A Study of Array Remote Field Eddy Current for Hole Edge Defect Location and Evaluation of Aircraft Fasteners

Hi-lock bolt fasteners are widely used in aircraft multilayer structures, and it is especially important to detect hole edge defects of hi-lock bolt connection structures. In this study, an array remote field eddy current (EC) testing system based on a field-programmable gate array (FPGA) and an array remote field EC testing probe based on multiplexing the principle were designed and developed. Using this system, experiments were carried out on fastener test blocks, and the effects of defect length and buried depth on the detection signal were studied. The experimental results show that the defect length has a significant impact on the amplitude of the detection signal, but a small impact on the phase of the detection signal. The buried depth of a defect has a significant impact on the amplitude and phase of the defect signal. The attenuation law of the detection signal in the weak magnetic field region between channels was studied, and a rotation angle- {w} model was proposed. The model was used to locate the defect, and the detection signal when the defect was located directly below the coil was deduced. A method for defect evaluation using the phase and amplitude of the defect signal was proposed. It was verified that the defect location error was within 2°, and the positioning error was small, which provided a reference for the defect evaluation of array remote field EC.