Magnetic Particle Testing, also known as Magnetic Particle Inspection (MPI), is a quick, simple, sensitive and inexpensive non-destructive testing method employed across various industries to detect surface and near-surface defects in ferromagnetic components and products. This highly effective technique relies on the principles of magnetic flux and magnetic flux leakage to identify discontinuities that may compromise the integrity and performance of critical components.
The Science Behind Magnetic Particle Testing
During Magnetic Particle Testing, the specimen is magnetised to produce a magnetic line of force in the material and if these lines of flux meet a discontinuity such as a crack, secondary magnetic poles are created at the face of the crack and at the surface which creates a flux leakage across the crack. This flux leakage field attracts finely divided ferromagnetic particles, either in dry powder or wet suspension form, which accumulate at the discontinuity, outlining their location, size, shape, and extent.
Methods of Magnetization
The equipment used for MT can be divided according to size and purpose. The magnetising force may be supplied by anything from a small permanent magnet to a highly sophisticated fixed installation, utilising high values of rectified current and finely calibrated meters. Primary methods are used to generate magnetic flux within the component under investigation. External magnetic fields, generated by permanent magnets, electromagnets, or current-carrying conductors, can be introduced into the component. Alternatively, an electric current can be passed directly through the component, inducing a magnetic flux perpendicular to the current flow. The choice of method depends on many factors such as the type of defects being targeted, the sensitivity required and the size, and shape of the component.
Dry vs. Wet Magnetic Particles
The magnetic particle test method may be classified as a wet or dry technique based upon the detecting media used. While magnetic inks consist of finely divided colored or fluorescent particles suspended in a suitable carrier fluid, forming a uniform suspension when agitated, in dry powder there is no carrier present. Powders are usually applicable to site work such as welds and castings, often as an initial check on a weld root pass, where wet materials would cause contamination. Generally, they can be used for testing hot components up to 300°C. In contrast, wet particles, suspended in substances like water or oil, offer increased sensitivity to flux leakage sites due to their mobility in suspension, covering larger surface areas and detecting more defects.
Applications Across Industries
Magnetic Particle Testing finds widespread application in industries such as aerospace, automotive, marine, power generation, oil and gas, engineering, and fabrication. It is utilized to assess the integrity of various components, including castings, forgings, and weldments, ensuring they meet stringent quality and safety standards.
TWI's Expertise in Magnetic Particle Testing
With over 40 years of experience, TWI has established itself as a leader in non-destructive testing (NDT) techniques, including Magnetic Particle Testing. TWI's services encompass the inspection of diverse components and structures, aiming to identify defects and ensure the fitness-for-service of critical assets.
Examples of TWI's work include the inspection of mechanical systems, investigation into the effects of warm prestressing on component integrity, and examining welded manifold valves. Through meticulous testing and analysis, TWI assists clients across industries in maintaining the reliability and safety of their assets.
Conclusion
Magnetic Particle Testing is a cornerstone of non-destructive testing, providing a reliable means of detecting defects in ferromagnetic components. With its unparalleled sensitivity and versatility, this method plays a vital role in ensuring the quality and integrity of critical assets across industries worldwide.
To learn more about TWI’s training and certification programmes on NDT, visit https://www.twi-global.com/locations/middle-east/training/non-destructive-testing or contact us at Enquiries@twime.com.