Failure Mechanisms and Corrosion

Question 1

Brittle Fracture

Answer 1

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

Question 2

Ductile Fracture

Answer 2

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

Question 3

Transgranular fracture

Answer 3

cracks pass through grain boundaries

Question 4

Intergranular Fracture

Answer 4

crack propagation along grain boundaries

Question 5

Ductile to brittle transition

Answer 5

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

Question 6

Stress concentration

Answer 6

applied stress is amplified at stress raisers

Question 7

stress raisers

Answer 7

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

Question 8

fatigue

Answer 8

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

Question 9

3 stages of fatigue

Answer 9

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

Question 10

Low cycle fatigue

Answer 10

high loads, plastic and elastic deformation

Question 11

High cycle fatigue

Answer 11

low loads, elastic deformation

Question 12

Fatigue limit

Answer 12

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

Question 13

Fracture strength

Answer 13

stress at which fracture occurs after a specified number of cycles

Question 14

Fatigue life

Answer 14

number of cycles to fail at a specified stress

Question 15

ways to increase fatigue life

Answer 15

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

Question 16

Factors that affect fatigue life

Answer 16

magnitude of stress

quality of the surface

Question 17

Thermal fatigue

Answer 17

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

Question 18

corrosion fatigue

Answer 18

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

Question 19

corrosion fatigue solutions

Answer 19

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

Question 20

Creep

Answer 20

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

Question 21

1st stage of creep

Answer 21

Intantaneous deformation: mainly elastic

Question 22

second stage of creep

Answer 22

Primary/transient creep:

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

Question 23

third stage of creep

Answer 23

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

Question 24

tertiary

Answer 24

rapidly accelerating strain rate up to failure:

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

Question 25

temperature and stress increase in creep

Answer 25

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

Question 26

mechanisms of creep

Answer 26

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

Question 27

Creep is minimized in materials with

Answer 27

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

Question 28

creep resistant materials

Answer 28

stainless steels refractory metals superalloys

Question 29

processes that enhances creep resistance

Answer 29

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

Question 30

anode

Answer 30

gives electron oxidation

Question 31

cathode

Answer 31

accepts electron, reduction

Question 32

corrosion in metals

Answer 32

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

Question 33

fluid velocity increases

Answer 33

corrosion rate enhances

Question 34

increasing temperature in corrosion

Answer 34

increases corrosion rate

Question 35

increasing concentration of corrosive species

Answer 35

faster rate of corrosion

Question 36

cold hardening

Answer 36

makes metals prone to corrosion because dislocation density is higher

Question 37

forms of corrosion

Answer 37

``` uniform galvanic crevice pitting intergranular selective leaching erosion corrosion stress corrosion hydrogen embrittlement ```

Question 38

uniform corrosion

Answer 38

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

Question 39

galvanic corrosion

Answer 39

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

Question 40

crevice corrosion

Answer 40

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

Question 41

pitting

Answer 41

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

Question 42

intergranular corrosion

Answer 42

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

Question 43

Selective leaching

Answer 43

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

Question 44

erosion corrosion

Answer 44

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

Question 45

stress corrosion

Answer 45

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

Question 46

hydrogen embrittlement

Answer 46

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

Question 47

oxidation

Answer 47

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

Question 48

corrosion in ceramics

Answer 48

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

Question 49

degredation of polymers

Answer 49

swelling and dissolution bond rupture weathering

Question 50

swelling and dissolution

Answer 50

the polymer is completely soluble in liquid. | Polymers are more resistant to this than metal

Question 51

bond rupture

Answer 51

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

Question 52

weathering

Answer 52

occurs in polymers exposed to outdoor conditions

Question 53

concentration polarization

Answer 53

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

Question 54

activation polarization

Answer 54

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