Recent Developments in Techniques to Minimize Lightning Current Arcing Between Fasteners and Composite Structure

Hasim Mulazimoglu

Alcoa New Product Development, Aerospace Products 900 Watson Center Road, Carson, CA 90745

Luke Haylock

Alcoa New Product Development, Aerospace Fasteners 3000 W. Lomita Blvd., Torrance, CA 90505

Abstract Lightning protection of composite structures, which are extensively used on newer composite aircrafts, is more complex due to the inherent high resistance of carbon fibers and epoxy, the multi-layer construction and the anisotropic nature of the structure. On the other hand, the intrinsic high conductivity of metallic fasteners and the large number of fasteners used in aircraft construction combine to create a condition of a high probability of lightning attachment to fasteners when they are used for joining composite structures. It is well know that lightning currents may create detrimental ignition sources by attaching to a fastener and flowing through the fastener to some point, which may result in arcing between fasteners and the composite structure. The danger from the lightning strikes is particularly worrisome near the composite fuel tanks since fastener arcing can create an ignition source for fuel vapor. In this paper, the basic mechanisms pertaining to the fastener arcing are described including the lightning current flow behavior at fastener/composite interface before discussing how the lightning protection and countermeasure techniques evolved over the years. In addition, modeling and simulation methods to predict the current flow pattern between fasteners and composite structure are presented. Finally, the recent techniques to mitigate the fastener arcing are reviewed in details, including the use of sleeved fasteners for joining carbon fiber composite structures and how these newly developed fasteners can be incorporated into the design of the aircraft joints. I. Introduction

Continuous fiber reinforced composites are seeing an expanded use in the design of aircraft components for variety of applications where the light weight, higher strength and corrosion resistance are primary concerns. Composites are typically made up of fine fibers such as carbon or glass that are oriented at certain directions and surrounded in a supportive matrix material. Although a wide variety of matrix materials are commercially available, elevated temperature cured epoxy resins are by far the most commonly used. In most component design, the plies of the composite material are arranged at a variety of angles depending on the direction of major loading. This manufacturing technique produces a stacked laminated structure which is highly anisotropic and structurally inhomogeneous. It is well established that the composite structures in aircrafts are more susceptible to the lightning damage compared to metallic structures [1-4]. Metallic materials such as aluminum are excellent conductor of electricity and able to dissipate the high currents resulting from a lightning strike. The high conductive properties of aluminum allow lightning currents to conduct through structure with relatively few adverse effects since

most of the lighting current remains on the exterior skin of the aircraft. On the other hand, lightning current cannot dissipate fast enough throughout a composite skin resulting in significant current penetration into the substructures such as ribs and spars. This may lead to voltage drops across connected structures high enough that potentially can result in arcing. Additionally, since composite airframes cannot readily conduct lightning current away, as the traditional metal ones do, they are more prone to severe damage from lighting if proper protection methods are not in placed. Carbon fibers reinforced plastics (CFRP) are 2000 times more resistive than aluminum to the flow of current. Similarly epoxy, which is often used as a matrix in conjunction with carbon fibers, is 1 million times more resistive than aluminum [1].The main danger for airplane designers guard against is sparking inside the wings which serve as the main fuel tank. Consequently, lightning protection of composite structure is more complex due to the intrinsic high resistance of carbon fibers and epoxy, the multi-layer construction and the anisotropic nature of the structure [5]. It is a well established that fasteners are primary pathways for the conduction of the

lightning currents from skin of the aircraft to supporting structures such as spars or ribs and poor

electrical contact between the fastener body and the parts of the structure can lead to

fastener arcing or sparking from the lightning strikes. It is more worrisome if the lightning occurs near the composite fuel tank since the fastener arcing may occur, creating a ignition sources for the fueled volume. Generally, lightning current and fasteners interact in two different means. The most common mode is the direct attachment of the lighting to the exposed head of the individual fasteners as illustrated in figure 1a. In this case, the struck fastener experiences very high levels of current up to 200,000 amps and it carries the bigger portion of this lighting current than the neighboring fasteners in the same row and transfers the current to the surrounding structure. If the surrounding structure is aluminum, the current in the struck fastener is usually only 10 to 30 % higher than the adjacent fasteners and the current density is typically low enough to cause any sparking at fasteners [1]. If the skin is made of CFRP, however, the struck fasteners may experience substantially more current than the neighboring fasteners due to

high resistivity of CFRP. Lightning current that flows along the body of the struck fastener could be high enough to generate hot particles or gases that may be ejected from the struck fastener into the fuel tank thereby creating a hazard. The second mean of fastener/lighting interaction is that fasteners could be in the pathway of flowing lightning currents that enter to the aircraft structure at various points on the skin other than fasteners as shown in figure 1b. In this case, the fasteners located in the same row or nearby share lighting current. Although this lowers the current density per fastener, it still can be sufficiently high to cause arcing/sparking of the fasteners. As a result, there is a potential susceptibility to sparking/arcing inside the composite fuel tank from these fasteners as very high lightning currents can enter the skin and substructures components of the fuel tank via the fasteners. Figure 2 illustrates the potential effects of direct lightning attachment to the head of a fastener installed on a composite “T” joint resulting in sparking.

a) Direct attachment b) Current transfer via fastener Figure 1: Lightning and fastener interaction modes.

Figure 2: Photographs showing the fastener sparking during lightning test. One of the methods to improve lightning current protection of the composite material is to introduce metal meshes and/or foils near the outer surface of composite structures during the fabrication of panels. Aluminum, copper or bronze foils or meshes

are adhered during the process for manufacturing the composite structures to the outer face which will receive the direct lightning strike. Current technology already exists for fabricating panels with this protection technique [6,7]. Although this technology

assists maintaining the lightning current closer to external surface of the skin, it does not diminish the risk of fastener arcing. It is important to emphasize here that composite parts still need to be drilled in order to facilitate joining the components using mechanical fasteners. Drill holes can create disruption to the flowing lightning current throughout the CFRP structure and lead to increased current path and higher resistance around the drill holes. In addition to their lightning challenges, machining fastener holes in composite materials can be quite problematic. While machining ductile materials such as steel or aluminum results in shearing of the material and the formation of consistent chips, machining of composites involves a more complex mechanism with fracture of the fibers under compression or bending combined with shearing of the matrix material [8,9]. Drilling fastener holes in composite does not compare to the uniformity of aluminum or steel, which can be expected to remain consistent from a drill's entry point to its exit. Individual carbon fibers fracture at irregular angles and form microscopic voids between the sleeve and the hole. As the cutting tool wears there is an increase of surface chipping and an increase in the amount of uncut fibers or resin and delamination [10]. The most common defects observed in drilled

composite holes are fiber fractures, fiber pull-outs, chipping, drill entry and exit delaminations, voids and pitting [11-14]. The composite microstructure containing such defects is referred to as machining–induced micro texture. Although there have been substantial amount of works done on the effects of machining induced defects on the mechanical strength of the composite structures there is not much research done on the effects of such defects on the lightning current behavior of a composite joint, fastener arcing in particular [15-17].Enhanced machining techniques could improve machining-induced micro texture and reduce the development of microscopic voids [18, 19]. In recent years, there have been a large number of techniques developed in order to minimize or mitigate fastener sparking as the use of composite material in commercial aircrafts was dramatically increased. Figure 3 presents the data from our recent survey of the number of patents issued and/or applied by decades since 1980. It is obvious that continuing surge of the issued/applied patents in this field since the start of the new century coincides with the beginning of design activities of mostly composite commercial aircraft programs. These new research studies are expected to continuity with similar rapid pace for the next couple of years.

Figure 3: Survey of data on the number of patents issued and/or applied by decades since 1980 In this paper, first the basic mechanisms of the fastener arcing are described including the lightning current flow behavior at fastener/composite interface before discussing how the lightning protection and countermeasure techniques evolved over the years. Furthermore, the recent techniques to mitigate the fastener arcing are reviewed in details, including the

use of sleeved fasteners for joining carbon fiber composite structures and how these newly developed advanced fasteners can be incorporated into the design of the aircraft joints. Finally, an example of modeling and simulation methods to predict the current flow pattern between fasteners and composite structure is presented.

II. Discussion 2.1. Macrostructure analysis of lightning damage

When the lightning attaches to a fastener head, the evidence of lightning damage is typically limited to the areas surrounding the struck fastener. This is consistent with the fact that there is a current conductance path from fastener head into the skin of the composite laminate. Typical damage to the test panel after the lightning testing is shown in Figure 4. As it can be seen in this figure, there is a significant burn of the composite panel around the fastener and partial melting of the fastener head at the top side of the test panel whereas no damage is found at the collar/nut side. In some cases, there is visual soot marks radiating from the collar/nut base. The

damage area on the top plate is typically about 25 mm in diameter at the strike zone. The delamination of the composite and burning of epoxy and copper mesh are the main types of damage found. Such damage is expected due to the fact that the composite panels have a poor electrical conductivity and can be damaged structurally by high current flow through the fibers and overheating the epoxy matrix. Some efforts have been made to quantify this damage to CFRP panels due to lightning strike using non-destructive inspection techniques [2,4, 20].

Figure 4: Typical view of the test panels after lightning strike 2.2. Microstructure analysis and arcing phenomena The sparked fasteners generally exhibit heavy deposits around the base of the nut/collar as shown in figure 5. These deposits consist principally of polysulfide sealant with small amount of fastener droplets and carbon fiber particles as determined by energy dispersion spectrometer (EDS) chemical analysis [21]. The presence of fastener material and fiber particles in these deposits is a clear evidence that these deposits are originated from arcing between the fastener and the hole surface and carried by the hot gases and ejected from fasteners by the arcing pressure.

a) Deposit b) Fastener OD surface

c) Residual Sealant d) Arcing

Figure 5: SEM micrograph of the collar deposit and arc spots along the fastener outer surface Another factor that is believed to be playing an important role during occurrence of fastener arcing is the drilling induced damages on the machined fastener hole. Surface topography examination of the fastener holes showed that average surface roughness of drilled aluminum hole is 5-6 µin whereas in composite it is 120-150 µin, indicating substantially rougher and irregular finish for composite holes. SEM micrographs taken from the surface of the representative holes drilled on unidirectional CFRP are shown in Figures 6 and 7. As can be seen in these figures, subsurface delamination and pitting due to fiber pull-outs are evident along the hole axis resulting from the fiber fracture during drilling. Delamination and the pitting were the predominant damage forms observed and were found to be of about 100 micron in size. As reported in earlier studies the comparatively low

interfacial shear strength of CFRP results in delamination along the fiber-matrix interface and bending induced fiber fracture beneath the drilled surface [11-14]. Pitting and craters were created on the worked surface of the matrix of CFRP at positions shown by where the cutting direction is 45 degrees relative to the extending direction of carbon fibers. During a previous work [22], it has been found that such defective hole texture indeed leads to the arcing between sleeve and the CFRP panels. Photos presented in Figure 5 illustrate typical arcing evidence between the fastener and the CFRP structure. Arc pits are randomly distributed and do not appear to be related to any geometric feature of the fastener. Arcing appears to be associated with residual sealant on the fastener outer surface and the drilling induced voids along the axis of the composite hole.

a) Composite b) Aluminum

Figure 6: Drilling texture of the bore surface of composite versus aluminum

a) Delamination b) Pitting due to fiber pull-out

Figure 7: SEM micrographs from the surface of the fastener hole drilled on the directional CFRP. In addition, individual carbon fibers fracture at irregular angles and form microscopic voids between the fastener and the hole. As the cutting tools wear there is an increase of surface chipping and an increase in the amount of uncut fibers or resin and delamination. The photographs shown in Figure 8

illustrate the contact between a conventional fastener and the composite structure. It shows the typical voids formed between the fastener and the composite structure with entrapped sealant, marked by a red circle in the micrographs.

Figure 8: SEM pictures taken from the cross section of the installed fastener showing the voids Although enhanced machining techniques [18,19] could improve the drilling induced defects, as a result of the drilling technology available today and

the inherent heterogonous structure of the CFRP, such micro level damage described above is still unavoidable in the current manufacturing practice.

2.3 Strategies to mitigate arcing The protection of aircraft fuel systems against fuel vapor ignition due to lightning is one of the primary task for designers and engineers. Since commercial aircraft contain relatively large amounts of fuel and also include very sensitive electronic equipment, they are required to comply with a specific set of requirements related to the lightning strike protection

in order to be certified for operation [23]. Generally multilayered approaches are taken to lightning protection of the fuel tank during the design stages of aircraft program. Several of these protection strategies related to the fasteners used in composite fuel tank applications are discussed in following sections.

fastener fastener

2.3.1 Fastener isolation Lightning is known to be attracted to external fasteners on the skin and the struck fastener and neighboring fasteners carry the lighting current to the substructures which can spark at the gaps. Fasteners themselves may also arc while they are transferring the current. In this protection approach, the fastener heads are isolated to avoid any direct lightning attachment and the lightening is diverted away from the fasteners to other structural members which can tolerate lighting currents. Since the introduction of the idea by Amason in 1975 [24], more works has been done in this area to develop various methods for electrically isolating the fasteners [25-27]. The most common method involves covering the head of each fastener using a barrier dielectric patch as described by Covey [27] and others [28,30]. The dimensions of this patch which is shown in figure 9, is chosen sufficiently large enough to cause the lighting current to diffuse mostly before reaching the fastener thereby minimizing the risk for fastener arcing. Further improvement can be achieved by filling the gaps between the fasteners head and the countersink hole with dielectric filler material as demonstrated by Heeter et al. [31] and Winter et al. [32]. Le et al. [33, 34] has also shown that adhesively bonded multi-layer arrangements of conductive and dielectric plies are used to cover metal surface features such as skin fasteners completely wherein the current from

lightning strike is dispersed away from the fasteners with the aid of conductive plies over the dielectric plies. Additional protection methods described by Pearson [35] and Bannink Jr. et al.[25,26 ] include further electrically isolating metallic fasteners from skin. These are accomplished by applying a dielectric coating to any surface portions of fasteners that substantially contact with skin or applying a glass fiber insert around the fastener shank to fill the gap between fasteners and composite. It has been shown that such non-conductive insert keeps the fastener becoming a preferred lightning current path and prevent current from entering composite through metallic fastener. This electrical isolation prevents arcing between an external skin and metallic fastener. The protection against arcing provided by the electrical isolation is in addition to substantially redundant with protection provided by confining lightning strikes to portion of the outer surface of the skin and diffusing current flow from such strikes. It was also proposed to use of fasteners made entirely from dielectric materials where the load on fasteners is sufficiently low [35]. This approach, however, may not be practical since the load levels are quite high in most airframe joints, requiring stronger metallic fasteners.

Figure 9: Use of dielectric patch to isolate fasteners 2.3.2. Enhanced current conduction Uses of sufficiently large size fasteners and/or large number of fasteners were suggested by A. Plumer to lower the current density of individual fastener below a predetermined arcing threshold [1, 36]. Experimental studies were performed to determine this threshold current level of typical aerospace fasteners. These tests indicated that the spark threshold current levels of individual fastener are

about 5 kA [1]. Current densities in fasteners installed on aluminum aircraft structures are below this threshold level due to high conductivity of aluminum structure and large number of fasteners employed in those structures thereby sharing the lightning current more efficiently. It is important to note that the lightning current tends to concentrate in the fasteners closest to point of attachment,

especially in the case of fasteners installed in a CFRP. For this reason additional improvement in electrical conductivity by using a thicker wire mesh or strip such as bronze or copper material overlapping the LSP wire mesh along the rows of the fasteners was suggested by De La Fuent De Ana et al. [37]. These thicker mesh/strips (Figure 10) which are referred as bolt-line mesh are placed up to a minimum of 50 mm to both sides of the rows of the

fasteners. It has been suggested that bolt-line meshes allow increasing the exterior metallic cross section and optimizing electrical continuity around the fasteners by providing a better bonding between fasteners and bolt-line. Additionally, bolt-line meshes improve electrical current dispersion among the fasteners hence reducing the current density of each fastener and minimizing the risk of fastener sparking.

Figure 10: Fasteners with bolt-line mesh configuration As mentioned earlier, drilled holes cause the disruption of electrical continuity of the LSP mesh. In order to re-establish the electrical continuity of the composite skin and to ensure the presence of electrical contact between fastener heads and mesh, a metal spraying process was described by Sanchez-Brunete Alvarez et al. [38]. According to their description, this process ensures the electrical continuity allowing that most part of the discharged current is conducted over the surface, limiting the

current conducted into substructures throughout the fasteners and reducing the subsequent risk of sparks or hot spots. Brown et al. [39] has proposed use of a conductive hot-melt fastener cap that can be placed over countersunk fasteners for electrical continuity and smooth surface finish. It was also claimed that this type of fastener filler permits quick maintenance and field-level repairs because of relatively short cure time of the filler material as compared to the dielectric patches.

2.3.3. Sleeved fasteners

a) Flush head b) Protruding head

Figure 11: Sleeve fasteners with different head configurations

Sleeved fasteners were specifically developed for composite application where they are installed with close-fitting sleeves in the drilled holes to improve electrical conductivity and fatigue life of joints. Figure

11 presents sleeves fasteners with two different head configurations. After the sleeve is placed in the hole, the interference-fit pin pulled into the sleeve as illustrated in figure 12.

a) after sleeve insertion b) after completed installation Figure 12: Installation sequence of sleeved fasteners This expands the sleeve radially to provide an interference fit between sleeve and composite and minimizes the risk of damage to the composite structure around the holes which can occur if the interference-fit pins are forced into un-sleeved holes. Sleeve fasteners do not have to conduct the full lightning current. Instead, they ease the flow of current into CFRP since the interference fit provides more evenly distributed contact between the fastener and the wall of the hole and distribute electrical current more uniformly through the wall and/or internal structure. Since the interference fit

also minimizes the air gap between the fastener and structure it may reduce or prevent arcing between the fastener and the wall and/or the internal structure. This fastener configuration was shown to reduce the intensity of arcing at fastener/panel interface and allows increasing the arc threshold levels of the fasteners installed on composite structures [21,22]. When the contact area is increased, the current density through the fastener is reduced which in turn decreases the intensity of the arcing at fastener/wall interface and potential of ejection of any hot particles from the struck fasteners.

2.3.4. Arc containment The techniques explained above may prevent ignition at low to moderate current levels, but at higher current levels fastener arcing may still occurs. For this reason, it is generally necessary to employ as barriers between the arc source and fuel vapor areas. There has been several different techniques

were reported in literatures[1, 40-43] but the most common method of containing the arc products generated at the fasteners is to coat the colar/nut side of exposed fasteners with fuel tank sealant. Figure 13 shows the basic principle of arc containment with the use of sealant.

Figure 13: Arc containment method using fuel tank sealant. It is important to note that sealant itself does not mitigate fastener arcing but contains the arc products, so that they cannot reach to flammable fuel vapor inside fuel tank. Another way to reduce this danger is to use a special type of fasteners that provides a pressure seal. Examples of this type of fastening systems were described in several different patents. Morrill et al. [42,43] has described a washer sealing assembly for internal lightning protection. The washer may also include dielectric rings on both sides. When it used with a nut, the washer seals the fastener hole and contains any sparking and hot gasses that may arise along the fastener hole from entering the

structures. Additionally, the dielectric rings electrically isolates the metal collar or nut from underlying metal or CFRP substructure thereby preventing arcing between nut/substructure interface due to poor contact. Jones [41] has developed a fastener system comprising separate nut and bolt portions and a cage member. The nut is captively mounted inside to cage. When it is seated against a substructure, cage provides adequate seal and prevents arc products entering into fuel tank. Covey [40] has suggested use of a specially design gas filled-cap around installed fastener. The volume of gas is sufficiently large enough to absorb arc products and hot gases that may be present resulting from fastener arcing.

3.3.5 Conforming fasteners The newly developed fastener system is a sleeved fastener consisting of a core pin and a conformable sleeve. The sleeve OD surface is plated with low hardness but highly conductive materials such as gold, silver or nickel. The finish texture of the sleeve surface is adjusted to provide a surface roughness (Sa) value greater than 0.34 micron in order to increase the level of conformity. The sleeves are specifically designed to conform to the machine–induced micro texture inherent in fastener holes

drilled in composite. It has been reported that the conforming sleeved fasteners were spark free during the lightning testing where they were exposed to 100 kA current [ 21, 22]. Figure 14 illustrates the typical evidence spark-free conforming fastener detected by the chamber cameras. It is important note here that the orange dot on the spark-free fastener photograph is not an actual spark but it is the reference light placed in the dark chamber to ensure the proper operation of the light detection systems.

Figure 14: Photographs showing the spark-free conforming fastener during lightning test

The following photographs shown in Figure 15 illustrate macro level conformance between the sleeve and the composite structure. Figure 16 shows the imprints of individual carbon fibers clearly visible on the OD of installed sleeves indicating micro-level conformance between the sleeve and

the panel hole as a result of micro level deformation of the conforming sleeve. It is believed that the conforming sleeve provides a much better intimate contact between the sleeve and individual carbon fiber as it deforms to fill the microscopic machining induced voids.

a) sleeve head b) sleeve shank

Figure 15: SEM micrographs showing the macro level conformity between the sleeves on the hole

a) perpendicular to fibers b) tangent to fibers

Figure 16: SEM micrograph showing the carbon fiber imprints on the OD of sleeves It is suggested that the conformable sleeve deforms into the small voids that are created during drilling of the composite. As the sleeve deforms into the void it displaces the entrapped sealant. The insertion of the core pin causes the excess sealant to be extruded outside the sleeve/composite interface hence the conformable sleeve excavates excess entrapped sealant during installation of the fastener while bringing the sleeve in intimate electrical contact with the composite structure. This increased contact surface decreases current density and the voltage drop across the sleeve/composite interface. It is

obvious that the current will only flow through the graphite fibers embedded inside CFRP structure since it has a much higher conductivity compared to the epoxy matrix. Therefore enhanced contact to graphite fiber provided by confirmed sleeves have a profound effect on the current transferring capability of the fasteners. This leads to more efficient current transfer from fastener to the panel and minimizes the dielectric effect caused by the sealant thus reducing the possibility of arcing between the sleeve and the composite panel. That means that conforming fasteners can tolerate significantly higher

threshold current than 5 kA. The conforming sleeve can be achieved in a variety of ways, which some are more suitable for particular structures. The conformable sleeve could simply consist of a soft sleeve such as titanium sleeves while in another conforming the sleeve may consist of a hard base material with a soft conformable coating. The coating can be selected from a group of relatively

soft, conductive, metallic materials which are known to be galvanically compatible to composite structure. These materials include gold, silver, nickel, copper and titanium. It is also possible to use various alloys and the best material for a particular application generally involves a trade-off between cost and performance.

3.3.6. Conductive Fastener coatings Another important subject to discuss is the effect of coating on fastener sparking during the event of lighting strike. Although the best approach for enhanced electrical current flow is to have metal-to-metal contact. This is quite often not practical since fasteners needs to be coated for installation purposes and/or for corrosion protection. Since most of these coatings are dielectric they can be problematic with respect to current conduction within the fastener components and/or fastener to structure. Therefore, there have been some efforts to develop alternative conductive fastener coating for further improvement of the current threshold of fastener systems. Recently, Alcoa has successfully

developed high conductivity low friction coating by adding a small concentration of carbon nano tubes (CNTs) [44]. This recent study showed that as low as 1% carbon nanotube addition reduces the surface resistivity of typical fastener coating by 10 million times without sacrificing the desired low frictional properties of the conventional non-conductive coating. It has also been demonstrated that this newly developed conductive coating can be easily incorporated to the sleeved fasteners to further increase the threshold current levels that fasteners can tolerate without sparking during a lightning strike event [22].

III. Concluding Remarks It is now well established that fasteners installed in CFRP are more likely to spark than the ones installed on aluminum structures during the event of lightning channel attachment to aircraft due to the inherent high resistance of composite materials and anisotropic nature of the structure. The present review has demonstrated that extensive new developments have already been made and still more research efforts are being carried out to mitigate potentially hazardous fastener arcing/sparking in composite fuel tanks. The review has attempted to describe the mechanism and the source of fastener sparking and outline the contributing factors including the effect of machining induced micro-texture of the drilled composite holes. It was explained that fastener arcing occurs along the fastener/carbon fiber interface and the drilling induced mico-texture defects are associated with the occurrence of fastener arcing. It has shown that new fastener systems based on conforming sleeve technology provides an excellent gap filling at both macro and micro levels and an intimate contact between the sleeve and the composite structure and makes the fastener less sensitive to the hole quality. Consequently, these new fasteners enhance the current transfer from fastener to structure and

mitigate the fastener arcing as demonstrated during lightning strike tests. Furthermore, conforming fasteners with conductive CNT coating are superior to typical aerospace fasteners with respect to their lighting current carrying capabilities and they have the ability to carry a Zone 2A strike of 100,000 amperes without sparking. When these advanced fasteners are incorporated into joining composite structures and fuel tanks, they can play an important role for facilitating to dissipate the lightning strike energy over the CFRP structures and doing that without sparking they can offer additional lightning protection for composite airplanes. In recent years, numerical analysis techniques based on finite element analysis (FEA) has been employed to predict current distribution and dissipation in mechanically fastened composite joints [45-49]. However, design validation of fuel tanks still largely involves actual testing of representative test panels. In addition, Federal aviation regulations require that the aircraft fuel tank system to be designed and verified to prevent the ignition of fuel vapor [ 23,50]. An example of the use of FEA technique for predicting the lightning current distribution within fastener and surrounding structure is illustrated in Figure 17.

Figure 17: FEA model predicting lightning current distribution within fastener and CFRP structure 1 microsecond after lightning attachment. In future, it is reasonable to expect that FEA techniques will be used more widely for designing composite fuel tanks at least for screening purposes

such as the effects of different joint configurations on lightning current density and distribution.

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CONTACT

Hasim Mulazimoglu, Ph.D., is Manager, New Product Development Center at Alcoa Fastenings Systems. He can be reached at 900 Watson Center Road, CA 90745

Tel: 310 847 8100; e-mail: hasim.mulazimoglu@alcoa.com; web address: http://www.alcoa.com/fastening_systems/