Fluorescent magnetic particle inspection

Fluorescent Magnetic Particle Inspection

Fluorescent Magnetic Particle Inspection (FMPI) is a non-destructive testing (NDT) technique that has become increasingly popular in various industries due to its ability to detect surface and subsurface defects with high accuracy.

This technique uses magnetic particles that are coated with a fluorescent dye to identify cracks, voids, and other surface irregularities in metallic materials.

The particles are applied to the surface of the material being tested, and a magnetic field is then applied, causing the particles to gather at areas of magnetic flux leakage.

These areas can then be visualized using UV light, allowing for easy identification and measurement of any defects present.

In this article, we will explore the principles of FMPI, its advantages and limitations, as well as its applications in various industries.

What is Fluorescent Magnetic particle inspection?

Fluorescent Magnetic Particle Inspection (FMPI) is a non-destructive testing (NDT) technique that is used to detect surface and subsurface defects in metallic materials.

What is Fluorescent Magnetic particle inspection

This technique involves the use of magnetic particles that are coated with a fluorescent dye.

These particles are applied to the surface of the material being tested, and a magnetic field is then applied, causing the particles to gather at areas of magnetic flux leakage.

The particles are then illuminated with ultraviolet (UV) light, causing them to emit a visible light that can be observed by the inspector.

The areas where the particles have accumulated can be easily identified, making it possible to detect and measure any surface irregularities or defects in the material being tested.

FMPI is a highly effective technique that is widely used in industries such as aerospace, automotive, and manufacturing.

It is commonly used to detect cracks, voids, and other surface irregularities in materials such as steel, iron, and other ferromagnetic alloys.

FMPI is a cost-effective and efficient way to identify defects and ensure the quality of materials before they are put into service.

Advantage and disadvantages of Fluorescent Magnetic particle inspection

Fluorescent Magnetic Particle Inspection (FMPI) is a highly effective non-destructive testing technique used to detect surface and subsurface defects in metallic materials.

Like any other testing technique, FMPI has its advantages and disadvantages.

Advantages of FMPI:

High sensitivity: FMPI can detect very small surface and subsurface defects in ferromagnetic materials, making it highly sensitive and accurate.

Reliability: FMPI is a reliable and repeatable testing technique that can provide consistent results.

Cost-effective: FMPI is a cost-effective testing technique that is less expensive than other methods, such as X-ray or ultrasonic testing.

Fast: FMPI is a fast testing technique that can be performed on large areas in a short amount of time.

Portable: FMPI equipment is portable, making it easy to perform testing in the field or on-site.

Disadvantages of FMPI

Limited to ferromagnetic materials: FMPI can only be used on ferromagnetic materials, such as steel and iron, which limits its applicability.

Surface preparation: FMPI requires careful surface preparation, including cleaning and demagnetizing the surface, which can be time-consuming and labor-intensive.

Environmental conditions: FMPI requires specific environmental conditions, such as low humidity and no UV light, which can be challenging to maintain in some environments.

Inspector expertise: FMPI requires a skilled inspector to perform the testing and interpret the results accurately.

Principle of Fluorescent magnetic particle testing

Magnetic particle testing, also known as fluxing, is a method for detecting cracks in or near (0.5 mm) the surface of ferromagnetic materials.

In the magnetization of a ferromagnetic material, the magnetic field lines, since they seek the least resistance, are guided in the magnetically conductive medium.

If the magnetic field lines hit a fault in a magnetically poorly conductive area, a change in flow is caused by the high magnetic resistance.

This creates a leakage flux on the surface, which causes an accumulation of ferromagnetic particles, whereby superficial defects become visible.

There are two basic methods for magnetic particle crack testing: It can be carried out in daylight or in the dark – using fluorescent test equipment.

Fluorescent magnetic particle inspection procedure

In magnetic particle testing, a ferromagnetic component is magnetized with the help of mostly artificially generated magnetic fields.

During magnetization, the component is simultaneously rinsed with a test medium. The magnetic powder, which is located in the test medium, accumulates at surface defects that are open to the surface.

Due to the color contrast between magnetic powder and component surface, the surface defects become visible and can be documented.

It is considered a highly reliable method in professional circles. In the case of large workpieces where complete magnetization is not possible, only the sub-area to be tested is magnetized.

How does Fluorescent magnetic particle testing work?

Magnetic particle testing (MT) – also known as MP testing – is a simple but sensitive method (surface crack testing) for detecting irregularities on the surface of ferromagnetic materials (e.g. iron, cobalt and nickel and their alloys) with a relative permeability μ>100. Near-surface irregularities can also be detected.

The depth of detectable errors depends on the type of field used:

  • Alternating field – verifiable error depth approx. 1 – 2 mm
  • DC field – verifiable error depth approx. 2 – 3 mm

Magnetic particle testing uses the formation of leakage fluxes. This effect is produced by magnetization of a ferromagnetic material over a flat material separation (e.g. crack).

Stray flows occur when the material separation has an extent perpendicular to the direction of the field lines.

The leakage fluxes attract the ferromagnetic particles distributed in the magnetic particle suspensions. These then accumulate over the material separations.

For this purpose, the surface of the test object is first cleaned and then magnetized.


Magnetization of materials

There are several methods for the magnetization of a material, such as: current flow in clamping device, coil magnetization, field flooding with inner conductor or touchdown electrons.

When magnetizing, direct current, alternating current or permanent magnets are often used.

For smaller materials, shock magnetization (discharge by condenser discharge) is often used so that the material does not heat up too much.

Which magnetization method is used depends on the material geometry, the size of the test specimen and the type of probable defects that can occur.

Demagnetization after the test

In magnetic particle testing, the test material remains magnetized as long as it does not come into contact with other materials or is demagnetized.

To ensure that no residual magnetization remains, the material is usually demagnetized after testing, as the residual magnetic effect can lead to problems during further processing or to undesirable effects during use.

These are, for example: the adhesion of metal chips in the factory, the deflection of the arc during welding or effects on electrical instruments in machines. To demagnetize, the material must pass through a variable magnetic field.

This is achieved using the so-called low-frequency magnetization or counter-pole method. In the low-frequency method, an alternating current with a low Hz number is applied, whereby the magnetic field changes slowly.

The opposite pole method is based on the fact that the test specimen is moved back and forth by a magnetic field.

In order to achieve a real final demagnetization, the material must be turned out of the Earth’s field direction, north-south, i.e., in the east-west direction.

Application examples of magnetic particle testing

Application of Fluorescent magnetic particle inspection are:

  1. weld seam inspections, 
  2. castings and forgings, 
  3. all magnetic materials, 
  4. single & series part tests and 
  5. rolled products.

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