Magnetic Particle Examination
Definition - What does Magnetic Particle Examination mean?
Magnetic particle examination is a highly popular, cost-effective method that is used to facilitate non-destructive examination (NDE) involving ferromagnetic objects. A ferromagnetic material is any material that is capable of magnetization or those highly attracted by magnetic fields. This method is used to detect surface as well as subsurface ferromagnetic discontinuities existing in materials like:
- Other alloys
Magnetic particle examination is also known as magnetic particle inspection (MPI).
Corrosionpedia explains Magnetic Particle Examination
The magnetic particle examination process involves placing a magnetic field into action. Pieces can become magnetized by direct or indirect magnetization. In cases of direct magnetization, the electric current travels through the material being tested, and a magnetic field builds in that object. Indirect magnetization takes place when there is no current passing through the material, but a magnetic field is applied from an outside source.
There are many techniques involving magnetic particle examination, and the most recognized magnetization techniques include:
- Longitudinal magnetization
- Prod technique
- Yoke technique
- Circular magnetization
- Multidirectional magnetization
In MPI a non-defective ferromagnetic material will transfer magnetic flux throughout a material without interruption. However, in cases of discontinuity or cracks present in the material, magnetic flux will promote leakage out of the object. As the leakage occurs, the magnetic field will gather all the ferromagnetic particles (such as iron powder), making the shape and size of the discontinuity highly visible.
The magnetic field will only escape the object if the existing discontinuity is perpendicular to the flow. If a crack or any form of discontinuity is parallel to the magnetic flux lines, leakage is less likely to occur, so there will be no noticeable indications.
In order to solve this problem, all areas need to be subjected to the test at least twice. The second test should be vertical to the initial examination so that the discontinuities in all directions are identified. In such a case, the examiner must make sure that an adequate overlap of magnetic flux areas is sustained during the entire examination in order to detect any discontinuity.