Development and Experimental Validation of Magnetic Saturation Pulsed Eddy Current Testing for Thick Ferromagnetic Structures

The detection of outer-wall defects in thick ferromagnetic structures by conventional pulsed eddy current testing is limited by the shallow penetration depth caused by the high magnetic permeability of ferromagnetic materials. In this study, a magnetic saturation pulsed eddy current testing method is proposed to improve the detectability of such defects. The analytical dependence of the eddy current skin depth on magnetic permeability was first clarified, and finite element simulations were carried out to visualize the effect of magnetic saturation on the magnetic field and eddy current distributions. Pulsed eddy current responses under different relative permeabilities were then numerically analyzed, followed by experimental validation using Q345B steel plates with different thicknesses and cubic Q345B specimens containing flat-bottom hole defects of different depths. The influence of the direction of the saturation magnetic field on testing performance was also investigated. The results demonstrate that magnetic saturation effectively increases eddy current penetration depth and significantly improves the sensitivity of pulsed eddy current testing to wall-thinning defects in thick ferromagnetic structures. Signal separation for different defect depths was markedly enhanced under saturated conditions. In addition, the optimal testing performance was achieved when the saturation magnetic field was parallel to the probe axis. These results, obtained on uncoated specimens under laboratory conditions, provide a validated physical basis and a preferred field-probe configuration for applying magnetic saturation pulsed eddy current testing to thick-walled ferromagnetic components.