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DPCNews 020 - The wipe-off technique

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Written by Administrator
Friday, 01 January 2010 16:21

January 2010

I- Introduction

For every Non Destructive Testing (NDT) Method identifying signals or indications due to a discontinuity is a recurrent problem.

The Level 1 operator who sees the signal or the indication shall report to a Level 2 who decides after acceptance criteria.

Sometimes the basic question is: "does the signal/indication come from a real discontinuity or is it a non-relevant indication"? (The word "artefact" is sometimes used in French, but not in English, as it has a different meaning).

In Penetrant Testing a non-relevant indication is often due to scratches or other not-to-worry about tiny surface anomalies.

As often when on the border-line, stating "yes" or "no" may have costly consequences.
Penetrant Testing (PT) method, a comprehensive NDT method, is the one method which allows for a very simple "recheck", called "wipe-off", when other methods such as Magnetic Testing (MT), ultrasonic inspection (UT), etc. would need a more complex recheck. This is especially true for MT in areas such as elbow joints, diameter sudden changes, thread roots, etc.

II- Penetrant Testing: Indications classification

Indications may be classified as follows:

1 - Acceptable indications.
2 - Out-of-acceptance-criteria indications.
3 - Dubious indications.

Nbr 1 and 2 indications are "relevant indications". Non-relevant indications are those that need not to be considered. Dubious ones...are dubious and need more care!

Another important classification refers to the indication pattern:

• Linear indications: those with a length/width ratio of 3 or more.
• Non linear, or rounded, indications: those with a ratio less than 3.

Linear indications shall always be thought of as potentially dangerous, and then more thoroughly inspected.

III- Definition

The "wipe-off" wording is absent from the ISO 12706:2009 standard, titled "Non-destructive testing- Vocabulary of terms used in penetrant testing".

As a matter of fact, in ASTM E270–78 "Standard definitions of terms relating to Liquid Penetrant Inspection", the "bleed-back" wording comes with the following explanation: "ability of a penetrant to bleed out of a discontinuity subsequent to removal of the indication without (repenetration) reapplication of penetrant''.

Note that ASTM E270-78 is superseded since 1991 by ASTM E1316-08 "Standard Terminology for Nondestructive Examinations".

Further keep in mind that the English "wipe-off technique" sometimes become the American "rebleed process".

A more complete definition could be as follows:

Bleed back, rebleed or Wipe-off technique: This step is intended to check and to interpret indications beyond acceptance criteria. Wipe the indication smoothly using a soft paintbrush or a swab lightly moistened with a volatile solvent such as acetone. Then spray a light coating of a solvent-based (non-aqueous wet) developer for linear indications (or no developer for non linear indications). If, within two minutes, the indication reappears, under ultraviolet radiation, the indication is confirmed.

IV- Principle

Even after developing time has elapsed, when removing the developer layer we are sure some penetrant is left INSIDE the discontinuity: capillary effect, Van der WALLS forces make it impossible to draw out all the penetrant with the developer.

Taking out all the penetrant will need at least one, often more than one, more application of developer.

It is assumed, and daily use confirms, that the different developers forms do not show the same sensitivity. By DECREASING order:

• Form d: Non-aqueous Type 1 (solvent-based).
• Form a: Dry powder.
• Form c: Water suspendible
• Form b: Water soluble.

Using a "Form d" developer helps to draw out more penetrant than with any other form. This is the basics of the "wipe-off" technique.

V- Scope

As per the previous paragraph, it is easy to understand that this technique is used after a dry developer was applied. And as dry developers are used exclusively in conjunction with fluorescent penetrants, one may conclude that the wipe-off technique deals only with fluorescent penetrant inspection (FPI): that's true! It may also occur when no developer at all was applied: not a recommended way of doing.

Wipe-off is only for inspection of small surfaces. It is especially useful when inspecting areas with a lot of background due to porosities (foundries, castings) in which some difficult-to-see indications may be due to discontinuities.

VI- APPLICABLE DOCUMENTS

Interestingly the ISO 3452-1:2008, previously EN 571-1:1997, does not specifically address this technique. The EN 571-1:1997 only states in its paragraph 8.9 it only specifies a cleaning and retesting if any doubt. But many aerospace primes and subcontractors require the wipe-off test when needed.

In our DPCNewsletetr N°008 titled "Report and comment on ASNT fall conference and quality testing show in Charleston, South Carolina, USA, November 10/14, 2008" issued on our Website in January 2009, we wrote: "The American specification AMS-SAE 2647 at paragraph 3.4.12.12 deals with the wipe-off technique. A new revision will be available in January 2009."
The American specification AMS 2647, Revision Number C named "Fluorescent Penetrant Inspection Aircraft and Engine Component Maintenance" was issued in July 2009.

VII- Procedure

Users shall follow the applicable documents supplied by the primes or written by the Company Level 3. The document written underneath is purely for help and IS NOT a procedure.

1- Wipe across the indication with a cotton-swab or a fine brush damped with acetone, isopropanol, methylethylketone or a similar volatile nonhalogenated solvent. Do not allow solvent to flow over the part surface. For this press excess solvent from brush prior to wipe-off.

Using a non-overturn microvolume dispenser (see pictures underneath) allows for getting very small amounts of solvent.

2- Allow surface to dry (solvent should evaporate rapidly).

3- Apply a light film of Form d
: non-aqueous Type 1 (solvent-based) developer packed in spray can. Where there is a quite important bleed back, it is a good idea to use Form a: dry developer prior to using Form d: non-aqueous Type 1 (solvent-based) wet developer packed in spray cans. In this case and if the cause of the indication cannot be decided, the dry developer powder shall be removed by a light air blow at low pressure (30 kPa approx) and the non-aqueous wet (solvent-based) developer is applied.

4- Inspect immediately
for indications' bleed back.

5- Allow 10 minutes
to develop and re-inspect.

6- Evaluate the indications
which reappeared according to the acceptance and rejection criteria of the relevant standard or specification.

7- If no indication reappears, examine under white light using a suitable magnifier lens to find out the cause of the original indication. If this cannot be achieved, carry out locally a "control test" as per paragraph 8.9 of EN 571-1:1997 Standard.

> Note 1: With linear indications or linear chain of dots, no bleedback (repeat indication) does not mean non-relevant indication; thorough care shall nevertheless be given to the original information.

> Note 2: There are diverse alternative procedures to this method, such as:
The different procedure according to the original linear/non linear indications:

• In the case of a linear indication, wipe across the indication (with a cotton-swab or a fine brush damped with the solvent).
• After solvent evaporation:
→ If the indication was seen as linear before wipe-off: apply a solvent-based developer (non-aqueous for type 1) packed in spray can.
→ If the indication was seen as non linear, inspection is carried out without any developer.

In both cases, should indications reappear within three minutes when inspected under ultraviolet (UV-A) radiation, these indications are viewed as being due to surface discontinuities which must be taken into account and interpreted according to acceptance criteria and their size before wipe-off. If indications do not show up they are not seen as indications of a discontinuity detectable by PT; nevertheless they are marked for a further visual assessment.

> Note 3: If interpretation is difficult or impossible, other NDT methods need to be used for investigation purpose.

> Note 4: Depending on the applicable document the inspector may be allowed to wipe off the indication once, twice or 3 times. It is of the UTMOST IMPORTANCE to check this point especially during an audit: wiping twice instead of once if once is specified is cause for a major non-compliance report. That's a VERY COMMON OCCURENCE.

VIII- Conclusion

As one may understand, this wipe-off technique is highly useful. But it relies upon three main human factors:

• Proceed with care, exactly follow the procedure, use clean items (brush, cotton-swabs, solvent, gloves, etc.).
• Check for re-emergence of the indication.
• Acceptance/Non-acceptance is your responsibility.

We are yet far away from any "expert system" replacing the old but invaluable system (sensor+signal processing device) known as: human eyes+human brain--once again!


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.

Last Updated ( Wednesday, 18 May 2011 16:45 )