project abstract revised
TRANSCRIPT
GOVT.OF INDIA
DEFENCE R&D ORGANISATION (DRDO)
ADVANCED COMPOSITES CENTER (ACC)
ADVANCED SYSTEMS LABORATORY
P.O. Kanchanbagh, Hyderabad -500 058, A.P.
Phone.No. 040-24583403, FAX.No. 0040-24583405
S
ABSTRACT
i) Aim of the project
Determination of mechanical properties like tensile, flexural and inter laminar shear
strength for carbon-epoxy (LY556+HT972), bi-directional composites with different
process parameters to fiber volume fraction with low thickness.
ii) Why we have chosen this project?
1. Metals are not suitable materials for aerospace applications due to their high
density and therefore composite materials are used as substitute due to their light
weight and high strength characteristics.
2. Composite materials especially carbon-epoxy composites are especially strength
bearing materials useful in aero space applications like reentry vehicle structures
of missiles, aircraft structures etc.
3. Filament winding process and tape wound processes are used for fabrication of
Re-entry vehicle structure capable to mechanical loads when compared to other
materials.
4. Bi-directional carbon-epoxy composites are representative materials to filament
tape wound components of Re-entry vehicle structure.
5. We tried to improve the fiber volume fraction of the composites with effective
consolidation (low thickness) of layers by applying vacuum and pressure during
curing of the laminate.
6. Hence we have taken the project topic as “ Mechanical characterization of
carbon-epoxy composites for light weight and high strength application for
Re-entry vehicle structure(RVS)”
iii) Definition of the Problem
DRDO, Hyderabad is the organization dealing with design and development
of missile systems to support the army with the weapon systems. In this context
Advanced Systems Laboratory is involved in developing the AGNI-1, AGNI-2,
AGNI-3 and AGNI- 5 missile systems and has been tested successfully.
The typical missile system is shown in Fig.1, contains pay load in the Re-entry
vehicle structure (RVS) followed by propulsion. The re-entry vehicle structure is
made by two composites shells, the internal one is made by carbon-epoxy
composite( for mechanical strength bearing purpose) and where as the outer shell is
made by carbon-phenolic composite( for heat resistant purpose).
The internal carbon-epoxy shell is made by filament wound process or tape
wound process in the RVS. The exact thermal and mechanical loads are simulated
on the large size of the components by substituting the mechanical properties
obtained from laminate level experiments. Based on the simulation studies the
thickness of the C-E structure will be decided. Bi-directional carbon-epoxy
composites are representative materials to filament tape wound components of Re-
entry vehicle structure. Hence we tried to improve the fiber volume fraction of the
composites with effective consolidation (low thickness) of layers by applying
vacuum and pressure during curing of the laminate. Hence we have selected the
above topic.
Introduction
to
Composites
Introduction
to
Composites
ABSTRACT
1. Introduction
Material have been classified into four categories based on their
applications to achieve particular physical, mechanical and thermal characteristics.
1. Metals
2. Organic materials (polymers)
3. Ceramic materials
4. Composite materials
1.1 Metals:
1. Metals are materials that are easily shaped by forming, malleable, reflective,
electrically conductive, thermal conductors posses an orderly arrangement of
atoms, resulting in a crystalline structure.
2. Metals have useful properties like strength, ductility, high melting points and
posses considerable toughness.
1.2. Polymers:
1. Polymers/plastics are made up of repeated chains to make a long molecule,
typically 10 to 20 nm that have developed as a consequence of the linking of
many smaller molecules. The carbon atoms may be attached to other carbon,
oxygen, nitrogen, and hydrogen atoms. But polymers may or may not have
an orderly arrangement of atoms.
2. A polymeric solid can be thought of as a material that contains many
chemically bonded parts or units which themselves are bonded together to
form a solid.
3. Polymers classified as three types and one is thermoplastics, thermosets and
rubbers.
4. Thermoplastics are available as solids at room temperature and by heating
they will be converted as liquids and where as by cooling they will be
converted as solids.
5. Thermosets will be available as liquids at room temperature and by heating
process they turns as solids.
6. Rubbers are partially thermoplastic and partially thermo set in nature but by
heating they achieve hardness.
1.3 Ceramics:
1. A ceramic has traditionally been defined as “an inorganic, nonmetallic solid
that is prepared from powdered materials, is fabricated into products through
the application of heat, and displays such characteristic properties as
hardness, strength, low electrical conductivity, and brittleness." which are
basically a mixture of metal oxide powders like structural clay products,
glasses, abrasives, etc. Ceramic is a crystalline material, inorganic in nature.
2. There are different types of ceramic based products manufactured for
domestic, industrial and commercial purposes. Many common ceramics are
made up of oxides or nitride compounds and are crystalline with long range
molecular order. Other ceramics are partially or fully amorphous, with no
long range
molecular
order; these are
typically
classified as
glassy materials.
3.
4. Ceramics are lighter, stiffer and corrosion resistant. They are brittle because
of strong directional bonds. So they shatter rather than deform. The electrons
in ceramics are tightly held because of covalent bonds. There are no mobile
electrons to conduct current. Therefore ceramics are good insulators.
1.4. Composites
A composite is commonly defined as a combination of two or more distinct
materials, each of which retains its own distinctive properties, to create a new
material. The two distinct materials. Composites are the mixture of two materials,
which in combination, offer superior properties to the materials alone. Structural
composites usually refer to the use of fibers which are embedded in a plastic. These
composites offer high strength with very little weight.
2. The two distinct materials are one is matrix and another is reinforcement
embedded in the plastic. Matrix surrounds the reinforcement and protects the
reinforcements impart their special mechanical and physical properties to enhance
the matrix properties. A synergism produces material properties unavailable from the
individual constituent materials.
Wood is a natural composite
5. The matrix material can be introduced to the reinforcement before or after the
reinforcement material is placed into the mold cavity allows the designer of
the product or structure to choose an optimum combination.
Composites are having the following advantages in terms of light weight ,
weight distribution, high strength to weight ratio, directional strength and stiffness,
corrosion resistance, weather resistance, low thermal conductivity, low coefficient of
thermal expansion, high dielectric strength, non-magnetic and radar transparency.
1.5 Types of Resins
i) Epoxy resin
1. Epoxy or poly epoxide is a thermosetting polymer formed from reaction of
an epoxide "resin" with polyamine "hardener". Epoxy term refers to a
chemical group consisting of an oxygen atom bonded to two carbon atoms
that are already bonded in some way. The resin consists of monomers or
short chain polymers with an epoxide group at either end. The structure of
molecule is shown in the figure below.
2. Epoxies generally out-perform most other resin types in terms of mechanical
properties and resistance to environmental degradation, which leads to their
almost exclusive use in aircraft components.
3. The epoxy molecule contains two ring groups at its centre which are able to
absorb both mechanical and thermal stresses better than linear groups and
therefore give the epoxy resin.
4. Epoxies are used as adhesives, caulking compounds, casting com- pounds,
sealants, varnishes and paints, as well as laminating resins for a variety of
industrial applications.
5. Epoxies are widely used as a primary construction material for high strength
performance boats or as a secondary application to sheath a hull or replace
water-degraded polyester resins and gel coats.
ii) Polyamide resin
1. Polyamides (nylon) are the oldest and largest volume engineering polymers.
Commonly used products are designated as nylon 6; 6,6; 6,9; 6,12; 11; and
12, with the nomenclature designating the number of carbon atoms that
separate the repeating
2. Most widely used nylon polymers are semi crystalline products with
molecular weights of 10,000-40,000 and chemical structures in which amide
linkages connect aliphatic chain segments can be operated at higher
temperatures can stand is required
3. Typical applications include missile and aero-engine components, extremely
expensive resin, which uses toxic raw materials in its manufacture.
4. The polyamide analogs exhibit good chemical resistance and low moisture
absorption at the expense of heat resistance, impact properties in wet
environments, and stiffness.
5. All polyamides are hygroscopic to some extent because of that water acts as
a plasticizer in polyamides, reducing most mechanical and electrical
properties while improving toughness and elongation.
iii) Phenolic resin
1. Phenol formaldehyde resins include
synthetic thermosetting resins such as
obtained by the reaction of phenols with formaldehyde Phenol-formaldehyde
resins, as a group, are formed by a step-growth polymerization reaction that
can be either acid- or base-catalyzed.
2. Paper phenolics are used in manufacturing electrical components such as
punch-through boards and household laminates.
3. Primarily used where high fire-resistance is required, phenolics also retain
their properties well at elevated temperatures. For room-temperature curing
materials, corrosive acids are used which leads to unpleasant handling.
1.6 Types of reinforcements
i) Carbon fiber
1. Carbon has the highest strength and highest price of all reinforcement
fibers used in composites the size or thickness of carbon tows is measured
in terms of number of filaments.
2. Carbon fibers exhibit substantially better strength and stiffness values than
all the others, outstanding temperature performance, high electrical and
low thermal conductivity. Impact or damage tolerance of pure carbon
composite products can be from relatively low to very poor, and greatly
depends on processing method.
ii) Glass fiber
1. Glass fiber reinforcement in composite construction is accounting for
over 90% of worldwide similarity in shape between the fiberglass and the
asbestos fibers, fiberglass was able to effectively replace asbestos in
many applications such as electrical, thermal, and acoustic insulation and
structural reinforcement.
2. It possesses good strength to weight characteristics with low cost and
glass filaments are made relatively easily by extruding molten glass.
iii) Boron fiber
1. Carbon or metal fibers are coated with a layer of boron to improve the
overall fiber properties. The extremely high cost of this fiber restricts it
use to high temperature aerospace applications and in specialized
sporting equipment.
2. A boron/carbon hybrid, composed of carbon fibers interspersed among
80-100m boron fibers, in an epoxy matrix, can achieve properties greater
than either fiber alone, with flexural strength.
1.7 Types of composites
Composite material is composed of at least two elements like matrix and
reinforcement combinations where matrix holds the fibers and transfers the
mechanical load among the reinforcements whereas the strength and stiffness is
achieved by reinforcement. Composites are divided mainly as three groups.
i). Polymer Matrix Composites (PMC): Glass, carbon and aramid type of different
reinforcements are embedded in the plastic structures made by thermoset resins.
These composites are also known as Fiber Reinforced Plastics (FRP)
ii). Metal Matrix Composites (MMC): Variety of fibers can be embedded as
reinforcing elements in metallic matrix materials like Aluminum, steel and titanium.
These composites are used extensively in automobile industry.
iii). Ceramic Matrix Composites (CMC): Wide variety of reinforcements used in
silicon carbide and boron nitride matrix to achieve sustainability of composites at
high temperature environments. The ceramics are used as matrix and short fibers or
whiskers are used as reinforcements. Ceramic-metal matrix and carbon-phenolic
composites can sustain to high temperature applications but mechanical strength is
inferior. Carbon-polyimide composites are most suitable for sustaining up to 3200C
with superior mechanical properties.
A). Carbon-Epoxy Composites
1. The carbon fibers are first placed in the mould and then semi-
liquid epoxy resins are sprayed or pumped in to form the object. Pressure
may be applied to force out any air bubbles, and the mould is then heated
to make the matrix set solid.
2. Carbon epoxy resins have an infinite service lifetime when
protected from the sun, but, unlike steel alloys, have no endurance limit
when exposed to cyclic loading.
B. Glass-Epoxy Composites
1. Glass-reinforced plastic also known as glass fiber-reinforced plastic is a
composite material an individual structural glass fiber is both stiff and strong
in tension and compression—that is, along its axis.
2. Made of a plastic matrix reinforced by fine fibers made of glass. GRP is a
lightweight, strong material.
C). Carbon-Phenolic Composites
1. Phenolics resin composites are used as ablative layers under thermal
environments to protect the structure and posses good thermal resistivity
under thermal environments
1.8 Applications of composites in different fields
1. Carbon fiber-reinforced polymer is used extensively in high-end automobile
racing.
2. The high cost of carbon fiber is mitigated by the material's unsurpassed
strength-to-weight ratio, and low weight is essential for high-performance
automobile racing.
3. For the same strength, a carbon-fiber frame weighs less than a bicycle tubing
of aluminum or steel.
4. Carbon fiber-reinforced polymer frames, forks, handlebars, seat posts, and
crank arms are becoming more common on medium- and higher-priced
bicycles.
5. Sporting goods applications include rackets, fishing rods, long boards, and
rowing shells.
6. CFRP has also found application in the construction of high-end audio
components such as turntables and loudspeakers, again due to its stiffness.
7. Shoe manufacturers may use carbon fiber as a shank plate in their
basketball sneakers to keep the foot stable.
8. It is common now to find wing and tail sections, propellers and rotor
blades made from advanced composites, along with much of the internal
structure and fittings.
9. The airframes of some smaller aircraft are made entirely from composites,
since composites are less likely than metals to break up completely under
stress. A small crack in a piece of metal can spread very rapidly with very
serious consequences.
10. The fibers in a composite act to block the widening of any small crack and
to share the stress around. Wing, tail and body panels of large commercial
aircraft
Production Methods
of
Composites
Production Methods
of
Composites
2. Fabrication methods of composites
Different methods of fabrication are employed to manufacture the composite
structures based on the nature of resin and fiber used. Spray lay up is used for
stacking of different layers one on another for making sheet molded composites.
Autoclave curing method is used for fabrication of composites with volatile evolving
resins like phenolics. Match die molding is used for all types of composites
manufacturing.
2.1 Spray Technique
Spray technique is used for fabrication of composites with low viscosity like
polyester with glass rovings. The method of fabrication is shown in Fig.2. The
mould of the required shape of the component is selected release agent is first
applied to the mold and then a layer of gel coat is applied. The gel coat is left for two
hours, until it hardens and a spray gun is used to deposit the fiber resin mixture onto
the surface of the mold. The spray gun chops the incoming continuous roving to a
predetermined length and impels it through the resin/catalyst mixture. Resin/catalyst
mixing can take place inside the gun (gun mixing) or just in front of the gun. The
mixture of resin and fiber is sprayed on to mold and compacted with help of rollers
and to remove entrapped air as well as to ensure good fiber wetting. Fabric layers or
continuous strand mats are added into the laminate, depending on performance
requirements. The curing process of the resin is done at room temperature so that
the fibers will be embedded in the solid structure. This method is cheap and used in
fabrication of simple enclosures, lightly loaded structural panels, e.g. caravan bodies,
truck fairings etc.
Fig.2. Spray technique of composites
2.2. Vacuum bag technique
A process for molding reinforced plastics in which a sheet of flexible
transparent material such as nylon plastics is placed over the lay-up on the mold and
sealed. Vacuum is applied between the sheet and the lay-up. The entrapped air is
removed by the vacuum and the part is placed in an oven or autoclave. Addition of
pressure further results in higher fiber concentration and provides better adhesion
between layers of sandwich construction. The entrapped air is removed by the
vacuum and the part is placed in an oven or autoclave. The addition of pressure
further results in higher fiber concentration and provides better adhesion between
layers of sandwich construction. Fig.2.1 shows vacuum bag technique set up.
Fig.2.1. Vacuum bag technique
In vacuum bag processing can produce laminates with a uniform degree of
consolidation, while at the same time removing entrapped air, thus reducing the
finished void content. Structures fabricated with traditional hand lay-up techniques
can become resin rich and vacuum bagging can eliminate the problem. Complete
fiber wet-out can be accomplished if the process is done correctly. Improved core-
bonding is also possible with vacuum bag processing.
2.3. Resin transfer molding technique
Resin transfer molding is an intermediate volume molding process for producing
composites. The RTM process is to inject resin under pressure into a mold cavity.
RTM can use a wide variety of tooling, ranging from low cost composite molds to
temperature controlled metal tooling. This process can be automated and of is
capable producing rapid cycle times. Fig.2.2 shows typical RTM process for
composites manufacturing.
The reinforcement is positioned in the mold and the mold is closed and
clamped. The resin is injected under pressure, using mix/meter injection equipment,
and the part is cured in the mold. The reinforcement can be either a perform or
pattern cut roll stock material. Preforms are reinforcement that is pre-formed in a
separate process and can easily be placed in a mold. RTM can be done at room
temperature; however, heated molds are required to achieve fast cycle times and
product consistency. Clamping can be accomplished with perimeter clamping or
press clamping.
Fig.2.2. RTM process of composites
This closed molding process produces parts with two finished surfaces. By
laying up reinforcement material dry inside the mold, any combination of materials
and orientation can be used, including 3-D reinforcements. Fast cycle times can be
achieved in temperature controlled tooling and the process can range from simple
to highly automate.
2.4. Compression molding technique
Counter mould and other mould in the required shape of the component is
taken by making the mould with cast of forged steel, cast iron, and cast aluminum.
The molding material is preheated and placed in a open mold cavity and the molding
material becomes soft. A counter mold is used for applying pressure, for the
compaction of fibers and resins. Heat and pressure are maintained until the molding
material has cured. Stages of compression molding are shown in Fig.2.3.
Fig. 2.3. Compression molding set up
It is one of the lowest cost molding methods compared with other methods
such as transfer molding and injection molding and wastage is relatively
low.Compression molding produces fast molding cycles, high part uniformity and
the process can be automated.
In the present study the composite samples are fabricated by compression
molding method and unidirectional composite laminates with 0-900 orientations of
fibers in the epoxy matrix is used. The UD laminates are prepared by epoxy LY556+
HT972 epoxy based resin with carbon T-300 fiber. Carbon-epoxy laminates of
320x320x3 mm dimensions are made by compression mold.
Mechanical testing
of
Composites
Mechanical testing
of
Composites
3.
Mechanical testing of Composites
Specimen sizes are marked on the laminate and the specimens were cut on
the cutting machine with circular diamond cutter as per marking. The Specimen
sizes required for performing different tests are shown in the Table.1 as per ASTM
standards. Care was taken, such that laminates are perfectly cut with out any
delamination and change in fiber orientation. Cutting was carried out in warp
direction.
Table.1: Specimen sizes as per ASTM standards
Type of Test ASTM Standard
Dimensions(mm)
Number of Specimens
Tensile D3039 250x25x3 6Flexural D790 78x12.7x3 6
ILSS D2344 48x10x3 6
3.1 Mechanical testing on Universal testing machine(UTM)
Make : INSTRON, UK
Model : 1185
Load capacity of the cell : 100 kN
The Universal Testing Machine is shown in the Photo 3.3.1. Comprised of the
following [4]:
1. Fixed Member: A fixed or essentially stationary Member supports the load
fixture.
2. Movable Member: Capable of applying the load
3. Drive Mechanism: A drive imparts to the movable member controlled velocity
with respect to the stationary member.
4. Control panel with MTest software: A total control of UTM with loading and a
software tool to receive the data and plot them as graphs
`````````````````
Fig.3 Universal Testing Machine
3.2 Tensile test
Determination of the tensile strength and tensile modulus of Carbon Epoxy
composite samples using ASTM D3039 standard.
Tensile tests provide different measures of the material mechanical
properties. The tensile test gives a measure of the Young’s modulus of the
material as well as the tensile strength. In order to give reproducible results, the
tensile bars should not have notches or burrs on their edges and should be free of
scratches. The experiments must be carried out as per ASTM D3039 specifications.
Apparatus
1. Vernier Caliper: Suitable for measuring the cross sectional
Dimensions of the test specimen.
2. Testing Machine: Universal Testing Machine – INSTRON 1185.
2. Wedge Action Grips: For holding the specimen under load
3. Number of sample : 6
Procedure
1. Turn on the computer, control unit & Universal Testing Machine.
1. Open M-test Control Software of UTM.
2. Measure and enter the Dimensions of the specimens, required Cross head
speed as per ASTM Standard.
1. Place the sample between the upper grip jaws & tighten.
1. Similarly tighten the lower grip firmly so that your specimen is
secure with in the grips.
1. Start the test. Load on the specimen is increased gradually. At the
same time graph is generated for load and elongation of the
specimen.
1. After the specimen fails, load vs. elongation graph and peak load is
obtained through M-Test software.
1. Tensile strength and Modulus are calculated from the results obtained
through MTest Software.
1. Next specimen is mounted and above procedure is repeated.
1. The calculations were carried out from the data as follows.
Calculations
1. Tensile Strength: Tensile Strength is calculated as follows
FL+ = P / A
Where,
FL+ = Longitudinal Tensile Strength, MPa
P = Maximum Load at Failure, N
A = Minimum Cross-Sectional Area, mm2
2. Tensile Modulus: The Tensile Modulus is calculated as follows
E= P/e
Where,
E = Tensile Modulus, GPa
P = Load with in elastic limit (linear portion of material), N
e = Strain
A = Original cross sectional area, mm2
3.3 Flexural test
Determination of the flexural strength and flexural modulus for Carbon
Epoxy composite samples using ASTM D790 standard.
Flexural strength is the ability of the material to withstand bending forces
applied perpendicular to its longitudinal axis. The stresses induced due to the
flexural load are a combination of compressive and tensile stresses in case of an
isotropic beam. In anisotropic composite materials, additional shear forces also
come into play, the relative properties of bending and shear forces being function of
span to depth ratio of the beam and E/G (young’s modulus/modulus of rigidity) ratio
of the material. A bar of rectangular cross-section is tested in flexure as a beam, on
a three point loading system, with-center loading on a simply supported beam is
used. The specimen rests on two supports and is loaded by means of specimen
loading nose midway between the supports.
Apparatus
The specimens were cut from the laminates to the desired dimensions.
According to the standard (ASTM D790) the support span is 16 times of the
thickness of the specimen. Thickness of the specimen is 3 mm and the width of the
specimen is 12.7 mm
1. Vernier Caliper: Suitable for measuring the cross sectional
dimensions of the test specimen
2. Testing Machine: Universal Testing Machine – INSTRON 1185.
2. Three Point Bending Fixture: As per ASTM standard
3. Number of samples : 6
Procedure
1. Turn on the computer, control unit & Universal Testing Machine.
2. Open Mtest Control Software of UTM.
3. Measure and enter the Dimensions of the specimens, required Cross head
speed as per the Standard.
4. Determine the support span, which should be sixteen times the thickness
of the specimen.
5. Mount the specimen on three point bending fixture by aligning the sample
midway between the supports.
6. Lower the upper roller using the jog controls so that it almost touches the
Compression arm.
7. Start the test. Load on the specimen is applied gradually. At the same time
graph is generated through M-Test software for load and deflection of the
specimen.
8. After the sample has failed, load vs. deflection graph and peak load is
obtained through which flexural strength and modulus are calculated.
9. Next specimen is mounted and above procedure is repeated.
Calculation
1. Flexural Strength: The flexural strength was calculated from the data as
follows
Flexural Strength = 3PL / 2bd2
Where,
P = maximum load at failure, N
L = support span, mm
b = width of specimen, mm
d = thickness of specimen, mm
2. Flexural Modulus: The Flexural Modulus is the ratio (within the elastic
limit) of stress to corresponding strain. It is calculated from linear portion of
the load-deflection curve by using the following equation
Flexural Modulus = (L3 / 4bd3 ) x (∆P/∆l)
Where,
L = support span, mm
b = width of specimen, m
d = thickness of specimen, mm
∆P = change in Load, N
∆l = Deflection, mm
3.4 Inter laminar shear strength (ILSS)
Determination of the Inter Laminar Shear Strength(ILSS) for
Carbon Epoxy composite samples using ASTM D2344 Standard.
The stresses acting on the interface of two adjacent lamina are called
inter laminar stresses. The inter laminar stresses are illustrated in the below Figure.
where T is the inter laminar normal stresses on plane ABCD and TL and TT are
the inter laminar shear stresses. These cause relative deformations between the
lamina 1 and 2. If these stresses are sufficiently high, they will cause failure along
plane ABCD. It is, therefore, of considerable interest to evaluate inter laminar shear
strength through tests in which failure of laminates initiates in a shear (delamination)
mode. If the span to depth ratio is short enough, failure initiates and propagates by
inter laminar shear failure, and the test can be used to evaluate inter laminar shear
strength.
A bar of rectangular cross section is used for ILSS test. As show in
the Fig No 42 three point loading system, with center loading in a simply supported
beam is used. The specimen rests on two supports and is loaded by means of
specimen loading nose midway between the supports. ILSS test was conducted as
per ASTM D 2344.
Apparatus
The specimens were cut from the laminates to the desired finished dimensions, the
specimen is shown in the Fig.
1. Vernier Caliper: Suitable for measuring the cross sectional
dimensions of the test specimen
2. Testing Machine: Universal Testing Machine – INSTRON 1185.
3. Three Point Bending Fixture: As per ASTM
4. Number of samples : 6
5. Cross head speed : 2 mm/min cross head speed.
Procedure
Turn on the computer, control unit & Universal Testing Machine.
2. Open M-test Control Software of UTM.
3. Measure and enter the Dimensions of the specimens, required Cross head
speed as per the Standard.
4. Determine the support span, which should be Five times the thickness of
the specimen.
5. Mount the specimen on three point bending fixture by aligning the sample
midway between the supports.
6. Lower the upper roller using the jog controls so that it almost touches the
Compression arm.
7. Start the test. Load on the specimen is applied gradually. At the same time
graph is generated through M-Test software for load and deflection of the
specimen.
8. After the sample has failed, load vs. deflection graph and peak load is
obtained through which flexural strength and modulus is calculated.
9. Next specimen is mounted and above procedure is repeated.
10. Calculation was carried out from the data as follows.
Calculation
The inter laminar shear strength was calculated according to the following
formula
ILSS = 3P/4bd
Where,
ILSS = Inter laminar shear strength, MPa
P = Maximum load, N
b = Width of the specimen, mm
d = Thickness of the specimen, mm
L = Span, mm
4. Chemical Analysis of samples for chemical and physical
properties
Chemical analysis is carried out to measure the following physical and
chemical properties for assessing the quality of composites like solid resin content,
fiber volume fraction, density on the specimens as per the ASTM standards. The
details of the test procedures are given below
4.1 Density Measurements
This method is employed to determine the density of composite samples by
Archimedes principle.
Apparatus: Balance 0.0001 gmSuspension wire one end of which tied to sinker
Reagents : Distilled water, Paraffin wax with ceresin
Procedure
1. Attach the Balance-beam vertically to the weighing pan of the balance.
2. Place the Balance-Bridge across the pan without touching the sides of the pan and
the beam attached to it.
3. Hook and suspend the wire with sinker on to the Balance-Beam and weigh in air
(A gm).
4. Take approx. 1.0 - 2.0 gm of cut specimen (with regular sides) and tie along with
the sinker of the suspension wire, weigh in air (B gm) using Balance-Beam and take
care that it suspends to the same height as in step (3).
5. Separate the above specimen from sinker and immerse into the molten paraffin
wax to obtain uniform coating of wax and cool in ambient temperature.
6. Tie the wax coated specimen onto the sinker with suspension wire and weigh in
air (C gm) using Balance-Beam.
7. Remove wax coated specimen + sinker with wire from Balance-Beam.
8. Place distilled-water filled beaker onto the Balance-Bridge. Care should be taken
that the beam attached to the pan does not touch the beaker.
9. Hook and suspend the wire containing wax coated specimen + sinker onto the
Balance-Beam into distilled water and weigh in distilled water (W4 gm)
10. Remove the suspension wire containing wax coated specimen + sinker from the
beaker.
11. Separate the wax coated specimen tied to the sinker.
12. Take weight of suspension wire + sinker in distilled water (W3 gm).
The average density is calculated from six samples by using the equation
W1Density = -------------------------------
(W2+W3-W4) (W2-W1) ----------------- --------------- D d
Where W1 = Wt. of specimen in air (B-A)
W2 = Wt. of specimen + wax in air (C-A)
W3 = Wt. of specimen + wax in water
W4 = Wt. of specimen + wax + sinker in water
D = Density of water
d = Density of paraffin wax with ceresin
4.2 Fiber volume fraction measurements by Acid digestion method
This testing method involves the digestion of resin matrix[54] in a hot
digestion medium like Nitric Acid which doesn't attack the fibers excessively. This
test is mainly carried out to determine the fiber volume fraction of the composite
material.
Apparatus: Balance 0.0001gm accuracy
Hot water bath
Measuring Cylinder, 50ml
Sintered Glass Filter, 50/30ml Gr.I
Vacuum filter flask 1000ml fitted with funnel
with funnel attached to vacuum pump
Reagents: Conc. Nitric Acid AR Grade
Acetone (AR grade)
Procedure
3. Take a clean dry beaker and record the initial weight (W1gms)
4. Take approx. 1.0gm sample into the beaker and weigh (W2gms)
5. Put 30ml of Conc. Nitric Acid(70% aqueous) into the beaker and cover with
watch glass
6. Heat the beaker with contents on hot-water bath for 5hrs or till the digestion is
complete
7. Remove the beaker from hot-water bath and cool to room temperature. Wash
the fibers three times with distilled water followed by Acetone once. Ensure that
no traces of acid is left on the fiber
6. Dry sintered-glass filter with specimen in Hot-air circulating oven at
100C for 1hr to remove water and acetone
8. Cool the sintered-glass filter containing specimen tat room
temperature in a desiccator and weigh(B gm). By using the below
equation the following fiber volume fraction is calculated.
(B - A)
% Fiber Content = ------------- X 100
(W2-W1)
Results
and
Discussions
Results
and
Discussions
5. Results and Discussion
The present work has been categorized in to
1. Preparation of laminates by varying the process parameters.
2. Determination of physical properties by chemical methods
3. Specimen Preparation and mechanical testing of samples
5.1. Preparation of laminates by varying the process parameters
i). Raw materials used in the present study
In the present study polyimide resin is used as matrix which is purchased from
M/s. ANABOND Ltd, Chennai, and which is synthesized from a soluble polymeric
precursor. Carbon fabric is used as reinforcing agent, purchased from M/s. Nikunj
Eximp Enterprises, Mumbai (Torayca make) with 12k roving or in the form of bi-
directional fabric. The fibers/ fabric is fabricated from poly acrylo nitrile (PAN)
precursor.
ii).Laminate preparation
Epoxy resin (LY556) and Hardener (HT972) are mixed in the 27:10 ratio and
the solution is applied on the bi-direction carbon fabric and 320x320x2mm thickness
of the layers have been developed by stacking 10 layers in the mould.
Carbon fabric layers of size 320mm x 320mm are taken from fabric role by
cutting with the help of template of size 320x320 mm. Epoxy resin (LY 556) is
mixed with hardener (HT972) in the weight ratio 27:10 in a beaker. The moulds are
cleaned with acetone and then wax is applied to the moulds for easy removal of the
cured laminate. Resin impregnated carbon fabric layers are placed in the mould by
hand lay up technique. The laminate preparation sequence is shown in Fig.5. Three
laminates are prepared by different process parameters i). No vacuum and pressure
ii). With vacuum iii). With vacuum and pressure. The corresponding cure cycles,
vacuum levels and pressure application steps are given as cure cycle-1, curecycle-2
and cure cycle-3.
iii). Cure cycle Selection
Cure cycle-1.
i). Keep component temperature at 1400C for 4 hours
ii). Vacuum= - 960mbar
iii). Pressure = At 1400C, 1 bar up to 2 hours, 2.0 bar up to 4 hours.
Cure cycle-2
i). Keep component temperature at 1400C for 4 hours
ii). Vacuum= - 960mbar
iii). Pressure = At 1400C, 0.5 bar up to1 hour, 1.0 bar up to 2 hours, 1.5 bar up to 3
hours and 2 bar up to 4 hours.
Cure cycle-3
i). Keep component temperature at 1400C for 4 hours
ii). Vacuum= - 960mbar
iii). Pressure = NIL
The prepared laminates are designated as i). Vacuum and pressure
(Laminate-1), ii). Vacuum and pressure (Laminate-2) and Vacuum only (Laminate-
3). The prepared laminates are tested for mechanical properties.
5.2. Determination of physical properties by chemical methods
The material properties are standardized based on the epoxy resin content,
fiber volume fraction and void content of the samples. Density, resin content and
fiber volume fractions are determined by acid digestion and Archimedes principles
and the values are tabulated in Table.5.
5.3 Specimen preparation and mechanical testing of samples
The specimens are cut in the required dimensions as per the ASTM standards
using a diamond wheel cutting machine as shown in the Fig.5.1. The samples are
tested for tensile, flexural and inter laminar shear strength properties using universal
testing machine (UTM) as per the ASTM standards ASTM 3039, ASTM7264 and
ASTM D2344 respectively. The specimen samples are shown in Fig.5.2 to 5.4. For
each test 6 samples are tested and the average value of the test results is considered
as the material properties. Table.5.1-5.3 shows the mechanical properties of the
laminates, cured under different cure cycles.
5.4 Effect of fiber volume fraction on Mechanical properties
All mechanical properties were compared in Table.5.4, with respect to
consolidation thickness, density and fiber volume fraction. From table it is obvious
that the laminate-3 cured at vacuum condition only is exhibiting improvement in
tensile strength, flexural strength due to high fiber volume fraction. The effect on
ILSS properties is marginal because it is resin dependent property. ILSS value
depends on the interfacial strength among the fabric layers only.
5.5 Effect of resin content on the mechanical properties
If the resin content is more, the strength of the composite will be lower
because strength depends on the fiber volume fraction. Hence Laminate-1 cured
with vacuum and pressure condition is having high resin content due to vacuum
suction and pressure application. Therefore it exhibits good interfacial strength due
to high resin content. Therefore it is exhibiting high ILSS value.
5.6 Effect of void content on the process parameters
Vacuum phenomenon during process is removing the volatile gases hence
the trapping of gases in the component is minimum in laminate-3. Hence the voids
formed in the component are low. If the component is having low void content, it
exhibit high tensile, flexural and ILSS values. Pressure application may cause
trapping of volatiles between the fabric layers. Hence high void content is observed
in the laminate-1.
Finally it is concluded that to minimize the weight of the structure, fiber
volume fraction should be high and which reduces the final component thickness
due to good tensile and flexural properties. The effect of three different processes on
ILSS is marginal.
ConclusionsConclusions
6. Conclusions
1. High fiber volume fraction is achieved in the laminate-3 with only vacuum curing
process of the laminate.
2. High fiber volume fraction in Laminate-3 is exhibiting improved tensile and
flexural strength of the laminate.
3. The effect of vacuum and pressure application on ILSS properties is marginal.
4. Low void content in the laminate-3 is attributed to vacuum suction phenomenon,
it in turn improves the tensile and flexural properties.
5. Finally it is suggested to implement vacuum curing process instead of oven curing
for C-E shells of the Re-entry vehicle structure.