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Magnetic Particle Testing (MPT)
Magnetic particle inspection is an inspection method used to identify defects on the surface of ferromagnetic materials by running a magnetic current through it.
It can also be used to detect defects just beneath the surface of materials. The types of defects it can detect include cracks, pores, cold lap, and the lack of sidewall fusion in welds.
Magnetic particle inspection (MPI) is also commonly called magnetic particle testing (MT), magnetic testing, or particle inspection.
In this guide, we will use the terms magnetic particle inspection and magnetic particle testing and other permutations like magnetic particle inspection test interchangeably, following the alternate terms listed above.
Magnetic particle inspections work by running a magnetic current through the material that is being inspected. When the current is interrupted by a defect magnetism spreads out from that point, indicating its presence and allowing inspectors to identify its location in the material.
Magnetic testing is one of the more commonly used non-destructive testing (NDT) methods because it is quick and relatively inexpensive.
However, it only works on materials that can be magnetized—called ferromagnetic materials—so its applications are somewhat limited. Some examples of ferromagnetic materials include steel, cobalt, iron, and nickel.
What Is Magnetic Particle Testing?
To conduct a magnetic particle test, inspectors start by magnetizing the material they want to inspect.
If the magnetized object has no defects, the magnetic field will transfer throughout the material without any discontinuities or interruptions. But when the current encounters defects in the material it will be interrupted, causing it to spread out from that point and create what is called a flux leakage field where the defect is located.
Once the material is magnetized and defects have created these secondary flux leakage fields, inspectors spread magnetic particles over the surface. The particles will be drawn to the secondary field, gathering around it and making it visible to the naked eye.
The particles inspectors use are typically either black or coated with some kind of fluorescent dye to make them easier to see. These particles can be used in the form of powder or put into a liquid.
THE PROS AND CONS OF MAGNETIC PARTICLE INSPECTION
Magnetic particle testing is quick and fairly inexpensive, but it does have some limitations.
Here’s a list of pros and cons for MPI:
Pros
It is very portable and quick
Results of the test immediately visible on the surface of the material
No strict pre-cleaning regiment is required and post-cleaning can also generally be avoided
Generally inexpensive, and does not need a stringent pre-cleaning
Sensitive—it can detect shallow/fine cracks in a surface
Can detect both surface and near-surface indications.
Easy to use, without a lot of training required
Flexibility—it can be used with strangely shaped objects, even on surfaces that have other materials on them
Can inspect parts with irregular shapes (external splines, crankshafts, connecting rods, etc.)
Cons
Only ferromagnetic material can be tested with MPI
Only surface and subsurface (to a depth of about 0.100" in most conditions) defects can be detected
After the test is complete the material has to be demagnetized, which can pose challenges
Inspectors must achieve an alignment between indications and magnetic flux
Only small sections of a surface can be examined at one time
Paint must be removed if it is thicker than about 0.005" for MPI to work
Conventional NDT methods
Advanced NDT methods
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