Magnetic particles (NDT)
Introduction
Magnetic particle inspection is a type of non-destructive testing that allows detecting surface and subsurface discontinuities in ferromagnetic materials, which can lead to their future failures.
Physical basis
The application of the Magnetic Particle test basically consists of magnetizing the piece to be inspected, applying the magnetic particles (fine powder of iron filings) and evaluating the indications produced by the grouping of the particles at certain points. This process varies depending on the materials used, the defects to look for, and the physical conditions of the object of inspection.
Use
It is used when a faster inspection is required than that achieved using liquid penetrants. There are 32 variants of the method, and each one serves different applications and sensitivity levels.
This method is used on ferromagnetic materials such as iron, cobalt and nickel. Due to its low magnetic permeability, it is not applied to paramagnetic materials (such as aluminum, titanium or platinum) or diamagnetic materials (such as copper, silver, tin or zinc).
The defects that can be detected are only those that are on the surface or at shallow depths. The smaller the size of the defect, the lower the depth at which it can be detected.
Magnetic field
The magnetic field can be generated by a permanent magnet, an electromagnet, a coil or the circulation of electrical intensity over the piece. Permanent magnets are rarely used because they can only be achieved with weak magnetic fields.
In an elongated piece, coil magnetization generates a longitudinal magnetic field, which is why it shows transverse defects. Instead, an electric current between the ends of the piece generates a transverse field, which detects longitudinal defects.
Particle size, shape and application
Magnetizable particles must be small in size to have good resolution, that is, to detect small or deep defects. This is because the larger the size of the particle, the larger the field needed to rotate it. However, they should not be too small so that they do not accumulate on surface irregularities, causing erroneous readings. Therefore, it is usual to combine small particles (between 1 μm "Micrometer (unit of length)") and 60 μm "Micrometer (unit of length)") and large particles (from 60 μm "Micrometer (unit of length)") to 150 μm "Micrometer (unit of length)") in the same test).