Surface examination of ferromagnetic materials which is standardized in DIN EN 1369, Founding – Magnetic particle inspection and serves for detection of cracks and/or defects in or close to the surface of castings.
For this test, a magnetic material is magnetized, a magnetic field is established, and a magnetic flow is generated in the component. The magnetic lines of force flow through the material and are introduced and/or emerge at the poles. Defects such as cracks or cavities cannot support the magnetic flow to the same extent and when the flow lines meet the defect part of the flow will scatter to the outside air. Finely dispersed magnetic particles which are applied to the component are attracted to the areas of emerging flow and produce a visible indication. Application of the magnetic particles can be dry or wet, distributed in solution, or died. For highly sensitive applications, particles with a fluorescent coating are used and inspection is then made under UV light.
Current flow detection:
With this method the object is flooded with electric current. This current produces a circular magnetic field. Cracks which run parallel to the object can therefore be detected due to their alignment perpendicular to the field lines and produce the necessary magnetic leakage field, i.e. current flow detection makes “longitudinal cracks” visible.
Field flow detection:
In contrast to the current flow detection the field flow detection produces a magnetic field in the tested object without a flow of electric current.
The magnetic field is produced in a U-shaped iron yoke of one or several energized coils. The tested component is mounted into the iron yoke. Thus a magnetic field longitudinal to the tested component is produced.
Transverse cracks build a magnetic flux leakage and are therefore indicated.
In many tested objects cracks cannot be expected in a specific direction. In this case either several different techniques have to be performed consecutively or combined techniques can be used.
The simplest combination is a direct current yoke magnetisation and an alternating current flow. The testing equipment is conceived to the effect that the current is induced in the tested sample via the magnetic poles simultaneously. The magnetic yoke has to be insulated once in order to prevent a shunt.
Magnet particle inspection can be used for the detection of cracks, cavities, overlaps, non-metallic inclusions etc. The sensitivity of the magnet particle inspection depends on the type of magnetization, the electromagnetic characteristics, and the surface conditions of the material to be tested and equally on the size, shape, and orientation of the defect.