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The potential of FRP materials in a passenger aircraft structure
Fibre Reinforced plastic/polymer (FRP) represents a composite material described as a polymer matrix reinforced with fibres. The FRP composite is no longer a new material, but a material of choice in the airtech industry, as long as constructors require lightweight amenities with enhanced mechanical features.
The fibres used are various, such as carbon, glass, basalt, aramid, wood, asbestos or even paper. The polymer can be polyester thermosetting, epoxy or vinylester. These materials can change their properties in compliance with their structural assignments. A FRP is a combination between a matrix (binder) of low modulus material and some reinforcing elements. Those elements are 10 to 1000 times stronger and stiffer than the matrix (Nelson, 2010).
Figure 1: “The properties of common reinforcing fibre” (Charles Sturt University, 2009)
Advantages and Disadvantages of FRP
There are two kinds of processes in manufacturing the FRP:
Forming the fibrous material: this can be done by aligning the material in an x-direction or in a y-direction; the costs and efforts are increased because a large amount of skilled labour is required (disadvantage). If the material is aligned in three dimensions (x, y and z), the mechanical properties and impact damages are increased and costs of manufacturing are decreased (advantage).
Bounding with the matrix (by moulding): this should be a chemical process designed to strengthen the fibres, envelop and protect them; must be stable from the chemical and physical points of view.
Figure 2: Composite Materials in Aircraft: “CFRP = Carbon-fiber-reinforced; GFRP = GFRP - Glass Fiber Reinforced Polymer” (Composite Materials in Aircraft, n.d.)
The best suitable FRP combinations for airplanes are the ones made out of polymer with:
Glass - is manufactured in a filament form; this is the most popular way to reinforce a polymer and has a great advantage in reducing weight and improving the aerodynamics of a surface (Smallman, 2002).
Carbon - is meant to replace aluminium as a solution for reducing weight and improving the mouldering process; its use also reduces costs in airplane production.
Aramid - can be manufactured in various forms and designed after specific needs; there are various types from the strength and rigidity point of view (Baker, 2002).
The mechanical properties of FRP can be increased depending of the fabrication process. The resistance of the composite material is given by the matrix and the way it protects the fibre from the action of the environment and prevents the reduction of strength due to fatigue or abrasion (Kivits, Charles and Ryan, 2010).
The Use of FRP containing glass can be regarded as disadvantageous at first glance, but one must consider the fact that there are parameters that count in the way FRP can be designed to suit a certain part of an airplane. An advantage is that FRP is lighter than steel, but it costs more to design.
The resistance at corrosion for FRP with carbon is excellent and there are also remarkable physical and thermal properties for FRP with polyester or vinyl (advantage). When the composite contains a metal, it is much more likely to be corrosive (disadvantage), but the FRP can be improved by adding paint or a coat from an environmentally friendly material (Marsh, 2009).
Aramids have a high impact resistance and great damage tolerance; therefore, they are to be used for the engine pylons and fuel lines. Corrosion represents a problem that can be avoided by keeping dissimilar metals out of contact with a wet environment (Reichhold.com, 2009).
FRP Strength: Failure and damage resistance of composite structures
The composite materials used for aircraft should be the ones that absorb impact energy by plastic deformation - a process very different from the way metal reacts on impact. Studies concerning the way composite materials react in case of damage had shown that the strength of FRP is tested when the tension applies more on the matrix – in which case, it causes fractures between the fibres. Also, the fibres can end up separated from the matrix and their fracture can damage the whole composite. In this case, that part of the plane can be torn apart (Reichl, 2007).
The main purpose of the FRP repairing technique is to restore the qualities of any damaged component as much as it can in order to fulfil the tasks it was design for. There are several ways to repair FRP composite materials, such as resin injection, doubler or scarf repair, regarding the kind of damage and where it is.
As the injection is mainly used for stopping the damage spreading as a temporary measure, double repairs will permanently restore the structural strength regardless of the surface's aerodynamics (Katnam, Silva and Young, 2003).
The scarf repair will offer all the advantages mentioned above, thus offering more benefits, especially for external panels. On the other hand, FRP repairs are adhesively bonded. Repairs using mechanically fastened procedures cannot be made on sandwich components or on thin laminates due to mechanical stress generated by fasteners. When adhesively bonded, the material will offer better joint efficiency and aerodynamics, this being a procedure of choice to obtain an effective stress transfer mechanism (Nelson, 2010).
The bond quality, which is based on adhesive type, curing factors, surface treatment and joint design, has to be sturdy enough to safely acquire the stress transfer between the two parts being repaired. Generally speaking, adhesively bonded patches, with zero load eccentricity and minimum peel stresses, represent the most efficient way of FRP repairing.
On the other hand, there are some issues for in place scarf repair: bondable surfaces, precise damage evaluation, accurate scarfing, controlled pressure and temperature (Nelson, 2010).
The idea of recycling arose in the mid-70s, nourished by the oil crisis of 1974, 1978 and 1979, which led to important increases in raw material prices. The recycling of FRP dates back to the late 80s and is still an issue nowadays. In any case, with the increasing usage of composite polymers, the idea of recycling them is becoming more stringent than ever. Incineration and landfills have always been the most at hand methods for disposing of composites waste, being responsible for 98% of the disposed waste, while re-use and recycling through mechanical is limited to only 2% of the rejected material. In order to comply with the legislation in use, FRP manufacturers will need to develop materials with real recycling potential (Faruk, 2012).
There are on-going trials that evaluate processes for products and energy recovery from FRP, such as pyrolysis, grinding, incineration/co-incineration and fluidised-bed processing. Other processes can use mixed polymer waste with cement-like composites in the construction industry (Farrow and Young, 1988). At the present time there are no plants for FRP recycling in the UK, but they can be found in Germany (ERCOM), France (MCR) and Norway (Miljotek), which proves that large scale recycling of FRP is feasible, but these facilities need supportive markets for their recycling. Due to these facts, researchers need to develop new FRP materials and write new recycling standards. In addition, they need to address minimalising manufacturing waste, dismantling and collecting end-life waste (Faruk, 2012).
The matter of re-use, recycling and safe disposal should be evaluated at the material's design stage. European legislation is forcing FRP industry towards re-use, mechanical or chemical recycling, sets limitations on incineration and in several countries even forbids direct landfill of FRP (Faruk, 2012).
FRP recycling is a viable option, but the industry needs to develop new FRP materials to be more eco-friendly. Due to these challenges, an alternative to FRP will be green technology, which will hopefully develop reliable biocomposites. However, the biggest issue in dealing with natural fibre reinforced plastic composites is their feature variability, which at the moment renders them unreliable (Haliwell, 2006).
FRP and Aircraft Structure
At the beginning, FRP materials were used in secondary aircraft structures, like control surfaces, small doors or fairings. Now, as the technology has improved, the demand has increased and FRP usage in primary structures such as fuselages and wings is more common (Deo, Starnes and Holzwarth, 2001). In the picture below, different types of composites on different aircraft structures are shown.
Figure 3: Types of composites on aircraft (Kumar, 2013)
In comparison with conventional materials, advanced composites usage in aircraft and helicopters construction have achieved 20-30% in weight savings. Advanced composites with metallic and non-metallic honeycomb cores and metal are now used for landing gears, fin boxes, fairngs, floor boards, rudders, engine cowls, doors and several others interior components. A recently launched prototype, Advanced Light Helicopter (ALH), is having 60% of the surface area made up of FRP, including advanced fibre materials and metal sandwich panels (Kumar, 2013).
Given the composite materials used in the aircraft and helicopters industry, up to 20-30% of the materials' weight can be diminished, when compared to conventional construction materials. Fairings, engine cowls, landing gears, fins doors, floors and a number of other interior components are made of advanced composites combined with metallic and non-metallic honeycomb structures. The recently developed experimental Advanced Light Helicopter (ALH) has up to 60% of its surface area constructed out of composite components such as advanced fibre components or metal sandwich structures (Kumar, 2013). Composites are used for lowering the weight of rudders, elevators, ailerons, forewings, flaps, fillets, wings or fuselage (Zagainov, 1996). For the parts of an airplane that are prone to impact by stones or birds, such as tails and wings, woven composites are recommended instead of carbon composites (Baker, 2002).
The Future of FRP Composites in Passenger Aircraft Structures
The future of composites relies on three directions: the development of new materials, the improvement of existing materials composition and the usage of current materials in new structures.
These are tested materials, but which are not yet applied in aircraft industry. Some of these are ceramic matrix composites, metal matrix composites, shape memory metals and nano-composites.
The possible usage of ceramic matrix composites are mainly in the high-temperature sector of the engines (combustor, transition duct convergent flags, turbine disks, turbine aerofoils, acoustic liners) (Kivits, Charles and Ryan, 2010). The possible application of metal matrix composites may be in heavily loaded surfaces (floor supports, turbine fan blades, helicopter rotor blades).
Whenever shape memory metals are heated, they will return to their shape prior to the deformation, a property which renders them suitable for hybrid applications (variable jet intake), so metals can compete with ceramics in previously unsuitable applications (Park and Kong, 2013). Nano-composites use the high length to width ratio and colossal surface area per mass ratio of nano-scale components to enhance the material's features.
A technique that has a lot in common with the bruising and healing process of human wounds has been employed by Bristol University's aerospace engineers on fibre-reinforced polymer composites (FRP) (ScienceDaily, 2008). The technique deals with filling hollow glass fibres in the FRB using a mixture of resin and a hardener material; whenever the fibres are damaged, the resin-hardener mixture flows into the damaged area, the composite consequently recovering up to 90% of its strength, which allows a plane to function normally even with damaged materials.
"This approach can deal with small-scale damage that's not obvious to the naked eye but which might lead to serious failures in structural integrity if it escapes attention," says Dr Ian Bond, who has led the project. "It's intended to complement rather than replace conventional inspection and maintenance routines, which can readily pick up larger-scale damage, caused by a bird strike, for example." (ScienceDaily, 2008)
Improving the safety of FRP composites using this technique will hopefully lead to a greater number of aircraft manufacturers employing the use of these materials, leading to a wider use of lighter aircraft, which means fuel savings and an extended lifetime for components (ScienceDaily, 2008).
Several structures have been tested or are in different stages of development. Some are already on the market, such as fibre metal laminates. Lattice weighs about 15% of a traditional solid plate with the same dimensions, having good strength and damage architecture. It is believed that foam structures can be an alternative to honeycomb structures, due to their higher performance and lower costs.
Laminate structures, such as fibre metal laminates, prevent catastrophic failure and exhibit enhanced impact features (King, Inderwildi and Carey, 2009). With uninterrupted incremental fuel price and environmental lobbying, the passenger aircraft industry needs to improve its efficiency, and weight reduction is necessary in this regard.
While technology is advancing, new types of carbon and basalt nano-tube forms will surely extend composite utilisation. Thus, in this industry, composite materials will certainly be employed widely in the near future (King, Inderwildi and Carey, 2009).
Whenever goals such as weight savings, finite tolerances, precision engineering and the pursuit of a good weight/strength ratio are sought, fibre-reinforced plastics are the proper answer, as they are cheaper, faster and easier to manufacture than conventional metal components without sacrificing strength and stress tolerance.
The increasing popularity of these lightweight, high-performance materials is not limited to the aircraft industry, as automotive, wind turbine and space industries have also manifested interest towards their use. Revolutionary techniques such as the self-repair system devised by the engineers at Bristol University might set the future standards for these industries, given the ability of FRP to suit a wide range of design programs.
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