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Materials Science > Topic 4- Mechanical properties of materials > Flashcards

Topic 4- Mechanical properties of materials Flashcards

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1
Q

What are the British/European standards for test methods?

A

ISO

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2
Q

What American group set standard test methods?

A

American Society for Testing and Materials (ASTM)

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3
Q

How can we measure strain and deformation as a material is stretched?

A

Using a strain gauge and clamps.

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4
Q

Why is it important to use a strain gauge when testing materials?

A

Without the strain gauge we would end up measuring the displacement of everything, not just the sample, e.g. grips.

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5
Q

How do we measure stress?

A

Most commonly using a tensile test with a uniform static load applied.

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6
Q

What are the units for stress?

A

N/m^2 or N/mm^2

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7
Q

What is the equation for stress? What do each of the symbols stand for?

A

Ft/Ao.
Ft = tensile force.
Ao = original area before loading.

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8
Q

What is the difference between tensile strain and lateral strain?

A

Tensile strain is applied, lateral strain is resulting.

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9
Q

What is the equation for tensile strain?

A

Extension / original length

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10
Q

What is the equation for lateral strain?

A
  • extension x original length / original width
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11
Q

What is the gauge length?

A

The length of the material that will be subject to stretching.

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12
Q

Is elastic deformation reversible or non-reversible?

A

Reversible

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13
Q

What type of materials have a high E (Young’s Modulus)?

A

Ceramics

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14
Q

Which materials have a low E?

A

Polymers

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15
Q

How does Young’s Modulus relate to stiffness?

A

Higher E = more stiff.

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16
Q

What does the slope of a stress-strain graph depend on? (Slope proportional to elastic modulus)

A

Bond strength of material.

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17
Q

What is E for Aluminium (Al)?

A

70 GPa

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18
Q

What is E for steel?

A

210 GPa

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19
Q

What is E for aluminium oxide (Al2O3)? (alumina ceramic)

A

380 GPa

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20
Q

What is E for Red Oak wood (anisotropic) along the grain and across the grain?

A

Along the grain: 12 GPa.

Across the grain: 0.6 GPa.

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21
Q

What is Poisson’s ratio, v?

A

The ratio of lateral to axial strains.

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22
Q

What is Poisson’s ratio for metal?

A

0.33

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23
Q

What is Poisson’s ratio for ceramics?

A

0.25

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24
Q

What is Poisson’s ratio for polymers?

A

0.4

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25
Q

What is Poisson’s ratio for cork? What does this mean?

A

0

It doesn’t expand as you push down, hence you can get cork in a wine bottle.

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26
Q

What are auxetic materials? Give two applications.

A

Materials that have a negative poisson’s ratio when stretched.
Explosion curtains and comfortable beds.

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27
Q

What does plywood allow for?

A

Get around problem of low GPa across the grain.

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28
Q

When does plastic deformation occur?

A

If we exceed the limits of elasticity.

29
Q

Until what strain do metals elastically deform?

A

Strain of 0.5%

30
Q

What is plastic deformation?

A

Permanent, non-recoverable deformation.

31
Q

What happens during plastic deformation?

A

Breaking bonds, reforming bonds with new neighbours and large numbers of atoms move relative to one another.

32
Q

What plays a key role in plastic deformation?

A

Dislocations

33
Q

What law to materials obey in the elastic region?

A

Hooke’s Law.

34
Q

What three features occur during the plastic region?

A
  1. Hardening occurs.
  2. Dislocation motion and generation.
  3. Strain is uniform across material.
35
Q

What happens to the sample beyond the tensile stress?

A

Necking occurs, deformation in non-uniform plastic.

36
Q

What is ‘necking’? How does it affect a stress plot?

A

When cross-section gets smaller and smaller, stress plots aren’t the true stress. Not due to Poisson’s ratio.

37
Q

What does proof stress eliminate?

A

Misreading of yield point.

38
Q

What is proof stress?

A

Stress at which noticeable plastic deformation has occurred. Measured at strain = 0.002.

39
Q

What is yield strength?

A

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

40
Q

What occurs beyond yield strength?

A

Permanent deformation.

41
Q

What is yield strength a measure of?

A

Resistance to plastic deformation (onset of dislocation movement in most metals).

42
Q

What is tensile strength?

A

Maximum stress that a structure can sustain.

43
Q

Where is tensile strength measure on a stress-strain curve?

A

Maximum stress.

44
Q

When does tensile strength occur in metals?

A

When noticeable necking starts.

45
Q

When does tensile strength occur in polymers?

A

When polymer backbone chains are aligned and about to break.

46
Q

Why do we engineer to yield strength not tensile strength?

A

Want to avoid dangerous region of permanent deformation and necking.

47
Q

What is ductility?

A

The ability of a material to be stretched. It is the degree of plastic deformation before failure.

48
Q

How do we measure ductility?

A

Plastic tensile strain at failure.

49
Q

What is the equation for ductility?

A

%EL = (Lf - Lo)/Lo x 100

50
Q

What are the two definitions of toughness?

A
  1. Measure of the ability of a material to absorb energy up to fracture.
  2. Materials resistance to fracture when a crack is present (fracture toughness).
51
Q

How can we easily approximate toughness?

A

Area under stress-strain curve.

52
Q

How do we measure toughness?

A

Energy to break a unit volume of material.

53
Q

In general are ductile or brittle materials tough?

A

Ductile materials are usually tough (e.g. aluminium), brittle materials usually have a low toughness (e.g. Al2O3).

54
Q

Which materials have high tensile strength but small toughness?

A

Ceramics.

55
Q

Which materials have medium tensile strength but large toughness?

A

Metals.

56
Q

Which materials have low tensile strength and very small toughness?

A

Unreinforced polymers.

57
Q

What is resilience?

A

Capability of a material to absorb energy during elastic deformation and then on unloading recover the energy.

58
Q

What do resilient materials have?

A

High yield strengths and low elastic modulus.

59
Q

Give two examples of resilient materials?

A

Spring materials, diving boards.

60
Q

What is the modulus of resilience, Ur?

A

Strain energy per unit volume required to stress a material to the yield point.

61
Q

How can we measure modulus of resilience?

A

Area under stress-strain curve. Easier if we assume linear stress-strain curve.

62
Q

Who devised a scale of hardness by arranging 10 minerals so that one mineral could only scratch those below it?

A

Friedrich Mohrs, 1812.

63
Q

What is Mohrs scale of hardness (1-10)?

A
  1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase 7. Quartz 8. Topaz 9. Corundum 10. Diamond
64
Q

What is hardness?

A

Resistance of materials to localised plastic deformation (dents or scratches).

65
Q

What is hardness related to?

A

Yield strength and Young’s Modulus.

66
Q

How do we measure hardness?

A

Quantitative analysis - small indenter forced into the surface and depth of indent is a measure of hardness.

67
Q

What is an arbitary property?

A

Hardness.

68
Q

Give two example of hardness tests and the indenter used.

A

Brinell - 10mm sphere of steel or tungsten carbide.

Vickers microhardness - Diamond pyramid

69
Q

How do we calculate safety factor, N?

A

Working stress = yield stress / N

Materials Science (8 decks)

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