Magnetic Particle Testing: Reliable Defect Detection Method

Magnetic Particle Testing (MPT), also referred to as Magnetic Particle Inspection (MPI), is a fast, straightforward, sensitive, and cost-effective non-destructive testing technique. It is widely used in various industries to identify surface and near-surface flaws in ferromagnetic materials. This technique operates on the principles of magnetic flux and flux leakage to reveal discontinuities that could affect the safety and functionality of key components.

• How Magnetic Particle Testing Works

In Magnetic Particle Testing, the material under inspection is magnetized to create magnetic lines of force. When these lines encounter a discontinuity, such as a crack, they form secondary magnetic poles, leading to a leakage of magnetic flux. This leakage field attracts ferromagnetic particles, which can be applied in dry powder or wet suspension form. These particles gather at the discontinuity, making it visible and outlining the defect’s size, shape, and location.

• Magnetization Techniques

MPT equipment varies in complexity and size, depending on the specific requirements. Magnetizing force can come from simple permanent magnets to advanced setups utilizing high rectified current and precise instruments. Primary magnetization methods involve inducing magnetic flux within the component, either through external magnetic fields generated by permanent magnets or electromagnets, or by passing an electrical current through the component to produce a perpendicular magnetic flux. The choice of technique depends on factors like the type of defects, sensitivity needed, and the component’s size and shape.

• Dry vs. Wet Magnetic Particles

The detection media used in MPT can be classified into dry and wet techniques. Wet methods involve magnetic inks, where fine colored or fluorescent particles are suspended in a fluid, allowing them to form an even distribution when agitated. Dry powders, used without a carrier fluid, are more suited for certain site applications, like testing welds or castings, and can withstand higher temperatures (up to 300°C). Wet particles, on the other hand, offer greater sensitivity due to their mobility, making them more effective at identifying flux leakage over larger areas.

• Applications Across Industries

MPT is widely used in sectors such as aerospace, automotive, marine, power generation, oil and gas, and manufacturing. It is applied to inspect critical components like castings, forgings, and welds, ensuring they comply with rigorous quality and safety standards.

• TWI’s Expertise in Magnetic Particle Testing

With more than 40 years of experience, TWI is recognized as a leader in non-destructive testing methods, including MPT. TWI’s expertise covers inspecting various components to identify defects and ensure they are fit for service. Notable projects include inspecting mechanical systems, studying the effects of warm prestressing on component integrity, and examining welded manifold valves, all contributing to maintaining the reliability and safety of client assets.

Conclusion

Magnetic Particle Testing remains a crucial non-destructive testing technique, offering a dependable way to detect flaws in ferromagnetic components. Its high sensitivity and adaptability make it essential for ensuring the quality and integrity of critical assets across many industries worldwide.