A technician performing a magnetic particle inspection on a welded joint, holding a yoke in one hand while spraying the surface with the other.

Magnetic Particle Testing: An Essential Method for NDT

Discover the advantages of the magnetic particle testing (MT) method, understand the basic principles of MT, and explore the variety of techniques for applying this highly adaptable method in nondestructive testing across industries.

Table of Contents
OverviewAdvantages and LimitationsBasic PrinciplesTechniquesIndustry Applications

What Is Magnetic Particle Testing and How Is It Used in NDT?

Magnetic particle testing is an NDT method used to detect surface and near-surface discontinuities in ferromagnetic materials. MT involves magnetizing a ferromagnetic material and then applying fine ferromagnetic particles to the surface. Discontinuities in the material, such as cracks or voids, disrupt the magnetic field, creating a leakage field. The ferromagnetic particles are attracted to these leakage fields, forming visible indications of the discontinuities.

The primary objectives of MT in industrial applications include:

  • Detecting Surface and Near-Surface Defects: By applying magnetic particles to a magnetized part, MT helps identify and measure surface and near-surface defects.

  • Ensuring Material Integrity: MT is used to ensure the integrity of critical components in various industries, such as aerospace, automotive, and manufacturing.

Image of a welded surface with a visible crack.

Advantages and Limitations of Magnetic Particle Testing

MT is a versatile method applicable in many industries. Its ability to detect surface and near-surface flaws makes it invaluable for ensuring the integrity and safety of equipment and structures.

Advantages of Magnetic Particle Testing

  • Surface and Near-Surface Inspection: MT is highly effective at detecting surface and slightly subsurface discontinuities.

  • Quick and Relatively Simple: The process is relatively quick, making it suitable for time-sensitive projects.

  • Cost-Effective: Compared to some other NDT methods, MT is relatively low-cost.

  • Versatility: MT can be used on a variety of ferromagnetic materials, including iron, nickel, cobalt, and their alloys. It is applicable to different shapes and sizes of test objects, from small components to large structures.

  • Immediate Results: The results of an MT inspection are visible immediately, allowing for quick decision-making regarding the integrity of the test object.

  • Portable Equipment: Many MT equipment setups are portable, making it possible to perform inspections in the field or in remote locations.

Limitations of Magnetic Particle Testing

  • Limited to Ferromagnetic Materials: MT can only be used on materials that can be magnetized.

  • Surface Preparation: The surface of the test object must be clean and free of contaminants, which can add time and effort to the inspection process.

  • Depth Limitation: MT is primarily effective for detecting surface and near-surface discontinuities. It may not be able to detect deeper flaws that are located further below the surface.

  • Demagnetization Required: After the inspection, the test object often needs to be demagnetized to remove any residual magnetism, requiring additional time and specialized equipment.

  • Environmental Conditions: Extreme temperatures or high humidity, can affect the performance of MT. For example, magnetic particles may not adhere properly in very humid conditions.

  • Health and Safety Concerns: Certain magnetic particles may pose health and safety concerns. Proper handling and disposal procedures must be followed to mitigate these risks.

How Magnetic Particle Testing Works: Basic Principles

To understand how MT works, it’s helpful to be familiar with the principles of magnetism. Magnets attract or repel one another, and they attract some metals but not all. Magnets come in all shapes and sizes with varying degrees of power; even the Earth is a huge spherical magnet.

In the MT method, a magnetic field is introduced into the part being inspected. Technicians use various equipment such as yokes, coils, or prods to magnetize the material. The magnetic field can be applied using either alternating current (AC) or direct current (DC), depending on the inspection requirements.

Once the part is magnetized, ferromagnetic particles (either dry powder or suspended in a liquid) are applied to the surface. These particles are attracted to areas where the magnetic field is disrupted by discontinuities, such as cracks or voids, forming visible indications.

By analyzing these indications, skilled operators can identify the location and size of surface and near-surface discontinuities.

Magnetic Field Interaction with Materials

The magnetic field used in MT interacts with the material's internal structure. When the magnetic field encounters a discontinuity, it creates a leakage field that attracts the ferromagnetic particles, revealing the discontinuity's presence.

MT operates on the principles of magnetic flux leakage and particle attraction to accurately locate and identify discontinuities within the material.

Magnetic Flux Leakage: Similar to water flowing around a rock in a stream, the magnetic field flows through the material but leaks out at discontinuities. This leakage field attracts the ferromagnetic particles, forming visible indications.

Particle Attraction: Comparable to iron filings aligning along the lines of a magnetic field, ferromagnetic particles are drawn to areas of magnetic flux leakage, highlighting discontinuities.

A diagram showing magnetic flux leakage, illustrating how magnetic lines of force behave when encountering a defect in a material. The image includes a U-shaped magnet with clearly marked north (N) and south (S) poles, and a block of material below it with a crack. The magnetic lines curve outwards around the crack, demonstrating how the magnetic flux leaks from the surface due to the defect, altering the usual flow of the magnetic field within the material.

Types of Materials in MT

All materials react in some way to magnetic fields. Every material is made up of tiny particles called atoms, which have a core (nucleus) that is positively charged and surrounded by electrons that move around it. These electrons are always in motion. When a material is exposed to a magnetic field, the movement of the electrons is affected or distorted. How much this distortion happens helps us group materials into three categories:

Diamagnetic Materials

These materials are very weakly repelled by a magnetic field and do not retain any magnetism when the external magnetic field is removed
Examples: Copper, gold, and bismuth.

Paramagnetic Materials

These materials are weakly attracted to a magnetic field and do not retain magnetism once the external magnetic field is removed.
Examples: Aluminum, platinum, and some stainless steels.

Ferromagnetic Materials

These materials are strongly attracted to a magnetic field and can become permanently magnetized. They can retain their magnetism even after the external magnetic field is removed, which is why they are used to make permanent magnets.
Examples: Iron, nickel, and cobalt.

Types of Magnetic Fields in MT

By using different types of equipment, NDT practitioners can create magnetic fields that propagate through materials in different ways, each with unique characteristics and applications.

Circular Magnetization: Generated by passing current through the part or a central conductor, creating a magnetic field that encircles the part. Ideal for detecting longitudinal discontinuities.

Longitudinal Magnetization: Produced by placing the part in a coil or using a yoke, creating a magnetic field that runs along the length of the part. Effective for detecting transverse discontinuities.

Alternating Current (AC) Magnetization: AC is used for surface inspections as it creates a strong surface field but does not penetrate deeply. Suitable for detecting surface cracks.

Direct Current (DC) Magnetization: DC provides deeper penetration, making it suitable for detecting subsurface discontinuities. Often used in conjunction with wet particles for better visibility.

A horseshoe magnet with iron filings around it, showing the magnetic field lines curving from one pole to the other.

Particle Types in MT

Different types of ferromagnetic particles are used in MT, each suited for specific applications and inspection conditions.

Visible particles

Dry particles are applied to the surface and are ideal for rough surfaces or when portability is required.

Fluorescent Particles

Suspended in a liquid carrier, wet particles are used under ultraviolet light for more sensitive inspections. They provide better coverage and visibility, especially for fine discontinuities.

Dual-Use Particles

Particles are coated with both visible and fluorescent pigments for inspection using visible and UV-A light.

A technician wearing safety gear performs magnetic particle testing on a large metal surface in an industrial setting.

Magnetic Particle Testing Techniques Used in NDT

Magnetic particle testing (MT) employs a variety of techniques to detect and measure discontinuities in materials. These techniques can be categorized based on how the magnetic field is introduced into the material and the specific methods used to enhance flaw detection and measurement.

Techniques can be applied in three ways.

Contact Testing

  • Yoke Testing: Uses a handheld yoke to create a magnetic field in the part. Ideal for field inspections and irregularly shaped parts.

  • Prod Testing: Uses two electrodes (prods) to pass current through the part, creating a circular magnetic field. Suitable for localized inspections.

Coil Testing

  • Circular Coil Testing: The part is placed inside a coil, creating a longitudinal magnetic field. Effective for inspecting cylindrical parts.

  • Solenoid Coil Testing: Similar to circular coil testing but used for larger parts or assemblies.

Portable Testing

Use a portable yoke or coil to conduct field inspections, providing flexibility and ease of use.

Add Magnetic Particle Testing Certification to Your Qualifications

ASNT certifications enable you to become a qualified Level II or Level III in MT.

What Certification Is Right for Me?

Application of Ultrasonic Testing in NDT Across Industries

Magnetic particle testing (MT) is widely used across various industries to detect surface and near-surface discontinuities in ferromagnetic materials.

Energy

MT is used in the oil and gas industry to inspect pipelines, pressure vessels, and storage tanks for surface cracks, corrosion, and other defects that could lead to leaks or failures. It’s also used to check drilling tools, rigs, and other equipment for wear and fatigue cracks to ensure safe and efficient operation. In power generation, MT is used to inspect turbine blades, rotors, and other components in power plants for surface defects that could affect performance and safety. MT is used in the nuclear industry for regular inspections of critical components in nuclear power plants to detect and address any surface discontinuities that could lead to failures.

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"A composite image showcasing various energy sources: solar panels in the foreground, oil pump jacks in the middle ground, and wind turbines and a power plant in the background. The scene illustrates the diversity of energy production methods at sunset.

Aerospace

MT is used to inspect critical aircraft components such as landing gear, engine parts, and structural elements for cracks, seams, and other defects that could compromise safety. MT is also used for regular inspections of aircraft during maintenance and overhaul processes to ensure the integrity of components and prevent in-service failures.

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A technician performing maintenance or inspection work on the landing gear of a large commercial airplane inside an aircraft hangar. The scene is illuminated with a blue tint, highlighting the aircraft's engines and the structural details of the hangar.

Transportation

MT is used in the automotive industry to inspect engine blocks, crankshafts, camshafts, and other critical engine components for manufacturing defects and fatigue cracks. It can also be used to check gears, shafts, and other drivetrain components for surface discontinuities that could lead to failure. In the rail industry, MT is used to inspect railroad tracks for surface cracks and other defects that could lead to derailments, as well as to inspect wheels, axles, and other components of trains to ensure they are free from surface defects and safe for operation.

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A modern high-speed train moving swiftly through a train station at sunset. The motion blur effect emphasizes the train's speed, with vibrant colors in the sky and station lights creating a dynamic and futuristic atmosphere.

Manufacturing

MT is used to inspect welds in various manufacturing processes to ensure they are free from surface cracks and other defects. It’s also used for quality control in routine inspections of raw materials, semi-finished, and finished products to ensure they meet quality standards.

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A modern manufacturing facility with robotic arms working on an automated assembly line. The scene is well-lit with blue overhead lighting, showcasing advanced machinery and precision engineering in a clean, industrial environment.

Infrastructure

MT is applied to inspect structural steel components used in buildings, bridges, and other infrastructure projects to ensure they are free from surface cracks and other defects. It’s also used to check heavy machinery and equipment for surface discontinuities that could affect performance and safety.

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A large infrastructure project featuring a highway under construction. Several cranes are positioned along the unfinished sections of the elevated roadway and bridge. The scene is set on a clear, sunny day with blue skies and some scattered clouds.

Example: UT in the Real World

MT is often used in tandem with the liquid penetrant testing (PT) method to ensure the safety and reliability of objects that are made of different materials. For example, an aerospace manufacturing company that produces landing gear for commercial aircraft will routinely conduct NDT during both the manufacturing process and as part of regular maintenance checks to ensure the safety and reliability of these critical components. MT could be used to detect any surface or near-surface defects in the ferromagnetic steel parts of the landing gear.

Through the use of MT coupled with PT, the aerospace company ensures that both ferromagnetic and nonferromagnetic components of the landing gear are free from defects, thereby guaranteeing the highest levels of safety and performance for their aircraft. This comprehensive inspection approach enables the identification and remediation of potential issues before they lead to failures, aligning with industry safety regulations and standards.

Two inspectors examine a large aircraft engine in a hangar, using a flashlight and notes for detailed inspection.

Deeper Learning About Magnetic Particle Testing

ASNT offers both members and nonmembers learning opportunities and resources for NDT specialists certifying in and using MT.

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ASNT Magnetic Particle Testing (MT) Courses

Advance your skills and knowledge with courses and webinars on magnetic particle testing.

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Programmed Instruction Series: Introduction to NDT

A comprehensive self-study resource for Level I and II candidates covering 16 NDT methods. Includes theory, principles, applications, quizzes, and an online interactive training program.

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Nondestructive Testing Handbook, Vol. 8: Magnetic Particle Testing (MT), 3rd ed.

A practical guide on MT for Level II and III inspectors, featuring industry-specific applications including metals, welding, chemical, petroleum, and electric power.

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