composite materials unit ii ssn

8/8/2019 COMPOSITE MATERIALS UNIT II SSN

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UNIT II

POLYMER MATRIX COMPOSITES

- B. Anand Ronald


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MANUFACTURING PROCESSESOF COMPOSITES

Composite materials have succeeded remarkably in theirrelatively short history. But for continued growth,especially in structural uses, certain obstacles must beovercome. A major one is the tendency of designers torely on traditional materials such as steel and aluminumunless composites can be produced at lower cost .

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


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Fig. Classification of manufacturing processes for

fiber reinforced polymer (FRP) composites

Classification of FRP Processes


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Types of Fibers

Fiber Glass

Graphite Fiber

Kevlar Fiber

Kevlar/Carbon Hybrid


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MATERIAL FORMS

Textile formsBraiding or weavingTubular braided form

can be flattened and cut for non-tubularproducts


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Fabric StructuresWoven: Series of Interlaced yarns at 90 to each other

Knit: Series of Interlooped Yarns

Braided: Series of Intertwined, Spiral Yarns

Nonwoven: Oriented fibers either mechanically,chemically, or thermally bonded


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Woven Fabrics

Basic woven fabrics consists of two systems of yarns interlaced atright angles to create a single

layer with isotropic or biaxialproperties.


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Components of a Woven Fabric


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Basic Weave TypesPlain Weave


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Basic Weave TypesSatin 5HS


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Basic Weave Types2 x 2 Twill


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Basic Weave TypesNon-Crimp


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Braiding

A braid consists of two sets of yarns, which are helicallyintertwined.

The resulting structure is oriented to the longitudinal axisof the braid.

This structure is imparted with a high level of conformability, relative low cost and ease of manufacture.


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Braid Structure


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Types of Braids


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Triaxial Yarns A system of longitudinal yarns can be introducedwhich are held in place by the braiding yarns

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

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


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The processing of fiber reinforced laminates can be

divided into two main steps:

LayupCuring

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

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


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2000 The McGraw-Hill Companies , Inc., Irwin/McGraw-Hill


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Open Mold ProcessesOnly one mold (male or female) is needed and may be made of anymaterial such as wood, reinforced plastic or , for longer runs, sheet metalor electroformed nickel. The final part is usually very smooth.

Shaping. Steps that may be taken for high quality

1. Mold release agent (silicone, polyvinyl alcohol, fluorocarbon, orsometimes, plastic film) is first applied.

2. Unreinforced surface layer (gel coat) may be deposited for best surfacequality.


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Hand Lay-Up : The resin and fiber (or pieces cut fromprepreg) are placed manually, air is expelled withsqueegees and if necessary, multiple layers are

built up. Hardening is at room temperature but may be improved by heating. Void volume is typically 1%.

Foam cores may be incorporated (and left in the part) for greatershape complexity. Thus essentially all shapes can be produced.

Process is slow (deposition rate around 1 kg/h) and labor-intensive

Quality is highly dependent on operator skill. Extensively used for products such as airframe components, boats,

truck bodies, tanks, swimming pools, and ducts.


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SPRAY UP


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SPRAY-UP

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


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Filament Winding Characteristics Because of the tension, re-entrant shapes cannot be produced. CNC winding machines with several degrees of freedom (sometimes 7)

are frequently employed. The filament (or tape, tow, or band) is either precoated with the polymer

or is drawn through a polymer bath so that it picks up polymer onits way to the winder.

Void volume can be higher (3%) The cost is about half that of tape laying Productivity is high (50 kg/h).

Applications include: fabrication of composite pipes, tanks, and pressurevessels . Carbon fiber reinforced rocket motor cases used forSpace Shuttle and other rockets are made this way.

FILAMENT WINDING PROCESS


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A FILAMENT WINDING MACHINE


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PREPREG PRODUCTION PROCESSES

Prepreg is the composite industrys term for continuous fiber reinforcementpre-impregnated with a polymer resin that is only partially cured. Prepreg is delivered in tape form to the manufacturer who then molds and fully

cures the product without having to add any resin.

This is the composite form most widely used for structural applications

P P P


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Manufacturing begins by collimating a series of spool-woundcontinuous fiber tows.

Tows are then sandwiched and pressed between sheets ofrelease and carrier paper using heated rollers(calendering).

The release paper sheet has been coated with a thin film ofheated resin solution to provide for its thorough

impregnation of the fibers.

PrePreg Process

P P g P


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The final prepreg product is a thin tape consisting ofcontinuous and aligned fibers embedded in apartially cured resin

Prepared for packaging by winding onto a cardboardcore.

Typical tape thicknesses range between 0.08 and 0.25mm

Tape widths range between 25 and 1525 mm. Resin content lies between about 35 and 45 vol%

PrePreg Process

PrePreg Process


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The prepreg is stored at 0 C (32 F) or lower because thermoset matrix

undergoes curing reactions at room temperature . Also the time inuse at room temperature must be minimized. Life time is about 6months if properly handled.

Both thermoplastic and thermosetting resins are utilized: carbon, glass,

and aramid fibers are the common reinforcements. Actual fabrication begins with the lay-up. Normally a number of plies are

laid up to provide the desired thickness. The lay-up can be by hand or automated.

PrePreg Process


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Pultrusion Fibers are impregnate with a prepolymer, exactly positioned with guides,

preheated, and pulled through a heated, tapering die where curingtakes place.

Emerging product is cooled and pulled by oscillating clamps Small diameter products are wound up Two dimensional shapes including solid rods, profiles, or hollow tubes ,

similar to those produced by extrusion, are made, hence its name

pultrusion

PULTRUSION PROCESS


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Composite Production Methods


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PultrusionContinuous fibers pulled through resin tank, thenpreforming die & oven to cure

Adapted from Fig.16.13, Callister 7e .

Production rates around 1 m/min. Applications are to sporting goods (golf club shafts), vehicle drive shafts

(because of the high damping capacity), nonconductive ladder rails forelectrical service , and structural members for vehicle and aerospaceapplications .


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RESIN TRANSFER MOLDING (RTM) process


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Used to make large parts.Pre preg : short form of Preimpregnated fiber / thin

AUTOCLAVE BASED METHODS:


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sheet or lamina of unidirectional fiber / polymercomposite protected on both sides with easilyremovable separators.Prepregs can be made by:

1) Solution dip2) Solution spray3) Direct hot melt4) Film calendering

AUTOCLAVE :- is a closed vessel (round / cylindrical)- process ( physical / chemical ) occur under

simultaneous application of high Temp. and Pressure .- results in high quality product- The combined action of heat & pressure,

(i) consolidate s the laminae(ii) removes the entrapped air(iii) helps cure the polymeric matrix.


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Vacuum Bagging


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M P Groover, Fundamentals of Modern Manufacturing 3/e

Use atmospheric pressure to suck air from undervacuum bag, to compact composite layers down andmake a high quality laminate.Layers from bottom include: mold, mold release,composite, peel-ply, breather cloth, vacuum bag,also need vacuum valve, sealing tape.


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Vacuum Bagging Process


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PRESSURE-BAG MOLD NG


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Pressurebag process is virtually a mirror image of vacuumbag molding. Applications are sonar domes,

antenna housings, aircraft fairings, etc.

CLOSEDMOLD


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(1) Matcheddie molding : As the namesuggests, a matcheddie mold consistsof closely matched male and female

dies (shown in figure). Applications arespacecraft parts, toys, etc.(2) Injection molding : The injectionprocess begins with a thermosetting(or sometimes thermoplastic) material

outside the mold. The plastic maycontain reinforcements or not. It is firstsoftened by heating and/or mechanicalworking with an extrusiontype screw.It is then forced, under high pressure

from a ram or screw, into the coolmold. Applications are auto parts,vanes, engine cowling defrosters andaircraft radomes.

I j i M ldi PMC P


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Injection Molding PMC ProcessesInjection molding is noted for lowcost production of plastic parts inlarge quantitiesAlthough most closely associated

with thermoplastics, the processcan also be adapted to thermosetsProcesses of interest in the context

of PMCs:Conventional injection moldingReinforced reaction injection molding

Conventional Injection Molding


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Used for both Thermoplastic and Thermosettingtype FRPsVirtually all Thermoplastics can be reinforcedwith fibersChopped fibers must be used

Continuous fibers would be reduced by theaction of the rotating screw in the barrel

During injection into the mold cavity, fiberstend to become aligned as they pass the nozzle

Part designers can sometimes exploit thisfeature to optimize directional properties inthe part

Reinforced Reaction Injection Molding (RRIM)


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Reaction injection molding (RIM) - two reactiveingredients are mixed and injected into a

mold cavity where curing and solidificationoccur due to chemical reactionReinforced reaction injection molding (RRIM) -

similar to RIM but includes reinforcing fibers ,typically glass fibers, in the mixtureAdvantages: similar to RIM (e.g., no heatenergy required, lower cost mold), with the

added benefit of fiber-reinforcementProducts : auto body, truck cab applicationsfor bumpers, fenders, and other body parts


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INJECTION MOLDING:


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INJECTION MOLDING:

SHEET MOLDING COMPOUND (SMC)


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Chopped glass fiber added to polyester

resin mixture

DIAPHRAGM FORMING


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Tape-Laying Machines

(Automated Lay-Up)


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Cut and lay the ply or prepreg under computer control and without tension;

may allow reentrant shapes to be made. Cost is about half of hand lay-up Extensively used for products such as airframe components, boats, truck

bodies, tanks, swimming pools, and ducts.

(Automated Lay Up)

Automated Tape-Laying Machines


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Automated Tape-Laying MachinesAutomated tape-laying machines operate by dispensing

a prepreg tape onto an open mold following a

programmed pathTypical machine consists of overhead gantry towhich the dispensing head is attachedThe gantry permits x-y-z travel of the head, forpositioning and following a defined continuous path


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Fig. Automated tape-laying machine (photocourtesy of Cincinnati Milacron).

AUTOMATED TAPE LAYING PROCESS


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Compression Molding PMC Processes


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A charge is placed in lower mold section, andthe sections are brought together underpressure, causing charge to take the shapeof the cavityMold halves are heated to cure TS polymer

When molding is sufficiently cured, themold is opened and part is removed

Several shaping processes for PMCs based

on compression moldingThe differences are mostly in the form of the starting materials

COMPRESSION MOLDING:


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composite materials unit ii ssn

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