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18th World Conference on Non destructive Testing, 16-20 April 2012, Durban, South Africa
Magnetic Particle Inspection: Characterisation of the magnetic field for various
magnetization techniques
Ephraim MOTUKISI1
More info about this article: http://www.ndt.net/?id=12825
1
Southern African Institute of Welding; Johannesburg, Gauteng, South Africa
motukisie@saiw.co.za; www.saiw.co.za
Abstract
Magnetic particle inspection is one of the most commonly used non destructive testing methods for
detecting surface and limited subsurface indications in ferromagnetic materials. The magnetic field
provides the means by which indications are formed and detected. It is thus essential that the magnetic
field be optimized, not only in magnitude, but also in direction when detection of specific indications is
required. For example, too low a field strength would not result in adequate particle build-up and would
thus reduce the probability of detection for small indications.
This paper discusses the magnitude, direction and special distribution of the magnetic field for various
different magnetization techniques utilized. Special attention is given to the correct usage of various field
detection instruments.
Keywords: Magnetic Particle Testing (MPI/MPT), field strength, field direction, Nondestructive
Test (NDT)
Measurement of magnetic field characteristics for MPI
1. Introduction
Magnetic Particle Inspection, a surface method used to inspect Ferromagnetic
Materials, is one of the most commonly used methods in NDT.
In order for the test method to be used effectively, it is essential that the strength of the
magnetic field be appropriate for the task. If the strength of magnetic field is too low,
the particle build-up identifying a defect may be inadequate and a defect may go
undetected. If the magnetic field is excessive there can be a heavy accumulation of
particles around superficial irregularities and real indications may be masked
(furring), especially those indications that stem from in-service failure mechanisms,
which result in tight cracks.
This paper discusses some of the factors, which influence the quality of test results
including:1. The importance of determining the direction of the induced magnetic field
when affected by residual stresses.
2. The use of instruments correctly to measure magnetic field characteristics.
3. Using reference samples for optimizing test parameters.
2. Determining the direction of induced magnetic field
2.1 Equipment commonly used to verify magnetic field in MPI
1.
2.
3.
4.
Gauss/Tesla Meter (Field Strength Meter)
Pie Gages
Shims Gages
Reference samples
2.2 Common errors
1. Incorrect placement of Field Strength Meter Probe
2. Using instruments incorrectly to verify magnetic field
3. Making assumptions that the magnetic field is in the desired direction
2.3 Tesla meter
Fig. 1 Tesla meter connected with a Tangential Probe
Fig. 2 Tesla meter Probes
[1]
It has become a norm to place the instrument’s probe on the surface of the material to be
inspected and record the readings. The direction of the probe and area of placement, are
important things to consider when verifying magnetic field.
Fig. 3 Probe output versus Flux Angle
[2]
A
B
C
The plane of the probe must be perpendicular to the surface of the part at the location of
measurement to within 5°. See figure 3A. The direction and magnitude of the tangential
field on the part surface can be determined by placing the Hall-Effect Tangential-Field
Probe on the part surface in the area of interest.
The direction of the field can be determined during the application of the magnetizing
field by rotating tangential field probe while in contact with the part until the highest field
reading is obtained on the instrument.
The orientation of the probe, when the highest field is obtained, will indicate the field
direction at that point. Various specifications call for the use of different Field Strength
Meter values in particular applications. [3]
Fig. 4 A Correct placement of the probe
Fig. 4 B Probe placed at an angle
X
Fig. 4 C Hall Effect sensor not perpendicular
X
2.4 Pie gages
Fig. 5 Pie gage
Copper side
Triangular segments on the opposite side
Pie gages are disks of high permeability material divided into triangular segments
separated by known gaps. The gaps are typically filled with a nonmagnetic material, to
protect the integrity of the gap and to strengthen the disk. The testing surface is coated
with a nonmagnetic (copper) layer.
This device is used to determine the approximate orientation and, to a limited extent,
indicate the adequacy of field strength. However, they do not measure the internal field
strength of the test object. It merely detects external fields in the vicinity of test object.
The presence of multiple gaps at different orientations helps reveal the approximate
orientation of the magnetic flux. Slots perpendicular to the flux lines produce the distinct
indications. Slots parallel to the flux lines produce little or nothing (see figure 6). [4]
Fig. 6 Orientation of magnetic flux
Pie gage used in Head Shot
with slot direction
perpendicular to flux lines.
Copper side must face up.
Magnetic field direction in and
around the article
2.5 Shims Gages (Indicators)
Shim indicators are thin foils of high permeability material containing well-controlled
notch discontinuities (see fig. 7). Frequently, multiple shims are used at different
locations and different orientations on the article to examine the magnetic field
distribution.
Fig. 7 Shim Gages
[1]
One popular version of the shim indicator is a strip of high permeability magnetic
material containing three slots of different widths. The strip is placed in contact with the
testing surface and shares flux with the test object. For the purposes of producing test
indications, the slots in the strip act as if they were cracks in the test object.
Shims are sometimes called paste-on discontinuities because they must be attached to the
test object with pressure sensitive tape. Shims are used most often during the
development of test procedures, where they help indicate the relative strength and
direction of a magnetic field for a particular test configuration.
Because shims are often made of high permeability foils, they are generally small and
flexible enough to fit into complex test object geometries to help determine the adequacy
of field strength in these critical areas. Once the field distribution is found adequate, the
testing procedure is recorded and the components are tested with the parameters
established by the shims. [4]
Fig. 8 Shim gage used in a coil shot
[1]
2.6 Reference samples
DISTORTION OF FIELD DUE TO SHAPE. If there is an upset section along the length
of the article as shown in Fig. 9, the field tends to flow out into the upset portion, but
does not do so uniformly. The larger the relative diameters of the upset portion, the
farther will the field in this section depart from a strictly longitudinal direction.
When attempting to magnetize a part of irregular shape for the first time, such an analysis
of the probable path of the field should be made. As the shape of the part becomes more
complex, the problem becomes correspondingly more difficult. Sometimes separate coil
magnetization must be applied to various projections of the part to ensure proper field
direction at all locations. A satisfactory procedure can usually be worked out for the most
complicated shapes after some experimentation.
In predicting the direction, the field will take when magnetizing with a coil, it is well to
remember that flux lines always must close upon themselves to form a complete circuit,
and that tend to follow the path of lowest total reluctance. Further, because of the very
low reluctance of iron as compared to air, they will follow the iron path as far as possible.
CIRCULAR FIELDS. In predicting or determining the distribution and intensity of fields
produced by passing current through the part or through a central conductor threaded
through an opening in the part, a different set of rules applies. Some of these rules are
simple, and when applied to relatively simple shapes, they enable the operator to carry
out the inspection with the assurance that he has the correct fields. However, often in
complicated cross-sections, cut-and-try methods must still be resorted to. [5]
Fig 9 Complex shape shaft
South
Pole
Fig. 10a Steeples on a rotor body
North
Pole
Fig. 10b A 20mm crack was found in a steeple groove
Phased Array
Ultrasonic
Inspection was
the method used
to find a crack in
figure 10b. The
article’s
configuration
made the sizing
of the crack
difficult, so,
Eddy Current
Inspection was the method used to estimate the length of the flaw. A visual image of the
crack was obtained using Magnetic Particle inspection and a dental mirror.
3. Conclusions
It is easy to omit one fundamental stride when using the above-mentioned instruments.
Proper understanding or usage of these gages must, be inculcated in NDT personnel to
make optimum use of this NDT method in order to get indications that are sought.
Different codes give specific instructions on the usage of these instruments to quantify
field adequacy.
We can also conclude that articles of complex shape and dimensions are not only difficult
to inspect with Magnetic Particle Testing, but exhibit equal difficulty in measuring field
adequacy used prior to inspection. A combination of different NDT Methods is essential,
and when verifying the presence of an indication in complex shape articles. No NDT
method stands alone. They all work together to eliminate the chances of missing
detrimental indications that could otherwise be missed with one method.
Acknowledgements
For compilation of this paper, I feel a deep sense of gratitude to my colleagues at
Southern African Institute of Welding who shared their experience with me.
References
1. NDT Resource Centre web site, http://www.ndt-ed.org, Introduction to Magnetic
Particle Inspection.
2. Pacific Scientific-OECO, Gauss/Tesla meter Instruction Manual UN-01-229 Rev. D;
ECO 12745, Model 5060, pp 3-19, Figure 3-14.
3. ASME section v, Article 7, Appendix A, pp 153 A-750, 2011a
4. Nondestructive Testing Handbook, Second Edition, Volume Six Magnetic Particle
Testing, pp 345-346, 1989.
5. C. E. Betz, ‘Principles of Magnetic Particle Testing’, First Edition, February 1, 1967.
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