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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!
