design against failure - materials groupthere must be compressive stress to balance the tensile...

1 IM Hutchings, November 2012

ENGINEERING TRIPOS PART IIA MANUFACTURING ENGINEERING TRIPOS PART IIA

2012-13

Module 3C1: Materials processing and design Module 3P1: Materials into products

Design against failure The failure of engineering components and structures often involves fracture, which can result from changes in the level of applied stress, toughness of the material, corrosion and other environmental factors, or temperature. 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

aYKIC πσ=

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. We shall look at examples of all these effects, recognising that in some cases more than one effect plays a role. 1. Local stress The stress to which the crack is exposed may be quite different from the stress estimated from the external loads on an ‘ideal’ structure because of - stress concentration by local geometrical features - residual stress


tensile load producing stress σ

3σ

1.1 Elastic stress concentration

Example of stress concentration. The local stress immediately beside a circular hole in a plate is 3 times the mean stress far from the hole. Sharp notches have much greater stress concentrations. The local stress at a distance r from the end of a sharp edge crack of length a is given by:

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

ra2

1local σσ

Local stress concentrations can lead to initiation of failure. They may be - pre-existing (e.g. design features, results of manufacturing processes)

or - develop during service (e.g. by corrosion, fatigue crack growth). Some examples of pre-existing stress concentrators: holes for fasteners (e.g. rivets or bolts), notches, changes of section, porosity, welding defects. Welding generally involves a section change (even with butt welds between identical sections unless the weld is ground smooth) which results in stress concentration.

Examples of stress-concentrating geometry in welds: undercutting at the toe of the weld or abrupt convex profile.

Eliminate by improved welding practice and design. Butt welds are often machined smooth (‘dressed’). NB grinding direction is important as grinding leaves marks which can act as surface cracks. Examples: keyway in shaft; punched lettering; Comet Mk.1


1.2 Residual Stresses

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 this model system the tensile forces in the outer springs are balanced by the compressive force in the inner spring. There is no external force on the system, but there can be a large amount of elastic energy stored in the springs. In any component containing residual stress,

there must be compressive stress to balance the tensile stress. Residual stress due to deformation 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. (a) The shot (which does not itself deform) strikes the surface of the metal. Under load, the material immediately under the shot flows plastically. The material below the plastic region remains elastic. (b) The shot rebounds. The elastically deformed material relaxes but cannot recover its original shape completely because it is constrained by the plastically deformed region. The elastic stresses in the lower regions are insufficient to cause further plastic flow to restore the surface material to its original state, and 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. (c) Multiple impacts of shot all over the surface thus produce a near-surface region containing compressive residual stress, which enhances the fatigue life of the component by reducing the tensile stress acting on short surface cracks.


coldhot

coldhot

coldcold

(a)

(b)

(c)

coldhot

coldhot

coldcold

(a)

(b)

(c)

Thermal residual stresses

The top surface of a 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, and the stress in the hot region is small (b). As the hot region then cools, it contracts, but as its yield stress is now higher, it cannot deform plastically and thus exerts a force on the neighbouring cold region. The final tensile residual stress in the heated region can be up to the material’s yield stress, and is balanced by compressive stress elsewhere (c).

Thermal residual stresses can arise in hot working of metals where different regions of the part cool at different rates – e.g. in an I-beam the central web may cool faster than the (thicker) flanges, and there may also be a contribution to residual stress from the deformation. Example: rolled steel I-beam 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.

heat flow

(a) (b) (c)

heat flow

(a) (b) (c)


Welding involves intense local heating and thus causes residual stresses. Example: Distribution of residual stresses across a butt weld in a plate Distribution of residual stresses along the weld 2. Growth of defects Fatigue crack growth (revision from Part IA: see also Ashby and Jones I ch. 15) Fatigue involves crack growth under a fluctuating stress, well below the expected failure stress and generally below the yield stress of the material. If the component does not initially contain macroscopic cracks, then fatigue fracture is controlled primarily by the initiation of cracks. If it does contain cracks or flaws (e.g. many welded structures) the failure follows growth of the cracks: it is propagation-controlled.


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.) Example of S-N curves for high-cycle fatigue of uncracked specimens: S = stress range N = number of cycles to failure R=min stress/max stress. R=0 means minimum stress zero and applied stresses are tensile. R=-1 means reversed tension/compression. S0 is the endurance limit; below this stress, the fatigue life of an uncracked specimen is infinite. 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.


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 (see below) • inability of the weld metal to contract during cooling (similar to

hot tearing of castings)

Some measures to minimize cracks and residual stresses are:

• 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

Example: gauge-corner cracking in railway rails 3 Environmental factors: metals Environmental factors are a common cause of failures because (i) environments cannot always be fully controlled; and (ii) there may be unexpected interactions between different parts of the system. Examples include environmentally-assisted crack growth, and defect formation by localised corrosion. Environmental effects become particularly significant when they result in localised attack. We will mainly consider corrosion under damp or wet conditions because this is where most failures occur. Dry corrosion (e.g. high temperature oxidation) is on the whole less unexpected - and so less dangerous. 3.1 Wet corrosion of metals (largely revision: see IA notes or Ashby & Jones I ch. 23, 24) 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.

Key features of wet corrosion:

• 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 reactions liberate electrons, e- (called oxidation, although oxygen need not be involved); a metal atom M is oxidized to form an ion Mn+ by the removal of electrons:

M = Mn+ + n e- 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, with various possible processes e.g. : 2 H+ + 2 e- = H2 (gas) or O2 + 2 H2O+ 4 e– = 4 OH- Electrochemical couples. We shall look at three ways in which electrochemical couples can form: by different metals in contact, and two ways in which separate anodic and cathodic regions can form on a single metal. (a) Bimetallic corrosion: 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. We need both types of reactions before corrosion can happen. (For relative reactivities of materials see Materials Data Book, p37 which lists standard electrode potentials for the various reactions. These SEPs provide a measure of the tendency for the reaction to occur). Oxidation (the anodic process) will occur for the reaction with the lowest SEP, while reduction will occur for the reaction with the highest SEP. For example, consider zinc and iron in contact with each other in water in the presence of air. The standard electrode potentials are as follows: Zn = Zn2+ + 2 e- Eo = - 0.76 V Fe = Fe2+ + 2 e- Eo = - 0.44 V O2 + 2H2O + 4e- = 4(OH-) Eo = + 0.40 V


The 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 presence of the zinc protects the iron. This is why iron is often galvanised (coated with zinc), and why sacrificial anodes of reactive metals (such as zinc, magnesium etc) are used to protect ship hulls, lock gates etc. Galvanised iron will not corrode until most of the zinc has dissolved, even if the zinc covering is incomplete. 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, with a high concentration of hydrogen ions) in the absence of oxygen, the major cathodic reaction is reduction of hydrogen ions: 2H+ + 2 e- = H2 Eo = 0 V This process, which produces hydrogen gas, is important in the internal corrosion of domestic central heating systems, in which water is continuously circulated through steel radiators connected by copper pipes. In the presence of oxygen (e.g. when the system is initially filled with water), corrosion of the iron occurs rapidly since the SEP for oxidation of copper is +0.34 V, considerably higher than that for iron. However, the oxygen dissolved in the water is used up by the cathodic reaction, and the corrosion rate therefore drops to a tolerable level, with a cathodic reaction which now reduction of hydrogen ions. Because the hydrogen ion concentration is low, the corrosion rate is also low. The gas which is bled off from radiators is predominantly hydrogen, for this reason. (b) Differential aeration in steels (leading to crevice corrosion; differential concentration corrosion) Corrosion of steel or iron in water in the presence of oxygen takes place by the reactions (as above): Fe = Fe2+ + 2 e- 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. Examples of good and bad joint design

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 when 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.


(c) Differential energy corrosion 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. An example of localised corrosion caused by composition variation is weld decay in austenitic stainless steels (e.g. type 304: Fe-18%Cr-8%Ni). 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’. Localised corrosion occurs, with the grain boundary region being anodic. Sensitization can be avoided by using a low-carbon alloy (e.g. 304L), or a ‘stabilized’ stainless steel containing carbide-formers such as Ti or Nb which react with the carbon in preference to chromium. Examples: localized corrosion, corrosion of sensitized steel Protection against corrosion: corrosion inhibitors We have seen (Part IA) that corrosion of steels can be prevented (or slowed down) by using protective coatings (e.g. paints, or the natural layer of chromium oxide which grows on a stainless steel), or by reducing the potential of the metal to ensure it is the site for the cathodic reaction (i.e. cathodic protection by an applied potential or by galvanising). It is also possible to reduce corrosion by blocking one of the reaction processes, either anodic and 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.) 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.


Cathodic inhibitors work by forming surface layers which inhibit the cathodic reaction, and are intrinsically safer – 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.

(a) Cathodic inhibition: cathode area reduced progressively by chemical reaction with inhibitor, which forms an impermeable layer. Total corrosion rate and intensity of attack on anode areas (i.e. local corrosion) both fall steadily as concentration of inhibitor rises. (b) Anodic inhibition: anode area reduced progressively by chemical reaction with inhibitor. However, corrosion becomes increasingly intense at remaining anode areas, until finally all the anode is completely blocked at the critical concentration and corrosion stops abruptly. Maps of aqueous corrosion behaviour: Pourbaix diagrams Most metals show a range of behaviour when in contact with corrosive environments, depending on such factors as ion concentration or pH. In some regimes, metals form protective (passivating) coatings by reaction with the corrosive medium, so that the reaction is stifled and the metal does not apparently corrode. It is not in fact immune, but only a small amount of corrosion takes place until layers have built up so the system is ‘safe’. In other regimes, the coating dissolves, and the metal starts to corrode rapidly. 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. Note that the form of the 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.


NB: There is no rate information on these diagrams: some reactions may be very slow.

‘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. The broken lines represent the two most important cathodic reactions (see labelled diagram top right).


3.2 Stress Corrosion Cracking (SCC) SCC is characterised by cracks propagating at a stress well below the normal failure stress with little macroscopic plastic deformation, even in a material which is normally ductile, in certain types of enviroment. Cracking can be transgranular or intergranular and is associated with particular metal/environment combinations. SCC is a common cause of industrial failures (particularly in the chemical industry). It is, however, still liable to lead to unexpected failures mainly because of the range of possible effects and the specificity of the metal/environment combinations. Essential ingredients:

• a susceptible material • sufficient tensile stress (which can be residual stress) • a specific environment

Examples: boiler explosions; ‘season cracking’ in brass; steam turbine SCC can be caused by three different mechanisms: - active path dissolution; - film-induced cleavage; or - hydrogen embrittlement. 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 is discussed separately below.


General features of SCC Although SCC is found in a wide range of alloys, the effects tend to be more severe in high-strength materials (NB that high strength tends to be associated with limited ductility). Failures can be intergranular or transgranular, but there is always only very limited ductility and the fracture surfaces have the appearance of brittle fracture. Tensile stress and the presence of some specific chemical (may be very low concentration) in the environment are required. The elimination of either removes the problem. In uncracked components there may be a threshold stress below which SCC does not occur. E.g. for austenitic stainless steels. Cracked material would NOT show such behaviour and would always be 'unsafe'.

Some environments that can cause SCC in metals and alloys

Al alloys Chloride solutions, seawater

Cu alloys (e.g. brass, bronzes)

Ammonia vapour and solutions, other nitrogen sources

Carbon steels Sodium hydroxide solution, nitrates, mixed acids (sulphuric/nitric), hydrogen sulphide, seawater, chlorides

Stainless steels Chloride solutions, seawater, hydrogen sulphide

Titanium alloys Concentrated nitric acid, seawater


3.3 Hydrogen embrittlement 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. Crack growth rates can be rapid – up to 1 mm/s in some cases. Hydrogen diffuses much more rapidly in ferritic iron than in austenite; austenitic steels are therefore almost immune to hydrogen embrittlement. - Caused by hydrogen in high-strength alloy or plain carbon steels

(principally). - 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. 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

Intergranular failure from hydrogen embrittlement in high-carbon steel

Preventing hydrogen cracking - 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). Examples: electroplated steel coupling, wheel bolts


3.4 Liquid metal embrittlement (LME) 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 the metal component which is in contact with the 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. Because it is particularly easy for metal to diffuse along grain boundaries, this type of LME is nearly always intergranular. As with SCC, particular combinations are important, and prediction of effects is not easy. One of the clearer factors is that the liquid metal should not dissolve too readily in the solid metal (otherwise it will simply diffuse into the host metal, rather than forming a surface film at the crack tip). Common scenarios leading to LME

• 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).

Example: mercury in aircraft


3.5 Bacterial contamination Micro-organisms (generally bacteria) can be responsible for corrosion of metals by various different mechanisms. For example Desulfovibrio desulfuricans thrives under damp or wet anaerobic (i.e. no oxygen) conditions. It produces sulphide ions which accelerate anodic dissolution and stress corrosion cracking. This kind of corrosion is becoming increasingly common in container and tanker ships - partly because of cross-contamination, and partly because of reduction in maintenance and consequent falling standards of cleanliness. Examples: bacterial corrosion in ship fuelling 3.6 Effect of temperature on fracture toughness Temperature has a strong influence on the fracture toughness of some materials and the fracture stress therefore depends on the testing temperature. Ferritic steels undergo a transition from ductile behaviour at high temperature to brittle behaviour at low temperature. The transition also occurs with increasing strain rate (brittleness being encouraged under impact loading). Such behaviour is common in metals with bcc crystal structures: dislocation mobility falls as the temperature drops, and the yield stress rises rapidly; the stress at the crack tip then reaches a level at which cleavage between grains occurs in preference to ductile void formation and coalescence, with a lower dissipation of energy, and hence lower toughness. The fracture surface shows characteristic crystalline facets. The temperature at which the transition occurs depends on the specimen geometry and the strain rate. In thick plates (plane strain), the enhanced stress required for yielding at the crack tip leads to the ductile-brittle transition occurring at a higher temperature than in thin sheets. Austenitic steels (with an fcc structure, not bcc) remain ductile to very low temperatures.


Because of the difficulties of performing fracture toughness measurements over a wide range of temperatures, the effect is often described in terms of energy absorbed in impact tests. Note that polymers also show ductile-brittle transitions (as shown here for nylon, a thermoplastic): here the transition is associated with the reducing mobility of the polymer chains below the glass transition temperature (Tg for nylon is ~50 °C) Examples: fracture of large welded steel structures 4. Failures in non-metals 4.1 Corrosion of glasses and ceramics Many glasses contain sodium as a network modifier (breaks up 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. The phenomenon of 'static fatigue' occurs when glass under stress suddenly breaks under static load. (n.b. the name is misleading: no cyclic stress is involved.) Cracks grow slowly in the moist environment, and the material at the crack tips, which is under higher stress because of stress concentration, is particularly susceptible to corrosion. Once one crack reaches critical length, fast fracture takes place.


4.2 Degradation of polymers Polymers are affected by their environment in several ways. As with metals, there are some very specific material/environment combinations which cause dramatic loss of strength or toughness. Some common causes of degradation are as follows: (a) Photo-degradation 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. Often some discolouration. Permanent damage; mechanical properties affected in various ways specific to the damage type. Polymers can be made UV-resistant either by surface coatings or by introducing scattering features into the structure (crystalline regions of the polymer, or white pigment particles), or absorption features (coloured or black pigment particles). (b) Oxidising atmospheres (e.g. air, various fluids, ozone (= O3)). Similar effects to photo-degradation. Permanent damage and changes to mechanical properties. (c) High temperatures Reversible effects: softening of thermoplastics. Irreversible effects: increase in crystallinity; covalent bond damage as above; discolouration; charring (due to oxidation or chemical decomposition = ‘pyrolysis’). (d) Solvent damage 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 (e.g. a problem with thermosets such as epoxy if they have been incompletely cured); chemical reactions with polymer (e) Environmental Stress Cracking (ESC) 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.

The environments responsible for ESC vary between different polymers, and as with metals there can be some unexpected reactions with specific polymer-environment combinations.

Examples: fracture and crazing in polymer components 4.3 Degradation of polymer-matrix composites Polymer-matrix composites are often used in preference to metals in aggressive environments, but although visible signs are less obvious (e.g. no corrosion) degradation still takes place. The composite can be divided into three regions for consideration of the effects: the fibres; the matrix; and the fibre-matrix interface. We will here confine the discussion to degradation effects in long-fibre composites: the effects are more dramatic than with discontinuously reinforced composites, although the same basic principles apply. Matrix degradation: As for bulk polymers (section 4.2). 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. 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. Interface degradation: 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


Fatigue failure of composites (a) Due to external stress cycling Composites are full of 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. (b) Due to thermal cycling If composites are heated, the fibres and matrix expand by different amounts, often resulting in elastic or plastic deformation, sometimes accompanied by fracture. Thermal expansion

coefficient α (10-6 K-1) Softening temperature oC

Carbon fibres –0.1 to –0.5 (axial) 7 to 12 (radial)

>2000

E-glass fibres 4.9 850; but properties deteriorate rapidly above 250

Kevlar fibres –2 (axial) 59 (radial)

Degrades above about 250

Epoxy resin matrix 60 50–300 Polyester resin matrix

100–200 50–110

Nylon 6.6 90 75 Polycarbonate 70 110-140 The following effects may be expected in composites which have suffered thermal cycling. Exactly which combination of effects is seen depends on the fibre-matrix mix; the lay-up sequence; and whether the temperatures involved are above the softening temperature of the matrix:

(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

The effect of all of these factors (except possibly (b)) is a steady degradation in the mechanical properties of the composite. The tensile strength and the toughness will decrease (because of damage accumulation in the material), and the elastic modulus will also fall.


Factor (e) in particular can lead to increased corrosion (environmental degradation) rates because of increased ‘wicking’ of liquid along the cracks at the fibre-matrix interface. We can use a table to try to analyse what damage will occur in a composite. Here two different composites are cycled between –50 and +100o C: Fibre Matrix Cycle What happens

to fibres? What happens to matrix?

Consequences?

Heat from room temp. to

100oC

Expand 0.4x10-3 Fibres are brittle

Expand 5x10-3 (i.e. much more than fibres). Matrix is brittle at this temperature

Fibres resist expansion of matrix so are put into tension. Fibres may crack, as may fibre-matrix interface, with some fibre pull-out.

E-glass Epoxy

Cool to

-50oC

Try to contract to slightly shorter than original length. Brittle.

Try to contract to much smaller than original size. Brittle.

Matrix put into tension by fibres. Matrix may crack, or fibre-matrix interface may crack.

Heat from room temp. to

100oC

Expand 0.4x10-3 Fibres are brittle

Expand 7x10-3 (i.e. much more than fibres). Matrix is ductile at this temperature.

Expansion of matrix resisted by fibres, so matrix is put into compression, and will creep around fibres to relieve stress.

E-glass Nylon

Cool to

-50oC

Try to contract to slightly shorter than original length. Brittle.

Try to contract to smaller than original size, and much smaller than size at

100oC. Now brittle.

Fibres prevent contraction of matrix, so matrix put into tension. Matrix may distort or crack, or fibre-matrix interface may crack.

design against failure - materials groupthere must be compressive stress to balance the tensile...

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