Reliably visualizing near-surface defects
Magnetic particle testing is based on the targeted magnetization of a ferromagnetic component. To do this, a magnetic field is induced into the workpiece – for example, by passing an electric current through it or using an applied magnetic yoke. This creates a directed magnetic flux within the material.
As long as the component is free of defects, this magnetic flux flows largely uniformly through the material. However, if there is a surface-open defect or a defect close to the surface in the material, the magnetic flux is locally disrupted at this point. Due to the interruption, magnetic field lines emerge from the surface and form a so-called stray field.
While the component is magnetized, extremely fine ferromagnetic particles – either as a dry powder or as a suspension in a carrier fluid – are applied to the surface. These particles react to the escaping stray fields and accumulate specifically in the area of the disturbance. This creates a visible particle indication that reflects the path and orientation of the defect.
The sensitivity of the test depends largely on the alignment of the magnetic field relative to the potential crack direction. To generate a stray field of sufficient strength, the magnetic flux must intersect the defect as perpendicularly as possible. For this reason, multiple magnetization directions are often used in practice – for example, longitudinal and transverse fields – to reliably detect defects of varying orientations.
The evaluation of the indication is performed while maintaining magnetization, as the stray field is effective only in this state. The shape, length, and intensity of the particle accumulation provide clues to the location and extent of the defect in the edge region of the component.
In this way, magnetic particle testing enables a reliable examination of surface-open and near-surface cracks in ferromagnetic materials – non-destructively and with immediate visual feedback.
