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Materials Science 1 > Mechanical Properties of Materials > Flashcards

Mechanical Properties of Materials Flashcards

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1
Q

what is resilience?

A

the capacity to absorb and recover elastic energy

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2
Q

what are the two main standardised test methods for testing the mechanical properties of materials?

A
  • British/European standards (ISO)

- American society for testing and materials standards (ASTM)

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3
Q

what are the main mechanical properties of a material (the standardised language)?

A
  • strength
  • hardness
  • ductility
  • stiffness
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4
Q

what is the most simple mechanical test?

A

applying a load to a sample which will make it extend

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5
Q

what is normal stress defined as (in terms of testing materials)?

A
  • using a uniform static load applied over a certain area
  • stress = force / area
  • units: N/m^2
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6
Q

what is strain defined as (in terms of testing materials)?

A
  • strain = extension / original length

- dimensionless

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7
Q

what is the difference between tensile/longitudinal and lateral strain?

A

tensile/longitudinal/axial strain = change in the length

lateral strain = change in the width

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8
Q

what does elastic deformation mean?

A
  • the deformation is temporary
  • the sample will return to its original shape and size when the force has been removed
  • obeying Hooke’s law
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9
Q

what is another name for Young’s modulus?

A

elastic modulus, E

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10
Q

what is the equation that defines Hooke’s law?

A

E = stress / strain

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11
Q

what does the gradient of a stress-strain graph tell you?

A

E, elastic modulus (which relates to the stiffness of the material) - a higher gradient = more stiff material

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12
Q

what is the area under the curve of a stress-strain graph?

A

the toughness of the material

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13
Q

how does bond strength of the material relate to the stress-strain graph of the material?

A

the slope (gradient is proportional to stiffness) depends on the bond strength of the material

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14
Q

what is poisson’s ratio?

A
  • symbol: v
  • ration of lateral to axial strains
    v = - Ey / Ex
  • dimensionless
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15
Q

what is poisson’s ratio for most metals?

A

0.33

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16
Q

what is poisson’s ratio for most ceramics?

A

0.25

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17
Q

what is poisson’s ratio for most polymers?

A

0.40

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18
Q

what is poisson’s ratio for cork?

A

0

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19
Q

what is an auxetic material?

A
  • materials that have a negative Poisson’s ratio when stretched
  • as we stretch the material, it gets wider
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20
Q

what are auxetic materials used in?

A
  • they are developed in the UK (Exeter)

- applications range from explosion curtains to comfortable mattresses

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21
Q

what materials tend to have the biggest elastic modulus?

A

metals, ceramics, composites

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22
Q

what material tends to have a low elastic modulus?

A

polymers

23
Q

what is E, elastic modulus a measure of?

A

measure of stiffness and bond strength

24
Q

what happens when we exceed the limits of elasticity when deforming a material?

A

plastic deformation will occur (permanent, non-recoverable deformation)

25
Q

what is plastic deformation?

A
  • when Hooke’s law is not being obeyed
  • stress is not proportional to strain
  • bonds are no longer being stretched, bonds are being broken and bonds are being reformed with new neighbours
  • dislocations play a key role here
  • metals will undergo elastic deformation first
26
Q

how far will metals undergo elastic deformation in terms of strain until they start plastically deforming?

A

elastic deformation until strain of 0.5%

27
Q

what does UTS stand for?

A

ultimate tensile stress/strength

28
Q

generally what are the 3 main regions of a stress-strain curve?

A

region 1 - straight line, undergoing elastic deformation, obeying Hooke’s law
region 2 - uniform plastic deformation, strain is uniform across material
region 3 - non-uniform plastic deformation, deformation is limited to “neck” region
FAILURE/FRACTURE

29
Q

where can the yield point be found?

A

where the strain deviates from being proportional to the stress (the proportional limit) - where the line is not straight anymore

30
Q

what is proof stress?

A

the convention we use so we can identify a point at which noticeable plastic deformation has occurred because some materials do not exhibit a distinct transition from the elastic to plastic region - a straight line is drawn parallel to the stress strain curve with 0.2% strain offset

31
Q

what is the yield strength σy?

A

the point at which stress is no longer proportional to strain, after this permanent deformation occurs - it is a measure of resistance to plastic deformation

32
Q

what is the maximum tensile strength or UTS?

A

it is the maximum stress that a structure can sustain before the material fails or fractures…
for metals: occurs when noticeable necking occurs
for polymers: occurs when polymer backbone chains are aligned and about to break

33
Q

what is the definition of ductility?

A

degree of plastic deformation or plastic strain before failure

34
Q

what type of structure (unit cell) are usually ductile materials?

A

gold, silver, copper which are FACE CENTRED CUBIC

35
Q

what is the definition of toughness?

A
  • toughness is the energy to break a unit volume of material
  • the level of resistance to crack propagation
  • a measure of the ability of a material to absorb energy up to fracture (from a sudden impact)
36
Q

what is the area under a stress-strain curve approximately equal to?

A

the toughness of the material

37
Q

what type of energy is involved in a brittle fracture?

A

elastic energy

38
Q

generally what kind of materials are tough and not tough?

A

generally:

  • ductile materials are tough e.g Al
  • brittle materials have low toughness e.g Al2O3 ceramic
39
Q

what is resilience (Ur), or modulus of resilience?

A

the ability of a material to store energy

40
Q

in what region of a stress-strain graph is energy best stored?

A

in the elastic (straight) region, the area under the elastic region is equal to the modulus of resilience

41
Q

what is the equation for resilience?

A

approx = 0.5 σy x εy
= (σy)^2 / 2E (USE THIS)
units: Pa or Nm^-2 or Jm^-3

42
Q

what do resilient materials have in terms of yield strengths and elastic modulus? what are resilient materials used for?

A

resilient materials have…
- high yield strengths
- low elastic modulus
(they are used in spring materials and diving springboards)

43
Q

what is hardness?

A

hardness is the resistance of a material to localised plastic deformation (dents or scratches)

44
Q

what was Mohs Scale of Hardness?

A
  • he devised a scale of hardness by arranging 10 minerals so that one mineral could only scratch those below it
  • it goes from 1 to 10 in intervals of 1
  • starts at 1 with talc
  • ends with 10 with diamond
45
Q

what is hardness related to?

A

σy and E

46
Q

how do we measure hardness?

A

qualitatively - we use a diamond indenter to indent the surface and the depth of the indent is a measure of the hardness

47
Q

what does a large hardness mean?

A
  • high resistance to plastic deformation or cracking in compression
  • better wear properties
48
Q

what can welding do to a material?

A

affects the hardness

49
Q

what is the equation relating to the factor of safety we use when working with materials?

A

σworking = σy / N

where N is the factor of safety (usually between 1.2 and 4)

50
Q

to minimise deformation what type of material should you pick?

A

a material with a large elastic modulus (E)

51
Q

when does plastic deformation occur?

A

when σy has been reached

52
Q

what is the equation relating to ductility?

A

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

53
Q

what type of energy is involved in a ductile fracture?

A

elastic & plastic energy

Materials Science 1 (10 decks)

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