Investigation of the Magnetization Mechanism and Optimal Axial Length of Ring-Type Permanent Magnet Magnetizers Based on Coupled Demagnetization Effects

Open-loop permanent magnet magnetizers are widely employed in magnetic flux leakage (MFL) testing of steel wire ropes due to their structural simplicity and operational convenience. However, the underlying magnetization mechanism remains inadequately understood, and systematic investigations into their optimal structural configurations are still lacking. In this study, a combined approach of finite element simulation and experimental validation is utilized to systematically examine the influence of axial length on the magnetization performance of ring-type permanent magnet magnetizers for steel wire ropes modeled as equivalent steel tubes. The results reveal a nonlinear relationship between axial length and the peak axial magnetic flux density in air: the intensity initially increases and then decreases with length. In contrast, both the magnetic field uniformity and the effective magnetization region improve monotonically with increasing length. These two mechanisms jointly modulate the internal magnetization state of the tube, giving rise to an optimal axial length. Further analysis confirms that for a steel wire rope with a diameter of 50 mm (or its equivalent tube), the optimal axial length range of the open-ring magnetizer lies between 95 mm and 110 mm, within which an optimal balance between field strength.