Magnetic Particle Inspection (MPI), also known as Magnetic Particle Testing (MPT), is a non-destructive testing (NDT) method used to detect surface and near-surface flaws in ferromagnetic materials. Ferromagnetic materials are those that can be magnetized, such as iron, steel, and certain alloys.
The basic principle behind MPI involves creating a magnetic field in the material being inspected and then applying magnetic particles (usually iron or iron oxide particles) to the surface of the material. These magnetic particles are typically suspended in a liquid or applied as a dry powder. When applied to the surface, these particles are attracted to areas where there are flaws or defects in the material.
The process typically involves the following steps:
- Magnetization: The component or structure to be inspected is magnetized by passing an electric current through it or by using a permanent magnet. This magnetization causes magnetic field lines to flow through the material.
- Application of Magnetic Particles: Either dry magnetic particles or a liquid suspension containing the particles is applied to the surface of the material. The particles adhere to the surface because of the magnetic field.
- Inspection: An inspector examines the surface of the material under proper lighting conditions. The presence of flaws, cracks, or defects in the material disrupts the magnetic field, causing the magnetic particles to gather at these locations. This makes the flaws visible as distinct lines or patterns.
- Interpretation: The inspector interprets the patterns of the magnetic particles to determine the location, size, and nature of any defects in the material.
MPI is a valuable method for identifying surface and near-surface defects like cracks, seams, and weld discontinuities in ferromagnetic materials. It is widely used in industries such as aerospace, automotive, construction, and manufacturing to ensure the integrity and safety of critical components and structures. One of the advantages of MPI is its ability to detect defects on or just below the surface, making it a valuable tool for quality control and safety assurance.
