Magnetic particle testing is used for the detection of surface defects in ferromagnetic materials (mainly steel) and offers the highest sensitivity for the detection of surface cracks.
A magnetic field is introduced into the test object by means of various magnetization techniques. Close to and in the surface of the test object, where the magnetic properties (relative permeability) of the material change significantly (e.g. cracks), the magnetic field emerges from the surface as magnetic flux leakage.
This magnetic flux leakage can be visualized by coloured (usually black or fluorescent) magnetisable particles (magnetic powder) applied to the test object during the test.
With magnetic particle inspection, only defects close to the surface can be detected up to a maximum depth of approx. 2 mm (standard value) below the surface. In the case of wide open or flat surface irregularities, procedural limitations exist with regard to the detectability of these displays under UV light. These can only be reliably detected by means of a visual inspection (VT) under appropriate VT light conditions (no UV light). Furthermore, it is generally not possible to determine a defect depth within the framework of a magnetic particle inspection (MT) or a visual inspection (VT).
Compared to other non-destructive testing methods, magnetic particle testing can also be used for complicated material geometry and unmachined surfaces.