In this article we will discuss about:- 1. Introduction to Composites 2. Influence of Fibre Orientation and Concentration 3. Fibre Phase 4. Various Matrix Materials 5. Natural Fibres 6. High Performance Fibres 7. Aspect Ratio and Fibres 8. Glass Fibres 9. Stress-Strain Behaviour of Fibres, Matrix and Composites 10. Bulk Moulding Compounds.
Introduction to Composites:
A composite is a multiphase material that exhibits a significant proportion of properties of both constituent phases. It is based on the principle of combined action, better properly combination are achieved through liquidious combination of two or more distinct materials.
One phase is termed as matrix which is continuous and surrounds the other phase often called dispersed phase.
Properties of composites depend upon concentration, size, shape, distribution and orientation of the dispersed phase.
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Ceramics (refractories) are high temperature resisting brittle material and metal are ductile and melting point varying from low to high.
Composites possess the nature of orthotropy and anisotropy. They can be homogeneous or heterogeneous.
A composition of ceramics and metal in different proportion gives desired thermal and mechanical properties in cermet.
Influence of Fibre Orientation and Concentration:
(i) Continuous and Aligned Fibre Composites:
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Stress strain Behaviour when tensile stress is applied in the direction of alignment for the composite in stage 1 both fibre and matrix behave elastically.
Typically matrix deformed plastically while fibre continues to stretch elastically. Tensile strength of fibre is much higher than matrix from stage 1 to stage 2 the proportion of applied load that is borne by the fibre increases. The onset of composite failure begins as fibre starts to fracture at strain ϵf. Composite failure is not sudden as all fibres do not fracture at the same time and even if all fibres fail, the matrix remains intact as long as ϵm > ϵf.
In the elastic region, condition of equal strain exists in fibre and matrix (Isostrain condition).
Fig: When Tensile Strength is applied in Transverse Direction.
Tensile stress in longitudinal direction corresponds to fibre fracture stress and marks onset of composite failure. Generally Ef > Em, so when fibers get fractured the entire load is taken by the matrix.
Properties of continuous and aligned fibrous composited are highly anisotropic (including strength). Under actual circumstances some transverse loading may also be there which may lead to premature failure of composite as its transverse strength is very low.
(ii) Discontinuous and Aligned Fiber Composites:
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Their reinforcement efficiency is lower than continuous fibre composites and have strength and moduli of elasticity that approach 50% and 90% respectively of their continuous fibre counterparts.
(iii) Discontinuous and Randomly Oriented fibre Composites:
These fibre are used in:
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a. Applications involving multi directional stresses use discontinuous fibres randomly oriented in matrix.
b. Production rates of short fibre composites are rapid.
c. Intricate shapes can be formed that are not possible with continuous fibre reinforcement.
d. Fabrication costs are considerably lower compared to continuous and aligned fibre composites.
Fibre Phase:
Fibre are classified according to their diameter and character into:
(i) Whiskers:
These are very thin single crystals that have extremely large length to diameter ratios. As a result of their small size they have a higher degree of crystalline perfection and are virtually flaw free.
This accounts for their exceptionally high strengths, they are strongest materials.
Whiskers are not extensively used as reinforcement medium because they are extremely expensive. Moreover, it is very difficult to incorporate whiskers into a matrix.
Examples are:
SiC, Al2O3, graphite, Silicon nitride.
(ii) Fibers:
These are not single crystals but polycrystalline amorphous and have small diameters. These are either polymer or ceramics.
e.g. are Al2O3, Aramid, Carbon, Boron, SiC.
(iii) Wires:
Relatively large diameters. Typical materials are steel, Mo, W. They find application in radial steel reinforcement in automobile tyres, filament winding of rocket motors, and wire windings of higher pressure hoses.
Matrix Phase:
It may be metal, polymer or a ceramic. Generally metals are preferred because of their higher ductility and strength.
It performs the following functions:
It binds the fiber together and act as a medium by which externally applied stress is transmitted and distributed to fibers, only a very small portion of applied stress is carried by a matrix phase. Elastic modulus of fiber material should be much higher than matrix material.
It protects individual fibers from surface damage as a result of chemical reactions with environment and mechanical abrasion. Such interactions may introduce surface flaws on composites surface and may lead to crack initiation.
It separates individual fibers and prevents the propagation of crack between fibers, which could result in sudden failure.
Various Matrix Materials:
i. Mylar: A form of Flake:
Mylar, (polyester) is used as thin flake (sheet of 25 to 150 nm thickness) in magnets and as an insulating material.
Aluminium diboride is a new material for planer reinforcement in flaked composites. Planer stiffness of its flake measures 265 GPa which is four times higher than the cross-plied planer reinforced graphite fibres.
The flakes have excellent damage tolerance and are easy to process, and holes can be drilled into them for attachments. However, they suffer from limited stretch-ability under application of pressure.
ii. Wood-Plastic Composite:
Wood-plastics composite is made by impregnating natural wood with liquid monomers such as methyl methacrylate, acrylonitrile, or styrene.
Fibres are transformed to other forms for reinforcement. Various forms obtained from continuous filaments.
Usually the transformed forms are the following:
a. Strands, and
b. Yarns
a. Strands:
In order to minimise self-abrasion and mechanical damage to the fibres, they are collected and size (starch) is applied. Then sized fibres are binded together to form a strand. Thus a strand is combination of large number of fibres which can be termed as unified multi-fibres.
b. Yarn:
Yarn is produced from sized cake by first twisting the strand then followed by playing a number of twisted strands together to form a doubled balanced yarn. It is designated by its ‘Count’ which is expressed in the unit of Tex’. One Tex is a measure of linear density and is defined as weight in gms per km. Yarns are used for manufacturing woven fabrics.
Natural Fibres:
i. Ramie:
It is one of the strongest natural fibres. It possess greater strength when wet. It is not as durable, is usually used as a blend with other fibres such as cotton or wool.
It is stiff and brittle, and breaks if folded repeatedly in the same place. Low resilient and is low in elasticity and elongation potential.
ii. Sisal Fibre:
It is fairly coarse and inflexible. It is used for its strength, durability, ability to stretch, affinity for certain dyestuffs, and resistance to deterioration in saltwater.
iii. Coir Fibres:
These are gained from coconut husks. Coir fibres are light in weight, strong and elastic.
iv. Flax:
It is obtained from flax fibre plant. It generally known as ‘patsan’ which is a substitute of jute’. The flax fibre is strong and wiry, longer and finer in nature. Flax fibre is used for its strength, luster, durability and moisture absorbency.
v. Jute:
It is a bast and one of the cheapest natural fibres. It possesses a poor resistance against moisture, brittles under influence of light, absorbs paint easily.
vi. Hemp Fibre:
It is a bast fibre, and is yellow-brown in colour.
vii. Palmyra Fibre:
It is obtained from palm (toddy) plants. It is a bast fibre.
High Performance Fibres:
High-performance fibres are fibres based on polyester, nylon, aramid and polyfoam. Fibres such as aramid (kevlar and nomex), polybenzimidazole (PBI), sulfur and spectra have increased the range of choice for materials.
Glass developed for use as continuous fibres. It is composed of 55% silica, 20% calcium oxide, 15% aluminum oxide, and 10% boron oxide.
Types of Glass Fibres:
i. A – glass fibre for acid resistance.
ii. C – glass fibre for improved acid resistance.
iii. D – glass fibre for electronic applications.
iv. E – glass fibre for electrical insulation.
v. S – glass fibre for high strength. (65%, SiO2, 25%, Al2O3, 10%, Mgo)
Boron Fibre:
Boron fibres are composites of the substrates tungsten, silica coated with graphite or carbon filaments upon which boron is deposited by a vapor-deposition process (CVD).
The final boron fibre has a specific gravity of about 2.6, a diameter between 0.01 and 0.15 mm, a tensile strength of about 3.5 GPa, and a tensile modulus of around 415 GPa.
It is more expensive than graphite and requires expensive equipment to place the fibres in a resin matrix with a high degree of precision.
Carbon and Graphite Fibres:
Carbon fibre usually has a modulus of less than 345 GPa and a carbon content between 80% and 95%. Graphite fibres have a modulus of over 345 GPa and a carbon content of 99% or greater.
Another distinguishing feature is the pyrolyzing temperature. For carbon, this temperature is around 1315°C, and for graphite it is around 190° to 280°C. Pyrolysis is the thermal decomposition of a polymer.
Kevlar Fibre:
Kevlar, is an organic fibre termed as an aramid or aromatic polyamide fibre. This organic fibre is melt-spun from a liquid polymer solution.
The aromatic ring structure results in high thermal stability. The rod-like nature of the molecules classifies Kevlar as a liquid-crystalline polymer characterized by its ability to from ordered domains in which the stiff, rod-like molecules line up in parallel arrays.
Grades of Kevlar Fibres:
(i) Kevlar 29
(ii) Kevlar 49, and
(iii) Kevlar 149 (ultra high tensile strength)
Synthetic fibres produced in industries these are cheaper and more uniform in cross- section than the natural fibres. Their diameters very between 10µm to 100µm.
Examples- Glass, Boron, Carbon, Graphite, Kevlar
Organic fibres carbon and graphite fibres are light in weight, flexible, elastic and heat- sensitive.
Commercial carbon fibres are available by the trade names such as- Hyfil, Grafil, Fortafil, Thornel, PAN etc.
Inorganic fibres, have high strength, low fatigue resistance and good heat resistance.
Their examples are-Glass, Tungsten, Ceramic
Aspect Ratio and Fibres:
Composites may be termed as particulate composite, continuous fibre composite, or short fibre composite depending on the aspect ratio (I/d) as mentioned here width-
Where, l = length, d = die or shorten dimension in e non-circular section fibre.
Glass Fibres:
Glass fibres manufactured by molten ‘glass drop’ through minute orifices and then lengthening them by air jet.
The standard glass fibre used in glass-reinforced composite materials is E-glass, a borosilicate type of glass.
The glass fibres produced, with diameters from 5 to 25 µm, are formed into strands having a tensile strength of 5 GPa. Chopped glass used as a filler material in polymeric resins for moulding consists of glass fibres chopped into very short lengths.
Stress-Strain Behaviour of Fibres, Matrix and Composites:
Manufacturing Processes for Thermoset Composites:
1. Hand lay-up (or wet lay-up) process
2. Spray-up process
3. Filament winding process
4. Pultrausion process
5. Resin transfer moulding process
6. Structural reaction injection moulding (SRIM) process
7. Compression moulding process
8. Roll wrapping process
9. Injection moulding process
Manufacturing Processes for Thermoplastic Composites:
1. Thermoplastic tape winding process
2. Thermoplastic pultrusion process
3. Compression moulding of glass mat thermoplastic (GMT)
4. Hot press technique
5. Autoclave processing
6. Diaphragm forming process
7. Injection moulding process.
Bulk Moulding Compounds (BMCS):
These are premixed material of short fibre (Chopped glass stands) pre-impregnated with resin and various additives.
Dough moulding compound (DMC) is on alternative of BMC. BMC mode are limited to 400 mm in their longest dimension due to problems with separation of the components of moulding compound during molding.
Sheet moulding compounds (SMC). These are non-metallic plastic rein-forced- composite laminate mode-up by pressing together many unidirectional (U/D) laminc.
One over the other. Desired mechanical properties achieved by playing U/D laminae in different orientation.