Composites Introduction: Modern technology require materials with combinations of properties that can’t be met by conventional materials Composite : any multiphase material that exhibits a significant proportion of properties of both constituent phases such that a better combination of properties is realized. This can be made with two or more distinct materials It is a multiphase material that is artificial made. The constituent phases must be chemically dissimilar & separated by a distinct interface Many composites are composed of two phases, the matrix which is continuous & surrounds the other phase, the dispersed phase

Classification for various types of composites

Particle Reinforced Composites Sub classified as large particle & dispersion strengthened composites Large Particle Composites The term large indicate that particle-matrix interactions can’t be treated on the atomic or molecular level The particulate phase is stiffer & harder than the matrix These particles tend to restrain movement of the matrix phase in the vicinity of each particle due to its small size & even distribution The volume fraction of the two phases influences the behavior; mechanical properties are enhanced with particulate content increment The rules of mixtures equations predict that elastic modulus should fall between upper bound : Ec (u)=EmVm + EpVp & lower bound, E= elastic modulus, V= volume fraction, c=composite, m=matrix, p=particulate phases

Concrete A common example of large particle composite in which both matrix & dispersed phases are ceramic materials Concrete is a composite material consisting an aggregate of particles that are bound together in a solid body. The binding medium is the cement Example: Portland cement concrete The ingredients-portland cement, fine aggregate (sand), coarse aggregate (gravel) & water The aggregate particles act as filler material The aggregates comprise 60%-80% of total volume The amount of cement-water paste should be sufficient to coat sand & gravel particles, if not cementious bond will be incomplete Addition of correct quantity of water will ensure a complete bonding & sufficient porosity Another example :reinforced concrete The concrete is reinforced with steel rods, wires or mesh Capable to support greater tensile, compressive & shear stresses

Dispersion Strengthened Composites Particles normally smaller. Diameters between 0.01 & 0.1 m Particle matrix interactions that lead to strengthening occur on the atomic or molecular level Whereas, the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations. Plastic deformation is restricted. Yield, tensile strength, hardness improved Metal & metal alloy can be strengthened by this way by involving interactions between particles & dislocations within the matrix The strengthening is retained at elevated temperatures & for extended time periods due to the dispersed particles chose to be unreactive with the matrix phase

Fiber Reinforced Composites The dispersed phase in fiber form Designed to have high strength &/or stiffness on weight basis. Expressed in specific strength & modulus These correspond to the ratios of tensile strength to specific gravity & modulus of elasticity to specific gravity Short fiber don’t produce significant improvement Influence of fiber length Some critical fiber length is necessary for effective strengthening & stiffening lc , critical length, dependent on d , fiber diameter & tensile strength, & c, fiber –matrix bond strength

Influence of Fiber Concentration and Orientation Fibers which l>> lc (normally l> 15 lc) are termed continuous. Shorter than this are discontinuous or short fibers For discontinuous fibers of lengths significantly less than lc, the matrix deforms around fiber. This results in no stress transference & little reinforcement by the fiber. Thus, for strength improvement of composites, the fibers must be continuous Influence of Fiber Concentration and Orientation Continuous fibers are normally aligned as in figure (a) !

Tensile Stress-Strain Behavior-Longitudinal Loading Continuous & Aligned Fiber Composites Tensile Stress-Strain Behavior-Longitudinal Loading In this figure, fracture strength in tension for fiber & matrix & , & their corresponding fracture strains, & ; It is assume that  However, a fiber reinforced composite consist of these fiber & matrix materials will exhibit the uniaxial stress strain response

In the initial Stage I region, both fibers & matrix deform elastically; normally this portion of the curve is linear. Typically, for a composite of this type, the matrix yields & deforms plastically at while the fibers continue to stretch elastically, in as much as the tensile strength of the fibers is significantly higher than the yield strength of the matrix. This process constitutes Stage II which stage is ordinarily very nearly linear, but of diminished slope relative to Stage II. In passing from Stage I to Stage II, the proportion of the applied load that is borne by the fibers increases. The onset of composite failure begins as the fibers starts to fracture, which corresponds to a strain approximately . Composite failure is not catastrophic. The reason: Not all fibers fracture at the same time These fractured fibers are still embedded within the intact matrix, thus capable of sustaining diminishing load as the matrix continues to plastically deform

Elastic behavior-Longitudinal Loading It is assumed that the fiber-matrix interfacial bond is very good, such that deformation of both matrix & fibers is the same The total load sustained by the composite Fc is equal to the loads carried by the matrix phase Fm & the fiber phase Ff or From the stress formula, F=A, the substitution of these into above formula yields: Then, the equation is divided by Ac , where Am/Ac & Af/Ac are the area fractions of the matrix & fiber phases. If the composite, matrix, & fiber phase lengths are all equal, Am/Ac is equivalent to the volume fraction of the matrix Vm, & for the fibers, Vf=Af/Ac If an isostrain state means: c=m=f, by dividing the above equation with its respective strain:

If composite, matrix, & fiber deformations are all elastic, then c/c=Ec, m/m=Em, f/f=Ef, the E’s being the moduli of elasticityfor respective phases. Then we substitute these into the previous equation which yields an expression for the modulus of elasticity of a continuous & aligned fibrous composite in the direction of alignment, Ecl, as Since the composite consists of only matrix & fiber phases; that is Vm+Vf=1 Ecl is equal to the volume-fraction weighted average of the moduli of elasticity of the fiber and matrix phases. Other properties including density, also have this dependence on volume fractions.

Longitudinal Tensile Strength Failure of this type of composite material is a relatively complex process. Several different failure modes are possible The mode that operates for a specific composite will depend on fiber & matrix properties, & the nature & strength of the fiber-matrix interfacial bond If , then fibers will fail before the matrix Once the fibers have fractured, the majority of the load that was borne by the fibers is now transferred to the matrix From equation: , we obtain the following expression for the longitudinal strength of composite, :

Transverse Tensile Strength Transverse tensile load might be present during in service applications Under these circumstances, premature failure may result inasmuch as transverse strength is usually extremely low Whereas, longitudinal strength is dominated by fiber strength, a variety of factors will influence on the transverse strength: Properties of both fiber & matrix Fiber-matrix bond strength Presence of voids

Discontinuous & Aligned Fiber Composites Advantage : short fiber composites can be produced having moduli of elasticity & tensile strength that approach 90% & 50% of continuous fiber counterparts. For a discontinuous & aligned fiber composite having a uniform distribution of fibers & in which l>lc. The longitudinal strength ( ) is given by the relationship: Where, & represents, the fracture strength of the fiber & the stress in the matrix when the composite fails. If the fiber length is less than critical (l<lc), then the longitudinal strength is given by: d=fiber diameter c=the smaller of either fiber matrix bond strength or matrix shear yield strength

Discontinuous & Randomly Oriented Fiber Composites For these kinds of fiber orientation, an expression for the elastic modulus similar to rule of mixture may be utilized: K=fiber efficiency parameter that depends on Vf & Ef/Em ratio

The Fiber Phase An important characteristic of most materials, i.e. brittle one, small diameter fiber is much stronger than the bulk material On the basis of diameter & character, fibers are classified to: Whiskers Fibers Wires Whiskers-very thin single crystals that have extremely large length-to-diameter ratio Due to their small size, they have high degree of crystalline perfection, flaw free, high strengths, known as strongest material Not utilized as reinforcement medium because they are extremely expensive. Impractical to incorporate into matrix Whisker materials include graphite, silicon carbide, silicon nitride & aluminum oxide Fibers-are either polycrystalline or amorphous & have small diameter. Generally polymer or ceramics Fine wires-relatively large diameters. Typical materials include steel,molybdenum & tungsten

The Matrix Phase The matrix phase of fibrous composites can be a metal, polymer or ceramic Metal & polymers are used as matrix materials due to their ductility For ceramic-matrix composites, reinforcing component is added to improve fracture toughness For fiber reinforced composites, matrix phase serves as: To bind the fibers together & acts as the medium by which an externally applied stress is transmitted & distributed to the fibers; only a very small proportion of an applied load is sustained by the matrix To protect individual fibers from surface damage as a result of mechanical abrasion or chemical reactions with the environment Matrix phase serves as barrier to crack propagation. Although some individual fibers fail, total composite fracture will not occur until large numbers of adjacent fibers have failed

Polymer-Matrix Composites Consist of a polymer resin as the matrix, with fibers as the reinforcement medium Glass Fiber Reinforced Polymer Composites Fiberglass is a composite consisting of glass fibers, either continuous or discontinuous, contained within a polymer matrix Produced in largest quantities.Fiber diameters range between 3 & 20 m Glass is popular because: Easily drawn into high strength fibers from molten state Readily available & maybe fabricated into glass-reinforced plastic economically using wide variety of composite manufacturing techniques As a fiber, it is relatively strong & when embedded in a plastic matrix, it produces a composite having a very high specific strength When coupled with various plastics, it possesses a chemical inertness that renders the composite useful in a variety of corrosive environments

Carbon Fiber Reinforced Polymer Composites Newly drawn fibers are coated during drawing. This is made from a thin layer of substance that protects the fiber surface from damage & undesirable environmental interactions Coatings are desirable to avoid surface flaws which are easily introduced by rubbing the surface with harder materials Limitations: Not very stiff, do not display rigidity that is necessary for some applications Limited to applications below 200oC Carbon Fiber Reinforced Polymer Composites Commonly used for reinforced in advanced (i.e.nonfiberglass) polymer-matrix composites Reasons of application: Carbon fibers have the highest specific modulus & specific strength of all reinforcing fiber materials Retain high tensile modulus & high strength at elevated temperatures; high temperature oxidation At room temperature; not affected by moisture & wide variety of chemicals Relatively inexpensive & cost effective

Aramid Fiber Reinforced Polymer Composites The stable form of crystalline carbon at ambient temperature is graphite Carbon fibers are not totally crystalline but are composed of graphitic & non crystalline region The classification scheme is based on tensile modulus: standard, intermediate, high & ultrahigh moduli Range of fiber diameters between 4-10 m. Available in continuous & chopped form Usually coated with protective epoxy to improve adhesion with polymer matrix Aramid Fiber Reinforced Polymer Composites Known chemically as poly paraphenylene terephtalamide High strength, high modulus, tough; resistance to impact, creep & fatigue failure. High longitudinal tensile strength & tensile moduli Desirable for their outstanding strength to weight ratios (superior than metal)

Schematic representation of mer & chain structures for aramid Schematic representation of mer & chain structures for aramid. Chain alignment with the fiber direction & hydrogen bonds between adjacent chains are shown Aramid are thermoplastic, but combustion resistant & stable to relatively high temperature. They can retain their high mechanical properties between -200oC & 200oC. Chemically susceptible to degradation by strong acids & bases, but relatively inert in other solvents & chemicals Most often used in composites have polymer matrices; common matrix materials are epoxies, & polyesters. The fibers are relatively flexible & somewhat ductile, they maybe processed by common textile operations

Other Fiber Reinforcement Materials Common fiber reinforcements incorporated in polymer matrices: Glass, carbon & aramids Others (but not that common): boron, silicon carbide, aluminum oxide Polymer Matrix Materials The matrix determines the maximum service temperature, since it normally softens, melts, or degrades at much lower temperature than the fiber reinforcement Polyesters & vinyl esters are widely utilized due to inexpensiveness Epoxies are more expensive but have better mechanical properties & resistance to moisture Polyimide resins can be used for high temperature application i.e. up to 230oC. Other materials-polyetheretherketone (PEEK), polyphenylene sulfide (PPS), & poly etherimide (PEI).

Metal Matrix Composites The matrix is a ductile metal Maybe utilized at higher service temperatures than their base metal counterparts. The reinforcement may improve specific stiffness, strength, abrasion resistance, creep resistance, thermal conductivity & dimensional stability Advantage over polymer-matrix composites: higher operating temperature, non flammability, greater resistance to degradation by organic fluids. However, more expensive Superalloy, as well as alloys of aluminum, magnesium, titanium, & copper, are employed as matrix materials Reinforcement might be in particulates, continuous & discontinuous fibers, whiskers. Concentrations normally range between 10 & 60 vol%. Continuous fiber materials: carbon, silicon carbide, boron, alumina & refractory metals. Discontinuous-silicon carbide whiskers, chopped fibers of alumina & carbon, & particulates of silicon carbide & alumina

Some matrix-reinforcement combinations are highly reactive at elevated temperatures Composite degradation maybe caused by high temperature processing or by subjecting the metal-matrix composites to elevated temperature during service This is resolved by applying protective surface coating to the reinforcement or by modifying the matrix alloy composition

Ceramic Matrix Composites The fracture toughness of ceramics have been improved significantly by these composites where particulates, fibers, or whiskers of one ceramic materials have been embedded into a matrix of another ceramics Ceramic matrix composite materials have extended fracture toughness between 6 & 20 MPam The improvement in the fracture properties results from interactions between advancing cracks & dispersed phase particles Crack initiation normally occurs with the matrix phase whereas crack propagation is impeded or hindered by the particles, fibers or whiskers

In general, increasing fiber content improves strength & fracture toughness There is also a considerable reduction in the scatter of fracture strengths for whisker-reinforced ceramics relative to their unreinforced counterpart In addition, these ceramic matrix composites exhibit improved high temperature creep behavior & resistance to thermal shock

Carbon-Carbon Composites Both reinforcement & matrix are carbon Newly developed & expensive. Not extensively utilized Desirable properties include: High tensile moduli & tensile strength that are retained up to 2000oC Resistance to creep Large fracture toughness value Low coefficients of thermal expansion High thermal conductivities Low susceptibility to thermal shock Hybrid Composite Obtained by using two or more different kind of fibers in a single matrix Hybrids have better all around combination of properties than composites containing only single fiber type A variety of fiber combinations & matrix materials are used, usually are from carbon & glass fibers which incorporated into polymeric resin The glass-carbon hybrid is stronger & tougher, has higher impact resistance, maybe produced at lower cost

Structural Composites When hybrid composites are stressed in tension, failure is usually non catastrophic The carbon fibers are the first to fail, at which time the load is transferred to the glass fibers Upon failure of the glass fibers, the matrix phase must sustain the applied load Eventual composite failure concurs with that of the matrix phase Structural Composites Laminar Composites Composed of 2 dimensional sheets or panels that have a preferred high strength directions as in wood & continuous & aligned fiber reinforced plastics The layers are stacked & subsequently cemented together such that the orientation of the high strength direction varies with each successive layer

Laminar composite has relatively high strength in a number of directions in 2D plane, but the strength in any given direction is lower than it would be if all the fibers were oriented in that direction

Sandwich Panel Considered to be a class of structural composites Consist of 2 strong outer sheets or faces, separated by a layer of less dense material, core which has lower stiffness & lower strength The faces bear most of the in-plane loading & also any transverse bending stresses Typical face materials- aluminum alloys, fiber reinforced plastics, titanium, steel, plywood Structurally, the core serves 2 functions. Separates the faces & resists deformations perpendicular to the face plane Provides a certain degree of shear rigidity along planes that are perpendicular to the faces Core materials-foamed polymers, synthetic rubbers, inorganic cements Another popular core consists of honeycomb structure-thin foils that have been formed into interlocking hexagonal cells The material maybe the same as face material