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What is a Magnetic Particle Inspection?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 30 November 2016
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Magnetic particle inspection is a nondestructive testing method which can be used in the evaluation of all ferrous materials. Like other forms of nondestructive testing, this method has the advantage of not damaging or compromising the materials being tested during the testing process. This method is one of the fastest and least expensive ways to test ferrous materials before certifying them as safe and ready for use.

In a magnetic particle inspection, the object being tested is surrounded by a magnetic field. Particles suspended in a carrier fluid are sprayed onto the object. If there is a defect in the object, it will cause a corresponding distortion in the magnetic field which will be readily visible as particles are pulled towards the distortion. If there are no defects, the particles should remain evenly distributed.

Ferrous metals at a variety of stages of production can be subjected to magnetic particle inspection to check for fatigue cracks, pitting, and other problems. This testing can be used to evaluate finished parts and products before they are released to the market in addition to being used in the inspection of slabs of raw materials to check for flaws which could compromise future performance.

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Ferrous metal parts in service can also be inspected with the use of this technique. Equipment, such as aircraft and manufacturing parts, needs to be routinely inspected to check for signs of fatigue, strain, and other issues, to confirm that it is safe for use. Magnetic particle inspection is one technique which can be used during an inspection. If a part or machine fails the inspection test, it indicates that additional work is needed to render it safe.

Dyes can be added to the particles to make them easier to see, including dyes which will react to ultraviolet light. With ultraviolet-sensitive particles, testers can conduct a magnetic particle inspection under UV light and clearly visualize the distribution of the particles. The highlighted particles will also photograph well, allowing people to create easily-read documentation of inspection results.

There are other inspection techniques available to evaluate ferrous metals. Some of these techniques may be used in addition to magnetic particle inspection for a complete inspection, while others may be used instead in some circumstances. All of these techniques require extensive training in nondestructive testing to confirm that they are conducted properly. If a test is done wrong, a false negative result may be obtained, leading people to believe that something is safe when it is not.

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Mammmood
Post 5

@SkyWhisperer - It’s too bad that the method only works on metal however. It would be neat if a similar testing method could be crafted which could determine cracks in structural concrete foundations.

Just surround the house with a field and see where the disruptions occur. I think that structural engineers mainly use site and physical inspection to determine if foundation problems exist, but it would be cool if they could use a magnetic field. I think the results would be more precise too.

SkyWhisperer
Post 4

@everetra - Yeah, there is a lot riding on making sure that the components are structurally sound.

I can think of things like aircraft, for example. They may be made of thousands of bolts. One hairline crack in a bolt somewhere, and the next thing you know the bolt may come flying off and your airplane wing may start coming apart, bolt by bolt.

everetra
Post 3

@MrMoody - I agree. The magnetic particle inspection method is ideal for these kinds of components.

It’s kind of an indirect method, from what I can tell, kind of like determining the volume of a cube by sinking it in water and measuring the displacement of the water. It’s clever, yet precise.

MrMoody
Post 2

Magnetic particle testing is an ingenious method, in my opinion.

Basically what you’re doing is creating a magnetic field that acts like a circuit. If there is discontinuity or a defect in the material you’re testing, then the circuit is more or less broken.

I believe that it’s a brilliant approach, far better than a visual inspection or any other method for that matter. When you’re dealing with some metal components I think you have very precise tolerance levels that you have to deal with, and it would be difficult for a typical instrument to detect variances in the seams at these low levels, in my opinion.

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