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Y1 Materials and Manufacturing - Sem 1 > Lectures 5-6 > Flashcards

Lectures 5-6 Flashcards

1
Q

What is used in the tensile test to measure extension? Why use this piece of equip?

A

Extensometer, measure extension accurately

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2
Q

Elastic region for the material means?

A

It can return to its original length, when the force is removed.

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3
Q

Plastic region for the material means?

A

Test piece cannot return to its original length

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4
Q

What does the elastic region look like on a force extension graph?

A

Straight line with constant gradient

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5
Q

What does the plastic region look like on a force extension graph?

A

Straight line ends and it starts to curve (excl. polymers as they do curve from the start)

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6
Q

On a stress strain graph, what is the gradient of the elastic region?

A

Young’s Modulus (modulus of elasticity)

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7
Q

What does a neck forming in the material look like on a force extension graph?

A

The highest point of the plastic region curve

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8
Q

σ eng =

Engineering Stress =

A

F / A (CSA)

Force / cross sectional area

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9
Q

ε eng =

A

Strain =

change in l / l

change in length (extension) / original length

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10
Q

Alternative units for stress

A

Pa (Nm-2)

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11
Q

On a stress strain graph, a material that fails in the elastic region means?

A

It is brittle and no plastic deformation occurs

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12
Q

On a stress strain graph, if the material has a long curve after it enters the plastic region, what does it say about its properties?

A

Ductile material

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13
Q

Brittle materials show no what before failure?

A

Plastic deformation

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14
Q

At a higher temperature what happens to the young’s modulus?

A

It decreases

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15
Q

E =

Young’s Modulus =

A

𝜎/ε

Stress/strain

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16
Q

Young’s modulus units

A

Nm-2 or Pa

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17
Q

What does the initial gradient of polymers look like on a stress strain graph?

A

Curved

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18
Q

Two ways to find the young’s modulus from a curved stress strain graph?

A

Secant and tangent modulus

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19
Q

How to use the secant modulus?

A

Origin to the point of stress that the material is under

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20
Q

How to use the tangent modulus?

A

Draw a tangent at the point of stress that the material is under

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21
Q

What tends to happen to a material which is compressed? And when it is stretched?

A

Compressed - spread wider/outwards

Stretched - contract in

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22
Q

Poisson’s ratio is represented with the letter…

A

v

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23
Q

Another word for limit of proportionality

A

Yielding

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24
Q

In the elastic region two equations for v=

A
  • (ε x / ε z) or -(ε y / ε z)
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25
Q

Tensile strength is where on the graph of stress strain?

A

At the top

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26
Q

What is tensile strength also known as?

A

Ultimate tensile strength - UTS

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27
Q

Offset yield strength is?

A

When a metal does not have a clear point where the elastic and plastic region meet, so 0.02 away from the origin, a line is drawn parallel to the gradient at 0.02 strain.

The line crosses where the offset yield stress is.

28
Q

When do dislocation slips occur in a metal?

A

When the metal starts to yield

29
Q

In a polycrystalline metal, what happens more easily sometimes and why?

A

Slip can happen more easily as some of the crystals are oriented in the same direction as the shear force applied.

30
Q

In a polycrystalline material, can a single crystal deform by itself and why?

A

No, because the neighbour crystals must deform as well

31
Q

What happens to the crystals in a polycrystalline metal when it is put under a load?

A

Elongates the grains

32
Q

What is true stress?

A

Shows the force divided by the actual CSA of the test piece at each stage.

33
Q

2 reasons why its difficult to tensile test a ceramic

A
  • hard to grip the specimen
  • must ensure it is aligned perfectly, to avoid bending
34
Q

Ceramics are —- in tension and —- in compression

A

Weak in tension, stronger in compression

35
Q

Flexural strength is

A

ability for the material to withstand bending forces

36
Q

Glass transition temperature, for polymers, is

A

the temp where the polymer turns from brittle glassy material to a more rubbery ductile material

37
Q

Deformation in a polymer involves what sliding past eachother?

A

Molecules

38
Q

Hardness is

A

Ability for a material to resist plastic deformation (surface indentation or scratching)

39
Q

Four ways to test material hardness

A
  • Brinell test
  • Vickers test
  • Rockwell test
  • Knoop test
40
Q

Two hardness tests used for polymers

A
  • Rockwell
  • Shore
41
Q

Letters and units in the hardness indentation formula? F, D, d

A

F = load (force) in kgf
D = ball indenter diameter in mm
d = diameter of indentation in mm

42
Q

Brittle materials exhibit a sudden what?

A

Fracture

43
Q

Ceramic and glass materials are mostly brittle or ductile

A

brittle

44
Q

Metals can become brittle after they have undergone what process and why?

A

Alloyed or heat treated which can restrict dislocation

45
Q

Toughness is

A

Ability for the material to absorb energy during deformation and fracture

46
Q

Toughness is represented where on a stress strain graph?

A

Area under the graph

47
Q

Toughness can be considered as the —- required to break the material

A

Energy or work

48
Q

Metals with high —- and —- are tough

A

Strength and ductility

49
Q

What does toughness of a material depend on?

A

The speed at which the load is applied

50
Q

Can a material have a higher toughness?

A

Yes, if the load is applied slowly

51
Q

Two methods to test toughness.

A
  • Charpy
  • Izod

Both use a swinging pendulum

52
Q

For many materials, increasing temperature leads to what? 5 points

A

Reduced;
- Young’s modulus
- Yield strength
- Tensile strength
- Hardness
- Brittleness (increased ductility)

53
Q

What transition do materials show when going from a high to low temperature?

A

Ductile to brittle transition

54
Q

Two steps in fracture

A
  • Crack initiation
  • Propagation
55
Q

In ductile materials, crack propagation proceeds with significant what?

A

Plastic deformation, warning before failure

56
Q

In brittle materials, crack propagation happens how?

A

Very little to no plastic deformation and the crack spreads quickly

57
Q

Transgranular means

A

through crystal grains

58
Q

Cleavage means

A

along specific crystal directions

59
Q

Intergranular means

A

through grain boundaries

60
Q

Two defects why most materials tensile strengths are lower than predicted theory

A
  • Crystal defects - vacancies and dislocations
  • Manufacturing defects - porous castings and surface scratches
61
Q

Where would a crack be initiated in a material and why?

A

At the flaw/defect due to a locally increased stress

62
Q

What are defects and flaws usually referred to as?

A

Stress raisers

63
Q

Two example features which are stress raisers in a material?

A
  • Sharp curves
  • Notches
64
Q

What is the alternative test to measure toughness and what does it consider?

A

Fracture toughness - measures the ability of a material CONTAINING flaws to withstand a an applied load

65
Q

To obtain a plane strain fracture toughness of a material…

A

Do the tensile test, with a sample of the material with flaws of known size and shape

66
Q

Few ways t reduce the spread of a crack?

A
  • Ductile materials
  • Thinner materials
  • Slow application of loads (dislocations have time to slip)
  • Increasing the temperature (dislocations can slip faster)
  • Fewer defects
67
Q

What feature needs to be considered when choosing materials for an engineering project?

A

The operating temperature

Y1 Materials and Manufacturing - Sem 1 (8 decks)

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