MPI testing - Magnetic Particle Inspection

I’ve been curious about this process, and what exactly it does… I’ve known it’s good for parts to be MPI tested (BCGs and such), but I never understood what it was testing FOR… If anyone else was curious like I was, I found this link that explains it pretty well.

http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Introduction/introduction.htm

Here you go. http://en.wikipedia.org/wiki/Magnetic_particle_inspection