United States Department of Transportation
i
Research on Lightning Strike of Composite Structures
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2022-06-01
[PDF-25.92 MB]
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Abstract:As aircraft industry moves from traditional metal construction to advanced composite materials, there is a concern that current techniques used to detect lightning-caused sparks within fuel tanks may ignite fuel vapor due to the difference in physics involved. Existing standard SAE Aerospace Recommended Practice (ARP) 5416A defines two test methods for detection of voltage sparks in metallic structures. The first is ignition of a standard hydrogen/oxygen/argon gaseous mixture and the second is detection of light, which is a simpler and less hazardous approach. Both techniques require a 200 microjoule (μJ) minimum fuel ignition threshold induced by discharge of a standard voltage spark source. The standard defines the pass criterion either by demonstrating the absence of a minimum light detected by camera or by no ignition of the flammable gas mixture. These 200 μJ voltage sparks are generated based on sparking occurring between metallic components. However, when carbon fiber composite materials are involved, other sources of sparking appear, which have not yet been properly characterized in regard to ignition of aircraft fuel. The light emission and sparking sources include a voltage phenomenon called edge glow, incandescent particles, and hot gases ejected from fastener joints. The existing photographic test method outlined in the SAE ARP 5416A is not closely linked to the heat energy of these ignition sources, resulting in a large number of false failures, or cases in which light is detected, but ignition of the flammable mixture will not occur. This project investigates feasibility of utilizing time-integrated digital photography imaging (photographic method) for predicting the ignition conditions of the standardized flammable gas mixture imposed by an incandescent heat source. The study showed that the ignition could be predicted by analyzing the hue histogram of the detected light emission source. Thus, based on the research findings to date, appearance of the yellow hue alongside the defined incandescent heat signature has been observed to be coincident with ignition of standardized gas mixture for several investigated materials. A round robin test was conducted during year three of this research with the purpose of validating the work done in the first two years of the program. As an outcome, development of a new or augmented test method suitable for characterization of composites and metal ignition sources simultaneously will be developed with the purpose of supplementing or superseding the existing standard. The project development is carried out under the continuous monitoring by the Federal Aviation Administration (FAA) and in a close partnership with the SAE AE-2 and EUROCAE WG-31 Lightning Committees.
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