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Material Science > Failure Mechanisms and Corrosion > Flashcards

Failure Mechanisms and Corrosion Flashcards

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1
Q

Brittle Fracture

A

Little plastic deformation before fracture,
Crack is unstable - it will keep growing rapidly even when a stress is not applied.
Ceramics, cold metals
Low Toughness(energy absorption before failure)
Clean breaks
Breaks by cleavage(breaking of Atomic bonds
Crack propagation is fast

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2
Q

Ductile Fracture

A 
extensive plastic deformation
high toughness(energy absorption) before fracture
metals, not too cold
crack is stable 
rough breaks
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3
Q

Transgranular fracture

A

cracks pass through grain boundaries

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4
Q

Intergranular Fracture

A

crack propagation along grain boundaries

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5
Q

Ductile to brittle transition

A

As temp. decreases a ductile material can become brittle
Alloying increases the ductile to brittle transition temperature
FCC ductile at very low temperatures
Ceramics have much higher temperatures of transition

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6
Q

Stress concentration

A

applied stress is amplified at stress raisers

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7
Q

stress raisers

A

tips of micro cracks, voids, notches, surface scratches, and corners

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8
Q

fatigue

A

Brittle like
cyclic stresses
loads lower than tensile or yield strengths of material

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9
Q

3 stages of fatigue

A

1 Crack initiation around stress raisers
2 incremental crack propegation
3 Catastrophic failure

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10
Q

Low cycle fatigue

A

high loads, plastic and elastic deformation

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11
Q

High cycle fatigue

A

low loads, elastic deformation

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12
Q

Fatigue limit

A

a maximum stress amplitude below which the material never fails, no matter how large the number of cycles is

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13
Q

Fracture strength

A

stress at which fracture occurs after a specified number of cycles

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14
Q

Fatigue life

A

number of cycles to fail at a specified stress

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15
Q

ways to increase fatigue life

A

polishing(removes machining flaws)
Introduce compressive stresses (shot peening)
ion implantation
laser peening
case hardening(makes harder outer layer and introduces compressive stresses)

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16
Q

Factors that affect fatigue life

A

magnitude of stress

quality of the surface

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17
Q

Thermal fatigue

A

thermal cycling causes expansion and contraction
solution: eliminate restraint by design
use materials with low thermal expansion coefficient

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18
Q

corrosion fatigue

A

chemical reactions induce pits which act as stress raisers and enhance crack propagation

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19
Q

corrosion fatigue solutions

A

decrease corrosiveness of medium
add protective surface coating
add residual compressive stresses

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20
Q

Creep

A

Time dependent and permanent deformation, subjected to a contant load at high temperature
> 0.4Tm

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21
Q

1st stage of creep

A

Intantaneous deformation: mainly elastic

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22
Q

second stage of creep

A

Primary/transient creep:

slope of strain vs. time decreases with time. Work hardening

23
Q

third stage of creep

A

secondary/steady state creep: rate of straining is constant: balance of work hardening and recovery

24
Q

tertiary

A

rapidly accelerating strain rate up to failure:

formation of internal cracks, voids, grain boundary separation, necking

25
Q

temperature and stress increase in creep

A

instantaneous strain increases
steady state creep rate increases
the time to rupture decreases

26
Q

mechanisms of creep

A

stress assisted vacancy diffusion
grain boundary diffusion
grain boundary sliding
dislocation motion

27
Q

Creep is minimized in materials with

A

High melthing temperature
high elastic modulus
large grain sizes(inhibits grain boundary sliding)

28
Q

creep resistant materials

A

stainless steels
refractory metals
superalloys

29
Q

processes that enhances creep resistance

A

1 anneal to increase grain size
2 solid solution alloying
3 directional solidification

30
Q

anode

A

gives electron oxidation

31
Q

cathode

A

accepts electron, reduction

32
Q

corrosion in metals

A

the destruction of a material by chemical or electrochemical reaction to its environment

33
Q

fluid velocity increases

A

corrosion rate enhances

34
Q

increasing temperature in corrosion

A

increases corrosion rate

35
Q

increasing concentration of corrosive species

A

faster rate of corrosion

36
Q

cold hardening

A

makes metals prone to corrosion because dislocation density is higher

37
Q

forms of corrosion

A 
uniform
galvanic
crevice
pitting 
intergranular
selective leaching
erosion corrosion
stress corrosion
hydrogen embrittlement
38
Q

uniform corrosion

A

slow predictible electrochemical reaction
rusting of steel, tarnished silver
prevent with coating, sacraficial anode,
regular maintenance

39
Q

galvanic corrosion

A

2 dissimilar metals or alloys are electrically coupled while exposed to an electrolyte
prevent by choosing metals close in galvanic series having large anode to cathode ratios, by insulating, or using cathode protection

40
Q

crevice corrosion

A

concentration differences of ions or dissolved gases in the electrolyte solution and between two regions of the same metal piece
prevent by using weld joints, using non abrasive gaskets, and designing to avoid stagnant areas

41
Q

pitting

A

localized corrosion, starts on the top of a horizontal surface and progresses downward
prevent by polishing the surface to avoid localized surface deposits
stainless steels are prone to this type of corrosion

42
Q

intergranular corrosion

A

metals and alloy specimens that experience disintegration along the grain boundaries
occurs often in welded samples due to the localized heating resulting in enhanced diffusion and formation of the CrC phase
prevent by annealing, lowering the C concentration, or alloying with a different metal

43
Q

Selective leaching

A

common in solid solution alloys
occurs when one element or constituent is preferentially removed
results in mechanical compromised material
prevent using coatings and alternative metals

44
Q

erosion corrosion

A

result of abrasive fluids and bubble impinging on surfaces
commonly found in pipes, propellers, turbine blades, valves, and pumps
minimize by changin the design to reduce fluid turbulence and impingement effects, use more erosion resistant material,or removing particulates from fluids

45
Q

stress corrosion

A

cracks grow along grain boundaries as a resulr of residual or applied stresses
can result even when stress levels are low
prevent by reducing the stress levels, heat treatments, and atmospheric control

46
Q

hydrogen embrittlement

A

metals lose and ductility when hydrogen is absorbed through the surface
high strength steels are prone to hydrogen embrittlement
reverse by baking the alloy
prevent by using hydrogen embitterment resistant alloy

47
Q

oxidation

A

leo
called scaling, tarnishing, or dry corrosion
can be parabolic, linear, or exponential
formation of stable metal oxide layer when exposed to gaseous atmosphere

48
Q

corrosion in ceramics

A

immune to corrosion at room temperature
due to chemical dissolution instead
resistant mostly even at high temperatures

49
Q

degredation of polymers

A

swelling and dissolution
bond rupture
weathering

50
Q

swelling and dissolution

A

the polymer is completely soluble in liquid.

Polymers are more resistant to this than metal

51
Q

bond rupture

A

radiation effects caused by electron beams, x- ryas, gama rays, ultraviolet rays, etc.
chemical reactions modify polymer chains at elevated temperatures
results in outgassing and weight loss of material
prevent using sablizers

52
Q

weathering

A

occurs in polymers exposed to outdoor conditions

53
Q

concentration polarization

A

reaction is limited by diffusion in the solution (mass-flow limited reactions)

54
Q

activation polarization

A

reaction is controlled by the one step in the series that occurs at the slowest rate(rate limited reactions)

Material Science (1 decks)

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