Textile Structural Composites
College of Textiles
Donghua University
Spring, 2006

Reading Assignment



Textbook chapter 1 General Information.
High-Performance Composites: An Overview,
High-Performance Composites , 7-19, 2003
Sourcebook.
FRP Materials, Manufacturing Methods and
Markets, Composites Technology , Vol. 6(3) 6-20,
2000.


Expectations
At the conclusion of this section, you should be able to:
 Describe the advantages and disadvantages of fiber reinforced composite materials vs. other materials


Describe the major applications of fiber reinforced composites
Classification of composites

Introduction




What is a composite material?
 Two or more phases with different properties
Why composite materials?
 Synergy
History
Current Status

Introduction






Automotive
Marine
Civil engineering
Space, aircraft and military
Sports

Applications in plane

Fiber reinforced composite materials
 Classifications according to:
 Matrices
 Polymer

Thermoplastic

Thermoset

Metal

Ceramic
 Others

Fiber reinforced composite materials
 Classifications
 Fibers

Length
 short fiber reinforced
 continuous fiber reinforced
 Composition

Single fiber type

Hybrid
 Mechanical properties

Conventional
 Flexible

Fiber reinforced composite materials
 Advantages
 High strength to weight ratio
 High stiffness to weight ratio
 High fatigue resistance
 No catastrophic failure
 Low thermal expansion in fiber oriented directions
 Resistance to chemicals and environmental factors

Comparison of specific gravities
8
6
4
2
0 materials

1400
1200
1000
800
600
400
200
0
Comparison of tensile strength
Materials

16
12
8
4
0
Materials

Fiber reinforced composite materials
 Disadvantages
 Good properties in one direction and poor properties in other directions.
 High cost due to expensive material and complicated fabrication processes.
 Some are brittle, such as carbon fiber reinforced composites.
 Not enough data for safety criteria.

Design of Composite Materials



Design of Composite Materials


W

V
 
AL

 
W
AL

W
A
when L

1
 However, for a given tensile sample, tensile stiffness has nothing to do with length or L = 1 may be assumed
 E




F
A


F

W
  

F

W


E


F
W


Design of Composite Materials


Major components for fiber-reinforced composites
 Reading assignment:

Textbook Chapter 2 Fibers and matrices
 Fibers
 Share major portion of the load
 Matrix



To transfer stress between the fibers
To provide a barrier against an adverse environment
To protect the surface of the fibers from mechanical abrasion

Major components for fiber reinforced composites
 Coupling agents and coatings
 to improve the adhesion between the fiber and the matrix
 to protect fiber from being reacted with the matrix or other environmental conditions such as water moisture and reactive fluids.
 Fillers and other additives:
 to reduce the cost,
 to increase stiffness,
 to reduce shrinkage,
 to control viscosity,
 to produce smoother surface.

Materials for fiber reinforced composites


Fibers
Matrices

Materials for fiber reinforced composites

 Influences:





Specific gravity,
Tensile and compressive strength and modulus,
Fatigue properties,
Electrical and thermal properties,
Cost.

Materials for fiber reinforced composites

 Fibers used in composites
 Polymeric fibers such as
 PE (Spectra 900, 1000)

PPTA: Poly(para-phenylene terephthalamide) (Kevlar
29, 49, 149, 981, Twaron)
 Polyester (Vectran or Vectra)
 PBZT: Poly( p -phenylene benzobisthiozol)

Materials for fiber reinforced composites
 Fibers
 Inorganic fibers:
 Glass fibers: S-glass and E-glass
 Carbon or graphite fibers: from PAN and Pitch


Ceramic fibers: Boron, SiC, Al
2
O
3
Metal fibers: steel, alloys of W, Ti, Ni, Mo etc.
(high melting temperature metal fibers)

Materials for fiber reinforced composites






Glass
Carbon/graphite
PPTA (Kevlar, etc.)
Polyethylene (Spectra)
Polyester (Vectra)

Materials for fiber reinforced composites



Manufacturing processes
Structure and properties

Materials for fiber reinforced composites

 Manufacturing processes
 Thermal decomposition of fibrous organic precursors:
 PAN and Rayon
 Extrusion of pitch fibers

Materials for fiber reinforced composites



Thermal decomposition of fibrous organic precursors
Rayon fibers
 Rayon based carbon fibers
 Stabilization at 400 ° C in O
2
, depolymerization & aromatization
 Carbonization at 400-700 ° C in an inert atmosphere
 Stretch and graphitization at 700-2800 ° C (improve orientation and increase crystallinity by 30-50%)

Materials for fiber reinforced composites

 Thermal decomposition of fibrous organic precursors
 PAN (polyarylonitrile) based carbon fibers
 PAN fibers (CH2-CH(CN))
 Stabilization at 200-300 ° C in O
2
, depolymerization & aromatization, converting thermoplastic PAN to a nonplastic cyclic or ladder compound (CN groups combined and CH2 groups oxidized)
 Carbonization at 1000-1500 ° C in an inert atmosphere to get rid of noncarbon elements (O and N) but the molecular orientation is still poor.
 Stretch and graphitization at >1800 ° C, formation of turbostratic structure

Materials for fiber reinforced composites




 pitch high molecular weight byproduct of distillation of petroleum heated >350 ° C, condensation reaction, formation of mesophase (LC) melt spinning into pitch fibers conversion into graphite fibers at
~2000 ° C

Materials for fiber reinforced composites

 Advantages
 High strength
 Higher modulus
 Nonreactive
 Resistance to corrosion
 High heat resistance
 high tensile strength at elevated temperature
 Low density

Materials for fiber reinforced composites

 Disadvantages
 High cost
 Brittle

Materials for fiber reinforced composites

 Other interesting properties
 Lubricating properties
 Electrical conductivity
 Thermal conductivity
 Low to negative thermal expansion coefficient

Materials for fiber reinforced composites


 heat treatment below 1700 ° C
 less crystalline
 and lower modulus (<365 GPa)
 heat treatment above 1700 ° C
 More crystalline (~80%) and
 higher modulus (>365GPa)

Materials for fiber reinforced composites



Compositions and properties
Advantages and disadvantages

Materials for fiber reinforced composites
 Glass fibers
 Compositions and Structures
 Mainly SiO
2
+oxides of Ca, B, Na, Fe, Al
 Highly cross-linked polymer
 Noncrystaline
 No orientation
 Si and O form tetrahedra with Si centered and O at the corners forming a rigid network
 Addition of Ca, Na, & K with low valency breaks up the network by forming ionic bonds with O
  strength and modulus

Microscopic view of glass fiber
Cross polar
First order red plate

Materials for fiber reinforced composites

 Types and Properties
 E-glass (for electric)
 draws well
 good strength & stiffness
 good electrical and weathering properties

Materials for fiber reinforced composites

 Types and Properties
 C-glass (for corrosion)
 good resistance to corrosion
 low strength

Materials for fiber reinforced composites

 Types and Properties
 S-glass (for strength)
 high strength & modulus
 high temperature resistance
 more expensive than E

Materials for fiber reinforced composites fibers

E-glass
Tensile strength
(MPa)
3450
Tensile
Modulus
(GPa)
72.5
Coeff. Of
Thermal
Expension
10
-6
/K
5.0
Dielectric
Const. (a)
6.3
S-glass 4590 86.0
5.6
5.1

Materials for fiber reinforced composites

 Production
 Melt spinning

Materials for fiber reinforced composites

 sizing:
 purposes
 protest surface
 bond fibers together
 anti-static
 improve interfacial bonding
 Necessary constituents
 a film-forming polymer to provide protecting
 e.g. polyvinyl acetate
 a lubricant
 a coupling agent: e.g. organosilane

Materials for fiber reinforced composites

 Advantages
 high strength
 same strength and modulus in transverse direction as in longitudinal direction
 low cost

Materials for fiber reinforced composites

 disadvantages
 relatively low modulus
 high specific density (2.62 g/cc)
 moisture sensitive

Materials for fiber reinforced composites

 Structure
 Polyamide with benzene rings between amide groups
 Liquid crystalline
 Planar array and pleated system

Materials for fiber reinforced composites

 Types
 Kevlar 29, E = 50 GPa
 Kevlar 49, E = 125 GPa
 Kevlar 149, E = 185 GPa

Materials for fiber reinforced composites

 Advantages
 high strength & modulus
 low specific density (1.47g/cc)
 relatively high temperature resistance

Materials for fiber reinforced composites

 Disadvantages
 Easy to fibrillate
 poor transverse properties
 susceptible to abrasion

Materials for fiber reinforced composites



Structure: (CH
2
CH
2
) n
 Linear polymer - easy to pack
 No reactive groups
Advantages
 high strength and modulus
 low specific gravity
 excellent resistance to chemicals
 nontoxic for biomedical applications

Materials for fiber reinforced composites

 Disadvantages
 poor adhesion to matrix
 high creep
 low melting temperature

Materials for fiber reinforced composites

 SiC and Boron
 Production

Chemical Vapor Deposition (CVD)

Monofilament

Carbon or Tungsten core heated by passing an electrical current

Gaseous carbon containing silane

Materials for fiber reinforced composites
 SiC
 Production
 Polycarbosilane (PCS)

Multi-filaments

 polymerization process to produce precursor
PCS pyrolised at 1300 º C

Whiskers

Small defect free single crystal

Materials for fiber reinforced composites



 small aspect ratio high strength and modulus mostly cheap

Materials for fiber reinforced composites




Compressive strength
Fiber fracture and flexibility
Statistical treatment of fiber strength

Materials for fiber reinforced composites

 Compressive strength
 (Mainly) Euler Buckling
P
 c
EI
L
2

* b


2
E
16 

 d
L 

 2

Materials for fiber reinforced composites

 Factors determining compressive strength
 Matrix material
 Fiber diameter or aspect ratio (L/d)
 fiber properties
 carbon & glass >> Kevlar

Materials for fiber reinforced composites

 Fiber fracture
 Mostly brittle
 e.g. Carbon, glass, SiC
 Some ductile
 e.g. Kevlar, Spectra
 Fibrillation
 e.g. Kevlar

Materials for fiber reinforced composites

 Fiber flexibility
 How easy to be bent

Moment required to bend a round fiber:
M

EI
 
E
 d
4
64

E
= Young’s Modulus d = fiber diameter

= curvature

Materials for fiber reinforced composites

 Fiber failure in bending
 Stress on surface

Tensile stress:
 
E
 d
2
E
= Young’s Modulus d = fiber diameter

= curvature

Materials for fiber reinforced composites

 Fiber failure in bending
 Stress on surface

Maximum curvature
  max
2

Ed
*

*
= fiber tensile strength

Materials for fiber reinforced composites

 Fiber failure in bending
 When bent, many fibers fail in compression
 Kevlar forms kink bands

Materials for fiber reinforced composites



Brittle materials: failure caused by random flaw
 don
’ t have a well defined tensile strength
 presence of a flaw population
Statistical treatment of fiber strength
 Peirce (1928): divide a fiber into incremental
L lengths
 
L
1
 
L
2
 
L
3
   
L
N

Materials for fiber reinforced composites

 Peirce
’ s experiment
 Hypothesis:
 The longer the fiber length, the higher the probability that it will contain a serious flaw.

Longer fibers have lower mean tensile strength.

Longer fibers have smaller variation in tensile strength.

Materials for fiber reinforced composites

 Peirce
’ s experiment
 Experimental verification:

/
 
1

4 .
2 ( 1
 n

1 / 5
) CV nl l

 l nl

Strength

Strength of fiber with of fiber with a length a length of nl of l
CV

Coefficien t of variation

Materials for fiber reinforced composites

 Weakest Link Theory (WLT)
 define n

= No. of flaws per unit length causing failure under stress

.
 For the first element, the probability of failure
P f 1
 n


L
1
The probability for the fiber to survive
P s

( 1

P f 1
)( 1

P f 2
)  ( 1

P fN
)

Materials for fiber reinforced composites

 Weakest Link Theory (WLT)
 If the length of each segment is very small, then
P fi
 are all very small,
Therefore (1P fi
)
 exp(P fi
)
 The probability for the fiber to survive
P s
 exp[

( P f 1

P f 2
  
P fN
)]
 exp[

( n


L
1
 n


L
2
   n


L
N
)]
 exp(

Ln

)

Materials for fiber reinforced composites

 Weibull distribution of fiber strength
 Weibull
’ s assumption:
L
0 n






0

 m m = Weibull shape parameter (modulus).

0
= Weibull scale parameter, characteristic strength.
L
0
= Arbitrary reference length.

Materials for fiber reinforced composites

 Weibull distribution of fiber strength
 Thus
P f

1
 exp






L
L
0






0

 m




Materials for fiber reinforced composites

 Weibull distribution of fiber strength
 Discussion:
 Shape parameter ranges 2-20 for ceramic and many other fibers.




The higher the shape parameter, the smaller the variation.
When

<

0
, the probability of failure is small if m is large.
When
  
0
, failure occurs.
Weibull distribution is used in bundle theory to predict fiber bundle and composite strength.

Materials for fiber reinforced composites

 Weibull distribution of fiber strength
 Plot of fiber strength or failure strain data
 let ln( P s
)
 


L
L
0






0

 m ln


1
P s





L
L
0






0

 m ln

 ln



1
P s





 ln
 
 ln
 
0
 m ln
 
 m ln
 
0

Statistical treatment of fiber strength



Estimate number of fibers fail at a gage length twice as much as the gage length in single fiber test
L/L
0
= 2

Matrices
 Additional reading assignment:
 Jones, F.R., Handbook of Polymer-
Fiber Composites, sections:
 2.4-2.6, 2.9, 2.10, 2.12.

Matrices



Polymer
Metal
Ceramic

Matrices
 Polymer
 Thermosetting resins
 Epoxy
 Unsatulated polyester
 Vinyl ester
 high temperature:

Polyimides

Phenolic resins

Matrices
Polymer
Target net resin properties
Properties minimum
Tensile strength
(MPa)
70
Modulus (GPa) 2.0
Ultimate Strain
(%)
Glass transition temperature (

C)
5
121 desired
>100
>3.0
>10
>177
Typical epoxy
---
3.8
1 - 2
121

Epoxy resins
 Starting materials:
 Low molecular weight organic compounds containing epoxide groups

Epoxy Resins


Epoxy resins
 Types of epoxy resin
 bifuctional: diglycidyl ether of bisphenol A
 a distribution of monomers
 n is fractional:
 effect of n

 molecular weight
  viscosity
  curing temp.

 distance between crosslinks
 
Tg &
 ductility


-OH
  moisture absorption

Epoxy resins
 Types of epoxy resin (cont.)
 Trifunctional (glycidyl amines)
 Tetrafunctional
 higher functionality
 potentially higher crosslink densities

 higher Tg
Less -OH groups
  moisture absorption

Epoxy resins
 Curing
 Copolymerization:


A hardener required: e.g. DDS, DICY
Hardeners have two active “ H ” atoms to add to the epoxy groups of neighboring epoxy molecules, usually from -
NH
2


Formation of -OH groups: moisture sensitive
Addition polymerization: No small molecules formed
 no volatile formation
 Stoichiometric concentration used, phr: part per hundred
(parts) of resin

Epoxy resin
 Major ingredients: epoxy resin and curing agent


Epoxy resin


Epoxy resin

Epoxy resins
 Curing
 Homopolymerization:


Addition polymerization: a catalyst or initiator required: eg. Tertiary amines and BF
3 compounds
Less -OH groups formed

Typical properties of addition polymers
 Combination of catalyst with hardeners

Epoxy Resins


Epoxy resins

 Mechanical and thermomechanical properties
 Effect of curing agent on mechanical properties
 Heat distortion temperature (HDT)

 measured as temperature at which deflection of 0.25 mm of 100 mm long bar under 0.455 MPa fiber stress occurs.
related but

Tg
 Moisture absorption: 1% decrease Tg by 20
º
K

Polyimides
 Largest class of high temperature polymers in composites
 Types
 PMR (polymerization of monomeric reactants)
 polyimides are insoluble and infusible.
 in situ condensation polymerization of monomers in a solvent
 2 stage process:

 first stage to form imidized prepolymer of oligomer and volatile by-products removed using autoclave or vacuum oven.
Second stage: prepolymer is crosslinked via reaction of the norbornene end cap under high pressure and temperature (316ºC and 200 psi)

Polyimides
 Types
 bis-imides (derived from monomers with 2 preformed imide groups).


Typical BMI (bismaleimides)
Used for lower temperature range ~ 200
º
C

Polyimides
 Properties
(show tables)

Polyimides



Advantages:
 Heat resistant
Drawbacks:
 toxicity of constituent chemicals (e.g. MDA)
 microcracking of fibers on thermal cycling
 high processing temperature
Typical Applications
 Engine parts in aerospace industry

Phenolic resins



Phenolic resins


Advantages:
 High temperature stability



Chemical resistance
Flame retardant
Good electrical properties
Typical applications


Offshore structures
Civil engineering



Marine
Auto parts: water pumps, brake components pan handles and electric meter cases

Time-temperature-transformation diagrams for thermosets resins

 reserved: Gillham, J.K., Formation and
Properties of Thermosetting and High Tg
Polymeric Materials, Polymer Engineering and Science , 26, 1986, p1429-1431

Time-temperature-transformation diagrams for thermosets resins

Time-temperature-transformation diagrams for thermosets resins



Gelation
 formation of an infinite network
 sol and gel coexist
Vitrification
 Tg rises to isothermal temperature of cure
 T cure
> Tg, rubbery material
 T cure
< Tg, glassy material
 After vitrification, conversion of monomer almost ceases.

Time-temperature-transformation diagrams for thermosets resins



Devitrification
 Tg decreases through isothermal temperature of cure due to degradation
 degradation leads to decrosslink and formation of plasticizing materials
Char or vitrification
 due to increase of crosslink and volatilization of low molecular weight plasticizing materials

Time-temperature-transformation diagrams for thermosets resins

 Three critical temperatures:
 T g
 - Tg of cured system
 gel
T g -
Tg of gel
 T go
- Tg of reactants

Time-temperature-transformation diagrams for thermosets resins






Ungelled glassy state is good for commercial molding compounds
 T go
> T processing
, processed as solid
 T go
< T processing
, processed as liquid
Store temperature < gel
T g to avoid gelation
Resin fully cured when T g = T g

Tg > T cure about 40
º
C
Full cure is achieved most readily by cure at
T > T g
 and slowly at T < T g
 .

Unsaturated polyester



Reading assignment
Mallick, P.K., Fiber Reinforced Composites .
Materials, Manufacturing and Design, pp56-64.
Resin:


Products of condensation polymerization of diacids and diols
 e.g. Maleic anhydride and ethylene glycol
Strictly alternating polymers of the type A-B-A-B-A-B
 At least one of the monomers is ethylenically unsaturated

Unsaturated polyester

Unsaturated polyester

Unsaturated polyester
 Cross-linking agent




Reactive solvent of the resin: e.g. styrene
Addition polymerization with the resin molecules: initiator needed, e.g. peroxide
Application of heat to decompose the initiator to start addition polymerization an accelerator may be added to increase the decomposition rate of the initiator.

Unsaturated polyester


Unsaturated polyester
Factors to control properties
 Cross-linking density:
 addition of saturated diacids as part of the monomer for the resin: e.g phthalic anhydrid, isophthalic acid and terephthalic acid
 as ratio of saturated acids to unsaturated acids increases, strength and elongation increase while HDT decreases


Unsaturated polyester
Factors controlling properties



Type of acids
 Terephthalic acids provide higher HDT than the other two acids due to better packing of molecules
 nonaromatic acid: adipic acid HOOC(CH
2
)
4
COOH, lowers stiffness
Resin microstructure:
 local extremely high density of cross-links.
Type of diols
 larger diol monomer: diethylene glycol
 bulky side groups

Unsaturated polyester
 Factors to control properties
 Type of crosslinking agent
 amount of styrene: more styrene increases the distance of the space of neighboring polyester molecules
 lower modulus
 Excessive styrene: selfpolymerization
 formation of polystyrene
 polystyrene-like properties

Unsaturated polyester


Advantages



Low viscosity
Fast cure
Low cost
Disadvantages

 lower properties than epoxy large mold shrinkage
 sink marks
 an incompatible thermoplastic mixed into the resin to form a dispersed phase in the resin
 “ low profile ” system

Vinyl ester


Resin:


Products of addition polymerization of epoxy resin and an unsaturated carboxylic acid (vinyl) unsaturated C=C bonds are at the end of a vinyl ester molecule
 fewer cross-links
 more flexible
Cross-linking agent
 The polymer is dissolved in styrene



Addition polymerization to form cross-links
Formation of a gigantic molecule
Similar curing reaction as unsaturated polyester resin

Vinyl ester

Vinyl ester



Vinyl ester
Advantages


 epoxy-like:
 excellent chemical resistance
 high tensile strength polyester-like:
 Low viscosity
 Fast curing
 less expensive good adhesion to glass fibers due to existence of -OH
Disadvantages:
 Large volumetric shrinkage (5
–
10 %)

Vinyl ester

Advantages of thermosetting resins




High strength and modulus.
Less creep and stress relaxation
Good resistance to heat and chemicals
Better wet-out between fibers and matrix due to low viscosity before cross-linking

Disadvantages of thermosetting resins





Limited storage life
Long time to cure
Low strain to failure
Low impact resistance
Large shrinkage on curing

Thermoplastic matrices
 Reading assignment:

Mallick, P.K., Fiber Reinforced Composites . Materials,
Manufacturing and Design, section 2.4 pp 64-69.
 Types:

Conventional: no chemical reaction during processing

Semi-crystalline

Liquid crystal

Amorphous

Pseudothermoplastics: molecular weight increase and expelling volatiles

Thermoplastic matrices

 Conventional
 Nylon
 Polyethylene
 Polypropylene
 Polycarbonate
 Polyester
 PMMA

Thermoplastic matrices

 Advanced (e.g.)

Thermoplastic matrices

 Advanced (e.g.)
 Polyimide

Thermoplastic matrices



Main descriptors:
 Linear
 Repeatedly meltable
Properties and advantages of thermoplastic matrices
 High failure strain
 High impact resistance
 Unlimited storage life at room temperature
 Short fabrication time
 Postformability (thermoforming)
 Ease of repair by welding, solvent bonding
 Ease of handling (no tackiness)

Disadvantages of thermoplastic matrices




High melt or solution viscosity (high MW)
Difficult to mix them with fibers
Relatively low creep resistance
Low heat resistance for conventional thermoplastics

Metal Matrices


Examples
 Al, Ti, Mg, Cu and Super alloys
Reinforcements:
 Fibers: boron, carbon, metal wires
 Whiskers
 Particulate

Metal Matrices
 Fiber matrix interaction
 Fiber and matrix mutually nonreactive and insoluble
 Fiber and matrix mutually nonreactive but soluble
 Fiber and matrix react to form compounds at interface

Metal Matrices

 Versus unreinforced metals
 higher strength to density ratio
 better properties at elevated temperature
 lower coefficient of thermal expansion
 better wear characteristics
 better creep performance

Metal Matrices

 Versus polymeric matrix
 better properties at elevated temperature
 higher transverse stiffness and strength
 moisture insensitivity
 higher electrical and thermal conductivity
 better radiation resistance
 less outgassing contamination

Metal Matrices

 higher cost
 high processing temperature
 relatively immature technology
 complex and expensive fabrication methods with continuous fiber reinforcements
 high specific gravity compared with polymer
 corrosion at fiber matrix interface (high affiliation to oxygen)
 limited service experience

Ceramic Matrices




Glass ceramics
 glass forming oxides, e.g. Borosilicates and aluminosilicates
 semi-crystalline with lower softening temperature
Conventional ceramics
 SiC, Si
3
N
4
, Al
2
O
3
, ZrO
2
 fully crystalline
Cement and concrete
Carbon/carbon

Ceramic Matrices


Increased toughness through deflected crack propagation on fiber/matrix interface.
Example: Carbon/carbon composites