Magnetic Particle Inspection / Flux Testing

Magnetic particle testing is a method of nondestructive material testing. It is also known as magnetic particle crack testing, flux testing, or flux inspection. Surface or near-surface cracks in ferromagnetic materials can be detected using this testing method.
To perform the test, the workpiece or a section of it is first magnetized. This is done by passing an electric current through the workpiece. When current is passed through the workpiece, a ring-shaped magnetic field is generated. When a field is applied perpendicular to the workpiece, a magnetic field is created along the length of the component. A combination of both methods is possible.
The magnetic field lines run parallel to the surface. Cracks or defects cause a magnetic stray field, which can be made visible using fluorescent ferromagnetic powders dispersed in suitable liquids. UV-A or blue light sources are typically used for this purpose.
This method is only suitable for defects near the surface or extending to the surface. Defects running parallel to the surface (e.g., material separations) cannot be detected.

Magnetic particle testing, often referred to as magnetic particle crack testing or flux testing, works exclusively on ferromagnetic workpieces, such as those made of steel, cobalt, or nickel. At the start of the inspection, the metal part must be magnetized. If the workpiece is too large to be fully magnetized, only the areas to be inspected are magnetized.
Magnetization creates field lines within the component that run parallel to the component’s surface. Cracks originating from the surface, as well as hidden defects up to approximately one millimeter deep within the component, disrupt these field lines by generating magnetic stray fields. In practice, these stray fields are often referred to as stray flux.
In the case of a crack or other defect, the field lines exit the magnetic material on one side of the defect and re-enter on the opposite side. This creates magnetic poles. When iron powder is distributed over this stray field, the powder accumulates at the defective location due to magnetic attraction.

The methods for magnetizing test specimens can generally be divided into current-flushing and field-flushing. If cracks in different directions are expected in a specimen, the two methods must be performed either sequentially or as a combined magnetic particle inspection procedure.

• Current Flux: This method is suitable for detecting cracks in the longitudinal direction. To do this, the test piece is connected to the current source in such a way that a ring-shaped magnetic field is created around the test area.
• Field magnetization: This method makes transverse cracks visible. In field magnetization, the magnetic field is generated in the longitudinal direction. To do this, the test piece is clamped in a so-called iron yoke. Cracks lying transverse to the magnetic field generate a stray flux, which is indicated by the magnetic particle testing agent.
• Combined Magnetization: For many workpieces, the location of cracks or defects cannot be predicted in advance. Therefore, testing machines have been developed that can generate current magnetization and field penetration either individually or in combination with one another. This allows cracks of any orientation to be detected.

The specification of which magnetization method (flux method) is to be used in a specific case is always found in the workpiece’s inspection instructions.