July 2009

Tools to check MT materials performance

1-Introduction

Performance check of MT materials is carried out by the manufacturers, and by users when putting the product in service, and also as a regular check of in-use products: every shift, or every 4 hours, etc. The ISO 9934-2 requires ‘‘Type and Batch testing’’ to be carried out by the manufacturer or the supplier and ‘‘On-Site control testing’’ to be carried out by the user.

Items then used come in two types:
- Those needing an MT equipment (continuous magnetization technique) such as KETOS RING and the similar steel rings.
- Those which we could call as self-sufficient, such as Types 1 & 2 of the ISO 9934-2 standard or the magnetic stripe cards, which need no MT equipment.
All these items will be explained in this paper: advantages/disadvantages balance, fields of use.

2- Ketos Ring

The KETOS RING is ring-shaped, as could be guessed from its name, 127 mm (5’’) diameter, 22.22 mm (7/8’’) thick with a central hole 31.75 mm (1-1/4’’) diameter. It comprises 12 holes of 1.77 mm (0.07’’) diameter.

The SAE-AS5282 (Tool Steel Ring for Magnetic Particle Inspection) specification details manufacturing and requirements.

The KETOS RING is used to check overall performance of an MT installation (i.e. equipment + product). It may also be used to check the MT materials sensitivity. The American ASTM E 444 ‘‘Standard Practice for Magnetic Particle Testing’’ specification states the requirements both for dry powders and wet MT materials.

Its very first purpose was to check dry powder. Its ability to classify liquid products quality is still disputed.

Strange but true: there was a time when magnetic inks were used to check the KETOS RING which then was used to ... check magnetic inks! A very ‘‘righteous circle’’ which, today,
would make our beloved qualiticians’hair raises on their heads!

Now magnetic probes are used to measure the field atop holes of brand-new KETOS RING.
How to use the KETOS RING?

1. Slip the KETOS RING on a copper bar, between 25.4 and 31.75 mm (1’’ and 1¼’’) diameter, 40 to 60 cm (16-24’’) long.
2. Clamp the copper bar (also known as ‘‘central conductor’’) between the headstock and the tailstock of the magnetic bench.
3. Put the KETOS RING in the middle of the bar.
4. While spraying the detection medium apply the current as defined in the underneath Table (in paragraph 6).
5. Record the number of seeable indications.
6. Refer to the Table (from ASTM E 1444) to know the minimum number of indications to be seen according to the detection medium (dry powder or wet technique).
The KETOS RING and similar test parts are suitable for the overall check of MT installations using the current flow (head shot) technique; hence their frequent use on magnetic benches.

Each KETOS RING is identified by its serial number and comes with a verification (our UK and US friends call that ‘‘calibration’’) certificate. The ring shall be recertified every 12 months.

Indications as they appear are compared to the KETOS RING reference photograph. If any discrepancy is seen the MT installation shall be checked.

Also keep in mind that some industries, such as the aerospace one, require only a measure of the settlement volume of the detection medium using the pear-shaped centrifuge tube as per ASTM D 96.

This test is not required in the ISO 9934-2 standard.

Quite easy to understand why: this method gives the total volume of all the solids in 100 mL of the detection medium, without any means to know which percentage is magnetic particles! Solid impurities come from parts (metallic, sand, whatever they are) may be almost 100% of the volume: then what about the sensitivity, the ability to detect a discontinuity?

3- Self-sufficient items

As seen above the KETOS RING is used to make an overall check of the installation, this comprising the bench and the detection medium.

If the results got on the KETOS RING are lower than anticipated, no way to know whether this is due to a malfunction of the equipment, to the detection medium...or even both!

The current waveform is one of the important parameters to get the right sensitivity from the detection medium.

So the best way is to be able to check the performance of the detection medium itself, the magnetizing current parameters being side-stepped.

Several items are designed to meet this need.

3.1- MTU test block

This test block comes as a black ring 50 mm (2’’) diameter, 10 mm (ca 3/8’’) thick, a central hole 10mm (ca 3/8’’) dia. Many cracks are visible on both sides of the ring. These cracks are:

- Grinding cracks.
- Stress corrosion cracks.
This ring is magnetized by the central conductor technique. After magnetization the residual
magnetism is strong enough to allow for numerous uses. The test block SHALL NOT BE SUBMITTED TO ANY MAGNETIC FIELD.
We think this test block was designed in the ‘60s. It is now the Type 1 reference block of the ISO 9934-2 standard.

This test part allows only for a qualitative assessment of the detection media.

A certificate supplied with every test block shows a photograph of both sides. This test block may be verified by the manufacturer.

3.2- One-permanent-magnet-based item giving magnetic field gradient

Karl DEUTSCH must be credited for designing this unit known under the trade name
FLUXA® test block (see figure).

Once again we think it became available in the ‘60s; it was the only reference block allowing
for a quantitative assessment of the detection media sensitivity. This till 1988.

The FLUXA® test block is a one-permanent-magnet- based test block giving rise to a magnetic field gradient. It comes as two precisely ground steel blocks with an artificial discontinuity between them. On one end there is a permanent magnet under a brass cover. This magnet gives birth to a magnetic field whose intensity lowers as the distance increases.

If a detection medium is applied on the test block one sees a linear indication just on top of the artificial discontinuity. The longer the indication, the more sensitive is the detection medium.

As for the MTU item every FLUXA® test block comes with a certificate and a photograph. The test block may be verified by the manufacturer.

The FLUXA® test block is not included in the ISO 9934-2 standard.

3.3- Two-permanent- magnet-based item giving a magnetic field gradient

This is the Type 2 Test Block as mentioned in the ISO 9934-2 standard.

As the Type 1 Test Block (MTU test block), the Type 2 Test Block is described in the ISO 9934-2 standard; you may find additional information on our Website (1).

A French Engineer, Michel TOITOT, close to the Nobel Prize Louis NÉEL designed this test block in 1988. Initially known as the ‘‘Témoin C’’ in the French standards, it came as the ‘‘Type 2 Test Block’’ when introduced in the ISO 9934-2 standard.

The Type 2 test block, is especially designed to make a quantitative assessment of the detection media sensitivity along with an increased stability when compared to the FLUXA® test block. Nevertheless all these units SHALL NEVER COME CLOSE TO A MAGNETIC FIELD.

The Type 2 test block comes with a verification certificate. It should be recalibrated every 12 months.

Not yet well known in aerospace industry, this test block is nevertheless used on a large scale in different industries: nuclear, automotive, railways, service companies, etc.

A separate paper (2) on our website gives you a thorough technical explanation about this test block.

3.4- Note on magnetized test blocks

Apart the warning stated above (these test blocks SHALL NEVER BE SUBMITTED TO A MAGNETIC FIELD), except naturally to the unavoidable Earth magnetic field without any influence because too weak, note that different test blocks, even from the same type/manufacturer, give different results. This means the figure the user gets are valid only for the test block with the specified serial number.

For sure all similar test blocks manufactured by the same manufacturer should give the same figures with the same detection medium/batch (different batches of the same detection medium may give different readings). But the impossibility to perfectly control all the manufacturing and calibration parameters may lead to important differences in the way discontinuities are seen. Typically ± 25% of the detectable length.

That’s why a user shall always use the SAME serial number, why the test block shall ALWAYS be tilted at the same angle (say: 45°).

The orientation of the test block has no influence on the results because the Earth magnetic field is weak. Note however that the Earth magnetic field can magnetize some ferromagnetic parts if they are directed and stored in the North-South direction while by positioning them in the East-West direction this phenomenon is minimized. Such is the case of the tubes which, because of their geometry (very long parts with a small section), channel particularly well the Earth magnetic field. Welders know this problem very well.

That’s why also, when a test block comes back from the supplier after recalibration, it is a good idea to ‘‘recalibrate’’ the readings got with brand new detection medium. Then, as the in-use specification should give an acceptable percentage of decrease of the reading (and not directly and only a figure!) for an in-use detection medium, it is very easy to calculate the real figure which makes a change of detection medium mandatory in the bench.

Further permanent magnets shall not be submitted to shocks or high temperatures. These test blocks shall then be considered as precise, and costly, equipment, used by competent people only.

4- Other tools comprising magnetic fields gradients

4.1- The Paris-metro ticket

The metro ticket cannot be considered as a reference test piece for MT, for sure! But we think it is useful to give some clues on this ‘‘tool’’.

Many of French MT users had the opportunity to apply a magnetic ink, either back or fluorescent, on a ticket used in the Paris Metro. On the magnetic strip straight lines perpendicular to the strip are visible, quite similar to those of a bar-code, black or shiny yellow depending on the detection medium/lighting.

The metro ticket may be seen as the first magnetic card, the simplest one - - and the cheapest one! It was first issued in 1967. Deep brown in colour the magnetic strip comprises myriads of iron oxide particles each having a North pole and a South pole: they are called magnetic dipoles. When they are submitted to an external magnetic field these particles are magnetized and turn upwards or downwards. A binary encoding is then done (for example ‘‘0’’ in the former case, ‘‘1’’ in the latter one); this information is fixed on the magnetic strip unless to a ‘‘magnetic disruption’’.

When the ticket comes into the turnstile slot it goes through a magnetic field sensor. As the ticket stripe is magnetized the sensor ‘‘reads’’ the stored information (expiry date, route limit, validity, etc.), passes them on to the computer which confirms the ticket (or not!) stamps seeable indications (date/hour, metro station code number, etc) and encodes the magnetic stripe with additional information. The entire process needs about 2 seconds.

Statistical data from the computer allow for optimising train frequency and traffic control according to the hours, the days, months and special events such as the World Car Show or any football or rugby match.

We may assume that a magnetic ink could be checked using this ticket. Nevertheless not seeing indications or seeing them as faint lines is not a proof that the magnetic ink does not meet applicable specification.

In fact when the ticket is put into the slot the machine modifies magnetic information. The magnetic stripe on the ticket may probably be modified by applying a magnetic field of some kA/m to several tens of kA/m. As an example when measuring the magnetic field close to a ON or OFF cell phone it can be obtained a value of 7 kA/m (DC field probably produced by the loudspeaker or the vibrator. It is enough to interfere with the encoded information. Putting metro tickets in a plastic folder does not prevent them from surrounding magnetic field action. As a general rule these tickets should be kept at a certain distance from any magnetic source that one does not always think about: keys, bags and cases with magnetic clasp (cases for spectacles, MP3-like items, notebooks, etc.)

While magnetic metro tickets are still in use, weekly, monthly and annual tickets were replaced by RFID (Radio Frequency Identification-equipped) passes. Information is then stored in a chip which comprises a very small antenna. Then all the information is transferred trough radio-waves without any contact.

4.2- Magnetic stripe cards

In 1996, such cards were put on the market for detection media performance check.

These cards are encoded and the lines attract the magnetic particles.

A magnetic coil is used to encode the stripe:

- A combination of the card speed through the coil and the current frequency allows for the distance between two successive magnetic gradients.
- The different magnetic gradient figures are adjusted by the amperage of the current in the coil.

Though the stripe does not show any measurable magnetic field (at least using our NDT meters) the tiny magnetic particles are attracted by the magnetic field gradients; the card is then a very efficient test block.

Note that the US patent N°6310471 dated October 30, 2001 is titled: "Card with magnetic stripe and method for testing magnetic inspection particle fluid".

Two types of magnetic stripe cards are available:

- Those with high coercitive field: used to check dry magnetic powders and the liquid-based detection media. The stripe material displays a coercitive field of 218.6 kA/m.
- Those with a lower coercitive field: they are better suited to discriminate between high sensitivity detection media or to check in-use magnetic inks in benches. We do not know the field figure but may assume that they could be more sensitive to external magnetic fields.

Depending on the aspect of the lines the inspector knows a bit about the quality of the detection medium:

- If all the lines of the stripe are easy to see, are very clear: OK.
- If some lines are missing, the detection medium should be replaced.
- If the stripe is almost completely fluorescent this should be a sign that there are too many particles in the magnetic ink, or that the fluorescent dye has begun to dissolve in the liquid carrier.
- If no lines are shown this is probably due to the fact there are almost no more particles in the detection medium or that the liquid does not wet the card due to some pollutants; it may also come from a demag card, though this is unlikely (it is easy checked with an unused card).

The magnetic lines may be wiped if the card is in a magnetic field higher than the coercitive field of the stripe.

Just to give some clues: an audio cassette tape shows a 16 kA/m coercitive field and a video cassette tape come at 160 kA/m.

Though they are tough against external fields, these magnetic stripe cards, as per manufacturer’s instructions, should be used/stored:

- Far from strong magnetic fields.
- At ambient temperature avoiding extreme temperatures, though no figure is given!

The manufacturer’s instructions state that in the unlikely event of stripe demag the lines would then not be visible. In such an occurrence a second card shall be available.

A certificate may be supplied along the cards but we do not know which parameters are specified and checked, neither the acceptance range for the figures. The ISO 9934-2 Type 2 test block always come with a certificate of verification while it seems that the magnetic stripe cards are often supplied without.

Along the time a card may degrade (wrong handling, magnetic stripe aging, etc.). Without, say, a yearly verification using a defective card is more and more likely.

Everybody seems OK with such a peculiar situation as per Quality Assurance requirements.
When a thorough traceability requirement exists, up to the national reference meters, for equipments such as numerous meters (amperage, temperature, tangential magnetic field, UV-A irradiance, etc.) such a lack is difficult to understand!

Using a brand-new card every year is a kind of an answer.

5- Conclusion

We think the ISO 9934-2 Type 2 test block is the most reliable test block to check detection media performance on a regular basis, even daily or for every shift.

Magnetic stripe cards supplied without certificate of verification, without any proof of traceability to the national standards and any yearly verification do not meet the ISO 9001 Quality Assurance requirements.

(1) Patrick DUBOSC and Pierre CHEMIN: ‘‘A survey of Standard and Codes for Magnetic testing
WebSite: http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

(2)Stéphane GRAVELEAU: ‘‘Reference test block N°2 of the ISO 9934-2 standard How does it work?’’
WebSite: http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

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.