Fundamentals of Magnetic Particle Inspection

Magnetic Particle Inspection (MPI) stands as a non-destructive technique employed to detectsurface & sub surface defects in ferrous components. This method proves to be both swift and comparatively straightforward in its application, with less stringent demands on the preparatory condition of the surface compared to certain other non-destructive testing methods. These attributes have propelled MPI to become one of the most extensively employed techniques in non-destructive testing.

Magnetic particle inspection (MPI) is a fusion of two distinct NDT testing approaches: magnetic flux leakage testing and visual examination.

This method finds utility in scrutinizing diverse product forms, encompassing castings, forgings, and weldments. Multiple industries rely on magnetic particle inspection to assess the fitness-for-purpose of components. Illustrative sectors that make use of this technique encompass structural steel, automotive, petrochemical, power generation, and aerospace industries.

MPI harnesses magnetic fields in conjunction with minute magnetic particles, such as iron filings, to identify defects on surface of components. The sole prerequisite for its feasibility is that the component under inspection be constructed from a ferromagnetic material, encompassing substances like iron, nickel, cobalt, and their alloys.

Ferromagnetic Materials

A ferromagnetic material is a type of material that exhibits a strong and permanent magnetic behaviour. It possesses the ability to become highly magnetized when subjected to an external magnetic field and retains a significant portion of that magnetism even after the external field is removed. This property is due to the alignment of atomic magnetic moments within the material’s structure.

Ferromagnetic materials are characterized by their ability to exhibit spontaneous magnetization, meaning they can become magnets on their own without the need for an external magnetic field. Common examples of ferromagnetic materials include iron, nickel, cobalt, and some of their alloys. These materials are essential in various applications such as creating permanent magnets, transformers, and magnetic recording media.

Magnetic Flux Leakage

Magnetic flux leakage is a phenomenon in which a magnetic field “leaks” or deviates from its intended path due to the presence of defects, cracks, or changes in material properties within a ferromagnetic object. These anomalies disrupt the flow of the magnetic field, causing it to exit or re-enter the material, creating localized areas of magnetic imbalance.

By applying iron particles onto the surface of the object and magnetizing it, these leaked magnetic fields attract and cluster the particles, revealing the hidden defects. Magnetic particle inspection utilizes this principle to identify surface and near-surface flaws, making it a valuable non-destructive testing method in various industries.