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ENGEN 112 > Failure And Degradation > Flashcards

Failure And Degradation Flashcards

1
Q

How does fracture occur

A

Exceeding the UTS

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2
Q

How does fatigue occur

A

Failure due to repeated stress/strain

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3
Q

What is creep

A

Time dependent plastic deformation when subjected to a constant load

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4
Q

What is brittle fracture

A

Catastrophic failure
- very little plastic deformation before fracture
Rapid breaking of bonds
Little energy absorption up to fracture

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5
Q

What is ductile fracture

A

Slow failure
- significant plastic deformation before fracture
- “slow” breaking of bonds
- significant energy absorption can occur

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6
Q

Draw a graph of brittle fracture and ductile fracture

A

See notes

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7
Q

What is the process of ductile fracture

A
  • under stress
  • necking ( bonds start breaking)
  • formation of microvoids and cavities
  • coalescence of voids and cavities to form elliptical crack
  • rapid propagation of crack by shear deformation at 45 degrees to tensile axis
  • resulting in cup and cone + dimples
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8
Q

Process of brittle fracture

A
  • crack propagates rapidly and perpendicular to tensile axis
  • little plastic deformation around crack tip
  • fracture in the crystalline brittle materials = cleavage ( breaking bonds along an atomic plane)
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9
Q

What is fratography

A

Examination of fracture surfaces using a microscope

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10
Q

What signs show ductile fracture (microscopic)

A

“Fibrous” central regions consisting of multiple “dimples”
Extensive “tearing” of material

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11
Q

Brittle fracture at low magnification

A

V shaped “chevron” marks or radial fan shaped ridges
V’s point to where failure started
Lines meet together at a point where failure started

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12
Q

What are the names of brittle fracture at high magnification

A

Transgranular or intergranular

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13
Q

What are the signs of transgranular fracture

A

Fracture through crystal grain
Eg. polystyrene with a hot knife

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14
Q

What are the signs of intergranular fracture

A

Fracture along grain boundaries
Eg. pulling apart polystyrene

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15
Q

What surface level feature shows ductile fracture

A

Cup and cone

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16
Q

What surface feature shows brittle fracture

A

Flat, faceted fracture surface

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17
Q

What is fatigue

A

When materials fail due to repeated/fluctuating stresses (usually over a long period of time)

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18
Q

What are stress forms

A

Stress forms can cause fatigue failure

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19
Q

What is the equation for stress ratio

A

Stress ratio = stress min/ stress max

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20
Q

Equation for stress amplitude

A

Stress amplitude = stress range / 2

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21
Q

Equation for stress range

A

Stress range = stress max - stress min

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22
Q

What are the 3 general forms that a stress cycle can be

A

Reversed (mean stress = 0)
Repeated (mean stress ~= 0)
Random

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23
Q

How should fatigue testing be done

A

Duplicate as nearly as possible the service stress conditions
- alternations compression/tension
- rotating/bending test apparatus

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24
Q

What materials experience a fatigue limit

A

Most ferrous and titanium alloys

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25
Q

What does fatigue limit mean

A

Bellow a certain stress no fail true will occur

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26
Q

What does no fatigue limit mean

A

The material will eventually fail

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27
Q

What is the first stage of fatigue failure

A

Stage 1 propagation ( crack initially propagates very slow - along crystallographic planes)

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28
Q

What is the second stage of fatigue failure

A

Much more rapid
Crack propagation direction changes to perpendicular to stress direction
Occurs due to repeated opening and closing of the crack

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29
Q

What is the third stage of fatigue failure

A

Fatigue failure
Crack reaches critical size
Can either have ductile or brittle characteristics

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30
Q

What can be seen from fatigue fractography

A

Benchmarks
Then brittle or ductile marks

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31
Q

How is fatigue protected against (4 ways)

A
  • intelligent design
  • reduce the mean stress or stress amplitude
  • surface finishing
  • case hardening , just the outside harder
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32
Q

Why is high temperature needed in creep

A
  • creep is a temp activated process
  • temp greater than 40% of melting point, termed homologous temp
  • temp above which creep starts to occur
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33
Q

What is creep testing

A

Tested by applying a constant load to a specimen held at high temp

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34
Q

What are the 4 regions of a creep graph

A
  • instantaneous
  • primary
  • secondary
  • tertiary
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35
Q

What does the tertiary region of a creep graph show

A

Accelerating rate leading to rupture

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36
Q

What does the secondary region of a creep graph show

A

Linear creep, slowest rate, most important, gradient is calculated

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37
Q

What does increasing stress show on a creep graph

A

Increases steady state creep rate, decreases rupture time
Slope increase, breaking faster

38
Q

What situations can creep cause a problem in

A

Displacement limited - situations where precise measurements or small clearances are required, turbine blades
Failure limited - dimensions don’t matter but items must not break, power lines
Stress limited - when items must maintain a particular stress, tensioning cables

39
Q

How do we design against creep (3)

A
  • chose high temp materials
  • chose materials with a large grain size
  • alloy
40
Q

What is corrosion?

A

The destructive and unintentional electrochemical attack of a metal

41
Q

What is corrosion in metals (2 things)

A
  • loss of material by dissolution
  • formation of a non-metallic layer
42
Q

Examples of corrosion of metals

A
  • rust
  • tarnishing
  • verdigris
43
Q

What does electrochemical mean?

A
  • Corrosion in metals is normally electrochemical
  • a chemical reaction where there is a transfer of electrons from one chemical species to another
44
Q

What is an oxidation reaction?

A

Where electrons are lost by a species

45
Q

What is a reduction reaction?

A

Where electrons are gained by a species

46
Q

Where does oxidation occurs

A

The anode

47
Q

Where does reduction occur

A

The cathode

48
Q

What does the emf give?

A

Gives a metals tendency to corrode

49
Q

What are the 5 types of wet corrosion

A
  • uniform attack
  • galvanic corrosion
  • selective leaching
  • crevice corrosion
  • pitting
50
Q

What is uniform corrosion? *

A
  • small potential differences on the metal surface set up localised cells
  • the cells produce uniform corrosion of the surface
51
Q

What is galvanic corrosion?

A

When two different metals are in contact and exposed to an electrolyte
- if in question state which one preferentially corrodes

52
Q

Which metal is more likely to corrode (galvanic)

A
  • Metal which is more anodised on the galvanic series will preferentially corrode
  • Preferential corrosion protects the other metal
53
Q

What does the rate of galvanic corrosion depend on?

A

Relative areas of the anode and cathode
A large cathode area compared to the anode will accelerate the process

54
Q

Other name of selective leaching

A

De-alloying

55
Q

What is selective leaching?

A

Where one element is preferentially removed via corrosion

56
Q

What is an example of selective leaching

A

Dezincification of brass
Leaves behind weak and porous copper

57
Q

What is crevice corrosion?

A

Occurs in a situation where the electrolyte can become stagnant (oxygen depleted) in a localised area

58
Q

Example of crevice corrosion

A
  • riveted metal sheets
  • Under a washers
  • Under deposits of dirt
59
Q

What is pitting

A

Specialised version of crevice corrosion where pits travel down through horizontal sheet

60
Q

How does pitting occur (4)

A
  • Initiated by a surface flow, such as a scratch.
  • Density of solution in the pit increases as the concentration of corrosion products increases
  • Gravity acts on the solution causing the pit to grow downwards
  • can cause failure with very little loss of material
61
Q

What are the methods of protecting against corrosion

A
  • coating
  • electrochemical protection
  • Intelligent design
62
Q

How do coatings (protecting against corrosion) work?

A

Work by acting as a physical barrier between the metal and the electrolyte, preventing wet corrosion

63
Q

Examples of coatings (protecting against corrosion) 4

A

Painting, plastic coating, enamelling and plating

64
Q

Problems with coatings?

A
  • A potential problem with coatings is that if it is physically damaged the metal will be exposed
  • This will mean that corrosion will start occurring, possibly at a greater rate (due to crevice corrosion).
65
Q

What is passivisation (long answer)

A

Some metals naturally have a protective ceramic coating on their surface.
●This coating is an oxide of the metal, formed by uniform corrosion.
●If this layer is impermeable and adherent then it will passivate the metals surface – protecting against further corrosion.
- Passivating layers are self-healing, exposing fresh metal causes corrosion but the product of the corrosion is the protective oxide layer.

66
Q

What metals have passivating layers?

A

Aluminium, chromium, stainless steel, titanium and zinc

67
Q

What does a purbaix diagram show

A

Corrosion stability

68
Q

What happens if a passivating layer is is corroded?

A

Passivating layers are self-healing.
Exposing fresh metal causes corrosion but the product of that corrosion is the protective oxide layer

69
Q

What is erosion corrosion

A

Where the passivating oxide layer is being continuously removed
If in bends of ss pipe it’s due to water continuously removing it

70
Q

Example of erosion-corrosion

A

Bends in stainless steel pipe work

71
Q

What is weld decay

A

Heat from welding can cause the stainless steel on either sid of the weld to corrode

72
Q

Why does weld decay occur?

A

Due to chromium segregating at the grain boundaries due to heat treatment

73
Q

What is plating

A

Protective coating over another metal

74
Q

What r the two common metals for plating?

A

Chromium
Zinc

75
Q

What are the problems with coating (stainless steel)

A

If a scratch penetrates the coating
If steel is exposed along with an electrolyte, it will set up an electrochemical cell

For chromium = bad - less reactive than steel
For galvanising = not bad - more reactive than steel

76
Q

What is cathodic protection

A

Involves supplying electrons from an external source to the metal to be protected
Causes the metal being protected to now be the cathode

77
Q

Cathodic protection - two main ways electrons can be provided

A

Sacrificial anode
Impressed current

78
Q

What are inhibitors

A

Compounds that get dissolved in the water (low concentrations)
Used in closed systems

79
Q

Intelligent design?

A

Design of components should be considered to help minimise corrosion

80
Q

Why are ceramics extremely resistant

A

They are considered already corroded

81
Q

What are ceramics

A
  • Considered as having already corroded, therefore very resistant to corrosion
  • Combination of a metal and a non-metal
82
Q

What happens to SiC when it corrodes

A
  • Forms SiO2
  • Forms a passivating layer, protecting from further corrosion
83
Q

What are the 2 stages of glass corrosion

A

Stage 1 - leaching
Stage 2 - silicate attack

84
Q

What is stage 1 of glass corrosion

A
  • begins when water touches glass
  • sodium ions and hydrogen ions create NaOH
  • pH of water gradually increases
85
Q

Stage 2 glass corrosion

A
  • occurs when pH is greater than 9.0
  • initially causes microscopic pitting and translucent haze
  • although optical properties of the glass are destroyed, mechanical properties remain intact
86
Q

What are the ways polymers degrade (3)

A
  • swelling and dissolution
  • bond rupture
  • weathering
87
Q

What is swelling and dissolution

A
  • small solvent molecules are absorbed into the polymer, disrupting secondary bondings
  • due to this, polymers will become soft, rubbery weak
  • during dissolution the polymer chains totally dissolve in the solvent
88
Q

What is bond rupture?

A
  • polymers may also experience degradation from the scission of primary bond in the polymer chain
  • The scission can be in response to radiation, chemical reactions or thermal effects.
89
Q

What can lead to bond rupture

A
  • radiation
  • chemical reactions
  • thermal effects (elevated temps)
90
Q

What is weathering

A
  • complex process often involving a combination of all degradation sub types
91
Q

Draw a creep graph

A

Draw

ENGEN 112 (7 decks)

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