Introduction to Composite Materials

ByMd.Ahasan

Introduction to Composite MaterialsA composite material can be defined as a combination of two or more materials that results in better properties than those of the individual components used alone. In contrast to metallic alloys, each material retains its separate chemical, physical, and mechanical properties. The two constituents are a reinforcement and a matrix. The main advantages of composite materials are their high strength and stiffness, combined with low density, when compared with bulk materials, allowing for a weight reduction in the finished par

Composite materials IntroductionDefinition: any combination of two or more different materials at the macroscopic level. ORTwo inherently different materials that when combined together produce a material with properties that exceed the constituent materials. Reinforcement phase (e.g., Fibers)Binder phase (e.g., compliant matrix)

AdvantagesHigh strength and stiffnessLow weight ratioMaterial can be designed in addition to the structure

ApplicationsStraw in clay construction by EgyptiansAerospace industrySporting goodsAutomotiveConstruction

Types of CompositesMMCsCMCs PMCsMetal Matrix CompositesCeramic Matrix Comps. Polymer Matrix Comps

Costs of composite manufactureMaterial costs -- higher for compositesConstituent materials (e.g., fibers and resin)Processing costs -- embedding fibers in matrix not required for metals Carbon fibers order of magnitude higher than aluminumDesign costs -- lower for compositesCan reduce the number of parts in a complex assembly by designing the material in combination with the structureIncreased performance must justify higher material costs

Types of Composite Materials There are five basic types of composite materials: Fiber, particle, flake, laminar or layered and filled composites.

A. Fiber CompositesIn fiber composites, the fibers reinforce along the line of their length. Reinforcement may be mainly 1-D, 2-D or 3-D. Figure shows the three basic types of fiber orientation.1-D gives maximum strength in one direction.

2-D gives strength in two directions.

Isotropic gives strength equally in all directions.

Composite strength depends on following factors:Inherent fiber strength, Fiber length, Number of flawsFiber shape The bonding of the fiber (equally stress distribution)Voids Moisture (coupling agents)

B. Particle CompositesParticles usually reinforce a composite equally in all directions (called isotropic). Plastics, cermets and metals are examples of particles.Particles used to strengthen a matrix do not do so in the same way as fibers. For one thing, particles are not directional like fibers. Spread at random through out a matrix, particles tend to reinforce in all directions equally.Cermets (1) OxideBased cermets(e.g. Combination of Al2O3 with Cr) (2) CarbideBased Cermets(e.g. Tungstencarbide, titaniumcarbide)Metalplastic particle composites(e.g. Aluminum, iron & steel, copper particles) Metalinmetal Particle Composites and Dispersion Hardened Alloys(e.g. Ceramicoxide particles)

C. Flake Composites - 1Flakes, because of their shape, usually reinforce in 2-D. Two common flake materials are glass and mica. (Also aluminum is used as metal flakes)

C. Flake Composites -2A flake composite consists of thin, flat flakes held together by a binder or placed in a matrix. Almost all flake composite matrixes are plastic resins. The most important flake materials are:

Aluminum Mica Glass

C. Flake Composites -3Basically, flakes will provide:Uniform mechanical properties in the plane of the flakesHigher strengthHigher flexural modulus Higher dielectric strength and heat resistanceBetter resistance to penetration by liquids and vaporLower cost

D. Laminar Composites - 1 Laminar composites involve two or more layers of the same or different materials. The layers can be arranged in different directions to give strength where needed. Speedboat hulls are among the very many products of this kind.

D. Laminar Composites - 2Like all composites laminar composites aim at combining constituents to produce properties that neither constituent alone would have.

In laminar composites outer metal is not called a matrix but a face. The inner metal, even if stronger, is not called a reinforcement. It is called a base.

D. Laminar Composites - 3We can divide laminar composites into three basic types:Unreinforcedlayer composites (1) AllMetal (a) Plated and coated metals (electrogalvanized steel steel plated with zinc) (b) Clad metals (aluminumclad, copperclad) (c) Multilayer metal laminates (tungsten, beryllium) (2) MetalNonmetal (metal with plastic, rubber, etc.) (3) Nonmetal (glassplastic laminates, etc.)Reinforcedlayer composites (laminae and laminates)Combined composites (reinforcedplastic laminates well bonded with steel, aluminum, copper, rubber, gold, etc.)

D. Laminar Composites - 4A lamina (laminae) is any arrangement of unidirectional or woven fibers in a matrix. Usually this arrangement is flat, although it may be curved, as in a shell.

A laminate is a stack of lamina arranged with their main reinforcement in at least two different directions.

E. Filled CompositesThere are two types of filled composites. In one, filler materials are added to a normal composite result in strengthening the composite and reducing weight. The second type of filled composite consists of a skeletal 3-D matrix holding a second material. The most widely used composites of this kind are sandwich structures and honeycombs.

F. Combined CompositesIt is possible to combine several different materials into a single composite. It is also possible to combine several different composites into a single product. A good example is a modern ski. (combination of wood as natural fiber, and layers as laminar composites)

Forms of Reinforcement PhaseFibers cross-section can be circular, square or hexagonalDiameters --> 0.0001 - 0.005 Lengths --> L/D ratio100 -- for chopped fibermuch longer for continuous fiberParticulatesmall particles that impede dislocation movement (in metal composites) and strengthens the matrixFor sizes > 1 mm, strength of particle is involves in load sharing with matrixFlakesflat platelet form

Fiber ReinforcementThe typical composite consists of a matrix holding reinforcing materials. The reinforcing materials, the most important is the fibers, supply the basic strength of the composite. However, reinforcing materials can contribute much more than strength. They can conduct heat or resist chemical corrosion. They can resist or conduct electricity. They may be chosen for their stiffness (modulus of elasticity) or for many other properties.

Types of FibersThe fibers are divided into two main groups:Glass fibers: There are many different kinds of glass, ranging from ordinary bottle glass to high purity quartz glass. All of these glasses can be made into fibers. Each offers its own set of properties. Advanced fibers: These materials offer high strength and high stiffness at low weight. Boron, silicon, carbide and graphite fibers are in this category. So are the aramids, a group of plastic fibers of the polyamide (nylon) family.

Fibers - Glass Fiberglass properties vary somewhat according to the type of glass used. However, glass in general has several wellknown properties that contribute to its great usefulness as a reinforcing agent:Tensile strengthChemical resistanceMoisture resistanceThermal propertiesElectrical properties

There are four main types of glass used in fiberglass: AglassCglassEglassSglass

Fibers - GlassMost widely used fiberUses: piping, tanks, boats, sporting goodsAdvantagesLow costCorrosion resistanceLow cost relative to other composites: DisadvantagesRelatively low strengthHigh elongationModerate strength and weightTypes:E-Glass - electrical, cheaperS-Glass - high strength

Fibers - Aramid (kevlar, Twaron)Uses: high performance replacement for glass fiberExamplesArmor, protective clothing, industrial, sporting goodsAdvantages: higher strength and lighter than glassMore ductile than carbon

Fibers - Carbon2nd most widely used fiberExamplesaerospace, sporting goodsAdvantageshigh stiffness and strengthLow densityIntermediate costProperties: Standard modulus: 207-240 GpaIntermediate modulus: 240-340 GPaHigh modulus: 340-960 GPa Diameter: 5-8 microns, smaller than human hairFibers grouped into tows or yarns of 2-12k fibers

Fibers -- Carbon (2)Types of carbon fibervary in strength with processingTrade-off between strength and modulusIntermediate modulusPAN (Polyacrylonitrile) fiber precursor heated and stretched to align structure and remove non-carbon materialHigh modulus made from petroleum pitch precursor at lower costmuch lower strength

Fibers - OthersBoronHigh stiffness, very high costLarge diameter - 200 micronsGood compressive strengthPolyethylene - trade name: Spectra fiberTextile industryHigh strengthExtremely light weightLow range of temperature usage

Fibers -- Others (2)Ceramic Fibers (and matrices)Very high temperature applications (e.g. engine components)Silicon carbide fiber - in whisker form.Ceramic matrix so temperature resistance is not compromisedInfrequent use

Fiber Material PropertiesSteel: density (Fe) = 7.87 g/cc; TS=0.380 GPa; Modulus=207 GPaAl: density=2.71 g/cc; TS=0.035 GPa; Modulus=69 GPa

Fiber Strength

Matrix MaterialsFunctions of the matrixTransmit force between fibersarrest cracks from spreading between fibersdo not carry most of the loadhold fibers in proper orientationprotect fibers from environmentmechanical forces can cause cracks that allow environment to affect fibersDemands on matrix Interlaminar shear strengthToughnessMoisture/environmental resistanceTemperature propertiesCost

Matrices - PolymericThermosetscure by chemical reactionIrreversibleExamplesPolyester, vinylesterMost common, lower cost, solvent resistanceEpoxy resinsSuperior performance, relatively costly

PolyesterPolyesters have good mechanical properties, electrical properties and chemical resistance. Polyesters are amenable to multiple fabrication techniques and are low cost.Vinyl EstersVinyl Esters are similar to polyester in performance. Vinyl esters have increased resistance to corrosive environments as well as a high degree of moisture resistance.

Matrices - Thermosets

EpoxyEpoxies have improved strength and stiffness properties over polyesters. Epoxies offer excellent corrosion resistance and resistance to solvents and alkalis. Cure cycles are usually longer than polyesters, however no by-products are produced.

Flexibility and improved performance is also achieved by the utilization of additives and fillers.

Matrices - Thermosets

Matrices - ThermoplasticsFormed by heating to elevated temperature at which softening occursReversible reactionCan be reformed and/or repaired - not commonLimited in temperature range to 150CExamplesPolypropylene with nylon or glass can be injected-- inexpensiveSoften layers of combined fiber and resin and place in a mold -- higher costs

Matrices - OthersMetal Matrix Composites - higher temperaturee.g., Aluminum with boron or carbon fibersCeramic matrix materials - very high temperatureFiber is used to add toughness, not necessarily higher in strength and stiffness

Important NoteComposite properties are less than that of the fiber because of dilution by the matrix and the need to orient fibers in different directions.

MANUFACTURING PROCESSES OF COMPOSITESComposite materials have succeeded remarkably in their relatively short history. But for continued growth, especially in structural uses, certain obstacles must be overcome. A major one is the tendency of designers to rely on traditional materials such as steel and aluminum unless composites can be produced at lower cost.

Cost concerns have led to several changes in the composites industry. There is a general movement toward the use of less expensive fibers. For example, graphite and aramid fibers have largely supplanted the more costly boron in advancedfiber composites. As important as savings on materials may be, the real key to cutting composite costs lies in the area of processing.

The processing of fiber reinforced laminates can be divided into two main steps:

Layup Curing

Curing is the drying and hardening (or polymerization) of the resin matrix of a finished composite. This may be done unaided or by applying heat and/or pressure.

Layup basically is the process of arranging fiberreinforced layers (laminae) in a laminate and shaping the laminate to make the part desired. (The term layup is also used to refer to the laminate itself before curing.) Unless prepregs are used, layup includes the actual creation of laminae by applying resins to fiber reinforcements.

Laminate layup operations fall into three main groups:

Winding and laying operationsMolding operationsContinuous lamination

Continuous lamination is relatively unimportant compared with quality parameters as not good as wrt other two processes. In this process, layers of fabric or mat are passed through a resin dip and brought together between cellophane covering sheets. Laminate thickness and resin content are controlled by squeegee rolls. The layup is passed through a heat zone to cure the resin.

A. Winding Operation

The most important operation in this category is filament winding. Fibers are passed through liquid resin, and then wound onto a mandrel. After layup is completed, the composite is cured on the mandrel. The mandrel is then removed by melting, dissolving, breakingout or some other method.

B. Molding OperationsMolding operations are used in making a large number of common composite products. There are two types of processes:

Openmold (1) Hand layup (2) Sprayup (3) Vacuumbag molding (4) Pressurebag molding (5) Thermal expansion molding (6) Autoclave molding (7) Centrifugal casting (8) Continuous pultrusion and pulforming.

1. Hand Lay-upHand layup, or contact molding, is the oldest and simplest way of making fiberglassresin composites. Applications are standard wind turbine blades, boats, etc.)

2. Spray-upIn Sprayup process, chopped fibers and resins are sprayed simultaneously into or onto the mold. Applications are lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubes, small boats, etc.

3. Vacuum-Bag MoldingThe vacuumbag process was developed for making a variety of components, including relatively large parts with complex shapes. Applications are large cruising boats, racecar components, etc.

4. Pressure-Bag MoldingPressurebag process is virtually a mirror image of vacuumbag molding. Applications are sonar domes, antenna housings, aircraft fairings, etc.

5. Thermal Expansion MoldingIn Thermal Expansion Molding process, prepreg layers are wrapped around rubber blocks, and then placed in a metal mold. As the entire assembly is heated, the rubber expands more than the metal, putting pressure on the laminate. Complex shapes can be made reducing the need for later joining and fastening operations.

6. Autoclave MoldingAutoclave molding is similar to both vacuumbag and pressurebag molding. Applications are lighter, faster and more agile fighter aircraft, motor sport vehicles.

Continuous pultrusion is the composite counterpart of metal extrusion. Complex parts can be made.7. Centrifugal CastingCentrifugal Casting is used to form round objects such as pipes. 8. Continuous Pultrusion and Pulforming

Pulforming is similar to pultrusion in many ways. However, pultrusion is capable only of making straight products that have the same volume all along their lengths. Pulformed products, on the other hand, can be either straight or curved, with changing shapes and volumes. A typical pulformed product is a curved reinforced plastic car spring. (shown in figure.)

B. Closedmold (1) Matcheddie molding: As the name suggests, a matcheddie mold consists of closely matched male and female dies (shown in figure). Applications are spacecraft parts, toys, etc. (2) Injection molding: The injection process begins with a thermosetting (or sometimes thermoplastic) material outside the mold. The plastic may contain reinforcements or not. It is first softened by heating and/or mechanical working with an extrusiontype screw. It is then forced, under high pressure from a ram or screw, into the cool mold. Applications are auto parts, vanes, engine cowling defrosters and aircraft radomes.

Material Forms and ManufacturingObjectives of material productionassemble fibersimpregnate resinshape productcure resin

Sheet Molding Compound (SMC)Chopped glass fiber added to polyester resin mixtureQuestion: Is SMC isotropic or anisotropic?

Manufacturing - Filament WindingHighly automated low manufacturing costs if high throughpute.g., Glass fiber pipe, sailboard masts

PrepregsPrepreg and prepreg layupprepreg - partially cured mixture of fiber and resinUnidirectional prepreg tape with paper backingwound on spoolsCut and stackedCuring conditionsTypical temperature and pressure in autoclave is 120-200C, 100 psi

Manufacturing - Layupscompressionmoldingvacuum bagging

Material FormsTextile formsBraiding or weavingTubular braided form can be flattened and cut for non-tubular products

Fabric Structures Woven:Series of Interlaced yarns at 90 to each otherKnit:Series of Interlooped YarnsBraided:Series of Intertwined, Spiral YarnsNonwoven:Oriented fibers either mechanically, chemically, or thermally bonded

Woven FabricsBasic woven fabrics consists of two systems of yarns interlaced at right angles to create a single layer with isotropic or biaxial properties.

Physical PropertiesConstruction (ends & picks)WeightThicknessWeave Type

Components of a Woven Fabric

Basic Weave TypesPlain Weave

Basic Weave TypesSatin 5HS

Basic Weave Types2 x 2 Twill

Basic Weave TypesNon-Crimp

BraidingA braid consists of two sets of yarns, which are helically intertwined.The resulting structure is oriented to the longitudinal axis of the braid.This structure is imparted with a high level of conformability, relative low cost and ease of manufacture.

Braid Structure

Types of Braids

Triaxial YarnsA system of longitudinal yarns can be introduced which are held in place by the braiding yarns

These yarns will add dimensional stability, improve tensile properties, stiffness and compressive strength.

Yarns can also be added to the core of the braid to form a solid braid.

Fabric effects on material properties

Resin transfer molding (RTM)Dry-fiber preform placed in a closed mold, resin injected into mold, then cured

Material FormsPultrusionFiber and matrix are pulled through a die, like extrusion of metals -- assembles fibers, impregnates the resin, shapes the product, and cures the resin in one step.Example. Fishing rods

Pultrusion

ManufacturingTube rolling - tubular productsExamplesfishing rodsgolf clubsoarsPrepreg tape typically used wrapped in 2 directions or spiral wrapped