Material Science. Prof. Satish V. Kailas Associate Professor Dept. of Mechanical Engineering, Indian Institute of Science, Bangalore India

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1 Material Siene Pro. Satish V. Kailas Assoiate Proessor Dept. o Mehanial Engineering, Indian Institute o Siene, Bangalore India Chapter 12. Coposites What are oposites? Why do we need oposites? How the oposites are ade? How the presene o dispersed phase iproves the properties? What are strutural oposites? Want to know how good you are in this hapter? What are oposites? Coposites are the ultiphase aterials, whih an be deined as any ultiphase aterial that is artiiially ade and exhibits a signiiant proportion o the properties o the onstituent phases. The onstituent phases o a oposite are usually o aro sized portions, dier in or and heial oposition and essentially insoluble in eah other. Why do we need oposites? Coposites are the lass o aterials with speial properties. Properties o the oposite an be tailored to eet the required purpose. So, it is iportant to have oposite aterials or the tehnologies whih deand speial properties. How the oposites are ade and lassiied? Coposites are ade by obining two distint engineering aterials in ost ases; one is alled atrix that is ontinuous and surrounds the other phase dispersed phase. The properties o oposites are a untion o the properties o the onstituent phases, their relative aounts, and size-and-shape o dispersed phase. Millions o obinations o aterials are possible and thus so nuber o oposite aterials. For ease o reognition, oposite aterials are lassiied based on dierent riteria like: (1) type o atrix aterial etal atrix oposites, polyer atrix oposites and erai atrix oposites (2) size-and-shape o dispersed phase partile-reinored oposites, iber-reinored oposites and strutural oposites. How the presene o dispersed phase iproves the properties?

2 Partile-reinored oposites They are again two kinds: dispersion-strengthened and partiulate-reinored oposites. These two lasses are distinguishable based upon strengthening ehanis dispersion-strengthened oposites and partiulate oposites. In dispersion-strengthened oposites, partiles are oparatively saller, and are o μ in size. Here the strengthening ours at atoi/oleular level i.e. ehanis o strengthening is siilar to that or preipitation hardening in etals where atrix bears the ajor portion o an applied load, while dispersoids hinder/ipede the otion o disloations. Exaples: thoria (ThO 2 ) dispersed Ni-alloys (TD Ni-alloys) with high-teperature strength; SAP (sintered aluinia powder) where aluiniu atrix is dispersed with extreely sall lakes o aluina (Al 2 O 3 ). Partiulate oposites are other lass o partile-reinored oposites. These ontain large aounts o oparatively oarse partiles. These oposites are designed to produe unusual obinations o properties rather than to iprove the strength. Mehanial properties, suh as elasti odulus, o partiulate oposites ahievable are in the range deined by rule o ixtures as ollows: Upper bound is represented by: E ( u) EV + E pv p EE p And lower bound is represented by: E ( l) E V + E V Where E and V denote elasti odulus and volue rations respetively while,, and p represent oposite, atrix and partiulate phases. A sheati diagra o these bounds is shown in the igure p p Figure-12.1: Sheati presentation o rule-o-ixture bounds. Partiulate oposites are used with all three aterial types etals, polyers and erais. Cerets ontain hard erai partiles dispersed in a etalli atrix. Eg.: tungsten arbide (WC) or titaniu arbide (TiC) ebedded obalt or nikel used to ake utting tools. Polyers are requently reinored with various partiulate aterials suh as arbon blak. When added to vulanized rubber, arbon blak enhanes toughness and

abrasion resistane o the rubber. Aluiniu alloy astings ontaining dispersed SiC partiles are widely used or autootive appliations inluding pistons and brake appliations.

3 abrasion resistane o the rubber. Aluiniu alloy astings ontaining dispersed SiC partiles are widely used or autootive appliations inluding pistons and brake appliations. Fiber-reinored oposites Most iber-reinored oposites provide iproved strength and other ehanial properties and strength-to-weight ratio by inorporating strong, sti but brittle ibers into a soter, ore dutile atrix. The atrix aterial ats as a ediu to transer the load to the ibers, whih arry ost o the applied load. The atrix also provides protetion to ibers ro external loads and atosphere. Figure Partiulate reinored oposite Figure Partiulate reinored oposite These oposites are lassiied as either ontinuous or disontinuous. Generally, the highest strength and stiness are obtained with ontinuous reinoreent. Disontinuous ibers are used only when anuaturing eonois ditate the use o a proess where the ibers ust be in this or. The ehanial properties o iber-reinored oposites depend not only on the properties o the iber but also on the degree o whih an applied load is transitted to the ibers by the atrix phase. Length o ibers, their orientation and volue ration in addition to diretion o external load appliation aets the ehanial properties o these oposites. Eet o iber length: Soe ritial length (l ) is neessary or eetive strengthening and stiening o the oposite aterial, whih is deined as:

4 * σ d l 2τ σ * ultiate/tensile strength o the iber, d diaeter o the iber, τ interae bond strength. Fibers or whih l >> l (norally l >15 l ) are tered as ontinuous, disontinuous or short ibers on the other hand. Eet o iber orientation and onentration: with respet to orientation, two extrees possibilities are parallel alignent and rando alignent. Continuous ibers are norally aligned, whereas disontinuous ibers are randoly or partially orientated. Two instants o loading are: longitudinal loading and transverse loading. (a) Continuous iber oposites: Under longitudinal loading, by assuing that deoration o both atrix and iber is the sae i.e. isostrain ondition, rule-o-ixtures results in the ollowing: A σ σ + A σ A A Where, A /A and A /A are the area rations o the atrix and iber phases respetively. In the oposite, i atrix and iber are all o equal length, area rations will be equal to volue rations. Thus, σ σ V + σ V When the isostrain assuption is taken into aount, the above equation transors into E E V + E V E ( 1 V ) + E l V The ratio o the load arried by the ibers to that arried by the atrix is given by F F E E V V In ase o transverse loading, it is assued that both atrix and iber will experiene the equal stress i.e. isostress ondition. Then the odulus o the oposite is given by: EE EE Et E V + E V E (1 V ) + E V Longitudinal tensile strength: as entioned earlier, atrix aterial is soter i.e. ibers strain less and ail beore the atrix. And one the ibers have ratured, ajority o the load that was borne by ibers is now transerred to the atrix. Based on this riterion the ollowing equation an be developed or longitudinal strength o the oposite:

5 σ σ ( 1 V ) + σ V * l ' * where σ stress in the atrix at iber ailure, σ * iber tensile strength. Whereas longitudinal strength is doinated by iber strength, a variety o ators will have a signiiant inluene on the transverse strength. These inlude properties o both the iber and atrix, interae bond strength, and the presene o voids. (b) Disontinuous and aligned iber oposites: Even though reinoreent eiieny is lower or disontinuous iber oposites than ontinuous iber oposites, disontinuous and aligned iber oposites are oerially gaining an iportant plae. The longitudinal strength o these oposites is given by: * * l ' σ d σ V (1 ) + σ (1 V ) when l > l and 2l * lτ ' σ V + σ (1 V ) when l < l d ' d where τ saller o either the iber-atrix bond strength or the atrix shear yield strength. () Disontinuous and randoly orientated iber oposites: Reinoreent eiieny o these iber oposites is diiult to alulate, and is usually haraterized by a paraeter known as iber eiieny paraeter, K. K depends on V and the E /E ratio. I rule-o-ixtures an be applied, elasti odulus o these oposites is given by: E K + l ( EV E V What are strutural oposites? ) Strutural oposites are speial lass o oposites, usually onsists o both hoogeneous and oposite aterials. Properties o these oposites depend not only on the properties o the onstituents but also on geoetrial design o various strutural eleents. Two lasses o these oposites widely used are: lainar oposites and sandwih strutures. Lainar oposites: there are oposed o two-diensional sheets/layers that have a preerred strength diretion. These layers are staked and eented together aording to the requireent. Materials used in their abriation inlude: etal sheets, otton, paper, woven glass ibers ebedded in plasti atrix, et. Exaples: thin oatings, thiker protetive oatings, laddings, bietallis, lainates. Many lainar oposites are designed to inrease orrosion resistane while retaining low ost, high strength or light weight.

6 Sandwih strutures: these onsist o thin layers o a aing aterial joined to a light weight iller aterial. Neither the iller aterial nor the aing aterial is strong or rigid, but the oposite possesses both properties. Exaple: orrugated ardboard. The aes bear ost o the in-plane loading and also any transverse bending stresses. Typial ae aterials inlude Al-alloys, iber-reinored plastis, titaniu, steel and plywood. The ore serves two untions it separates the aes and resists deorations perpendiular to the ae plane; provides a ertain degree o shear rigidity along planes that are perpendiular to the aes. Typial aterials or ore are: oaed polyers, syntheti rubbers, inorgani eents, balsa wood. Sandwih strutures are ound in any appliations like roos, loors, walls o buildings, and in airrat or wings, uselage and tailplane skins. Want to know how good you are in this hapter? 1. Coposite aterials are lassiied based on: (a) Type o atrix (b) Size-and-shape o reinoreent () Both (d) None 2. Major load arrier in dispersion-strengthened oposites (a) Matrix (b) Fiber () Both (d) Can t deine 3. Usually soter onstituent o a oposite is (a) Matrix (b) Reinoreent () Both are o equal strength (d) Can t deine 4. Usually stronger onstituent o a oposite is (a) Matrix (b) Reinoreent () Both are o equal strength (d) Can t deine 5. Last onstituent to ail in iber reinored oposites (a) Matrix (b) Fiber () Both ails at sae tie (d) Can t deine 6. Size range o dispersoids used in dispersion strengthened oposites

7 (a) μ (b) n () (d) None 7. Rule-o-ixture provides bounds or ehanial properties o partiulate oposites. (a) Lower (b) Upper () Both (d) None 8. Al-alloys or engine/autoobile parts are reinored to inrease their (a) Strength (b) Wear resistane () Elasti odulus (d) Density 9. Mehanial properties o iber-reinored oposites depend on (a) Properties o onstituents (b) Interae strength () Fiber length, orientation, and volue ration (d) All the above 10. Longitudinal strength o iber reinored oposite is ainly inluened by (a) Fiber strength (b) Fiber orientation () Fiber volue ration (d) Fiber length 11. The ollowing aterial an be used or illing in sandwih strutures (a) Polyers (b) Ceent () Wood (d) All 12. Not an exaple or lainar oposite (a) Wood (b) Bietalli () Coatings/Paints (d) Claddings Answers: a 3. a 4. b 5. a

8 6. a b 9. d 10. a 11. d 12. a

Material Science. Prof. Satish V. Kailas Associate Professor Dept. of Mechanical Engineering, Indian Institute of Science, Bangalore India

Material Science. Prof. Satish V. Kailas Associate Professor Dept. of Mechanical Engineering, Indian Institute of Science, Bangalore India Material Siene Pro. Satish. Kailas Assoiate Proessor Det. o Mehanial ngineering, Indian Institute o Siene, Bangalore 560012 India Chater 12. Coosites There is a great need or aterials with seial roerties