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Misconceptions within magnetic testing

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Written by Administrator
Saturday, 14 January 2012 12:19

February 2012

This document is based on a conference given by George HOPMAN, in Las Vegas, on 13 November 2007.
It is important to know a bit more about this man, well known - and renowned - in the American NDT world. You may find his résumé at the end of this paper.

Time goes by and some things may have changed since this conference. What we wish here is to show "the critical thinking" we all of us shall continuously have: this is not always because one does "according to the relevant standards or documents" that one shall perform a successful inspection, i.e. finding the discontinuities that must be detected.

This is the kind of conference that holds the audience spellbound, due to the topic, to the lecturer and to the way he entertains everybody.

We thank him for the time one of us spent then, avidly listening to him, laughing, and talking with him when his conference ended. We thank him also for the opportunity we have to give here some examples drawn from his conference.

He is the kind of auditor that every auditee would like to have, as he is ready to listen to explanations by the auditees, and to give some advice, free of charge … or almost!!!

Well, it is time now to go to more basic concerns.

"Of all the nondestructive testing methods, the magnetic particle method is apparently the least understood and least quantitative in terms of repeatability and test reliability, as well as being frequently misapplied."
Don Hagemaier and John Petty
McDonnell Douglas Aerospace
Materials Evaluation – May 1997

Quite a very good start, don’t you think so?

They cited two US Air Force studies (1973 and 1984) which showed that Magnetic Particle Inspection is only 47% effective in finding cracks in aerospace components.
They also cite current practices, which "provide an assurance that does not really exist."

What could be the main causes for such a situation?

According to George HOPMAN

1- Minimum Training to be a Level II

NAS 410 (Editor’s note: now aligned with EN 4179 and ISO 9712 standards): 32 hours in MT.

2- Bad Training

All training centers are not equal.
(Editor’s note: training centers should be accredited by national organizations such as: ASNT, COFREND, etc.).

3- Inaccurate Specifications and Procedures

This is a very common occurrence: documents written by people who do not thoroughly know the method, who do not know which equipment is available, documents that cannot be used on complex parts, etc.

4- Lack of Basic Research

5- The Industry Standard (in the USA, ASTM E 1444) has not kept up
. The same may be said for the ISO 9934 standards series, though they are more accurate. (Editor’s note: the ASTM E1444 standard has been revised in 2011 and the ISO 9934 standards series are under revision).

6: Transition from a mindset of empirical formulas to flux sharing devices for amperage determination.

Misconception N°1

MT finds all the defects.

Air Force "Have Cracks Will Travel" studies show that MT/MPI suffers from gross variability.

Other than a transition away from formulas and better equipment, not much has changed since then.

Misconception N°2

What’s about empirical formulae?

• "What’s the matter with formulas? That’s the way we’ve always done it."

• Formulae were only meant for "simple" configurations. Is the part below a "simple part"?

• Circular magnetism formula is inadequate:
300 to 800 A/inch depending on the material permeability. For in-service inspection, how does one know what the material permeability of the part is?
A 1997 paper by HAGEMAIER/PETTY, based upon SAE-AS 5371 QQI confirmation, showed that for AISI 4130 steel, 200 A/cm (500 A/inch) is adequate, 120 A/cm (300 A/inch) was too low and 320 A/cm (800 A/inch) saturated the part, leading to excessive background fluorescence.

• Longitudinal magnetism low fill factor formula (NI = 45,000 / L/D) is inadequate.
In the same 1997 paper, HAGEMAIER/PETTY demonstrate that the 45,000 constant is too high, saturating the part and leading to excessive background fluorescence.
Using 25,000 as the constant puts one in the empirically verified range of 3 to 6 mT (milliTesla, or 30 to 60 gauss).

Air Force NDI Manual TO33B-1-1:
"All studies agree "rule-of-thumb" formulas for estimating magnetizing currents, contained in ASTM E 1444, will usually produce field strengths well in excess of what is needed for adequate magnetization with the concurrent risk of producing a background that can hide defect indications. Always use a magnetizing force sufficient to minimize background and maximize the signal to noise ratio of the method."

• There is no formula for the induced current technique.

• Formula confusion:
> Effective diameter of hollow part.

> Low fill factor part.



> Intermediate fill factor part.



> High fill factor part.

• Using formulae is not that easy: not everyone is gifted with math skills.

• One may confirm that using formulae is not that simple by giving the same part to five inspectors: it is likely that the result will be five different techniques.

• Any idea of the amperages on this part? Who can answer within a minute?

Editor’s note: We are rather against using formulae (with the exception of the transverse magnetization by the current flow technique H = I/(π D)) because they are often misused (applicable in some cases only) and therefore  a source of errors. Therefore, many formulae should be prohibited, and, when not possible, their "informative" aspect should be emphasized.

• Even on simple parts, formulas do not account for varied waveforms.

CURRENT

METER
PEAK

Pure DC

1000

1000

AC

1000

1414

HWDC

1000

1570*

1FWDC

1000

11570

1FWDC

1000

1000

*Uncorrected meter reading would be ½ the value shown.

• Most MT amperemeters (ammeters) read the RMS waveform, not the peak waveform, which has the greatest effect on domain movement.

• Paragraph 6.3.1.3: Except with CEO approval, formulas may only be used if the amperages are confirmed with known or artificial defects (QQIs) or with the Hall effect probe gaussmeter (which should be called Teslameter).

• One may wonder why, if the calculated amperage shall be confirmed with a QQI or a Hall effect Teslameter, why would anyone bother to make a calculation?

Misconception N°3

• You can’t use a Hall effect probe Teslameter to determine coil (longitudinal magnetization) amperage.

The Hall effect probe Teslameter has always be allowed by the ASTM E 1444 specification. In its Appendix X4, it states, "The direction and magnitude of the tangential field on the part surface can be determined by two measurements made at right angles to each other at the same spot."

Hall effect probe Teslameter:

• The related misconception is: a tangential field is synonymous with a circular field.

• It is not! A tangential field may be circular or longitudinal.

• Let us check what Wikipedia writes on the topic: "In plane geometry, a straight line is tangent to a curve, at some point, if both line and curve pass through the point with the same direction; such a line is the best straight-line approximation to the curve at that point." In the following diagram, a line intersects the curve at two points. It is tangent to the curve at only one point; at the dot.

Use the Hall effect probe Teslameter the right way.

• What if I put a wood dowel inside the coil and get a measurement with the Hall effect Teslameter?

• Nice, but all you’re doing is measuring the flux density of an unloaded coil. The wood dowel is irrelevant. Once one inserts a ferromagnetic part into the coil, the flux lines take the path of least resistance through the part.

Comparison between the Hall effect probe Teslameter and QQIs.

• Empirical experiments proved that a minimum reading of 3 mT (30 gauss) on the part when it’s in the coil is sufficient to get an AS 5371 QQI to light up.

We have two valuable documents to help us in this comparison:

• "Evaluation of Shims, Gaussmeter, Penetrameter and Equations for Magnetic Particle Inspection," by Hagemaier/Petty – Materials Evaluation – May 1997.

- Hopman/Kleven, "The Use of the Hall effect probe Gaussmeter in the coil" – ASNT Fall Conference – 2000.
> Various L/D ratio bars had QQIs pasted on them at 2.54 cm (1”) increments over half the length of the bar.
> Induction readings were taken at these same locations.  The measurements are consistent except at the ends of the part where the flux entering and exiting the poles of the part skews the reading abnormally high.

Here are some guidelines to use a Hall effect probe Teslameter in a coil:
• Use a tangential probe and hold it upright within 50 from normal.
• The probe shall be positioned away from geometries such as the bottom of gear teeth, sharp corners, and keyways that will lead to non-relevant readings from non-relevant flux leakage.
• Take the reading away from the ends of the part where the normal field will skew the reading.
• The probe may be placed either inside or out to the side the coil – it does not matter.

Misconception n°4

100% of the part is inspected with MT for defects oriented in either direction.

100% coverage issues.

• Have we inspected the endcaps for cracks in either a transverse or circumferential direction?

•  It is important to understand that:
> complex parts have fields that cancel out, creating dead spots that require special techniques to overcome them.
> most technicians just perform the best they can with the equipment they’ve been given.
> geometric limitations restrict complete inspections on complex parts.

Misconception N°5

The central bar conductor cannot be hollow: it must be solid. This is heard so often!

Weld all these central bar conductors together and get a hollow conductor.

A pipe is a lighter and easier to use central conductor.

Misconception N°6

So often read and heard also: the central bar conductor must be non-ferrous.

• Not so - it can be steel. Small diameter steel bars are much harder than copper, so they won’t bend under pressure like copper and aluminum bars.

• However, they will heat up with use, and non-relevant indications where the steel parts touch are likely.

Misconception N°7

AC current can only find surface defects. This assertion is probably based on ASTM E 1444 Paragraph 6.2.4: "Alternating current is to be used only for the detection of defects open to the surface."

Indeed, AC can pick up the #1 hole (0.070” deep; 0.18 cm deep) on a Ketos ring consistently. AC can pick up near surface defects

Misconception N°8

When using AC in conjunction with a central bar conductor, one can only inspect the ID of the part. Once again, ASTM E 1444 is at the origin of this well entrenched idea, due to its Paragraph 6.3.6: (Central Conductor Circular Magnetization) "… In this case, alternating current is to be used only when the sole purpose of the test is to examine for surface discontinuities on the inside surface of the part."

Thus, if I have a 1” OD (2.54 cm), 0.040” (1 mm) wall pipe to inspect, I cannot inspect the OD, even if I have verified the amperage with an EDM Notch, QQI, or a Hall effect gaussmeter probe. Indeed, it is what ASTM E 1444 infers!

Further, what about the ID magnetizing current determination? Since ASTM E 1444 does not allow one to use formulas in isolation, how can I determine the ID field strength?

• I can’t put a QQI in a small diameter ID.

• I can’t put a probe inside the ID of the tube.

• I guess I can put a notch inside a scrap part for each part number I use a central bar conductor for.

Nonsense! As long as one demonstrates a flux density of at least 3 mT (30 gauss) or the illumination of a QQI on the OD of the part, one has a valid inspection – and this is easily demonstrated – try it yourself.

The ISO 9934-1 standard allows one to use AC with the "Threader Bar" technique.

One thinks that the ASTM E 1444 specification should be changed to accommodate the realities of AC inspection!

Misconception N°9

The parallel magnetism techniques work.

Parallel magnetism confusion.

ASTM E 1444-05 Paragraph 6.2.10 disallows this technique and states that the field "is more transverse than circular."

• Principles of Magnetic Particle by Betz (1960) disallows it.

• Air Force TO 33B-1-1 disallows it.

• ASNT Handbook on Magnetic Particle disallows it.

So, it seems that every recognized authority says it does not work. Does it Work?  Maybe yes - Maybe no.

• On a small round pin in a V-channel, I don’t think so.

• On a flat washer in a V-channel, I think it would work.

• Some demonstrations need to be performed to prove out what the strengths and weaknesses of this technique are.

Aluminum V-channel between headstocks
(a flat disc may be suitably magnetized, but other shapes may produce
a distorted circular magnetic field that could leave discontinuities undetected.)

Misconception N°10

The residual technique of particle application is inferior to the continuous technique of particle application

The residual technique relies upon the domain generated flux field only, whereas the continuous technique relies upon the combination of the flux field and the applied field.

Point 2 on the hysteresis curve represents the continuous technique field strength.

Underneath, the requirements stated in the ASTM E 1444-05 specification, Para. 6.4.3, Residual Particle Application are detailed:

• The magnetic particles are to be applied immediately after the magnetizing force has been discontinued (not later).

• The residual technique is "not as sensitive" as the continuous technique.

• It can be useful in detecting service induced fatigue cracks on the surface of materials with high retentivity.

• It can be useful on parts, which, because of geometric constraints, cannot be examined with the continuous technique.

• The residual technique shall only be used when approved by the CEO or when it has been documented that it can detect discontinuities or artificial discontinuities in parts under examination.

Why not design an experiment?

Three 1.0” (2.54 cm) diameter by 18” (46 cm) long test bars were selected with three varying material permeabilities, alloy steel, carbon steel, and ferromagnetic stainless steel.

In the middle of each bar is a milled area of 50% wall. This presents a flat surface (best geometry) as well as a curved surface for the test.

Residual field results of ferromagnetic stainless steel on flat surface using 1 phase FWDC with a QQI:

Residual field results of alloy steel on flat surface using 1 phase FWDC with a Pie Gage:

Residual field results of carbon steel on curved surface using 1 phase FWDC with a Pie Gage:

The conclusions of the DOE (Design of Experiment) on residual field are quite interesting:

• A QQI or a Pie Gage is capable of demonstrating the adequacy of a residual field to perform inspections using longitudinal magnetization.

• It follows that since indirect magnetism (coil shot) is weaker than direct magnetism (head shot), this methodology proves the adequacy of using the residual technique for circular magnetism on each specific part this is demonstrated on.

• The response indicated by the QQI or the Pie Gage with a residual field is a function of the material permeability and the geometry of the test piece. Aerospace parts are inherently low permeability/high retentivity type parts.

This type of demonstration can serve as documentation to satisfy the necessary requirements noted in ASTM E 1444 in order to enable the residual technique of particle application.

Misconception N°11

A very common misconception is that the procedure you are working to is technically accurate.

Let us take an explicit example:

Service Bulletin: Landing Gear Torque Knee Inspection.

Steps 1-4: Clean the part.

Step 5: Shoot a centrally located central conductor through the small holes at the small end of the torque knee at 500 A.

Step 6: Shoot a centrally located central conductor through the large holes at the larger end of the torque knee at 600 A.

Step 7: Demagnetize the torque knee.

Step 8: Shoot a direct contact shot between the left-hand large diameter hole ear and the right-hand small diameter ear at 600 amps.

Step 9: Demagnetize the torque knee.

Step 10: Shoot a direct contact shot between the right-hand large diameter hole ear and the left-hand small diameter ear at 600 A.

Step 11: Shoot a coil shot at 800 A with torque knee located near the inside diameter of the coil.

Step 12: Inspect the torque knee for any evidence of fatigue cracks.

Step 13: Demagnetize and clean the torque knee.

Well … very clear, very easy to understand, no risk to make a wrong inspection.

Misconception N°12

When proceeding from a high amperage shot to a lower amperage shot in the opposite direction, one must demagnetize the part between operations.

This is what could be called "the shot sequence myth".

• Most textbooks make the statement that one should proceed from a low amperage to a higher amperage. If not, one should demagnetize the part before proceeding with the next shot.

• Nonsense!  As long as one demonstrates a flux density of at least 3 mT (30 gauss) or the illumination of a QQI, it does not matter what happened in the preceding magnetizing operation.

Point 1 represents the field strength of shot 1 while Point 2 represents the field strength of shot 2.

Misconception N°13

When the previous shot was a head shot (e.g., 1200 A) and the next shot is a coil shot (e.g., 1000 A), one must demagnetize the part.

Again, the "shot sequence myth".

• Several auditors have stated that this is proceeding from a high amperage to a lower amperage and the coil shot will not overcome the previous domain alignment.

• More nonsense! That 1 000 A is multiplied by the 5 turn coil for 5 000 A/turns.  Moreover, as long as one demonstrates a flux density of at least 3 mT (30 gauss) or the illumination of a QQI, it does not matter what happened in the preceding magnetizing operation.

Misconception N°14

The demagnetization myth:

One has to hold the part to be demagnetized one foot past the coil and move it slowly through the AC coil in order to have the part properly demagnetized.

ASTM E 1444 Paragraph 6.7.1.1 states:

• "When using an AC demagnetizing coil, hold the part approximately 30 cm (1 ft) in front of the coil and then move it slowly and steadily through the coil and at least a meter (3 ft )beyond the end of the coil while the current is flowing."

• It is not necessary to hold it in front of the coil 30 cm (1 foot), nor to move it slowly (how slow is "slowly"?), nor to withdraw it "at least a meter (3 ft) beyond the coil."

• All that matters is that the residual field is reduced to +/- 0.3 mT (3 gauss). Sticking the part in the middle of the coil and withdrawing it about 60 cm (two feet) past the coil very quickly works!

Misconception N°15

Your magnetic testing equipment is calibrated accurately

• The meter used to check the MT amperemeter has no requirement for accuracy.

• The meter used to check the field indicator has no requirement for accuracy.

• The meter used to check the timer has no requirement for accuracy.

• The meter used to check the UV-A irradiance/visible light luminance has no requirement for accuracy.

Misconception N°16

Hall effect Teslameter measurements are better than AS 5371 QQI shims.

• Some believe that since QQIs will illuminate at 0.5 to 1 mT (5 to 10 Gauss), this is an inferior field strength to the minimum of 3 mT (30 gauss) that E1444 requires.

Hall effect gaussmeter measurements are better than AS 5371 QQI shims.

Each manufacturer of Hall effect probes is different:

• One has placed the sensor 1.2 mm (0.047”) off the tip of the probe.

• The QQI is approximately 50 µm (0.002”) thick.

• Applying the inverse square law to the two magnetic fields. If we have 3 mT (30 Gauss) at 1.2 mm (0.047”), we can calculate the corresponding field strength at 50µm (0.002”) to be 166 mT (1657 Gauss).

The Conclusion?

Being compliant with unreliable documents does not mean that an inspection is satisfactorily performed: the main reason for performing MT, or any other NDT, is to find the discontinuities that the method should detect, and to decide whether these discontinuities are defects or are acceptable. It is not to comply with documents known to be falsely assuring the quality of the inspection.

Nevertheless, it is obvious that many users, NDT department managers and auditors do not even know about the "complacency" that the MT method is benefiting. Is this because magnetic fields do not work as one would, sometimes going by unexpected paths? Because it is not a "high-tech" method, such as UT (Ultrasonic Testing) or ET (Eddy Current Testing), or AT (Acoustic Emission)? Because it seems to be "dangerous" (due to the high magnetic fields that may be found very close to powerful MT equipment), or, as Penetrant Testing (PT), which is also said, "old fashioned", "using pollutant products", etc …?

GEORGE M. HOPMAN’s Résumé

PO Box 30085
Phoenix, AZ 85046
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Home Office: 602-595-1033
Cell Phone: 480-225-0775

QUALIFICATIONS

ASNT Level III #15776
Magnetic Particle (MT) Since 2-1983
Radiographic (RT) 5-1983
Ultrasonic (UT) 8-1983
Liquid Penetrant (PT) 2-1984
Eddy Current (ET) 7-1988
Visual (VT) 10-1999
Magnetic Flux Leakage (ML) 11-2010
ASNT IRRSP #15776 – X-ray and RAM
AWS CAWI #11041014
Current Chairman:
ASTM E07.03 Liquid Penetrant / Magnetic Particle Subcommittee
Six Sigma Green Belt Certified
American Society for Quality - Certified Quality Engineer #37575
American Society for Quality - Certified Quality Auditor #15154
FAA Repairman’s Certificate #3361432
Boeing Approved Consultant PT, MT, RT, UT, ET (Vendor #657471)
Honeywell Certified Agent in PT, MT

AWARDS

ASNT Fellow – Class of 2011

EDUCATION

Moraine Valley Community College - Palos Hills, Illinois (5-82)
Degree: AAS in Nondestructive Evaluation (With honors)

EXPERIENCE

2-1996 to present
NDE Solutions Inc. – Phoenix, AZ
NDT Training, auditing, Nadcap preparation, certification testing, procedures, consulting
11-1990 to 8-2005
Honeywell Engines & Systems– Phoenix, AZ
Quality and Materials Engineer
Level III in UT, RT, PT, MT, ET
Responsible for performing domestic and international special process audits, vendor support, authoring written instructions for inspection personnel, webmaster for work instructions, authoring specifications for all nondestructive test methods, and providing technical support to all departments.
9-1983 to 11-1990
Boeing Commercial Aircraft (Modification Division) - Wichita, Kansas
Level III NDT inspector in Radiography, Liquid Penetrant, Eddy Current, Ultrasonics, and Magnetic Particle inspections
Responsible for performing in-service NDT on commercial/military aircraft (747, 737, 727, L-1011, KC135, C9, B52, F4), train/certify NDE personnel, develop new NDT techniques, and provide technical support to all departments.
1-82 to 9-83
Conam Inspection - Itasca, Illinois
Level II NDT inspector in UT, RT, MT, PT. Level I in ET and LT.
Experience in contact/immersion ultrasonic, x-ray/gamma-ray, eddy current, magnetic particle, and liquid penetrant inspections on various configurations of welds, forgings, castings, and tubing.
8-81 to 12-81
Calumet Testing Services - Highland, Indiana
NDT inspector performing x-ray, gamma-ray, magnetic particle, liquid penetrant, and visual inspections on welds, forgings, and castings.
5-81 to 8-81
Magnaflux Quality Services - Houston, Texas
NDT inspector performing RT and MT on weldments and castings.

TECHNICAL ORGANIZATIONS

ASNT #15776
ASME #100121075
ASTM #000187535


References

• Materials Evaluation is published monthly by the American Society of Nondestructive Testing, Inc (ASNT).
Materials Evaluation, 1711 Arlingate Lane, PO Box 28518, Columbus, OH 43228-0518, USA.

Normative references


• NA AS410, Certification & Qualification of Nondestructive Test Personnel, Aerospace Industries Association (AIA) 1000 Wilson Boulevard, Suite 1700, Virginia, 22209, USA.

• EN 4179, Aerospace series. Qualification and approval of personnel for non-destructive testing, Committee for Standardization, Brussels, Belgium, 2010.

• ISO 9712, Non-destructive testing -- Qualification and certification of personnel, International Organization for Standardization, Geneva, Switzerland, 2005.

• ASTM E1444 – 05: Standard Practice for Magnetic Particle Testing, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2005.

• ASTM E1444/E1444M - 11 Standard Practice for Magnetic Particle Testing, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2011.

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

• ISO 9934-2:2002 Non-destructive testing - Magnetic particle testing - Part 2: Detection media, International Organization for Standardization, Geneva, Switzerland, 2002.

• ISO 9934-3:2002 Non-destructive testing - Magnetic particle testing - Part 3: Equipment, International Organization for Standardization, Geneva, Switzerland, 2002.

• SAE-AS5371: Reference Standards Notched Shims for Magnetic Particle Inspection, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvanie 15096, USA, 1998.

• T.O. 33B-1-1 NAVAIR 01-1A-16 TM 1-1500-335-23, Technical Manual, Nondestructive Inspection Methods, Basic Theory, 2007.

Last Updated ( Saturday, 14 January 2012 16:13 )