Magnetic Particle Inspection (MPI / MT) in Practice
The magnetic testing (MT) method induces a magnetic field created with either AC or DC into ferromagnetic materials (alloyed nickel and iron, among others) and partially magnetic stainless steel. The aerospace industry typically uses de-aromatized kerosene as the carrier fluid for approved wet magnetic particles. The industry mandates using a Gauss meter to determine magnetic field strength, which must be between 30-60 gauss for aerospace applications. This range enables the detection of local field pattern distortion caused by discontinuities.
MT can locate cracks, seams, discontinuities, and impurity inclusions at the surface of and within ferromagnetic materials, making it a good candidate for incoming/final inspection and periodic maintenance. Due to the range in types of those potential failure points, the choice of current can improve detection. DC-generated fields penetrate the material cross-section, making it a better choice for gaps within the material. AC fields detect material gaps at the surface.