Making near-surface defects reliably visible
Magnetic particle testing is based on targeted magnetisation of a ferromagnetic component. A magnetic field is introduced into the part – for example by current flow or via an applied yoke. This produces a directed magnetic flux in the material.
As long as the component is free of defects, this magnetic flux passes largely uniformly through the material. If, however, a surface-breaking or near-surface defect is present, the magnetic flux is locally disturbed at that point. Because of the discontinuity, magnetic field lines emerge from the surface and form a so-called leakage field.
While the component is magnetised, very fine ferromagnetic particles – either as dry powder or as a suspension in a carrier fluid – are applied to the surface. These particles respond to the emerging leakage fields and accumulate selectively in the disturbed region. This produces a visible particle indication that reflects the course and orientation of the defect.
The sensitivity of the test depends significantly on the orientation of the magnetic field relative to the potential crack direction. For a leakage field of sufficient strength, the magnetic flux must intersect the defect as nearly perpendicular as possible. For this reason, several magnetisation directions are often applied in practice – for example longitudinal and transverse fields – to reliably detect defects of different orientation.
Evaluation of the indication is performed while magnetisation is maintained, because the leakage field is only effective in this state. The shape, length and intensity of the particle accumulation provide indications of the location and extent of the defect in the edge zone of the component.
In this way, magnetic particle testing enables reliable inspection of surface-breaking and near-surface cracks in ferromagnetic materials – non-destructively and with immediate visual feedback.
