What is MPI?
What is MPI?
Magnetic Particle Inspection (MPI) is a method of NTD used for the detection of surface and near-surface flaws in ferromagnetic materials and is primarily used for crack detection.
Why use MPI?
- High sensitivity when testing ferromagnetic components
- Ideal for crack testing - Especially weld testing
- Detect surface and near-surface flaws
- Detect flaws though thin coatings
- Aerosol spray cans and portable equipment make the process portable and convenient
- Easy to understand and requires minimal training
- Well-established method of NDT
- Quick and inexpensive - Inspect large areas and large volumes of components
MPI Theory
The process works by applying a magnetic field to the test component. This can be done directly by passing an electric current through the component which then induces a magnetic field perpendicular to the electric current. Or indirectly where no electric current is pass through the component but an external magnetic field is applied instead.
Since air cannot support as much magnetic field per unit volume as metals, the magnetic flux is normally heavily concentrated within the component. However, when there is a surface-breaking flaw the magnetic field is distorted, causing local magnetic flux leakage around the flaw. This leakage can be found by covering the surface with fine magnetically responsive particles. The particle can be applied either dry or suspended in a liquid and accumulate at the regions of flux leakage. This produces a build-up which can be seen visually even when the crack opening is very narrow.
Because MPI is most sensitive when the magnetic field is perpendicular to a crack, operators typically inspect components in 2 perpendicular directions - Often utilizing one direct and one indirection method of magnetisation in their inspection procedure.