composites m sc new class
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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
Factors influencing the mechanical properties of the composite
Factors influencing the mechanical properties of the composite
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.
Factors influencing the mechanical properties of the composite
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.
Factors influencing the mechanical properties of the composite
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
Factors influencing the mechanical properties of the composite
Where lc = minimum fibre length required for the fibre
stress to be equal to the fibre ultimate strength
f = ultimate fibre strength
Factors influencing the mechanical properties of the composite
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?
Factors influencing the mechanical properties of the composite
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
Factors influencing the mechanical properties of the composite
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
Proposed Reaction Mechanism During treatment
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
Proposed Reaction Mechanism During treatment
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.
Factors influencing the mechanical properties of the composite
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
Factors influencing the mechanical properties of the composite
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
Factors influencing the mechanical properties of the composite
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
Factors influencing the mechanical properties of the composite
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
Factors influencing the mechanical properties of the composite
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?
Factors influencing the mechanical properties of the composite
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.