As the name indicates, composite material is a material which is different from heterogeneous materials. It is a material which have strong fibers, it may be continuous or non continuous surrounded by a weaker matrix material. The matrix task is to distribute the fibers and also transmit the load to the fibers.
Modern composite materials were used about 30 years ago when boron reinforced epoxy composite was used for the skins of the empennages of the US F14 and F15 freighters. In the beginning composite materials were used only in secondary structures, but as knowledge and development of materials has improved, their use in primary structure has increased. Composite materials are attractive to aviation and aerospace applications because of their exceptional strength and stiffness to density ratios and superior physical properties. Better known man made composite materials used in aerospace industries are carbon and glass fibre reinforced plastic (CFRP and GFRP) respectively, both of which are stiff and strong. The fundamental design concept of composites is that the bulk phase accepts the load over a large surface area, and transfers it to the reinforcement material, which can carry a greater load. (2)
Fig 2: Use of Composite on the skins of empennage
However the main concern with the use of CFRP and GFRP is that the defects are difficult to detect. Defects may result from the raw product such as fibres, matrix and prepregs, as well as due to poor bonding between the fibres and matrices and between individual lamina. Defects may also occur from in-service use such as low velocity impacts in aircraft structures. The NDT technique which is used to detect cracks and defects in components are listed below:
(3)
Visual Inspection
Liquid Dye Penetrant
Ultrasonic Inspection
Thermography
Laser Shearography
Mechanical Impedance
Magnetic Particle Testing
Fibers
Fibers are produced from thousands of filaments, each filament having a diameter between 5 and 15 micrometers which allow them to be producible using textile machines.
Composite materials have their own special balance of properties which when combined with resin (matrix) and fibers (reinforcement) are able to produce the resulting material which is much stronger and stiffer than the original material, it is the fiber that is mainly accountable for the strength and stiffness.
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Fig 3: Natural Fibers
The most common fiber types are:
Glass fiber reinforced plastic (GFRP)
Aramid or Kevlar fiber reinforced plastic (AFRP)
Carbon fiber Reinforced Plastic (CFRP)
Fiber Glass
The filaments are obtained by securing the glass (silicon+ sodium carbonate and calcium carbonate with temperature more than 1000) through the small orifices of a plate made of platinum alloy
Fiberglass is a material made from very accomplished fibers made of glass which is used as a build-up agent for most of the polymer products and resulting composite material, properly known as fiber-reinforced polymer (FRP) or glass-reinforced plastic (GRP). Fibreglass is steady, durable, and impervious to many caustics and to maximum temperatures
Fiber Reinforced Polymer (FRP) is an effectively new class of composite material manufactured from fibers and resins and for the development and repair of new and deteriorating structures has proven efficient and economical. The mechanical properties of FRPs make them perfect for common applications
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Fig 4: Fiber glass in Plain Weave Design
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Fiber Reinforced Polymer Laminate structure
FRPs are typically ordered in a laminate structure in such a manner that each lamina (or flat layer) contains an alignment of unidirectional fibres or woven fibre fabrics embedded within a thin layer of light polymer matrix material. To provide the strength and stiffness, the fibres typically contains of carbon or glass,. The matrix is commonly made of polyester, Epoxy or Nylon, binds and secures the fibers, and transfers the stresses between fibers.
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Properties
A glass fiber would be stiff and strong with respect to tension and compression along it’s axes from both side if it is analyzed independently, but due to the long aspect ratio of fiber it is considered that fiber is weak in compression because a typical fiber is long and narrow, and it buckles easily. Across its axis a glass fiber is not stiff and strong in its shear strength but if a combination of fibers can be arranged definitely in a particular direction within a material and the fibers are avoided from buckling in compression, then the material is suppose to become strong in that particular direction
Uses
Fibreglass is used in following ways
Thermal Insulation
Electrical Insulation
Reinforcement of various materials
Tent pole
Sound absorption
Automobile bodies
Hockey sticks
Medical purposes
Kevlar Fiber
Kevlar is also known as Twaron and poly-paraphenylene terephthalamide, is a fiber which is also called synthetic fibre and is five times stronger than steel, weight for weight. Kevlar is a heat opposing and decomposes above 400 °C without melting. It is spin into ropes or fabric sheets that can be used as such or as constituent in composite material components. It is usually used in bulletproof vests, in sports equipment, and in aircraft construction. It is also used as a backup for steel cords in fire suits and in the tires of the vehicles. Kevlar was invented by the DuPont Corporation in the early 1960s, following the work of Stephanie Kwolek. Kevlar is a diplay trademark of E.I. du Pont de Nemours and Company. It is also used in the Military in the form of gloves, bulletproof vests or helmets.
Aramid/Kevlar Fiber Protective Glove, Aramid Glove, Protective Glove http://www.bulletstopper.net/wp-content/uploads/2009/06/vest.jpg http://www.bulletproofme.com/Images/Kevlar%20Helmet%20and%20Tactical%20Goggles.jpg
Fig 4: Use of Kevlar in Production of Military Equipment
Production
Kevlar is manufactured from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (Aramid) with shifting benzene rings and amide groups. These polymer strands are adjusted randomly, when they are produced. To make Kevlar, they are dissolved and spun, causing the polymer chains to allign in the direction of the fiber.
Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated sulphuric acid in its manufacture. The chemical synthesis of Kevlar from 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride is given below
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Three different grades of Kevlar
There are several modifications of Kevlar, developed for various applications:
Kevlar 29 : It has got high strength (520000 psi/3600 MPa) and low density (90 lb/ft³/1440 kg/m³) fibers used for manufacturing bullet-proof vests, composite armor reinforcement, helmets, ropes, cables, asbestos replacing parts.
Kevlar 49: It has got high modulus (19000 ksi/131 GPa) , high strength (550000 psi/3800 MPa) and low density (90 lb/ft³/1440 kg/m³) fibers used in aerospace, automotive and marine applications.
Kevlar 149:It has got high ultra high modulus (27000 ksi/186 GPa), high strength (490000 psi/3400 MPa), low density (92 lb/ft³/1470 kg/m³) highly crystalline fibers used as reinforcing dispersed phase for composite aircraft components.
The most popular matrix materials for manufacturing Kevlar (aramid) Fiber Reinforced Polymers are Thermosets such as Epoxies (EP), Vinylester and Phenolics (PF).
Properties
Kevlar fibers possess the following properties:
High tensile strength (five times stronger per weight unite than steel);
High modulus of elasticity;
Very low elongation up to breaking point;
Low weight;
High chemical inertness;
Very low coefficient of thermal expansion;
High Fracture Toughness (impact resistance);
High cut resistance;
Textile processibility;
Flame resistance.
High mechanical strength results due to these high properties and its remarkable heat resistance. Because it is highly unsaturated, i.e. the ratio of carbon to hydrogen atoms is quite high, it has a low flammability.
Kevlar’s main deficiency is that it crumble under alkaline conditions or when disclosed to chlorine. While it can support great tensile stress, like all fibers it tends to bend in compression. In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a composite
The disadvantages of Kevlar are their ability to absorb moisture, difficulties in cutting and low compressive strength.
Kevlar Fiber Reinforced Polymers are produced by open mold processes, closed mold processes and Pultrusion method
Carbon Fiber Reinforced Plastic (CFRP)
Carbon fiber-reinforced polymer or carbon fiber-reinforced plastic (CFRP or CRP), is a very secure, strong, light, and expensive composite material or fiber-reinforced polymer. It is woven into a textile material and resin such as epoxy resin is applied and then allowed to cure for some time. The resulting material is very strong as it has the best strength to weight ratio of all the construction materials. It is a development of glass fibre reinforced plastic, although much more expensive.
A carbon fiber is a long, thin strand of material about 0.0002-0.0004 in (0.005-0.010 mm) in diameter and is made mostly of carbon atoms. The carbon atoms are fixed together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The fibers are incredibly strong for its size because of the alignment of the crystal from which it is made. To form a yarn several thousand carbon fibers are weaved together, which may be used by it woven into a fabric. The yarn or fabric is combined with epoxy and wound or moulded into shape to form different composite materials
Carbon fibers were developed in the 1950s as reinforcement for high-temperature moulded plastic components. The first fibers were produced by heating strands of rayon until they carbonized. This development proved to be inefficient because the fibers produced as a result of this process contained only about 20% carbon and had low strength and stiffness properties. In the early 1960s, a process was developed using polyacrylonitrile as a raw material. A carbon fiber that contained about 55% carbons and had much better properties was produced as a result of this development. For producing carbon fibers the polyacrylonitrile conversion process quickly became the perfect method.
In 1970s, work was started to discover substitute raw materials which led to the introduction of carbon fibers made from a petroleum pitch produced from oil processing. These fibers had excellent flexural strength because they contained about 85% carbon. They were not broadly accepted because they had limited compression strength.
Classification of CFRP
Carbon fibers are arranged by the tensile modulus of the fiber. Tensile modulus is a measure of how much pulling force a certain diameter fiber can apply without breaking. Carbon fibers which is restricted to “low modulus” have a tensile modulus below 34.8 million psi
Carbon fibers are classified according to the manufacturing method
1. PAN-based carbon fibers (the most popular type of carbon fibers).
In this method carbon fibers are produced by a transfer of polyacrylonitrile precursor to the following stages:
Expanding filaments from polyacrylonitrile precursor and their thermal oxidation at 400°F. Stress is then applied on these filaments.
Carbonization in Nitrogen atmosphere at a temperature about 2200 °F for many. During this stage non-carbon elements volatilize resulting in improvement of the fibers with carbon.
Graphitization at about 4500 °F (2500°C).
2. Pitch-based carbon fibers.
Carbon fibers of this type are manufactured from pitch:
Filaments are spun from coal tar or petroleum asphalt (pitch).
The fibers are cured at 600°F (315°C).
Carbonization in nitrogen atmosphere at a temperature about 2200 °F (1200°C).
Carbon Fiber Reinforced Polymers (CFRP) are characterized by the following properties:
Light weight;
High strength-to-weight ratio;
Very High modulus elasticity-to-weight ratio;
High Fatigue strength;
Good corrosion resistance;
Very low coefficient of thermal expansion;
Low impact resistance;
High electric conductivity;
High cost.
The main use of Carbon Fiber Reinforced Polymers (CFRP) is to produce automotive, marine and aerospace parts, sport goods (golf clubs, skis, tennis racquets, fishing rods), bicycle frames.
Weaves
Fabric is produced in different collection and design and then various mechanisms is used to produce those design and effect on the fabric which helps to form different weaves and many design which enhances the look of apparels.
Plain Weaves
It is simplest of all patterns, in which each weft yarn goes alternately over and under one warp yarn. Each warp yarn goes from time to time over and under each weft yarn. Plain weave is a most simple style of weaves; it has basically got a check board effect which has the weft and warp threads interwined in an alternate way. It is also sometimes known as one-up-one-down weave or over and under pattern. In this type of design equal tension and spacing is applied to the warp and weft and are of equal tension and spacing and it is equally visible on the surface.
The plain weave may also have variations including the following:
Rib weave: In diameter the filling yarns are larger than the warp yarns. A rib weave produces fabrics in which very less yarns per square centimetre are clearly visible on the surface.
Matt Weave or Basket weave: Here, two or more yarns are used in both the warp and filling direction. These groups of yarns are woven as one, producing a basket effect
Characteristics of Plain Weave
It is snag resistant.
It wrinkles very easily.
It has lower elastic strength
Basket Weave
The opposite of the plain weave is usually basket or checkerboard pattern. In this type of weave design different types of colours are used and it also much cheaper, less durable than plain weave. Basket weave is the modification in height and width of plain weave. For each plain weave point two or more yarns are lifted or lowered over or under two or more picks. When the groups of yarns are equal in the whole weave design, the basket weave is then defined as regular; otherwise it is defined as irregular.
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Fig 5: Basket Weave
There two types of basket weave available in this type of design:
a) Regular basket weave: This is mainly used for edges in drapery, or as a bottom in very small weave repeats, because the texture is too loose-fitting for big weave repeats; moreover, yarns of different groups can slip, group and overlap, spoiling the appearance.
b) Irregular basket weave: This is mainly a combination of irregular warp and weft ribs.
Method of Construction: Two or more warps simultaneously interlaced with one or more fillings.
Twill Weave
In this type of weave it creates a design of diagonal, chevron, hounds tooth, corkscrew. The design is upgraded with coloured yarn due to which it becomes stronger and may develop a shine. Twill weave is defined by diagonal ridges formed by the yarns, which are exposed on the surface. These may vary in angle from a low slope to a very steep slope. Twill weaves are more closely woven, heavier and stronger than weaves of comparable fiber and yarn size. They can be produced in complicated designs.
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Twill weave is defined by the effect of parallel diagonal ribs formed left-to-right or right-to-left. It is created by the interlacing of 2 or 3 warp threads over 1 or 2 filling thread in rows. The fabric which is produced as a result of this design is more pliable and drapeable than the plain or basket weave but less pliable than satin
Fig 6: Twill Weave
Characteristics of Twill Weave
It has fewer interlacing than plain weave.
It is durable and heavier at the same time.
It is wrinkle resistant.
It is resistant to showing soil and soiling.
The twill direction is defined as left or right hand or variation.
It is more ravelling than plain weave.
Method of Construction: Three or more shafts; warp or filling floats over two or more counterpart yarns in progressive steps right or left
Satin Weave
Satin weave is a more adaptable type of weave than the plain weave but it is more complicated. In this weave construction, the interlacing of the threads is arranged in such a way that the face of the cloth is covered with the warp yarn or filling yarn and no twill line is apparent. It is made by “floating” warp or weft yarns across many yarns to bring them to the surface.
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Fig 6: Satin Weave
Characteristics of Satin Weave
With a smooth surface it is flat and lustrous.
The surface slides easily for linings.
Resin
The Resin is mainly accountable for the principle of composite structure. Resin binds the fibers together to allow equal distribution of loads and to save the notch sensitive fibers from self abrasion and externally induced scratches. It helps to protect the fibers from external moisture and chemical corrosion or oxidation which may spark to the cause of failure. Resin systems for composites which are advanced aim to behave in a brittle manner with low strain to failure and with a high modulus compared to systems designed for non reinforced applications.
Types of Resin
The main types of resin which are used in yielding of composite components are epoxy, polyester and vinyl ester. The most simple and common matrix is polyester resin, which is mostly used in composite applications for relatively low stress situations. Most of advanced composite applications call for the use of epoxy resins. The search for improved matrices continues, especially to allow production of composites suitable for use at higher temperatures and with lower moisture sensitivity.
Composite material for aircraft structures by Brian C.Hoskin and Alann A.Baker. Editors pg 47
Epoxy Resin
Epoxy Resins are a class of compounds which accomodate two or more epoxide groups. The major types are formed by reacting polyphenols with epichlorhydrin under basic conditions. If the polyphenol is diphenylol propane, the most usual epoxy resin is achieved. Trade names include such materials as Epikote. The resin is supplied as a viscous clear to light yellow liquid.
Composite material for aircraft structures by Brian C.Hoskin and Alann A.Baker. Editors pg 48-49
Properties
Although epoxy is expensive than other polymer matrices, it is the most popular matrix. In Aerospace Industry more than two thirds of the polymer matrices used is epoxy based. The main reasons why epoxy is the most used polymer matrix material are
High strength and stiffness
Low viscosity and low flow rates, which allow good wetting of fibers and prevent misalignment of fibers during processing
Low volatility during cure
Low shrink rates, which reduce the tendency of gaining large shear stresses of the bond between epoxy and its reinforcement.
Available in more than 20 grades to meet specific property and processing requirements.
Mechanics of composite materials by Author K.KAW second edition PG 28
Advantages and Disadvantages
The main advantages of epoxy resins are:
Ability to formulate for optimum properties for particular applications
Control of fracture toughness
Moderate convenience to use
The main disadvantages of epoxy resins are:
Expensive compared to polyesters
Less convenient than polyesters due to relatively slow cure and high viscosity
Limited resistance to some organic materials
Composite material for aircraft structures by Brian C.Hoskin and Alann A.Baker. Editors pg 52
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