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Indirect Exam 2 > Mechanical Properties > Flashcards

Mechanical Properties Flashcards

1
Q

What are mechanical properties associated with and depend on?

A
  • deal with internal structures
  • associated with response to externally applied forces
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2
Q

Some physical properties

A
  • density
  • coefficient of thermal expansion
  • optical
  • solubility
  • viscosity
  • electrical/thermal conductivity
  • melting/solidification point
    remember, independent of mechanical manipulation
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3
Q

What does testing mechanical properties tell us?

A

a useful predictor of clinical success especially when comparing materials

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4
Q

What is a force/load?

A
  • push/pull exerted on the material
  • units: pound, newton
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5
Q

What is stress? And the different types?

A
  • force per unit area

types:
- tension: 2 sets of forces directed away from each other in same straight line
- compression: 2 sets of forces directed toward each other in same straight line
- shear: 2 sets of forces directed toward each other but not in the same line

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6
Q

In the mouth, compression can be translated to:

A

tension
(especially in sharp line angles representing a wedge)

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7
Q

What are the biting forces of each tooth?

A

Biting Forces:
- molars: 130 lbs
- premolars: 70 lbs
- incisors: 40 lbs

Biting Stresses:
- molars: 130,000 psi

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8
Q

What is the tensile strength compared to compressive strength of amalgam?

A
  • tensile strength is much less (1/9-1/10 of compressive strength)
    EvoCeram/composite compressive strength is much less than amalgams compressive strength though. tensile strength is 1/5 less than its compressive strength
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9
Q

T/F: small flaws can cause local high stresses and unexpected fracture at low stress

A

True!
Even a notch or scratch

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10
Q

What is strain?

A
  • change in length/original length
  • amount of deformation per unit length
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11
Q

What is strength the measure of?

A
  • measure of level of stress required to make a material fail
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12
Q

Elastic Deformation:

A
  • initially= strain is proportional to stress
  • reversible
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12
Q

What is a stress- strain diagram?

A

graphic representation of deformation of a material as stress is applied
- shows how far a material can stretch out before it breaks when stress is applied

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13
Q

What is modulus of elasticity?

A
  • measures rigidity (ratio of stress to strain in straight line)
  • property can not be changed because it’s the measure of the interatomic bonding
  • higher number, more rigid
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14
Q

What is one of the most rigid materials?

A

Cobalt-chromium

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15
Q

The steeper the slope on diagram, the more ________ the material

A

rigid

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16
Q

What is plastic deformation?

A
  • permanent deformation of atoms so strain is NOT reversible
  • once it starts, strain will increase faster than stress (will fracture quickly after putting more stress on it)
17
Q

What is proportional limit?

A
  • maximum stress a material can withstand without deviation (if stretch beyond the limit, will not go back to original)
18
Q

increasing the proportional limit will __________ the resistance of the material to permanent deformation

A

increase
- high proportional limit= strong
- low proportional limit= weak

19
Q

What is elastic limit?

A

basically the same as proportional limit (same on the graph)

20
Q

What is yield strength?

A
  • stress at which a material exhibits a small amount of permanent deformation
  • just a little higher than proportional/elastic limit on the graph
  • arbitrarily selected by manufacture
21
Q

Any restoration/appliance that deforms in service is…

A

usually a clinical failure

22
Q

At what psi can a material’s yield strength be burnished?

A

below 30,000 psi
(over that can not be burnished)

23
Q

What is ultimate stress?

A

maximum stress before failing
- highest point on curve

24
Q

What is elongation/ductility?

A
  • how much a material will stretch out/tensile force before it fractures
  • indicates how much you can burnish a material
  • the length of the curve shows the ductility
25
Q

Total % elongation includes:

A

both elastic and plastic deformation
- an alloy with high value can be bent permanently with less risk of fracture
- value should be at least 10% elongation to be useful

26
Q

Difference between malleability and ductility

A
  • malleability: ability to be hammered or rolled into thin sheets
  • ductility: ability to be drawn into wire/stretching. related to burnish ability
27
Q

Wrought wires are….

A

flexibile

28
Q

Cast alloys are…

A

ductile

29
Q

What is resilience?

A
  • how much energy you can put into material until it permanently deforms (until it reaches its elastic/proportional limit)
  • measured by area under elastic portion of a stress-strain diagram
30
Q

What is toughness?

A
  • amount of energy necessary to fracture a material
  • area under the total curve
31
Q

What is the hardness of material?

A
  • resistance to permanent indentation
  • harder=harder to polish
32
Q

What is the Brinell Hardness test method?

A

-small steel ball that makes indentation in materal
- measure indentation… smaller indentation=higher hardness
- used for metals and alloys, not brittle materials like porcelain

33
Q

What is a diamond pyramid hardness test?

A

basically same as brine hardness just diamond shape

34
Q

What is knoop hardness?

A
  • diamond tooth with one long diagonal and one short diagonal that indents and cuts the material and then measure
  • used in all dentistry materials
35
Q

What does knoop hardness tell us clinically?

A
  • when opposing two materials, the weaker hardness will wear and the harder material will be fine (enamel vs porcelain… porcelain will win and enamel will wear)
36
Q

What is Rockwell hardness used for?

A
  • elastic materials
37
Q

What does yield strength tell us?

A

burnishability

38
Q

What does tensile strength tell us?

A

load-bearing ability

39
Q

What does elongation tell us?

A

similar to yield strength
- burnishability
- margin-finishing properties

40
Q

What does hardness tell us?

A
  • wear resistance
  • finishing/polishing
41
Q
A

Indirect Exam 2 (7 decks)

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