composites m sc new class

Sreekumar P.A

COMPOSITE: ?

Example of perfect partnership

Consists of fibre and matrix with distinct interface.

Each partner share the load according to one’s mite.

In case of exigencies, the stronger partner share the load

and prevent the weaker partner from failure.

The partners share a perfect bond.

But each partner retain ones physical and chemical identity

throughout the life time of the composite.

Combination produces properties that cannot be achieved

with either of the constituents alone.

Here the fibre is load carrying matrix keeps them in the desired position.

Composite materials

Fibre reinforced composites Particle reinforced composites

Single layer Multi layeredComposites(Angle ply)

Laminate Hybrid

Continuous fibre Discontinuous fibre

Unidirectional Bi-directional

Random orientation Preferred Orientation

Random Preferred

Fibre

Natural Man Made

Regenerated Synthetic

Mineral Plant Animal

Leaf Bast FruitWool Mohair Silk

Natural fiber classification

R ice s tra wW h e at s trawC o rn s tra w

S tra wfib e rs

F la xK e n a fH e m pJu te

B a st

H e n eq u enS isa l

P in e a pp le lea f f ib e r

L e a f

C o ttonC o ir

S ee d/fru it

B a m b oo f ib erE le p ha n t g ra ss

G ra ssfib e rs

N o n w oo d n a tu ra l / b io fib e rs S o ft w o odH a rd w o od

R e cyc ledw o o d fib e rs

R e in fo rc ing n a tu ra l / b io fib e rs

REASONS FOR THE USE OF NATURAL FIBERS

• Annual growing raw materialup to two crops/a

• Low costs0.5 to 0.9 €/kg compared to2 €/kg for glass fibers

• Low density1500 kg/m3, glass 2500 kg/m3

• Fibers act non-abrasive

• Low energy consumptionone-fifth of fiber glass production

• Good specific mechanical properties

• Physiological harmlessnessno skin irritation

• CO2-neutrality

• Residual free thermal utilization

• Safer crash behavior (high stability and absence of splintering)

LIMITS OF NATURAL FIBERS

Natural fibers vs glass, carbon, etc.

High moisture adsorption

Poor microbial resistance

Low thermal resistance

Local and seasonal variations in quality

Demand and supply cycles

• Fast absorption/desorption of water

• Good thermal conductivity Biodegradability

Factors influencing the mechanical properties of the composite

Strength, modulus and chemical stability of the fibre and the resin matrix

Choice of the material depend on final requirement of the product.

The function of the resin matrix in a fibrous composites will vary,

depending upon how the composite is stressed.

For compressive loading the matrix prevents form buckling, and

therefore a very critical part of the composite without it the reinforcement

could not carry any load.

A bundle of fiber can sustain high tensile strength without matrix.

Resin prevent the fibre from corrosion as well from the abrasion.

Resin also provides stores transfer medium so that when individual

fibre breaks it does not loses it load carrying capacity.

The physical properties of the resin influences thebehavior of the composites includes the following

Shrinkage during cure.

Modulus of elasticity

Ultimate elongation

Tensile or flexural strength

Compression strength

Ineterlaminar shear strength

Fracture toughness



Critical Fiber LengthThe minimum length per given fiber diameter essential for high tensile fracture stress.When the length of the fibre is below critical fibre length, the maximum fibre stress maynever reach the ultimate fiber strength

f l

l< lc l = lc l > lc

How to calculate critical fibre length?

Consider an infinitesimal length distance dx at a distance x form

one of the fibre ends the force equilibrium for this length is

(/4.d2f ) (f + df) – (/4. d2ff) - df dx. = 0 (1)

Which on simplification gives

df/dx = 4/ df (2)

Where

f= longitudinal stress in the fibre at a distance x from one of its ends.

= Shear stress at the fibre/matrix interface.

df= Fibre diameter.


Assuming no stress transfer at the fibre ends, i.e f =0 at x=0, and integrating

equation (2) the normal stress distribution in the fibre ends as

f

For simple analysis, it is assumed that interfacial strength is constant.

Then equation becomes

f = (4I / df ) x

Where = interfacial shear stress.

The maximum fibre stress that can be achieved at a given load is

(f ) max = 2I (lt / df )

Where x= lt /2 = load transfer length at each fibre end.


0

4 =

df

xdx 3

4

5

lc = (f / 2I )df

For lf<lc, the maximum fibre stress may never reach the ultimate

fibre length. In this case either the fibre/matrix interfacial bond

or the matrix may fail before fibre achieve their potential strength.

For lf>lc, the maximum fibre stress may reach the ultimate fibre

strength over much of its length. Over a distance of lc/2 the fibre

remains ineffective


Where lc = minimum fibre length required for the fibre

stress to be equal to the fibre ultimate strength

f = ultimate fibre strength


Fibre content

As fibre content increases mechanical properties such as tensile and

flexural strength,Young’s an flexural modulus for the composite

also increases up to a optimum fibre load beyond that limit decreases.

At lower fibre loading dispersion of fibre is very poor so that stress

transfer will not occur properly. At higher fibre loading there is a

chance for fibre-fibre interaction and poor wetting of fibres and thereby

reducing the effective aspect ratio.

Crack initiation and its propagation will be easier at higher loadings.

How to calculate fibre content and composite density?


Where R is the resin content in composite, r is the sisal fibre vol%, D is the density of the resin

d is the density of sisal fibre.

Td=100/(R/D + r/d)

Vf = Wf /f

Wf /f + (1-Wf) /m

Where Wf is the fibre weight fraction

(1-Wf) is the matrix weight fraction

m is the resin density

f is the fibre density

Interfacial Adhesion

How to improve interfacial adhesion?

By Chemical MethodsBy Physical methodsBy using Coupling agents


fibrematrix

interface

Treatment Methods

NaOH treatment

Sisal fibres were fully immersed in 5, 10 and 15 % of NaOH solution for

30 minutes. After that fibre is taken out, washed several times with

distilled water. Finally it is washed with water containing little acid and dried

Heat treatment

Sisal fibres were heated at 1500C in an air-circulating oven for

4hrs continuously. The fibre was then cooled to room temperature

Permanganate treatment

Sisal fibres were soaked in KMnO4 solution in acetone at a concentration

of 0.02% for 3 minutes. After that fibre is taken out, washed many times

with distilled water and dried in an air oven

Treatment Methods

Benzoylation

Sisal fibres were soaked in 5% of NaOH solution for half an hour

and agitated well with 50ml of benzoylchloride. The mixture was

kept for 15mts, filtered, washed thoroughly with water. The fibre

is then soaked in ethanol for 1hr to remove unreacted

benzoylchloride and finally washed with water and dried.

Silane treatment

Sisal fibres were dipped in alcohol water mixture (60:40)

containing Vinyl tris (2 ethoxymethoxy) silane as coupling agent.

The fibres were washed in distilled water and dried.

Gamma Irradiation

Sisal fibres were exposed to gamma radiation from 60Co

at a dose rate of 1 Mrad for 4 hours.

Proposed Reaction Mechanism During treatment

Fibre-OH + NaOH Fibre-O-Na+ + H2O

NaOH Treatment

Silane Treatment

CH2 = CH - Si -OC2H5

OC2H5

OC2H5

H2OCH2=CH-Si-O-H

O-H

O-H


Silane Treatment

FibreCelluloseHemicellulose

Lignin

O-H

O-HO-H CH2=CH-Si+

O-H

O-H

O-H

Fibre

Cellulose

Hemicellulose

Lignin

O--Si - CH=CH 2

OH

OH

O--Si - CH=CH2

OH

OH

O--Si - CH=CH2

OH

OH


Permanganate Treatment

Cellulose -H + KMnO4 Cellulose -H - Mn -O-K+

Cellulose -H -O - Mn -O-K+

O

O

Cellulose.

+ H -O - Mn -O-K+

O

O

Benzoylation Treatment

Fibre - O-Na+ + Cl -C -

O

Fibre - O-C-

O

+ NaCl

Fibre-OH + NaOH Fibre-O-Na+ + H2O

Influence Of Fibre Orientation

Longitudinal

Transverse

Longitudinally aligned fibrous composites are anisotropic

in that maximum strength is achieved in the direction

fibre alignment.Transverse direction fracture usually occurs

at low tensile stress.


Continuous and aligned fibre composites

Longitudinal loading

There are composites in which the fibers are aligned in the direction of

applied stress.Assume that all the filaments are perfectly bonded to the

matrix.

Where c = composite strain

f = fibre strain

m = Matrix strain


f = m = c

The total tensile force applied on the composite lamina is hared by the fiber and matrix

P =Pf +Pm

Since load = stress x area: then

Rule of mixtures


c. Ac = f. Af + m. Am

c. = f (Af/Ac) + m(Am/Ac)

Where c. = Average tensile strengthAf = Area of the fibreAm = Area of the matrix

Ac = Af + Am

Since Vf = Af/Ac and Vm = Am/Ac

c. = f Vf + m Vm

This equation is known as rule of mixtures

For transverse loading

In this type the load is applied at 900 angle. Under this situation Stress to both phases are exposed in the same time

Ec = Em.Ef/Vm.Ef + Vf. Em


Ec = elastic moduli of the compositeEf = elastic moduli of the fibreEm = elastic moduli of the matrixVf = elastic moduli of the fibreVM = elastic moduli of the matrix

For randomly oriented fibre composites are composed short and

discontinuous fibre. Under these circumstance the expression for

the elastic modulus

K= Fibre efficiency parameter which value is lees than unity

Modified rule of mixture


Ec = KEfVf+EmVm

Voids

During the incorporation of fibers into matrix or during the manufacturing, of laminates, air or other volatiles may be trapped in the material.

Voids destroy the integrity of the composite and as they grow and interact with each other, initiate cracks and promote specimen failure

How to calculate void content?


where Td is the theoretical composite density, Md is the measured composite density.

V= 100(Td-Md)/Td

Processing techniques

Thermoset Composites

•Hand Lay Up, Spray Lay up

•Vacuum Bag Moulding

•Compression Moulding

•Filament Winding

•Pultrusion

•Resin Transfer Molding

•Structural Reaction Injection

Moulding

Thermoplastic Composites

•Calendaring

•Sheet Moulding

•Film Casting

•Injection moulding

•Extrusion

•Blow Moulding

•Rotational Moulding

•Thermoforming

HAND LAY UP

Processing stages

The mould is cleaned and a mould releasing agent is applied.

A gel of UPE resin containing pigment and curing additives

is brushed over the mould surface.

After the gel coat becomes stiffened layers of glass reinforcement

and resin are applied.

The glass layers are fully wetted and impregnated with resin by

rollers.

When it is cured it is tripped from the mould an trimmed to size,

usually with power saw

FEATURES OF HAND LAY UP

1. Extremely large parts can be made in a single moulding

2. The moulds are not pressurized and extremely large parts

can be made from single moulding.

3. Moulds can be made from cheap materials

4. Thin areas in the moulding and sharp corners often become resin rich.

5. Only one surface is moulded, the other is being rough.

6. The process is very operator-dependent and a consistent resin-glass

ratio is difficult to achieve.

7. It may require post curing to develop optimum strength.

8. Void content is high.

9. Fibre damage can occur during processing.

10. Open Moulded method

SPRAY LAY UP

In this feeding a stream of chopped fibres into a spray of liquid in a mould cavity

A specialized spray gun is used to apply the chopped fibre and resin to the tool.

The direction of fiber is random.Uniformity for the surface occurs.Void content is lees when compared to handlay up.

COMPRESSION MOULDING

Moulding through the force of compression is another very common industrial process. The materials used are melamine,phenol and urea formaldehyde, Polyesters etc.

Process Description

The mould is held between the heated platens.

A 'slug' or piece of the plastic is placed into the mould.

The hydraulic press closes with sufficient pressure. The Compound softens and flows to shape. If necessary cooling is done. The press is opened and the moulding removed

Classification of Moulds

Positive Mould Semi-positive mould

Flash Mould

Filament Winding is the process of winding resin-impregnated fiber

or tape on a mandrel surface in a precise geometric pattern.

This is accomplished by rotating the mandrel while a delivery head

precisely positions fibers on the mandrel surface.

By winding continuous strands of carbon fiber, fiberglass or other

material in very precise patterns, structures can be built with properties

stronger than steel at much lighter weights.

Filament winding machines operate on the principles of controlling

machine motion through various axes of motion.

FILAMENT WINDING

The filament winding process was originally invented toproduce missile casings, nose cones and fuselage structures,but with the passage of time industries other than defense and aerospace have discovered the strength and versatility of filament winding.

Picture

Advantages

The highly repetitive nature of fibre placement.

The capacity to use continuous fibre over the whole component

area and to orient fibres easily in the load direction.

Ability to fabricate structures that are larger than most autoclaves.

Obtainablity of high fibre volume fraction.

Lower cost for large quantity of components.

Relatively low material cost.

Disadvantages

Difficulty in winding reverse curvature

Inability to change fibre path easily

Need for mandrel which can be complex or expensive

Poor external surface finish

Characteristics of resin

Low temperature for curing

Viscosity should be lower

Pot life should be as long as possible

Toxicity should be low

Resin Provides

Retaining the filament in proper position

Transferring the load form filament to filament

Protecting the filaments from abrasion

Controlling the electrical an thermal properties

Providing the interlaminar shear strength

Impregnation method

Prepreg:

Wet Rerolled

A controlled volume of resin is impregnated on the controlled

length of fibre reinforcement and then respooled.

Preservative and solvents are not required

Wet winding

This is accomplished by either pulling the reinforcement

through a resin bah or directly over a roller that contains

a metered volume of resin controlled by a blade.

Widely used in the case of epoxy and polyester resin

Winding Patterns

Helical

The mandrel rotates more or less continuously while the fibre feed

carriage traverses back and forth at a speed regulated to generate

the desire helical angle.

After the first circuit is applied fibre are not adjacent, additional

circuits must be traversed before the patterns..

The mandrel revolution s per

circuit vary with winding angle

band width and overall length

of the vessel.

Any combination of diameter and

length may wound by trading

off winding angle

Picture

Polar

The fibre passes tangentially in the

polar opening at one end of the

chamber.Reverses direction, and

passes tangentially to the opposite

side of the polar.It is simple and

winding speed can be maintained

Hoop patterns

High angle helical winding that

approaches an angle of 90o.

They are generally combined with

longitudinal windings to produce a

balance structure

Surface Considerations

Use thinner tows on the last few outer hoop over wraps

Do not squeegee the last hoop layers

Over wrap with shrink tape or teflon-coated cloth and remove after

curing

Confine the last few layers to hoops only.

Use high wind angles as opposed to low wind angles

How to avoid slipping?

Pins to control the movement of the fibre

Powders or tackfying agent on wet filament wound parts

A tacky prepeg or wet rerolled tow to control movement

Pultrusion is a manufacturing process for producing continuous lengths

of FRP structural shapes.

Raw materials include a liquid resin mixture (containing resin, fillers and

specialized additives) and reinforcing fibers.

The process involves pulling these raw materials through a heated steel

forming die using a continuous pulling device.

The reinforcement materials are in continuous forms such as rolls of

fiberglass mat or doffs of fiberglass roving.

As the reinforcements are saturated with the resin mixture ("wet-out") in

the resin impregnator and pulled through the die, the gelation.

(or hardening) of the resin is initiated by the heat from the die and a rigid,

cured profile is formed that corresponds to the shape of the die.

PULTRUSION

The reinforcement must be located properly within the

composite and controlled.

The resin impregnator saturates (wets out) the reinforcement

with a solution containing the resin, fillers, pigment, and

catalyst plus any other additives required.

The interior of the resin impregnator is carefully designed to

optimize the "wet-out" (complete saturation) of the

reinforcements.

On exiting the resin impregnator, the reinforcements are organized and positioned for the eventual placement within the cross section form by the preformer

The preformer is an array of tooling which squeezes away excess resin as the product is moving forward and gently shapes the materials prior to entering the die

Precautions to be taken

In the die the thermosetting reaction is heat activated

(energy is primarily supplied electrically) and the composite

is cured (hardened).

•On exiting the die, the cured profile is pulled to the saw for

cutting to length. It is necessary to cool the hot part before

it is gripped by the pull block (made of durable urethane foam)

to prevent cracking and/or deformation by the pull blocks.

Advantages

High strength to weight ratioDimensional Stability is highWire, Wood can be encapsulated on a continuous basisWide variety of reinforcement can be used.Pultured shape can be made as large as required Cost of die is less

Desired resin characteristics and Matrix used

Low Viscosity less than 200cps

must remain liquid as it is held in the reservoir prior to injection

must impregnate fiber preform quickly and uniformly without voids

must gel as quickly as possible once

impregnation occurs must possess sufficient hardness

to be demoulded without distortion• Matrix

•Vinyl ester•Polyester•Epoxy resin•Phenol formaldehyde resin

RESIN TRANSFER MOULDING

Preform

Tool

Injection

Cure

Demould

SCHEMATIC REPRESENTATION SCHEMATIC REPRESENTATION

Mold filling processProper mold designingResin characteristicReinforcement characteristicMold temperatureVaccum state of systemResin flow rate

Factors affecting the RTM Process

Different Aspects Of Mold Filling Process?

•Fibre washing

The unexpected movement or displacement of reinforcement in the

closed mold. It leads to the failure of RTM due to fibre displacement

and interrupt the uniformity of predetermined reinforcement distribution

•Edge flow

In RTM due to small clearance there exist a path for resin flow during

mold filling.This edge flow can create dry spots or spillage of resin.

•Mould filling

Is fibre washing is related to injection pressure ? How?

Fibre washing increases when pressure is increased

Is fibre washing is related to fibre content? How?

Fibre washing distances reduces with more number layers of fibre and virtually reduces to zero due to

-Higher clamping forces occurs. It can be increased by prelaying of narrow nonwoven strips along the edge to intimate contact with the mold

Schematic diagram of edge flow

Factors governing the edge flow?

Injection pressure

Only a marginal increase for mould filling at the edge with increasing edge pressure

Preform permeability

More layers of fibre layer results in high flow resistance

and slow flow and slow impregnation

Proper mold designingwhat it means?

•Shape of the mold

•Proper positioning of gate and vent

•Gate and vent should be opposite in direction

•Number of gate and vent

•Pressure at the vent must be lesser than that from the gate position

What is “Dry Spot”?How it forms ?

Region of composite part which is devoid of resin.

Due to the presence of inserts , ribs, cores, edge flow etc

the resin flow may branch and merge around the inserts or

low permeability areas .

Flow front merges in the absence of a vent air get entrapped

Voids can form in RTM process

During processing

In the rein before processing

During injection

During curing

Importance of Dry spot….

Reduces

Tensile strength

Compression strength

Flexural strength

Impair surface quality

Reduces the water resistance

Is there any way to prevent dry spot?

By vacuum assistance.

To use gas which is easily dissolved into the resin.

To keep the resin flowing through a completely filled mould.

High pressure in curing stage of processing.

Use of more homogeneous reinforcement

Better wetting between the resin and fibre must be done

To continue filling after reinforcement has been completely wetted by resin

Advantages of RTM

Low capital investment

Good surface quality

no air entrapment if properly designed (tooling, preform, and resin)low tooling cost

Large, complex shapes

Ribs, cores and inserts

Range of available resin systems

Range of reinforcements

Controllable fiber volume fraction

CALENDERING

It is employed to produce continuous film and sheets.

It consists of set of highly polished metal rollers rotating in opposite direction.There is provision of precise adjustment of the gap between them.The gap determines the thickness of the sheet. The sheet are maintained at an elevated temperature.Emerging sheet is cooled by passing through cold rollers.Finally it is wound up.

Eg: PVC, ABS, rubbers.

SHEET MOULDING COMPOUNDS

The material is composed of a filled , thermoset resin and a chopped or continuous strand of glass fibre.

Advantages

High volume production.Weight reduction.Excellent design flexibility.Minimum material Scrap.

Low labor requirement.

FILM CASTING

Used to produce polymeric films

In this polymer in a suitable solvent is allows to fall at a

precalculated rate on an endless metallic belt of high finish

moving at a constant speed.

Continuous sheet of polymer solution is formed.

The solvent is evaporated under controlled condition.

The film is removed by stripping.

Eg: Cellophane sheet, photographic films.

INJECTION MOULDING

It involves forcing or injecting a fluid plastic material into a closed mould

where it solidifies to give the product

Two basic categories:

Thermoplastic; Thermosetting

In former material is meltedand force through an orifice or gate into a cool mould.In later a reacting material isinjected into a warm mould in which the material further polymerizes into a solid part

Schematic Diagram

Process

Feeding of the compounded plastic as granules through

a hopper at definite interval of time.

It is softened and pressure is applied.

Through a hydraulically driven piston to push the molten

material through a cylinder into a mould fitted at the end

of the cylinder.

While moving the ‘torpedo’ helps to spread the plastic

material uniformly around the inside wall and ensures

the uniform heat distribution.

Screw move back and check valve opens

Stage 4

Stage 1

Stage 3

Stage 2

Mould open

Mould clamped cavities filling with melt.

Mould clamped cavities full, melt freezing.

Screw almost stationary

Screw move forward and check valve closed

Reservoir full

Frozen moulding in clamped mould.

MOULDING STAGES

MOULDING STAGES

EXTRUSION MOULDING

Blow Moulding

The plastic is fed in granular form into a 'hopper' that stores it. A large thread is turned by a motor which feeds the granules

through a heated section. In this heated section the granules melt and become a liquid

and the liquid is fed into a mould. Air is forced into the mould which forces the plastic to the sides,

giving the shape of the bottle.The mould is then cooled and is removed.

The process is similar to injection moulding and extrusion. The process is also known as injection or extrusion blow moulding.

Rotational Moulding

It is used t o produce small to large hollow items with very uniform wall thickness

Heating while rotating

Cooling while rotating Part removal

Processing stages

A hollow thin wall mould with good heat transfer characteristics

is first charged with an amount of plastic that is equal to desired

part weight.

Mould is then attached to a mechanism that generally rotating it

simultaneously about two axes that are at 90o angles to each other.

During rotation the material inside the the mould tumbles to the

bottom creating a continues path that covers the mould surfaces equally.

The material is normally heated by rotating the mould in an oven.

After the proper time and temperature, the mould is removed and are

cooled to room temperature.

The mould is then opened.

Rotational Moulding

Three arm indexing machine

VACUUM FORMING

Vacuum forming is a technique that is used to shape a variety of plastics. In school it is used to form/shape thin plastic, usually plastics such as; polythene and perspex. Vacuum forming is used when an unusual shape like a ‘dish’or a box-like shape is needed.

THE STAGES INVOLVED IN VACUUM FORMING

First, a former is made from a material such

as a soft wood. The edges or sides are shaped

at an angle so that when the plastic is formed

over it, the former can be removed easily.

The former is placed in a vacuum former

A sheet of plastic (for example, compressed polystyrene) is clamped in position above the mould.

The heater is then turned on and the plastic slowly becomes soft and pliable as it heats up. The plastic can be seen to

'warp' and 'distort' as the surface expands.

After a few minutes the plastic is ready for ‘forming’ as it becomes very flexible.

The heater is turned off and the mould is moved upwards by lifting the lever until it locks in position.

The 'vacuum' is turned on and this pumps

out all the air beneath the plastic sheet.

Atmospheric pressure above the plastic

sheet pushes it down on the mould. At

this stage the shape of the mould can

be clearly seen through the plastic sheet.

When the plastic has cooled sufficiently

the vacuum pump is switched off.

composites m sc new class

Business

maximum fibre

dry spot

void content

factors influencing

edge flow

resin impregnator

resin flow

mechanical