Electrical Characterizations of Lightning Strike ProtectionTechniques for Composite Materials

George N. Szatkowski, Truong X. Nguyen, Sandra V. Koppen, Jay. J. ElyNASA, Langley Research Center, Hampton, Virginia

John J. MielnikLockheed Martin, Hampton, Virginia

Abstract

The growing application of composite materials incommercial aircraft manufacturing has significantlyincreased the risk of aircraft damage from lightningstrikes. Composite aircraft designs require newmitigation strategies and engineering practices tomaintain the same level of safety and protection asachieved by conductive aluminum skinned aircraft.Researchers working under the NASA AviationSafety Program’s Integrated Vehicle HealthManagement (IVHM) Project are investigatinglightning damage on composite materials to supportthe development of new mitigation, diagnosis &prognosis techniques to overcome the increasedchallenges associated with lightning protection oncomposite aircraft. This paper provides an overviewof the electrical characterizations being performed tosupport IVHM lightning damage diagnosis researchon composite materials at the NASA LangleyResearch Center.

Introduction

NASA’s work in advanced aeronautics includesgrowing interest in environmentally responsiveaircraft, one component of which involves the use ofcomposite materials to significantly reduce weight,and hence fuel consumption. The aircraft industryshares this interest and is utilizing more compositematerials in each new generation of aircraft beingmanufactured for general aviation, business jet andjumbo jet aircraft applications. Boeing’s 787 aircraftis just one example of the growing use ofcomposites being incorporated in the construction ofthe next generation fleet. With this growth incomposite usage, new technical challenges arise.Composite skinned aircraft are far more vulnerableto lightning strikes than their aluminum skinnedpredecessors. The electrical current incident on anaircraft from a typical lightning strike can exceed200,000 amperes [1], occurring in less than afraction of a second.

Without proper lightning strike protection, the carbonfiber/epoxy composites can be significantlydamaged, particularly at the entry and exit points ofthe strike. Approaches have been developed to

protect the composite structures from lightning directeffects to reduce damage to acceptable levels byusing conductive foils or meshes in the outer layer ofthe composite system. When a known lightningstrike occurs, the points of attachment anddetachment on the aircraft surface are visuallyinspected and checked for damage to ensurecontinued flight safety. Repairs may be required toreplace damaged composite sections per FAAprocedures [2].

Even though lightning damage can occur on acomposite aircraft, the damage level and associatedrisk to flight safety is deemed acceptable by the FAAand does not compromise flight safety. However,existing lightning strike protection techniques incomposite systems are not always sufficient toadequately protect avionic installations frompotential upset due to induced electromagnetic fieldscoupled into the aircraft and require additionalprotections to improve avionic shielding to meetstandards for aircraft certification. These additionalprotections reduce the overall weight savings thecomposites provide.

NASA’s IVHM project is interested in supportingresearch to help the aircraft industry improveshielding effectiveness performance on compositelightning strike protection designs. Identifying themost promising technologies which are expected toemerge for future composite fuselage designs is animportant component to IVHM research. The statedgoal of the IVHM lightning research is to diagnoselightning damage based on a measurement of thelightning current transferred to the aircraft toestimate potential damage in flight. To accomplishthis goal, identifying the appropriate compositelightning strike protection technique expected to befielded in future aircraft designs is essential todevelop representative lightning damage at differentlevels of lightning current. This will allow meaningfulfatigue to failure analysis to be performed to supportdamage diagnosis and failure prognosis.

Electrical characterizations will be conducted toevaluate RF shielding effectiveness, currentpropagation and surface conductivity on various

composite lightning strike protection techniques bothbefore and after lightning direct effect testing. Thispaper provides a description of the ongoingelectrical characterizations and presents exampletest results.

RF Shielding Effectiveness Measurements

Efforts are under way in the NASA Langley HighIntensity Radiated Fields Laboratory (HIRF Lab) toestablish test methodologies to conduct RF shieldingeffectiveness (SE) measurements on compositepanels with integrated lightning strike protection inthe outer layer. Facility modifications completed inthe summer of 2007 included the addition of a 4 footsquare aperture in the adjacent wall between twofaraday enclosures to provide the capability toperform nested reverberation shielding effectivenessmeasurements. A test fixture plate was fabricated togo over the large aperture to reduce the size of theopening to 12” x 12” to accommodate 14” x14”composite panels allowing 1” overlap on each sideto ensure adequate electrical contact. The testfixture can accept panels up to a half inch thick.Figure 1 presents a photograph of a compositepanel mounted in the test fixture.

Figure 1. Composite Panel Mounted in Test Fixture

SE measurements in the HIRF Lab [3] will follow aNIST developed test technique [4] which requiresmode stirring in the chamber to expose thecomposite panel with varied polarizations andangles of incidence to determine the average SE.Unlike other reverberation techniques, the NISTapproach accounts for the aperture shape, cavitysize and chamber loading effects to improvemeasurement accuracy. Basic SE is determined bythe ratio of two independent measurements, withand without the panels, with the transmit antenna inone chamber and the receiver antenna in anotherchamber. The NIST developed first-order correctionsaccount for the change due to the presence of the

panels to the chambers’ Q, which can be measuredwith both transmit and receive antenna in the samechamber. Rather than measuring average receivepower as theorized, peak receive power wasmeasured to improve measurement sensitivity byabout 10 dB. The receive power ratios used shouldbe equal for peak and average values, however, theuse of peak values can result in slightly largervariability and uncertainties.

Initial test results show very good dynamic rangeperformance can be achieved in the HIRF facility upto about 4 GHz. Above 4 GHz, chamber lossesbegin to impact system performance and reduce theoverall dynamic range of the measurement. Methodsto improve measurement performance above 4 GHzwill be investigated. Figure 2 presents the shieldingeffectiveness data of a 7 layer carbon fiber weavecomposite panel with aluminum mesh lightning strikeprotection. The vertical axis is calibrated SE in dB.The horizontal axis represents frequency inmegahertz. The top trace is the calibrated shieldingeffectiveness of the composite panel. The bottomtrace is the calibrated noise floor of themeasurement. The tested panel is shown to possesabout 120 dB of isolation below 1.5 GHz. Thecomposite panel edges were not prepared in anyway to improve surface conductivity between thepanel and the test fixture. Techniques to increasethe surface conductivity at the composite paneledges to reduce RF leakage around the panel willbe explored and are expected to further improve themeasured performance of the composite panels.

Figure 2. SE Measurement of Composite Panel

Shielding effectiveness testing will be conducted onvarious composite panels with lightning strikeprotection techniques both before and after lightningdirect effect testing.

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Figure 3. Jentek Meandering Winding Magnetometer

The eddy current measurements were conducted at10 MHz to provide the best surface flaw detection.Both the composite panel upper surface with thelightning protection layer and the bottom surfacewere independently scanned.

Figure 4. Eddy Current C-scan Image of Composite PanelTop Surface with Aluminum Mesh

Figure 5 presents the eddy current measurementdata collected from the bottom surface on the samepanel showing the difference in conductivity. Theconductively intensity scale for the back side of thepanel is 1 x 105 (ohm*m)-1 to 1.3 x 105 (ohm*m) -1

Eddy Current Measurements conductivity of the panel and ranges in intensity from1.95 x 10 4 (ohm*m)-1 to 2.45 x 10 (ohm*m) -1

Eddy current measurements were performed on aninitial set of composite panels with lightning strikeprotection at Langley’s Non Destructive EvaluationsLab to evaluate surface conductivity on the upperand lower surfaces. Eddy current measurementshave been shown to be an effective test to finddefects in reinforced carbon-carbon materials [5].The initial measurements will serve as a baseline forrepeat measurements after the panels are exposedto direct effect lightning exposure.

The Jentek Meandering Winding Magnetometersystem was used to conduct the measurements andis shown in Figure 3. The flexible array coil ispositioned on the underside of the wheeled cart. Thecart position is recorded by an encoder wheel as it istranslated across the composite panel under testand the data is processed to generate a C-scanimage over the surface of the sample.

Figure 4 presents an example eddy current C-scanimage collected on the upper surface of a 7 layercarbon fiber weave composite panel with aluminummesh lightning strike protection. The horizontal andvertical axis are surface dimensions of the panel ininches and the image depicts the center 12” x 12”area. The color intensity depicts the surface

Figure 5. Eddy Current C-scan Image of Composite PanelBottom Surface

Surface Current Mapping Measurements

Personnel at Old Dominion University (ODU)Research Foundation are working collaborativelyunder a NASA Research Announcement

cooperative agreement to support Langley HIRF Labresearchers to investigate lightning currentpropagation on composite materials using surfacecurrent mapping techniques. The purpose of theseinvestigations is to determine the effects variouslighting strike protection techniques have on theoverall lightning current propagation. Surfacecurrent mapping techniques will also be used to helpevaluate and calibrate the performance of lightningcurrent measurement sensors currently underdevelopment.

The measurements are conducted by injecting asimulated lightning waveform on one edge of thecomposite panel. The current flows across the panelto the ground terminal located on the other edge ofthe panel. The current sensor is moved to variouslocations on the panel and records the electricalcurrent at its location. Surface current mapping willbe conducted on various composite lightning strikeprotection techniques both before and after lightningdirect effect testing. Research at NASA, Langley willbegin in this area in the late fall of 2009.

ODU personnel have conducted preliminary surfacecurrent mapping measurements using a vectormeasurement technique incorporating twoorthogonal Hall Effect sensors to determine thedirection of current flow. Preliminary measurementsto demonstrate a proof of concept have beencompleted and are presented in Figure 6.

Figure 6. Current Map using Hall Effect Sensors

Distance in inches are shown on the x and y axis.Current density is shown in the z axis. The imageclearly shows the current path along the length ofthe composite surface. Discontinuities in the imageare believed to be from improper grid spacing in themeasurement procedure.

Conclusions

Researchers working under NASA’s Aviation SafetyProgram’s Integrated Vehicle Health Management(IVHM) Project are investigating lightning damageon composite materials to support the developmentof new mitigation, diagnosis & prognosis techniquesto overcome the increased challenges associatedwith lightning protection on composite aircraft. Thestated goal of the IVHM lightning research is todiagnose lightning damage based on ameasurement of the lightning current transferred tothe aircraft to estimate potential damage in flight.Identifying the appropriate composite lightning strikeprotection technique expected to be fielded in futureaircraft designs is essential to developrepresentative lightning damage at different levels oflightning current to conduct meaningful fatigue tofailure analysis for damage diagnosis and failureprognosis. Electrical characterizations will beconducted to evaluate shielding effectiveness,current propagation and RF surface conductivity onvarious composite lightning strike protectiontechniques both before and after lightning directeffect testing.

References

[1] Pitts FL, Fisher BD, Vladislav V, Perala RA.Aircraft jolts from lightning bolts. IEEE Spectrum1988;25:34–8.

[2] FAA Aviation Maintenance General Handbook,FAA8083-30

[3]J. Ladbury, G. Koepke, and D. Camell,”Evaluationof the NASA Langley Research Center Mode-StirredChamber Facility”, U.S. Department of Commerce,National Institute of Standards and Technology,Boulder, CO, NIST Tech Note 1508, 1999

[4] C. Holloway, D. Hill, J. Ladbury, G. Koepke andR. Garzia, “Shielding Effectiveness Measurement ofMaterials Using Nested Reverberation Chambers”,IEEE Transactions on ElectromagneticCompatability, vol 45,pp. 350-356, 2003

[5] B. Wincheski and J. Simpson, “Application ofEddy Current Techniques for Orbiter ReinforcedCarbon-Carbon Structural HealthMonitoring,” CP820, Review of QuantitativeNondestructive Evaluation, Vol. 25, D. O. Thompsonand D. E. Chimenti, Ed., AIP, 1082, 2006.

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