A flexible permanent magnetic field perturbation testing probe based on through-slot flexible magnetic sheet

In the field of non-destructive testing (NDT), the detection of surface cracks using permanent magnetic field perturbation (PMFP) testing has emerged as a novel approach. It is particularly well-suited for detecting ferromagnetic materials due to its capability to identify open cracks, facilitated by a compact and straightforward probe structure. However, the existing rigid PMFP probes pose challenges in terms of shape-conforming, resulting in large lift-off fluctuations during sweeping, and are prone to wear and tear. Furthermore, their suitability is limited when inspecting specimens with various specifications and irregular shapes. To address these issues, this paper proposes a novel PMFP detection method based on the magnetization of a Flexible Magnetic Sheet (FMS). For further optimization, the machining of a through-slot in the FMS, with the magnetic sensor embedded within, serves to capture the magnetic field perturbation. This configuration is employed for the detection of surface defects. Compared to the conventional PMFP method that relies on rigid permanent magnetization, its flexibility makes the proposed method offer notable advantages such as streamlined processing, lightweight construction, and inherent adaptability to various shapes. Moreover, compared to the traditional PMFP structure where the sensor is placed beneath the permanent magnet, the through-plot FMS design exhibits a larger and more uniform magnetic field. Consequently, this feature significantly improves the signal-to-noise ratio by approximately 30%. The flexible testing method has a high application value and can effectively detect rectangular slots of size 1 mm * 1 mm* 12.5 mm and flat-bottom holes of size phi 2mm* 0.5 mm in pipeline testing.