Composite Materials

Composite Materials Definition: A composite material is a macroscopic combination of two or more distinct materials, having a recognisable interface between them

Composite materials contain a continuous matrix constituent that binds together and provides form to an array of a stronger, stiffer reinforcement constituent. The resulting composite material has a balance of structural properties that is superior to either constituent material alone

Composites are used not only for their structural properties, but also for electrical, thermal, tribological and environmental applications

Evolution of Composite MaterialsThe three key historical developments in composites:1. Commercial availability of fibreglass filaments in 1935 led to use of fibre reinforced plastics (FRP) in aircraft radomes in 1942, FRP translucent sheet in 1949 and in the 1950s to developments in FRP boat hulls, car bodies and truck cabs2. The search for lighter, stiffer structures led to the development of aramid (1965) & second generation carbon fibres (1963) along with the development of epoxies (1937), phenolics (1969) and other thermosetting resins.3. The development of methods of analysis for composite structures

Classification of CompositesClassification may be done in two ways: (a) by type of matrix and (b) by form of reinforcement

(a) Major types of composites include organic matrix composites (OMC), metal matrix composites (MMC), ceramic matrix composites (CMC). OMC includes polymer matrix composites (PMC) and carbon matrix or carbon-carbon composites (CCC)

(b) Types of reinforcement include particulate, whisker, continuous fibre laminated and woven composites (by weaving, braiding or knitting the fibre bundles or tows)

Advanced Composite Materials Modern structural composites or Advanced Composites are a blend of two or more components stiff, long fibres and a binder or matrix that holds the fibres in placeThe resulting composite is generally composed of layers (laminae) of the fibres and matrix stacked to achieve the desired properties in one or more directionsThe commercially available reinforcement materials include fibre-glass, carbon, aramid, polyethylene, boron, silicon carbide, silicon nitride, silica, aluminaThermoset matrix materials for composites include polyesters & vinylesters, epoxy, bismaleimide, cyanate, phenolic, polyimide. Thermoplastic resins include poly- ether ether ketone (PEEK), poly- phenylene sulfide (PPS)

MatricesRole of matrix in fibre reinforced composite:

To distribute or transfer loads between fibresTo provide barrier against adverse environmentTo protect surface of fibres from mechanical abrasion in the structure and before fabricationKeep fibres in place in the structureControl the electrical and chemical propertiesCarry interlaminar shear stresses under bending loads and in-plane shear stresses under torsional loads

Desirable Properties of MatrixMinimize moisture absorptionWet and bond to fibreFlow to penetrate completely and eliminate voids during compacting and curing processBe elastic in order to transfer loads to fibresHave strength at elevated and low temperaturesHave good chemical resistanceHave low shrinkage & coefficient of thermal expansionHave reasonable strength, modulus & elongation (>fibre)Be easily processibleHave dimensional stability (maintain shape)

Matrix Materials

Thermoset Matrices

Thermoset matrices form the most widely used class of matrix materials in advanced polymer based compositesA thermosetting matrix sets at some temperature and cannot be reshaped by subsequent heatingIn general, thermosetting polymers contain two or more ingredients, a resinous matrix and a curing agentSolidification of composite matrix starts when resin and curing agent are mixed or when matrix is heated, causing a reaction Thermosets include polyester, vinyl ester, epoxy, polyimide, bismaleimide, with epoxy having a dominant position in aircraft structuresTo increase damage tolerance in aircraft structures, thermoset matrix resins are toughened by incorporation of dispersed second phase of rubber or thermoplastic particles

Advantages of Epoxy MatricesWide range of properties due to availability of large number or starting materials, curing agents & modifiersAbsence of volatiles during curing processLow shrinkage during cureExcellent resistance to chemicals and solventsExcellent adhesion wide variety of fibres, fillersHigh or low strength and flexibilityResistance to creep and fatigueGood electrical propertiesSolid or liquid resins available in uncured stateWide range of curing options

Disadvantages of Epoxy MatricesRelatively high cost and long curing timeResins somewhat toxic in uncured stateHeat distortion point lowered by moisture absorptionChanges in dimensions & physical properties upon absorption or moistureUse (dry) limited to approx 200 deg C upper temperatureDifficult to combine toughness & high temp resistanceHigh coefficient of thermal expansionHigh degree of smoke liberation in a fireMay be sensitive to ultraviolet light degradation

Advantages of Thermoplastic CompositesEnhanced processability due to reduced cycle times, no low temperature storage requirement, less energy intensive, absence of solvents (most systems), recyclingHigh temperature stabilityHigh toughness and damage toleranceHigher strains to failureExcellent damping characteristicsGreatly reduced moisture absorptionLow outgassing and better radiation toleranceImproved solvent tolerance

Disadvantages of Thermoplastic Composites Prepreg tapes are stiff and lack good drape and tack of the thermosetsSemicrystalline thermoplastic materials require carefully controlled cooling schedule after consolidation of composite to develop required degree of crystallinityLong term performance data base for materials selection and composite design not availableThermoplastic prepregs and hybrid yarns and fabrics are high cost materialsHigher temperature consolidation requirement leads to higher temperature equipment capability, higher cost

Sandwich PanelsSandwich Panel is a panel consisting of two thin face sheets bonded to a thick, lightweight honeycomb or foam coreLight weight core acts as support for the thin composite skins and prevents them from buckling when the component is subjected to bending and twisting loadsSandwich panel is similar to an I section beam, where flanges take the tension and compression loads and the web takes the shearIn aerospace structures, skin materials used are carbon epoxy, Kevlar epoxy, glass epoxy compositesCore materials include aramid honeycomb (Nomex), PVC foam, polymethacrylimide foam (Rohacell), phenolic coated Kraft paper honeycomb, aluminium honeycomb (various foil grades 2024, 3003, 5052, 5056)

Manufacturing of Composites

Manufacturing of Composites Goals of the composite fabrication process:To achieve a consistent product by controlling fibre thickness, fibre volume fraction, fibre directionsTo minimise presence of voidsTo reduce internal residual stressesTo process in a cost effective manner

Manufacturing process planning involves optimum selection of :Composite material and its configurationFabrication processTooling

Composite Fabrication ProcessesProcesses for incorporating reinforcements into a polymer matrix can be divided into two categories:

1. The continuous and discontinuous fibres and matrix are processed directly into the finished structure, e.g., filament winding and pultrusion

2. The reinforcements are incorporated into the matrix to prepare ready-to-mould sheets that can be stored and later processed to form laminated structures, e.g., autoclave moulding and compression moulding. Ready- to-mould fibre reinforced polymer sheets are available in two basic forms, prepregs and sheet moulding compounds (SMC)

Advantages of Hand Lay-up ProcessFlexibility of designLarge, complex components can be producedMinimum equipment investment requiredLow tooling costMinimal start-up and lead-time and costDesign changes easily effectedMoulded-in inserts & structural reinforcements possibleSandwich construction possibleStandard prototyping and pre-production methodSemi-skilled workers, minimal training

Disadvantages of Hand Lay-up Process

Labour intensive processOnly one tooled (moulded) surface obtainedQuality of product related to skill of operatorLow production volume processLonger curing time requiredWaste factor can be high

Bag Moulding ProcessesMoulding methods used include vacuum bag, pressure bag, oven and autoclave mouldingBags are thin, flexible membranes of nylon or polyvinyl alcohol or silicone rubber sheets that separate the laid up construction from atmospheric pressure during the composite curing processConsolidation and densification of the lay-up is achieved by the pressure differential across the bag contentsConsolidation is achieved when the separate plies of prepreg are bonded togetherDensification results in reduction of voids and removal of excess resinOther advantages of bag moulding methods during cure include prevention of blistering in the composites, better control of pressure and heat application and control of the fibre/resin ratio

Vacuum Bag and Pressure Bag MouldingVacuum bag moulding is least limited as to size of partWet lay-up vacuum bag moulded composites can be room temperature cured or thermally cured to produce improved propertiesThermal cures are best attained in air-circulating ovens or autoclaves, but can also be done by infrared heatingPressure bag moulding methods are efficient for curing of deeply contoured structures and shallow compositesExamples of deeply contoured composites are sonar domes, radomes and antenna housings. Heavy moulds are built for the high temperatures and pressuresShallow contoured composite items such as door panels and aircraft fairings may be made in modified compression pressesPressure bag and autoclave moulding processes commonly attain 177 deg C (350 F) temperature and 1379 kPa pressure (200 psi)

Autoclave MouldingAn autoclave system comprises of a cylindrical pressure vessel equipped with subsystems for heating the moulds with the lay-ups, for circulating the hot gases (nitrogen / air), for pressurisation of the gases, for applying a vacuum on the bagged parts, for cooling the autoclave shell, for controlling the operating parameters and for loading the moulds into the autoclave The autoclave system allows a complex chemical reaction to occur inside the pressure vessel per a cure cycle that specifies a time, temperature and pressure profile in order to process a variety of materialsDevelopment of materials and processes has led to autoclaves that operate over a wide range of 120-760 deg C and 275-69,000 kPaHeating systems available include electric for small, superheated steam for medium temp and indirect gas for high temp autoclavesMaterials processed in autoclaves include metal bonding adhesives, reinforced epoxy laminates, thermoplastic laminates and metal ceramic and carbon matrix materials

CuringCuring is the irreversible change in the physical properties of a thermosetting resin brought about by a chemical reactionCure may be achieved by the addition of curing or cross linking agents, with or without the addition of heat and pressure. Curing can also be accomplished by using ultraviolet radiation or electron beams for specific applicationsThe relationship between resin viscosity and the cure cycle is used to obtain maximum performance in a composite structure. The viscosity can vary with a change in heat-up rate and temperature.The material supplier can provide information on specific applications, processing parameters, material properties, test data and fabrication methodology for the material. Using this information, a cure cycle is defined for a specific composite part Uniform temperature in the lay-up during curing is critical. Non-uniformity can lead to residual stresses, trapped volatiles and variation in propertiesProper vacuum and autoclave pressure control as per the cure cycle can help to control laminate porosity

Resin Transfer Moulding Resin Transfer Moulding (RTM) is a closed mould liquid moulding process in which matched male and female moulds, preplaced with fibre preforms, are clamped to form composite componentsPreform is dry reinforcement material which has been cut and/or shaped into a piece that has the required size and contoursAfter the mould is closed and shut, resin mix is transferred into the cavity through injection ports at a relatively low pressure of

Advantages of RTMSurface quality: Surface definition is superior to lay-up. All surfaces attain better finish with mould matched toolsClose tolerances: Parts can be made with better repeat- ability than with lay-upDesign tailorability: Combination of reinforcements can be used to meet specific propertiesFast cycles: Production cycles are much faster than with lay-upFiller: Fillers can be used to reduce cost, improve fire, smoke performance, surface appearance and crack resistanceGel coat: Mould surfaces can be coated to improve part surfacesGood properties: Mechanical properties of moulded parts are comparable with other composite fabrication processesGood mouldability: Large and complex shapes can be made efficiently and inexpensivelyInserts: Ribs, bosses, cores, inserts and special reinforcements can be added easilyLabour saving: The skill level of the operator is less critical

Advantages/Disadvantages of RTMADVANTAGES, cont:

Low tooling cost: Clamping pressure is low compared to other closed mould operations. Resin injection done at 2-10 bar pressureLow volatile emissions: Volatile emissions are low since RTM is a closed mould process

DISADVANTAGES:

Mould design: Mould design is critical and requires good tools or superior skillMould filling: Control of flow pattern or resin uniformity is difficult. Radii and edges tend to be resin richProperties are equivalent to those with matched-die moulding but not as good as with vacuum bagging, filament winding or pultrusionReinforcement movement during resin injection is sometimes a problem

Variant of RTM - Vacuum Infusion/VARTMVacuum Infusion/Vacuum Assisted Resin Transfer Moulding (VARTM)Initially developed as alternative to open-mould hand lay-up and spray-up techniques. Fibre reinforcements and core materials are laid-up dry in a one-sided mould and vacuum bagged. Liquid resin is introduced through one or more ports by a series of designed-in channels that facilitate fibre wet-outVARTM cure requires neither high heat nor high pressure. Low cost tooling makes it possible to inexpensively produce large, complex parts in one shotRecent developments include VARTM with one stiff mould half and a semi rigid glass fibre/polyester second mould half in which 1 bar pressure is applied on inlet during injection for improved part qualityDevelopments in technology have shown it can be alternative to prepreg compression moulding and prepreg vacuum baggingProcess is suitable for intermediate number of parts (1000/year), small and large parts, low and high performance parts Commonly used in marine, transportation and infrastructure markets

Filament WindingFilament Winding is a process in which a filamentary yarn or tow is first wetted by a resin and then uniformly and regularly wound about a rotating mandrel. The finished pattern is cured and the mandrel removed. Filament winding can be classified as Helical or Polar winding. Helical winding: Fibre is fed from a horizontally moving delivery head to a rotating mandrel. The angle of the roving band with respect to the mandrel axis is called the wind angle. By adjusting the carriage speed and the mandrel rotating speed, any wind angle between near 0 deg and near 90 deg (Hoop winding) can be obtained. The properties of the part depend strongly on the wind angle. Typical fibre volume fraction is 55-60% in helical winding and 60-70% in hoop winding Helical winding is suitable for long slender parts like pressure pipe and launch tubes where wind angles of 20 to 90 deg are used. Most pipe is wound at 54.7 deg, which assumes a 2:1 hoop to longitudinal stress (cylindrical closed pressure vessel). Since the carriage moves backward and forward, fibre bands criss-cross at plus and minus the wind angle and create a weaving or interlocking effect

Filament Winding, cont:Polar winding: The carriage rotates about the longitudinal axis of a stationary mandrel. After each rotation of the carriage, the mandrel is indexed to advance by one fibre bandwidth. Thus the fibre bands lie adjacent to each other and there are no fibre crossoversThe fibre passes tangentially to the polar opening at one end of the chamber, reverses direction and passes tangentially to the opposite side of the polar opening at the other end. Since the fibre is wound in a plane intersecting the mandrel ends, the polar wind angle is small and typically 5-15 degPolar winding is a simple, rapid technique for short, stubby items with a length : diameter ratio of

Filament Winding, cont:Reinforcement Fibres used include glass, carbon and aramidResins used include epoxy, vinyl ester, polyester, phenolic, polyimide and bismaleimide. Thermoplastics are used in specific applicationsWet and Dry Filament Winding: In Wet Winding, the fibres are drawn through a bath containing epoxy resin, curing agent and additives. The fibres are passed through an orifice to remove excess resin and then wound onto a mandrel. Curing is done in an oven or autoclave, if required and the mandrel removed Dry, Prepreg or Tape Winding is a blend of filament winding and prepreg technology. The prepreg roving, with the resin in a tacky, semi-solid condition is obtained from a supplier and wound onto the mandrel and cured by the manufacturer. Dry winding allows better control of the fibre : resin ratio and also reduces exposure of workers to toxic curing agents or additivesMandrels: The mandrel is the geometric basis of the part, supports it during winding and cure and can be permanent, removable and reusable. Permanent Metal mandrels for pressure vessels can be of aluminium, stainless steel, Inconel, titanium. Net Metal mandrels of aluminium can be disassembled, removed and reused. Single use mandrels can be of plaster, salt paste or ultrafine sand with PVA as a binder.

PultrusionPultrusion is a continuous manufacturing process used to produce high-fibre-content composite shapes and is ideal for high throughput of constant cross section products. Primary reinforcement and strength are in the longitudinal direction. More than 90% of all pultruded products are of E-glass roving reinforced polyester. Pultruded products have higher mechanical properties than other forms of plastic processing, thus preferredPultrusion is similar to extrusion except that the raw materials are pulled instead of pushed through the die. Almost any length of solid, open-sided or hollow shapes can be producedAccurate resin content can be maintained because of the fixed cross section of the die, excess resin is squeezed out Pultrusion machine consists of 6 stages: Material Feed, Resin Impregnation, Preforming, Forming & Curing, Clamping & Pulling and Cut-offCuring: The critical step of curing is a continuous polymerisation process that takes place within the die and tunnel oven which can range in length from 30-155 cm. Dies are heated electrically or with hot oil. The reinforced resin gels in the die and is fully cured in the free state, as it moves through the oven.Applications include structural building shapes, automobile & aircraft parts, electrical industry tools and sporting goods

Machining of Composites

Quality Assurance

Nondestructive Inspection

Material Qualification

Types of Defects (Flaws)

Nondestructive Testing Techniques for Evaluation of Quality of Composite PartsA number of non-destructive inspection techniques are available for checking manufacturing and service related defects. The following inspection methods are available:

Visual inspection Tap test Ultrasonic testRadiographyThermographyAcoustic Emission

Visual Inspection

Tap TestQuality of a thin composite material laminate can be assessed by tapping a succession of locations with a coin or a tapping hammerThe sounds of good and bad areas are qualitatively different to the human ear, with a clear ring at good positions and a dull sound in poor areasThe test method is sensitive to laminar type flaws, such as delaminations or disbonds and depends on the different acoustic resonance of the loose upper layer compared to the surrounding material The method does not require sophisticated or expensive equipment but depends on subjective interpretation and is not suitable for thick laminates The technique has been automated with a mechanical tapper and instrumentation to interpret the signals, which gives the advantage of improved repeatability in terms of tap impact and location

Ultrasonic Inspection

Radiographic (X-ray) Inspection

Acoustic EmissionAcoustic emission testing involves the detection of elastic energy that is released by materials when they undergo deformation under loadThe detected signals are frequently in the ultrasonic rather than audible regionThe method is capable of detecting matrix cracking, delamination and fibre breakageThe technique has been used in the proof testing of fibreglass pressure vessels and beams and also to monitor and characterize damage growth mechanisms in composites under cyclic loading Dye PenetrantThe technique can be used to detect surface breaking openings such as cracks, delaminations, exposed porosity or bondline defects in compositesLiquid penetrants rely on capillary action to enter the defects and are detected by inspection of the component under ultraviolet light

ThermographyThermography involves the contour mapping of regions of equal temperature on the surface, by an infrared camera, to investigate the state of material of a component into which thermal energy is introduced, typically by a radiant heat sourceDefects such as voids, delaminations, fibre orientation, disbonds in adhesive joints cause a variation in local thermal pattern and can be detectedThe method is noncontacting and can be used during testing or at high temperaturesThermography is attractive as a rapid means of inspecting large areas of a structureThe technique has been used for aerospace laminates of carbon epoxy as well as for marine applications of fibreglass upto 8 cm thick

Summary of Nondestructive Inspection Capabilities

Effects of Defects in Fibre Reinforced Composite Materials and Potential Effects on Structural PerformanceDelamination: Catastrophic failure due to loss of interlaminar shear strength. Typical acceptance criteria require the detection of delaminations with a linear dimension larger than 6.4 mm (0.25 in.)Impact Damage: Loss of compressive strength under static loadPly gap: Strength degradation depends on stacking order and location. For (0, 45, 90, -45) laminate, strength is reduced 9% due to gaps in 0 deg ply and 17% due to gaps in 90 deg ply

Effects of Defects in Fibre Reinforced Composite Materials and Potential Effects on Structural Performance, cont:Ply Waviness: For 0 deg ply waviness in (0, 45, 90, -45) laminate, static strength reduction is 10% for slight waviness and 25% for extreme waviness. Fatigue life is reduced at least by a factor of 10 Porosity: Degrades matrix dominated properties. 1% porosity reduces strength by 5% and fatigue life by 50%Surface Notches: Static strength reduction of upto 50%. Strength reduction is small for notch sizes expected in serviceThermal Over-exposure: Embrittlement and reduction of toughness up to complete loss of structural integrity