05 Composite Materials SZK

8/4/2019 05 Composite Materials SZK

1/39

Materials Science

05-Composite Materials

By Dr Sohaib Z Khan

[email protected]


2/39

Materials Science

Materials Science by Dr Sohaib Z Khan Spring 2011

Introduction

Main text book for this topic: Principles of Materials

Science and Engineering, Chapter 13 2ed by

William F. Smith. McGraw-Hill publishing

There is no widely accepted definition of composite

materials

A composite material is a materials system composedof a mixture or combination of two or more micro-or

macro-constituents that differ in form and chemical

composition and which are essentially insoluble in each

other

A Composite as something made-up of distinct parts (or

constituents


3/39

Materials Science


Introduction

The engineering importance of a composite is that two or

more distinctly different materials combine together toform a composite material which possesses properties

that are superior, or important in some other manner, to

the properties of the individual components

Examples: Fiber-reinforce plastics, concrete, wood


4/39

Materials Science


Fibers for Reinforced-Plastic Composite Materials

Generally three main types of synthetic fibers are used

to reinforce plastic material: glass,aramid, and carbon

Glass fiber is by far the most widely used reinforcement

fiber and is the lowest in cost. Aramid and carbon fibers

have high strengths and low densities and so are used in

many applications, particularly aerospace, in spite oftheir high cost


5/39

Materials Science


Glass fibers for Reinforcing Plastic Resins

Glass fibers are used to reinforce plastic matrices to

form structural components and molding compounds

Glass fiber plastic composite materials have the

following favorable characteristics: high strength to

weight ratio, good dimensional stability, good resistance

to heat, cold, moisture, and corrosion, good electricalinsulation properties, ease of fabrication, and relatively

low cost

The two most important types of glass used to produce

glass fiber for composites are E (electrical) and S (high-strength) glasses


6/39

Materials Science



E-glass: is the most commonly used glass for

continuous fibers

Basically, E-glass is lime-aluminum-borosilicate glass

with zero or low sodium and potassium levels

The basic composition of E-glass ranges from 52-

56%SiO2 12-16%Al2O3, 16-25%CaO, and 8-13%B2O3

E-glass has a tensile strength of about 3.44GPa and a

modulus of elasticity of 72.3GPa

Atomic internal structure of glass


7/39

Materials Science



Short glass fibre

Google images

SEM view

Optical view

Long glass fibre

In composite


8/39

Materials Science



Google images

A Continuous-strand mat

B Surfacing mat

C Chopped-strand mat

D Combination of woven roving and

chopped strand mat


9/39

Materials Science



E-glass: is the most commonly used glass for

continuous fibers

Basically, E-glass is lime-aluminum-borosilicate glass

with zero or low sodium and potassium levels

The basic composition of E-glass ranges from 52-

56%SiO2 12-16%Al2O3, 16-25%CaO, and 8-13%B2O3

E-glass has a tensile strength of about 3.44GPa and a

modulus of elasticity of 72.3GPa


10/39

Materials Science



S-glass: has a higher strength-to-weight ratio and is

more expensive than E-glass and is used primarily formilitary and aerospace applications

The tensile strength of S-glass is over 4.48GPa, and its

modulus of elasticity is about 85.4GPa

A typical composition for S-glass is about 65% SiO2,25% Al2O3, and 10% MgO


11/39

Materials Science


Carbon fibers for Reinforced Plastics

Composite materials made by using carbon fibers for

reinforcing plastic resin matrices such as epoxy arecharacterized by having a combination of light weight,

very high strength, and high stiffness (modulus of

elasticity)

These properties make the use of carbon-fiber-plasticcomposite materials especially attractive for aerospace

applications, such as the aircraft

Carbon fibers are produced from Polyacrylonitrile (PAN)

[polymer] precursor fibers by three processing stages:

(1) Stabilization

(2) Carbonization

(3) Graphitization

Chemical structureof the repeating unitof polyacrylonitrile


12/39

Materials Science



In the stabilization stage the PAN fibers are the first

stretched to align the fibrillar networks within each fiber

parallel to the fiber axis, and then they are oxidized in air

at about 200 to 220C while held in tension


13/39

Materials Science



The second stage in the production of high-strength

carbon fibers is carbonization, in this process thestabilized PAN-based fibers are pyrolyzed (heated) until

they become transformed into carbon fibers by the

elimination of O, H, and N from the precursor fiber

The carbonization heat treatment is usually carried out inan inert atmosphere in the 1000 to 1500C range

During the carbonization process turbostratic graphitelike

fibrils or ribbons are formed within each fiber which

greatly increase the tensile strength of the material


14/39

Materials Science



A third stage, orgraphitization treatment, is used if an

increase in the modulus of elasticity is desired at theexpense of high tensile strength. During graphitization,

which is carried out above 1800C, the preferred

orientation of the graphitelike crystallites within each

fiber is increased. The carbon fibers produced from PAN precursor material

have a tensile strength which ranges from about 3.1 to

4.45GPa and a modulus of elasticity which ranges from

about 193 to 241GPa.

The density of the carbonized and graphitized PAN

fibers is usually about 1.7 to 2.1gm/cm3, while their final

diameter is about 7 to 10 m


15/39

Materials Science



Carbon fiber

Human hair

Comparison of Carbon fiber (6m)

and human hairCarbon

fiber

products

Composite

samples tosee in the

class


16/39

Materials Science


Aramid Fiber for reinforcing plastic resins

Aramid fiber is the generic name for aromatic polyamide

fibers

Aramid fibers were introduced commercially in 1972 by

Du Pont under the trade name of kevlar, and at the

present time there are two commercial types:

kevlar 29 and kevlar 49 Kevlar 49 characterized by a low density and high

strength and modulus. The properties of kevlar 49 make

its fibers useful as reinforcement for plastics in

composites for aerospace, marine, automotive, andother industrial applications


17/39

Materials Science


Aramid Fiber for reinforcing plastic resins

The chemical repeating unit of the kevlar polymer chain is that of an

aromatic polyamide, as shown,

Hydrogen bonding bonds the polymer chains together in the

transverse direction. Thus collectively these fibers have highstrength in the longitudinal direction and weak strength in the

transverse direction. The aromatic ring structure gives high rigidity to

the polymer chains, causing them to have a rod-like structure

Kevlar aramid is used for high-performance composite applications

where light weight, high strength and stiffness, damage resistance,and resistance to fatigue and stress rupture are important. Of

special interest is that kevlar-epoxy material is used for various parts

of the space shuttle


18/39

Materials Science


Composites: Fibers


19/39

Materials Science


Matrix-Materials for Fiber-Reinforced-Plastic

Two of the most important matrix plastic resins for fiber-

reinforced plastics are unsaturated polyesterandepoxy resins

Some of the properties of unfilled cast rigid polyester and

epoxy resins are listed in Table.


20/39

Materials Science


Matrix-Materials for Fiber-Reinforced-Plastic

The polyester resins are lower in cost but are usually not

as strong as the epoxy resins

Unsaturated polyester are widely used for matrices of

fiber-reinforced plastics

Applications for these materials include boat hulls,

building panel, and structural panels for automobiles,aircraft, and appliances

Epoxy resins cost more but have special advantages

such as good strength properties and lower shrinkage

after curing than polyester resins Epoxy resins are commonly used as matrix materials for

carbon and aramid fiber composites


21/39

Materials Science


FiberGlass-reinforced Polyester Resins

The strength of fiberglass-reinforced plastics is mainly

related to the glass content of the material and thearrangement of the glass fibers

In general, the higher the weight percent glass in the

composite, the stronger the reinforced plastics is

When there are parallel strands of glass, as my be thecase for filament windings, the fiberglass for content may

be as high as 80wt% which leads to very high strengths

for the composite material


22/39

Materials Science



Figure shows a photomicrograph of a cross section of a

fiber glass-polyester-resin composite

material with

unidirectional fibers

Any deviation from the parallel alignment of the glass-strands reduces the mechanical of the fiberglass

composite. Composites made with woven fiberglass

fabrics because of their interlacing have lower strengths

than if all the glass strands were parallel (Table 13.3)


23/39

Materials Science



If the roving is chopped, producing a random

arrangement of glass fibers, the strength is lower for a

specific direction but equal in all directions


24/39

Materials Science


Carbon-Fiber-Reinforced-epoxy Resins

In carbon-fiber composite materials, the fibers contribute

the high tensile properties for rigidity and strength, whilethe matrix is the carrier for the alignment of the fibers

and contributes some impact strength

Epoxy resins are by far the most commonly used

matrixes for carbon fibers, but other resins such aspolyimides, polyphenylene sulfides, or polysulfones may

be used for certain applications

The major advantage of carbon fibers is that they have

very high strength and moduli of elasticity combined with

low density. For this reason, Carbon-fiber composites

are replacing metals in some aerospace applications

where weight saving is important


25/39

Materials Science



Table 13.4 lists some typical mechanical properties of

one type of carbon-fiber-epoxy composite material whichcontains 62% by volume of carbon fibers

In engineering-designed structures the carbon-fiber-

epoxy material is laminated so that different tailor-made

strength requirements are met


26/39

Materials Science



Fig.13.12 shows a photomicrograph of a five layer uni

and bi-directional carbon fiber-epoxy compositematerial after curing


27/39

Materials Science


Isostrain conditions

Consider an idealized lamellar composite test Sample

with alternate layers of continuous fibers and matrixmaterial, as in Fig. In this case the stress on the material

causes uniform strain on all composite layers. This type

of loading on the composite sample is called Isostrain

condition.

First, the load on the composite structure is equal to the

sum of the load on the fiber layers plus the load on thematrix layers,

Pc = Pf + Pm

cAc = fAf + mAm

Since the lengths of the layers of matrix and fiber are

equal, the areas Ac, Af and Am can be replaced by the

volume fractions Vc, Vf and Vm

cVc = fVf + mVm____________ (I)


28/39

Materials Science


Since the volume fraction of the total composite is 1,

then Vc = 1

c = fVf + mVm

For isostrain conditions c = f = m _______ (II)

Dividing equation (I) by (II)

c = fVf + mVm

c f m

Ec = EfVf + EmVm

This equation enables a value for the elastic modulus

of a composite to be calculated knowing the elastic

moduli of the fiber and matrix and their volume

percentages



29/39

Materials Science


Equations for the loads on the fiber and matrix regions

of a lmellar composite structure loaded under isostrainconditions:

Pc = Pf + Pm



30/39

Materials Science


Consider the case of an idealized lamellar composite

structure consisting of layers of fiber and matrix in

which the layers are perpendicular to the applied stress

as in Fig. The stress on the composite structure

produces an equal stress condition on all the layers.

We shall begin with an equation that states that the

stress on the total composite structure is equal to the

stress on the fiber layers and stress on the matrixlayers. Thus

c = f = m

The total strain for the composite in the directions of

the stresses is thus equal to the sum of the strains in

the fiber and matrix layers,c = f + m ___________ (I)

cVc = fVf + mVm

since Vc = 1

c = fVf + mVm

Isostress conditions


31/39

Materials Science


Equation (I) becomes

= Vf + Vm

Ec Ef Em

1 = Vf + Vm

Ec Ef Em

1 = Vf Em + Vm Ef

Ec Ef Em

Ec = Ef Em

Vf Em + Vm Ef

Isostress conditions


32/39

Materials Science


Composite Materials: More

Metal Matrix composites (MMCs):

Metal-matrix composite materials have been so intensely

researched over the past years that many new high

strength to weight materials have been produced. Most

of these materials have been developed for he

aerospace industries, but some are being used in otherapplications such as automotive engines. In general,

according to reinforcement, the three main types of

MMCs are continuous fiber, discontinuous fiber, and

particulate reinforced.


33/39

Materials Science



Continuous fiber reinforced MMCs:

Continuous filament provide the greatest improvement in

stiffness (Tensile modulus) and strength forMMCs. One

of the first developed continuous fiberMMCs was

aluminum alloy matrix-Boron fiber reinforced system.

The Al-B composite is made by hot pressing layers of Bfibers b/w aluminum foils so that the foil deform around

the fibers and bond to each other.


34/39

Materials Science



Particulate reinforced MMCs:

MMCs are low cost aluminum alloy MMCs made of

about 3-20m in dia. The particulate, which is sometimes

given a proprietary coating, can be mixed with the

molten aluminum alloy and cast into re-melt ingots or

extrusion billets for further fabrication. The ultimatetensile strength of Al-alloy 6061 can be increased from

310 to 496Mpa with 20% SiC addition while the Tensile

strength can be increased from 69 to 103Gpa.

Application of this material include sporting equipments

and automobile engine parts


35/39

Materials Science



Ceramic fiber composites (CMCs)

CMCs have been developed recently which have

improved mechanical properties such as strength and

toughness over the un-reinforced ceramic matrix. Two

kinds of continuous fibers that have been used for CMCs

are SiC, and Al2O3 . SiC fibers are woven into mat andthen chemical vapor deposition is used to impregnate

SiC into the fibrous mat


36/39

Materials Science


(a) (b)

Hand lay-up method for

molding fiber-reinforced

plastic composite materials

(a) pouring the resin over the

reinforcement in the mold (b)

use of a roller to density thelaminate to remove

entrapped air.

Smooth contour tooling mold

for forming a laminate frommultiple layers of prepreg

fiber-reinforced plastic

composite material.


37/39

Materials Science


Carbon-fiber-epoxy prepreg sheet being cut with computerized cutter at McDonnelDouglas composite facility.


38/39

Materials Science


Carbon-fiber-epoxy laminate of AV-8B wing section and tooling being put into autoclave

for curing at McDonnel Aircraft Co.


39/39

Materials Science


The outer panel of the front hood of 1984 Chevrolet Corvette. The panel is made of

sheet-molding compound by pressing at 300oF (149oC) at 1000psi (6.89Mpa) for 60 to

90s (Courtesy ofGeneral Tire and RubberCo.Southfield, Michigan)

05 Composite Materials SZK

Documents