Understanding Magnetic Particle Testing For Effective Inspection

When it comes to non-destructive testing (NDT), magnetic particle testing or inspection stands out as a pivotal method for ensuring the integrity of welds and components. This technique, often abbreviated as MPI, offers a reliable means of detecting surface and near-surface flaws without causing any harm to the tested material.

In this blog, we will dive into the depths of this subject, explore how MPI works, and what services you need to ensure effective inspection. Talk to our NDT and MPI experts directly. With 10+ years’ experience in the industry, we are fully qualified and capable of providing you with expert advice and quality NDT services. Call 01795 432 634 or email info@standwinspections.co.uk today.

What Is Magnetic Particle Testing?

Magnetic particle testing, also known as magnetic particle inspection (MPI), is a non-destructive method used to assess the integrity of materials. By employing magnetic fields and finely dispersed iron particles, it identifies flaws such as cracks or discontinuities in ferromagnetic materials.

This method is widely employed across various industries, including aerospace, automotive, and construction, to ensure the safety and quality of critical components.

How Does MPI Testing Work?

Magnetic particle testing relies on the principles of magnetism and the interaction between magnetic fields and iron particles to identify discontinuities that may compromise the integrity of the material. We have broken down the various steps of the process for your ease of understanding.

1.    Magnetisation Process

The inspection process begins with the magnetisation of the component under scrutiny. This is typically achieved using a handheld electromagnetic yoke, which generates a magnetic field around the test specimen.

Alternatively, a stationary magnet or a magnetic coil can also be employed depending on the size and geometry of the component. The magnetic field induces magnetic poles within the material, aligning the magnetic domains along the direction of the applied field.

This magnetisation process prepares the component for subsequent inspection by creating a magnetic flux within the material.

2.    Application Of Magnetic Particles

Once magnetised, the component is coated with a thin layer of white contrast paint, which enhances the visibility of defects against the background surface. This step is crucial for improving the contrast and facilitating the interpretation of inspection results.

A suspension containing finely divided iron particles is then applied to the surface of the component. These iron particles are typically mixed with a liquid carrier, such as oil or water, to ensure proper dispersion and coverage.

The presence of defects or irregularities in the material disrupts the magnetic field. It causes the iron particles to accumulate at these locations. This results in the formation of visible indications, which can be observed and evaluated by the inspector.

3.    Inspection Techniques

Depending on the specific requirements of the inspection and the characteristics of the material being tested, different techniques may be employed. Fluorescent magnetic particle testing, for instance, utilises fluorescent particles and ultraviolet (UV) light to enhance the visibility of indications in low-light conditions or on components with complex geometries.

Inspection can be performed using either a dry method, where dry magnetic particles are applied directly to the surface, or a wet method, where the particles are suspended in a liquid carrier. The choice between these methods depends on factors such as surface condition, accessibility, and the desired sensitivity of the inspection.

7 MPI Services That Are Crucial For Effective Inspection

1.    Detection Of Fine Irregularities

MPI excels in identifying minute flaws on the surface of components, including hairline cracks, fissures, and other imperfections that may compromise structural integrity. With its high sensitivity, it can pinpoint even the smallest irregularities, ensuring thorough inspection and quality assurance.

2.    Inspection Of Complex Geometries

One of the key advantages of magnetic particle inspection is its ability to inspect components with intricate shapes and configurations. Whether it is curved surfaces, welds, or irregularly shaped parts, MPI can effectively assess the integrity of these components and provide comprehensive coverage and reliable results.

3.    Compliance With Industry Standards

In industries where safety and regulatory compliance are paramount, MPI plays a crucial role in verifying that components meet the necessary standards and specifications. This ensures that components are safe, reliable, and compliant with industry regulations.

4.    Assessment Of Component Safety

MPI also evaluates the overall safety and reliability of components, which helps prevent catastrophic failures and mitigates potential risks. By detecting defects early on, this testing process enables proactive maintenance and ensures the continued safe operation of critical equipment and infrastructure.

5.    Quality Assurance And Risk Mitigation

MPI enhances quality assurance efforts and mitigates the risk of costly downtime, repairs, and accidents for a business. It provides valuable insights into the condition of components, allowing organisations to address issues proactively and maintain optimal performance and reliability.

6.    Customised Inspection Solutions

MPI can be tailored to suit the unique needs and requirements of various industries and applications. Whether it is performing routine maintenance inspections, evaluating newly fabricated components, or conducting post-repair assessments, this efficient testing process offers flexible solutions to meet a wide range of inspection challenges.

7.    Expert Consultation And Analysis

In addition to conducting inspections, MPI service providers often offer expert consultation and analysis to help businesses interpret inspection results, identify areas of concern, and develop effective maintenance strategies.

By leveraging the expertise of experienced professionals in the industry, you can make informed decisions and optimise the performance and longevity of your assets.


Magnetic particle testing or inspection offers a vital solution for non-destructive weld testing and component inspection. By harnessing magnetic fields and iron particles, this method provides a thorough examination of materials, thus identifying flaws with precision and efficiency.

At S.T & W Inspections, we specialise in providing comprehensive MPI services tailored to your specific needs. With state-of-the-art equipment and a team of certified technicians, we ensure accurate and reliable inspection results. All our testing processes are compliant with UKCA accreditation (UK Conformity Assessment) to further ensure your peace of mind. So, wait no longer – get in touch with us to discuss your NDT and MPI requirements.