Magnetic Particle Testing
Magnetic particle testing (MT) is a Non-Destructive Testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys.
Commonly also referred to as Magnetic Particle Inspection or MPI.
The process allows for the induction of a magnetic field into the part.
The test object can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside magnetising source.
The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).
The presence of a surface or subsurface discontinuity in the material allows magnetic flux leakage at the a defect that lies perpendicular to the lines of force.
Ferrous iron particles are then applied to the part. The particles may be either dry or in a wet suspension, of fluorescent or non-fluorescent. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication.
The indication forms an indication which is visible under white or ultra-violet light depending on the method utilized.