Part 2 Flashcards
What is cyclic saturation?
The shear stress/ shear strain are no longer altered
well annealed FCC single crystals (suitably oriented for single slip) are put through cyclic strains under fully reversed loading –> rapid hardening
What is the cyclic stress-strain curve?
Experiments with different plastic strains put together
Describe the different areas of the cyclic stress-strain curve BRIEFLY
A: low values of plastic shear strain, work hardening
B:Plateau (shear stress independent of shear strain)
C: Increase in shear stress
Why is there a
plateau?
1) The very formation of the PSBs appears to be closely related to the occurrence of the plateau.
2. )The plateau occurs when there’s an equilibrium between dislocation multiplication and annihilation.
Describe PSBs
PSB) forms through the bulk of the material, reappears at the same sites after polishing. PSBs are softer than the surrounding matrix (easier to deform without increasing the stress). Fatigue cracks are initiated along PSBs.
What is the reason for dislocation multiplication?
Dislocation multiplication is due to bowing out off edge dislocations between the walls.
What is the reason for dislocation annihilation?
Dislocation annihilation is due to climbing of edge dislocations of opposite signs.
What distinguishes cyclic loading? (7)
- higher dislocation density in cyclic loading
- no rotation of slip plane/direction towards tensile axis
- PSBs with wall structure made of edge dislocations
- plateau will occur (resolved shear stress independent of shear strain)
- high density of point defect clusters due to short range interactions among dislocations.
- surface roughness: extrusions and intrusion
- larger influence of strain rate and temperature under cyclic loading
What distinguishes monotonic loading? (3)
- rotation of slip plane/direction towards tensile axis
- surface roughness: staircase
- no PSBs, no plateau
Is strain hardening faster in cyclic or monotonic loading?
monotonic tension occurs much faster than that under cyclic loading (because you load and unload for cyclic)
Name the different dislocation structures that arise from cyclic loading of FCC single crystals
Veins
Walls/ladder
Cell
Labyrinth
In which area are veins present?
A
What are veins?
Networks of dislocation dipoles
What are dislocation dipoles?
When pos. and neg. dislocations attracts,the dislocations will be “trapped”, creating a dislocation dipole.
Only edge dislocations will form these dipoles since screw dislocations easily can cross slip and annihilate (if stacking fault energy is high enough).
Describe what happens first in region A
Dislocation forms on the primary glide plane
Approx. equal nbrs of positive and negative edge dislocations
How are veins separated?
Separated by almost dislocation free channels
What are walls mainly made of?
edge dislocations with its normal in the direction of the primary Burgers vector
How much of the volume in PSB is made of walls of edge dislocation?
10%
How much of the volume in matrix is made of veins of edge dislocation?
50%
Where does the transformation from veins to PSBs start?
center of the veins (dislocation-poor area)
Each vein transforms to two walls.
Where do cells form?
region C
Where do labyrinths form?
B-C
Why are cells/labyrinths formed?
At higher plastic shear strains > 2*10^-3 increase in secondary slip occurs
secondary slips starts in PSB/matrix interface and expands → “fills” the PSB
When does secondary hardening occur?
After 10^6cycles all PSBs have formed into cell structure = beginning of secondary hardening (region C)
Why is the cyclical behavior different for polycrystalline materials when compared
with single crystals?
grains in the middle are put through stresses/strains
Where can the mechanisms for cyclic damage in single crystals be applied in polycrystalline metals? Under what condition?
for the deformation in near surface grains
IF high purity
Have PSBs been found in polycrystalline metals?
Yes (and labyrinths + cells)
Name two general differences distinguishing polycrystalline FCC metals from single crystals oriented for single slip
1) Grains in a polycrystalline metal have many slip orientations
2) The incompatibility of elastic and plastic deformation between grains promotes local loading and multiple slip.
What happens for fine-grained FCC metals?
multiple slip deformation resembling the response of single crystals oriented for multiple slip.
What happens for coarse-grained FCC metals?
resembles that of single-slip oriented monocrystals with low strain hardening or a mild plateau.
Explain the Bauschinger effect
After a certain amount of plastic deformation, in tension or compression, the yield stress is lowered if the loading direction is reversed.
Why is the Bauschinger effect important to keep in mind?
to development models for complex cyclic deformation, understanding of work hardening and for rationalizing fatigue effects such as stress relaxation and cyclic creep
What happens in the material during the Bauschinger effect?
change in dislocation structure due to change in loading direction.
In polycrystalline materials
dislocation walls and subgrain boundaries forms during forward straining, dissolves under reversed loading
What is shakedown?
1.cyclic loads which build up residual stresses for example in ball bearings and railway rails
–> deformation entirely elastic –> no net accumilation of plastic strain = shakedown
Which materials are often subjected to shakedown?
Ductile metals
Elastic shakedown?
development of residual stresses results in a steady state that is purely elastic.
Plastic shakedown?
close cycle of alternating plasticity without accumulation of plastic strains, ratchetting or incremental collapse.
What is shakedown limit?
The limit value for the applied load for shakedown to occur
What happens if the shakedown limit i exceeded?
plastic strain continuous to accumulate in each cycle. This is commonly called ratchetting, cyclic creep or incremental collapse.
Why does surface roughening occur during fatigue?
cyclic straining of the materials with high purity leads to different amount of net slip on different glide planes.
The irreversibility of the shear displacement results in ‘roughening’ of the surface seen as microscopic hills and valleys.
Why do intrusions/extrusions occur?
irreversibility of the shear displacement
Describe protrusions (2 properties)
- Protrusions grow slower than extrusions
- The height of the protrusion increases in proportion to the width.
Why does the volume increase?
Screw dislocations of opposite sign annihilates through cross slip whereas edge dislocations form dipoles.
If close enough they annihilate and form a vacancy or an interstitial.
The vacancy generation is responsible for the swelling of the material which produces protrusions and extrusions in fatigue.
Why does the occurrence of surface roughening initiate cracks?Two mechanisms.
Notches (intrusons)
In the PSB/matrix interface there are abrupt gradients in density and distribution of dislocations. Preferable site for fatigue crack nucleation.
crack nucleation and early growth appear in the PSB (along the ladder structure)
Name 2 cons with a corrosive environment
- ) oxides are formed in slip steps –> transport of embrittling species in to the bulk –> crack nucleation
- ) surface roughening
How does GB cracking happen at low plastic strain ?
PSBs stuck att GBs can cause cracking (when they cross at the intercept)
Are grain boundaries good or bad regarding crack initiation?
Less GBs less creep/ cyclic fatigue
Name different places where crack initiation is likely to occur
metals and alloys of high purity: surface grains most likely location for crack initiation.
commercial alloys:also interior locations are feasible (ex. voids, inclusions, slag or gas entrapments, scratches, folds)
Name some different initiation mechanisms in comercial alloys
High strength steels containing MnS: debonding of inclusion from matrix
Al-alloys: crack nucleation at constituent particle sites
High strength Ni-based superalloys: pores, nonmetallic inclusions
What are corrosion pits?
If a component is exposed to a chemically aggressive medium preferential attack at selected points at the surface may provide nucleation sites for fatigue cracks, called ‘corrosion pits’
How can crack initiation occur during compressive loads?
related to the plastic zone at the tip of the crack
The growth rate decreases as the distance to the notch increases until stop at certain crack length a*
Why does the growth rate of a crack during compressive loads decrease?
crack grows –> crack surfaces come in more contact –> lower residual stresses lower growth rate
What can crack initiation during compressive loads be used for?
make pre-cracks, controlled crack propagation
How does the R-ratio influence the crack propagation rate and threshold value?
increasing R-ratio will decrease the fatigue crack initiation threshold
Why/when will a zigzag pattern happen (crack propagation)?
- )When the crack and the plastic zone around the crack is limited within a few grain diameters, crack growth dominated by single shear in the direction of the primary slip system.
- ) often short cracks
Can the zigzag pattern be observed for longer cracks as well?
if the plastic zone at the tip is smaller than the grain dimensions
Why/when will a flat pattern happen (crack propagation)?
At higher stress intensity values the plastic zone at the tip encircle several grains. The crack growth process involves simultaneous or alternating flow along two slip planes, called duplex slip.
What are striations?
In stage II fatigue striations are formed which can be seen as ripples /lines on the fracture surface.
What can one telll from striations?
the direction of the crack advance,
where final failure occurred and
if the failure was due to fatigue or not
Are striations always formed during
fatigue?
Not all engineering materials form striations during stage II fatigue. Striations are clearly seen in pure metals, many ductile alloys and engineering polymers
When are striations difficult to see?
barely visible in cold-worked alloys.
What is the striation formation strongly dependent on?
stress state, ΔK, alloy content and environment.
Name some concepts for development of striations
- Plastic blunting
- Alternating slip model : For ductile metal single crystals were work hardening of the primary slip plane leads to alternating shear on another slip plane.
- Environmental effect: brittle striations
What is generally the difference if a fatigue experiment is performed in vacuum
instead of air?
- )The formation of striations may be suppressed in vacuum in alloys which form striations in moist air due to irreversible slip
- )The crack growth rate can be a magnitude slower in vacuum than in air.
What characterizes stage A in the fatigue sequence? (briefly)
The average da/dN is smaller than the lattice spacing.
There exists a threshold stress intensity factor ΔK0 below which the crack remains “still” or to slow to be detected.
What characterizes stage A in the fatigue sequence? (bullet points)
- slow growth rate
- stage I single shear
- fracture surface: faceted or zigzag
- sensitive to load ratio effects
- large microstructural effects
- large environmental effects
What is dK0 dependent on?
microstructure, R-value, environment and crack length
How can you measure dK0?
load-shedding technique; precrack
Does the microstructure affect dK0?
Yes, example: overaged (straight)/underaged (zigzag)
What characterizes stage B in the fatigue sequence? (briefly)
Paris regime, linear variation of log(da/dN) vs. log(ΔK)
What characterizes stage B in the fatigue sequence? (bullet points)
- Mid growth rate (Paris regime)
- Stage II, striations and duplex slip
- fracture surface: planar with ripples
- small microstructural effects
- insensitive to test environment.
- sensitive to load ratio effect
How have people tried to predict the growth rate?
1) Geometrical models; crack tip displacement
2) critical values of strains/plastic work at crack tip
What characterizes stage C in the fatigue sequence? (briefly)
High ΔK values, the crack growth rate increases rapidly causing catastrophic failure.
What characterizes stage C in the fatigue sequence? (bullet points)
- high growth rate
- additional static modes
- additional cleavage or microvoid coalescence
- large microstructural effects
- small environmental effects
What is crack closure?
When a fatigue crack closes at a far-field tensile load.
How’s the extent of crack closure related to the crack length?
crack closure increases with increasing crack
length up to a saturation crack length.
Where are the mechanisms for crack closure present?
Both at tip and wake
Generally more crack closure in plane stress in..
…cyclic tension
Generally more crack closure in plane strain in..
…cyclic compression
What different types of crack closure exists?
– Oxide-induced crack closure
– Microscopic crack closure
– Viscous fluid-induced crack closure
– Transformation-induced crack closure
Describe the conditions of Plasticity induced crack closure?
Seen from experiments that not only the conditions ahead of the crack tip is important, but also the nature of the crack face contact behind the crack tip.
The conditions in the wake is a result of load history, length of the crack and stress state.
What is the mechanism behind Plasticity induced crack closure?
An atomically sharp notch or saw-cut closes at zero compressive load. However, the propagation of a fatigue crack gives rise to a wake of previously plastically deformed material. This results in residual tensile strains left in the material behind the advancing crack, closing the crack.
What is a compliance plot?
strain gages above and below crack –> meaure far field stress required to open up the surfaces completely
What is the mechanisms behind oxide-induced crack closure?
Corroded particles on the crack surface, like a wedge
+ surface roughness –> roughness-induced crack closure
In which region does oxide-induced crack closure have a major effect on the crack growth?
near dK0
How can oxide-induced crack closure be enhanced?
moist environment high temperature low R-ratios low ΔK values high cyclic frequencies lower strength and coarser-grained microstructures
What is Roughness-induced crack closure?
Plastic deformation ahead of the crack tip and slip irreversibility leads to mis-match between the two fracture surfaces.
Which stage growth occurs during roughness-induced crack closure?
stage 1
What is the effect of grains size in roughness-induced crack closure?
coarser grains better (moves the curve to the right)
How does crack closure influence the crack propagation rate and threshold value?
Crack closure can increase the threshold value while reducing the crack growth.
WHat is crack closure dependent on?
The influence is strongly dependent on microstructure, environment, and loading.
What is the connection between crack closure and R-value?
Crack closure is often more dominant at a lower R ratio and deltaK levels. This is due to smaller minimum crack opening displacements of the fatigue cycle
What happens during overloads?
A single tensile overload or high amplitude block loading sequence can result in the retardation or arrest in crack growth.
Describe the Retardation following tensile overload
1) accelerated growth –> stretch zone
2) crack growth (delay distance)
overload produced a larger plastic zone –> plasticity induced crack closure
blunting of crack tip
Describe the Compressive overloads
compressive overloads can increase the growth rate.
Compressive overloads also lead to flattening of fracture surface parts which reduces the effect of crack closure
What are small cracks, which different definitions exists?
1) Microstructurally small flaws
2) Mechanically small
3) Physically small
4) Chemically small
Describe microstructurally small flaws
Fatigue cracks for which the crack size is comparable to microstructural dimension, grain size, particle spacing.
Describe mechanically small flaws
When the plastic zone at the tip is comparable to the crack size
Describe microstructurally small flaws
Physically small around 1-2 mm.
Describe Chemically small flaws
Exhibit anomalies in da/dN below a certain crack size due to dependence of environmental effects on crack dimensions
Why is there a large interest for short cracks?
small flaws can grow significantly faster than long flaws for the same nominal driving force, ΔK.
Can lead to overestimations of fatigue life for short flaws.
How does the propagation of short and long cracks differ?
small flaws propagate faster than long cracks at the same ΔK
Short flaws can grow below ΔKth for long cracks.
Why do small cracks propagate quicker than large ones?
due to the limited wake existing for such short cracks, resulting in less crack closure and higher growth rates
What is corrosion fatigue?
Damage and failure of a material under the combination of cyclic stresses and embrittling medium.
What mechanisms for corrosion fatigue are available?
- Metal embrittlement
- Liquid metal embrittlement
- Stress corrosion cracking (SCC)
- Hydrogen embrittlement
Describe metal embrittlement
weakening of a higher melting point metal in contact with certain lower melting point metals.
Describe liquid metal embrittlement
metal embrittlement where the embrittling medium is a liquid metal.
Describe Stress corrosion cracking (SCC)
Embrittlement of alloys resulting from aqueous solutions.
Describe Hydrogen embrittlement
Hydrogen can be introduced (from hydrogenous gases) into the metal by dissociation of hydrogen molecules into atomic hydrogen or by release of hydrogen by metal dissolution.
How is the corrosion rate influenced?
test frequency, stress waveform, load ratio
time, temperature, environment in general (vacuum, air..)
How can we take into account the corrosion fatigue in models for crack propagation.
• Simple approaches uses superposition of crack growth rates for purely mechanical fatigue and stress corrosion crack growth rate
