Composite Materials*Callister

Two different materials that have different characteristics are combined together, Multi-phased materials. Both constituents contribute to the properties of the final material.

Wood is a cellular material where the cells have cellulose fibres that grow in a similar direction. It is a natural composite. Fibres will give the strength and the stiffness to the wood. Another example is reinforced concrete is very strong in compressions however it has poor behaviour when it is under tension. Another natural composite is bone. It includes fibres and minerals that give the toughness of our bones. Steel with the eutectic composition that includes pearlite: ferrite and cementite that are 2 dissimilar phases, however this is not considered as a composite. Composites are artificially made and they are not the result of a homogenous melt.

They are a new class of materials: fibreglass materials. These composite materials that are used for structural materials have a very high strength to weight ratio and a very high stiffness to weight ratio. The common problem is that strong materials are usually high in density and are heavier. The use of composites was quite restricted because little knowledge was known regarding such materials as opposed to metals. Composite materials even for the simple class the ones that are easy to produce are still very expensive. The idea was to use them where the cost of production would justify their use. In the car industry they are getting more popular due to their weight saving property. Are also used in the marine industry since they do not corrode and rot. A layer of gel coat is normally used to protect the composite from UV radiation.

The properties are generally anisotropic; their properties differ in the direction of the fibres. They are less ductile which can cause problems with large loadings. They are found not to be too susceptible to fatigue. When it comes to aerospace, their fatigue performance is not a critical design factor like in metals. They have advantages in corrosion and fatigue. However, they can be affected by moisture and their polymer resistance temperature is much less.

Fibre reinforced dental resins-polymer composite that is stronger and much more susceptible to failure. Dentures undergo a lot of masticating forces that is a lot of fatigue stress. Research is being conducted to improve dentures from failing such as tubes with liquid resin that leaks when the denture is damaged.

The main property determining factors of composites: The properties of the matrix and the dispersed phase The proportion of volume of each component - those that have a metallic

matrix, the dispersed phase is usually stronger and stiffer. The morphologies (the shape has a big effect: the higher length to

diameter ratio the better the properties, the orientation also has a big

effect together with the size and distribution) of the components (the dispersed phase)

The effectiveness of the bond between the components – if the bonding between the fibres and the matrix is not strong enough no stress transfer occurs -As soon as a tensile load is applied, the matrix transfers the load to the fibres so that they would take most of the load. If there is no bonding then no stress transfer can occur, => fiber pull out and they would reside within the composite w/o taking any load.

The presence of filler materials especially with the resin we add fillers to impart a certain property such as abrasion resistance, sometimes also to lower the cost by using cheap filler materials like silica.

Different reinforcements that one might find in composites – slide 8

The dispersed phase is usually stiffer and stronger: particulate and fibrous type of reinforcement. Particulate (Particle Reinforced Composites):

Large particle composite contribute to an increase in strength by interacting with the surrounding matrix in the macro level. They can be made from different dispersed phase. The particle is often macroscopic. The particles should be evenly distributed. The bond needs to be effective. *Slide 11 To compute the stiffness of such composites see slide 12-Rule of mixtures. The stiffness lies over a range: the upper bound and the lower bound. As the dispersed phase is increased the stiffness increases as well.Ceramic particles are hard but brittle and low in toughness. Typically have low fracture toughness values, they tend to fail by large impacts. Can be enhanced by embedding ceramic particles within a metallic matrix to hold the ceramic particles together and to prevent any crack from propagating from one particle to the next. To increase strength, hardness and wear resistance and it can have an effect on thermal conductivity and dimensional stability. The carbide cutting tools used on a lathe or drill bits used to drill in concrete with a carbide tip, tungsten carbides particles are embedded within a metallic binder usually the metallic binder is cobalt and sometimes it is also nickel. Another example is tungsten carbide in steel for wear resistance. Carbon black is usually brought from the result of rich combustion of oil or gas. During combustion we get very fine particles less than 100nm that are used as a filler material to rubber. Gives high toughness, tear and abrasion resistance. It is so popular as an addition to rubber since these particles can bond very well to the rubber matrix. Silica is not used with rubber even though it is very cheap and has a lot of good characteristics since it does not bond very well with rubber. Concrete is a mixture of cement with high amounts of dispersed phase. Up to around 80% of concrete, which consists of fine sand and coarser gravel. Adding too much water results in porosity. The particulate phase should be free from clay in order to bond well with the cement. It is a very common composite and it is only good in compression.

Dispersion strengthened – the dispersed phase is very small and the strengthening is done by the dislocation mobility in the dispersed phase- very similar to age hardening in aluminium. The dispersed phase in these types of composites are usually oxide materials. With precipitation hardening if the material is exposed to a high temp the coherent precipitates can easily grow (coarsen) or dissolve and lose the strength. However with this process we can select strengthening particles that are more stable with temperature and do not change in size.

Fibrous Composites (Fibre-Reinforced Composites) - the strengthening phase consists of fibres that have a high length to diameter ratio. The Fibres increase strength and stiffness in polymeric or metallic matrices. When fibres are added to ceramics the idea is to increase toughness by crack bridging that prevents the crack from propagating.

Single layer - One lamina usually consists of a sheet of fibres oriented in one direction. So the properties of a single lamina in 2D are highly anisotropico Continuous Fibres

Can be aligned in one direction or randomly oriented. They are much stiffer than discontinuous fibres.

Fibre lengths >15Lc

o Discontinuous Fibres Easier to produce than the continuous fibres Random Orientation and Preferred Orientation Fibre Lengths < 15Lc

Multi-layered laminates – various laminas that are attached to each other with the fibre orientations being in the desired direction. Can be also considered as structural composites.

*Slide 22- stresses in fibres and matrix. The fibre can take most of the load due to its stiffness. The strength and stiffness are highly dominated by the fibre. Lc is the length of fibre required to reach the maximum stress before fracture. If the matrix reaches the shear strength it will shear. *Slide 24 critical length equation*Slide 26 4 different composites and their respective critical lengths and length to diameter ratios. Longer fibres result in more difficult and costly processing.

A fibre composite where the fibres are continuous and aligned: The fibre reinforcement is at its maximum in one direction and zero in the transverse direction. The properties are highly anisotropic.

*Slide 28- the matrix is ductile, the stiffness of the matrix is lower than that of the fibre, and the fracture strain of the matrix is higher than the fracture strain of the fibre. The fibre is brittle and higher in tensile strength and stiffness. Initially during stage 1 of deformation the composite is pulled and the matrix and fibres are both being strained at equal amounts: isostrain in an elastic fashion.

Then in stage 2 as the load is increased the fibres will continue to stretch elastically while the matrix will start to deform plastically. A certain strain is reached which is equivalent to the maximum fibre strain and the fibres start to break. This is the onset of composite failure. The matrix can take the load and redistribute it to other fibres; therefore the failure would not be catastrophic. At failure point the total stress within the composite is the stress multiplied by the volume fraction of the matrix. *See slide 29 and worked examples in Callister. The modulus of the composite when loaded in a transverse direction to the fibres see slide 30. Weak bonding will translate into lower strength-slide 31. The properties of the composites of slide 32 are highly anisotropic. An epoxy matrix and the fibre is a polymer-Kevlar. In the transverse direction the Kevlar is acting as a negative reinforcement.

Discontinuous and Aligned Fibre Composites less than 15 times the critical length are easier to process and there are more techniques, which can produce them and can yield cheaper products. Less strength than the continuous aligned fibre composites: since the fibres are short and therefore fewer fibres are available to carry the maximum amount of load. A continuous fibre equal to the critical length the tensile strength would be around half that of a continuous fibre which exceeds the critical length by 15 times. If the length of the fibre is smaller than the critical length, the fibre won’t manage to reach the tensile strength. The load carrying capability of the composite will be very limited.

Discontinuous Random oriented fibre composites – the fibre efficiency parameter depends on the relative volume of the fibres and the volume per cent of the matrix.

Polycarbonates are known for their impact toughness. Also they are considered to be the strongest amongst polymers. When the volume fraction of the fibres is increased the tensile strength increases. Doubling the reinforcement almost doubles the modulus. The impact strength is reduced when volume fraction is increased.

Consider a composite where all fibres are parallel, are aligned in one direction and the properties are anisotropic the reinforcement efficiency would be 1(parallel to fibres) and 0(perpendicular to the fibres). If the fibres are randomly and uniformly distributed within a specific plane than in any direction the reinforcement efficiency is 3/8. If the fibres are randomly and uniformly distributed in all directions (3D) the fibre reinforcement will be around 1/5 with the advantage that it would cater for stresses in all directions.

Structural Composites – Sandwich panels consist of two stiff strong plates, which are sandwiching a core material such as foam. They are increasing the moment of area; the stronger material is being furthered away from the neutral axis. Laminate composites are made from various layers containing fibres in various directions and are considered to be structural composites. In these laminates during production some problems could occur such as residual stresses which can lead to warpage and even to cracking or delamination from one layer to the

next. Also, residual stress could occur during processing. Another problem is chopped fibre laminates. *See slide 36

The matrix in composites could be made either from a metallic, polymeric or ceramic material. Usually the matrix is a tough material not always, in metals and polymers it is tough. The role of the matrix is to act as a binder with the fibre ensuring they are held in the desired position and ensuring that a proper stress transfer is obtained. It also protects the fibres from degradation. Fibre material have a high strength because fibres consist of material having very small volume of material, this reduces drastically the chance that a defects lies within the material. The surface of the fibres is coated with a sizing compound to protect the fibres until they are laid in the composite. In ceramic composites reinforcements are added to increase toughness. The selection of the matrix is very important since it also has a big effect on the composite. The matrix influences the mechanical properties and the surface temperature: how the composite is going to behave at different temperatures. The matrix ideally is there to help achieve a high strength to weight ratio and a high stiffness to weight ratio. In Polymer matrices usually the most commonly used are the polyester resins. These are cheap and have a good adhesion to the glass fibres. Epoxies are somewhat of better properties than polyester, are much more expensive. The Polyimides have a high temperature resistance and are also fire resistant. Even higher performance polymers include PEEK and PPS are highly resistant to high temperatures.

Fibre materials are materials that have a high aspect ratio. The fibres can be crystalline for example most metals or they can be amorphous where the atoms are randomly oriented- example glass. They can also have a combination of amorphous and crystalline and by changing the amount of crystalline and amorphous a very big change in properties: strength and stiffness can result.

Whiskers are materials, which have a very high length to diameter ratio, the diameters can be submicron in size. They are single crystals and very small therefore they have a very high degree of crystalline perfection, the grain boundary will contain a mismatch of grains, no amorphous regions. They are flaw free. Any flaw in the material can act as a stress concentrator. They include a variety of materials such as Alumina and Silicon Carbide. These materials are very expensive and therefore are not as used as the fibre composites.

Wires also have a very high length to diameter ratio. Are used quite a lot for example in the production of car tyres, hydrogen hoses. These wires are produced from a variety of high strength materials.

GFRP- glass fibres can be produced quite easily. The idea of adding other oxides to Silica is to reduce the melting temperature of the glass making it cheaper and easier to produce. Very fine glass fibres made from E glass are very cheap but it can operate up to lower temperatures since it can easily creep and soften. A composite, which has to resist high temperatures glass fibres need to consist of almost pure Silica. Advantages of glass fibres are easily to produce and very versatile and the raw material is available. They are chemically resistant, they

are coated to avoid moisture. Disadvantages include low stiffness and low rigidity; they can be prone to osmotic degradation. Applications include the marine industry, storage containers and industrial floorings. They can be found in various forms, having a low modulus allows them to be form in various forms. Can be woven, stitched, loose and joined together to form a yard.

CFRP –the techniques to produce carbon fibres are more expensive than the techniques to form glass fibres. The higher the temperature the higher the crystallinity and the fibre are more expensive. Carbon fibres are very commonly coated with epoxy. The modulus of these fibres is very higher. The higher the treatment temp the higher the modulus. Chemical Resistant however they are very prone to oxidation above 400 deg C. If Oxygen is present their useful temperature would be up to a maximum of 400 deg C. Include continuous fibres and chopped fibres.

Aramid Fibre such as Kevlar and Nomex. These fibres are called Aramids and are polymeric materials. Aromatic Polyamides- an amide means that we have a carboxic combined with Nitrogen. Various types of Kevlar the amides are linked to the 1 and 4 positions. In Nomex the amides are linked to the 1 and 3 positions. These polymers can form a high degree of crystallinity when drawn. Advantages a very high stiffness to weight ratio, …* see slide 52.

Hand Lay Up Process: a mold tool produced from a cheap material-wood and this mold tool is usually is first covered with a gel coat that protects ingress of water and has the characteristics that it can be set to have a very good finish. A mixture of resin that is not too viscous so that it could penetrate the fibre. Rollers are used to press and impregnate the resin in the fibres. This process can use any resin such as epoxy. Fabric can be made out of glass, Kevlar. If the fibre is too heavy it might be difficult to impregnate. Advantages of such process: cheap tooling, can be accustomed for a few products, has been used for many years, high skills not required to do the job, plenty of supplies makes the material competitive, reasonably high fibre content.Disadvantages: the resin and catalyst (a substance added to make a reaction faster to convert resin to polymer) are mixed by hand that might result in quality problems (quality depends in skill), health and safety - volatile constituents, the low viscosity resin can also easily penetrate clothing – one must be properly dressed when doing such a process, expensive extraction systems, the low viscosity resins (resins with low molecular weight) will translate to lower mechanical and thermal properties, since it utilises fabric it makes it more expensive.

Spray Lay-Up process: is used almost exclusively with polyester resins. Styrene is added to reduce viscosity and thus makes it easier to spray. It is principally used with glass fibres. A composite is formed where the fibres are randomly oriented and relatively short therefore less ability to carry high loads. Inner side can have a limited surface finish. Advantages: cheap tooling, experience, faster than the up layout, in terms of fibre content these techniques would have lower fibre content and therefore more rich in resin.

Disadvantages: chopped fibres that are limited in strength, quality depends in skill, problems with styrene levels going in the atmosphere therefore health and safety issues, low viscosity resin even less than before-low properties. Typical applications are applications were light weight is required and adequate strength: shower trays, bathtubs, small dinghy’s or boats.

Vacuum bagging process: A coating is applied to laminate. The pressure helps to further impregnate the laminate. We surround polymer with thick plastic sheet so that after we are finished with the lay up we vacuum the air and by doing so we have one atmosphere of pressure acting on the composite which will help to impregnate the laminate. The release film is a thin layer of polymer where it allows the composite to breathe without sticking to the upper layers. The absorption fabric absorbs an excess resin. The maximum pressure that can be applied in this case is the atmospheric pressure. Advantages: less porosity, better fibre wet out, higher fibre content, the composite is enclosed and it is easier to control the volatile constituents. Disadvantages: requires more skilled people, expensive, the mixture is also done by people which can reduce the quality, Applications: Race cars components

Pultrusion: similar to drawing in the production of metallic wires. The starting material is fibre making it cheaper. These fibres are made to pass through a resin bath. The die is designed to control the fibre resin ratio. From there it goes through a curing die, heats the polymer to make curing faster. The resin is cured using temperature. Then pullers pull the composite out and the process can be continuous ==> continuous composite being produced. Advantages: fast process, economic to produce composites which have a constant cross section, the fibre cost is minimised since we are not using fabric, laminates which give a good structural properties, it is possible to get high volume fractions with this fibre, the resin impregnation area is in a closed room which will limit any volatile constituents in the work place. Disadvantages process is very limited to have fibres oriented in other directions: Typical Applications: structural beams, frameworks such as ladders.

Pre-impregnated tapes: very thin tapes with different widths, these tapes contain fibres in one direction with very accurate fibre to resin content and with a resin, which is only partially taped. These tapes are used to build laminates. They are produced from partially cured resins and then they are stored in a cold environment in order to ensure that the semi-cured resin does not set-stop curing. Pre-peg tapes are moulded on the tool and this is applied in any direction requires. Once they are laid they are fitted in a similar configuration to vacuum bagging. Materials usually used- any resin and any fibre. The cores have to withstand the temperature at the autoclave. Advantage: the inital fibre cost is relatively cheaper because we are not using fabric but continuous fabric. however prepreg tapes are by themselves an expesive raw material to use. We can use high viscosity resins because of high pressure => even better properties.are by themselves an expensive raw material to use , high viscosity resin-better properties, Disadvantages: pre-pegs are expensive, pre-impregnated fabrics are even more

expensive, autoclaves are expensive as well, less choice in core materials due to the fact that they need to withstand high temperatures and pressures.Typical applications: structural part of an aircraft

Filament Winding: a mandrel and continuous fibres that go into a resin bath, nip rollers control the amount of resin, there are many ways how to lay the fibres on the mandrel. This technique is very suitable to produce pressure vessels, missile casings and also pipe lines. An alternative to fibres pre-pegs can also be used. Any resins can be used: epoxies. Cores can be used however usually they are single skill composites. Advantages: very fast process, very good structural properties. Disadvantages: limited to convex shapes and oval products, difficult to lay the fibres parallel to the longitudinal axis, external surface is not moulded the internal surface is moulded; a lower viscosity resin is used which can limit the properties of the matrix.

Resin Transfer: a fabric is used most of the time a stitched fabric to allow space for the resin to go through. The mould is expensive since it has to resist pressure. Resin is injected under pressure to fill the cavity. This technique can produce components with good surface finish on the side. Typical applications would be composite bus and train seats. Advantages: high volume fraction of fibre, can be automated, both components have a moulded surface finish that leads to a good surface finish.Disadvantages: More expensive tooling, unimpregnated parts

Metal matrix composites are more expensive however they produce a material with high resistance to temperature, more resistant to fire. Use a variety of matrices: al, mg, cu, ti. *see slide 67 During processing that involves high temperatures one must be careful so that we do not have unwanted reactions between the fibre and the matrix.

Ceramic matrix composites such as concrete are reinforced mainly to increase the toughness of the ceramic matrix and also make the yield strength or the tensile strength less scattered around. The fracture toughness of ceramics ranges from 1-5 MPa and by reinforcing this rises to around 20MPa. Zirconia increases the toughness of the ceramic matrix. At high temp it changes structure, stabilisers are added to stabilise the high temp phase at room temp. It hinders crack propagations making the material tougher. Another way to toughen is to add whiskers or fibres. As soon as a crack forms and starts propagating, the whiskers can bridge the crack and thus hindering the crack propagation.