Ocena wątku:
  • 0 głosów - średnia: 0
  • 1
  • 2
  • 3
  • 4
  • 5
Carbon Fibre : Sweeping technical world off its feet
In 1879, the world experienced a novel level of innovation that peacefully took over the reins of technical textile. It was in this year that Edison took out a patent for the manufacture of carbon filaments suitable for use in electric lamps. However, it was in the early 1960s when successful commercial production was started, as the requirements of the aerospace industry - especially for military aircraft - for better and lightweight materials became of paramount importance.

    The use of carbon fiber fabric is now widespread and it is in everything from tennis rackets to bicycle frames. It has become a big part of Formula 1 cars and many supercars as well. Several cruisers use carbon fibre extensively. However, as carbon fibre is not part of daily textile needs of consumers, many are unaware that this fibre has become an undeniable part of everyday lives. The truth is that it is everywhere to be found in the technological world.

    Carbon fibre is an exceptionally lightweight strengthening fibre derived from the element carbon. Sometimes known as graphite fibre, when this extremely strong material is combined with a polymer resin, a superior composite product is produced. Also called graphite fibre or carbon graphite, carbon fibre consists of very thin strands of the element carbon. Carbon fibres have high tensile strength and are very strong for their size. In fact, carbon fibre might be the strongest material there is. Speaking in technical terms, each fibre is 5-10 microns in diameter. One micron (um) is 0.000039 inches.

    In Textile Terms and Definitions, 3k carbon fiber fabrics has been described as a fibre containing at least 90 percent carbon obtained by the controlled pyrolysis of appropriate fibres. The term graphite fibre is used to describe fibres that have carbon in excess of 99 percent. Large varieties of fibres called precursors are used to produce carbon fibres of different morphologies and different specific characteristics. The most prevalent precursors are polyacrylonitrile (PAN), cellulosic fibres (viscose rayon, cotton), petroleum or coal tar pitch and certain phenolic fibres. Based on modulus, strength, and final heat treatment temperature, carbon fibres are classified into different categories.

    As far as carbon fibre cloth is concerned, spools of carbon fibre are taken to a weaving loom, where the fibres are then woven into cut resistant fabric. The two most common types of weaves are plain weave and twill. Plain weave is a balanced checker board pattern, where each strand goes over then under each strand in the opposite direction. Whereas a twill weave looks like a wicker basket. Here, each strand goes over one opposing strand, then under two. Both twill and plain weaves have an equal amount of carbon fibre going each direction, and their strengths will be almost same. The two are aesthetically different.

    Carbon fibre is primarily used for producing sporting goods, which account for nearly 11 million lb of that material. Currently, the United States of America consumes nearly 60 percent of the world production of carbon fibres, while the Japanese represent for almost 50 percent of the world capacity for production. The world production capacity of pitch-based carbon cloth is almost totally based in Japan. The key to further carbon fibre market expansion is continued development of high-rate manufacturing methods and considering this, it is predicted that demand of the fibre will increase by 235 percent by 2020.

    Carbon fiber is composed of carbon atoms bonded together to form a long chain. The fibers are extremely stiff, strong, and light, and are used in many processes to create excellent building materials. Carbon fiber material comes in a variety of "raw" building-blocks, including yarns, uni-directional, weaves, braids, and several others, which are in turn used to create composite parts. The properties of a carbon fiber part are close to that of steel and the weight is close to that of plastic. Thus the strength to weight ratio (as well as stiffness to weight ratio) of a carbon fiber part is much higher than either steel or plastic. Carbon fiber is extremely strong. It is typical in engineering to measure the benefit of a material in terms of strength to weight ratio and stiffness to weight ratio, particularly in structural design, where added weight may translate into increased lifecycle costs or unsatisfactory performance.

    Carbon fibers or kevlar fabrics are fibers about 5–10 micrometres in diameter and composed mostly of carbon atoms. Carbon fibers have several advantages including high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion. These properties have made carbon fiber very popular in aerospace, civil engineering, military, and motorsports, along with other competition sports. However, they are relatively expensive when compared with similar fibers, such as glass fibers or plastic fibers.

    The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile. The remaining 10% are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms. The exact composition of each precursor varies from one company to another and is generally considered a trade secret. During the manufacturing process, a variety of gases and liquids are used. Some of these materials are designed to react with the fiber to achieve a specific effect. Other materials are designed not to react or to prevent certain reactions with the fiber. As with the precursors, the exact compositions of many of these process materials are considered trade secrets.

    Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers. Commercially, the stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

    Once the fibers are stabilized, they are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures. The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering. As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber. In some processes, two furnaces operating at two different temperatures are used to better control the rate de heating during carbonization.

    After carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized. The addition of oxygen atoms to the surface provides better chemical bonding properties and also etches and roughens the surface for better mechanical bonding properties. Oxidation can be achieved by immersing the fibers in various gases such as air, carbon dioxide, or ozone; or in various liquids such as sodium hypochlorite or nitric acid. The fibers can also be coated electrolytically by making the fibers the positive terminal in a bath filled with various electrically conductive materials. The surface treatment process must be carefully controlled to avoid forming tiny surface defects, such as pits, which could cause fiber failure.

    Resistance to Fatigue in Carbon Fiber Composites is good. However when carbon fiber fails it usually fails catastrophically without much to announce its imminent break. Damage in tensile fatigue is seen as reduction in stiffness with larger numbers of stress cycles, (unless the temperature is hight) Test have shown that failure is unlikely to be a problem when cyclic stresses coincide with the fiber orientation. Carbon fiber is superior to E glass in fatigue and static strength as well as stiffness.

    Carbon Fiber has good Tensile Strength

    Tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, or failing. Necking is when the sample cross-section starts to significantly contract. If you take a strip of plastic bag, it will stretch and at one point will start getting narrow. This is necking. It is measured in Force per Unit area. Brittle materials such as carbon fiber does not always fail at the same stress level because of internal flaws. They fail at small strains.

    Testing involves taking a sample with a fixed cross-section area, and then pulling it gradually increasing the force until the sample changes shape or breaks. Fibers, such as carbon fibers, being only 2/10,000th of an inch in diameter, are made into composites of appropriate shapes in order to test.

    Fire Resistance/Non Flamable

    Depending upon the manufacturing process and the precursor material, carbon fiber can be quite soft and can be made into or more often integreted into protective clothing for firefighting. Nickel coated fiber is an example. Because carbon fiber is also chemically very inert, it can be used where there is fire combined with corrosive agents. Carbon Fiber Fire Blanket excuse the typos.

    Thermal Conductivity of Carbon Fiber

    Thermal conductivity is the quantity of heat transmitted through a unit thickness, in a direction normal to a surface of unit area, because of a unit temperature gradient, under steady conditions. In other words its a measure of how easily heat flows through a material.

    Because there are many variations on the theme of carbon fiber it is not possible to pinpoint exactly the thermal conductivity. Special types of Carbon Fiber have been specifically designed for high or low thermal conductivity. There are also efforts to Enhance this feature.

    Low Coefficient of Thermal Expansion

    This is a measure of how much a material expands and contracts when the temperature goes up or down. Units are in Inch / inch degree F, as in other tables, the units are not so important as the comparison. In a high enough mast differences in Coefficients of thermal expansion of various materials can slightly modify the rig tensions. Low Coefficient of Thermal expansion makes carbon fiber suitable for applications where small movements can be critical. Telescope and other optical machinery is one such application.

    Non Poisonous, Biologically Inert, X-Ray Permeable

    These qualities make Carbon fiber useful in Medical applications. Prosthesis use, implants and tendon repair, x-ray accessories surgical instruments, are all in development. Although not poisonous, the carbon fibers can be quite irritating and long term unprotected exposure needs to be limited. The matrix either epoxy or polyester, can however be toxic and proper care needs to be exercised.

    Carbon Fiber is Relatively Expensive

    Although it offers exceptional advantages of Strength, Rigidity and Weight reduction, cost is a deterrent. Unless the weight advantage is exceptionally important, such as in aeronautics applications or racing, it often is not worth the extra cost. The low maintenance requirement of carbon fiber is a further advantage.

    It is difficult to quantify cool and fashionable. Carbon fiber has an aura and reputation which makes consumers willing to pay more for the cachet of having it. You might need less of it compared to fiberglass and this might be a saving.

    Carbon Fibers are brittle

    The layers in the fibers are formed by strong covalent bonds. The sheet-like aggregations readily allow the propagation of cracks. When the fibers bend they fails at very low strain.

Skocz do:

Użytkownicy przeglądający ten wątek: 1 gości