Havacılık kompozitlerinde elektrik iletkenliğinin incelenmesi
Investigation of electrical conductivity in aerospacecomposites
Yazar: SERRA EFNAN GENÇ ÜSTÜNER
Danışman: DOÇ. DR. UMUT KIVANÇ ŞAHİN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Tekstil Mühendisliği Ana Bilim Dalı / Tekstil Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Tekstil ve Tekstil Mühendisliği = Textile and Textile Engineering

Özet:
Havacılık ve uzay bilimi, düşük ağırlıkları, yüksek mukavemetleri ve dayanıklılıkları nedeniyle giderek artan oranda kompozit malzemelere yönelmektedir. Yapısal parçalarda çokça kullanılan alüminyuma karşı alüminyumun yaklaşık yarısı kadar ağır olan kompozit yapılar, uçakların daha az yakıtla uçmasını sağlayarak havacılık uygulamalarında verimi arttırırlar. Kompozitler esas olarak iki veya daha fazla bileşen parçalardan oluşan, havacılık sektöründe ağırlıklı olarak karbon fiberlerle güçlendirilmiş plastiklerden meydana gelen malzemelerdir. Takviye malzemesi olarak karbon fiberlerin yanı sıra cam elyaf, kevlar lifleri de ağırlıklı olarak kullanılmaktadır. Mukavemet artışı için takviye malzemeleri kumaş formunda kullanılarak, farklı katmanlama teknikleriyle her yönde kompozite dayanım kazandırılabilir. Hava araçlarında en çok karşılaşılan dış etken kaynaklı sorunlar; yıldırım çarpması, hava sürtünmesinden dolayı uçak yüzeylerinde oluşan statik elektrik ve uçak içinde bulunan elektronik ekipmanlardan kaynaklı elektrik ark oluşumudur. Uçak yapılarında kullanılan kompozit malzemeler alüminyum gibi çok tercih edilen metallere göre çok az elektriği iletmektedirler. Bu sorunların üstesinden gelebilmek için uçak üzerinde elektrik yolunun oluşturulup, elektriğin uçak yapısından uzaklaştırılması gerekmektedir. Elektriğin uzaklaştırılması için uçak dış yüzeyine iletken boya uygulaması, cam kumaştan oluşmuş kompozit yapılar için yıldırım şeridi uygulaması, kanat ve kuyruk uç kısımlarına statik boşaltıcı fitil montajı, kompozit yapıların dış yüzeylerine metalik kumaş ya da folyo entegresyonu, bağlayıcıların etrafında ark oluşumunu engellemek için macun uygulaması, elektrik hattının ilerleyebilmesi için birbirine bağlanan yapılar arasında metalik plaka uygulaması yapılmaktadır. Bu çalışmada numune üretimine uygun, kolay ilerleyen, üretim sırasında müdahale edilebilirliği sağlanan vakum torbası yöntemi kullanılarak karbon kumaşların en üst katmanına metalik kumaş/folyo uygulaması yapılmıştır. RF geçirgenliği istenen kompozit malzemeler için cam kumaştan serim yapılarak dış yüzeye yıldırım şeridi uygulamasıyla birlikte elektrik yol oluşturulması amaçlanmıştır. Üretilen parçalar üzerinde boyut, fırınlama etkisi, farklı metalik malzeme bileşenleri, metalik malzemenin farklı boyutları ve serim için düz-eğimli yüzey kullanılarak elektrik dirençleri ölçülmüştür. Metalik malzeme olarak bronz kumaş, büyük boşluklu gerdirilmiş bakır folyo, küçük boşluklu gerdirilmiş bakır folyo ve farklı boyutlarda yıldırım şeritleri kullanılmıştır. Kompozit malzemeye metalik kumaştan elektriğin aktarılması için çukurlu rondela, cıvata, farklı yapılarla bağlantıyı sağlayan alüminyum plaka ve somun bağlantısı yapılmıştır. Elektrik direnç-akım ilişkisine ve havacılık standartlarına bakılarak fırınlamanın ve yüzey eğiminin direnç üzerinde büyük etkiye sebebiyet vermediği, metalik malzemeler arasında büyük boşluklu gerdirilmiş bakır folyonun en optimum malzeme olarak seçilmesi gerektiği ve cam fiber takviyeli polimer kompozitlere uygulanan yıldırım şeritlerinin boyutsal olarak istenen elektrik direnç değerini sağladığı sonucuna varılmıştır.

Abstract:
Aerospace science is increasingly choosing composite materials for their low weight, high strength, and durability. Composite structures, which are about half as heavy as aluminum, increase efficiency in aviation applications by enabling airplanes to fly with less fuel, against aluminum, which is widely used in structural parts. Composites are materials that are mainly composed of two or more parts, and in the aerospace industry, they are mainly made of plastics reinforced with carbon fibers. In addition to carbon fibers, glass fiber and kevlar fibers are mainly used as reinforcement material. By using reinforcing materials in the form of fabrics to increase strength, the composite can be strengthened in all directions with different layering techniques. Mechanical and physical properties of materials; rigidity, strength, corrosion resistance, toughness, hardness, UV resistance, heat transmission ability, electrical conductivity ability, fatigue, low density, heat resistance. It is not possible to find the best conditions for most of these properties in the same material. The main goal of composite material production is to create a new material with the desired technical properties, by combining two or more different material structures with technological mixturecombination methods, and to create a new material whose technical properties are improved in the expected direction.
Composite materials consist of two different components. These are the matrix and fiber reinforcement elements dispersed in the matrix. The matrix protects the brittle and brittle reinforcing elements against environmental and external effects, and prevents the composite material from breaking, while the reinforcement material increases the load-bearing and strength of the composite material. Matrix materials, which are the binders of the composite material prevent the movement of the fibers independently within the composite structure and keeps the fiber structures together. It is responsible for transferring the loads from the composite structure to the fiber reinforcement elements. It creates the form of the composite material and the matrix phase wraps the fibers and protects them from environmental influences.
Carbon composite materials also have a very important place in composite materials. This material, also called carbon fiber reinforced polymer composite, gains extraordinary durability and hardness when heat treatment is applied at very high temperatures.
Carbon-containing composites have advantages as well as disadvantages in aircraft. The most important of those features is electrical conductivity. The importance of electrical conductivity in an aircraft is also related to how the aircraft is exposed to an electrical charge. Exposure of the aircraft to friction under the influence of weather conditions causes static electricity to accumulate on the outer surface. Static electricity occurs in the structure originating from the equipment in the aircraft. A lightning strike is the most dangerous electrical charge for aircraft. The listed situations can have serious or even fatal consequences for aircraft structures if they are not avoided or removed from the structure. Especially in critical areas such as the fuel system, electric arcs may cause the aircraft to explode.
Some of the ways to create conductive paths and protect from electrostatic charges in aircraft are; wire bundle shields, grounding strips, using expanded foil in the composite structure, using metallic meshes (woven, knitted) in the composite structure, conductive paint application, metal frame using, to use lightning diverter strips, using metallic bonding strips between components, to choose the appropriate fastener for the materials, to apply sealing paste to prevent arc formation in the fastener areas, to use static dischargers on aircraft end components. Metal is applied to the outside of the composite laminate to protect aircraft parts from electrical damage. Due to its low
electrical resistance, this additional layer conducts the high current on the laminated surface and generally does not damage the underlying CFRP structure at all. Metal is typically used in the form of woven fabrics or expanded foils. Metal application in composite structures can be done in three ways. Adhering aluminum foil to the structure as an outer layer, bonding aluminum, copper, or bronze mesh to the structure as an outer layer or embedded one layer down, and incorporating strips of conductive material into the laminate. The metal materials used in the composite are generally
used in the form of foil, expanded foil mesh, woven or non-woven mesh. When used in plain foil form, it is not preferred as it will cause delamination in the structure and prevent resin adhesion. In addition, if the resin under the foil evaporates during a large electrical load, such as lightning, the build-up of pressure will further damage the foil.
The electrical path can be created using metal mesh. Two types of metal mesh can be created. A woven mesh is produced by machine weaving, while a non-woven mesh is formed by punching a foil. In woven mesh, metal wires are woven diagonally to provide electrical conductivity. Woven metal meshes require higher mechanical strength than perforated meshes due to the stress on the wires during weaving and therefore use alloyed metals. In addition, woven meshes have higher tenacity and double thickness due to the localized double thickness at the cross points.
If the current following the electrical path finds a gap, it can create an arc. Frequent contact between shielding layers and metal fasteners is required to transfer current to adjacent parts or substructures. This contact can be made either by direct transmission between the CFRP laminate and the in-hole fastener, or by attaching the fasteners to overlapping washers, sometimes called dimpled washers, or to metal folding plates. To provide better contact between fasteners and CFRP, countersunk holes and connection heads are used. However, testing has shown that significant arcs can occur around the fasteners.
At the same time, there are composite structures made of glass fiber in aircraft structures. In order for the equipment used for communication in aircraft to work properly, the outer shell structure in the area should not be conductive. The composite structure in these regions should be glass fiber reinforced and should not contain metallic mesh-foil. Instead, lightning diverter strips applied along the outer surface will protect this area. These strips may consist of solid metal rods or a series of closely spaced conductive metal buttons. 
There are many production methods for aerospace composites. In this study, the vacuum bagging method was chosen in terms of both ease of production and applicability to prototype parts. Vacuum bagging improves the mechanical properties of open molded laminates. This process can produce composites with the same properties and also remove the air trapped between the reinforcement-resin during
production, thus reducing the number of voids in the product and increasing the strength. Fiber wetting can be fully achieved when structures produced by hand layup techniques are vacuumed. With the full realization of the bonding between the resin and fiber, laminate separation is prevented. Vacuum bagging can be used with wetlaid fabrics or prepreg fabrics. After the manual lay-up steps, the vacuum bag is mounted on the mold to compress the laminate and remove air pockets. In the case of pre-impregnated advanced composite molding, the prepreg material is laid on the mold, the vacuum bag is installed. In the curing process, the mold is heated or the mold is placed in an autoclave that applies external pressure, which contributes to both the heat and the atmospheric pressure force. The prepreg-vacuum bag-autoclave method is mostly used to create advanced composite aircraft and military products.
In this study, metallic fabric/foil application was made on the top layer of carbon fabrics by using the vacuum bag method, which is suitable for sample production, easy to use, and can be intervened during production. For composite materials with RF transmission, it is aimed to create an electrical path with the application of lightning diverter strips on the outer surface by laying out glass fabric. The electrical resistances were measured on the produced parts using the size, curing effect, different metallic material components, different sizes of the metallic material, and flat&curved surfaces for laying. Bronze mesh, the large hole expanded copper foil, the small hole expanded copper foil and lightning strips of different sizes were used as metallic materials. To
transfer electricity from metallic fabric to the composite material, a dimpled washer, bolt, aluminum plate, and nut connection are made that provide the connection with different structures. Considering the electrical resistance-current relationship and aerospace standards, it was concluded that curing and surface inclination do not cause a great effect on the resistance, that the expanded copper foil with large gaps should be selected as the most optimum material among the metallic materials, and that the lightning diverter strips applied to the glass fiber reinforced polymer composites provide the desired electrical resistance value in terms of dimensions has been decided.