composite text book

7/30/2019 composite text book

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COMPOSITE

MATERIALS

By:Dr. Mark V. Bower, P . E.

Copyright 1992-2000

The Un ivers ity of Alabama in Hunt sville

Hun tsville, Alabama


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ii

Composite Mat eria ls

INTRODUCTION...........................................................................................................1

In t roduct ion ................................................................................................................1

Definition of Terms.....................................................................................................1

CONSTITUE NTS AND F ABRICATION TE CHN IQUE S ............................................8

Constituents................................................................................................................8

Reinforcemen t For ms ...............................................................................................17

Fabrication Techniques............................................................................................21

Ha zards .....................................................................................................................42

LAMINA MECHANICS ...............................................................................................48

Preliminar ies ............................................................................................................48

Constitutive Relations..............................................................................................53

Engineering Properties for Orthotropic Materials..................................................57

Pla ne Str ess Orthot ropic Const itu tive Relat ion .....................................................58

Off-Axis properties of orthotropic lamina................................................................60

STRE NGTH OF LAMINA ...........................................................................................65

Tens or Polynomia l Fa ilur e Cr iter ion.......................................................................65

Qua dr at ic Fa ilur e Crit er ion .....................................................................................66

R-Factor analysis......................................................................................................73

CLASSICAL LAMINATION THEORY.......................................................................75

His tory ......................................................................................................................75

Preliminar ies ............................................................................................................76

Force -- Moment Resultants.....................................................................................77

Equ ilibrium of a Pla te Elem ent ...............................................................................79

Displacement Field Model........................................................................................83


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iii

Ort hotr opic Const itu tive Relat ion ...........................................................................86

The Lam ina ted Pla te E qua t ions ..............................................................................91

LAMINATES ................................................................................................................97

In t roduct ion ..............................................................................................................97

Alternate Expressions for Laminate Stiffnesses.....................................................97

Simp lifying Assu mpt ions on Lam ina te Str uctur e ..................................................99

St res s Dist r ibut ion in a Lam ina te .........................................................................112

Lam ina te Fa ilur e Theories ....................................................................................113


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iv

Preface

This docum ent is a work in progress, as are most books. Every effort h as

been made t o ensur e the a ccur acy of th e inform at ion cont ained in it. That does not,

un fort un at ely, guar an tee that every equat ion is without error. Ha ving said th is,

th e reader is encour aged to consu lt oth er text s on composite mat erials. A few ar elisted on th is page. Fu rt her, neither t he aut hor nor The University of Alabama in

Huntsville is responsible for the application of the information contained in this

docum ent . Good engineer ing pr actice requ ires th e ap plicat ion of sound engineer ing

ju dgmen t .

The author acknowledges the support of The University of Alabama in

Huntsville, the Microsoft Academic Support Program, and the Dell Corporation

Academ ic Sup port Pr ogra m. I acknowledge th e support an d assist an ce of my wife,

Peggy, an d children, Renae, Amber, Elizabeth , an d Matt hew. Fu rt her, I

acknowledge the support, inspirat ion, and a noint ing of J esus Christ . With out H is

help, I could n ot h ave come t his far .

References:

1. Me c h a n i c s o f C o m p o s i t e Ma t e r i a l s , R. M. Jones, McGraw-Hill Book

Compa ny, Wash ington, D. C., 1975.

2. P r i m e r o n C o m p o s i t e Ma t e r i a l : An a l ys i s, J. E. Ashton, J. C. Halpin, and

P. H . Pet it, Techn omic Pu blishing Co., Inc., Westport , CT, 1969.

3. I n t r o d u c t i o n t o C o m p o s i t e M a t e r i a l s , S. W. Tsai and H. T. Hahn,

Techn omic Pu blishing Co., Inc., Westport , CT, 1980.

4. F u n d a m e n t a l s o f C o m p o s i t e s Ma n u f a c t u r i n g : Ma t e r i a l s , Me t h o d s a n d

Appl ica t ions , A. B. Strong, Society of Manufacturing Engineers Dearborn,

MI, 1989.


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Revised: 10 Febr ua ry, 2000 Pa ge 1

INTRODUCTION

INTRODUCTION

A composite material is defined as a material composed of two or more

constituents combined on a macroscopic scale by mechanical and chemical bonds.

Typical composite ma ter ials ar e composed of inclusions sus pended in a m at rix. The

const ituent s reta in th eir ident ities in t he composite. Norm ally th e components can

be physically identified an d ther e is an int erface between them. Composite

ma ter ials ar e classified based on th e sha pe an d r elative dimensions of the inclusion

an d the stru ctu res. Composite mat erials are classified as:

P a r t i c u l a t e F i l a m e n t a r y

L a m i n a t e d

In a particulate composite, the major dimension of the inclusion is small compared

to th e str uctur al dimensions. Pa rt iculat e composites may be ma de with small

par ticles, such as glass beads, or with chopped fibers. In filam ent ar y composites,

one dimension of the inclusion is of the sa me order of ma gnitude a s t he st ru ctu ra l

dimensions. Filament ar y composite ma terials ma y be made from uni-directiona l

ta pe or cloth. In lam ina ted composite mat erials, two of th e major dimen sions of th einclusions are of the same order of magnitude as the structural directions.

San dwich sections ar e examples of a lamina ted composite ma terial.

Two additional distinctions are made in the classification of composite

materials: advancedcomposite materials are those composites which are made with

inclusions that have a modulus greater than that of steel (30 Mpsi, 207 GPa) and

volume fraction of inclusions greater than fifty percent, and hybrid composite

materials are those composites which are made with two or more different inclusion

materials.

DEFINI TION OF TERMS

ANGLEPLY LAMINATE

Containing plies alternately oriented at plus and minus a fixed angle other

th an 90 degrees t o the r eference direction.


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COMPOSITE MATERIALS 2

INTRODUCTION

Revised: 10 Febr ua ry, 2000 M. V. Bower

ANISOTROPIC

Not isotropic; exhibiting different properties when tested along axes in

differen t d irections.

AUTOCLAVE

A pressur ized heat ed cha mber used to cur e composite mat erials. An

autoclave is pressurized with gas, typically air or nitrogen.

BALANCED LAMINATE

A composite laminate whose lay-up is symmetrical with relation to the mid-

plane of th e lam inat e.

BLEEDER CLOTH

A nonstructural layer of material used in manufacture of composite parts to

allow th e escape of excess gas a nd r esin du rin g cur e.

B-STAGE

An intermediate stage in the polymerization reaction of certain

thermosetting resins; the state in which most prepregs are stored and

shipped.

CAUL PLATE

A smooth meta l plate used in cont act with t he lay-up du ring cur e to tran smit

norma l pressur e an d to provide a smooth sur face to th e finished lamina te.

COLLIMATED

Rendered pa ra llel, applies t o filam ents.

COUPLING AGENT

That part of a sizing or finish, which is designed to provide a bonding link

between the reinforcement an d the lamina ting resin.

CRAZING

Fine r esin cra cks a t or un der t he sur face of a plast ic.


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INTRODUCTION


CROSSPLY LAMINATE

Cont aining lamina alter na tely oriented a t 0 degrees and 90 degrees.

CURE

To irreversibly change the properties of a thermosetting resin by chemical

rea ction, i.e., conden sat ion, rin g, closur e, or addit ion. Cur e ma y be

accomplished by addition of curing (cross-linking) agents, with or without

heat.

DELAMINATION

The separa tion of th e layers of ma terial in a lam inat e.

DRAPE

The a bility of broadgoods t o conform t o an irr egular sh ape.

ELONGATION

The amount of deformation of the fiber caused by the breaking tensile force,

expressed as t he per centa ge of the origina l length.

FIBER PLACEMENT

An au tomat ed fabrication process in wh ich th e ma chine places fiber bundles

along predetermin ed paths t o build up th e stru ctu re.

FILAMENT

A long, cont inu ous length of fiber, mea sur ed in yar ds.

FILAMENT WINDING

An automated fabrication process typically used to produce cylindrical or

spher ical shape. The machine winds fiber bundles ont o a ma ndr el th at is

removed after the cure process.

FILL

Yar n r un ning from selvage to selvage at right a ngles to the war p in a woven

fabric.


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INTRODUCTION


FILLER

A relatively inert material added to a plastic to modify its strength,

perm an ence, working pr operties, or oth er qu alities, or to lower costs .

FINISH

A mixtu re of ma ter ials for tr eat ing glass fibers. It cont ain s a coupling agent

to improve the bond of resin to glass; and usually includes a lubricant to

prevent abra sion an d a binder to promote str an d int egrity. With gra phite or

other filamen ts , it m ay perform eith er or all of th e above fun ctions.

FLASH

Excess plastic material which forms at the parting line of a mold or which is

extr uded from a closed m old.

GEL COAT

A quick-setting resin used in molding processes to provide an improved

surface for composites; it is the first resin applied to the mold after the mold-

release agent.

HAND LAY-UP

The process of placing and working successive plies of the reinforcing

ma teria l or r esin impregnat ed reinforcement in position on a mold by ha nd.

HYBRID COMPOSITE

A composite structure composed of more than two different materials, for

example, a lamina te with out er lam inae of glass/epoxy an d inner lamina e of

graphite/epoxy.

HYDROCLAVE

Similar to an autoclave except that the chamber is pressurized using heated

water or other liquid.

INTERLAMINAR SHEAR

The shear strength at rupture in which the plane of fracture is located

between the layers of reinforcement of laminate.


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INTRODUCTION


ISOTROPIC

Ha ving un iform properties in all directions. The measu red properties of an

isotropic material are independent of the axis of testing.

LAMINA

A single layer or ply of ma ter ial. The fun dam ent al building block of a

laminate.

LAY-UP

A laminate that has been assembled, but not cured; or a description of the

component m at erials an d geometr y of a lamina te.

NON-WOVEN FABRIC

A fabric, usually resin-impregnated, in which the reinforcements are

continuous and unidirectional; layers may be crossplied.

ORTHOTROPIC

Ha ving th ree mu tu ally perpendicular planes of elastic symmetr y.

PARALLEL LAMINATE

A laminate of woven fabric in which the plies are aligned in the same

position a s origina lly aligned in t he fabr ic roll.

PLASTICIZER

For epoxy, a lower molecular weight material added to reduce stiffness and

brittleness; it results in a lower glass- transition temperature for the

polymer.

PULTRUSION

A fabrication process used to produce a highly collimated composite shape

(rod, bar, etc.).

POSTCURE

Additional elevated temperature cure, usually without pressure, to improve

fina l propert ies an d/or complet e th e cure. In cert ain resin s, complet e cur e

and ultimate mechanical properties are attained only by exposure of the

cur ed resin to higher temper at ur es tha n t hose of cur ing.


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INTRODUCTION


POT LIFE

The length of time th at a resin system reta ins viscosity low enough to be used

in pr ocessing.

PREPREG; PREIMPREGNATED

A combination of mat, fabric, non-woven material, or roving with resin,

usually in the B-stage, ready for molding.

QUASI-ISOTROPIC LAMINATE

A laminate approximating isotropy by orienting plies in several directions.

RESIN TRANSFER MOLDING (RTM)

A ma nu factur ing process used to produce large composite st ru ctu res. In t hispr ocess, a dry lay-up is infused with r esin in a molding process. May be

found in various forms such as vacuum assisted resin transfer molding

(VARTM) or Seeman Composite Resin Infusion Molding Process (SCRIMP).

Not in wide spr ead u se for adva nced composites.

ROVING

A multiplicity of single ends of continuous filament with no applied twist

drawn t ogether as par allel stra nds.

STACKING SEQUENCE

The sequence of angles a nd possibly ma terials t ha t describes th e orient at ion

of the individual lamina in a laminate from top to bottom, e. g.,

+45/-45/+45/-45, or 0/90/90/0, or 0/+60/-60/0/+60/-60/-60/+60/0/-60/+60/0.

Most laminates are composed of a large number of laminae, frequently in

repeat ed pat tern s, which leads to th e use of shorth an d notat ion. Using

shorth an d notat ion t he first example is written : 2[45]. The second an d thirdsequences are symmetric about the mid-plane, and thus can be written:

[0/90]S an d 2[0/60]S, where th e subscript S indicates symmetr y.

SIZING

On glass fibers, the compounds which, when applied to filaments at forming,

provide a loose bond between the filaments, and provide various desired

ha ndling an d processing properties.


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INTRODUCTION


SYMMETRIC LAMINATE

A lamina te th at is symmet ric in both geometr y and ma terial properties about

th e mid-plane.

TACK

With prepreg ma terials, th e degree of stickiness of th e resin.

TAPE LAYING

An au tomat ed process in which th e ma chine lays a composite t ape, eith er wet

or prepr eg, on predeter mined pat hs.

TOW

A loose, un twist ed bun dle of filamen ts .

TRANSVERSELY ISOTROPIC

Ha ving uniform pr operties in one plane. The measur ed properties of a

transversely isotropic material are independent of the axis of testing within

the plane.

UNI-DIRECTIONAL LAMINATE

A laminate with non-woven reinforcements that are all laid up in the same

direction.

WARP

The yarn ru nn ing length wise in a woven fabric.

WET LAY-UP

A reinforced plastic which has liquid resin applied as the reinforcement is

being laid u p.


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CONSTITUENTS AND

F ABRICATION TECH NIQUES

For the purpose of this discussion, composite materials are defined as a

ma rr iage of two or more const itu ent ma ter ials on a macroscopic scale. To

understand the fabrication techniques associated with composite materials it is

important to discuss the types of constituent materials and the fabrication

techniques used to produce composite structures.

CONSTITUENTS

MATRIX MATERIALS

Polyester Resins

Polyester is a thermoset polymer that is formed from a condensation

polymerizat ion. Polyester h as been widely us ed in comm ercial app licat ions with

fiberglass . Applicat ions include:

Boat hu lls, Shower st alls, Bath tu bs, Car bodies,

Building and roof panels, Molded fur nitu re, an d Pipes.

Advan ta ges for t he u se of polyester r esin include:

Low cost (genera lly lowest foun d in composite m at eria ls) and A wide assort men t of diacids an d diols can be used t o give physical a nd

chem ical pr opert ies.

Disadvantages for the use of polyester resin include:

Poor tempera tu re capabilities,

Poor weat her resistan ce, Shelf life ma y be limit ed, an d Poor mechanical pr operties (stiffness a nd str ength) as compa red t o

adva nced composites.


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CONSTITUENTS AND FABRICATION TECHNIQUES


Epoxy Resins

Epoxy is a thermoset polymer that forms a strong rigidly crosslinked network

of polymer chain s. Epoxy ha s been widely used in comm ercial applicat ions with

fiberglass, graph ite, and ar oma tic fibers. Applicat ions include:

Aircraft componen ts, Pr essure vessels, Rocket motor cases, an d Car bodies.

Advan ta ges for th e use of epoxy resin include:

Excellent adhesion Excellent mechanical properties (strength and stiffness), Excellent chemical resist an ce, Excellent weath er r esistance, Low shr inkage, Good fatigue st rength , Good corrosion protection, and Versa tility in pr ocessing.

Disadva nt ages for t he u se of epoxy resin include:

Poor h igh t empera tu re capabilities, Un cur ed resin is t oxic, Poor ha ndling properties (uncured), an d Relatively expensive.

Epoxies are available in multi-component and single component systems.

The cure of epoxy may be through the application of hardeners, a catalytic agent

that activates or facilitates crosslinking between the polymer chains, (a two-part

system), or through the application of heat or ultra-violet light (a one-part system).

Epoxies may be stored at freezer temperatures, which prompts long storage/shelf

life. Wide ran ges of cure cycles ar e available.

Polyimide and Polybenzimidaole Resins

Polyimide and polybenzimidaole (PBI) are thermoplastic polymers with

excellent high tem pera tu re (600 to 700) propert ies. Polyimide and PBI ha s been

used in comm ercial app licat ions with graph ite, an d ar oma tic fibers. Applicat ionsinclude:

Aircraft componen ts.

Advan ta ges for t he u se of polyimide and P BI resin in clude:

Excellent mechanical properties (strength and stiffness), Excellent th erma l propert ies, an d Good pr ocessa bility on convent iona l molding equipmen t.


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Disadvantages for the use of polyimide and PBI resin include:

Var iable res ista nce to solvents depend ing on specific compoun d, Difficult synt hesis process, Difficult fabr icat ion met hods, Resin is t oxic and sh ould be ha ndled with grea t car e,

Expensive raw mat erials, an d Very expens ive (more t ha n epoxies).

Use of Polyimide and PBI compounds is growing as the knowledge base

increases.

Phenolic Resin

Phenolic is a thermoset polymer with good high temperature properties.

Phenolic has had a long history of commercial applications as a general

un reinforced plast ic an d is now being used a s a composite r esin with gra phite, an d

ar oma tic fibers. Applicat ions include: Aircraft componen ts, Rocket nose cones an d n ozzles, an d Aut omotive a pplicat ions .

Advan ta ges for th e use of phen olic resin include:

Good mechanical properties (strength and stiffness), Good ther ma l properties with a n ablat ive nat ur e, an d Good pr ocessa bility.

Disadvan ta ges for t he u se of phen olic resin include:

Absorbs m oistu re ea sily, Brittle behavior, an d Relatively expensive (more t ha n epoxies).

Carbon Matrices

Carbon matrices are produced from polymeric resins that are carefully

cha rr ed in a processed called pyrolysis. Car bon ma tr ices may also be produced by

vapor deposition, but the process is limited to structures less that 3/16 thick.

Applicat ions include:

Aircraft componen ts, Rocket nose cones an d n ozzles, an d Aut omotive ap plicat ions, esp ecially bra ke componen ts .

Advan ta ges for th e use of car bon m at rices include:

Very h igh specific hea t capa city (highest kn own), Good mechanical properties (stiffness and strength), Good t ough ness , Good r esista nce to sh ock,


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Excellent t herm al properties, and Excellent th erma l sta bility.

Disadvanta ges for th e use of car bon ma tr ices resin include:

Absorbs m oistu re ea sily,

Poor wear resistan ce, and Very expensive (more t ha n five times t he cost of a phen olic abla tivecomposite).

Thermoplastics Matrices

Thermoplastic polymers have long chain molecules that are loosely

int erconn ected by weak chem ical bond s and mechan ical tan gling. Becau se of th e

structure of thermoplastic polymers they do not require reactive cure cycles or have

a distinct melting temperature, displaying fluid like (viscoelastic) behavior at even

room tempera tu re. Consequent ly, th ese ma terials lend themselves to molding

processes. Ther moplast ics include: Polyethylene, Nylon, Polystyrene, Polyester, Polycarbonate, Polyvinylchloride (PVC), Acrylonit rile but adien e st yren e (ABS), Acrylic, Polyethylene t erepht ha late (PET),

Polyethereth erketone (PE EK), Polyphen ylene oxide, et ceter a.

Advantages for the use of thermoplastics resin include:

Large number of processing methods, Lower fabrication tim es, (compa red t o th erm osett ing polymers), Good compression strength after impact, Good hot/wet compr ession st ren gth , Resista nt to moistur e absorpt ion, an d Ea sy dyed or given special propert ies (e.g. flam e ret ar da nt ).

Disadvanta ges for th e use of therm oplastic resins include: High viscosity impa irs wet -out of reinforcement , High consolidation pr essures a re r equired, and Mechanical, chemical, thermal, and electrical properties depend on

specific selected .


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Ceramic Matrices

Organic resins are characterized by the prevasive presence of covalent bonds.

Ceramic matrices, by contrast, are characterized by the predominance of ionic

bonds , however silicon-car bide (SiC) ha s covalent bondin g. Cera mic solids ma y be

cryst alline, vitr eous (glass-like), or mixed.

Advan ta ges for th e use of cera mic mat rix includ e:

Dimensiona lly stable at h igh temper at ur es, High chemical stability, High thermal stability, Excellent mechanical properties (strength and stiffness), Resista nt to moistur e absorpt ion, an d Applicable t o extr eme t emper at ur es (2000 to 4000).

Disadvantages for the use of ceramic matrix include:

Very britt le, Very high consolidation pressu res a re r equired, and Very expensive to produce an d ma inta in.

Phenolic and carbon matrice are sometimes classified as ceramic matrices.

Metal Matrices

In comparison to organic resins and ceramic matrices, metal matrix

composites (MMCs) ar e cha ra cter ized by th e predomina nce of met allic bonds . In

MMCs discontinous or continuous metal fibers are suspended in a matrix of a

differing m etal (e.g. alum inum , tita nium , ma gnesium, copper, et cetera ).

Advant ages for th e use of metal m at rix include:

Outst an ding mecha nical pr operties (stiffness a nd str ength) forcont inu ous fiber MMCs,

Good wear resistance, Ther ma lly conductive, Good fractu re t ough ness for cont inu ous fiber MMCs, Good fatigue st ren gth for cont inu ous fiber MMCs, Resista nt to moistur e absorpt ion, an d

Applicable t o extr eme t emper at ur es (2000 to 4000).Disadvanta ges for t he u se of metal ma tr ix include:

Significant difficulties associated with the inherent non-wetability offibers,

Very high consolidation pressu res a re r equired, and Very expensive to produce.


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REINFORCING MATERIALS

While there is no restriction as to the material used as the reinforcing

element for modern composites there are generally three materials that are

commonly used: glass, graph ite, and organic. These mat erials ar e discussed in t his

section.

Glass Fibers

Glass fibers ha ve long been used as reinforcing element s. Owens-Illinois an d

Corn ing Glass developed a fiberglass ma nu factu rin g facility in 1937. Glass is

produced from silica sa nd, limest one, boric acid, and other element s. Types of glass

include:

E-glass, S-glass (and the variation S2-glass), C-glass, and Quartz.

These are t he four prima ry types of glass u sed in composite ma terials. The

type of glass is defined by the chemical composition

Advan ta ges for t he u se of glass fibers include:

Applicable t o wide r an ge of geomet ries a nd sizes, Seamless construction, Good str ength a nd du ra bility, Lower tooling costs,

Increa sed design flexibility,

Minimal m aintenan ce, and Corr osion resistan t.

Disadva nt ages for t he u se of glass fibers include:

Mechanical properties are not as good as metals or other reinforcingfibers.

The fiber glass pr oduction processes ar e shown in th e following figure. Note

th at th e process ma y begin from stock (ma rbles) or directly from melt. The use of

stock h as ha d bett er cont rol over th e properties.


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F i b e r g la s s p r o d u c t i on p r o ce s se s ,

(a ) m a r b l e p r o c e s s a n d ( b ) d i r e c t -m e l t p r o c e s s .

Carbon/Graphite Fibers

Carbon or graphite fibers for structural applications began production insignifican t qua nt ities in t he 1950s. Graph ite fibers a re am ong th e highest st iffness

an d highest str ength m at erial known today. Types of graph ite fibers include:

Polyacrylonit rile (PAN)-Based Fibers Pitch-Based Fibers Rayon-Based Fibers

Advan ta ges for t he u se of gra phit e fibers include:


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Excellent st rength , Excellent st iffness, Excellent specific strength and stiffness, and Corr osion resistan t.

Disadvantages for the use of graphite fibers include: Significantly more expensive than glass fibers, and Brittle behavior.

Two graphite fiber production processes (PAN-based and pitch-based) are

sh own in th e following figur e. Note tha t both pr ocesses use a two step

carbonization/graphitization process to convert the raw fiber into graphite.

Char acteristic properties for graphit e fibers from th e th ree processes is listed in th e

following t able.

G r a p h i t e fi b e r p r o d u c t i on p r o c es s es ,

(a ) P AN -b a s e d p r o c e ss a n d (b ) P i t c h - b a s e d p r o c e s s .


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C a r b o n F i b e r M e c h a n i c a l P r o p e r t i e s4

P AN-ba sed f ibe rs

Low Modulu s High Modu lus

Tensile Modulu s (Mpsi) 33 56

Tensile Str engt h (kpsi) 480 350Elonga t ion (%) 1.4 0.6

Den sit y (g/cc) 1.8 1.9

Carbon Assay (%) 92-97 100

P i t c h -b a s e d fi b e r s

Low Modulu s High Modu lus

Tensile Modulu s (Mpsi) 23 55

Tensile Str engt h (kpsi) 200 350

Elonga t ion (%) 0.9 0.4

Den sit y (g/cc) 1.9 2.0

Carbon Assay (%) 97 99R a y o n -b a s e d fi b e r s

Ten si le Modu lus (Mps i) 5 .9

Tens i le S t r en g th (kp s i ) 150

Elon gat ion (%) 2 .5

Den sit y (g/cc) 1.6

Car bon Assa y (%) 99

Organic Fibers

Organic fibers for structural applications were introduced for commercial

applicat ions in 1971. Graph ite fibers a re am ong th e highest st iffness an d highest

st ren gth ma ter ial known today. Types of orga nic fibers include:

Kevlar Fibers Nomex Fibers, an d Spectr a (ultr a highly orient ed polyethylene) Fibers.

Advan ta ges for th e use of orga nic fibers in clude:

Very high st rength , Very h igh st iffness, Very high specific st ren gth a nd st iffness,

Excellent impact resistance, High toughness, and Corr osion resistan t.

Disadva nt ages for t he u se of orga nic fibers include:

Significantly more expensive than glass fibers, and Pr opert ies ma y be affected by environm ent al factors (e.g. ultr a violet

radiation).


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Specialty Reinforcements

Specialt y reinforcement s include:

Boron, Silicon Ca rbide, an d

Other.

These rein forcement s were origina lly developed in the 1960s. Advan ces in

graphite and organic reinforcing materials coupled with lower costs associated with

th em h as impacted th e growth in a pplicat ions for specialty ren iforcements.

Advan ta ges for th e use of boron a nd silicon car bide fibers include:

Very high str ength, an d Very h igh st iffness.

Disadva nt ages for t he u se of boron a nd s ilicon car bide fibers include:

Extremely expensive.

REINFORCEMENT FORMS

The form of the reinforcements used in composite materials spans a wide

range and has a direct impact on the mechanical properties of the structural

componen t. The form of th e reinforcing element s also impa cts the fabr icat ion

techniques tha t can be used. As discussed in the first chapter , composites ar e

classified based on t he geometr y of the r einforcing element .

Fiber Terminology

Fiberous reinforcements have several specific terms used to describe the

ma ke-up an d geometr y. These terms include:

FilamentSingle fiber produced from a single port in the spinning process.

Diameters for common filaments (glass and graphite) range from

0.000015 inches to 0.0005 inches.

FibersA gener al t erm comm only u sed t o refer to a collection offi la m en ts.

Strand

Commonly a bundle or group of untwisted, collimated fi la m ents. Usedintercha ngeably with fiberand fi la m en t.

TowA bundle or group of untwisted, collimated fi la m en ts usually with a

specific coun t .

YarnA twist ed bun dle of contin uous fi lam en ts.

Rovin g


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A number of tows or ya rns collected into a parallel bundle without

twisting.

TapeA collection of collimated (parallel) fi la m en ts usually made from tows

held t ogeth er by a binder , which is t ypically a B-sta ge resin.

Woven FabricA two-dimensional material made by interlacing yarn s or tows in

various patt erns.

Braid in gA three-dimensional material made by interlacing ya rns or tows in

various patt erns.

MatA two-dimensional material made of randomly oriented chopped fibers

or swirled continuous fibers that may be held together loosely by a

binder.

Weave types

The textile industry has developed a number of different weaves that are

commonly used in app licat ions from cloth ing an d uph olster y to composite ma ter ials.

The specific weave used in a structure may impact the drape in the fabrication

process and t he mechanical properties of th e stru ctu re. Typical weaves used in

composite materials include:

Plain weave Basket weave Crowfoot satin weave

Long-shaft satin or harn ess weave Leno w eave

The typical weaves are shown in the following figure with the machine directions as

indicat ed in t he figure.

The plain weave is the simplest weave that has uniform strength in two

directions wh en t he yar n size and coun t a re similar in t he war p an d fill directions.

Pla in wea ve fabr ics a re comm only us ed for:

flat laminates, print ed circuit boards,

na rr ow fabrics, an d tooling.

The basket weave is similar t o plain weave except t ha t wa rp yar ns a re woven

as one over and u nder t ow fill yar ns. The weave is less stable tha n plain weave.

Consequent ly th e weave is more pliable and dr ape is bett er. Basket weave fabrics

ar e str onger t ha n a n equivalent weight/coun t plan e weave fabr ic. Applicat ions for

basket weave fabrics are similar to those for plain weave.


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Crowfoot satin weave has improved unidirectional quality with more

str ength in t he fiber directions th an pla in weave fabrics. The crowfoot satin weave

is more pliable than pla in weave fabrics and can comply to complex contours and

sph erical sh apes. Applicat ions include:

Fishing r ods,

Diving Boards , Skis, Aircraft du cts, Chann el, and Conduit.

Long-shaft satin or harness weave has a high degree of drape and stretch in

all directions. The weave is less st able th an in pla in weave fabrics. Applications

include:

Aircraft housings, Radomes, Ducts, an d Other cont our ed su rfaces.

Leno weave produces hea vy fabr ics for r ap id build-up of plies. Leno weave

fabrics are used:

As inn er cores of thin coat ings, Tooling, and Repairs.

The choice of weave for a particular application will generally be a

compr omise between str uctur al and fabrication requirement s. Unidrectiona l ta pe

will pr oduce higher st ren gth plies but ar e more difficult t o fabricate. Dra pe of th e

cloth can be a ma jor consider at ion in st ru ctu res with complex contour s.


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C o m m o n w e a v e s u s e d i n c o m p o s it e m a t e r i a l s.

C lo t h D i r e c t i o n s a n d

N a m e C o n v e n t i o n sP l a i n We a v e

B a s k e t We a v e C r o w f oo t S a t i n

L o n g -s h a f t S a t i n L e n o We a v e


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FABRICATION T ECHNIQUES

For the purposes of this presentation the fabrication techniques for composite

materials are arbitrarily divided into three catagories they are: manual processes,

ma chin e processes, and ma ss production pr ocesses. The type of techn ique used in apar ticular applicat ion will depend on a mong oth er t hings:

th e nu mber of par ts t o be produced, th e facilities a vailable, th e repeat ability of th e par ts to be produced, th e mecha nical properties required in the finished pa rt s, th e mat erials (resin a nd r einforecment) to be used, and th e size of th e part .

Unlike metal manufacturing processes, composite fabrication processes can

ha ve significan t impa ct on par t qua lity. Composite ma ter ials have received undo

criticism in some a rena s due in par t t o ina dequat e quality cont rol in t he fabricat ion

processes. Consequent ly, it is importa nt for t he designer an d ana lyst to underst an d

th e composite fabrication pr ocesses a nd t o develop an app reciation for t he imp act of

th e fabr icat ion pr ocesses on system beh avior.

CURE PROCESSES

The majority of composite materials in production today are made with

th ermoset polymeric resins. Consequent ly, the str uctur e requires some kind of a

cur e process to produce the fina l part . A generic cur e cycle is shown in th e

following figure .

0

100

200

300

0 50 100 150 200 250 300

t (minutes)

T(F)

0

50

100

150

p(psi)

2/min Heating

5/min Heating

Pressure

Three hour hold at 250F

with one hour at 150psi.

G e n e r i c c u r e c y cle w it h t e m p e r a t u r e a n d p r e s su r e r e q u i r e d .


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Vacuum Bag Processes

Whether or not a cure cycle requires additional pressure to achieve

consolidation of part the vast majority of all composites produced require use of a

vacuu m bag. In t he vacuum bag process the pa rt is covered by a release ply, next a

barr ier film, then th e appr opriat e nu mber of bleeder ply to absorb t he excess resinfrom the lay-up, a breather ply to provide a flow path for trapped gasses and

volitols released du rin g the cur e cycle, an d fina lly a bag. The vacuum ba g ma y be a

molded bag or sh eet of polymer fitted t o th e par t. Molded bags are more expensive

but r equire less manu al labor to inst all. They ar e comm only used in high

production applicat ions . A generic lay-up with t he various vacuu m bag componen ts

is sh own in th e following figur e.

Typical Vacuu m Ba g Componen ts .

Bagging is an import an t pa rt of processing thermoset composite par ts. It h as

a direct impact on par t qua lity. It is possible for a pa rt , carefully laid-up, to be

scra pped due to poor bagging. It is essential tha t a ba g be tight ly sealed and leakfree and be in perfect cont act with t he work piece. A leak free bag is necessar y to

achieve consolidation of the lay-up and to provide the necessary path to exhaust

evolved gasses th at ma y be tra pped in t he lay-up during t he fabricat ion pr ocess or

th ose tha t ar e produced by th e chem ical reactions in th e cure cycle. Pr oducing a

leak free bag can be cha llenging, but is not in genera l impossible. Ther e may be

times th at a ba g loses its seal during the cur e cycle. When th is ha ppens it is


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typically unrepairable, but the part may not be lost, depending on when the leak

develops a nd wha t type of cur e cycle is in us e.

Perfect contact with the part must be established in the bagging processes.

Fa ilur e to establish perfect cont act will produce a flawed part th at ma y or ma y not

ha ve to be scra pped due to the failure. Perfect cont act appear s in two points in t hebagging process, bridges and dar ts . These problems ar e shown in the following

figur es. Dart s are used in bag const ru ction when flat ba g ma ter ial is used inst ead

of a molded bag. The seams produced on t he finished pa rt s due t o use of dar ts is a

resin rich point that is not likely to adversly affect the mechanical properties of the

par t. Bridges on the oth er hand are more than un sight ly belmishes. It is quite

comm on t o have intern al voids an d delamina tions in th e finished workpiece in th e

vicinity of bridges. Rubbing tools are u sed to compact th e bag and r emove wrink les.

Vacuum Bag

Mold

Lay-up

Gap due to bridging

Mold

Lay-up

Vacuum Bag

Gap due to dart

Dart

Bridging in a vacuu m bag process. Dart used in vacuu m bag process.

Auto- and Hydroclave processes

Autoclave and hydroclave processes use additional pressure to consolidate

th e lamin at e. Vacuum bags ar e used with both of th ese processes. The use of a

hydroclave may produce a superior cure cycle due to the improved heat transfer

from the liquid medium to the workpiece as compared to the gas used in an

au toclave. Typically th e added pressu res us ed in th ese processes ra nge from 50 psi

to 200 psi. Research on t he applicat ion of pressur e and du ra tion of th e vacuu m

held on the part for phenolic composites has indicated that part quality can be

significantly impacted by these steps.

MANUAL TECHNIQUES

Manual fabrication techniques for composite materials include manual

lay-up and man ua l spra y-up. Of th ese processes, th e ma nu al techniques ar e

domina ted by th e man ua l lay-up process. Advant ages and disadvant ages for th ese

processes a re list ed in t he following ta ble.


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Design considera tions for m an ua l techniqu es include t he following:

Minimum inside ra dius: 0.1875 to 0.25 inches. Tight er ra dii ar epossible but not desirable.

Minimu m dra ft recomm ended: 2. Split molds can h ave 0. Under cut s: should be avioded but can be made by usin g split or r ubber

molds.

Molded in h oles: lar eg diamet er only. Minium practical thickness: 0.03 inches for manual lay-up, 0.06 inches

for ma nu al spray-up.

Maximum pra ctical t hickness: un limited t ota l, 0.25 inches per cure. Normal thickness variation: +0.03/-0.015 inches for manual lay-up.

0.025 inches for ma nu al spr ay-up. Special const ru ction p ossible: built-in cores, m eta l insert s, met al oroth er edge stiffener s

Bosses: must be ta pered. Fins: special ha ndling required. Limiting size factor: none, oth er t ha n m old size, oven s ize (if requ ired)

an d ha ndling considera tions.

Sha pe limitat ions: none.

Ad v a n t a g e s a n d D is a d v a n t a g e s of Ma n u a l F a b r i c a t i on T e ch n i q u e s

A d v a n t a g e s * D i s a d v a n t a g e s *

Design flexibility Labor -intens ive pr ocess

Large an d complex part s can be

produced

Only one good (molded) su r face is

obtained

Pr oduction ra te requ iremen ts ar e low Low-volume pr oduction processMinimum equipment investment is

necessary

Quality is related t o the skill of the

operator

Tooling cost is low Longer cur e tim es requ ired

Any ma terial t ha t will hold its sha pe can

be used a s a m old form

Pr oduct u niform ity is difficult to

mainta in with in a single part and from

part t o part

Sta rt -up lead time an d cost are minima l Waste factor is high

Design chan ges ar e easily effected

Molded-in inserts and structural

reinforcements are possibleSandwich constructions are possible

Pr ototyping a nd pre-production meth od

for high volum e m olding pr ocesses

Semi-skilled worker s ar e needed an d ar e

easily tr ained

Hazards associated with handling the

mat erials ar e higher

* Note: the h orizont al alignment in t he t able is not int ended to imply a relat ionsh ip

between the points.


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Fin ished su rfaces: one (two with special t ooling). Gel-coat sur face: only one sm ooth sur face, reverse can be coat ed a fter

molding

Molded in la bels:

Manual Lay-up

In a ma nu al lay-up process each ply of a lamin at e is placed by ha nd. The

advan ta ge to th ese processes is th at litt le in the way of equipment is required. The

disadvantages to these processes include: high degrees of variablitiy between parts,

even th ose produced by a single technician an d genera lly inconsistent quality even

when per form ed by a h ighly qualified technician. As always the advan ta ges mu st

be weighed a gainst t he disadvan ta ges in a given applicat ion.

Wet Processes

In wet lay-up pr ocesses the lamina or ply must be satu ra ted with r esin beforebeing laid-up in th e tooling. Following satu ra tion, excess resin mus t be rem oved to

avoid having a part th at is una ccepta bly resin rich. The following ar e the basic

steps in a wet lay-up pr ocess:

1. Prepare patterns for each ply of the laminate locating all darts and folds

requ ired to accur at ely follow the m old. Minimize the nu mber of overla ps an d

never superimpose overlaps. If overlapping plies are required, keep th e

overla p width t o 0.75 inches, +0.25 inches/-0.0 inches. A pat ter n ma y be used

for m ultiple ply when t he pat ter n is repeat ed in the st acking sequence.

2. Optiona l, prepar e a laminat ion kit, cut ting all plies to th e required patt ern,ma rkin g their order in t he st acking sequence with Teflon t ape.

3. Coat tooling with releas e film or place releas e ply on tooling. If Gel-coat is

desired it should be applied at th is time.

4. Pr epare th e resin pot. Mix th e resin components as required and place in a

cont ainer th at is sufficiently large to lay th e individua l plies for th e lamina te.

5. Sat ur at e th e first ply of ma ter ial with resin.

6. St rip excess resin from ply.

7. Place first ply on tooling with the orientation specified in the stacking

sequen ce, film side-up. Usin g a n on-stick (Teflon or st eel) tool press t he ply

ont o th e tooling. Work out all air bubbles and wrinkles. En sur e th at t he ply

is in total contact with the tooling, working out all bridges that occur at

fillets an d gaps at corn ers t ha t occur at rounds. Draw excess resin from ply.


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8. Remove Teflon tape mar king lam ina sequence. (This is very import an t.)

9. Sat ur at e th e next ply of ma terial with resin.

10. St rip excess resin from t he ply.

11. Apply th e next ply with th e orien ta tion specified in t he s ta cking sequence,

film side-up. Again , usin g a n on-stick (Teflon or st eel) tool press t he ply ont o

th e lay-up. Work out all air bubbles and wrinkles. En sur e that t he ply is in

total contact with the previous layer, working out all bridges that occur at

fillets an d gaps at corn ers t ha t occur at rounds. Draw excess resin from ply.

12. Repeat st eps 8-11 unt il the s pecified st acking sequ ence is completed.

13. Dependin g on th e r esin syst em u sed, if th e fabrication pr ocess cann ot be

completed in a single operation t he lay-up sh ould be bagged with a vacuu m

bag a nd held u nder a cont rolled environm ent (especially low hu midity) until

th e operat ion can resum e.

14. Pr epar e th e par t for t he cure pr ocess following th e bagging procedur e.

Manu al scissors, power shea rs, an d semi-au tomat ic an d au tomat ic machines

ma y be used to cut th e plies for the lay-up. When compu ter n um erically cont rolled

au tomat ic cut ting machines are used, pat tern s are not necessary.

Prepreg Processes

1. Prepare patterns for each ply of the laminate locating all darts and folds

requ ired to accur at ely follow the mold. Minimize the nu mber of overla ps an d

never superimpose overlaps. If overlapping plies are required, keep th e

overla p width t o 0.75 inches, +0.25 inches/-0.0 inches. A pat ter n ma y be used

for mu ltiple ply when t he pat tern is repeated in the sta cking sequence. As

with th e wet pr ocess, if au tomat ic cut ting machines ar e used patt erns a re not

required.

2. Coat tooling with releas e film or pla ce relea se ply on t ooling.

3. Optiona l, prepar e a laminat ion kit, cut ting all plies to th e required patt ern,

ma rkin g th eir order in th e stacking sequence on th e backing film. Retur n

th e kit t o stora ge as quickly as possible to ma inta in qua lity.

4. Bring the prepr eg to room tem perat ur e for t he lay-up pr ocess.

5. Place first ply on tooling with the orientation specified in the stacking

sequen ce, film side-up. Usin g a n on-stick (Teflon or st eel) tool press t he ply

ont o th e tooling. Work out all air bubbles and wrinkles. En sur e th at t he ply

is in total contact with the tooling, working out all bridges that occur at


31/121




fillets and gaps at corn ers th at occur a t rounds. A hot air gun can be used to

increas e ply flexibility a nd t ack t o place th e ply.

6. Remove th e backing film from the prepr eg. (This is very import an t.)

7. Apply th e next ply with t he orien ta tion specified in th e stacking sequen ce,film side-up. Again , usin g a n on-stick (Teflon or st eel) tool press t he ply ont o

th e lay-up. Work out all air bubbles an d wrinkles. En sur e that t he ply is in

total contact with the previous layer, working out all bridges that occur at

fillets and gaps a t corn ers th at occur at rounds.

8. Repeat st ep 7 un til the specified st acking sequence is complet ed.

9. If th e fabr icat ion pr ocess can not be complet ed in a single operat ion t he lay-up

should be bagged with a vacuum bag and placed in cold storage under

vacuum until the operation can resume or held under a controlled

environm ent (especially low hu midit y). When lay-up resu mes, if th e par twas placed in cold storage, the part must be brought to room temperature

before fabr icat ion can contin ue.

10. Pr epar e th e par t for t he cur e process following th e bagging procedur e.

Spray-up

In the typical spray-up process chopped fibers, usually glass, and resin are

simu lta neously spr ayed ont o or int o an open mold. Fiber roving is fed thr ough a

chopper and injected into a resin str eam th at is man ua lly directed at t he mold. The

resin system ma y be pre-mixed or mixed in the spra y-up nozzle. After t hecomposite is sprayed into the mold it is hand rolled to remove air, compact the

fibers, an d smooth th e interior su rface. Becau se of th e nat ur e of th e process the

fibers are randomly oriented within the laminate and the behavior is transversely

isotr opic. Dependin g on th e resin system us ed the work piece will be bagged and

cures in the sam e mann er as lay-up par ts.

MACHINE PROCESSES

Machine processes are typically superior to manual processes in quality,

qua nt ity, an d production tim e. However, th ey ar e also significan tly more expens ive

th an m an ua l processes to implement. Simple winding ma chines are in the tens ofthousands of dollars, while the most advanced, sophisticated fiber placement

ma chin es ar e millions of dollar s. With capit al equipment cost s of th is ma gnitu de

th e decision t o use ma chin e processes is n ot a caviler d ecision.

At this writing there are four principal types of machines used for composite

fabrication. They ar e: filamen t winding, ta pe placement or tape laying, fiber

placement , and pultr usion. Of th ese four , only the pultr usion pr ocess incorporat es


32/121




a cur e cycle in th e process. Pa rt s produced by filamen t winding, tape laying, an d

fiber placemen t m ust be bagged and cur ed with t he specified cur e cycle.

Filament Winding

Filam ent win ding is the oldest of th e machine processes. The process isrelat ed to th e tur ning processes used in ma chining operat ions. Filament winding is

used to produce axisymm etric str uctur es. Pa rt s produced by filamen t winding

include:

Tubes or pipes, Cylindr ical pr essur e vessels (Rocket motor cases), an d Spherical pressur e vessels.

Fundamentally the process involves winding roving or tow around a

ma ndr al. The winding an gle ran ges from near ly axial, or longitudina l (0, axial,

can be obtain ed in special winding opera tions) to hoop, or circum feren tia l (90). In

th e winding opera tion dr y roving is pulled thr ough a resin bat h wh ere th e roving issat ur at ed with r esin. The excess is stripped from the roving and th e roving is

dra wn through a generat ion ring. The winding head with its generat ing ring

tr averses t he longitudina l direction of the workpiece riding on a car riage an d laying

th e roving on th e man dra l. The roving follows a helical pat h ar oun d the ma ndr al,

see th e figures below. As th e car ria ge rea ches t he end of th e workpiece it reverses

direction an d lays down anoth er layer in the opposite direction. The process

continues until the mandral is completely covered and then the machine moves to

th e next ply. Becau se th e roving does not complet ely cover t he work piece in a

single pass (except in the hoop direction) the roving is laid down in stripes that

alternate in direction (

). The resu lt is someth ing app roaching a woven cloth ,

similar to a plain weave fabric, except that the fiber (roving) directions are not

perpendicular . A typical filament winding machine is shown in t he figure.


33/121




Fu nda ment al components in a filam ent winding process.

Hoop or circumferen tia l winding. Typical polar winding.

Mult i-circuit h elical winding. Note th e overlap of windings.


34/121




Typical filament winding machine.

One of th e most importa nt components in su ccessful filament winding is t he

determina tion of th e relat ive speed between the man dra l an d winding head. These

motions determ ine the wrapping an gle and overlap of th e roving. The winding

an gle ma y be approxima ted from:

arctanR

v

=

,

where is the winding an gle,R is the radius of the workpiece, is the rotation r ateof the workpiece, and v is the longitudina l speed of th e winding head. Fr om th is

equation you can observe that if the workpiece has a change in diameter along the

length the rotation rate of the workpiece and/or the speed of the carriage must be

adju sted t o hold th e winding an gle cons ta nt . The complexity of th e pr oblem is

further complicated by polar winding at the dome of pressure vessels.

Another important component in successful filament winding is tension

contr ol. Tension affects resin cont ent , void cont ent , an d st ru ctu ra l propert ies.

Roving tension ra nges from 0.25 lbs. to 1 lb. per bun dle or t ow. Tension is provided

by guide eyes in line, center rotating guide eye, rotating scissor bars, drum-type

brakes (which may be electromagnetically controlled), and/or drag through theresin bat h. The first t hr ee of th ese tensioners a re shown in a figure below.


35/121




Guide eyes in line. Cent er rota tin g guide eye Rota tin g Scissor bar s.

Advances in filament winding technology include spherical winding

ma chines and multi-hea d winders. Spher ical winding ma chines can producevessels tha t a re cylindr ical or sph erical with a single opening. Multi-hea d winding

ma chines can produce vessels with a qua si-braided str uctur e. A spherical winding

ma chine is sh own in th e following figur e.

Ad v a n t a g e s a n d D is a d v a n t a g e s fo r F i la m e n t Wi n d i n g P r o c e s se s .

Ad v a n t a g e s D is a d v a n t a g e s o r L im i t a t i o n s

Applicable t o par ts of widely var ying

size.

Resin viscosity a nd p ot life mu st be car efully

chosen an d m onitored.

Par ts with strength in several

directions can be ea sily ma de.

Pr ogra mm ing of the windin g can be difficult .

Excellent ma terial usa ge. Not all sha pes can reasonably be ma de byfilam ent winding.

Forming after winding and other

techn iques a llow noncylindr ical

sha pes to be made.

Opera tional cont rol of severa l key

param eters is importa nt.

Flexible mandr els can be reta ined in

th e str uctur e to serve as liners for

tanks.

Ability t o ana lyze (design) impa ired du e to

invalidation of key assum ptions in

lamina tion th eory.

Panels and fittings for reinforcement

or a tt achment can be easily

included du ring t he winding process.

Par ts with high pressure rat ings can

be made.


36/121




A spherical winder.

Tape Lay-up

Semi-au tomat ic an d au tomat ic ta pe laying ma chines h ave been developed to

reduce production t imes, impr ove consistency with in par ts a nd between pa rt s, and

improve part qu ality. Tape lay-up ma chines use prepreg ta pe, un idirectiona l an d

cloth , and ar e compu ter n um erically cont rolled. Through appr opriat e program ming

it is possible to eliminate the patterns use in the manual lay-up processes.

Semi-automatic and automatic tape laying machines are used to produce flat and

cont our ed lam inat es. Tape laying ma chines ar e limited in th eir capa bilities to

sur faces with lar ge radii of cur vatu re. Tape laying machines can not produce highly

geomet rically complex pa rt s.

Typical automatic tape laying machines are described by the number of

degrees of freedom (DOF) or axes of the tape laying head, for example, a head may

have three translational degrees of freedom, two rotational degrees of freedom, theability to sta rt a ta pe, an d the ability to cut t he ta pe. This machine is described as

a seven-DOF machine. Figures below show a tape laying head, a flat a ut oma tic

tape laying machine, a coutoured laminate in a tape laying machine, and a

mu lti-axis ta pe laying machine.


37/121




Grumm an t ape laying head.

Flat a ut omat ic lay-up ma chine.


38/121




A cont our ed laminat e on a n a ut oma tic lay-up m achine.

A ten DOF au tomat ic lay-up ma chine.

Fiber Placement

Automatic fiber placement machines combine and extend the capability of

filam ent winding an d tape laying ma chines. Fiber placement machines place

individual fiber bundles onto a mold. This is reminescent of the filament winding

process and in contrast to the laying of a tape with a tape laying machine.

However, the fiber placement machine places the fiber bundles in parallelth roughout a layer, without t he overlapping in a helical winding process. Becau se

of the ability to place fiber bundles the fiber placement process can produce highly

geometr ically complex sha pes with sma ll ra dii of cur vatu re. Int erna l radii ar e

limited by the size of the placement head, approximately 6 inches on a Viper

Placement Machine and external radii are limited to the minimum bending radius

of th e fiber bund les, ap proximat ely 0.1875 to 0.25 inches. A fiber placement

ma chine is shown in th e figure below.

Tape

Laying

Carriage


39/121




NASA Viper fiber placement ma chine.

Fiber placement is a compar ably ra pid process. Fiber placement m achines

can place up t o 700 inches of fiber per minu te. Fu rt her, t he complexity of th e par ts

produced by fiber placement is extrordinar y. For example, th e inta ke duct for t he

F-16 is a geometr ically complex pa rt with m ult iple compoun d curves is produced by

fiber placement with r elative ease.

Pultrusion

Pu ltru sion is an ada pta tion of the dr awing process to composites fabricat ion.

This process produces long relatively narrow cross section with highly ordered and

compacted reinforcement s. Cross sections produced by pult ru sion ra nge fromcircular to L-cha nn el to ha t sections. The r einforcing fibers, which ma y be glass,

graphite, or arimid, are generally all oriented along the major direction of the

pultrusion.

In general the process begins with dry roving that is drawn through a resin

bath an d into a compa ction die. Fr om th e compa ction die the mat erial is drawn int o

a curing die where th e excess resin is st riped an d th e par t is cured with significan t

press ur es. It is possible to draw th e work piece over a bench before the fina l cur e to

produce cur ved (inst ead of str aight ) sections. Aut omotive composite leaf springs a re

an example of curved pultruded parts, a pultrusion forming operation and

au tomobile spr ings are shown in th e figur es. A microwave cur ing pr epreg basedpultr usion system is sh own in th e figure below.


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Schematic of a Pultrusion forming system.

Aut omobile leaf springs pr oduced by Pult ru sion form ing.

Schema tic of a Pu ltru sion system u sing microwave energy to cur e th e resin system .


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MASS PRODUCTION TECHNIQUES

Manual production techniques by their very nature are limited in their

production rat es. The ma chine processes ar e typically more expens ive an d do not

lend th emselves to high pr oduction volumes . Pr oduction of composite par ts for

applicat ions su ch as th ose foun d in t he a ut omotive indust ry n ecessitat e production

processes th at h ave very high production ra tes. Molding processes ar e able to meet

these demands.

Molding

Molding of composite m at erials ha s its foun dat ion in t he m etals casting a nd

form ing processes. These processes include: sheet molding, bulk molding, th ickmolding, an d liquid molding (resin tra ns fer molding). The advan ta ges of th ese

processes include lower per part tooling costs and higher production rates.

S he et Mol d i n g

Sheet molding processes were developed in response to a request from the

au tomotive indust ry. Their desire was for a composite ma terial process tha t

allowed them to use the metal bending and stamping equipment and techniques

with which t hey were fam iliar. The ma ter ial used in sheet molding is called sheet

molding compound (SMC).

In the sheet molding process chopped roving, usually glass fibers, is mixed

with r esin an d deposited between plast ic films, usu ally polyethylene. The mat erial

is then u sed in a sta mpin g like process. The schem at ic below shows a typical SMC

machine.

P u l t r u s i o n


High mat erial usage compar ed with

lay-up

Part cross-sections must generally be

uniform.

High th roughput ra te Pr oblems can ar ise when resin or fibersaccum ulat e and build up at t he die opening

Can give high resin conten ts When dies ru n resin rich to accoun t for fiber

anomalies, strength is sacrificed.

Close to fiber tow propert ies Voids can resu lt if dies ar e ru n with too

mu ch openin g for th e fiber volum e.

When quick cur ing systems a re u sed,

mechan ical pr opert ies ar e often sa crificed.


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Schema tic of th e SMC fabr icat ion pr ocess. Note th at th e flow is from left t o right .

T h i c k a n d B u l k M ol d in g

Thick and bulk molding operations are related to the closed die or

ma tched-die cas tin g an d forming processes. In th ese processes the bulk molding

compound (BMC) or the thick molding compound (TMC) is prepared similar to the

SMC compound, however it is mixed in buld ra th er th an sh eet. Advant ages and

disadva nt ages of th e process ar e listed below. The TMC pr ocess is shown in th efigure.

Ma t c h e d -d i e Mo ld i n g


Both interior an d exterior sur faces

ar e finished.

More equipment is needed tha n for lay-up.

Complex shapes including ribs a nd

th in deta ils ar e possible.

Molds an d t ooling ar e costly compa red t o

lay-up m olds.

High production ra tes ar e possible. Tran spar ent products ar e not possible with

SMC and BMC.Labor costs ar e low. Molding pr oblems (tr app ed wat er, etc.) ma y

cau se su rface imperfections such a s pitt ing

or waviness.

Minimum t rimm ing of par ts in

needed.

Products have good mechanical

propert ies and close par t toleran ces.

Good consolidation of parts.

SMC an d BMC ha ve limit ed shelf-lives.


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Schema tic of th e TMC fabrication pr ocess.

R esin T r a n sfer Mold in g

Resin transfer molding is used in a number of variations with a number of

differen t na mes to ident ify th e var iat ions. Among th ese processes are: Resin

Transfer Molding (RTM), Structural Reaction Injection Molding (SRIM) Resin

Injection Molding (RIM), Vacuum-Assisted Resin Injection (VARI), Thermal

Expansion Resin Transfer Molding (TERTM), Vacuum Assisted Resin TransferMolding (VARTM), and Seamans Composites Resin Injection Molding Process

(SCRIMP) to na me a few. All of th ese processes involvethe sam e basic st eps. The

basic steps in RTM are:

Place preform in mold. Close m old. In fu se/ in ject liqu id resin in to m old . Cure part in m old. Open m old. Rem ove part from m old . Clean u p part.

One of th e significan t pr oblems in t he u se of th ese liquid molding pr ocesses is

adequa te sett ing of th e stru ctu ra l preform by th e liquid resin. To produce a high

quality composite part it is essential that the reinforcing fiber structures be

th roughly impregnat ed with th e resin. The resin or ma tr ix in a composite tr an sfers

th e load from one fiber to th e next. If th ere is no resin p resen t, th e loads do not get

tr an sferred from fiber to fiber, which result s in an ina dequat e str ucture. Pr oper

infusion or impregnation of the fiber preform requires a low viscosity resin and an


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extend ed pot life (two hour s or more depend ing on t he size of th e par t). In a ddition

it is very important to provide a leak path for all gasses that are native to the

preform a s the resin is injected. Advant ages and disadvanta ges for th e RTM

process a re listed below.

Schematics for the resin transfer molding process and the resin infusion

process ar e shown in th e following figures. Note in the RTM process th at a pum p is

used t o force th e resin int o th e mold an d a pr ess is used t o hold the mold closed. It

ma y be advan ta geous t o use pum ps to force the resin int o th e mold. However, high

resin flowra tes m ay cause t he pr eform to be dislocated from th e desired position.

R e s i n T r a n s fe r Mo ld i n gAd v a n t a g e s D is a d v a n t a g e s o r L im i t a t i o n s

Very lar ge and complex sha pes can

be ma de efficient ly and

inexpensively.

The mold design is critical and requires

great sk ill

Pr oduction t imes are m uch shorter

tha n lay-up.

Pr operties a re equivalent t o mat ched-die

molding (ass um ing proper fiber wet-out ) but

ar e not generally as good as with vacuum

bagging, filament winding, or Pu ltru sion.

Clamping pressure is low compared

to ma tched-die molding.

Cont rol of resin un iform ity is difficult .

Radii and edges tend to be resin rich.

Surface definition is superior to

lay-up.

Reinforcement movement dur ing r esin

injection is sometimes a problem.

Insert s a nd special reinforcements

can be added ea sily.

The sill level requir ed for t he

opera tor is low.

Many mold ma teria ls can be used.

Par ts can be made with better

reproducibility th at with lay-up.

Work ers a re n ot exposed t o chem icals

an d vapors a s with lay-up.


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Schema tic of th e Resin Tra nsfer Molding P rocess.

Schema tic of the Resin Infusion P rocess.

Automated Spray-up

The automated spray-up processes are simply an automation of the manual

spra y-up processes. In some ways the process is related to the au tomated pa inting

processes used in indu str ies such a s th e aut omotive industr y. In th is applicat ion arobotic arm is programmed to spray chopped reinforcement and resin into a mold.

A schematic of the automated spray-up process is shown in the following figure.

The various letters in th e figur e designa te components of th e spra y-up m achine.


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Schema tic of an au tomat ed spra y-up m achine.

HAZARDS

The hazards associated with the production and fabrication of composite

materials is higher than in the production of parts from conventional materials.

Some of the more exotic conventional ma teria ls, such a s tit an ium, ma gnesium, an d

berelium, have significant healt hazards associated with their production and/or

fabrication processes. Composite mat eria ls in gener al are produced from orga nic

chemical compounds that in their uncured state may pose significant health

hazards unless han dled with great care.

HEALTH INFORMATION TERMINOLOGY

To begin th e discussion we mu st a gain define some ter ms in th e field. Fir st

of all we must define the difference between toxicity and hazard. Toxicity is an

inh eren t ha rm ful effect of a chemical. It is a physical propert y of th e chem ical.

Hazard is cont rolled by exposur e. Exposur e to a t oxic chem ical requ ired for a

ha zar d to exist. A chem ical with Acu te Toxici ty has a harmful effect after single

an d/or sh ort term exposur e.

Toxicity is measu red in lethal doses and letha l concentra tions. The Mean

Lethal Dose LD50 is expressed a s a r at io in mg of chem ical t o kg of body weight . Itis the amount of chemical administered by a specific route that is expected to kill

50% of a group of experimen ta l an ima ls. The Mean Lethal Con cen tration LC 50 is

expressed in mg/m 3 or par ts per million (ppm) in air. It is th e concent ra tion of

chemical in air that is expected to kill 50% of a group of experimental animals.

Ther e ar e for s ome chem icals levels below which th ere is n o observa ble effect. This

level is defined as th e No Observable E ffect Level, NOEL.


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The toxicity of a chemical is further characterized by the local effects it

cau ses. These effects include:

Irr it ationLocalized reaction resulting from either single or multiple exposures to

a ph ysical or chemical ent ity at th e sam e site.

CorrosionTissue destru ction in su ch a way tha t n orm al hea ling is not possible.

SensitizationAllergic reaction to a substance that develops upon repeated exposure.

Chron ic ToxicityChar acterized by adverse hea lth effects in a n a nima l or person, which

has been caused by exposure to a substance of over a significant

portion of that animals or persons life, or by long-term effects

resu lting from a sin gle or a few doses.

Two oth er h ealth term s t ha t ar e used t o describe the t oxicity of chemicals ar e

used comm only today but h ave specific definit ions in t he hea lth field. The first iscarcinogenicity . This is th e ability of a subst an ce to cause tu mors. Long ter m

test ing requ ired to deter min e if a subst an ce is car cinogenic. The resu lts of th ese

test s ar e conclusive. The second is mutagenicity . This is th e ability of subst an ce to

cau se cha nges in the genetic ma terials of cells. Short t erm t esting can be used to

determine if a substa nce is mu ta genic. The results of th ese short term t ests are

speculative (non-conclusive).

To minimize the hazards associated with working with toxic substances

exposure limits ar e defined. Exposur e limits t ha t a re defined in ter ms of Thresh old

Limit Valu es (TLV). TLVs assu me th at th e exposed populat ion is composed of

norma l, health y adu lts, an d does not a ddress a ggra vation of pre-existing conditions

of illness es. These limits ar e not fine lines between safe an d da nger ous

concentrations and should not be used by anyone untrained in the discipline of

indust rial hygiene. Four importan t TLVs are:

Th reshold Lim it Value Tim e Weighted Average (TLV -TWA)The time weighted average for a normal 8 hour workday and 40 hour

work week, to which nearly all workers may be exposed, day after day

without adver se effect.

Th reshold L im it Value S hort Term E xposure Lim it (TL V-ST EL )The concentration to which workers can be exposed continuously for a

short period of time (15 minutes) without suffering from (1) irritation,(2) chronic or irreversible tissue damage, or (3) narcosis of sufficient

degree to increase the likelihood of accidental injury, impair

self-rescue, or ma ter ially reduce work efficiency, an d pr ovided tha t t he

da ily TLV-TWA is n ot exceeded.

Th reshold Lim it Value Ceiling (T LV -C)The concentration that should not be exceeded during any part of the

workday.


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Perm issible Exposure Lim its (PEL )PELs are legal binding airborne exposure limits, which are issued by

th e Occupa tiona l Safety and Health Administr at ion (OSHA).

Proper industrial hygiene is essential to minimize the hazards of working

with toxic substa nces and to ensu re th at t he TLVs are not exceeded. Pr operindustrial hygiene requires controlling the routes of exposure to the toxic

subst an ces. Rout es of cont act include:

S kin an d E ye ContactAt risk through skin and eye contact are: Hands, Lower Arms, and

Fa ce. Cont act with liquids, gases, vapors, or par ticulat es sh ould be

minim ized to redu ce th e risk of cont act.

Inhala tionInhalation can be a significant route of exposure to toxic substances in

composite fabr icat ion. Solvent s and oth er volat iles ma y be releas ed

from the resin systems during the manufacturing and curing of

composites. Fu rt her, dusts may be generat ed in the machining ofcured composite materials.

Inges tionIngestion is not typically not a major problem in the fabrication of

composite ma ter ials provided th at th ere is sufficient cont rol.

Proper industrial hygiene requires control of the processes in five areas:

Administrative, Engineering, Operations/Process, Safety, and personal.

Administrative Controls include proper: handling of materials, training, isolation of

operations, personal protective equipment, personal hygiene, warnings and labels,

housekeeping, dispensing and storage of chemicals, and emergency instructions.

The Engineering Controls include proper: plant layout, design and use of

equipment, an d exha ust ventilation. Operat ions/Process Cont rols include proper:

mixing of resins (personal protective equipment as appropriate, and specific mixing

instructions -- available and followed), curing operations (use product specific cure

cycle), an d han dling of cur ed resin system s (as ap pr opria te). Safety Cont rols ar e as

appr opriat e. Personal Cont rols include proper tr aining of all personnel and a

commit ment by all personnel to mainta in a safe, hazard free workplace. This

includes a commitment by employers to effectively instruct the employees on site

hazards, warning labels, and ma terial safety data sheets. Fur ther, mana gement

and employee are responsible for knowing about hazards and taking measures for

minimizing exposure.

TOXICOLOGICAL PROPERTIES OF COMPONENTS

The following section lists some of the toxicological properties of components

of composite mat eria ls. The informa tion present ed below is generic an d fur th er

specific information should be obtained regarding the specific compounds with

which you ar e dealing.


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Epoxy R esinsEpoxy resins a re a lways used with curing a gents a nd comm only with a

series of oth er additives. Generally, more deman ding ha ndling

procedures an d cont rols a re r ecommen ded for th e curing agent or oth er

additives. Epoxies ar e prima ry skin and mucous membra ne irr itan ts.

Some epoxies h ave s ensit izing effects. H a r d e n i n g a n d C u r i n g Ag e n t s

Arom atic Am in e H ardenersThese hardeners have slight irritating effect on skin and mucous

membr an es. They ha ve been shown to cau se damage to liver and

may decrease ability of blood to transport oxygen to tissues.

Exposur e should be minim ized or a voided.

Aliphatic and Cycloaliphatic Am in e H ardenersThese har deners are strong bases. It is a sever irritant an d is

corr osive. Exposur e should be avoided.

Polyaminoamide HardenersThis hardener produces mild irritation of skin and mucous

membr an es. It ma y cau se sensitization. Exposur e should be

min imized or a voided.

Am id e H ard enersThis har dener ha s a slight irrita nt effect. Avoid inh aling dust.

Anhydrid e Curin g Agen tsThis hardener is a sever eye irritant and a strong skin irritant.

Exposur e should be minimized or avoided.

P o l yu r e t h a n e R es in s Isocyanates

Most commercial isocyanates are highly toxic due to skin andrespiratory sensitization, or skin absorption and systemic toxicity.

They produce strong irritation of skin and mucous membranes of

eyes an d respir at ory tr act. Ext rem e care is necessar y! Good

ventilation is r equired!

T oluene d iisocyana te (TDI)Toluene is a mutagen . TLVs for toluen e are: TLV-TWA of 0.005

ppm a nd TLV-STEL of 0.02 ppm. Toluene h as n o odor below TLV

levels. At th is tim e th ere is no car cinogenic dat a. It is, however,

class ified a s potent ially carcinogenic.

PolyolsThese ar e cur e agents. At this time no par ticular hea lth hazar d is

indicated.

Phenolic and Am ino Resins Phenol-Form aldehyde R esins

These resins ha ve low ha zard levels. Ph enol an d form aldehyde

ma y be absorbed thr ough skin. Good vent ilation is recomm ended

an d skin sensitization is possible.

Urea- and Melam ine-Form aldehyde Resins


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These resins have acute toxicity similar to the phenol-formaldehyde

resin s. Skin sensitizat ion is possible.

B ism aleim id esNo extensive studies have been performed on bismaleimides at this

time. They ma y cau se skin irr itat ion or sensitizat ion. Dust or vapors

ma y irr itat e eyes, nose, an d thr oat . Thermoplastics

Generally thermoplastics are not considered harmful to workers

hea lth . Skin irr ita tion is not observed an d no toxic effects are known

to be associated with inh alat ion of dust s. Burn s may present sever

hazard with thermoplastics.

S tyrene MonomerStyr ene vapors can cau se eye irr ita tion. The liquid will cau se eye,

skin, and mucous membra ne irritat ion. Styrene ha s systemic effects

on cent ra l ner vous system , liver, and kidneys ha ve been observed. It

is possibly car cinogenic to hum an s.

Rein forcin g Materia lsMost reinforcing materials in and of themselves are non-toxic.

However, inha lation of filler may be detriment al to health. Inh alat ion

ma y produce effects similar to asbest osis.

Carbon and Graphite FibersThreshold limits have been established for carbon and graphite

fibers. The limit s ar e: (TLV-TWA) 10mg/m3 (OSHA) and 3

fibers/cm 3 (U. S. Navy).

Ara m id FibersThe exposure limit (TLV-TWA) is set by manufacturers at 5

fibers/cm3

. No app ar ent effects from inh ala tion are observed. Fiber Glass

The exposure limit (TLV-TWA) for fibrous glass is 10mg/m 3.

NIOSH recommends 3 fibers/ cm 3. Exposur e ma y cau se

mecha nical irr ita tion of eyes, nose, an d thr oat . It is classified as

possible human carcinogen.

Solven tsContact with most organic solvents causes drying and defatting of skin

an d dermat itis. Some solvents ar e directly absorbed thr ough inta ct

skin; absorption is enha nced if skin abraded or irr itat ed. An

additional concern is the ability of a solvent to carry other substances

th ough skin with it. AcetoneAcetone is a comm on labora tory solvent. It wa s placed on t he

ha zardous list. However, it ha s been more recently removed.

The threshold limits are: a TLV-TWA of 750ppm and a TLV-

STEL of 1000ppm.

Methyl ethyl ketone (M EK)


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In addition to being a solvent, MEK is also used as an

accelerat or for Gel-coat . It cau ses eye, nose, an d th roat

irr ita tion. The th resh old limit s ar e: a TLV-TWA of 200 ppm a nd

a TLV-STEL = 300 ppm.


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LAMINA ME CH ANIC S

PRELIMINARIES

A lamina is a flat or near ly flat t hin layer of ma terial. In t his applicat ion th e

ma terial is a composite ma terial eith er ta pe or cloth . In pra ctical engineering

applicat ions th e lam ina is the fun dam ent al building block of th e str ucture. To

understa nd t he mechan ics of laminated st

composite text book

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