Ferrite-Core Four-Quadrant Rotating Eddy Current Probe for Enhanced Subsurface Defect Detection of Any Orientation

Deep subsurface defect detection (due to the so-called skin effect) and sensitivity in the detection of any orientation defects remain two of the main challenges in Eddy Current Testing (ECT). Although low-frequency excitation increases the penetration depth of the induced eddy currents, it also reduces the intensity of the generated magnetic field, significantly degrading the inspection capability. Rotating eddy current probes overcome the dependence on defect orientation, but their performance remains limited for deep subsurface defects. This paper proposes a ferrite-core four-quadrant probe that combines orientation-independent inspection with magnetic flux concentration provided by ferrite cores integrated inside the excitation coils. The proposed probe is first investigated through finite element analysis and then experimentally validated through a direct comparison with the corresponding air-core configuration. Both numerical and experimental results demonstrate that the ferrite-core configuration systematically enhances the inspection capability, with improvements ranging from approximately 22% for shallow defects to over 100% for the deepest investigated ones. Moreover, the proposed probe successfully detects defects buried up to 6 mm below the inspected surface, whereas the corresponding air-core configuration fails under the same operating conditions. These results demonstrate the potential of ferrite-core integration for extending the applicability of REC probes to deep subsurface inspections.