FUEL TANK SAFETY - CONDITIONING ASPECT FOR SPARKING PHENOMENOLOGY ON METALLIC ASSEMBLIES

摘要

The recent evolution of the regulation on fuel tank safety requires new justifications related to the prevention of ignition sources within fuel tanks in case of a lightning event. Fasteners are a potential location source of ignition, above a threshold current flowing inside the complexity of the arrangement in fuel tanks. Therefore it is useful to understand the mechanism of sparking on fastener assembly. The focus point presented in this paper concerns the "conditioning effect" for conduction aspects. The conditioning represents the modification of the sample after the first lightning strike; the current path could produce modifications of some characteristic of the sample regarding the sparking phenomenon for the following strikes. The classical methodology to prevent sparking consists to perform systematically one lightning strike on three identical samples (named family of samples). These families of samples can represent "in tolerance" design with implemented fault. In this methodology the second lightning strike on the sample is not usable due to possible conditioning effect. Indeed in certain cases, the first lightning strike can modify part of the protection feature and increase the sparking sensitivity of the sample. This paper gives some preliminary understanding based on tests and analysis. These tests are performed with classical lightning facilities used for lightning indirect domain. Possible adjustments of the lightning generator are used (injected level, waveforms types, rise time, etc.) to evaluate the sparking behavior of different types of fasteners with dedicated variations (torque or load, surface contact, type of contact, over-diameter...) and the effect on conditioning. Thanks to the capability of doing successive strokes, these tests result in a statistical study showing that conditioning aspect can be assumed for certain amounts of lightning current, and for certain conditions.