Magnetic Particle Inspections: A Guide

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.

[NDT magnetic particle testing is just one of the non-destructive testing methods that inspectors use. Learn more about NDT and the other methods used in this guide.]

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 History of Magnetic Particle Testing

1868 was the first recorded time that magnetism was used to check the integrity of a material.

At the time, it was used to test cannon barrels for defects by magnetizing the barrels and then following its length with a magnetic compass, looking for any signs of discontinuity in the magnetic current. When a discontinuity appeared—indicating the presence of a defect in the barrel—the compass needle would move, allowing people to identify the location of flaws that weren’t visible to the naked eye.

Fifty years later in the 1920s inventor William Hoke found that he could use metallic shavings to form patterns on a magnetized ferromagnetic surface. These patterns would cluster around the location of defects on the surface, showing their location—just as magnetic particles are used to identify defects today.

In the 1930s, the railroad industry began using Hoke’s findings to inspect its ferromagnetic materials—namely steel—and the method soon became a standard way to identify flaws in materials.

The principles of the tests used today remain fundamentally the same as when they were first developed. At the time, MPI were used to test steel materials by magnetizing them in order to produce lines of flux. If these lines were interrupted by a defect in the material it would become clear by the creation of a second magnetic field, or flux leakage field, at the point where the defect is located.

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

Magnetic Particle Testing Techniques

As we’ve covered above, inspectors can use either a powder or water suspension to conduct magnetic testing.

Using a powder is called Dry Magnetic Particle Testing (DMPT) and using water suspension is called Wet Magnetic Particle Testing (WMPT).

Inspectors can choose to use either fluorescent or non-fluorescent materials for both the power and the water suspension methods, allowing them to use an approach that will make defects most highly visible for the environment.

Two-Step Overview

Here is the basic two step process for how inspectors do both the wet and the dry methods of magnetic testing:

Magnetize the object. Run a magnetic current through the material. If defects are present they will create a secondary magnetic field, or flux leakage field.
Spread metal particles on the object. Spread metal particles over the material or object in the form of a powder or liquid. The secondary field(s) will attract these particles to the location of defects, allowing them to be made visible.

Although the basics of the process are fairly straightforward, there are several considerations to how each step is performed. These are covered in the next section of this guide, entitled Magnetization Considerations.

Some of the most common techniques for on-site magnetic testing include:

Electromagnetic yoke
Current flow probes
Permanent magnet
Flexible coil
Adjacent cable

Magnetization Considerations

Here’s an overview of the most common considerations inspectors make when conducting magnetic particle testing.

Ways to Magnetize the Material

There are several different techniques for magnetizing a material when conducting a magnetic particle inspection. Here are the five techniques most commonly used, which are also recognized by various standards bodies, including the ASME (American Society of Mechanical Engineers).

Longitudinal magnetization technique
Multidirectional magnetization technique
Yoke technique
Prod technique
Circular magnetization technique

Perpendicular Application

Magnetic lines of force should be applied perpendicularly to the direction of the electric current. The current can either be Direct Current (DC) or an alternating current (AC).

To conduct a thorough MPI, inspectors need to inspect a material twice. This is because the defect will only interrupt the magnetic flux (or line of force) if the flux is perpendicular to the defect. If the two aren’t perpendicular then there won’t be an interruption in the flow, and the defect won’t be identified.

Therefore, inspectors must conduct their magnetic testing twice in order to ensure that they’ve gotten coverage—once in one direction, and once more in a direction perpendicular to the first direction.

Direct vs. Indirect Magnetization

Inspector can magnetize materials either by indirect or direct magnetization.

Direct magnetization refers to passing an electric current directly through the material, creating a magnetic field in it.
Indirect magnetization refers to creating a magnetic field in the material from an outside source instead of passing an electric current through it.

Electrical Current Considerations

Inspectors use several types of electrical current when doing a magnetic test.

To choose the right current for a given inspection, inspectors must consider:

The object’s shape
The types of defects they are looking for
The object’s material
How deeply the magnetic field needs to go into the object to achieve the goal of the inspection

Here’s a list of electrical currents and associated considerations for MT:

AC (Alternating Current). AC is used to detect flaws on the surface of materials—not ideal for subsurface flaw detection because it can be subject to the “skin effect,” in which the electrical current runs only along the surface and doesn’t penetrate it.
DC (Direct Current)—full wave. Also called FWDC, full wave DC is used to identify flaws that are just underneath the surface of materials, since it can magnetize materials more deeply than AC. The depth of magnetic penetration for DC is dependent on the amount of current running through the material.

DC (Direct Current)—half wave. Also called pulsating DC or HWDC, half wave DC can achieve similar results as full wave DC but it can achieve deeper magnetic penetration.

Magnetic Particle Inspection Equipment

There are several different types of magnetic particle testing equipment that inspectors use in their work. In general, this equipment is used to create magnetic currents and fields for inspection purposes.

Here are some of the most common types of particle inspection equipment:

Magnetic wet benches

Magnetic benches allow inspectors to create circular and longitudinal magnetic field outputs for magnetic particle testing.

A magnetic wet bench | Credit: Magnaflux

Power packs / electromagnetic current generators

Power packs give inspectors a quick, easy way to generate a magnetic current for MPI.

A portable power pack | Credit: Magnaflux

Magnetic Yokes

Inspectors use magnetic yokes to generate a magnetic field for magnetic particle inspections.

An electromagnetic AC/DC yoke | Credit: Magnaflux

Enclosures, hoods, and curtains

Enclosures, hoods, and curtains are used to sufficiently darken the magnetic particle examination area to required levels.

An enclosure | Credit: Magnaflux

Demagnetizers

Demagnetizers help inspectors remove residual magnetism after a magnetic particle inspection has been conducted.

A table-top demagnetizer | Credit: Magnaflux

Magnetic Particle Inspection Standards and Codes

For certain inspections inspectors are required by law to follow specific steps when conducting magnetic particle examination. In addition, the inspector conducting the inspection must be certified to do so by the relevant standards body.

Here are some of the internationally recognized standards for magnetic particle inspection:

Magnetic Particle Inspections: A Guide