Composites Groover

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COMPOSTES

Groover

2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e

Composite Material Defined

A materials system composed of two or moredistinct phases whose combinationproduces aggregate properties that aredifferent from those of its constituents

Examples: Cemented carbides (WC with Co binder)

Plastic molding compounds with fillers

Rubber mixed with carbon black

Wood (a natural composite asdistinguished from a synthesizedcomposite)


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The Reinforcing Phase Function is to reinforce the primary phase

Imbedded phase is most commonly one of thefollowing shapes:

Fibers

Particles

Flakes

Also, secondary phase can take the form of an

infiltrated phase in a skeletal or porous matrix Example: a powder metallurgy part

infiltrated with polymer


Physical Shapes of Imbedded Phase

Figure 9.1 Possible physical shapes of imbedded phases incomposite materials: (a) fiber, (b) particle, and (c) flake.


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FibersFilaments of reinforcing material, usually circular

in cross-section Diameters from less than 0.0025 mm to about

0.13 mm, depending on material Filaments provide greatest opportunity for

strength enhancement of composites Filament form of most materials is

significantly stronger than the bulk form

As diameter is reduced, the materialbecomes oriented in the fiber axis directionand probability of defects in the structuredecreases significantly


Continuous vs. Discontinuous Fibers

Continuous fibers - very long; in theory, theyoffer a continuous path by which a load can becarried by the composite part

Discontinuous fibers (chopped sections of

continuous fibers) - short lengths (L/D =roughly 100)

Whiskers = discontinuous fibers of hair-likesingle crystals with diameters down to about0.001 mm (0.00004 in.) and very highstrength


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Fiber Orientation Three Cases One-dimensional reinforcement, in which

maximum strength and stiffness are obtained inthe direction of the fiber

Planar reinforcement, in some cases in theform of a two-dimensional woven fabric

Random or three-dimensional in which thecomposite material tends to possess isotropicproperties


Fiber Orientation

Figure 9.3 Fiber orientation in composite materials: (a)one-dimensional, continuous fibers; (b) planar,continuous fibers in the form of a woven fabric; and (c)random, discontinuous fibers.


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Materials for Fibers Fiber materials in fiber-reinforced composites

Glass most widely used filament

Carbon high elastic modulus

Boron very high elastic modulus

Polymers - Kevlar

Ceramics SiC and Al2O3 Metals - steel

Most important commercial use of fibers is inpolymer composites


Particles and Flakes

A second common shape of imbedded phaseisparticulate, ranging in size from microscopicto macroscopic

Flakes are basically two-dimensional

particles - small flat platelets Distribution of particles in the composite matrix

is random

Strength and other properties of thecomposite material are usually isotropic


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The Interface There is always an interface between

constituent phases in a composite material

For the composite to function, the phases mustbond where they join at the interface

Figure 9.4 Interfaces between phases in a composite material: (a)direct bonding between primary and secondary phases.


Other Composite Structures

Laminar composite structure conventional

Sandwich structure

Honeycomb sandwich structure


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Two or more layers bonded together in anintegral piece

Example: plywood, in which layers are thesame wood, but grains are orienteddifferently to increase overall strength

Figure 9.7 Laminar composite

structures: (a) conventionallaminar structure.

Laminar Composite Structure


Relatively thick core of low density foambonded on both faces to thin sheets of adifferent material

Figure 9.7 Laminarcomposite structures:(b) sandwich structureusing foam core.

Sandwich Structure: Foam Core


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Alternative to foam core

Foam or honeycomb achieve high ratios ofstrength-to-weight and stiffness-to-weight

Figure 9.7 Laminarcomposite structures:(c) sandwich structure

using honeycomb core.

Sandwich Structure: Honeycomb Core


Other Laminar Composite Structures

Automotive tires - multiple layers of rubberbonded together with reinforcing agent

FRPs - multi-layered, fiber-reinforced plasticpanels for aircraft, boat hulls, other products

Printed circuit boards - layers of reinforcedcopper and plastic for electrical conductivityand insulation, respectively

Snow skis - layers of metals, particle board,and phenolic plastic

Windshield glass - two layers of glass oneither side of a sheet of tough plastic


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Metal Matrix Composites (MMCs)Metal matrix reinforced by a second phase

Reinforcing phases:

1. Particles of ceramic

These MMCs are commonly calledcermets

2. Fibers of various materials

Other metals, ceramics, carbon, and

boron


Cermets

MMC with ceramic contained in a metallic matrix

The ceramic often dominates the mixture,sometimes up to 96% by volume

Bonding can be enhanced by slight solubility

between phases at elevated temperaturesused in processing

Cermets can be subdivided into

1. Cemented carbides most common

2. Oxide-based cermets less common


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Figure 9.8 Photomicrograph (about 1500X) of cementedcarbide with 85% WC and 15% Co (photo courtesty ofKennametal Inc.).


Ceramic Matrix Composites (CMCs)

Ceramic primary phase imbedded with asecondary phase, usually consisting of fibers

Attractive properties of ceramics: high stiffness,hardness, hot hardness, and compressive

strength; and relatively low density Weaknesses of ceramics: low toughness and

bulk tensile strength, susceptibility to thermalcracking

CMCs represent an attempt to retain thedesirable properties of ceramics whilecompensating for their weaknesses


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Polymer Matrix Composites (PMCs)Polymer primary phase in which a secondary

phase is imbedded as fibers, particles, orflakes

Commercially, PMCs are more important thanMMCs or CMCs

Examples: most plastic moldingcompounds, rubber reinforced with carbonblack, and fiber-reinforced polymers (FRPs)

FRPs are most closely identified with the termcomposite


Fiber-Reinforced Polymers (FRPs)

PMC consisting of a polymer matrix imbeddedwith high-strength fibers

Polymer matrix materials:

Usually a thermosetting plastic such as

unsaturated polyester or epoxy Can also be thermoplastic, such as nylons

(polyamides), polycarbonate, polystyrene,and polyvinylchloride

Fiber reinforcement is widely used in rubberproducts such as tires and conveyor belts


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Fibers in PMCs Various forms: discontinuous (chopped),

continuous, or woven as a fabric

Principal fiber materials in FRPs are glass,carbon, and Kevlar 49

Less common fibers include boron, SiC, andAl2O3, and steel

Glass (in particular E-glass) is the mostcommon fiber material in today's FRPs

Its use to reinforce plastics dates fromaround 1920


Common FRP Structure

Most widely used form of FRP is a laminarstructure

Made by stacking and bonding thin layers offiber and polymer until desired thickness is

obtained By varying fiber orientation among layers, a

specified level of anisotropy in propertiescan be achieved in the laminate

Applications: boat hulls, aircraft wing andfuselage sections, automobile and truck bodypanels


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FRP Applications Aerospace much of the structural weight of

todays airplanes and helicopters consist ofadvanced FRPs Example: Boeing 787

Automotive some body panels for cars andtruck cabs Low-carbon sheet steel still widely used due

to its low cost and ease of processing Sports and recreation

FRPs used for boat hulls since 1940s Fishing rods, tennis rackets, golf club shafts,

helmets, skis, bows and arrows


Figure 9.11 Composite materials in the Boeing 757 (photo courtesyof Boeing Commercial Airplane Co.).


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Guide to Processing Composite Materials The two phases are typically produced

separately before being combined into thecomposite part

Processing techniques to fabricate MMCand CMC components are similar to thoseused for powdered metals and ceramics

Molding processes are commonly used forPMCs with particles and chopped fibers

Specialized processes have beendeveloped for FRPs


SHAPING PROCESSES FORPOLYMER MATRIX COMPOSITES

Apolymer matrix composite (PMC) is a compositematerial consisting of a polymer imbedded with areinforcing phase such as fibers or powders

1. Starting Materials for PMCs2. Open Mold Processes

3. Closed Mold Processes

4. Filament Winding

5. Pultrusion Processes

6. Other PMC Shaping Processes


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PMC Shape Processing Many PMC shaping processes are slow and

labor intensive In general, techniques for shaping composites

are less efficient than for other materials -Why? Composites are more complex than other

materials, consisting of two or more phases For FRPs, there is the need to orient the

reinforcing phase Composite processing technologies have

not been the object of refinement over asmany years as processes for other materials


Categories of FRP Shape Processes

Open mold processes - some of the originalFRP manual procedures for laying resins andfibers onto forms

Closed mold processes - much the same asthose used in plastic molding

Filament winding - continuous filaments aredipped in liquid resin and wrapped around arotating mandrel, producing a rigid, hollow,cylindrical shape

Pultrusion - similar to extrusion only adapted toinclude continuous fiber reinforcement

Other - operations not in previous categories


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Starting Materials for PMCs In a PMC, the starting materials are:

A polymer

A reinforcing phase

They are processed separately beforebecoming phases in the composite


Polymer Matrix

Thermosetting (TS) polymers are the mostcommon matrix materials

Principal TS polymers are:

Phenolics used with particulate

reinforcing phases Polyesters and epoxies - most closely

associated with FRPs

Thermoplastic molding compounds includefillers or reinforcing agents

Nearly all rubbers are reinforced with carbonblack


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Reinforcing Agent Possible geometries - fibers, particles, and

flakes

Possible materials - ceramics, metals, otherpolymers, or elements such as carbon or boron

Particles and flakes are used in many plasticmolding compounds

Of most engineering interest is the use of fibersas the reinforcing phase in FRPs


Combining Matrix and Reinforcement

1. The starting materials arrive at the fabricationoperation as separate entities and arecombined into the composite during shaping

Filament winding and pultrusion, in which

reinforcing phase = continuous fibers2. The two component materials are combinedinto some starting form that is convenient foruse in the shaping process

Molding compounds

Prepregs


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Molding CompoundsFRP composite molding compounds consist of

the resin matrix with short randomly dispersedfibers, similar to those used in plastic molding

Most molding compounds for compositeprocessing are thermosetting polymers

Since they are designed for molding, they mustbe capable of flowing

Accordingly, they have not been cured prior

to shape processing Curing is done during and/or after final

shaping


Prepregs

Fibers impregnated with partially cured TS resinsto facilitate shape processing

Available as tapes or cross-plied sheets orfabrics

Curing is completed during and/or aftershaping

Advantage: prepregs are fabricated withcontinuous filaments rather than choppedrandom fibers, thus increasing strength andmodulus


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Open Mold ProcessesFamily of FRP shaping processes that use a

single positive or negative mold surface toproduce laminated FRP structures

The starting materials (resins, fibers, mats, andwoven rovings) are applied to the mold inlayers, building up to the desired thickness

This is followed by curing and part removal

Common resins are unsaturated polyesters

and epoxies, using fiberglass as thereinforcement


Open Mold FRP Processes

1. Hand lay-up

2. Spray-up

3. Automated tape-laying machines

The differences are in the methods of applying

the laminations to the mold, alternative curingtechniques, and other differences


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Hand Lay-Up MethodOpen mold shaping method in which successive

layers of resin and reinforcement are manuallyapplied to an open mold to build the laminatedFRP composite structure

Labor-intensive

Finished molding must usually be trimmed witha power saw to size outside edges

Oldest open mold method for FRP laminates,

dating to the 1940s when it was first used forboat hulls


Figure 15.4 Hand lay-up : (1) mold is treated with mold release agent; (2) thin gelcoat (resin) is applied, to the outside surface of molding; (3) when gel coat haspartially set, layers of resin and fiber are applied, the fiber is in the form of mator cloth; each layer is rolled to impregnate the fiber with resin and remove air;(4) part is cured; (5) fully hardened part is removed from mold.

Hand Lay-Up Method


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Products Made by Hand Lay-Up Generally large in size but low in production

quantity - not economical for high production

Applications:

Boat hulls

Swimming pools

Large container tanks

Movie and stage props

Other formed sheets The largest molding ever made was ship hulls

for the British Royal Navy: 85 m (280 ft) long


Spray-Up Method

Liquid resin and chopped fibers are sprayed ontoan open mold to build successive FRPlaminations

Attempt to mechanize application of resin-fiber

layers and reduce lay-up time Alternative for step (3) in the hand lay-up

procedure


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Figure 15.5 Spray-up method

Spray-Up Method


Products Made by Spray-Up

Boat hulls, bathtubs, shower stalls, automobileand truck body parts, recreational vehiclecomponents, furniture, large structural panels,and containers

Movie and stage props are sometimes madeby this method

Since products made by spray-up haverandomly oriented short fibers, they are not asstrong as those made by lay-up, in which thefibers are continuous and directed


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Automated Tape-Laying MachinesAutomated tape-laying machines operate by

dispensing a prepreg tape onto an open moldfollowing a programmed path

Typical machine consists of overhead gantry towhich the dispensing head is attached

The gantry permits x-y-z travel of the head, forpositioning and following a defined continuouspath


Figure 15.6 Automated tape-laying machine (photocourtesy of Cincinnati Milacron).


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Curing in Open Mold Processes Curing is required of all thermosetting resins

used in FRP laminated composites

Curing cross-links the polymer, transforming itfrom its liquid or highly plastic condition into ahardened product

Three principal process parameters in curing:

1. Time

2. Temperature

3. Pressure


Closed Mold Processes

Performed in molds consisting of two sectionsthat open and close each molding cycle

Tooling cost is more than twice the cost of acomparable open mold due to the more

complex equipment required in theseprocesses

Advantages of a closed mold are: (1) goodfinish on all part surfaces, (2) higher productionrates, (3) closer control over tolerances, and(4) more complex three-dimensional shapesare possible


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Classification of Closed Mold Processes Three classes based on their counterparts in

conventional plastic molding:

1. Compression molding

2. Transfer molding

3. Injection molding

The terminology is often different whenpolymer matrix composites are molded


Compression Molding PMC Processes

A charge is placed in lower mold section, and thesections are brought together under pressure,causing charge to take the shape of the cavity

Mold halves are heated to cure TS polymer

When molding is sufficiently cured, the moldis opened and part is removed

Several shaping processes for PMCs based oncompression molding

The differences are mostly in the form of thestarting materials


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Transfer Molding PMC ProcessesA charge of thermosetting resin with short fibers is

placed in a pot or chamber, heated, andsqueezed by ram action into one or more moldcavities

The mold is heated to cure the resin

Name of the process derives from the fact thatthe fluid polymer is transferred from a pot into amold


Injection Molding PMC Processes

Injection molding is noted for low costproduction of plastic parts in large quantities

Although most closely associated withthermoplastics, the process can also be

adapted to thermosets Processes of interest in the context of PMCs:

Conventional injection molding

Reinforced reaction injection molding


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Filament WindingResin-impregnated continuous fibers are wrapped

around a rotating mandrel that has the internalshape of the desired FRP product; the resin isthen cured and the mandrel removed

The fiber rovings are pulled through a resin bathimmediately before being wound in a helicalpattern onto the mandrel

The operation is repeated to form additional

layers, each having a criss-cross pattern with theprevious, until the desired part thickness hasbeen obtained


Figure 15.8 Filament winding.

Filament Winding


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Figure 15.10 Filament winding machine (photo courtesyof Cincinnati Milacron).

Filament Winding Machine


Pultrusion Processes

Similar to extrusion (hence the name similarity)but workpiece is pulled through die (so prefix"pul-" in place of "ex-")

Like extrusion, pultrusion produces continuous

straight sections of constant cross section Developed around 1950 for making fishing

rods of glass fiber reinforced polymer (GFRP)

A related process, calledpulforming, is used tomake parts that are curved and which mayhave variations in cross section throughouttheir lengths


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PultrusionContinuous fiber rovings are dipped into a resin

bath and pulled through a shaping die wherethe impregnated resin cures

The sections produced are reinforcedthroughout their length by continuous fibers

Like extrusion, the pieces have a constantcross section, whose profile is determined bythe shape of the die opening

The cured product is cut into long straightsections


Figure 15.11 Pultrusion process

Pultrusion Process


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PulformingPultrusion with additional steps to form the length

into a semicircular contour and alter the crosssection at one or more locations along thelength

Pultrusion is limited to straight sections ofconstant cross section

There is also a need for long parts withcontinuous fiber reinforcement that are curvedrather than straight and whose cross sections

may vary throughout length Pulforming is suited to these less regular

shapes


Figure 15.12 Pulforming process (not shown in thesketch is the cut-off of the pulformed part).

Pulforming Process


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Other PMC Shaping Processes Centrifugal casting

Tube rolling

Continuous laminating

Cutting of FRPs

In addition, many traditional thermoplasticshaping processes are applicable to FRPs withshort fibers based on TP polymers

Blow molding Thermoforming

Extrusion

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Composites Groover