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Engineering materials > Mechanical testing > Flashcards

Mechanical testing Flashcards

1
Q

What type of test is used for brittle materials and why?

A

3 point bend test clamping would damage the specimen reducing the measured specimens

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2
Q

Define stiffness

A

How much a material deflects under a given load

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3
Q

Define strength

A

a measure of how much force is needed to permanently deform or break a material

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4
Q

Define toughness

A

the ability of a material to resist fracture or to withstand impact

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5
Q

Define hardness

A

the ability of a material to resist local plastic deformation

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6
Q

Define true stress

A

It is defined as the force divided by the instantaneous cross sectional area true strain differs due to the change in cross sectional area of the specimen during the test

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7
Q

True strain equation

A

equation

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8
Q

True stress and true strain graph

A
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9
Q

Magneto- striction

A

materials strained by magnetic fields

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10
Q

Piezo-electric materials(uses)

A

materials which respond to electric fields Uses: piezoelectric actuators fuel injectors tyre pressure sensors engine knock sensors keyless door entry

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11
Q

Thermal strain

A

Thermal strain can cause stress in a constrained object

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12
Q

Isotropic mechanical properties

A

properties the same in the each direction regardless of the load applied

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13
Q

Number of properties of composites can be understood using the rule of mixtures. The density is given by…

A

equation

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14
Q

Electrical conductivity of a fibre reinforced composite along the fibres

A

equation

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15
Q

Thermal conductivity

A

equation

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16
Q

Modulus of elasticity along the fibres (Rule of mixtures)

A

equation the equation generally overestimates the yield strength as the matrix will not be fully extended when the fibres fail

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17
Q

Modulus of elasticity perpendicular to the fibres

A

equation

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18
Q

Define cohesive energy

A

It is defined as the energy per atom when solid bonds together

As the cohesive energy increases, the bond strength between the atoms increases

Cohesive energy is a quadratic around the minimum

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19
Q

Stress caused by an atom when a force is applied

A

equation

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20
Q

Types of point defects

A
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21
Q

Interstitial point defects

A

an extra atom is wedged into the crystal causing structural expansion

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22
Q

Substitutional point defects

A

a different atom i aadded into the crystal replacing an original atom

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23
Q

Self-interstitial point defects

A

an atom from the crystal jumps from its original position to elsewhere-remains close to a vacancy

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24
Q

Calculating number of vacancies

A

equation

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25
Q

Why are vacancies present?

A

they are present due to the increase in entropy of the system there more different ways in which the energy is distributed therefore the free energy of the system decreases

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26
Q

Hume-Rothery rules

Alloying elements will only form solid solutions if:

A

the atomic radii of the atoms differ by no more than 15% the atoms have

  • similar electronegativity
  • similar crystal structures
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27
Q

Number of self interstitial defects

A

equation

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28
Q

Difference between the interstitial and vacancy formation energy

A

Interstitial formation energies are generally 2-5x bigger than the vacancy formation energy

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29
Q

FCC interstitial sites

A

4 octahedral interstitial sites per fcc unit cell 8 tetrahedral interstitial sites per fcc unit cell

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30
Q

BCC interstitial sites

A

6 octahedral interstitial sites per bcc unit cell 12 tetrahedral interstitial sites per bcc unit cell

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31
Q

Types of dislocations

A

edge screw dislocations are characterised by the Burgers vector

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32
Q

Edge dislocation

A

additional half plane of atoms

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33
Q

Screw dislocation

A

when a perfect crystal is cut and sheared

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34
Q

Define a slip

A

process by which a dislocation moves

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35
Q

Define a slip plane

A

plane defined by the dislocation line and Burgers vector the plane on which the dislocation moves

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36
Q

Peierls-Nabarro stress

A

the stress required to move the dislocation

the stress which resists slip

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37
Q

Dislocation density

A

the length of the dislocation line contained in a volume of material

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38
Q

A dislocation will always want to be as short as possible.

The line tension associated with a unit length of the dislocation line is given by…

A

equation

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39
Q

Number of obstacles touching per unit length of the dislocation line

A

equation

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40
Q

Pinning force

A

equation

41
Q
A
42
Q

Different ways of strengthening a material

A

solution hardening precipitation hardening work hardening grain boundaries

43
Q

Total stress required to move a dislocation

A
44
Q

Solution hardening

A

isolated substantial atoms in a lattice the solute atoms pin the dislocation line increasing the force that needs to overcome to shear the crystal

45
Q

Solid solution strengthening

A

equation

46
Q

Precipitation hardening

A

occurs when strong particles are dispersed in the path of dislocations particles much larger obstacles than solute atoms

47
Q

Precipitation strengthening

A
48
Q

Work hardening

A

it occurs when a dislocation line is stuck between dislocation lines in different directions dislocation lines intersect with the plance along which the slip occurs there is a jump in the dislocation line when it passes through

49
Q

Work strengthening

A
50
Q

Averaging spacing between dislocations

A

equation

51
Q

Pining force of each dislocation that is cut through

A

equation

52
Q

Grain boundaries

A

occurs when two crystalline regions meet at different orientation

53
Q

Grain boundary strengthening

A
54
Q

How is yield strength measured?

A

it is measured when tension is applied to a specimen and measurements are taken when plastic deformation starts this depends on the inclination of the slip planes relative to the applied load

55
Q

How does increased yield strength affect ductility?

A

increasing the yield strength of a metal results in the reduction of the ductility therefore the maximum elongation is reduced

56
Q

Define proof stress

A

It is a stress needed to cause a small amount of plastic deformation it corresponds to 0.1-0.2% of plastic strain used

57
Q

What happens when the temperature of a polymer is less than 0.75 glass transition temperature?

A

it will be brittle

58
Q

Work hardening stress

A

equation

59
Q

What happens when the temperature of a polymer is greater than 0.75 glass transition temperature?

A

the polymer chains realign under tension this process is known as drawing the drawn object is stronger and stiffer than before due to the alignment, by a factor of about 8

60
Q

Define crazing

A

it occurs when a polymer is at too low of a temperature small cracks form over where the polymer stretches this leads to light scattering and reduced transparency

61
Q

Additives in polymers

A

they are used to improve strength and ease of processing

62
Q

Plasticisers in polymers

A

they are added to lower glass transition temperature and they consist of low molecular weight polymers

63
Q

Reinforcements in polymers

A

they are added to polymers to improve their strength and are usually some form of filament

64
Q

Define delomation

A

It is the separation of layers and it is difficult to detect

65
Q

During a bend test, how is the flexural strength calculated?

A

equation

66
Q

Identify the different methods for hardness testing

A

vickers method rockwell method

67
Q

Benefits of hardness testing

A

a smaller sample is used than in tensile testing the specimen requires no prior preparation it is a non-destructive test

68
Q

Define hardness

A

It is defined as force over an area

69
Q

How is vickers hardness calculated

A

equation It is very useful as it correlates well with wear resistance

70
Q

Impact tests

A

they measure the response of a material to a very high rate of deformation impact specimens are notched as to control where and how they break

71
Q

How do impact tests work?

A

they work by releasing a hammer from a known height the hammer breaks the specimen and the final height of the hammer is measured

72
Q

Impact energy

A

it is measured using the conservation of energy

73
Q

Define fracture toughness

A

It is a measure off the ability of a material containing a flaw to withstand an applied load

74
Q

How is fracture toughness measured?

A

a specimen is prepared with a flaw of known size and geometry and then a load is applied

75
Q

Stress around the crack tip is higher than in the rest of the specimen

How is the local stress calculated?

A

equation

76
Q

Stress near the tip

A
77
Q

Mode 1 stress intensity factor

A

if the stress intensity factor is above a critical value, the crack propagates

78
Q

Fracture toughness calculation

A

equation

79
Q

How are microvoids formed?

A

they are usually formed when ductile metals are under stress they form at grain boundaries or at boundaries with inclusions under stress, the formation of cracks at flaws leads to the failure of brittle metals the crack propagates along the lattice planes leading to characteristic fracture surfaces(often flat with separated cracks visible)

80
Q

How do ceramics and glasses fail?

A

Brittle fracture ceramics fracture along their lattice planes leading to smooth flat fracture surfaces glasses are amorphous and therefore a conchoidal fracture structure is observed

81
Q

Probability of failure is governed by Weibull statistics

A

equation

82
Q

Polymers fail through either ductile or brittle mechanisms What happens below and above the glass transition temperature

A

Below thermoplastics fail through brittle fracture due to the amorphous structure, sane as thermoset polymers above thermoplastic polymers fail through ductile mechanism due to the sliding of the polymer chains deformation can be observed before failure

83
Q

Define fatigue

A

It is a reduction in strength or failure of a material due to repeated application of stresses

84
Q

3 stages of fatigue

A

small cracks occur near the surface close to scratches, pitting, grain boundaries and inclusions localised stress near the crack pit is higher than in the rest of the material. the crack propagates at each loading cycle crack eventually propagates through the whole material and it fractures

85
Q

Fatigue tests

A

a load is applied to the end of a bar and then rotated

86
Q

Endurance limit

A

It exists for most materials below which fatigue will not occur regardless of the number of cycles

no endurance limit = no safe stress below which lack of failure can be guaranteed graph

87
Q

Define fatigue strength

A

it is the maximum stress for which fatigue will not occur for a given number of cycles

88
Q

Define fatigue life

A

It is the lifetime of a component for a given stress

89
Q

Mean stress a component experiences

A

equation

90
Q

Stress amplitude a component experiences

A

equation

91
Q

Goodman relation

A

equation

92
Q

Define creep

A

The process at which material at a high temperature can fail even when the load is below the yield stress

93
Q

Which factors contribute to creep?

A

diffusion dislocation movement grain boundary movement

94
Q

Define stress rupture

A

when a material breaks after creep

95
Q

Creep performance of materials

A

It is measured by applying a load to a material in a furnace and monitoring its deflection over a long period of time

96
Q

Brittle stress rupture

A

voids can form at the intersections of grain boundaries and diffuse along the boundaries

97
Q

Ductile stress rupture

A

necking and many cracks don’t grow to fracture

98
Q

Strain rate of the material

A

equation

Engineering materials (7 decks)

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