Magnetic Particle Inspection (NDT)

by | Mar 21, 2023

The aerospace is a multi-billion dollar industry with the potential for even more growth. With billions of dollars invested in aircrafts, the effects of manufacturing errors and faults in parts can be rather disastrous. If the functionality and accuracy of these parts are not properly analyzed, the lives of operators who work with them may be in danger. 

Not only that, but a significant portion of the money used to produce those essential components may be lost. 

Equipment maintenance then includes testing as a necessary step in order to avoid these catastrophic failures. Assessing the components, design, structure, and raw materials of your assets and products is crucial. The procedures can be categorized as destructive testing or nondestructive testing, depending on the condition of the tested component after the testing is complete.

Destructive testing is a testing technique used when a component is damaged or destroyed during the testing process. Nondestructive testing, in contrast, is performed without harming the equipment being tested. Magnetic Particle Inspection (MPI) is a Non-Destructive Testing (NDT) technique used to detect surface and subsurface defects in ferromagnetic materials. 

In this article, we will discuss magnetic particle inspection, the difference between magnetic inspection and non-destructive testing, if any, its benefits, disadvantages, and different techniques in carrying it out. 

If you’re looking for a flawless NDT service for your business, you can contact us or check out our different non-destructive testing services here.

Magnetic particle inspection (NDT)


What Is Magnetic Particle Inspection?

Magnetic Particle Inspection (MPI) is a Non-Destructive Testing (NDT) technique used to detect surface and subsurface defects in ferromagnetic materials.  By using a magnetic field to find discontinuities in the material, MPI can assess the material’s condition quickly and precisely.

This method of  inspection is popular, trustworthy, and economical. It is used to find surface and subsurface flaws in ferromagnetic materials, such as fractures, laps, seams, and porosity. In addition to the production of electrical and medical equipment, MPI is frequently utilized in the nuclear, aerospace, and automobile industries.


What Is NDT? 

Non-Destructive Testing (NDT) is a technique of testing used to assess a material’s, component’s, or system’s characteristics without causing damage. With NDT, it is possible to find faults, cracks, corrosion, and other defects that might cause failure or poor performance. Many materials, such as metals, polymers, composites, and ceramics, can be subjected to NDT.

The aerospace, automobile, and energy sectors are just a few that employ it. NDT techniques include visual inspection, eddy current testing, radiography, ultrasonic testing, and magnetic particle testing. NDT is a useful technique for quality control and safety since it makes it possible to find flaws before they cause problems.

What is NDT?


What Is The Difference Between NDT And MPI? 

The main difference between NDT and MPI is that MPI is specifically used to detect flaws in ferromagnetic materials, while NDT is a general term used to describe a variety of techniques used to evaluate the condition of a material or component without causing damage or destruction. 

Additionally, MPI is a non-invasive technique, meaning that it does not require the removal of material from the object being inspected.


The History Of Magnetic Particle Inspection 

The concept of utilizing a magnetic field to find discontinuities in ferromagnetic materials was created by German physicist Heinrich Barkhausen in the early 1900s. This is how Magnetic Particle Inspection (MPI) got its start. Morris Cohen, an American scientist and inventor, expanded on this method in the 1920s. 

A technique for magnetizing ferromagnetic materials and subsequently dispersing iron filings in it was created by Cohen. Then he illuminated the filings with a light source to look for any material discontinuities.

Since then, MPI has gained popularity as a non-destructive testing technique (NDT). It is frequently used to find faults in parts, welds, and components in the manufacturing, aerospace, and auto industries. Moreover, MPI is employed in the medical sector to identify bone fractures and in the oil and gas sector to identify faults in pressure vessels and pipes.

Today, MPI is one of the most widely used NDT techniques. It is used to detect surface and subsurface defects in a wide variety of materials, from metals to plastics. It is a fast, accurate, and cost-effective way to evaluate the condition of a material without destroying it.


How Does Magnetic Particle Inspection Work? 

The magnetic particle inspection technique makes use of a magnetic field to find material discontinuities, enabling quick and precise evaluation of the material’s state. The object must first be magnetized before an MPI can be performed. A powerful magnetic field is used to do this on the object. Metal dust, such as iron filings, is applied to the surface of the object after it has been magnetized. Any discontinuities in the material will be drawn to these particles, making them obvious to the inspection.

The magnetization process is important in order to ensure that the magnetic field is strong enough to attract the particles. Several magnetization techniques may be utilized, depending on the kind of material being tested. For steel or iron, for instance, a direct current (DC) magnetization method may be employed; however, for aluminum or stainless steel, an alternating current (AC) method may be utilized.

The metal flakes are applied to the surface of the object after it has been magnetized. To do this, either use a spray system or sprinkle the particles across the surface. Any discontinuities in the material will then be drawn to the particles, making them obvious to the inspection. The inspector will then be able to spot any flaws in the substance, like inclusions, porosity, or fissures.

It is important to note that the magnetic field created by the magnetization process must be strong enough to attract the particles.The particles might not be drawn to the discontinuities if the field is insufficient, which would make them challenging to find. In order to identify any discontinuities, it is also crucial to make sure the particles are distributed equally across the object’s surface.

Once the inspection is complete, any remaining particles must be removed from the object. This is typically done using a vacuum system or a wet-bench. Finally, the object must be demagnetized in order to remove any residual magnetism.

How does magnetic particle inspection work


Magnetize The Object 

The first stage in the Magnetic Particle Inspection method is to magnetize the object. To create a magnetic field, an electric current must be run through the material. Any material discontinuities, such as cracks, porosity, or other defects, are found using the magnetic field.

Depending on the substance being inspected and the intended outcomes, many methods of applying the magnetizing current might be used. Typically, an electrical current generator, like a DC power supply or an AC generator, is used to apply the current. Either the current is applied directly to the material or it is induced using a flexible coil or an electromagnetic yoke.

When magnetizing the material, it is important to consider the direction of the current.  To guarantee optimal sensitivity, the current should be applied perpendicular to the material’s surface. To make sure that the magnetic field is powerful enough to detect discontinuities, the current should also be applied with the proper duration and intensity.

It is also important to take into account the kind of content being examined. Aluminum and copper, which are soft materials, require less current than steel and iron, which are tougher materials. The present intensity and length will also be impacted by the kind of discontinuity being looked for. For instance, a longer time period might be necessary to find cracks, whereas a shorter time period might be adequate to find porosity.


Spread Metal Particles On The Object 

Once the object has been magnetized, metal particles are spread over the surface of the object. These granules may be dry or suspended in a liquid, such as water or oil. The magnetic field attracts the particles, making it simple for them to detect any magnetic anomalies or discontinuities in the substance. The particles will gather close to the flaws, making it simple to spot and measure them. The type of flaw can also be determined by looking at the size and form of the particles. For instance, if the particles are lengthy, that can be a sign of a material crack.


Why Is Magnetic Particle Inspection Important? 

Magnetic Particle Inspection (MPI) is an important Non-Destructive Testing (NDT) technique used to detect surface and subsurface defects in ferromagnetic materials. A useful tool for many sectors, MPI provides a quick, precise, and economical way to assess the state of materials.

MPI is utilized in a wide range of sectors, including the aerospace, automotive, and medical fields. MPI is employed in the aerospace sector to find flaws in aircraft parts, such as fatigue in fuselage frames or cracks in turbine blades. MPI is employed in the automotive sector to check welds, engine blocks, and other parts. MPI is used in the medical sector to find flaws in implants and other medical equipment.

For many sectors, MPI is a critical tool due to its precision and affordability. MPI is a quick and reliable way to find material flaws, enabling accurate and timely assessment of the state of the material. This lessens the possibility of pricey repairs or replacements and helps verify that components fulfill safety and quality standards.


Benefits Of Using Magnetic Particle Inspection 

MPI offers numerous advantages over other NDT methods, making it an ideal choice for many applications. The primary benefits of using MPI include:

Quick and Accurate Results 

MPI can detect defects quickly and accurately, allowing for fast and reliable evaluation of the material’s condition.


MPI is a cost-effective method of testing compared to other NDT techniques.


MPI is a non-destructive method of testing, meaning that the material being tested is not altered or damaged in the process.


MPI requires minimal equipment and can be easily transported to different locations, making it a great choice for on-site testing.


MPI can be used to detect a wide range of defects, including cracks, voids, inclusions, and other discontinuities.


Disadvantages Of Magnetic Particle Inspection 

  • MPI is limited only for ferromagnetic materials like steels, cast irons, etc. Non-ferrous materials cannot be inspected.
  • The inspection is limited to small sections only. The examination of large parts may require the use of special equipment.
  • Access may be a problem for the magnetizing equipment.
  • Magnetic flux and indications must be aligned for proper results.
  • Equipment must be calibrated, with no permanent record of the result.

Disadvantages of magnetic particle inspection

Magnetic Particle Inspection Techniques 

The inspection process involves the application of a magnetic field to the material, which can then be used to detect discontinuities in the material. In order to properly detect these discontinuities, it is important to understand the various techniques used in MPI.            
            Magnetic particle inspection techniques

Electromagnetic Yoke 

Electromagnetic Yoke is a type of Magnetic Particle Inspection (MPI) technique used to detect surface and subsurface defects in ferromagnetic materials. Since it is convenient and economical, it is the most extensively used MPI technique. 

When powered by an alternating current, the electromagnetic yoke, a U-shaped device, produces a magnetic field. By using the magnetic field to find discontinuities in the material, the state of the material may be assessed quickly and precisely.

The ease and affordability of employing an electromagnetic yoke for MPI are its primary advantages. It is perfect for non-destructive testing of huge things because it is very simple to operate. The yoke is a flexible NDT instrument since it may also be used to examine items of various sizes and shapes.


Current Flow Probes 

One of the most popular methods for Magnetic Particle Examination is using Current Flow Probes (MPI). With this technique, surface and subsurface flaws can be found by magnetizing the part being examined with a low-frequency AC current.

In cases where the part being investigated is either huge or too challenging to be magnetized by other techniques, current flow probes are frequently utilized. Since the current may be supplied in any direction, this technique is also excellent for parts with complicated geometry.


Adjacent Cable 

Adjacent Cable Magnetic Particle Inspection (MPI) is a technique that uses an alternating magnetic field to detect discontinuities in ferromagnetic materials.  This method is frequently employed to find surface and subsurface flaws in ferromagnetic alloys such as steel, iron, and other.

The conventional Yoke MPI method has a variation called Adjacent Cable MPI. This technique involves passing a current across two nearby cables that are capable of transporting current. The magnetic field generated by the current between the wires is used to find any discontinuities in the material under inspection.


Flexible Coil

Flexible Coil is a type of Magnetic Particle Inspection (MPI) technique that uses a flexible coil to detect discontinuities in ferromagnetic materials. An alternating current is passed through the flexible coil as it is wrapped around the object to be examined. The material is penetrated by the coil’s magnetic field, which enables the detection of any potential flaws. 

The flexible coil can be easily wrapped around huge, oddly shaped objects, such pipelines or turbine blades, making it perfect for inspecting them. The flexible coil technique is also relatively fast and cost-effective, making it a popular choice for many MPI applications.


Magnetization Considerations 

Magnetization Considerations are an important part of Magnetic Particle Inspection (MPI). The type of magnetization used, the direction of the applied magnetic field, and the electrical current used all play a role in determining the accuracy of the MPI results.

Magnetization Technique 

Magnetization technique is an important consideration to make in Magnetic Particle Inspection (MPI). This technique is used to create a magnetic field in the material being inspected. There are two primary approaches to magnetizing a material: perpendicular application and electrical current. 

During perpendicular application, a magnetic field is produced in the material using a permanent magnet. For ferromagnetic materials, such as iron and steel, this technique is frequently employed. The north and south poles of the magnet, which are positioned perpendicular to the surface of the material, produce the magnetic field. This technique is frequently used to find cracks, pits, and other surface discontinuities.

Electrical current magnetization is used to create a magnetic field in a material by passing an electric current through it. For non-ferromagnetic materials like titanium and aluminum, this technique is frequently employed. A magnetic field is produced in the material as a result of the electric current being applied to it in a certain pattern. This technique is used to find cracks, inclusions, and other subsurface discontinuities.

The choice of magnetization technique depends on the type of material being inspected and the type of discontinuities being sought. It is important to use the appropriate technique to ensure accurate results.


Magnetic Particle Inspection Equipment 

Magnetic Particle Inspection (MPI) requires specialized equipment to detect defects in ferromagnetic materials. This equipment includes enclosures, hoods, and curtains to contain the particles used in the inspection process. 

It also includes electromagnetic current generators to magnetize the object, magnetic wet benches to hold the object while the particles are applied, and demagnetizers to remove any residual magnetism after the inspection. 

Additionally, MPI requires magnetic yokes to produce a magnetic field around the object to detect any discontinuities.              Magnetic Particle Inspection Equipment


Enclosures, Hoods, And Curtains 

Enclosures, hoods, and curtains are essential components of a Magnetic Particle Inspection (MPI) system. They are designed to contain the magnetic field and prevent any interference from external sources. Enclosures, hoods, and curtains also provide a safe environment for the operator and prevent any damage to the object being inspected.

Enclosure, Hoods, and curtains


Electromagnetic Current Generators 

Electromagnetic current generators are an essential part of Magnetic Particle Inspection equipment. These generators are used to create the magnetic field necessary to detect surface and subsurface defects in ferromagnetic materials. Electromagnetic current generators come in a variety of shapes and sizes, and can be used for both alternating current (AC) and direct current (DC) applications. 

        Electromagnetic current generators


Magnetic Wet Benches 

In Magnetic Particle Inspection (MPI), magnetic wet benches are used to demagnetize the test object. The liquid demagnetizing solution is contained in a stainless steel tank that serves as the wet bench. To lessen the residual magnetism, the test object is put in the tank and exposed to a changing magnetic field. 

The demagnetizing procedure is often carried out in stages, with the magnetic field’s intensity steadily increasing. This guarantees that the object’s residual magnetism is diminished to a level that is suitable for the MPI process.

             Magnetic wet benches 



Demagnetizers are an important component of Magnetic Particle Inspection (MPI) equipment. After the inspection procedure is finished, they are used to lower the magnetic field in a ferromagnetic material. Demagnetizers can be used to lessen a material’s residual magnetism, which could interfere with other machinery or processes. 

Also, they aid in lowering the danger of corrosion brought on by the accumulation of magnetic fields. Demagnetizers can be powered by electricity, compressed air, or a combination of the two and come in a range of sizes and designs.



Magnetic Yokes 

Magnetic yokes are a type of equipment used in Magnetic Particle Inspection (MPI). They are used to create a magnetic field of a known strength and direction, which can then be used to detect discontinuities in ferromagnetic materials. Magnetic yokes come in a variety of shapes and sizes, ranging from small handheld units to large industrial-grade systems.

There are different types of magnetic yokes. They include electromagnetic yokes, flow probes, adjacent cable yokes, flexible coil yokes, and magnetic yokes.

                                    Magnetic Yokes  


Magnetic Particle Inspection Standards And Codes 

Magnetic Particle Inspection (MPI) is widely used in many industries, and as such, there are a variety of standards and codes that govern its use. These standards and codes ensure that MPI is conducted in a safe and reliable manner, and provide guidelines for inspectors to follow when performing MPI. 


ASTM (American Society Of Inspection And Materials) 

ASTM (American Society of Inspection and Materials) is an international standards organization that develops and publishes technical standards for a wide range of materials, products, systems, and services. The ASTM has a subcommittee on Magnetic Particle Inspection (MPI), which is responsible for developing standards for the use of MPI in various industries. 

The ASTM Subcommittee on MPI has developed several standards for the use of MPI, including ASTM E1444, ASTM E1444-17, ASTM E1444-20, ASTM E1445, and ASTM E1445-20. These standards provide guidance on the proper use of MPI, including the types of materials that can be inspected, the types of defects that can be detected, and the methods of magnetization and demagnetization that should be used. The ASTM also provides guidance on the requirements for personnel who are certified to perform MPI, as well as the requirements for equipment used in MPI.


ISO (International Standards Organization) 

The International Standards Organization (ISO) is an international body that sets standards for products, services, and processes.To guarantee the security and caliber of goods and services, ISO standards are applied. International committees of specialists create ISO standards, which are then periodically revised and updated.

The testing procedure for Magnetic Particle Inspection (MPI) is governed by a number of ISO standards. The most popular MPI standard is ISO 9934-2. This standard addresses the conditions for carrying out MPI in line with the tenets of the ISO 9934 series. It covers the specifications for the tools, workers, and protocols needed to carry out MPI.

ISO 9934-3 is the standard for the evaluation of the results of magnetic particle testing. ISO 9934-4 is the standard for the reporting of the results of magnetic particle testing.
ISO 9934-5 is the standard for the calibration of the magnetic particle testing equipment. Finally, ISO 9934-7 is the standard for the qualification of personnel performing magnetic particle testing. 


CEN (European Committee For Standardization) 

The European Committee for Standardization (CEN) is an organization that develops and maintains standards for products, services, and systems in Europe. CEN works to ensure that products, services, and systems are safe, reliable, and of high quality. In terms of Magnetic Particle Inspection (MPI), CEN has developed a standard for the performance and evaluation of MPI. 

This standard, EN 4179:2016, outlines the requirements for personnel, equipment, and procedures for the MPI of metallic materials. The standard also outlines the requirements for the acceptance and rejection of components based on the results of the inspection. In addition, the standard outlines requirements for the documentation of the inspection process and results. By adhering to the CEN standard, organizations can ensure that their MPI process is safe, reliable, and of high quality.


Final Thoughts 

At Valence, we take care in ensuring your parts are flawless. We offer a range of inspection services, including NDT as a standalone service, as well as all pre- and post-NDT finishing services.

With multiple in-house level 3 certified technicians, we expertly process everything from small parts to large structural parts up to 30.’ Our technicians complete exceptional training and exceed standards throughout the entire production process.

We offer extensive maintenance and fracture critical approvals for all major primes, including Boeing, Airbus, and many more. We hold AS9100 Nadcap approval as well as unique approvals for NASA, United Launch Alliance, and many more.

At Valence, we understand the importance of meeting industry requirements. We are committed to quality assurance in all of the products we produce. Ask us about our on-location NDT services, including magnetic particle inspection..



What factors can affect the accuracy of a magnetic particle inspection? 

The accuracy of Magnetic Particle Inspection can be affected by several factors, including the type of material being inspected, the size and shape of the object, the type of magnetic particles used, the magnetizing current, the type of magnetizing technique used, and the sensitivity of the equipment. Additionally, environmental conditions such as temperature and humidity can also affect the accuracy of the inspection.


Does magnetic particle inspection require specialized training? 

Yes, Magnetic Particle Inspection requires specialized training to ensure the accuracy of the results. The inspector must be familiar with the equipment and techniques used in the inspection, as well as the standards and codes that apply.


What types of flaws can be detected using magnetic particle inspection? 

Magnetic Particle Inspection can detect surface and subsurface discontinuities such as cracks, laps, seams, inclusions, and other surface defects.


What is the difference between magnetic particle inspection and visual inspection?

Magnetic Particle Inspection is a Non-Destructive Testing technique that uses a magnetic field to detect discontinuities in the material. Visual Inspection is a more basic form of inspection that involves simply looking at the object to identify any visible defects.


Is magnetic particle inspection safe? 

Yes, Magnetic Particle Inspection is a very safe procedure. The magnetizing current used is very low, and the particles used are not harmful. However, it is important to follow safety procedures when performing the inspection, such as wearing protective clothing and avoiding contact with the magnetic field.

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