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Composites Why Study Composites? With the knowledge of various types of composites, and the factors (constituents, their relative amounts, geometry & distribution) that affect their performance (properties), we can design materials with desired properties. These desired properties are better than those of available materials, metals, polymers and ceramics. What is a Composite? 1. A composite is a multi-phase material that exhibit a significant proportion of the properties of both constituent phases such that a better combination of properties is realized. Better property combinations are fashioned by judicious combination of two or more distinct materials. 2. A composite is artificially made, as opposed to one that occurs or forms naturally. In addition, the constituent phases must be chemically dissimilar and separated by a distinct interface. a. Multi-phase alloys (as Pearlite, combination of α Ferrite & Cementite) are composites? b. Wood is a natural composite (core is covered by cellulose) c. Bone is a natural composite (protein collagen and mineral apatite) Do a, b, c fit to definition of composites??? Why Need Composites? Many of our modern technologies require materials with unusual combinations of properties that can not be met by conventional metal alloys, ceramics and polymeric materials. This is especially true for aerospace, underwater and transportation applications. For example, aircraft engineers are increasingly searching for structural materials that have low densities, are strong, stiff, and abrasion and impact resistant, and are not easily corroded. This is a rather difficult combination of characteristics. Frequently, strong materials are relatively dense. Also, increasing the strength or stiffness generally results in a decrease in impact strength (& ductility). This is why, we need to engineer new materials, called composites, by combining various existing ones. Two Phase Composites Many of composites are composed of just 02 phases: 1. Matrix phase Continuous phase and surrounds the other phase 2. Dispersed phase This phase is dispersed in the matrix phase • The properties of composites depend on : - Properties of constituent phases - Relative amounts of constituent phases - Geometry of dispersed phases - shape of particles - particle size - distribution of particles - orientation of particles Geometry of Dispersed Phase - Classification of Composites - A. Particle Reinforced Composites 1. Large-Particle Composites • Particle-Matrix • Mostly, Particulate phase (dispersed) is harder & stiffer than Matrix phase • Matrix transfers some of applied load to Particles 2. Dispersion-strengthened Particles • Particle-Matrix • Strengthening mechanism is similar to precipitation hardening • Particles impede the dislocation motions and restrict plastic deformation • Matrix bears the major portion of load • Particle size is b/w 0.01-o.1μ 1. Large Particle Composites P-R Composites • Particles can have a variety of geometries (Ref Prev Fig), but they should be equi-axed • For effective re-inforcement the particles should be small and evenly distributed throughout the matrix • Mechanical properties are enhanced with increasing particulate contents • The elastic modulus of composite can be predicted using 02 expressions, called Rule of Mixture Equations • The experimental Ec should fall between these 02 upper and lower limits Cu-W composite: W is dispersed phase Experimental 1. Large Particle Composites: Examples P-R Composites Cermet: Ceramic-Metal Composite • Most common cermet: Cemented Carbide • Extremely hard particles of refractory carbides (TiC, WC) are embedded into matrix of Co or Ni metals. - These hard composites are used as cutting tools for hardened steels - Hard carbide particles provide the cutting surface , however, being brittle cannot withstand heavy machining loads/stresses - Necessary toughness to withstand stresses is achieved by adding Co, Ni. Matrix also isolates carbide particles and prevents particleparticle crack propagation - Abrasive action of the composite is increased by keeping vol. fraction of particles above 90%. WC-Co cemented carbide 1. Large Particle Composites: Examples P-R Composites Tyre: Polymer-Carbon Composite • Vulcanized Rubber (polymer) is mixed with carbon black - Carbon black, an inexpensive material, enhances UTS, toughness and tear & abrasion resistance - Tyres contain carbon black ranging from 15-30% - For C black to provide significant reinforcement, the particle size must be extremely small with diameter b/w 20 & 50nm. - The particles must be evenly distributed - The particles must form strong adhesive bond with rubber matrix Carbon black reinforced Rubber: A composite -Water marks show air pockets 1. Large Particle Composites: Examples Concrete: Ceramic-Ceramic Composite Dispersed phase: ceramic particles & Matrix phase: cement a. Plain Concrete: - Dispersed phase: stones and Silica - Matric phase: Cement b. Reinforced Concrete: - Dispersed phase: stones, Silica, Steel bars - Matric phase: Cement c. Pre-stressed Concrete: - Dispersed phase: stones, Silica, Pre-stressed Steel bars - Matric phase: Cement P-R Composites 2. Dispersion Strengthened Composites: Examples P-R Composites • Metal & metal alloys may be strengthened and hardened by the uniform dispersion of several volume percent of fine particles of a very hard and inert material. Examples: a. The high-temperature strength of nickel alloys may be enhanced significantly by the addition of about 3 vol% of thoria (ThO2) as finely dispersed particles b. The same effect is produced in the aluminum–aluminum oxide system. Flakes of Al2O3 (0.1 to 0.2 m thick)are dispersed within an aluminum metal matrix; this material is termed sintered aluminum powder (SAP). B. Fiber Re-inforced Composites • In these composites, the dispersed phase is in the form of fiber • Design goals of FRC often include high strength/stiffness on weight basis, called as specific strength and specific modulus • FRC are technologically the most important composites. Types: Continuous (long fiber) FRC Discontinuous (short fiber) FRC Factors Affecting Mechanical Properties of FRC FR Composites Mechanical characteristics of FRC depend on: a. Properties of fiber b. Degree to which an applied load is transferred to fibers by matrix phase - Load transmittance depends on the magnitude of interfacial bond b/w the fiber-matrix phase - Under an applied load (Fig), the fiber-matrix bond ceases at the fiber ends, yielding a matrix deformation pattern as shown. - The matrix does not transmit any load to each fiber extremity c. Fiber size (length/dia) d. Orientation e. Concentration Deformation pattern of matrix in FRC a. Influence of Fiber Length FR Composites • Some critical fiber length is necessary for effective strengthening NOTE: For PRC, smaller the dispersed particles, and stiffening of FRC: greater the strength of composite lc= Critical length For FRC, larger the dispersed fiber, higher will be the f * Tensile strength of fiber composite strength d= diameter of fiber Shear yield strength matrix c For a no of glass and carbon fiber combinations, lc is of the order of 1mm. This ranges b/w 20 and 150 times the fiber diameter 1. Continuous Fibers: l >>lc (l > 15lc) 2. Discontinuous Fibers: (l < 15lc) OR Short Fibers If l < < lc, the matrix deforms around the fiber such that there is virtually no stress transference and little reinforcement by the fiber b. Influence of Fiber Orientation& Concentration FR Composites With respect to orientation, 02 extremes are possible: a. Parallel alignment b. Random alignment - Continuous fibers always have parallel alignment - Discontinuous fibers could have parallel or random alignment Better the fiber distribution, better will be overall properties of composite Schematic representations of (a) continuous and aligned (b) Discontinuous and aligned (c) discontinuous and randomly oriented fiberreinforced composites. Parallel Continuous Parallel Discontinuous Oriented Discontinuous b. Influence of Fiber Orientation Continuous & Aligned Fiber Composites - - - - - FR Composites Tensile Stress-Strain Behavior- Longitudinal Loading Depends on: a. Stress-strain behavior of fiber phase & matrix phase b. Phase volume fraction c. Direction of load/stress: aligned fiber composites are highly anisotropic, so the properties depend on the measuring direction Stress–strain Fiber phase is brittle (say), has fracture strength but lower fracture strain than matrix phase Composite properties are in b/w fiber & matrix phases Stage 1 is elastic region Composite enter into stage (plastic) when composite strain equals to yield strain of matrix phase In stage 2, major part of load is borne by fiber phase Failure in composite occurs when strain in composite equals to fracture strain of fiber phase Failure in this kind of composite is not catastrophic. Because, 1. there is always a variation in fiber strength , 2. even all fiber are broken, they remain in tact with matrix, as matrix has higher fracture Schematic tensile stress–strain curves for brittle fiber and ductile matrix materials. curve for composite with fiber-matrix superimposed b. Influence of Fiber Orientation Continuous & Aligned Fiber Composites FR Composites Tensile Strength- Transverse - The strength of continuous and aligned fiber is highly anisotropic - These composites are designed for longitudinal loading, however, during service some transverse loading also act - Transverse tensile strength of such composites is very low, sometimes even lower than tensile strength of matrix - Traverse strength is affected by : - Properties of fiber & matrix Fiber-matrix bond strength Presence of voids - Transverse strength can be improved by modifying matrix properties Longitudinal and Transverse Tensile Strengths. The Fiber Content for Each Is approximately 50 Vol% b. Influence of Fiber Orientation Dis-Continuous & Randomly Oriented Fiber Composites FR Composites - Aligned Fibrous Composites are inherently anisotropic in that the maximum strength and reinforcement are achieved along the alignment direction. In Transverse direction, virtually zero enforcement exists - The fiber aligned in other directions will demonstrate strength b/w these 02 extremes • When multidirectional stresses are imposed within a single plane, aligned layers that are fastened together one on top of another at different orientations are frequently utilized. These are termed laminar composites • When multidirectional stresses are imposed in 3D, composites with fibers randomly oriented & uniformly distributed in 3D space are used. b. Influence of Fiber Orientation Selection of Fiber Length & Orientation Depends on: • FR Composites Level of stress Nature of applied stress (unidirectional/Random) Fabrication cost Production rates for short-fiber composites (both aligned/randomly oriented) are rapid • Intricate shapes can be easily made with short fibers • Fabrication techniques applied to short fibers include compression, extrusion and injection molding, similar to those applied for POLYMERS Fiber Phase FR Composites • Small diameter fiber is much stronger than bulk material, especially for brittle materials • The presence of critical flaws dangerous, that could cause failure, reduces with reducing volume of material • Composites make use of these 02 facts Types of Fibers: 1. Whisker: Common whisker materials are: - Single crystals Graphite, silicon carbide, silicon nitride, and aluminum oxide - Thin - Extremely large length/diameter ratio - As a consequence of their small size, they have a high degree of crystalline perfection and are virtually flaw free, which accounts for their exceptionally high strengths - They are among the strongest known materials. - Whiskers are not utilized extensively as a reinforcement medium because they are extremely expensive. Moreover, it is difficult and often impractical to incorporate whiskers into a matrix Fiber Phase - Whiskers have the highest properties Fiber and metallic wires have almost same order to properties Both whisker and Fibers are non metals Only Metallic wire uses metal fibers FR Composites Fiber Phase FR Composites Types of Fibers: 2. Fiber: - Fibers are either polycrystalline or amorphous - Fiber materials are polymers and ceramics, not metals - The polymer aramids, glass, carbon, boron, aluminum oxide, and silicon carbide 3. Wire: - Fine wires have relatively large diameter - Typical materials include steel, molybdenum, and tungsten - Wires are utilized as a radial steel reinforcement in automobile tires, in filament-wound rocket casings, and in wire-wound high-pressure hoses In general, the elastic modulus of the fiber should be much higher than that of the matrix. Matrix Phase FR Composites - Matrix phase of fibrous composites can be metal/polymer/ceramic - Generally, metals and polymers are used as matrix materials when ductility is desirable - However, if matrix is ceramic, fiber phase must be relatively ductile to provide higher facture toughness - Matrix serves several purposes: - binds the fibers together - acts as the medium by which an externally applied stress is transmitted and distributed to the fibers - very small proportion of an applied load is sustained by the matrix phase - matrix protects the individual fibers from surface damage as a result of mechanical abrasion or chemical reactions with environment - the matrix separates the fibers and, by virtue of its relative softness and plasticity, prevents the propagation of brittle cracks from fiber to fiber, which could result in catastrophic failure. In other words, the matrix phase serves as a barrier to crack propagation. Matrix Phase FR Composites - Even though some of the individual fibers fail, total composite fracture will not occur until large numbers of adjacent fibers, once having failed, form a cluster of critical size. Fiber- Matrix Bonding - It is essential that adhesive bonding forces between fiber and matrix be high to minimize fiber pull-out. - In fact, bonding strength is an important consideration in the choice of the matrix–fiber combination. - The ultimate strength of the composite depends to a large degree on the magnitude of this bond - Adequate bonding is essential to maximize the stress transmittance from the weak matrix to the strong fibers. Polymer-Matrix Composites FR Composites - Matrix phase: Polymer resin - Dispersed phase: Fiber (used as reinforcement medium) - Extensively used in a variety of applications due to: - Their good room-temperature properties, - ease of fabrication, - and cost - Common Types of PMCs - Glass Fiber Reinforced Polymer Composites Carbon Fiber Reinforced Polymer Composites Aramid Fiber Reinforced Polymer Composites Type of Fiber used in these PMC’s is called Fiber Polymer-Matrix Composites FR Composites 1. Glass-Fiber Reinforced Polymer Composites - Long/short fibers of glass are used as fibers. - Fiber diameters normally range between 3 and 20 μm. - Glass is popular as reinforcement medium because: 1. It is easily drawn into high-strength fibers from the molten state. 2. It is readily available and may be fabricated into a glass-reinforced plastic economically using a wide variety of composite-manufacturing techniques. 3. As a fiber it is relatively strong, and when embedded in a plastic matrix, it produces a composite having a very high specific strength. 4. When coupled with the various plastics, it possesses a chemical inertness that renders the composite useful in a variety of corrosive environments. Polymer-Matrix Composites FR Composites 1. Glass-Fiber Reinforced Polymer Composites Limitations of GFRPC: • In spite of having high strengths, they are not very stiff and do not display the rigidity that is necessary for some applications (e.g., as structural members for airplanes and bridges). • Most fiberglass materials are limited to service temperatures below at higher temperatures, most polymers begin to flow or to deteriorate. • Service temperatures may be extended to approximately by using highpurity fused silica for the fibers and high-temperature polymers such as the polyimide resins. Polymer-Matrix Composites FR Composites 1. Glass-Fiber Reinforced Polymer Composites Applications of GFRPC: Many fiberglass applications are familiar: - Automotive and marine bodies - Plastic pipes, - Storage containers, - Industrial floorings. - The transportation industries are utilizing increasing amounts of glass fiber-reinforced plastics in an effort to decrease vehicle weight and boost fuel efficiencies. - A host of new applications are being used or currently investigated by the automotive industry. Polymer-Matrix Composites FR Composites 2. Carbon-Fiber Reinforced Polymer Composites Carbon is a high-performance fiber material that is the most commonly used reinforcement in advanced polymer-matrix composites. It is used because: 1. Carbon fibers have the highest specific modulus and specific strength of all reinforcing fiber materials. 2. They retain their high tensile modulus and high strength at elevated temperatures; high-temperature oxidation, however, may be a problem. 3. At room temperature, carbon fibers are not affected by moisture or a wide variety of solvents, acids, and bases. 4. These fibers exhibit a diversity of physical and mechanical characteristics, allowing composites incorporating these fibers to have specific engineered properties. 5. Fiber and composite manufacturing processes have been developed that are relatively inexpensive and cost effective. Polymer-Matrix Composites FR Composites 2. Carbon-Fiber Reinforced Polymer Composites - The properties of C fiber depends on the type of pre-cursor used - Based on tensile modulus, C fibers are classified as standard, intermediate, high, and ultrahigh moduli. - Both continuous and chopped forms are available. - • • • • Applications of CFRPC: Sports and recreational equipment (fishing rods, golf clubs) Filament-wound rocket motor cases, Pressure vessels Aircraft structural components—both military and commercial, fixed wing and helicopters (e.g., as wing, body, stabilizer) • Rudder components Polymer-Matrix Composites 3. FR Composites Aramid-Fiber Reinforced Polymer Composites • The aramid fibers are most often used in composites having polymer matrices; common matrix materials are the epoxies and polyesters. • Since the fibers are relatively flexible and somewhat ductile, they may be processed by most common textile operations. Applications: - Typical applications of these aramid composites are in ballistic products (bullet proof vests and armor) - Sporting goods, - Tires, Ropes, - Missile cases, - Pressure vessels - And replacement for asbestos in automotive brake and clutch linings, and gaskets Polymer-Matrix Composites Comparison of FR-Polymer Composites Fiber is Continuous and Aligned Matrix: Epoxy All Cases the Fiber Volume Fraction is 0.60 CFRPC shows the best properties followed by Aramid and Glass FR Composites Metal-Matrix Composites FR Composites - Ductile metal is employed as a Matrix - These materials (MMCs)may be utilized at higher service temperatures than their base metal counterparts - The reinforcement may improve specific stiffness, specific strength, abrasion resistance, creep resistance, thermal conductivity, and dimensional stability - Some of the advantages of these materials over the polymer-matrix composites include higher operating temperatures, non-flammability, and greater resistance to degradation by organic fluids - Metal-matrix composites are much more expensive than PMCs, and, therefore, their (MMC) use is somewhat restricted - The super-alloys, as well as alloys of aluminum, magnesium, titanium, and copper are employed as matrix materials - The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers - Reinforcement concentrations normally range between 10 and 60 vol%. - Continuous fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. - Discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, and particulates of silicon carbide and aluminum oxide. Metal-Matrix Composites FR Composites Normally the processing of MMCs involves at least two steps: - Consolidation or synthesis (i.e., introduction of reinforcement into the matrix), followed by a shaping operation. A host of consolidation techniques are available, some of which are relatively sophisticated - Discontinuous fiber MMCs are amenable to shaping by standard metalforming operations (e.g., forging, extrusion, rolling) Applications - Some engine components have been recently introduced consisting of an aluminum-alloy matrix that is reinforced with aluminum oxide and carbon fibers. This MMC is light in weight and resists wear and thermal distortion. - Drive shafts (that have higher rotational speeds and reduced vibrational noise levels), extruded stabilizer bars, and forged suspension and transmission components. - Aerospace structural applications include advanced aluminum alloy metalmatrix composites; boron fibers are used as the reinforcement for the Space Shuttle Orbiter, and continuous graphite fibers for the Hubble Telescope. Metal-Matrix Composites FR Composites Applications - The high-temperature creep and rupture properties of some of the superalloys (Ni- and Co-based alloys) may be enhanced by fiber reinforcement using refractory metals such as tungsten. - Excellent high-temperature oxidation resistance and impact strength are also maintained. Designs incorporating these composites permit higher operating temperatures and better efficiencies for turbine engines. Properties of Several Metal-Matrix Composites Reinforced with Continuous and Aligned Fibers Ceramic-Matrix Composites FR Composites • Ceramics are noted for high strength, hot hardness and for sustaining high temperature under high stress. Resistance to oxidation and degradation at high temeperatures • However, they lack fracture toughness, which can be improved in the form of ceramic-matrix composite (about to 5 times of ceramics) - In essence, this improvement in the fracture properties results from interactions between advancing cracks and dispersed phase particles. - Crack initiation normally occurs with the matrix phase, whereas crack propagation is impeded or hindered by the particles, fibers, or whiskers. Ceramic-Matrix Composites FR Composites Processing of CMC: Ceramic-matrix composites may be fabricated using: Hot pressing, Hot iso-static pressing, Liquid phase sintering techniques. Applications: SiC whisker-reinforced aluminas are being utilized as cutting tool inserts for machining hard metal alloys. Tool lives for these materials are greater than for cemented carbide tools (Metal-Matrix Composites; Particles of TiC/WC in matrix of Co/Ni metal matrix) Processing of Continuous FRC FR Composites To fabricate continuous fiber-reinforced plastics that meet design specifications - the fibers should be uniformly distributed within the plastic matrix and, - in most instances, all oriented in virtually the same direction. - Important manufacturing processes for Continuous FRC are: - Pultrusion - Pre-preg production processes - Filament winding 1. Pultrusion: • Pultrusion is used for the manufacture of components having continuous lengths and a constant cross-sectional shape (i.e., rods, tubes, beams, etc.) Fibers of glass, carbon & aramid - Continuous process Easily automated High production rate Cost effective Wide variety of shapes No practical limit on length of part Polyester, Vinyl ester & Epoxy -Controls -machined die to impart final fiber/resin ratio - Resin ranges b/w shape on FRC - Die is heated to 40 & 70% Vol cure the FRC Processing of Continuous FRC FR Composites 2. Pre-preg Production Processes Prepreg is the composite industry’s term for continuous fiber reinforcement pre-impregnated with a polymer resin that is only partially cured. This material is delivered in tape form to the manufacturer, who then directly molds and fully cures the product without having to add any resin. It is probably the composite material form most widely used for structural applications Calendering: The fibers are sandwiched and pressed between sheets of release and carrier paper using heated rollers - Typical Pre-peg tape thickness ranges Used to spread a thin film of between 0.08 and 0.25 mm polymer resin on - Tape widths range between 25 and release paper 1525 mm - Resin content lies between about 35 and 45 vol% At room temperature the thermoset matrix undergoes curing reactions; therefore, the prepreg is stored at 0C or lower. Also, the time in use at room temperature (or “out-time”) must be minimized. If properly handled, thermoset prepregs have a lifetime of at least six months and usually longer. Processing of Continuous FRC FR Composites 2. Pre-preg Production Processes • Both thermoplastic and thermosetting resins are utilized; as matrix • Carbon, glass, and aramid fibers are the common fiber reinforcements. Fabrication of Final Product Using Pre-peg - Actual fabrication begins with the “lay-up”: pre-peg sheets are piled up to make desired thickness, after removing carrier paper - The lay-up arrangement can be uni-directional. But often they are stacked in different orientation to make a cross ply or angle ply - Final curing is accomplished by simultaneous application of heat and pressure - Lay-up is can be done with hand, in which sheets are cut and positioned on tool by hand. Alternatively ply pattern can be cut with machine also and positioned with hand - Lay up Process can be automated also (as filament winding) to reduce product cost. Processing of Continuous FRC FR Composites 3. Filament Winding - Filament winding is a process by which continuous reinforcing fibers are accurately positioned in a predetermined pattern to form a hollow (usually cylindrical) shape. - The fibers, either as individual strands or as tows, are first fed through a resin bath and then are continuously wound onto a mandrel, usually using automated winding equipment. - After the appropriate number of layers have been applied, curing is carried out either in an oven or at room temperature, after which the mandrel is removed. - As an alternative, narrow and thin prepregs (i.e., tow pregs) 10 mm or less in width may be filament wound. Processing of Continuous FRC 3. Filament Winding Techniques Applications: - Filament-wound parts have very high strength-to-weight ratios. - Rocket motor casings, storage tanks and pipes, and pressure vessels. - I-beams. - This technology is advancing very rapidly because it is very cost effective. FR Composites Structural Composites St Composites A structural composite is normally composed of both homogeneous and composite materials, the properties of which depend not only on the properties of the constituent materials but also on the geometrical design of the various structural elements. Laminar - Laminar composites composites - Sandwich panels A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength direction Laminations may also be constructed using fabric material such as cotton, paper, or woven glass fibers embedded in a plastic matrix. Thus a laminar composite has relatively high strength in a number of directions in the two-dimensional plane; however, the strength in any other given direction is, of course, lower than it would be if all the fibers were oriented in that direction. One example of a relatively complex laminated structure is the modern ski (see the chapter-opening illustration for this chapter). For example, adjacent wood sheets in plywood are aligned with the grain direction at right angles to each other. Structural Composites St Composites Sandwich panels - Consists of 02 faces and one core - The outer sheets called faces are made of a relatively stiff and strong material, typically aluminum alloys, fiber-reinforced plastics, titanium, steel, or plywood They (faces) impart high stiffness and strength to the structure, and must be thick enough to withstand tensile and compressive stresses that result from loading. The core material is lightweight, and normally has a low modulus of elasticity. Core materials typically fall within three categories: rigid polymeric foams (i.e., phenolics, epoxy, polyurethanes), wood (i.e., balsa wood), and honeycombs. - - Structurally, the core serves several functions. - First of all, it provides continuous support for the faces. - In addition, it must have sufficient shear strength to withstand transverse shear stresses, - and also be thick enough to provide high shear stiffness (to resist buckling of the panel) Tensile and compressive stresses on the core are much lower than on faces Structural Composites St Composites Sandwich panels Honey comb cores - - The honeycomb material is normally either an aluminum alloy or aramid polymer. Strength and stiffness of honeycomb structures depend on cell size, cell wall thickness, and the material from which the honeycomb is made. Applications of Sandwich Panels: - They are stiffer, strong and light wieght - Roofs, floors, and walls of buildings - In aerospace and aircraft (i.e., for wings, fuselage, and tail plane skins).