or 11700MPa

for enamel 235/.007= 33.6 GPa or 33600 MPa

The higher the elastic modulus the stiffer a material

looking at the graph aboove dentin is a tougher material more able to withstand flexion withut breaking

Question 27

How is shear modulus calculated?

Answer 27

Shear modulus = elastic modulus divided by 2 x( 1 + Poisson’s ratio)

Poisson’s ratio is typically 0.25-0.3

subsequently Shear modulus is typically 38% of elastic modulus.

Question 28

What is resilience? how is ist determined on a stress strain curve?

Answer 28

resilience is the sponginess of a material and is determined on a stress strain curve as the area under the curve in the period of elastic strain

it is the amount of energy absorbed within a unit volume of structure when it is stressed to its proprtional limit

Demarcated as the green area on this graph.

Question 29

What is proportional limit?

Answer 29

The point at which stress is no longer proprtional to strain.

Question 30

What is elastic limit?

Answer 30

The maximum stress a material can withstand before it becomes plastically deformed.

Question 31

What is yield strength?

Answer 31

The amount of stress required to produce a given amount of plastic strain.

Ie if want 0.2% strain then straight line to curve and across to stress and have yield strength for that strain

Question 32

What is ultimate tensile strength?

Answer 32

the amount of stress required to fracture the material.