August 2009

Colour-contrast PT: How to understand ISO 3452-2 Standard?

Sensitivity of PT systems

I-Introduction

The 2006 revision of the ISO 3452-2 standard supersedes the 2000 version. Both of them are titled: ‘‘Non destructive testing- Penetrant Testing- Part 2: Testing of penetrant materials’’.

Sometimes a standard revision fails to clear ambiguous points.

It seems this is exactly what happens with these two versions.

II-ISO 3452-2 Ambiguity

In the 2000 version Chapter 7.2 ‘‘Penetrant system sensitivity’’, paragraph 7.2.2 ‘‘Colour contrast penetrants’’, indent 7.2.2.3 ‘‘Interpretation of results’’ we may see:
‘‘The number of unbroken lines covering more than 80% of the panel width, clearly visible to the unaided eye (with glasses if usually worn) shall be recorded ...’’

In the 2006 version Chapter 6.2 ‘‘Penetrant system sensitivity’’, paragraph 6.2.2 ‘‘Colour contrast penetrants (Type II)’’, indent 6.2.2.4 ‘‘Interpretation of results’’ it comes as:
‘‘The number of unbroken indications covering at least 80% of the panel width, clearly visible to the unaided eye (with glasses if usually worn)’’.

III-Reference test blocks used for sensitivity test of colour contrast PT materials

Colour contrast PT materials shall be tested on the so-called 50 µm and 30 µm reference test blocks, Type 1 as described in the ISO 3452-3:1998 ‘‘Non-destructive Testing - Penetrant testing- Part 3: Reference test blocks.’’

IV-flaw indications on Type 1 reference test blocks described in THE ISO 3452-3

The underneath pictures show 50 µm and 30 µm panels processed with a degreaser, a colour contrast penetrant washed with water, dried and then with a non-aqueous wet developer. Note that we think the developer was applied as too wet and needed too long a time to dry: that's why indications are a bit "slobbery", i.e. less contrasted and larger than they should.
Indications are displayed as continuous lines and sometimes as broken lines.

These pictures supplied by BABB CO give the result as seen on the test panels UNDER COLD WHITE LIGHT.

V-The ambiguous point

The ambiguity comes from this tiny difference between the two sentences written in our Chapter II.

This could lead many people, including auditors, to a wrong understanding of the standard's requirements.

Let us have a thorough look at that:
- ‘‘The number of unbroken lines covering more than 80% of the panel width’’ 2000 version.
- ‘‘The number of unbroken indications covering at least 80% of the panel width’’ 2006 version.

Given that the panel width is 35 ± 1 mm:
- As per the 2000 version only lines continuous on more than 28 mm should be recorded. That means one continuous segment of more than 28 mm.
- As per the 2006 version indications with a combined length of at least 28 mm shall be recorded.

As seen on the pictures some lines may comprise several legs. If we "translate" the two ISO versions:
- Say a 28 mm-long leg is seen. As per the 2000 version, this line is not considered as seen. 28 mm comes exactly as 80% of 35 mm; therefore 28 mm is not MORE than 80%.  As per the 2006 version the line is seen as 28 mm is AT LEAST 80% of 35mm.
- Say the indication is displayed as 3 legs, 14, 12 and 5 mm long respectively. As per the 2000 version, the line is not recorded, though the total length is 31 mm: no leg is more than 28 mm long.

As per the 2006 version some people may require that only indications showing at least one 28 mm-long leg be recorded.

Nevertheless one of us has been involved in the European standardisation for more than 25 years. He has been very active in the team who wrote the EN standard which is now the ISO 3452-2. He knows very well what was in the minds of the team's members, and he recently got confirmation from some of them.

The idea was that any indication displaying one or several legs whose combined length is at least 80% of the panel width shall be recorded. All the members know that the flaws' width is MAXIMUM 1/20 of the depth. This leads to some areas where the flaw is so tight that a colour contrast penetrant, though as efficient as any fluorescent one to enter the crack and to go out, may be almost undetectable by the naked eye. So a Level 3 or 4 fluorescent penetrant may quite easily show up the crack when a colour-contrast one may display a line with several legs.

We suggest you have a look to a paper issued by Patrick DUBOSC (*) giving the calculus of the quantity of dye detected by the eyes + brain (‘‘obsolete system’’) of inspectors while performing a PT. Unfortunately, this paper is only in French. It may help specifications writers, users, auditors better understand the limits of the process.
Another point under discussion is the inspection: ‘‘to the unaided eye (with glasses if usually worn).’’

This is a boring and very tiring work. The standard does not allow for using any photograph for the sensitivity test when it allows it in paragraph B.4.2 ‘‘System performance’’ in Annex B ‘‘Process control tests".

Digital pictures would allow now for high quality pictures which would ease the sensitivity test ... and probably make it more reliable!

True, digital pictures may be "adjusted"; but if sensitivity tests are carried out by an independent laboratory, this should not happen!

VI-The right reading of ISO 3452-2

The right reading is to record as ‘‘seen’’ any indication, be it in one leg or in several legs, as soon as the combined lengths lead to assess: ‘‘at least 80% of the panel width are visible’’.

VII-An idea for the next revision

We suggest the following wording to be written in the next revision:
‘‘The number of any indication comprising one or several legs whose combined length is at least 80% of the panel width.’’

Further, as a general rule members of the TC 138 Working Group dealing with a standard should read the translations of standards so as to be sure that the three official versions (English, German and French) say the same thing before the formal vote. These experts, who worked in the meetings, sometimes for many years, before issuing a draft, are prone to know exactly why this word, this sentence have been used, for instance to prevent any discrepancy in translation or misunderstanding.

(*) Patrick DUBOSC ‘‘Ressuage : Lui en demande-t-on trop ?’’(editor’s note ‘‘Penetrant Testing: is more too much?’’) - CONTRÔLES-ESSAIS-MESURES Journal- January 2007, pages 83 to 85.

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.