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lightweight materials carbon fibre full report
Post: #1

Light weight material-Carbon Fibre

Steel and other hard construction materials have revolutionized the field of industry. Now, a stage has come that there is a need of a better material to catch up with the growing needs and demands of the modern society. This need has bought up a newer material to the field which is now known as Carbon Fibres.
Carbon fibre is one of the latest reinforcement materials used in composites. It's a real hi-tech material, which provides very good structural properties, better than those of any metal. Carbon fibre has a tensile strength almost 3 times greater than that of steel, yet is 4.5 times less dense. Carbon fibers are carbon fibres with values of Youngâ„¢s modulus between 150 and 275 to 300 GPa.
When you go to a sports shop you are inundated with new "graphite" based materials for sports equipment: golf clubs, tennis rackets, bicycles (frames and wheel disks), ultra light airframes feature these new lightweight materials. But, we are also familiar with graphite as being a very common and mundane substance. Graphite has long been a component of pencil lead, and is used as a basic lubricant. How is it that graphite is both a hi-tech and low-tech material? Could we take a bunch of pencil leads and epoxy them together into a cutting edge tennis racquet? Anyone who has used mechanical pencils knows that the leads break far too easily to provide a strong frame. It would seem as if there are two different kinds of graphite. In fact, this is true. When vendors market "graphite fibre" products they are usually selling a "carbon fibre" product. The correct name for the fibres used in all strengthening and reinforcing applications is carbon fibres. But, there is more to the story than just a general misconception over the term "graphite fibres." Surprisingly, if we look at a small section of graphite and carbon fibres on the atomic level they appear to be identical.
What is Carbon Fibre?
Carbon Fibre is one of the most recent developments in the field of composite materials and is one of the strongest fibers known to man. It is usually the first choice of fibre if something very strong and very light is required. Carbon fibre was originally developed in space technology, but has now been adopted in many other areas of manufacture. Racing car monocoques and aero plane wings are usually constructed of carbon. Generally the term "carbon fibre" is used to refer to carbon filament thread. Carbon fibre is one of the latest reinforcement materials used in composites. It's a real hi-tech material, which provides very good structural properties, better than those of any metal. This material is known for its high specific stiffness and strength. The material has an advantageous combination of good mechanical properties and low weight. With the decrease in its cost over recent years, it is fast becoming one of the leading materials in many areas, including performance sport equipment, transport, scientific experiments and even wallets and watches!
Key Benefits
Property Fine Grained Graphite Unidirectional Fibres 3-D Fibres
Elastic Modulus (GPa) 10-15 120-150 40-100
Tensile Strength (MPa) 40-60 600-700 200-350
Compressive Strength (MPa) 110-200 500-800 150-200
Fracture Energy (kJm-2) 0.07-0.09 1.4-2.0 5-10
Oxidation resistance Very low poor better than graphite

CARBON FIBRE 3.50 1.75 2.00
STEEL 1.30 7.90 0.17

Carbon fibre has a tensile strength almost 3 times greater than that of steel, yet is 4.5 times less dense.
Some other properties of carbon fibre are:
¢ high tensile strength
¢ low thermal expansion
¢ Resistance to corrosion and fire
¢ High stress tolerance levels
¢ electrically and thermally conductive
¢ Chemical inertness
¢ light weight and low density
¢ very hard and brittle
¢ high abrasion and wear resistance
Carbon fibres are long bundles of linked graphite plates, forming a crystal structure layered parallel to the fiber axis. This crystal structure makes the fibers highly anisotropic, with an elastic modulus of up to 5000GPa. Fibres can be made from several different precursor materials, and the method of production is essentially the same for each precursor: a polymer fibre undergoes pyrolysis under well-controlled heat, timing and atmospheric conditions, and at some point in the process it is subjected to tension. The resulting fiber can have a wide range of properties, based on the orientation, spacing, and size of the graphite chains produced by varying these process conditions.
Precursor material is drawn or spun into a thin filament. The filament is then heated slowly in air to stabilize it and prevent it from melting at the high temperatures used in the following steps. The stabilized fibre is placed in an inert atmosphere and heated to approximately 1500°C to drive out the non-carbon constituents of the precursor material. This pyrolysis process, known as carbonization, changes the fibre from a bundle of polymer chains into a bundle of "ribbons" of linked hexagonal graphite plates, oriented somewhat randomly through the fibre. The length of the ribbons can be increased and their axial orientation improved through further heating steps up to 3000°C, a process called graphitization. Because the graphite ribbons are bonded to each other perpendicular to the fibres only by weak Van der Waals bonds, the ribbons must be reoriented to increase the tensile strength of the fibre to a useful level. This is accomplished through the application of tension at some point in the stabilization or pyrolysis phases, the exact time depending on the precursor material. Increased axial orientation increases the fibre's tensile strength by making better use of the strong covalent bonds along the ribbons of graphite plates.
Polyacrylonitrile (PAN) and rayon are the most commonly used precursors. PAN is stretched during the stabilization phase, and heated to 250°C in air. The tension is then removed, and the fibre is heated slowly in an inert nitrogen atmosphere to 1000-1500°C. Slow heating maintains the molecular ordering applied by tension during the stabilization phase. Graphitization at temperatures up to 3000°C then follows. Applying tension at 2000°C further increases the proper ordering of graphite ribbons. Rayon, a cellulose-based fibre made from wood pulp, is spun into a filament from a melt, and stabilized without tension up to 400°C. It is then carbonized without tension up to 1500°C, and is stretched in the graphitization phase up to 2500°C

Manufacturing of Carbon fibre parts
A wide range of different processes have developed for moulding of composites parts ranging from very simple manual processes such as hand lay to very sophisticated highly industrialized processes Each process has its own particular benefits and limitations making it applicable for particular applications. The choice of process is important in order to achieve the required technical performance at an economic cost
The main technical factors that govern the choice of process are the size and shape of the part, the mechanical and environmental performance and aesthetics. The main economic factor is the number of identical parts required. Most processes will have an initial investment or set up cost. This is a major factor in the choice of process. Some of the common methods are:
¢ Open moulding - hand and spray lamination
¢ Vacuum Infusion
¢ Resin injection
¢ Vacuum Bag and Press Moulding
¢ Pultrusion
¢ Very low weight
¢ High impact tolerance
¢ Insensitive to climate and temperature changes
¢ Reduced maintenance costs
¢ Long service life
The chief drawback of carbon fibre composites is that they oxidize readily at temperatures between 600-700°C, especially in the presence of atomic oxygen. A protective coating (usually silicon carbide) must be applied to prevent high-temperature oxidation, adding an additional manufacturing step and additional cost to the production process. The high electrical conductivity of airborne graphite particles creates an unhealthy environment for electrical equipment near machining areas. Carbon fibre composites are currently very expensive and complicated to produce, which limits their use mostly to aerospace and defense applications.
Carbon fibres are cutting edges in:
¢ Aerospace and aircraft industry
¢ Sports equipment
¢ Automotive parts
¢ Small consumer goods like laptops, watches etc.
¢ Air filtration
¢ Fishing rods and tripods
¢ Acoustics
¢ As a microelectrode in extracellular recording in medicine
Carbon Fibre is now an engineering material that must be designed, engineered and manufactured to the same standards of precision and quality control as any other engineering material. Carbon fibre thus has revolutionized the field of light weight materials. This can be used as a substitute for steel without the most of latterâ„¢s difficulties like high weight, lack of corrosion resistance etc. This is thus one of the future manufacturing materials.
Material Science and Engineering, van Black
Post: #2
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