1)From the examples of fibres given, suggest ONE classification in of fibres that you can make
2)Ceramic fibres are produced for high temperature applications. Give examples of such application and identify the two families of fibres which have been developed
3)How are glass fibres produced? Explain in detail with assistance of approppriate sketches.
Glass fibers are produced by first drawing molten glass from a furnace as monofilaments and gathering large numbers of the filaments into glass fiber strands. The strands are then used to create glass-fiber yarns or rovings. A glass-fiber roving is a collection of bundles of continuous filaments which may be manufactured as continuous strands or as a woven material.
The basic material for making glass is sand, or silica, which has a melting point around 1750◦C, too high to be extruded through a spinneret. However combining silica with other elements can reduce the melting point of the glass which is produced. Fibers of glass are produced by extruding molten glass, at a temperature around 1300◦C, through holes in a spinneret, made of a platinum-rhodium alloy, with diameters of one or two millimeters and then drawing the filaments to produce fibers having diameters usually between 5 and 15 m. The spinnerets usually contain several hundred holes so that a strand of glass fibers is produced.
Drawing takes place at high speed and as the glass leaves the spinneret it is cooled by a water spray so that by the time it is wound onto a spool its temperature has dropped to around 200◦C in between 0.1 and 0.3 seconds. An open atomic network results from the rapid cooling and the structure of the glass fibers is vitreous with no definite compounds being formed and no crystallization taking place. Despite this rapid rate of cooling there appear to be no appreciable residual stresses within the fiber and the structure is isotropic. The glass fibers which are produced have slightly lower densities than the equivalent bulk glass. The difference is approximately 0.04 g/cc. The higher the draw speed used the lower the density of the glass fiber which is produced. Heating glass fibers above around 250◦C will produce an increase in density
4)What are the processing steps for the production of carbon fibers from polyacrylonitrile (PAN)? What reactions take place at each step?
PAN PROCESS
Carbon fibers are produced form polyacrylonitrile (PAN) precursor fibers through a three step process: (1) stabilization, (2) carbonization, and (3) graphitization. During the stabilization stage, stretched PAN fibers are held in tension and oxidized in air at about 200 to 220°C. Next, the fibers are subjected to a carbonizing heat treatment in which the PAN-based fibers become transformed into carbon fibers through the elimination of O, H, and N from the precursor fiber. Finally, an optional graphitization treatment increases the fibers’ modulus of elasticity by increasing the preferred orientation of the graphitelike crystallites within each fiber.
FIGURE 1:Two important production processes for carbon fibres
Pitches derived from coal tar, petroleum residues, or PVC can be used as relatively cheap precursors for carbon fibres. Pitch is a thermoplastic material and can therefore be extruded directly via melt spinning processes to precursor fibres. These can be subsequently transferred to carbon fibers in processes similar to the one described for the PAN route. This includes a stabilization step between 250 and 300 ° C and then carbonization and graphitization at temperatures between 1000 and 2500 ° C [31] (Figure 1-lower). In untreated pitches, condensed aromatic structures are present, which are isotropic and randomly distributed. This leads to fibres with low orientation of the carbon plains along the fibre axis, and moderate mechanical properties. In order to achieve enhanced fibre properties, the pitches are thermally treated at 400 to 450° C by which liquid crystalline, anisotropic structures are formed, the so - called mesophases. Precursor fibres, produced from mesophase pitch, exhibit high orientation values of the carbon plains in the direction of the fibre axis and can therefore be transformed to carbon fibres with very good mechanical properties. The yield of carbon from pitches can be above 75 wt. - %.
5)Why do the world’s tennis players use carbon-reinforced epoxy as tennis rackets? What are the important material properties of these material? (Smith pg 706)
Both glass and carbon fibre composites have a higher specific stiffness (modulus/density) than aluminium, so rackets made from composites can be much lighter, particularly in the case of carbon fibre. Continuous fibres can be woven into a variety of weave styles, giving increased control of the racket's characteristics. For example, unidirectional fibres are incorporated along the main racket axis for high bending stiffness, and 0/90° weaves are stacked at ±45° for high shear strength and stiffness. A variety of fibre grades are used, each with different levels of strength and stiffness. These fibres are coupled with epoxy resin matrices that often contain one or more property modifiers, such as rubber particles and thermoplastics that increase the toughness of the resin.
On top of these advantages the fatigue performance of the composite rackets was superior to aluminium constructions. Tests on aluminium rackets have shown that a marked decrease in stiffness occurs at around 6000 impacts, compared with a change in stiffness for carbon fibre rackets of around 4% after 50,000 impacts. Another important factor in aluminium's decline was the comparative damping properties of the frame materials, aluminium has a lower damping capacity than composite materials, and this has implications for the health of players.
6)The tensile strength of bulk borosillicate glass is 57 MPa. The tensile strength for the corresponding glass fibre (with diameters ranging from 3-20µm) is 3.4 GPa. Can you explain why? (Smith, pg706)
The inherent structure of a drawn glass fibre will be different than the corresponding annealed bulk glass structure of identical composition. The quenching rate and the directional strain applied to glass fibre during drawing will cause a stretching of the structural units within the fibre and alter its physical properties. In addition, the surface of a glass is quenched at a much higher rate than a comparable bulk specimen, resulting in some measure of compressive stress on the glass surface. As a result, the fibre will have higher tensile strength, lower density, lower Young’s modulus, lower refractive index, lower thermal conductivity, lower specific heatand higher chemical reactivity.
7)How does the amount and arrangement of the glass fibres in fiberglass reinforced plastics affect their strength? (Smith, pg 703)
FIBER LENGTH & DIAMETER
Fibers can be short, long or even continuous. Their dimensions are often characterized by aspect ratio l/d, where l is the fiber length and d is the diameter. The strength of the composite is improved when the aspect ratio is large. Fibers often fracture because of surface imperfections. Making the diameter as small as possible gives the fibers less surface area and, consequently, fewer flaws can propagate during processing or under a load6. The ends f a fiber carry less of the load than the remainder of the fiber; consequently. The fewer the ends, the higher the loadcarrying ability of the fibers
AMOUNT OF FIBER (VOLUME FRACTION)
The elastic modulus of the composites is given as rule of mixture
EC = EmVf + EfVm
Where ‘E’, ‘V’, and subscripts ‘c’,‘m’ and ‘f’ stands for young’s modulus, volume fraction, composite, matrix and fibers respectely. Composite with greater volume fraction of fibers increases the strength and stiffness of the composite, as we would expect from the rule of mixtures. However, the maximum volume fraction is about 80%, beyond which fibers can no linger be completely surrounded by the matrix
ORIENTATION OF FIBERS
The reinforcing fibers may be introduced into the matrix in a number of orientations. Short. Randomly oriented fibers having a small aspect ratio--typical of fiber glass—are easily introduced into the matrix and give relatively isotropic behavior in the composite. Long or even continuous, unidirectional arrangements of fibers produce anisotropic properties.
8)Explain the significance of the numbers in ‘polyamide 66’. Give the chemical formula for the polymer. Under what name is this fibre commonly known?
Numeric suffixes refers to number of carbon atoms present in the molecular structure of the amine and acid respectively. Numbers refers to six carbon atoms found from the parent molecules and which are found in the repeat unit of the molecular structure which is prodiced. Polyamide 66 is equals to Nylon 66. In which the monomers are 1,6-Hexamethylene diamine and adipic acid
9)On which minerals are glass fibres based and approximately what percentage of it makes up the composition? Why are the other minerals added to their composition?
All glass fibres are based on SiO2 of approximately 50% - 70% wt%. Other materials are added to create specialty glass.
Compared to S glass, E glass has higher percentages of SiO2 (65 percent versus 52 to 56 percent) and Al2O3 (25 percent versus 12 to 16 percent). Also, E glass contains about 10 percent MgO as an additional compound whereas S glass contains 16 to 25 percent CaO and 8 to 13 percent B2O3. S glass is both stronger and more costly than E glass.
10)The first high performance fibres to be made were produced by CVD. Explain what is this process in details and give examples of such fibres.
In the CVD process, ceramic fibers are formed via gas phase deposition of ceramic materials on carrier fibers. The carrier fiber usually forms the core of the ceramic fiber. Examples of core materials are carbon fibers and tungsten wires.
CVD is a process in which one material is deposited onto a substrate to produce near-theoretical density and small grain size for the deposited material. CVD onto an appropriate fine filamentary substrate produces a thicker filament predominantly composed of materials which otherwise could not be made by drawing or other conventional processes. The fine and dense microstructures of the deposited material ensures maximum strength and Young’s Modulus.
The oldest commercial process for producing non-oxide fibers is CVD of a ceramic material (typically SiC) onto a heated core monofilament (e.g., carbon fiber or tungsten wire). This process is still used by Textron Company. These fibers are used primarily to reinforce metal matrix composites (MMCs) and intermetallic matrix composites (IMCs). SiC fibers prepared by CVD, however, are typically large-diameter (generally = 75 µm [0.30 mils]) monofilaments that are stiff and unsuitable for weaving or other preforming techniques. In addition, commercially available CVD SiC mono-filaments are prohibitively expensive for use in CMCs. Recent attempts to deposit SiC (via CVD) on fine-diameter, multifilament carbon fibers are described in the literature (Lackey et al., 1995; Kowbel, 1997), but high strength, multifilament fiber tows have yet to be demonstrated. A key requirement for this potentially lower-cost approach is the development of a technique for spreading the core fiber tow to achieve uniform CVD of the individual filaments without causing interfilament bonding (which would compromise the mechanical properties of the fiber tow).
11)Explain how the above high performance fibres can be used to reinforce metal matrices.
When a load is applied parallel to the fiber axis, all of these fibers are much stronger and more rigid (per given mass of a material) than traditional metals such as steel or aluminum. However, each of these high-performance fibers has certain additional advantages. For example, in oxygen-free environments, carbon fibers can retain their strength at extremely high temperatures. Polymeric fibers are much lighter than carbon and ceramic fibers and transparent to radar. Ceramic fibers, on the other hand, are resistant to oxidation but lose strength at high temperatures
SiCfibres made on carbon core and destined to reinforce light alloys are produced with a surface coating the composition of which is made to vary from being carbon-rich to silicon carbide at the outer surface. Fibres which are to be used to reinforce Ti have a protective layer which varies from being rich in carbon to rich in silicon to a composition which is again rich in carbon at the surface. Outer sacrificial layer protects the fibres during contact with the molten and highly reactive Ti during composite manufacturing
12)Explain why aramidfibres are not used in primary loading bearing structures but for the same reason are used for improving the impact resistance of composite structures.
Within the aramid fibers, covalent bonding provides high strength in the longitudinal direction of the fibers whereas weak hydrogen bonding bonds the polymer chains together in the transverse direction. Their highly anisotropic behavior means the fibres are not used in primary loading structures subjected to compressive force. The process which limits the compressive properties of aramidfibres makes them absorb more energy than brittle fibres when broken. In compression, the fibre deform plastically in compression. As a result of this plasticity, neutral axis is displaced from the fibre axis towards the convex surface of the fibre structure. The tensile stresses on the convex surface do not attain the failure stress of the fibre, which can be bent to a zero radius of curvature. This give results to resistance to impact loading.
13)Discuss the implications of the different microstructures of alumina based fibres on the behaviour of the fibres, both at room temperature and high temperature.
Microstructure development in alumina-based fibers is strongly influenced by transformation sequences that occur as alumina precursors crystallize. The stable phase of alumina at all temperatures is a-Al2O3. However, a series of cubic alumina spinels, commonly called transition aluminas, form during heat treatment of alumina precursors.
Heating the precursor fibres induces the sequential development of transition phases of alumina, which if heated to high enough temperature all convert to the most stable form which is alpha alumina. However this transformation is followed by a rapid growth of porous alpha alumina grains giving rise to weak fibres