Omnidirectional crack detection in welded structures using a Rotating Three-Pole Magnetization system with a flexible coil array sensor

This study introduces a novel Alternating Current Field Measurement (ACFM) probe based on a Rotating Three-Pole Magnetization (RTPM) configuration integrated with a flexible magnetic coil array sensor. Conventional Eddy Current Testing (ECT) and Magnetic Flux Leakage (MFL) techniques face inherent limitations, ECT is highly sensitive to lift-off and coating thickness, while MFL requires strong magnetization and is restricted to ferromagnetic materials. In contrast, ACFM requires only a small alternating magnetic field to induce surface currents, enabling non-contact and coating-tolerant inspection with direct measurement of magnetic field perturbations (B z ) caused by surface-cracks. However, conventional ACFM systems lose sensitivity to cracks parallel to the induced current direction. To overcome this limitation, the proposed RTPM design generates a uniform in-plane rotating magnetic field, enabling orientation-independent crack detection. Three-dimensional finite element simulations confirm homogeneous induced currents and a clear B z response at defect sites. Experimental validation on aluminum and steel specimens demonstrates reliable detection of surface cracks as small as 1 mm in any orientation, with stable signals maintained at lift-off distances up to 2 mm. Moreover, the system effectively identifies defects in weld zones, where conventional ACFM, ECT, and MFL methods often struggle due to magnetic and conductivity variations. These results highlight the superior sensitivity, robustness, and practical applicability of the proposed RTPM-based ACFM system for rapid non-destructive inspection of complex industrial structures.