MPI: Reliable Flaw Detection with Magnetic Particles

Magnetic Particle Testing (MPI) is a widely used nondestructive testing method for detecting surface and shallow subsurface discontinuities in ferromagnetic materials, such as iron, nickel, cobalt, and their alloys. This testing process involves magnetizing the material and applying magnetic particles to its surface, which then reveal any discontinuities through a process known as flux leakage.

Principle of Operation
In MPI, the test object is magnetized either through direct or indirect magnetization. Direct magnetization occurs by passing an electric current through the material, creating a magnetic field. The direction of the magnetic lines of force is perpendicular to the electric current, and alternating or direct current (DC) can be used depending on the depth of the area to be examined. Indirect magnetization, on the other hand, involves applying a magnetic field from an external source without using electric current through the part itself.

When a discontinuity is present in the material, it causes the magnetic flux to leak, as air cannot support the magnetic field as effectively as metals. Magnetic particles are applied to the surface of the material, and these particles are attracted to areas of flux leakage, forming visible indications. These indications help to identify the presence, nature, and cause of the discontinuity.

Magnetic Particle Powders and Carriers
The detection process involves using magnetic particles, typically iron oxide, which can be applied dry or wet. Wet systems use very fine particles that fluoresce under ultraviolet light, making them ideal for detecting small cracks. Dry powder systems are applied in white light conditions and are typically used in non-wet environments. The particles are often suspended in oil or water-based carriers, ensuring the particles adhere effectively to the test surface.
Magnetic Particle Testing is a reliable and efficient method for detecting surface and near-surface defects in ferromagnetic materials, providing crucial insights into the integrity and safety of components in various industries such as manufacturing, automotive, and aerospace.