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Misconceptions within magnetic testing
News
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
rsum 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 reliab ility, as
well as being frequently misapplied."
Don Hagemaier and John Petty
McDonnell Douglas Aerospace
Materials Evaluation May 1997
Quite a very good start, dont 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
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NAS 410 (Editors note: now aligned with EN 4179 and ISO 9712
standards): 32 hours in MT.
2- Bad Training
All training centers are not equal.
(Editors 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. (Editors 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 N1
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 N2
Whats about empirical formulae?
"Whats the matter with formulas? Thats the way weve 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
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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.
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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?
Editors note: We are rather against using formulae (with the
exception of the transverse magnetization by the current flow
technique H = I/( D)) because they are often misused (applicable
in some cases only) and therefore a source of errors.
Therefore, many formulae should be prohib ited, and, when not
possib le, their "informative" aspect should be emphasized.
Even on simple parts, formulas do not account for varied
waveforms.
CURRENTMETER PEAK
Pure DC1000
1000
AC1000
1414
HWDC1000
1570*
1FWDC1000
11570
1FWDC1000
1000
*Uncorrected meter reading would be 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?
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Misconception N3
You cant 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 youre 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.
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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 its 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 n4
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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 theyve been given.
> geometric limitations restrict complete inspections on
complex parts.
Misconception N5
The central bar conductor cannot be hollow: it must be solid.
This is heard so often!
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Weld all these central bar conductors together and get a hollow
conductor.
A pipe is a lighter and easier to use central conductor.
Misconception N6
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 wont 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 N7
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 N8
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!
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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 cant put a QQI in a small diameter ID.
I cant 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 N9
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 dont 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.
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Aluminum V-channel between headstocks
(a flat disc may be suitab ly magnetized, but other shapes may
produce
a distorted circular magnetic field that could leave
discontinuities undetected.)
Misconception N10
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.
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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:
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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 N11
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.
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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 N12
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 N13
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 N14
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:
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"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 N15
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 N16
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 m (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 50m (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
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found very close to powerful MT equipment), or, as Penetrant
Testing (PT), which is also said, "old fashioned", "using
pollutant products", etc ?
GEORGE M. HOPMANs RsumPO Box 30085
Phoenix, AZ 85046
[email protected]
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 Repairmans 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
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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 )