MT/PT units: follow the rules, stop the mess!

August 2010

Stéphane GRAVELEAU from SREM TECHNOLOGIES sent us a letter by the end of June 2010 :

‘‘I took a careful look at your paper on the measurement units and I have some comments to make, in particular dealing with the magnetic units. I feel that this section is at best incomplete and even could lead to confusion.

Indeed, in MT, the right parameter for a reliable inspection is, in fact, the magnetic flux density in the material (as defined in the ISO 9934-1:2001 standard in § 8.1: ‘‘the minimal magnetic flux density in the component surface shall be 1T’’) which should be close to the saturation level (saturation is around 1.2 to 1.5 T for current steels). Since this parameter is very difficult to measure, it is easier to measure the magnetic field which is tangential relative to the surface of the part; this field is identical on both sides of the surface (internal Ht = external Ht) and gives us an idea of this magnetic flux density ... but, doing so, we forget a characteristic of the material: the magnetic permeability (B = µ.H). We may guess that the figure of the tangential magnetic field should be different on different parts according to their respective materials. For practical purposes, this is impossible, and we are then not surprised to find that different ranges of tangential magnetic field are given depending on the industries (Automotive/Railways/Aerospace); this could be understood as due to differences in materials.

As for the residual magnetization, it is in fact a magnetic flux density and not a magnetic field, even if by misuse of language and due to history, it is often measured in A/m. This parameter may be used as measurement is carried out without any external magnetic field, and Bn ( component of the magnetic flux density normal to the surface) is conservative (internal Bn = external Bn).

A Hall effect probe allows measuring of a magnetic field or a magnetic flux density, it depends on the way we use it.’’

Our answer:

"We agree that the magnetic fields just under the surface of the part and just above, in air, are similar.   But, as you wrote, and as we say in our training courses, the magnetic permeability is no longer a “topic”; that’s why we may have tangential magnetic field figures “in the right range for magnetization” on wood parts, as we will explain in a future paper.

Question: on the other hand, the Hall effect sensor is not right on the surface of the part, but generally some few millimetres apart. Then, at such a distance, is the magnetic field in the air really similar to that just on the surface? We call this into question.

The Stéphane GRAVELEAU’s answer:

"Obviously there is an error in measurement, which depends on the part geometry and on the magnetization conditions. For example, let us take the very simple case of a cylindrical part magnetized by direct current flow technique and on which we measure the magnetic field using an Analysse(*). In these conditions, the field value is underestimated and the error of measurement is about 3% for a diameter 100 mm part ... but about 25% for a diameter 10 mm part."

Note

(*) Pierre CHEMIN and Patrick DUBOSC, Magnetic Particle testing history, on our Website http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

Reference

• ISO 9934-1:2001 Non-destructive testing - Magnetic particle testing - Part 1: General principles, International Organization for Standardization, Geneva, Switzerland, 2001.