Alternating field or AC flux leakage testing uses two encircling magnetization yokes through which an alternating current flows. These yokes magnetize the test material contactlessly in the transverse direction. The magnetic flux is concentrated on the material surface and is thus particularly sensitive for the detection of µm-sized surface defects from approx. 100 µm in depth. The distance between the yoke and the test material is a few millimeters. Between the yoke legs there are test shoes with protected probes that scan the surface with contact.
The intermittently operating probe only detects a very small part of the entire surface at any one moment. A very minor material defect thus produces a large disturbance in proportional terms. In the case of defect-free material, the magnetic flux produces a closed circuit in the material.
The actual leakage flux results at exposed defects. The higher the frequency of the yoke is set, the closer to the surface the magnetization current flows (skin effect). At defects in the material, the magnetic flux emerges from the material surface above the defect. This is referred to as leakage flux and can eventually be seen as a signal on the user interface.
Operating principle of the constant field leakage flux method
DC leakage flux testing
The constant field or DC leakage flux method magnetizes the entire material cross-section. This makes it possible to detect defects on the inside and outside surface of a tube. The detection of internal defects decreases as the wall thickness increases.
The magnetic flux is structured circularly with two rotating yokes to detect longitudinally oriented defects (see figure on the left). The leakage flux that occurs at the defective locations is captured by rotating magnetic field sensors.
The magnetic flux is produced by two stationary encircling coils arranged in the longitudinal direction to detect transversely oriented defects (see figure on the right). Several stationary magnetic field sensors along the circumference capture the occurring leakage flux.
Source: https://www.foerstergroup.de/en/deu/technology/flux-leakage-testing/
