Eddy Current Imaging Using Multi-Frequency Mixing Methods for Aircraft Structural Integrity Verification

Abstract

Automated eddy current scanning of military or civilian aircraft structures is rarely performed in the field. Manual tests usually are performed by simply placing or sliding a probe across the interrogated surface, while an eddy current instrument’s impedance plane is observed for flaw responses. Scanning and imaging technologies, however, have sparked considerable interest and investigations in the feasibility of using automated eddy current nondestructive testing (NDT) methods in the aircraft community. The scanning system’s intent is to provide the inspector with a rapid and sensitive method to identify locations of potential or immediate concern. Automated imaging techniques offer several advantages over conventional methods, including better reproducibility, reportability and detectability. Recent strides in these areas have dealt mostly with the detection of corrosion in thin skin structures between the fasteners. Wing structures, however, typically consist of much thicker material, where sub-surface corrosion in overlapping joints or fatigue cracks can occur under the fasteners. These flaws may not extend to the surface nor beyond the fastener before they become critical, thus making visual (including enhanced) techniques ineffective.