June 2009

Dear Reader,

As an information which may be useful we are happy to let you know that:

- Since April 2008 the site has been visited by 2,246 IP addresses as of May 10, 2009.

- Since the 1st of January 2009 we received a total of 10,446 visits as of May 10, 2009: an average figure of 2,089 per month.

- The peak figure till now is 144 visits on the same day (April 22, 2009).

- In April/May 2009 the visitors are mainly from: Algeria, Australia, Bahrain, Barbados, Brazil, Cameroon, Canada, Chile, France, Gabon, Germany, Greece, Hong Kong, India, Indonesia, Italy, Ivory Coast, Japan, Kuwait, Libya, Luxembourg, Madagascar, Malaysia, Mexico, Netherlands, New Caledonia, Norway, Oman, Pakistan, Philippines, Portugal, Qatar, Romania, Russia, Saudi Arabia, Singapore, Spain, Sweden, Switzerland, Thailand, Tunisia, United Arab Emirates, United Kingdom, USA, etc.

Once again, we thank every reader and welcome any question, suggestion, technical paper or "Oldies but Goodies" mini-story proposal.

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CEN Meeting in Hannover Proceedings

A CEN TC 138 WG 4 (Technical Committee 138, Working Group 4, in charge of PT) meeting was held in Hannover end of April. Experts from Germany, UK, Finland, Italy and France made a thorough review of several documents and exchanged views about the future of UV-A and blue light to excite fluorescence of fluorescent penetrants, with a thinking to the WG 5 (dealing with MT) Experts which are on the same topic.

An ISO TC 135 (Technical Group 135, in charge of all the NDT methods) meeting is planned in Moscow during the 10th European Conference on NDT.

The first main talking point was about the ISO 3452-1, to be adopted from the EN 571-1 (PT. General Principles) through the "fast-track" of the Vienna accord. Some adjustments are needed to the EN 571-1. Several proposals by the French delegation were accepted while some were rejected.

Agreed: a proposal about "Personnel Qualification"; replace current Table 1 by the Table 1 of the ISO 3452-2 standard (Testing Products); some points about illuminance and irradiance (viewing conditions).

Not agreed: a proposal about penetrant removal with a solvent; a proposal about a first examination just after developer application.

A very clear technical paper was given by Members of the French delegation about blue light use, instead of UV-A radiation, when using fluorescent products. Experts' attention was drawn on the right wavelength (close to 450 nm) used with the right filter’s absorption curve. A German Expert concurred on many points, though tests in different labs were done with different parameters. Using the right conditions it was stated by the French Experts that fluorescence brightness from UV-A radiation or from blue light was very similar. This contradicts some American documents; but as we are at the beginning of understanding which are the right parameters when using blue light it seems more tests with well documented parameters should be carried out to confirm that blue light and UV-A radiation lead to a similar fluorescent brightness i.e. a similar sensitivity.

Measuring blue light is an other very important thing to think about. Meters' response curve will have to be well defined. The blue light source itself may give different results whether it is an LED (the likely future) or any other kind of source. Further the transmission curve of the goggles the inspectors shall wear will also be of the utmost importance.

If fluorescent brightness is very similar penetrant manufacturers will not have to redesign their penetrants. Otherwise fluorescent penetrants for use under UV-A radiation and fluorescent penetrants for use under blue light would come as parallel, and should not be mixed. Better to demonstrate that the same formula will give similar results in both lighting conditions!

An other discussion occurred about colour-contrast penetrants and light sources. Depending on the colour temperature of the source it has been shown that the seeability (the way they are seen) of red dyes may be dramatically enhanced or lowered: this has a tremendous importance on the contrast, due also to the fact that "white developers" may be not so white under low colour temperatures.

Next CEN 138 WG 4 meeting will be held on Oct 12 and 13 in the AFNOR building in Saint-Denis, close to Paris. All the Experts are invited.

Note also that the TC 138 General Meeting is planned on Sept 17/18 in Turku, Finland.

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MT: Do you know the reference test block N°2 of the ISO 9934-2 Standard?

We are pleased to publish on our Website a contribution by a French MT expert, Stéphane GRAVELEAU, from the R and D department of SREM TECHNOLOGIES (FRANCE).

Though it is supposed to be an international reference test part the Type 2 Reference Test Block as described in the ISO 9934-2 Standard is still hardly known and barely used. This block gives rise to a magnetic gradient perfectly suitable to check MT particles performance.

It seems that, till now, this topic has not been properly explained. While publishing it we think we fill in a gap; furthermore we hope it will be of some interest for you and it brings you a top - quality information.

Patrick DUBOSC et Pierre CHEMIN

French users know this part as the "Témoin C" displayed in the obsolete NF A 09-570 French Standard. We will call it "Type 2" in this paper.

The Type 2 is designed so as to get a discontinuity giving a well-characterised magnetic "leak" shifting along the discontinuity length. It may be thought of as a generator of a magnetic field gradient. For many years this reliable and efficient test part has been used to check MT powders and/or liquid performance. The length of the indications is an easy tool to measure performance. Indications are stronger near both ends and fainter when going to the middle of the part. A longer indication is directly related to a better performance.

The then "Témoin C" (*) has been designed by a well known MT French expert, Michel TOITOT in 1988.

In the early '90s, working in close cooperation with Michel TOITOT, SREM TECHNOLOGIES improved manufacturing while increasing its reliability. It then was introduced in the EN standards about MT. These standards got the ISO status later.

Manufacturing was streamlined; theorical calculus were compared to tests measures; a calibration procedure was written. And better computing tools made it easier to improve characterisation of every unit. In 2002 the "Témoin C" became the "Type 2 Reference Test Block" in the ISO 9934-2.

The "Type 2" comprises two ground mild steel blocks held together with a gap of 15 µm. On each side of these blocks two magnets are put head-to-tail so that each block has a North pole at one end and a South pole at the other one. By design the magnetic field in the gap is null in the middle and comes with an hyperbolic sine from one end to the other.

The magnetic particles of an MT medium respond mainly to two parameters:

- The field spatial layout above a defect;

- The magnetic field action on ferromagnetic particles.

 

The layout of a magnetic field leak from a defect was displayed in a paper by N.N. ZATSEPIN in 1966: "Calculation of the magnetostatic field of surface defects". Results from tests almost duplicate results got from the underneath formula:

Doing so, one may then draw the spatial distribution of the field above a defect.

The following pattern comes for instance for a 0.2 mm (7.8 mil) height above the part's surface at the -4 (minus 4) mark.

 

When plotting this distribution at several marks on the Type 2, all for the given 0.2 mm (7.8 mil) height, one may get the entire pattern of the magnetic field on the surface of the part.

Concerning the action of the field, the force applied to the ferromagnetic particle is proportional to the gradient of H². When looking at the spatial distribution as previously seen on the -4 mark of the Type 2 at a 0.2 mm (7.8 mil ) we go to the underneath forces distribution:

As per the above explanations it is obvious that an MT medium ferromagnetic particles are attracted by the discontinuity on a local level and that the force changes from one end to the other depending on the field spatial distribution above the discontinuity. As the field gradients are very high at the ends, that's why more particles are attracted there.

(*) Patrick DUBOSC and Pierre CHEMIN ‘‘A survey of Standard and Codes for Magnetic testing’’.

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We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.

We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.