COMPOSITES FACT SHEET Composites Fact sheet Introduction to Composites Composites are lighter, stronger and have more design shape freedom than aluminium.1 These advantages are reasons why aircraft manufacturers use more composite materials in their aircraft nowadays. Composites increase the design shape freedom. For example, the Boeing 787 Dreamliner weight consists of 50% composite materials2 and is more aerodynamic than previous models due to more design flexibility of composites in comparison with metals. Low weight and better aerodynamics contribute to 20 – 30% less fuel consumption than today’s similarly sized aircraft.2 However composite material has a few disadvantages as well. Composites are susceptible to different kinds of damage than metal structures such as micro-cracking and delamination.3 Conventional damage detection methods are not optimized to detect these kind of damages. That is why additional structural weight is necessary to provide safety at all times.4 Furthermore, damage Figure 1: A Boeing 787 Dreamliner from Arkefly assessments of composites (Figure 1) take more time nowadays than traditional metal structures (Example 1) due to the lack of routine with the repair of these large structures. Example 1: WILLEMSTAD, September 25th, 2014 A Boeing 787 Dreamliner from Arkefly was involved in an incident with a ground vehicle. The aircraft was hit by a high-loader, which was supplying the aircraft at the time. Due to a thorough assessment by Arkefly in cooperation with Boeing the passengers were delayed for almost a day. After inspection it turned out the aircraft was not damaged by the ground vehicle and Arkefly received approval to fly to Amsterdam. 1 Composites Fact sheet Properties of composites A composite material can be described as a combination of two or more materials having a recognizable interface between them.5 In this fact sheet the focus will be on the fibre-reinforced composites used in the aviation industry. This is a combination of fibres and a matrix (polymer resin). The two materials work together to combine the individual properties, which results in a lighter or stronger structure in comparison with traditional metal structures. It is possible to create a large part of an aircraft out of one piece composite. (Figure 2) Figure 3: Different materials used in the Boeing 787 Dreamliner With the design freedom of composites it is possible to improve the aerodynamics of an aircraft resulting in higher fuel efficiency. Furthermore it is possible to increase the humidity in the cabin since composite is not as sensitive for corrosion and fatigue as aluminium, resulting in a more comfortable flight for the passengers.2 (Table 2) Figure 2: Front section of Boeing 787 Dreamliner AdvantaGes Disadvantages Good strength to weight ratio Expensive production In the introduction it was stated that 50% of the total weight of the Boeing Corrosion resistance Labor intensive repair procedures 787 Dreamliner consists of composite materials (Figure 3). Carbon Fatigue resistance Labor intensive damage inspection composites are used more frequently because of their other good methods Excellent machining/ shaping properties in comparison with metals. Not only the low weight to strength capability to increase aerodynamics ratio is an advantage of composite materials, but fatigue and corrosion and save fuel resistance are important as well. The fatigue and corrosion resistance Possibility to increase humidity in results in less maintenance than metal structured aircraft.1 pressurized cabin Less maintenance Table 2: Advantages and disadvantages of composites in aviation 2 Composites Fact sheet Different sources of damaGe Nowadays the production of composites is rather expensive in comparison A lightning strike on a composite surface is approximately 1,000 times less with metals. However, mass production will probably decrease the price in conductive than on a metal surface6. The energy of the lightning strike is the future. The biggest disadvantage at the moment is the labor intensive restricted to a smaller area which results in a larger damage area damage inspection methods. This can be damage from foreign objects or (Figure 4). An aircraft surface made of a metal does not have to be better imperfections in the composite material. Damage of composite materials is in case of a bird strike, however the damage is harder to see with a different than damage in metals such as aluminium. After an analysis of composite structure. the types sources of these damages, the different kinds of inspection methods will be described. If the damage sources are not clear it can result Foreign object damage in unknown damaged parts and, in a worst case scenario, can lead to a Foreign object damage (FOD) is also a frequent damage source in the total failure of the aircraft structure. The most frequent sources can be current aviation industry. A FOD is damage caused by an object that is not divided into the following categories: impact in-flight, foreign object part of the flying aircraft. These items can be small rocks, screws, tools, damage (FOD) and damage caused by ground handling equipment. 6 debris etc. (Example 2) Impact in-flight Example 2: CHARLES DE GAULLE, July 25, 2000 The impact in-flight damage occurs when the aircraft is in movement. When the Concorde almost reached its take-off speed the aircraft struck a These impacts can be a lightning strike or a bird strike. The detection of thin metal strip on the runway. This strip had fallen from the underside of these impacts is better with a metal aircraft structure than a composite a DC-10 that had departed a couple minutes earlier, causing one of the structure. With a bird strike the damage is hard to see on the composite Concorde’s tires to burst. A large piece of the tire struck the aircraft resulting in a shockwave that caused a fuel tank to leak. When a spark surface while a metal surface shows a dent in case of a bird strike; a ignited this fuel both engines stopped functioning, causing the aircraft to composite structure, on the other hand, will return to its original shape. eventually crash. Ground handling damage The National Aerospace Laboratory (NLR) in the Netherlands has investigated the amount of ground handling incidents in 2008. They found out that 1 out of every 5000 flights has had a ground handling accident.7 This means that 61% of all incidents occurring to aircraft happen in combination with ground handling. A lot of ground handling equipment is needed to service a wide-body aircraft (Figure 5). The damage can differ from a small paint scratch to severe damage. However (especially with Figure 4: Damage of composite from a lightning strike 3 Composites Fact sheet composite materials) it is sometimes not visible on the outside how severe the damage is on the inside. This makes it very important that an incident is always reported. Delamination Figure 6: Cross sectional view of composite plate with a delamination Figure 5: Ground handling operations Types of damaGe Impacts on a composite aircraft structure can result in severe damage. Some damages (such as cracks) are the same as metal aircraft structures. However composites have one kind of damage that is significantly different in comparison with metals. This damage is called delamination. With delamination the layers of composite separate from each other. (Figure 6) Hereby the structure is seriously weakened. Barely Visible Impact Damage (BVID) is another version of delamination because this type of damage is barely visible from the outside. Therefore inspection methods had to be created to detect delamination.8 4 Composites Fact sheet DamaGe detection methods surface of the composite. The difference in echo time between an A visual inspection is often used as a damage detection method. However, undamaged and a damaged composite surface is used to identify internal to detect impact damage that is barely visible to the human eye other damage which cannot be detected with a visual inspection. A RDC is detection methods have been developed. The following three non- reliable for detecting internal damage, however this inspection method is 9 destructive inspection (NDI) methods are portable and practical in use in only usable for small areas. the aviation industry: visual inspection, ultrasonic pulse echo inspection Tap test inspection and a tap test inspection. For a more thorough investigation, radiography, For the tap test a special hand-held device is used. This device consists of a thermography and shearography are used. A relative new NDI technique hammer with an accelerometer built into the tip. This device measures the currently under development is ultrasonic verification (USV). contact time of the hammer tip with a composite surface (Figure 8). If a Visual inspection composite material is damaged, the contact time of the hammer tip will A visual inspection is used first when inspecting the surface of an aircraft. increase due to lower contact With a visual inspection it is possible to detect paint scratches and stiffness. A tap test is usable to wrinkles. Also discoloration can be detected. Discoloration can be caused detect impact damages on small due to a lightning strike. For internal damage of a composite material more surfaces. For larger areas or sophisticated NDI techniques are needed. delamination this device is not well 8 suited. Figure 8: Tap test inspection Ultrasonic pulse echo inspection A device which is often used for damage Radiography detection in composite aircraft structures X-ray is a synonym for radiography. With this is a “Ramp Damage Checker” (RDC) inspection method low energy x-rays pass (Figure 7). This NDI technique is based on through the composite material. A sensitive film ultrasonic pulse echo. In an undamaged for x-rays is placed under the material. This film composite plate an ultrasonic sound allows the inspector to analyze discolorations of wave will travel through the material the film.