teaching of composites brent strong brigham young university

Teaching of Composites

Brent StrongBrigham Young University

Course Details

• Title―Composites: Materials and Processes• Level―Beginning graduate/advanced

undergraduate• Credit―3 hours (2 lectures and 1 lab/week)• Prerequisites―General Engineering Materials

(no organic chemistry)• Text―Fundamentals of Composites

Manufacturing, 2nd Edition by A. Brent Strong

Curriculum Issues

• Required: Manufacturing students• Option: Mechanical, chemical, civil

engineering and occasionally others• Gateway course: Engineering Materials or

equivalent• Focus

– This course: Manufacturing methods including the chemistry (simplified) of crosslinking

– Typical ME course: Laminate design

Basic Composite Concepts

• Concept 1: Composite components – Composites are made of two materials

―a continuous phase (matrix) and a discontinuous phase (reinforcement)

– In the most common composites, the matrix is a polymeric resin and the reinforcements are fibers

– The resin must bond to the fibers– The properties of the components are

synergistic

Basic Composite Concepts

• Concept 2: Advantages and Disadvantages– Composites are not “super” materials without

some drawbacks

Composites:Advantages and Disadvantages

Advantages• Lightweight• High specific modulus• Tailored properties • Easily moldable• Part consolidation• Easily bondable• Good fatigue resistance• Good damping• Crash worthiness• Internal energy storage/release• Low thermal expansion• Low electrical conductivity• Stealth• Thermal transport (carbon fibers only)

Disadvantages• Cost of materials• Lack of well-proven rules• Metal and composite designs are

seldom interchangeable• Long development time• Manufacturing difficulties• Fasteners• Low ductility• Solvent/moisture attack• Temperature limits• Damage susceptibility• Hidden damage• EMI shielding sometimes required

Basic Composite Concepts

• Concept 3: Advanced and Engineering– Advanced composites optimize the relationship of

mechanical properties and weight, or optimize thermal performance and weight

• Usually made from long, high performance fibers (carbon and aramid) and advanced resins

• “Specific properties” (accounting for density) are important

– Engineering composites have good properties with a focus on cost

• Usually made from fiberglass, often chopped, and engineering resins

• Sometimes termed “Fiberglass reinforced plastics (FRP)”

Steel

Al Composites

Steel

Al

Composites

Weight

Thermal Expansion

AlSteel

Composites

Specific Stiffness

AlSteel

Composites

Specific Strength

SteelAl

Composites

Fatigue Resistance

Properties comparisons of metals and composites

Source: CFA (June 2000)

Est. 3.9 Billion Lbs of Shipments

Corrosion11%

Aircraft 1%

Construction20%

Consumer7%

Electrical10%

Marine11%

Appliance 5%

Other 3%Transport32%

U.S. Composites Shipments

Basic Composite Concepts

• Concept 4: Stealth– Stealth properties come from controlling radiation

detection

Stealth Radiation Considerations

• Radar• Infra-red• Visual from ground• Visual from above (satellite)• Visual from other aircraft• Sound• Con-trail• Radio transmissions• Ultraviolet• X-ray

B-2/B-52 Size Comparison

Radar Cross Section of Typical Airborne Objects

ObjectJumbo JetB-17 (WWII)B-47 (Korean War)B-52 (Cold War)B-1B (First stealth)Large jet fighterSmall jet fighterSmall single engine planeManSmall birdInsect F-117A (stealth fighter)B-2 (stealth bomber)

Radar Cross Section (m2)1008040101.05-62-31.01.00.010.000010.10.01

Basic Composite Concepts

Property Molecular Cause or Association

Thermal Resistance Chemical components and bonding

Resistance to Solvents or Water Polarity

Permeability Crystallinity

Fire Resistance Aromaticity or halogen content

Electrical Properties Polarity and filler content

Weather resistance Aliphatic content, additives and fillers

Toughness Aliphatic content, rubber tougheners

Wet-out of fibers Molecular weight, backbone stiffness

• Concept 5: Matrix-dominated properties

Resin Choices (Most common)

• Unsaturated polyesters– Advantages: low cost, room or elevated temperature

cure– Disadvantages: water absorption, low thermal

stability, relatively poor mechanical properties• Epoxies

– Advantages: good adhesion, good thermal stability, good mechanical properties

– Disadvantages: requires heat curing to develop properties, cost

Resin Choices (Common)

• Phenolics– Advantages: Excellent flame retardance and low

smoke emission, good adhesive, good thermal and electrical insulation

– Disadvantages: Brittle, difficult to cure• Vinyl esters

– Advantages: Easy to cure, good resistance to water absorption

– Disadvantages: More expensive than polyesters, fewer choices in types of resins available

Metal, Ceramic and Polymeric Composites

106-

105-

104-

103-

102-

10-

1-0 1000 2000 3000 4000-18 538 1093 1650 2204

Temperature

oFoC

ExposureTime(sec)

Ep

oxy

Com

pos

ites

Po

lyim

ide

s

Ad

van

ced

Met

allic

s

Carbon-Carbon

Experimental

Ablative Materials(such as phenolics)

Retention of properties of various composites with time and temperature

Metal and ceramic composites are for high temperatures

Metal, Ceramic and Polymeric Composites

Specific strengths versus specific stiffnesses of various composites

Specific modulus

Spe

cific

Str

engt

h

= non-reinforced metals

= continuous fibers

= whiskers

B4C in aluminum

Boron fibers in resin

Carbon fibers in resin

Beryllium

Boron fibers in aluminum

Beryllium fibers in aluminum

Boron fibers in nickel

Al2O3 in aluminum

Fiberglass in resin

Aluminum

Steel

Ti

Al2O3 in nickel

Basic Composite Concepts

• Concept 6: Reinforcement-dominated properties– In advanced composites, the reinforcements

typically carry over 90% of the load– Composites are non-isotropic materials (that is,

they have directionality)– Many composites are layered and are called

“composite laminates”

Resin Rules

• These rules allow for easy understanding of resin properties

• These rules require no previous knowledge of organic chemistry or plastics

• The rules allow prediction of trends in matrix-dominated properties

Resin Rules

• Resin Rule 1: Thermal Properties ― Thermal properties depend upon the difficulty with which polymer molecules separate from each other.– The separation of molecules the atoms remain

bonded together in long chains.– The separation of atoms occurs at a much higher

temperature than the temperatures at which molecules separate.

Thermal Properties• Thermal transitions and thermal stability are

linked– Thermal stability is the temperature at which a

polymer can be used and still have acceptable properties

– Failure to perform is usually associated with molecular motion and so the same concepts associated with thermal transitions apply to thermal stability

Resin Rules

Typical Polymer

Heat Deflection

Glass Transition Decomposition{Melting

Temperature

Tg

Tm

Td

TemperatureFl

exib

ility

Resin Rules

• Resin Rule 2: Mechanical Properties ― Mechanical properties depend upon the difficulty with which polymer molecules separate from each other.

Resin Rules• Impediment 1: Entanglement (molecular

weight)– Increases in molecular weight (length of the polymer

chain) result in increases in thermal and most mechanical properties

– Analogy: spaghetti

Measuring factors related to chain length

• As chain length increases, viscosity increases– Consider pouring liquids from a cup

Low viscosity fluid High viscosity fluid

The Great Dilemma in Polymers

• Polymers must have good properties– Good properties are favored by high molecular

weight

• Polymers must have good processing– Good processing is favored by low molecular

weight

Molecular Weight

Me

ch

an

ica

l Pro

pe

rtie

s

Molecular Weight

Ea

se

of

Pro

ce

ss

ing

The Great Dilemma In Polymers

• Thermoplastics meet the dilemma by compromise– High enough molecular weight to get adequate

properties– Low enough molecular weight to process OK

• Thermosets meet the dilemma by crosslinking– Low molecular weight initially (for wetout and

processing) followed by curing to increase molecular weight

– No compromise is required

Resin Rules• Impediment 2: Crosslinks

– Formed during curing

Crosslink bonds

Covalent bond (shared electrons)

Polymeric molecules

Thermal Properties

Typical Thermoplastic

Heat Deflection

Glass Transition Decomposition{

Melting

Typical Thermoset

Heat Deflection

Glass Transition MeltDecomposition X

Temperature

Visc

osity

Time/Temperature

Liquid-Solid LineSolids

Liquids

Region A Region B

Thermosetthinning due to temperatureThermosetcrosslinking

Thermosetcombination(What is seen)

Gel Point

Thermoplastic

Viscosity curves for typical thermoplastic and thermosets

Thermoplastics• Thermoplastics are not crosslinked and so they melt• Thermoplastics are molded as molten liquids • Thermoplastics are cooled to solidify• Thermoplastics can be re-melted repeatedly• Kitchen example:

candy

• Thermoplastics are tough rather than brittle• Examples of thermoplastics: polyethylene,

polystyrene, nylon, polycarbonate, acrylic, Teflon®, PET (thermoplastic polyester)

Thermosets• Thermosets are crosslinked and do not melt • Crosslinking is sometimes called curing• Thermosets are processed as room temperature

liquids • Thermosets are heated to solidify• Kitchen example:

cake

• Thermosets are often brittle • Examples of thermosets: polyesters, vinyl esters,

epoxies, phenolics, polyimides

Polyester polymerizationMonomers

Glycols G (di-alcohols)

Acids A (di-acids)-Two types: unsaturated and saturated-(In polyesters crosslinking occurs at unsaturation sites)

G

G

G

G

A

A

A

A

A

Polyester polymer

Polyesters − specific molecules

COCCOCCCCOH

O O O

COCCCCOCCOC

O O O

C OH

Iso (meta)

Isophthalic Polyester

unsaturationunsaturation

CCOCCOOCO

CCCOH

OCC

C CC

CO

O

CC C C O

OC OC

C

CO C C

OH

C

Bisphenol A Fumaric Acid Polyester

Crosslinking occurs at the carbon-carbon double bonds the number of which can be increased when the polymer is made

Acid Acid Acid

Acid AcidAcid Acid

CH2―CH2―CH―CH2

OCH2―CH2―CH―CH2

O

H2C―CH―CH2―CH2

OH2C―CH―CH2―CH2

O

―CH2―C―CH2―│CH3

CH3

│N― ―N

―CH2―C―CH2―│CH3

CH3

│―O― ―O― CH2―CH―CH2

OO

CH2―CH―CH2 ( )n

Epoxies

Crosslinking occurs only at the epoxy rings

The tetra-functional epoxy has much greater crosslinking

Thermoplastics and Thermosets

• Melting vs. decomposition

Melted

Decomposed

Resin Rules• Impediment 3: Crystallinity

– Formed when polymers pack tightly together

Crystalline RegionAmorphous

Region

Covalent Bond (shared electrons)

Polymeric Molecules

Amorphous and Crystalline

Amorphous(random entanglement)

Semi-Crystalline or Crystalline(regular packing)

CrystallineRegions

Three-dimensional representation of a crystalline polymer

Crystalline region

Amorphous region

Liquid Crystal Polymer

Liquid stateSolid state

Advanced Thermoset Advanced Thermoplastics

Engineering Thermoset Engineering Thermoplastic

High temperature capabilitiesHigh CostHigh strengthHigh modulusGood fiber wet-outBrittle

High cost Solvent resistance High toughnessPoor wet-outHigh strength

Low costExcellent fiber wet-out Moderate strengthBrittle

Low costStandard TP mfgShort fibersModerate strengthGood toughness

Comparison of advanced and engineering thermoplastics and thermosets

Resin Rules• Impediment 4: Polarity

– Occurs when F, O, N, and Cl are present

PolarityN S S N

S N S N

...C C O

O

C...

d-

d-

+d

+d

O

HH

O

HH

C O+d +d d-d-

C O

Attacked by water molecules

Polyester

Bonding between fiberglass and resin

Fiberglass

Sizing (alkylsilane)

...O Si O Si O...

OH

OH

OH

OH

....C C O C C C

O

C C...

CH3 Si O C C C

CH3

C C C C...

CH3 Nonpolar regions (weak attraction)

d-

d-

+d

+d

+d − A highly polar molecule

− Largely non-polar regionPolyester

− Mixed polar/non-polar

―

―

―

―

―

―

0

2

1

3

4

5

6

Wei

ght g

ain

(%)

││ │ │ │

0 50 100 150 200 250

Exposure time (hours)

□□ □ □ □

Vinyl ester resin

●

●

●● ●

Polyester resin

Weight gain in water of polyester and vinyl ester resins

Resin Rules• Impediment 5: Aromaticity

– Presence of aromatic groups (containing benzene molecule or similar group)

– Aromatic rings are hard, flat objects– Aromatic objects increase stiffness of the polymer,

especially as they are integrated into the backbone or into a network

CC...C C...

CC

CC

C

C

C...OC

C

C

C

C

CC

C

C

C

C

C

C

OCC C

O

OH OH

OHOHOH

C

C

CC

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

....C C...

C.......C

a) Aromatic group (benzene)b) Polystyrene (pendant aromatic)

c) Epoxy (aromatic backbone)

d) Phenolic (aromatic network)

CC...C C...

CC

CC

C

C

H

H

H

H

H

H

Aromatic molecules

CC...C C...

CC

CC

C

C

An imide-based epoxy

│CH3 C―O

CC

CN

C

O

O

C―

―O― C―C―C

O

O― C―C―C

O

C C

CN

C

O

O

O―C

―C

O

O

C―C―C―O―

C―C―C―O

Super high thermal stability

Aromaticity

• Aromatic content increases the flame resistance of the polymer

• Aromatic content decreases the weathering resistance of the polymer

• Materials with little aromatic character are called aliphatic

10 20 30 40

Vinyl Ester

Epoxy

FR Polyester

Phenolic

(ASTM E-162 for thermosetcomposites)

Vinyl Ester

Epoxy

FR Polyester

Phenolic

(ASTM E-662 for thermosetcomposites)

100

Specific Optical DensityFlame Spread Index

200 300 400 500 600

Flammability

700-

600-

500-

400-

300-

200-

100-

0 2 4 6 8 10 12 14 16 18 20 22

NBS Smoke Chamber (Smoldering)

Epoxy

Polyester

Phenolic

OpticalDensity

Time (min)

Resin Rules• Resin rule 3: Materials containing halogen

atoms (F, Cl, Br, I) have good flame retardance properties.– Smoke evolution is increased with halogens

Halogenated polymers

)(n

)( n

C

Cl

C...C C...

...C C C C...

F F

FF

C

C

C

C

C

C

Br

BrBr

C...

Br

OCCC

O

Polyvinyl chloride (PVC)

Polytetrafluoroethylene (PTFE)

Brominated Epoxy

Reinforcements

Fiberglass AramidCarbon/Graphite

UHMWPEBasalt

Ceramic whiskers

Reinforcement Rules• Reinforcement rule 1: Fiberglass is the least

expensive of the major types of reinforcement and is often about the same strength as other major reinforcement types.– Fiberglass composites are often called FRP (for

fiberglass reinforced plastics)– Fiberglass is used in most composite applications

4 2000 4 80 40

5 1500 3 60 30

6 1000 2 40 20

1 500 1 20 10

0% 10% 20% 30% 40% 50%

Coeffi

cien

t of T

herm

al

Expa

nsio

n, p

pm/o F

(ppm

/oC)

Flex

Mod

ulus

(ksi

)

Izod

Impa

ct, ft

-lb/i

n (J

/mm

)

Elon

gatio

n (%

)

Tens

ileSt

reng

th (k

si)

CTEFlex ModulusIzod ImpactElongationTensile Strength

Scales for each property

Effects of fiber content on properties of nylon

Reinforcement Rules• Reinforcement rule 2: Carbon/graphite is the

stiffest of the common fiber reinforcements and generally has the best specific strength and specific stiffness.

Reinforcement Rules• Reinforcement rule 3: Aramid is the toughest of

the major types of composite reinforcements.

Reinforcement Rules• Reinforcement rule 4: Reinforcement forms can

be of several types depending on the type of manufacturing process that is used.

Reinforcement FormsTow (or roving if fiberglass)

Cloth fabric Preform

Mat

Prepreg

Manufacturing Rules• Manufacturing rule 1: Put the fibers where the

loads are going to be.

Filament Winding

Pultrusion

Manufacturing Methods

• Hand Lay-up (wet and prepreg)

PrepregWet

Manufacturing Methods

• Spray-up– Fibers are chopped, coated with resin and sprayed

onto the mold

Manufacturing Rules• Manufacturing rule 2: Control the temperature,

viscosity, and crosslinking reaction.

Curing ProfileTe

mpe

ratu

re (°

F)

Time

0

100

200

300

400

Begin heat after full pressurization of autoclave

Vacuum

130

355±10

120 minutes

1-5°F/minute (critical for precipitate)

Autoclave

Manufacturing Rules• Manufacturing rule 3: Compact the composite

during cure to ensure that the layers are properly adhering and that the air bubbles are reduced.

Pump Hydraulic pressure

Mold

Heated platen

Material charge

Heated platen

Ejector system

Compression Molding

Vacuum Bagging

• Provides for increased part consolidation• Reduces matched die mold costs

Autoclaves

Thermoplastic − Thermoforming

Blanks

Oven

Clamp

Clamping Pressing

Press (in two modes)

FinishedPart

Manufacturing Rules• Manufacturing rule 4: Composites are often part

of assemblies and great care must be used to properly finish the structure.

Sandwich Materials

Summary

• Composites have succeeded in current products from automobiles to bathtubs

Summary

• Composites have great promise for the future

Space Plane

X

Spaceport

Circling radius for landing

SummaryCategory Concept/Rule

Composites ― general 1 Composites are mixtures of two materials in which both materials retain some of their individual properties but also combine in such a way that the combined materials have some properties that are superior to either of the materials individually.

Composites ― general 2 While composites are certainly unique among structural materials, they are not super materials that have no disadvantages.

Composites ― general 3 Composites can be conveniently divided into two categories―advanced and engineering.

Composites ― general 4 Stealth properties come from controlling radiation detection.

Composites ― general 5 Some properties of the composite are dominated by the matrix.

Composites ― general 6 Some properties of the composite are dominated by the reinforcement.

Resin ― Thermal properties Thermal properties depend upon the difficulty with which polymer molecules separate from each other.

Resin ― Mechanical Mechanical properties depend upon the difficulty with which polymer molecules separate from each other (impediments).

Resin ― Mechanical ― Impediment 1 Molecular weight increases entanglement

Resin ― Mechanical ― Impediment 2 Crosslinks inhibit molecular motions.

Resin ― Mechanical ― Impediment 3 Crystallinity restricts molecular movement.

Resin ― Mechanical ― Impediment 4 Polarity restricts molecular motion.

Resin ― Mechanical ― Impediment 5 Aromatic groups reduce molecular flexibility.

Resin ― Flammability Materials containing halogen atoms (F, Cl, Br, I) have good fire retardance properties.

SummaryCategory Concept/Rule

Reinforcement 1 Fiberglass is the least expensive of the major types of reinforcement and is often about the same strength as other major reinforcement types.

Reinforcement 2 Carbon/graphite is the stiffest of the common fiber reinforcements and generally has the best specific strength and specific stiffness.

Reinforcement 3 Aramid is the toughest of the major types of composite reinforcements.

Reinforcement 4 Reinforcement forms can be of several types depending on the type of manufacturing process that is used.

Manufacturing 1 Put the fibers where the loads are going to be.

Manufacturing 2 Control the temperature, viscosity, and crosslinking reaction.

Manufacturing 3 Compact the composite during cure to ensure that the layers are properly adhering and that the air bubbles are reduced.

Manufacturing 4 Composites are often part of assemblies and great care must be used to properly finish the structure.

Thank You

Composites are dynamic!