Failure And Degradation Flashcards
How does fracture occur
Exceeding the UTS
How does fatigue occur
Failure due to repeated stress/strain
What is creep
Time dependent plastic deformation when subjected to a constant load
What is brittle fracture
Catastrophic failure
- very little plastic deformation before fracture
Rapid breaking of bonds
Little energy absorption up to fracture
What is ductile fracture
Slow failure
- significant plastic deformation before fracture
- “slow” breaking of bonds
- significant energy absorption can occur
Draw a graph of brittle fracture and ductile fracture
See notes
What is the process of ductile fracture
- 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
Process of brittle fracture
- 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)
What is fratography
Examination of fracture surfaces using a microscope
What signs show ductile fracture (microscopic)
“Fibrous” central regions consisting of multiple “dimples”
Extensive “tearing” of material
Brittle fracture at low magnification
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
What are the names of brittle fracture at high magnification
Transgranular or intergranular
What are the signs of transgranular fracture
Fracture through crystal grain
Eg. polystyrene with a hot knife
What are the signs of intergranular fracture
Fracture along grain boundaries
Eg. pulling apart polystyrene
What surface level feature shows ductile fracture
Cup and cone
What surface feature shows brittle fracture
Flat, faceted fracture surface
What is fatigue
When materials fail due to repeated/fluctuating stresses (usually over a long period of time)
What are stress forms
Stress forms can cause fatigue failure
What is the equation for stress ratio
Stress ratio = stress min/ stress max
Equation for stress amplitude
Stress amplitude = stress range / 2
Equation for stress range
Stress range = stress max - stress min
What are the 3 general forms that a stress cycle can be
Reversed (mean stress = 0)
Repeated (mean stress ~= 0)
Random
How should fatigue testing be done
Duplicate as nearly as possible the service stress conditions
- alternations compression/tension
- rotating/bending test apparatus
What materials experience a fatigue limit
Most ferrous and titanium alloys
What does fatigue limit mean
Bellow a certain stress no fail true will occur
What does no fatigue limit mean
The material will eventually fail
What is the first stage of fatigue failure
Stage 1 propagation ( crack initially propagates very slow - along crystallographic planes)
What is the second stage of fatigue failure
Much more rapid
Crack propagation direction changes to perpendicular to stress direction
Occurs due to repeated opening and closing of the crack
What is the third stage of fatigue failure
Fatigue failure
Crack reaches critical size
Can either have ductile or brittle characteristics
What can be seen from fatigue fractography
Benchmarks
Then brittle or ductile marks
How is fatigue protected against (4 ways)
- intelligent design
- reduce the mean stress or stress amplitude
- surface finishing
- case hardening , just the outside harder
Why is high temperature needed in creep
- creep is a temp activated process
- temp greater than 40% of melting point, termed homologous temp
- temp above which creep starts to occur
What is creep testing
Tested by applying a constant load to a specimen held at high temp
What are the 4 regions of a creep graph
- instantaneous
- primary
- secondary
- tertiary
What does the tertiary region of a creep graph show
Accelerating rate leading to rupture
What does the secondary region of a creep graph show
Linear creep, slowest rate, most important, gradient is calculated
What does increasing stress show on a creep graph
Increases steady state creep rate, decreases rupture time
Slope increase, breaking faster
What situations can creep cause a problem in
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
How do we design against creep (3)
- chose high temp materials
- chose materials with a large grain size
- alloy
What is corrosion?
The destructive and unintentional electrochemical attack of a metal
What is corrosion in metals (2 things)
- loss of material by dissolution
- formation of a non-metallic layer
Examples of corrosion of metals
- rust
- tarnishing
- verdigris
What does electrochemical mean?
- Corrosion in metals is normally electrochemical
- a chemical reaction where there is a transfer of electrons from one chemical species to another
What is an oxidation reaction?
Where electrons are lost by a species
What is a reduction reaction?
Where electrons are gained by a species
Where does oxidation occurs
The anode
Where does reduction occur
The cathode
What does the emf give?
Gives a metals tendency to corrode
What are the 5 types of wet corrosion
- uniform attack
- galvanic corrosion
- selective leaching
- crevice corrosion
- pitting
What is uniform corrosion? *
- small potential differences on the metal surface set up localised cells
- the cells produce uniform corrosion of the surface
What is galvanic corrosion?
When two different metals are in contact and exposed to an electrolyte
- if in question state which one preferentially corrodes
Which metal is more likely to corrode (galvanic)
- Metal which is more anodised on the galvanic series will preferentially corrode
- Preferential corrosion protects the other metal
What does the rate of galvanic corrosion depend on?
Relative areas of the anode and cathode
A large cathode area compared to the anode will accelerate the process
Other name of selective leaching
De-alloying
What is selective leaching?
Where one element is preferentially removed via corrosion
What is an example of selective leaching
Dezincification of brass
Leaves behind weak and porous copper
What is crevice corrosion?
Occurs in a situation where the electrolyte can become stagnant (oxygen depleted) in a localised area
Example of crevice corrosion
- riveted metal sheets
- Under a washers
- Under deposits of dirt
What is pitting
Specialised version of crevice corrosion where pits travel down through horizontal sheet
How does pitting occur (4)
- 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
What are the methods of protecting against corrosion
- coating
- electrochemical protection
- Intelligent design
How do coatings (protecting against corrosion) work?
Work by acting as a physical barrier between the metal and the electrolyte, preventing wet corrosion
Examples of coatings (protecting against corrosion) 4
Painting, plastic coating, enamelling and plating
Problems with coatings?
- 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).
What is passivisation (long answer)
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.
What metals have passivating layers?
Aluminium, chromium, stainless steel, titanium and zinc
What does a purbaix diagram show
Corrosion stability
What happens if a passivating layer is is corroded?
Passivating layers are self-healing.
Exposing fresh metal causes corrosion but the product of that corrosion is the protective oxide layer
What is erosion corrosion
Where the passivating oxide layer is being continuously removed
If in bends of ss pipe it’s due to water continuously removing it
Example of erosion-corrosion
Bends in stainless steel pipe work
What is weld decay
Heat from welding can cause the stainless steel on either sid of the weld to corrode
Why does weld decay occur?
Due to chromium segregating at the grain boundaries due to heat treatment
What is plating
Protective coating over another metal
What r the two common metals for plating?
Chromium
Zinc
What are the problems with coating (stainless steel)
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
What is cathodic protection
Involves supplying electrons from an external source to the metal to be protected
Causes the metal being protected to now be the cathode
Cathodic protection - two main ways electrons can be provided
Sacrificial anode
Impressed current
What are inhibitors
Compounds that get dissolved in the water (low concentrations)
Used in closed systems
Intelligent design?
Design of components should be considered to help minimise corrosion
Why are ceramics extremely resistant
They are considered already corroded
What are ceramics
- Considered as having already corroded, therefore very resistant to corrosion
- Combination of a metal and a non-metal
What happens to SiC when it corrodes
- Forms SiO2
- Forms a passivating layer, protecting from further corrosion
What are the 2 stages of glass corrosion
Stage 1 - leaching
Stage 2 - silicate attack
What is stage 1 of glass corrosion
- begins when water touches glass
- sodium ions and hydrogen ions create NaOH
- pH of water gradually increases
Stage 2 glass corrosion
- 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
What are the ways polymers degrade (3)
- swelling and dissolution
- bond rupture
- weathering
What is swelling and dissolution
- 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
What is bond rupture?
- 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.
What can lead to bond rupture
- radiation
- chemical reactions
- thermal effects (elevated temps)
What is weathering
- complex process often involving a combination of all degradation sub types
Draw a creep graph
Draw
