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Henry Petroski famously said, “To engineer is human.” Mankind emphasizes ambitious engineering goals, often driven by aesthetics and convenience. In his book, he refers to engineering as a ‘puzzle’ with a ‘solution.’ Where one must analyze the study said solutions for potential ways they can fail. 

Non-destructive testing embodies this logic, wherein any new or pre-existing technology, material, or structure is thoroughly analyzed using scientific methodology to test for probable causes of failure.

The manufacturing processes of various brittle and rigid materials, metal or not, create defects and irregularities by themselves. Magnetic Particle Testing can be used to detect production defects and in-service damage.

Forging, casting, machining (lathe, milling cutter, indexing, drilling, etc.), and heat treatment processes leave residual stresses, cracks, embrittlement, hot tears, and many such irregularities that, with time and wear, create larger deformities, potentially causing structural failure and damage that can be detrimental to its operation and/or cause harm to the people and environment around it.

The ever-increasing technological demands and advancements make the criteria for material quality and the margin for errors and defects smaller. Magnetic Particle Inspection or Magnetic Particle Testing (MPI) is generally conducted on ferromagnetic materials which exhibit hysteresis. Such materials indicate magnetism and magnetic properties down to their atomic level, even without external magnetizing factors. Ferromagnetism is a rare property, often occurring in transition metals, their alloys, and rare-earth metal alloys.

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Modern-day Magnetic particle testing machines (Image credit: IndiaMart)

 History Of Magnetic Particle Testing

The concept of magnetism has been used even in Ancient Greece. A mineral called lodestone (naturally magnetized), made of magnetite, was used to attract iron. Primitive versions of Magnetic Particle Testing were used in the 1800s, where the barrels of cannons used in war were magnetized, and compasses ran through them to check for defects.

In the 1900s, metal shavings were used on magnetized materials to represent defects' locations visually. The usage of metal shavings graduated to using ferromagnetic power, which built up on defects, hence the location of the flaw sites.

Magnetic Particle Testing quickly caught on in various industries due to the sheer nature of the materials used and the resources available, and it is still used today.

Magnetic Particle Inspection Apparatus from the 1920s (Image credit: www.nde-ed.org)

How Magnetic Particle Testing Works

Magnetizing the test surface appropriately is of utmost priority for magnetic particle testing. Under magnetization will result in unreliable readings, wherein many flaws may go undetected. Over-magnetization may result in a build-up of ferromagnetic powder only on surface defects and cause furring. Furring will cause coverage of significant defects on a sub-surface level and instead cause irrelevant indications.

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Furring during an MPI (Image credits: BMFB 4283 NDT Failure & Analysis)

Clean, smooth surfaces must be used to perform Magnetic Particle Testing. Textured surfaces will have multiple points of flux leakage and will provide inaccurate results. Adequate magnetization must be verified, and errors must be rectified before Magnetic Particle Testing. This can be done using the following methods:

·        PIE Gauge: This is a flux-sharing magnetic field indicator that can be used to check the magnetic field strength.

·        Quantitative Quality Indicator Shims: These thin metal flux-sharing devices with patterned etches provide flux data in multiple directions. Care should be taken to install it correctly on test surfaces.

·        Hall Effects Gauss Meter: This Gauss meter detects the flux data by taking a record of the electrons emitted transversely from a conductor placed in a magnetic field.

·        Ketos Ring: A Ketos ring is a performance testing tool that replicates the presence of deformities under the surface. Used as a demonstration tool, it has evolved to become a commonly used magnetic particle testing accessory.

Magnetic Particle Inspection Equipment (Image credit: Anup Kumar De, whatispiping.com) 

Magnetic field lines produced by magnetizing a material have characteristic properties, making them ideal for detecting defects. Those proclivities are:

·        Magnetic field lines form closed loops.

·        They travel from north to south on the outside of a magnet, whereas they move from south to north inside the magnet. This property causes like poles to repel and unlike poles to attract.

·        Magnetic field lines do not intersect each other.

·        Magnetic field lines lean towards the paths of less resistance.

·        Every field line emitted from a magnet has uniform strength.

Specific apparatus or magnets are used to magnetize the subject to be tested. The magnetic field lines hence produced will be disrupted when they encounter defects.

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Magnetic particle testing and magnetization can be carried out by portable and static components, depending on material size and purpose. The methods used to magnetize any material may include the following:

·        Using the test subject as a bridge of a permanent magnet or electromagnetic by making it a part of a magnetic circuit.

·        By passing a current through the specimen, using Prods. These are electrodes connected to the material by hand. The operator must be mindful of their safety from the electric current and ensure sufficient physical contact between the electrode and material to prevent the formation of an electric arc. Electric arcs can change the physical state of the material and cause damage to the test specimen.

·        For looped specimens, the concept of transformers can be used by inducing a current and placing the specimen between magnets.

·         By electromagnetically inducing magnetization by looping a current-carrying conductor around the specimen, generating magnetic flux.        

Here, a leakage field will be created. Finely powdered iron particles are hence applied on this surface, and they are attracted to zones with leakage fields, causing a visible build-up. This aids the operator in detecting flaws and discontinuities.

In the case of large-scale testing units, stationary wet units can be found to magnetize the materials. They provide direct and indirect magnetization, and a slurry is applied to the test material. This slurry can be fluorescent to enable a better visual representation of defects.

In another method called ‘Headshot’, the test material is clamped and coated in a magnetic bath. On inducing a current in the test material, the magnetic particles are attracted to flux leakage at the points of defects and indicate the deformities.

Liquid carriers used to form the slurry in wet magnetic inspection can either be distillates of petroleum or

Post magnetization and particle coating of the test subject, the visibility of particle deposits of flux leakage needs to be considered. In the industry, the particles commonly used in the form of magnetic powder or slurry are fluorescent or visible to the naked eye. Visible particulate matter does not need dedicated lighting to highlight points of deformity. However, fluorescent magnetic material requires the presence of ultraviolet lighting to provide a contrast and give a clearer visual representation of defects.

The ambient white light of sufficient luminosity should be used even for visible magnetic particles to ensure thorough fault inspection of the material.

Fluorescent indication of a crack in the radius of a component (Image credit: Digital Research & Development)

Post magnetic particle testing, any residual magnetic fields are required to be demagnetized to prevent interference during the operation and usage of the materials.

 Types Of  Magnetic Particles  Used  For  Defect Indication

Magnetic particle inspection uses powders or liquids on the surface of the test materials, which respond to the material's magnetization and collect points of flux leakage. These materials need to possess low retentivity.

The particles are often colored in various light dyes (red, black, grey, or yellow). They are found in sizes ranging between 50mm and 150mm and are not merely fine powdered to fill discontinuities thoroughly. The course 150 mm grain particles are not subject to environmental factors and can accurately hold onto the discontinuities. The length-diameter ratio of particles is around 1:2 and is of oblong or spherical form.

Wet slurry mixes of magnetic particles in a liquid suspension usually have oil or water carriers, which make the fluid more mobile and accurate. The particles are either made of ferrous oxide, if visible or coated with fluorescent pigments to ensure visibility. The oil-based liquids pose fire risks, and safety measures must be taken beforehand. The magnetic particles in these mixes are smaller (approximately 10mm) and retain slight magnetism post-testing.

Water carriers may potentially cause corrosion of test surfaces. Hence thorough cleaning should be performed post-testing.

Dual-use particles are also available in the market, which contains pigments visible under normal white light and glows under Ultraviolet light, making them versatile and convenient for testing.

Advantages Of  Magnetic Particle Testing

Like every other effective non-destructive testing process, Magnetic Particle Testing has its merits, which are:

•Magnetic particle testing has withstood the test of time and advancement in technology because of its simplicity and accessibility. 

•It also provides a visible representation of defects often absent in various testing techniques.

•Stress concentrations are caused by the presence of sub-surface and surface defects, and magnetic particle testing helps to find these flaws and gauge if the component or machine is safe to be loaded and in operation.

•This testing method provides the option of mobility as well as large-scale testing for mega-production units, making it available for use across various industries and purposes.

•Magnetic particle testing also does not limit the test size and can be used in various applications and for tools and components of various sizes.

•Apart from thorough cleansing for dirt and debris, Magnetic Particle Testing does not require prior preparation for test surfaces. It also allows for the detection of minuscule deformities and larger deformities in the same test process with accuracy.

Disadvantage Of  Magnetic  Particle Testing

Despite being a vastly utilized testing methodology, Magnetic Particle Testing has demerits, such as:

•The magnetic nature of the entire test process does pose certain disadvantages, as this testing method cannot be used for nonmagnetic materials.

•A supply of electricity is necessary for almost all Magnetic Particle Testing methods that cause electricity-related hazards. The magnetic field obtained, in turn, is hard to gauge for strength and needs experienced operators to ensure an appropriate amount of magnetization.

•Non-relevant indications, due to the presence of surface texture, insufficient or over-magnetization can produce misleading results. The porosity of the surfaces needs to be inspected prior to testing.

•The magnetic powders or solutions used can be hazardous, and ventilation, chemical safety, and fire prevention measures must be considered. The inspectors need to be directly near the testing process and hence will be subjecting themselves to multiple safety hazards that need to be kept in check.

•Thick paints and coating pose another obstruction in the magnetic particle testing process and must be avoided.

Principle Used In Magnetic Particle Testing

Magnetic Particle Testing works on the basic principles of magnetization and leakage of magnetic flux due to the formation of defects on material surfaces, along with basic non-destructive techniques of visual detection of flaws.

The per unit magnetic field that metal can hold versus air is much greater. Hence due to the presence of cracks, the magnetic field spreads out into the atmosphere and appears to leak due to the lower air density. These crack regions act as individual magnetic poles and attract the magnetic particles imposed like individual magnetic regions. Hence causing a build-up of particles in defect regions.

Magnetic flux leakage (Image credits: Instructor notes, Dr. Ala Hijazi)

Industry standards are set for the Magnetic Particle Testing process to ensure conformity to principles, safety, and process consistency. Those standards are as follows:

·        ISO 3059- Viewing conditions.

·        ISO 9934- part I) Principles, part II) Detection media, part III) Equipment

·        ISO 18093-5, ISO 17638, ISO 23278 [ For various test subject types]

·        ASTM E1444/E1444M

·        ASTM E709 [ Process guide]

·        ASTM E229 [UV Light regulations]

·        AMS 3041, AMS 3043, AMD, 3042, AMD 3044 [Regulation for particle types]

Applications  Of Magnetic Particle Testing

The convenience provided by Magnetic Particle Testing, especially in detecting surface and sub-surface defects in ferromagnetic materials, makes it applicable to various industries for inspecting a variety of production machinery, process equipment, tools, casting, forging, and die casting materials.

This testing process can be carried out in environment-controlled laboratories, with a stationary inspection unit within a manufacturing process, or can even be carried out with a portable instrument on a functioning subject. It can ensure conformity to a standard, safety of usage, adherence of produced components to intended design, and fitness of a structure for continued use.

Magnetic Particle Testing has been used extensively in aerospace applications. It has even been used by NASA for Saturn I, IB, V, Apollo, Skylab, Space Shuttle Solid Rocket Booster (SRB), Space Shuttle Main Engine (SSME), and other Marshal Space Flight Centre projects.

Magnetic Particle Testing is also used in the automotive section in the inspection of engine parts, gears, crankshafts, suspensions, etc.

It is also used in manufacturing to inspect casting, forgings, and weld quality.

The petrochemical industry, power industry, and construction sector also use Magnetic Particle Testing.

This process also has applications in the Defense sector, having an entire industry code set for its applications in Military usage. The codes are as follows:

• Military Standard Specification - 1949A Magnetic Particle Inspection

• Military Standard Specification - 1907 Military Standard Inspection Liquid Penetrant and Magnetic Particle Inspection of materials, parts, and weldments

• Military Inspection Standard 83387- Magnetic Rubber Inspection Process.

This testing process has withstood time and advancements in technology. It is a good investment for any industry that requires monitoring of the status of the health of its machinery and/or the quality of its products.

Conclusion

Surface and sub-surface defects come hand in hand with using metallic materials in any shape or form. Production of the material creates defects and deformities often invisible to the naked eye. 

These minute defects can also arise on the application of load or when in operation. These defects enlarge or even travel into the material if disregarded on constant load application, vibration, or heating generated during operation.

Caution is necessary when using such tools, machinery, and structures as these invisible defects can cause spontaneous structural failure in the subject causing significant destruction.

 Magnetic Particle Inspection is one such process that saves millions of dollars that could have been potentially lost in revenue, damage, and faulty products and structures.

Industries and manufacturers should invest in these testing units and apparatus to ensure long-term safety and steer clear of losses.