7th March 2016
FIBER REINFORCED COMPOSITES
Composites refers to a material consisting of two or more individual constituents. The reinforcing constituent is embedded in a matrix to form the composite. Composite structures are quite common in nature where fiber and matrices are combined. For example a tree is composed from cellulose fibers in a matrix of lignin (a natural resin). The strength of wood is higher “along the grain” which defines the general direction of the cellulose fibers and it is weaker “across the grain”. Splitting a piece of wood along the grain is much easier because one just has to fracture the lignin matrix which is much weaker than the cellulose fibers.
The most common man-made composites are composed of glass or carbon fiber in a plastic resin. Resins can be of the form of thermoset or thermoplastic materials which each have their own unique advantages and disadvantages. The glass or carbon fibers are significantly stronger than the plastic matrix but they also tend to be brittle. A composite construction, therefore, allows one to take advantage of the excellent stiffness and strength properties of glass or carbon by embedding the fibers in a more compliant matrix.
When a composite structure is manufactured, a “dry” carbon fiber and a “wet” resin is used. The dry carbon fiber will be wetted with the resin and put inside a oven so that the resin can solidify and “block” the carbon fibers creating stiffness.
FIBER REINFORCED COMPOSITES IN INDUSTRY
The automotive industry has adopted composites slowly due to their higher costs. Particulate reinforced plastics have been used for some time but fiber reinforced composites have only really been used in high end sports cars but are starting to make their way into traditional vehicles. Race cars have used carbon fiber materials for many years.
Carbon fiber composites are now quite common in commercial and military aviation. The Boeing 787 and Airbus A350 XWB have roughly 50% of their structure from composite materials. Carbon fiber materials are also common in helicopter systems, rocket motors, satellite systems, and turbine engines.
Fiberglass has been the common material for recreational boats and yachts for many years. These are not traditional laminated composites, rather the glass fibers are oriented randomly giving the structure more uniform material properties. Being fairly cheap, glass fiber can be used in large structures; whereas carbon fiber would be too expensive.
Golf clubs and tennis rackets have used carbon fiber for many years as it provides light weight and high stiffness.
Dr Ralph Clague from Gordon Murray Design discusses the use of composite materials in the automotive industry
Carbon fiber is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are aligned parallel to the long axis of the fiber. The crystal alignment gives the fiber high strength-to-volume ratio (makes it strong for its size). Several thousand carbon fibers are bundled together which can then be woven into a fabric.
Advantages & Disadvantages
The advantages of carbon fiber materials are significant. First of all carbon fibers are very light fibers resulting in lightweight structures. Furthermore, one can chose between stiff or strong fibers depending on the composite part being produced and the type of carbon fiber material required. The exceptional impact properties make carbon fiber advantageous in various industry segments. During an impact carbon fibers disintegrates (metal instead would simply deform) which can make it is a very efficient energy dissipation mechanism. Although carbon fiber is fairly expensive compared to other more common fibers like fiberglass, the price is steadily decreasing due to the progress of production technology. Another major advantage is its thermal expansion is basically zero – this means that a metal for instance is expanding when heated, carbon fiber remains in its basic form. For specific projects where thermal stability is required carbon fiber can be a tremendous benefit. Moreover, the material can resist very high temperatures (1000 Celsius). Carbon fiber composite structures do not suffer any fatigue issues if designed and dimensioned properly. Finally, carbon fiber is permeable to X-ray and does not corrode, which is a huge issue with metals.
The material also has disadvantages that need to be taken into consideration when planning a project. For instance, carbon fiber is fairly expensive compared to other materials. Moreover, carbon fiber is an electric conductor and as such can reflect radio waves which can be a disadvantage in some cases. also, when the material brakes, the parts disintegrate which can cause some unpredictable results. For example in Formula One racing, debris from an impact can fly in multiple directions which could have safety implications for the driver. Finally, the manufacturing process for composite components can be more labor intensive than metallic parts.
Carbon Fiber Future Outlook
The manufacturing processes for fiber reinforced composites are improving steadily; however,it is still far away from the volume production achievable with metals or plastic. This holds true not only for carbon fibers but also for other kinds of fibers. The amount of carbon fiber used in various markets will simultaneously grow with the development of automated manufacturing techniques. A common view is that carbon fiber is a very stiff and brittle materials that breaks when a certain load is reached. Even though this might be true to a certain extend, carbon fiber should be known as carbon component structures that can be designed to work in a elastic and flexible way. In order to make the best use of the material and take advantage of its inherent properties, expertise in designing and manufacturing with carbon fiber is vital.
Carbon Fiber In Summary
Summarizing, carbon fiber is one of the most used fibers in many businesses simply because one of the lightest and strongest fiber available:
• High stiffness to weight ratio
• High strength
• Corrosion resistant
• Fatigue resistant
• Energy Absorption on Impact
• Tailored material properties
Dr Joerg Pohlman of SGL Automotive Carbon Fibers talks about the use of composite materials in BMW’s i3 and i8
Glass Fiber is one of the most common fibers in the composites industry. The main reason for its use is the relatively low costs involved as glass fiber is significantly cheaper than carbon fiber, for example. This makes glass fiber attractive for the production of large composite structures like boats, wind turbines and so on. Additionally, glass fiber is available in various grades. The E grade is the most common one, but also R and S are available. Both R and S grades have better mechanical properties but are more expensive compared to the E grade.
Advantages & Disadvantages
The material is fairly cheap, which can be very beneficial for large structures. Another benefit is the good chemical resistance to acids and solvents. Furthermore, glass fiber is electrically insulating, has a higher elongation at break compared to, for example, carbon fiber and has a high temperature resistance. Combining the properties of glass fiber, i.e. low moisture absorption, high strength, heat resistance and low dielectric constant makes the material ideal for various markets. The material is moisture resistant and has a high strength to weight ratio. Glass fiber can be processed in various ways and in combination with many resin systems: it is easy to trim after curing and processes such as wet layup and resin infusion are as well available and do not require high investments.
However, glass fiber also has some minor disadvantages: the material is not as strong as other materials or as stiff as many other materials such as carbon fiber.
Glass Fiber Future Outlook
Due to economic reasons, the material will remain the main choice in many sectors due to the significant price advantage in relation to higher performing fibers. The low stiffness issue can be easily overcome by designing sandwich structures where a lightweight core can be sandwiched between two glass skins and thereby making the end result stiffer. This kind of construction is already used in numerous market sectors. A typical example for the sandwich construction with glass fiber is the hull of many large boats.
Glass Fiber In Summary
Summarizing, glass fiber is a common fiber which is used in many businesses based on its properties:
• Cost (relatively cheap)
• Moisture resistance
• Chemical resistance
• Thermal conductivity
• Electrical properties
• High strength