Design against Failure

Question 1

Explain residual stresses

Answer 1

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.

In any component containing residual stress, there must be compressive stress to balance the tensile stress.

Question 2

Explain the process of shot peening

Answer 2

Localized plastic deformation is likely to leave residual stresses. If a region of the material is caused to yield (e.g. by rolling, forging, machining), there will be elastic ‘spring-back’ after the external force is removed which induces residual elastic stresses.

The process of shot-peening exploits this effect in a beneficial way. Shot peening involves the impact of small hard iron, steel or ceramic shot (~0.5 – 2 mm in diameter) on to the surface of a metal component, and is used to create a layer of compressive residual stress. It is often used to enhance the fatigue life of highly stressed components.

The shot strikes the surface of the metal. - material imediately under the shot deforms plastically

The shot rebounds -elastically deformed material relaxes but cannot recover its original shape completely because it is constrained by the overlying plastically deformed region​ -

results in a permanent indentation on the surface- with a compressive residual stress close to the surface, balanced by tensile residual stress deeper into the material

Question 3

Where do cracks in welds genrally result from?

Answer 3

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)

Question 4

What are some methods to minimize cracks and residual stresses?

Answer 4

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

Question 5

What are the key features of wet corrosion?

Answer 5

Promoted by electrochemical couples. 

Often occurs more rapidly in acids.

 Can be prevented by formation of protective layers on metals (e.g. Cr2O3 on stainless steel).

For corrosion to occur, we need two reactions, anodic and cathodic which transfer electrons between different chemical species.

Anodic reaction oxidise metals (removal of electrons)

Cathodic reaction have reduction reaction (gain of electrons)

Question 6

Explain Bimetallic Corrosion?

Answer 6

two dissimilar metals in contact under damp conditions

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

Oxidation (the anodic process) will occur for the reaction with the lowest Standard Electric Potential (SEP), while reduction will occur for the reaction with the highest SEP.

EG zinc is oxidized to form Zn2+ ions, releasing electrons. The electrons 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 cathodic process which takes place will depend on the relevant SEP and on the supply of reactive species. In neutral water (pH = 7) and in the presence of oxygen, the major reaction is reduction of oxygen. But in acid solution (low pH, i.e. with a high concentration of hydrogen ions) in the absence of oxygen, the major cathodic reaction is reduction of hydrogen ions:

Question 7

Explain Differential aeration in steels?

Answer 7

Corrosion of steel or iron in water in the presence of oxygen takes place by the reactions

Fe = Fe²⁺+ 2e^-

O2 + 2H₂O + 4e^- = 4(OH^-)

The two reactions occur at different regions of the steel surface, depending on the local oxygen concentration, and electrons are transported between the two through the metal

The oxygen levels are usually highest close to the surface of the water, and the lowest oxygen levels are deep inside cracks and crevices. This means that steels are particularly liable to form deep cracks or pits as a result of the presence of water, 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 (leading to ‘crevice’ or ‘pitting’ corrosion).

Question 8

Explain the dangers of ‘pitting’

Answer 8

Pitting is a particularly dangerous form of corrosion since it can lead to local perforation of sheet or plate (e.g. a tank, pipe, car body etc.) by the formation of deep pits while most of the rest of the object is relatively undamaged.

The conditions in a pit are ‘autocatalytic’ – i.e. they tend to further enhance the local corrosion rate within the pit.

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

Question 9

What is differential energy corrosion

Answer 9

Features that cause a local increase in energy in a metal (e.g. grain boundaries, dislocations, precipitate interfaces) will act as anodic regions. These areas dissolve rapidly, and other regions form cathodes. Problems can therefore be caused by (e.g.) cold-worked regions of a structure corroding.

Question 10

Explain Corrosion inhibitors

Answer 10

It is also possible to reduce corrosion by blocking one of the reaction processes, either the anodic or the cathodic.

Corrosion can be prevented by preventing either process, by the use of an anodic or cathodic inhibitor.

Inhibitors are chemicals added to the water (and so only applicable in closed systems where the presence of the inhibitor is acceptable, such as recirculating cooling or heating systems.

Question 11

Explain anodic inhibitors

Answer 11

An example of an anodic inhibitor is sodium nitrite.

This acts by forms a continuous protective film of iron oxide on the steel surface which acts as a barrier to further corrosion.

The process of forming a protective film is called passivation.

However, the inhibitor does this by encouraging oxidation of the steel, and unless the film is sufficiently thick and protective 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 therefore be kept above a 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

Question 12

Explain Cathodic inhibitors

Answer 12

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

Question 13

What do pourbaix diagrams show?

Answer 13

Pourbaix diagrams show the dominant behaviour on a plot of the electrochemical potential against the pH of the solution.

The Pourbaix diagram 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.

‘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.

Question 14

What are the three different mechanisms for stress corosion cracking?

Answer 14

Active path dissolution involves rapid corrosion along a narrow path (such as a grain boundary) with the rest of the material being passive. An example is the process of weld decay in unstabilized stainless steel discussed earlier. Cracks will be intergranular.

In film induced cleavage, a brittle surface film (e.g. an oxide) on a ductile metal cracks, and the crack then propagates a short distance into the metal (~ 1 μm) before it is blunted. The brittle film then reforms by corrosion at the crack tip, and the process repeats.

Hydrogen embrittlement

Question 15

Explain Hydrogen Embrittlement?

Answer 15

Hydrogen embrittlement is a special case of stress corrosion cracking and occurs when hydrogen atoms diffuse towards regions of high hydrostatic tension (e.g. just ahead of a crack tip)

Hydrogen atoms are very small and therefore can diffuse rapidly in some metals. The hydrogen lowers the fracture toughness.

Hydrogen diffuses much more rapidly in ferritic iron than in austenite; austenitic steels are therefore almost immune to hydrogen embrittlement.

Atomic hydrogen is necessary, usually generated chemically or electrochemically (‘nascent hydrogen’). Typically, damp/wet conditions in conjunction with electric currents or even small amounts of corrosion; e.g. acid pickling; electroplating; MMA welding with damp electrodes. -

Under tensile stress (applied stress or residual stress) failure occurs by brittle fracture. The fracture is often initially intergranular

Question 16

How can you prevent hydrogen cracking?

Answer 16

Use a lower-strength alloy, which is less susceptible. Steels with yield stress lower than about 700 MPa are generally resistant to hydrogen cracking.

• Avoid treatments (e.g. electroplating) and service conditions which promote hydrogen absorption
• Heat the component to remove any dissolved hydrogen (e.g. 150-200 oC for 1 - 2 hours).
• Reduce residual stresses.
• Reduce stress concentrators (e.g. reduce notch severity)

Question 17

Explain liquid metal embrittlement

Answer 17

Liquid metal embrittlement is another type of SCC. As with aqueous SCC it is favoured by a tensile stress, and exposure to a specific environment, in this case of a liquid (molten) metal.

The symptoms are that a metal component in contact with liquid metal suddenly fails by intergranular fracture.

In some metal-liquid metal combinations, the molten metal is able to penetrate along grain boundaries forming cracks. It then diffuses a limited distance into the host metal. It reduces the bond strength at the crack tip, and so the fracture toughness falls dramatically

Question 18

What are common scenarios that lead to LME

Answer 18

Exposure of welded carbon steel to molten metal (e.g. hot dip galvanising, tinning). Residual stresses in the weld can then cause initiation of cracks or propagation of existing cracks.

 Brazing (Cu-Zn filler) and soldering (Sn-Pb filler) of structures which contain residual stresses

 Process vessels containing liquid metal (e.g. baths for molten zinc made of carbon steel; baths for molten aluminium made of almost any metal; containment of mercury by carbon steels)

 Accidental contact between liquid metal and susceptible alloy (e.g. mercury in contact with aluminium alloys in aircraft structure)

 Overheating of components coated with a metal, above the melting point of the coating or plating (e.g. Cd - plated bolts made from high-chromium steel; galvanised steel bolts)

Question 19

Explain corrosion of glasses and ceramics

Answer 19

Many glasses contain sodium as a network modifier (which breaks up the silicate –O-Si-O-Si-O- network, so reduces softening temperatures and promotes a wide temperature range in which glass can be worked).

If the glass is in contact with moisture (particularly in an acid environment), the sodium atoms which terminate the silicate chains can be replaced by hydrogen atoms. This is associated with shrinkage of the surface layers of the glass. The glass is put into tension and surface cracks form.

Question 20

What are five types of polymer degradation

Answer 20

Photo- degradation

Oxidising atmospheres

high temperatures

Solvent damage

Enviromental stress cracking

Question 21

Explain Photo degradation

Answer 21

Very common. Polymers are susceptible to damage from light, particularly from energetic UV photons (e.g. in sunlight) which cause breakage of covalent bonds. May lead to three distinct effects:

reduction in polymer chain length; depolymerisation (i.e. chain breakage leading to monomer formation); or increase in cross-linking

Polymers can be made UV-resistant either by surface coatings or by introducing scattering features into the structure

Question 22

Explianthe effect high temperature and oxidising atmospheres have on polymer degredation

Answer 22

High temperatures Reversible effects: softening of thermoplastics. Irreversible effects: increase in crystallinity; covalent bond damage ; discolouration; charring

Oxidising atmospheres (e.g. air, various fluids, ozone (= O3)). Similar effects to photodegradation. Permanent damage and changes to mechanical properties.

Question 23

Explain Solvent damage

Answer 23

Some small solvent molecules can penetrate between the polymer chains, reducing the elastic modulus (plasticisation), and swelling the material. For example nylon can absorb >5% water, with a volume expansion of about the same amount. This effect is reversible.

Irreversible effects: leaching of additives, or in some cases of polymer

Question 24

Explain Enviromental stress cracking (ESC)

Answer 24

This is the polymer equivalent of SCC in metals. Polymers suffer premature failure under stresses below the conventional design stress in certain environments.

Amorphous polymers are particularly susceptible.

Early stages of ESC can lead to the formation of multiple very fine cracks: ‘crazing

The stress is often provided by residual stresses arising from the manufacturing process, e.g.:

 Uneven shrinkage in an injection moulding leading to internal stresses and often distortion;

 Parts which have been subjected to elastic stresses during joining, or assembly

Weld-lines within injection mouldings (where molten streams of polymer meet) are often sites of accelerated failure.

Question 25

Explain Matrix degredaton

Answer 25

Matrix degradation: As for bulk polymers . All can take place. Water/solvent swelling. Generally non-uniform (solvent entering and leaving via the surface) so internal stresses can result. Polymers can crack or craze (particularly at the surface). ESC at stress concentrations.

Question 26

Explain Interface degradation:

Answer 26

Swelling stresses can result in cracking of interfaces. Capillary flow (‘wicking’) of solvent along cracked interfaces can dramatically increase degradation rates, because it allows a ‘short-cut’ route for solvent to reach the interior of structures without having to diffuse through the matrix. ESC resulting in rapid and specific attack of fibre or matrix at the interface. Plasticisation of matrix at interface, leading to creep and distortion

Question 27

Explain Fibre degredation

Answer 27

Fibre degradation: Polymers: Kevlar may be affected by UV and oxidation, leading to loss of strength and toughness Glass suffers leaching (surface dissolution), so degrades in water. Carbon fibres do not degrade.

Question 28

What are the three types of polymer matirx composite degredation

Answer 28

Matrix

FIbre

Interface

Question 29

Explain Fatigue Failure of composites due to external stress cycling

Answer 29

Composites contain many stress concentrators and internal stresses.

When subjected to alternating load, damage builds up in the form of fractured fibres and failure at the fibre-matrix interface.

As a result, they show progressive degradation of mechanical properties. The elastic modulus falls, and the strength drops. Failure tends to be by the gradual linkage of many sub-critical cracks, rather than by the catastrophic fast fracture seen in metals

Question 30

Explain composite failure due to thermal cycling

Answer 30

If composites are heated, the fibres and matrix expand by different amounts, often resulting in elastic or plastic deformation, sometimes accompanied by fracture.

The following effects may be expected in composites which have suffered thermal cycling

(a) Build-up of internal stresses
(b) Distortion of composite (particularly for multi-ply composites)
(c) Fracture of fibres (reduction in effective fibre length; not for Kevlar which is tough)
(d) Fracture of matrix between fibres
(e) Fracture of fibre-matrix interface