Textile Structural Composites Yiping Qiu College of Textiles Donghua University Spring, 2006

Textile Structural Composites

Yiping Qiu

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 Applications

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


Classifications Fibers

Length short fiber reinforced continuous fiber reinforced

Composition Single fiber type Hybrid

Mechanical properties Conventional Flexible


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

0

2

4

6

8S

pec

ific

gra

vity

(g

/cc)

Ste

el

Al a

lloy

Ti a

lloy

Car

bon/

epox

y

Kev

lar/

epox

y

materials

Comparison of specific gravities

0

200

400

600

800

1000

1200

1400

Te

ns

ile

str

en

gth

(M

Pa

)

Ste

el

Al a

lloy

Ti a

lloy

Ca

rbo

n/e

po

xy

Ke

vla

r/e

po

xy

Materials

Comparison of tensile strength

0

4

8

12

16M

od

ulu

s t

o w

eig

ht

rati

o (

109

m)

Ste

el

Al allo

y

Ti allo

y

Carb

on/e

poxy

Kevla

r/epoxy

Materials

Comparison of modulus to weight ratio

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

Property Maps Merit index


Merit index Example for tensile stiffness of a beam

However, for a given tensile sample, tensile stiffness has nothing to do with length or L = 1 may be assumed

1 when

LA

W

AL

W

ALVW

W

FE

W

F

W

F

A

FE


How about for torsion beams and bending plates? Lets make the derivation of these our first homework.

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

Mainly two components: Fibers Matrices


Fibers Influences:

Specific gravity, Tensile and compressive strength and

modulus, Fatigue properties, Electrical and thermal properties, Cost.


Fibers 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)


Fibers Inorganic fibers:

Glass fibers: S-glass and E-glass Carbon or graphite fibers: from PAN and Pitch Ceramic fibers: Boron, SiC, Al2O3

Metal fibers: steel, alloys of W, Ti, Ni, Mo etc. (high melting temperature metal fibers)


Most frequently used fibers Glass Carbon/graphite PPTA (Kevlar, etc.) Polyethylene (Spectra) Polyester (Vectra)


Carbon fibers Manufacturing processes Structure and properties


Carbon fibers Manufacturing processes

Thermal decomposition of fibrous organic precursors: PAN and Rayon Extrusion of pitch fibers


Carbon fiber manufacturing processes Thermal decomposition of fibrous organic

precursors Rayon fibers

Rayon based carbon fibers Stabilization at 400°C in O2, 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%)


Carbon fiber manufacturing processes Thermal decomposition of fibrous organic

precursors PAN (polyarylonitrile) based carbon fibers

PAN fibers (CH2-CH(CN)) Stabilization at 200-300°C in O2, 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


Pitch based carbon fibers 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


Carbon fibers Advantages

High strength Higher modulus Nonreactive

Resistance to corrosion High heat resistance high tensile strength at elevated temperature

Low density


Carbon fibers Disadvantages

High cost Brittle


Carbon fibers Other interesting properties

Lubricating properties Electrical conductivity Thermal conductivity Low to negative thermal expansion coefficient


Carbon fibers heat treatment below 1700°C

less crystalline and lower modulus (<365 GPa)

Graphite fibers heat treatment above 1700°C

More crystalline (~80%) and higher modulus (>365GPa)


Glass fibers Compositions and properties Advantages and disadvantages


Glass fibers Compositions and Structures

Mainly SiO2 +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


Glass fibers Types and Properties

E-glass (for electric) draws well good strength & stiffness good electrical and weathering properties



C-glass (for corrosion) good resistance to corrosion low strength



S-glass (for strength) high strength & modulus high temperature resistance more expensive than E


Properties of Glass fibers

fibers Tensilestrength(MPa)

TensileModulus(GPa)

Coeff. OfThermalExpension10-6/K

DielectricConst. (a)

E-glass 3450 72.5 5.0 6.3

S-glass 4590 86.0 5.6 5.1


Glass fibers Production

Melt spinning

Materials for fiber reinforced composites Glass fibers

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


Glass fibers Advantages

high strength same strength and modulus in transverse direction

as in longitudinal direction low cost


Glass fibers disadvantages

relatively low modulus high specific density (2.62 g/cc) moisture sensitive


Kevlar fibers Structure

Polyamide with benzene rings between amide groups

Liquid crystalline Planar array and pleated system


Kevlar fibers Types

Kevlar 29, E = 50 GPa Kevlar 49, E = 125 GPa Kevlar 149, E = 185 GPa


Kevlar fibers Advantages

high strength & modulus low specific density (1.47g/cc) relatively high temperature resistance


Kevlar fibers Disadvantages

Easy to fibrillate poor transverse properties susceptible to abrasion


Spectra fibers

Structure: (CH2CH2)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


Spectra fibers Disadvantages

poor adhesion to matrix high creep low melting temperature


Other fibers SiC and Boron

Production Chemical Vapor Deposition (CVD)

Monofilament Carbon or Tungsten core heated by passing an

electrical current Gaseous carbon containing silane


SiC Production

Polycarbosilane (PCS) Multi-filaments polymerization process to produce precursor PCS pyrolised at 1300ºC

Whiskers Small defect free single crystal


Particulate small aspect ratio high strength and modulus mostly cheap


The strength of reinforcements Compressive strength Fiber fracture and flexibility Statistical treatment of fiber strength


The strength of reinforcements Compressive strength

(Mainly) Euler Buckling

22

* 16

L

dEb

2L

EIcP


The strength of reinforcements Factors determining compressive strength

Matrix material Fiber diameter or aspect ratio (L/d) fiber properties

carbon & glass >> Kevlar


The strength of reinforcements Fiber fracture

Mostly brittle e.g. Carbon, glass, SiC

Some ductile e.g. Kevlar, Spectra

Fibrillation e.g. Kevlar


The strength of reinforcements Fiber flexibility

How easy to be bent Moment required to bend a round fiber:

64

4dEEIM

E = Young’s Modulus

d = fiber diameter

= curvature


The strength of reinforcements Fiber failure in bending

Stress on surface Tensile stress:

2

dE

E = Young’s Modulus

d = fiber diameter

= curvature



Stress on surface Maximum curvature

Ed*

max

2

* = fiber tensile strength



When bent, many fibers fail in compressionKevlar forms kink bands


Statistical treatment of fiber strength Brittle materials: failure caused by random

flaw don’t have a well defined tensile strength presence of a flaw population

Statistical treatment of fiber strengthPeirce (1928): divide a fiber into incremental

lengths

NLLLLL 321


Statistical treatment of fiber strength 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.


Statistical treatment of fiber strength Peirce’s experiment

Experimental verification:

variationoft Coefficien

oflength a fiber with ofStrength

oflength a fiber with ofStrength

)1(2.41/ 5/1

CV

l

nl

CVn

l

nl

lnl


Statistical treatment of fiber strength Weakest Link Theory (WLT)

define n = No. of flaws per unit length causing failure under stress .

For the first element, the probability of failure

11 LnPf

The probability for the fiber to survive

)1()1)(1( 21 fNffs PPPP


Statistical treatment of fiber strength Weakest Link Theory (WLT)

If the length of each segment is very small, then Pfi are all very small, Therefore (1-Pfi) exp(-Pfi)

The probability for the fiber to survive

)](exp[ 21 fNffs PPPP

)exp()](exp[ 21 LnLnLnLn N


Statistical treatment of fiber strength Weibull distribution of fiber strength

Weibull’s assumption:

m

nL

00

m = Weibull shape parameter (modulus).

0 = Weibull scale parameter, characteristic

strength.

L0 = Arbitrary reference length.



Thus

m

f L

LP

00

exp1



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.



Plot of fiber strength or failure strain data let m

s L

LP

00

)ln(

m

s L

L

P

00

1ln

00 lnlnlnln1

lnln mmLLPs

Statistical treatment of fiber strength

Example Estimate number of fibers fail at a gage

length twice as much as the gage length in single fiber test

L/L0 = 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

Properties minimum desired Typicalepoxy

Tensile strength(MPa)

70 >100 ---

Modulus (GPa) 2.0 >3.0 3.8

Ultimate Strain(%)

5 >10 1 - 2

Glass transitiontemperature (C)

121 >177 121

PolymerTarget net resin properties

Epoxy resins

Starting materials: Low molecular weight organic compounds

containing epoxide groups

Epoxy Resins

Types of 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 -NH2

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 Chemical reactions

Epoxy resin Chemical reactions

Epoxy resins Curing

Homopolymerization: Addition polymerization: a catalyst or initiator required:

eg. Tertiary amines and BF3 compounds Less -OH groups formed Typical properties of addition polymers

Combination of catalyst with hardeners

Epoxy Resins Reaction of homopolymerization

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 20ºK

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 ~ 200ºC

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 ApplicationsEngine parts in aerospace industry

Phenolic resins Prepared through condensation

polymerization between phenol and formaldehyde.

Large quantity of Water generated (up to 25%) leading to high void content

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

Additional reading assignment: reserved: Gillham, J.K., Formation and

Properties of Thermosetting and High Tg Polymeric Materials, Polymer Engineering and Science, 26, 1986, p1429-1431



Important concepts Gelation

formation of an infinite network sol and gel coexist

Vitrification Tg rises to isothermal temperature of cure Tcure > Tg, rubbery material Tcure < Tg, glassy material After vitrification, conversion of monomer

almost ceases.


Important concepts 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


Important concepts Three critical temperatures:

Tg - Tg of cured system gelTg - Tg of gel Tgo - Tg of reactants


Discussion Ungelled glassy state is good for

commercial molding compounds Tgo > Tprocessing, processed as solid Tgo < Tprocessing, processed as liquid

Store temperature < gelTg to avoid gelation Resin fully cured when Tg = Tg Tg > Tcure about 40ºC Full cure is achieved most readily by cure at

T > Tg and slowly at T < Tg.

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 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 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(CH2)4COOH, 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: self-polymerization 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


examples: Conventional

Nylon Polyethylene Polypropylene Polycarbonate Polyester PMMA

Thermoplastic matrices examples:

Advanced (e.g.)

Thermoplastic matrices examples:

Advanced (e.g.) Polyimide



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

Advantage of metal matrix composites (MMC) 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

Advantage of MMC 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

Disadvantage of MMC 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 ceramicsSiC, Si3N4, Al2O3, ZrO2

fully crystalline Cement and concrete Carbon/carbon

Ceramic Matrices

Increased toughness through deflected crack propagation on fiber/matrix interface.

Example: Carbon/carbon composites

Documents

Textile Structural Composites Yiping Qiu College of Textiles Donghua University Spring, 2006