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March 2016 : Overhead magnetic particle inspection

Written by Laurence
Monday, 04 April 2016 22:57

Some of our readers asked us how to perform an overhead MT inspection: this is, indeed, a rarely mentioned topic.

Our point of view comes as follows:

The magnetic ink may be applied using spray cans propelled by liquefied petroleum gas (LPG), better than those propelled by carbon dioxide (CO2). CO2 propels the magnetic ink with a higher pressure than LPG. In alternative, the magnetic ink may be applied using a squeeze bottle.

In any event, if possible, provide an air extraction from the bottom, a fresh air intake from above or from a side.

The magnetic ink shall be applied during the magnetization and the examination of the surface under inspection shall be carried out during the magnetization. This requires maintaining the magnetization for few seconds, the time during which most of the droplets will drain. The speed of inspection is necessarily slower.

The magnetizing equipment to be used is a portable electromagnet. We must consider the fact that often the contact of the poles of the portable electromagnet with the work piece surface will not be perfect: therefore, put the legs of the portable electromagnet quite close to each other. Perform several magnetizations in the same area, if possible with small changes of the magnetization direction, looking at the surface to control, of course, after each magnetization.

This kind of overhead inspection was used by one of us when inspecting the internal structural parts in aircraft wings, around 2006, in maintenance. Therefore, this is not "Mission: impossible!"

But, why not consider an alternative technique?

We know that magnetic particle testing, like any other Non Destructive Testing (NDT) method, has its own limitations. In this case, others techniques of NDT methods may be considered; such as:

  • The "Alternating Current Field Measurement", also named ACFM, the limitations of which are known: the discontinuities shall have a length greater than 10 mm (circa ½’’) and a depth greater than 1 mm (circa 3/64’’). However, the signals are difficult to interpret in the case of complex geometry (edge effect, branched cracks, etc.);
  • Eddy current testing (ET) using array probes;
  • Infrared testing (TT).

These are NDT techniques and methods that obviously require specific skills and appropriate equipment. The conditions of access to the surfaces under inspection shall also be taken into account.

For example, when inspecting these aircraft wing structural parts, the discontinuities that were found were no longer than 3 to 4 mm (circa 1/8’’ to 5/32’’). for the longest ones, and barely 200 µm deep for the deepest ones. A light surface dressing* could be performed in the area of the discontinuities, provided that the remaining metal thickness was greater than a certain value. For some areas, a very small cutter has been used to remove the minimum amount of metal. A further MPI test was required to check whether the indication had been removed. On two of the indications, 3 and 4 actions respectively were necessary before complete elimination. Inspecting two wings took about an hour and a half one, including the setting up of the scaffolding.

*In French, this is often called “blanchir le métal”, i.e. removing a layer so thin that the metal becomes bright, as the thin oxide layer is removed.

Last Updated ( Tuesday, 05 April 2016 23:02 )