There are two primary methods of Magnetic Particle Testing (MT):
- Wet Method (Fluorescent): Uses fluorescent magnetic particles in liquid suspension, viewed under a LED UV-A / Black light lamps. This method is highly effective for detecting very fine surface flaws or shallow subsurface discontinuities, such as inclusions, seams, shrink cracks, tears, laps, flakes, and grinding, quenching, or fatigue cracks.
- Dry Method (Visible): Uses visible magnetic particles without the need for black light. It is more sensitive for detecting subsurface defects in rough-surfaced components, such as large forgings and castings.
Magnetic particle testing is a well-established, reliable method for inspecting magnetizable metals—primarily steel, and less commonly nickel or cobalt. The process requires magnetizing the part; however, larger workpieces can typically only be magnetized in partial areas.
When a component is magnetized, field lines run parallel to the surface. Cracks that are transverse to the field lines disturb them, creating a leakage flux (stray magnetic field). This flux causes magnetic poles to form on either side of the defect. When iron powder or magnetic particles are applied, they are attracted to these poles, making the defect visible.
- Cracks parallel to the field lines generally do not produce leakage flux and may remain undetected.
- Subsurface cracks may be detected to a limited depth, depending on the field strength and method used.
Magnetization techniques can be broadly divided into field flow and current flow methods. Using both methods in a single test cycle is referred to as combined magnetization, which improves flaw detection.
Field flow
If a component is exposed to a magnetic field, usually in a longitudinal direction, this is referred to as field flow. With the help of one or more current-carrying coils, a magnetic field is generated in a U-shaped iron yoke. The workpiece is clamped in this iron yoke The component creates a magnetic field in the longitudinal direction. Cracks at right angles to this, so-called transverse cracks, form a flux leakage and are displayed.
Current flow
Depending on the test task, however, cracks in the longitudinal direction of the component need to be detected as well. For this a second kind of magnetization is used, the so-called current flow. During the current flow, an electrical current passes through the workpiece to be tested generating a ring-shaped magnetic field. This allows longitudinal cracks to be displayed on the test specimen.
Combined magnetization
For many components, there is no bound crack orientation. Therefore, a preferred direction can not be expected. In the case of large quantities, for example for vehicle construction, testing takes place on testing machines. They allow field and current flow, which can be activated individually or combined (both simultaneously), so cracks of any orientation can be detected.
Testing agent
During magnetization, fluorescent magnetic particles are applied—typically through an aqueous suspension. These fine ferromagnetic particles, often iron oxides, align with and adhere to the leakage fields at discontinuities. To enhance visibility, the particles are combined with a fluorescent dye that glows yellow-green under ultraviolet (UV) light. In a darkened environment, this creates a bright, high-contrast indication, enabling even the finest flaws to be detected with accuracy.