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DS1102 > Week 4 - Material Properties > Flashcards

Week 4 - Material Properties Flashcards

1
Q

What 3 factors determine system performance

A
  • properties (strength, stiffness, toughness, optical)
  • design (size, shape)
  • Environmental (fatigue, creep, wear, corrosion)
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2
Q

What are the mechanical properties of materials (5)

A
  • Strength
  • Stiffness
  • toughness
  • fatigue
  • wear
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3
Q

What are the types of strength

A
  • static strength
  • Creep strength
  • Fatigue strength
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4
Q

What is static strength

A

the ability to resist a short-term steady load

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5
Q

What is creep strength

A

the ability to resist a continuous load over a long period

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6
Q

What is fatigue strength

A

the ability to resist cyclic loading (most relative) e.g. chewing

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7
Q

What are the types of load/force applications

A
  • tensile load
  • compressive load
  • shear load
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8
Q

What is tensile load

A

forces are applied away from the material (pulling it away)

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9
Q

What is compressive load

A

forces applied towards the material (pushing in)

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10
Q

What is shear load

A

2 forces in opposite directions but at different points of the material

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11
Q

What is stress

A

the force per unit cross-sectional area, that is acting on a material

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12
Q

What is strain

A

the fractional change in the dimensions caused by the force

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13
Q

What is tensile strength

A

a measure of how much pulling force a material can withstand before it breaks.

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14
Q

What does the linear segment of the graph represent

A

Elastic behaviour phase, which is where the material will return to its original state after tensile force.

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15
Q

What does the angle of the linear segment represent

A

elastic modulus, which shows the stiffness of the material, the higher the material the more stiff the material is (unit GPa)

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16
Q

What does the curved segment represent

A

Plastic flow, where the material starts to deform and is at its maximum strength

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17
Q

What does ultimate tensile strength mean

A

the maximum strength of the material, the max stress the material can handle

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18
Q

Materials with large plastic flow represents what

A

that the material is ductile or malleable

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19
Q

What are the 2 types of strength test

A
  • compressive strength test
  • flexural strength test
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20
Q

what is the flexural strength test and what forces does it generate

A
  • bending force test
  • produces compressive stress at the top layer
  • produces tensile stress at the bottom
  • In the middle shear stress is generated
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21
Q

The strength of a material is dependent upon

A
  • the type of stress which it needs to withstand (e.g. tensile, compressive, flexural)
  • The magnitude of the stress
  • The pattern of stress distribution/concentration which it will experience
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22
Q

What is stress concentration

A

a region in a material/structure, usually around a hole or a crack or a sharp angle, where the stress is highly magnified

Brittle materials (e.g. ceramic) concentrate stress more than ductile materials (gold)

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23
Q

What is the strength of a material dependent on

A
  • Flaw size
  • flaw distribution
  • volume of the sample
  • stress distribution in the sample
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24
Q

What is stiffness

A

the ability of a structure to maintain its shape when acted upon by a load.

25
Q

What does the elastic modulus represent

A

the direct representative of stiffness (not elasticity)

26
Q

What is the effect of stiffness on stress distribution

A

the applied load follows the direction of the more stiff material. Hence
- A low modulus(low stiffness) restorative material reduces the stress in the restoration but increases the stress in the tooth
- Mid range modulus (stiffness) restorative materials predict a stress distribution very similar to that of the natural tooth
- High modulus materials increase the stress in the restoration

27
Q

What is toughness

A

the resistance to crack propagation (or energy absorbed to fracture) - how long until the material fractures. Includes the area under the whole graph before the material fractures

28
Q

What is resilience

A

the measure of stored elastic energy (how much energy the material can stored before it becomes plastic) - the area under only the elastic part of the graph

29
Q

How to make a brittle solid stronger

A
  • reduce inherent flaw size (size of cracks)
  • reduce number of defects (number of cracks)
  • surface compression
  • resist crack propagation e.g. filler particles in composites (deflects the stress), rubber particles (absorb stress)
30
Q

What is wear

A

destruction of surface of materials due to mechanical or chemical attacks

31
Q

What are the 3 types of wear

A
  • Abrasive wear (Abrasion)
  • Fatigue wear
  • Corrosive wear
32
Q

What is abrasive wear (abrasion)

A

wear caused by indenting or scratching of surface by:
- abrasive toothpaste
- incorrect brushing technique
- food or foreign objects

33
Q

What is hardness

A

the resistance of a material against penetration of an indenter. (Depending on the shape of the indenter there are different methods of hardness)

34
Q

What is fatigue wear

A

the process of crack initiation and propagation under cyclic loads. Fatigue is a process of slow crack growth

35
Q

What is fatigue failure

A

the failure of structures at stresses well below the tensile strength of the material used when it has been subjected to repeated stress cycles. It occurs at low loads with no outward signs of a problem.

36
Q

What are characteristics of fatigue failure

A
  • Crack initiation

Where a crack forms at some point of high stress concentration

  • Crack propagation

During which the crack advances incrementally with each stress cycle

  • Catastrophic failure

Which occurs very rapidly once the crack reaches a critical size

37
Q

What is the equation for fatigue life

A
38
Q

What is corrosive wear

A

where both mechanical wear (abrasion) and chemical corrosion processes act together, leading to the accelerated deterioration of a material. Corrosive agents such as chemical attacks weaken the structure of materials and abrasive forces remove the structure.

In an oral environment most of the time combination of mechanical and chemical attacks causes wear (abrasion-erosion lesion)

39
Q

What is creep

A

time dependent deformation of a structure under prolonged load

a permanent deformation of materials when subjected to a constant or cyclic load for a period of time

40
Q

What are factors which influence creep

A
  • applied load
  • temperature
  • time
41
Q

How does low contact angle influence wetting

A

good wetting

42
Q

How does high contact angle influence wetting

A

poor wetting

43
Q

If a material has elastic behaviours what does that mean

A

Instantaneous and full recovery occurs after removal of load. The deformation and recovery are fast and is not time dependent e.g. spring

44
Q

If a material has plastic (viscous) behaviour what does that mean

A

Material retains deformation after removal of load.

45
Q

If a material has viscoelastic behaviour what does that mean

A

combination of viscous(plastic) and elastic behaviour
- The deformation under certain conditions can be time dependent
- The recovery after loading can be complete (elastic) or incomplete (viscose) meaning that it remains permanently deformed

e.g. the permanent distortion of dental impressions after a period of time and due to force applied

46
Q

What type of behaviour is this

A

elastic behaviour

47
Q

What type of behaviour is this

A

Plastic (viscous) behaviour

48
Q

What type of behaviour is this

A

Viscoelastic behaviour

49
Q

What is stress relaxation

A

the reduction of stress in the material when subjected to a constant strain.

50
Q

What are Dilatant fluids

A

viscosity increases with increasing shear rate (shear thickening)

51
Q

What are newtonian fluids

A

shear stress is directly proportional to shear rate and the viscosity f the material is constant and independent of shear rate

52
Q

What are pseudoplastic fluids

A

viscosity decreased with increasing shear rate (shear thinning)

53
Q

How does increasing temperature affect the viscosity of a non setting material

A

reduces viscosity

54
Q

How does increasing temperature influence the viscosity of a setting material

A

increases viscosity

55
Q

What are the different hardness tests

A
  • Vikers
  • Knoop
  • Brinell
  • Rockwell
56
Q

What does the diametral tensile strength test measure in dental materials

A

indirect tensile strength

57
Q

Define creep

A

time dependent deformation of a structure under prolonged load

58
Q

What is creep strength

A

the ability to resist a continuous load over a long period

DS1102 (30 decks)

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