Investigation of time-division multiplexing-based multi-mode eddy current testing method for directional defects

In traditional eddy current testing, crack orientation and direction of the eddy current are closely related. This becomes particularly problematic when cracks are oriented at specific angles, as the fixed transmitter–receiver mode creates an eddy current field that often results in signal blind spots, thereby reducing detection performance. To address this issue, this paper proposes a time-division multiplexing (TDM)-based multi-mode eddy current testing (ECT) method specifically designed for detecting directional defects in metallic materials. By combining multiple excitation modes, this approach gathers more comprehensive multi-angle, multi-frequency, multi-frequency and multi-mode information, which strengthens directional-crack detectability and improves accuracy and reliability. The study compares the variation in induced voltage amplitudes under single and mixed excitation conditions, then their correlation with various directional defects is analysed. Finite element simulations are used to evaluate the impact of key parameters such as coil excitation frequency, centre-to-centre distance and outer radius on detection sensitivity as well. Subsequently, the effects of defect length, width, depth and orientation on the induced voltage signal are investigated. Finally, an experimental platform based on time-division multiplexing is constructed to thoroughly validate the method with various directional defects in metal components.