Handout 9: Failures Flashcards
Explain the Griffith criterion.
- The failure of engineering components and structures often involves fracture. Recall that for a component containing a pre-existing crack of length a (edge crack) or 2a (internal crack), fast fracture will occur at a stress σ given by
- This is the Griffith criterion, where KIC is the fracture toughness of the material (for mode I crack opening) and Y is a geometrical factor (≈ 1).
- The component will fail by fracture if the criterion is satisfied – i.e if - the applied stress rises to the critical level;
- or - the crack grows and reaches the critical length; or
- the fracture toughness falls.

What is the equation that gives the local stress at a distance r from the end of a sharp edge crack of length a?

Explain what residual stresses are.
- Residual stresses are common in manufactured components – often caused by plastic deformation, thermal expansion or contraction, or phase changes with associated volume or shape change.
- This can be expressed as the spring model, where there are internal compressive forces which need to be balanced out.

Explain how the process shot-peening exploits the effect of residual stresses formed from deformation.
- A shot deforms metal surface, the metal yields in tension, material further below remains elastic.
- The shot is removed, the elastically deformed material relaxes, causing ‘springback’, it cant return to original shape.
- The result is a permanent indentation in the surface, with a compressive residual stress close to the surface, balanced by tensile residual stress deeper into the material.
- Multiple impacts of shot all over the surface produce a near-surface region containing compressive residual stress. This enhances the fatigue life of the component by reducing the tensile stress acting on short surface cracks, so making it more difficult for them to propagate.

Explain how thermal stresses arise.
- Occurs when a material cools unevenly .e.g Steel I beams (central web cools down faster).
- Example: The top surface of a metal slab is heated. The hot region expands (a). Because its yield stress is reduced when it is hot, it can deform plastically to accommodate the expansion, so the stress in the hot region falls. (b).
- (c) As the hot region cools, it contracts, but its yield stress has now gone back to its original higher value so it cannot deform plastically. It thus exerts a force on the neighbouring cold region. The final tensile residual stress in the region that had been heated can be up to the material’s yield stress, and is balanced by compressive stress elsewhere.

Explain how residual stresses arise in polymer injection moulding.
Also explain how this can cause problems.
- In polymer injection moulding, the material close to the mould wall solidifies and contracts first, with material in the centre still molten (a). This material then cools and shrinks (b), putting the surface regions into compression and leaving the core under tension (c).
- If cooling is asymmetric this can lead to warping and distortion of the part.
- The same process is done with glass, the outside is cooled fast leaving a low density glass structure, while the inside shrinks and puts the outside into compression.

Explain the different types of fatigues.

Where do fatigue cracks typically nucleate at?
- Fatigue cracks in an uncracked component typically nucleate at
- Stress concentrations (e.g. holes; welds; section changes; inclusions etc.)
- Surface features (especially machining marks; corrosion sites etc.)
Explain the term ‘endurance limit’ for steels.
- Steels typically show an endurance limit (So) of about half their tensile stress. Below this stress, the fatigue life of an uncracked specimen is infinite. Some alloys (e.g. Al alloys) do not show an endurance limit.

Why is the fatigue life a welded components dictated by crack growth rather than by crack initiation?
There are frequently cracks in and around welds (and perhaps slag inclusions which also act like cracks). The fatigue life of welded components is therefore controlled by crack growth rather than by crack initiation.
What factors initiate cracks in welds?
- Cracks in welds generally result from a combination of:
- temperature gradients causing thermal stresses
- variations in composition in the weld metal/HAZ giving differences in contraction
- segregation during solidification
- hydrogen embrittlement
- inability of the weld metal to contract during cooling (similar to hot tearing of castings)
What are measures that can be taken to reduce cracks and residual stresses due to welding?
- modify design of joint to minimize thermal stresses from shrinkage during cooling
- change welding process parameters, procedures and/or sequence
- preheat components being welded
- avoid rapid cooling after welding
- Induce residual compressive stress in weld metal by shot peening
What issues does corrosion cause?
Corrosion in metals occurs when metal reacts to form a corrosion product (often an oxide).
The product has inferior properties to the metal, so causes problems. e.g.
- Lower mechanical strength;
- Electrical insulator rather than conductor
For corrosion to occur, we need two reactions, anodic and cathodic, explain these two reactions.
- Anodic reactions liberate electrons, e- ; a metal atom M is oxidized to form an ion Mn+ by the removal of electrons.
- M = Mn+ + ne-
- The metal ions will often be soluble in water or react with oxygen ions or hydroxyl ions to form compounds. Removal of the metal in these ways constitutes corrosion.
- Cathodic reactions consume electrons (called reduction), with various possible processes e.g.
- 2 H+ + 2e- = H2
What is bimetallic corrosion?
- If two dissimilar pieces of metal are put in contact into an aqueous medium (e.g. water, or dilute acid) then one of the metals becomes the anode while the other becomes the cathode. We need both types of reactions before corrosion can happen.
- Oxidation (the anodic process) will occur for the reaction with the lowest SEP, while reduction (the cathodic process) will occur for the reaction with the highest SEP.
Explain why iron is galvanised zinc, giving the relevant equations.
Consider zinc and iron in contact with each other in clean water (pH neutral) in the presence of air. The standard electrode potentials and processes are as follows:
Zn = Zn2+ + 2e- Eo = - 0.76V (Anodic process)
[Fe = Fe2+ + 2e- , Eo = -0.76 V Alternative Anodic process - doesn’t take place]
O2 + 2H2O + 4e- = 4(OH-) Eo = +0.40V (Cathodic process).
- The zinc is oxidized to form Zn2+ ions, releasing electrons which flow into the iron, which becomes the cathode. On the iron surface, oxygen (dissolved in the water) reacts with water to form hydroxyl ions (OH- ). This uses up electrons, so the corrosion of the zinc continues. If the zinc (with the lower SEP) were not present, the anodic reaction would be oxidation of the iron, which would therefore corrode. The presence of the zinc protects the iron.
What is differential aeration in steels and how does it lead to crevice corrosion?
Fe = Fe2+ + 2e- (Anode)
O2 + 2H2O + 4e- = 4(OH- ) (Cathode)
- The two reactions occur at different regions, electrons are transported between the two through the metal.
- The lowest oxygen levels are deep inside cracks and crevices. This means that steels are liable to form deep cracks as a result of the waters presence, because once a crack has formed corrosion (the anodic process) will be concentrated at the growing tip of the crack where the oxygen concentration is lowest.
- Rust has lower density than the metal from which it forms, and is associated with expansion which can wedge cracks apart.
Why are marine conditions far more corrosion prone?
- Chloride ions (present in sea water) are particularly effective in assisting this mechanism, so that pitting corrosion is particularly prevalent in marine applications.
- The use of salt (sodium chloride) on roads in winter to avoid ice formation can also lead to severe corrosion of vehicle components. Local concentration corrosion can also result from broken or scratched paint coatings.
Describe how differential energy corrosion leads to sensitization(and hence weld in chromium steels.
- Features causing a local increase in energy in a metal (e.g. grain boundaries, dislocations, precipitate interfaces) act as anodic regions and can dissolve rapidly; other regions form cathodes.
- If the steel also contains carbon, heating during welding can cause the carbon to react with chromium to form precipitates of chromium carbide on grain boundaries in the HAZ, depleting the neighbouring metal of chromium. The steel is then said to be ‘sensitized’. The regions close to the grain boundaries do not contain enough chromium to be able to repair the protective surface film of chromium oxide when it is damaged in service.

Explain how Cathodic inhibitors work
- Cathodic inhibitors work by forming surface layers which inhibit the cathodic reaction, and are intrinsically safe – a reduction in concentration will lead to an increased corrosion, but it will still be less than the rate in the absence of the inhibitor.

Explain how sodium nitrite works as a anodic inhibitor.
- Acts by forms a continuous protective film of iron oxide on the steel surface which acts as a barrier to further corrosion.
- Process called passivation.
- Inhibitor encourages oxidation of the steel, and unless the film is sufficiently thick the corrosion rate of the steel is considerably greater than the corrosion rate with no inhibitor present at all.
- The concentration of the anodic inhibitor must be above the critical level – if it falls below this level then rapid corrosion will result.
- If the anodic film is incomplete, then this corrosion will be localised in the unprotected regions, and lead to pitting.

Explain what pourbaix diagrams are.
- It can be useful to represent the behaviour in aqueous solutions on a ‘map’. Pourbaix diagrams are used to plot the electrochemical potential against the pH of the solution.
- The Pourbaix diagrams can be altered dramatically by the presence of certain ions. e.g. stainless steel in aerated water shows a very large passive region because of the formation of a stable protective Cr2O3 layer. In the presence of chloride ions (e.g. as present in sea water) the film breaks down as a soluble complex chromium chloride forms, and no passive region is found. The diagrams below show the anodic reactions in water with and without oxygen, and the cathodic reactions for iron, titanium and aluminium.

Explain what ‘Immunity’ and ‘Passivation’ means in terms of Pourbaix diagrams.
- ‘Immunity’ is a range of pH and potential where corrosion of the metal is thermodynamically impossible.
- ‘Corrosion’ implies that there is a thermodynamic driving force tending to dissolve the metal as ions
- ‘Passivation’ shows that there is a driving force to form a stable film (e.g. oxide or hydroxide) on the metal surface, but this may or may not form an effective barrier to further corrosion.
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Explain what anodizing is in in aluminium and titanium.
- Aluminium and titanium can be ‘anodised’ to build up a thick protective layer of oxide.
- The hollow cells that are part of the structure of the thicker layers can be used to incorporate dyes or other surface modifiers such as PTFE (e.g. non-stick coating on saucepans).
- The tops of the cells are sealed after filling.
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