A Multiscale Electromagnetic Method for Damage Detection and Quantitative Characterization of Radar-Absorbing Coatings on Aircraft Skins

Abstract

This study proposes a multiscale electromagnetic method that combines far-field angular-domain analysis with local near-field imaging to detect damage in radar-absorbing coatings on aircraft surfaces. Using an intact state as a reference, continuous anomalous angular intervals and their principal anomalous azimuths are identified via robust statistical criteria, employing the electromagnetic scattering energy ratio (ESER), defined here as a normalized squared-field response ratio, and the azimuth difference factor. These angular characteristics, combined with target geometry and a predefined detection area, are used to determine the direction and azimuthal range of candidate near-field scanning windows. Subsequently, coherent back-projection is applied to the local near-field measurements. The centroid and equivalent electromagnetic dimensions of the differential response are estimated using complex differential imaging, multithreshold analysis, and squared-magnitude-weighted central second moments. Numerical analysis of a simplified aircraft-like model demonstrates angular-domain screening and local characterization of impact, coating loss and bulging. For the rectangular coating-loss simulation, the equivalent electromagnetic length and width deviate from the nominal side lengths by 8.6% and 2.3%, respectively. The corresponding experimental deviations for localized 1 cm × 1 cm coating loss on a coated conical specimen are 9.2% and 6.3%. These results support two-stage screening and local equivalent-scale extraction under the investigated geometry, material and reference-matching conditions.