Week 4

Question 1

Is the yield point high or low for stiff materials? Flexible materials?

Answer 1

stiff = high
flexible = low

Question 2

Describe the stress-strain curve for brittleness

Answer 2

Closer to 90 = strong
Further from 90 = weak

Question 3

Describe the stress-strain for brittle and ductile materials

Answer 3

brittle = no plastic deformation
ductile = plastic deformation

Question 4

Compare and contrast the stress-strain curves for stiff, ductile, flexible, and brittle materials

Answer 4

Question 5

The measure of energy required to permanently deform a material

Answer 5

resilience

Question 6

Where is resilience on a stress-strain curve?

Answer 6

up to the point of elastic limit

Question 7

the measure of energy required to fracture a material

Answer 7

toughness

Question 8

Where is toughness on a stress-strain curve?

Answer 8

the entire under the curve up to the point of fracture

Question 9

Which material is tougher? Black “ductile” or gray “brittle”?

Answer 9

ductile because the area under the curve will be larger

Question 10

the amount of deformation that a material can withstand before rupture

Answer 10

elongation

Question 11

Compare toughness for a brittle fracture and a ductile fracture

Answer 11

brittle = small toughness
ductile = large toughness

Question 12

What are the three different testing methods that can generate a stress-strain curve?

Answer 12

1. tension
2. compression
3. bending

Question 13

True or false: The tensile and compressive strength (yield) of a material may be significantly different.

Answer 13

True

Question 14

What do the behaviour of materials depend on?

Answer 14

loading mode

Question 15

What is Hooke’s Law?

Answer 15

strain in a solid is proportional to the applied stress within the elastic limit of that solid

Question 16

How is Hooke’s Law related to the force of a spring?

Answer 16

Question 17

When comparing ceramics and polymers for use in a hip prosthesis. What will become deformed more from a 20 kg load, a polymer or ceramics?

Answer 17

polymer since it is less stiff

Question 18

Regarding dental materials, which material has the highest ultimate tensile strength?

Answer 18

zirconia

Question 19

Weakening of a material caused by repeated applied loads, such as mastication.

Answer 19

cyclic loading (fatigue properties)

Question 20

During cyclic loading, maximum stress values that cause damage may be (much more/much less) than the strength of the material.

Answer 20

much less

Question 21

What’s an example of a material that has failed due to cyclic loading?

Answer 21

airplane engine failture

material failed due to repeated low magnitude loads

Question 22

Describe the general trend behind the endurance limit

Answer 22

the less stress you have = the more cycles you get before failure

Example of graph
each point corresponds to a failed specimen so you can see that when the specimen was subjected to a stress close to 480 MPa, after that the material fails (first 3 dots at the top)

if you lower the stress amplitude to 350 for 10,000 cycles, the material will hold for that long which is 9,000 more cycles than the first 3 dots.

Now if you reduce stress to 200 the sample will resist 1,000,000 cycles

Question 23

The resistance of a fluid to flow

Answer 23

viscosity

Question 24

Describe the relationship between viscosity and internal friction

Answer 24

higher viscosity = higher internal friction = greater resistance to flow

Question 25

Describe the behaviour of low and high viscosity materials.

Answer 25

Low viscosity = behaves like a fluid High viscosity = behaves like a solid

Question 26

Describe the graph of different viscosity behaviours: - shear thickening - normal - shear thinning

Answer 26

Question 27

What is shear thinning?

Answer 27

viscosity decreases after external force is applied *solid goes to liquid like

Question 28

What are some examples of shear thinning?

Answer 28

ketchup toothpaste blood

Question 29

What is shear thickening?

Answer 29

viscosity increases after external force is applied *liquid goes to solid like

Question 30

A material property that determines the strain that the material experiences when subjected to combinations of stress and temperature over time

Answer 30

vicoelastic behaviour

Question 31

True or false: Stress-strain is not time dependent for vicoelastic behaviours.

Answer 31

False, it is time depedent

Question 32

combinations of elasticity and viscosity in materials

Answer 32

vicoelastic

Question 33

What is the difference between load-unload cycles in fully elastic and vicoelastic properties?

Answer 33

Question 34

Describe the stress strain curves for elastic, viscous and viscoelastic properties.

Answer 34

Question 35

What are the two viscoelastic effects?

Answer 35

1. Creep = constant stress 2. Stress relaxation = constant strain

Question 36

What are 6 examples of viscoelastic biomaterials?

Answer 36

1. dentin 2. enamel 3. bone 4. cells 5. elastomers (i.e., dental impression materials) 6. polymers (i.e., some acrylic dentures)

Question 37

What are the 6 surface properties?

Answer 37

Question 38

measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion

Answer 38

hardness

Question 39

Commonly used for assessing mechanical properties because they are quick and convenient

Answer 39

hardness tests

Question 40

What are the 6 types of hardness tests?

Answer 40

1. brinell 2. knoop 3. vickers 4. rockwell 5. barcol 6. shore A

Question 41

What is the brinell hardness test (BHN)?

Answer 41

- used for low to medium hardness metals and non-metals - a hard ball is pressed into specie surface with a load of 500, 1500, or 3000 kg

Question 42

What is the rockwell hardness test?

Answer 42

- cone shaped indenter is pressed into specimen using an initial load of 10kg ("initial) - following, a major load f 150 kg is applied, causing indenter to penetrate beyond it's initial position ("major)

Question 43

What is the vickers hardness test?

Answer 43

Question 44

What can we calculate from hardness values?

Answer 44

elastic modulus

Question 45

When a drop of liquid is placed on a surface, it may either spread out or remain in the form of a droplet

Answer 45

contact angle

Question 46

What are the two types of wettability?

Answer 46

1. good wetting 2. bad wetting

Question 47

quantifies the wettability of a solid surface by a liquid

Answer 47

contact angle

Question 48

surfaces attracting water

Answer 48

hydrophilic

Question 49

surfaces repelling water

Answer 49

hydrophobic

Question 50

Describe the surface accuracy for hydrophobic and hydrophilic materials

Answer 50

Question 51

High degree of surface detail and surfaces get more "wet"

Answer 51

hydrophilic

Question 52

Low degree of surface detail and liquid doesn't "wet" the surface sufficiently

Answer 52

hydrophobic

Question 53

angle < 90

Answer 53

hydrophilic

Question 54

angle > 90

Answer 54

hydrophobic

Question 55

What is an example of superhydrophobic and self cleaning?

Answer 55

lotus leaf contact angle exceeds 150 degrees

Question 56

What happens to dirt on lotus leaves?

Answer 56

smooth surfaces - redistributed by water rough surfaces - adhere to the droplets and are removed when droplets roll off

Question 57

These surfaces have contact angles greater than 150 degrees

Answer 57

superhydrophobic

Question 58

Give examples of superhydrophobic surfaces

Answer 58

Question 59

When do you want a material to be hydrophobic?

Answer 59

antibacterial