Failure Mechanisms and Corrosion Flashcards
Brittle Fracture
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
Ductile Fracture
extensive plastic deformation high toughness(energy absorption) before fracture metals, not too cold crack is stable rough breaks
Transgranular fracture
cracks pass through grain boundaries
Intergranular Fracture
crack propagation along grain boundaries
Ductile to brittle transition
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
Stress concentration
applied stress is amplified at stress raisers
stress raisers
tips of micro cracks, voids, notches, surface scratches, and corners
fatigue
Brittle like
cyclic stresses
loads lower than tensile or yield strengths of material
3 stages of fatigue
1 Crack initiation around stress raisers
2 incremental crack propegation
3 Catastrophic failure
Low cycle fatigue
high loads, plastic and elastic deformation
High cycle fatigue
low loads, elastic deformation
Fatigue limit
a maximum stress amplitude below which the material never fails, no matter how large the number of cycles is
Fracture strength
stress at which fracture occurs after a specified number of cycles
Fatigue life
number of cycles to fail at a specified stress
ways to increase fatigue life
polishing(removes machining flaws)
Introduce compressive stresses (shot peening)
ion implantation
laser peening
case hardening(makes harder outer layer and introduces compressive stresses)
Factors that affect fatigue life
magnitude of stress
quality of the surface
Thermal fatigue
thermal cycling causes expansion and contraction
solution: eliminate restraint by design
use materials with low thermal expansion coefficient
corrosion fatigue
chemical reactions induce pits which act as stress raisers and enhance crack propagation
corrosion fatigue solutions
decrease corrosiveness of medium
add protective surface coating
add residual compressive stresses
Creep
Time dependent and permanent deformation, subjected to a contant load at high temperature
> 0.4Tm
1st stage of creep
Intantaneous deformation: mainly elastic
second stage of creep
Primary/transient creep:
slope of strain vs. time decreases with time. Work hardening
third stage of creep
secondary/steady state creep: rate of straining is constant: balance of work hardening and recovery
tertiary
rapidly accelerating strain rate up to failure:
formation of internal cracks, voids, grain boundary separation, necking
temperature and stress increase in creep
instantaneous strain increases
steady state creep rate increases
the time to rupture decreases
mechanisms of creep
stress assisted vacancy diffusion
grain boundary diffusion
grain boundary sliding
dislocation motion
Creep is minimized in materials with
High melthing temperature
high elastic modulus
large grain sizes(inhibits grain boundary sliding)
creep resistant materials
stainless steels
refractory metals
superalloys
processes that enhances creep resistance
1 anneal to increase grain size
2 solid solution alloying
3 directional solidification
anode
gives electron oxidation
cathode
accepts electron, reduction
corrosion in metals
the destruction of a material by chemical or electrochemical reaction to its environment
fluid velocity increases
corrosion rate enhances
increasing temperature in corrosion
increases corrosion rate
increasing concentration of corrosive species
faster rate of corrosion
cold hardening
makes metals prone to corrosion because dislocation density is higher
forms of corrosion
uniform galvanic crevice pitting intergranular selective leaching erosion corrosion stress corrosion hydrogen embrittlement
uniform corrosion
slow predictible electrochemical reaction
rusting of steel, tarnished silver
prevent with coating, sacraficial anode,
regular maintenance
galvanic corrosion
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
crevice corrosion
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
pitting
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
intergranular corrosion
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
Selective leaching
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
erosion corrosion
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
stress corrosion
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
hydrogen embrittlement
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
oxidation
leo
called scaling, tarnishing, or dry corrosion
can be parabolic, linear, or exponential
formation of stable metal oxide layer when exposed to gaseous atmosphere
corrosion in ceramics
immune to corrosion at room temperature
due to chemical dissolution instead
resistant mostly even at high temperatures
degredation of polymers
swelling and dissolution
bond rupture
weathering
swelling and dissolution
the polymer is completely soluble in liquid.
Polymers are more resistant to this than metal
bond rupture
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
weathering
occurs in polymers exposed to outdoor conditions
concentration polarization
reaction is limited by diffusion in the solution (mass-flow limited reactions)
activation polarization
reaction is controlled by the one step in the series that occurs at the slowest rate(rate limited reactions)
