ASNT NDT Handbook Volume 8 Magnetic Testing

PART 3. Fixed Coatings for Test Indications1

A helpful but seldom used means of interest of speed, drying equipment must
recording magnetic particle test often be provided.
indications is the fixing coating. These
coatings are usually bulk materials that Fixing for Removal from
are gently sprayed onto the test object Test Object
surface. The coatings can fix a
discontinuity indication in one of two The other type of fixing coating is
ways: (1) by binding the indication firmly designed to be stripped off the test object
in place on the test object or (2) by lifting so that the actual indication is preserved
the indication for preservation elsewhere, elsewhere as a record of the test. These
much like a tape transfer. coatings are special materials containing a
release agent that allows the dried coating
Only certain materials can be used as to be lifted off the object surface. Usually
magnetic particle fixing coatings. Because a specially prepared pressure sensitive
test surfaces typically contain bath or strip is pressed over the area and used to
other materials (scale, oil or water), a pick up the coating with its entrapped
coating must adhere to the object surface indication. The strip provides support for
in spite of contaminants. Specially the coating and allows easier removal and
formulated materials are required for this storage.
characteristic.
With fluorescent indications, the usual
Application of Fixing precautions apply. Strips and coatings
Coatings may be fluorescent and can compromise
indication visibility.
It is best to have the test object surface as
clean and dry as possible before applying Quality of Fixing Coatings
the coating. In production operations,
this normally requires special drying Test indications fixed to the test object
equipment to complete the recording in a retain their validity indefinitely while the
reasonable amount of time. Water baths test object is inactive. The characteristics
are probably the easiest to fix because of a normal service environment can
they have shorter drying times than other quickly erode or mask typical test results
vehicles and because water does not emit fixed on the object surface. Fixed
noxious or flammable fumes. indications removed from the test object
must be stored with the same precautions
Fixing on Test Object as tape transfer records. Humidity and
temperature controls are needed to extend
One function of fixing coatings is to the life of fixed records.
protect the test indications so that
abrasion from mechanical handling does The pressure sensitive strips used for
not damage the test result. Coatings removing a fixed indication are special
designed for this purpose must be durable grades of expensive materials. Their life
after drying. expectancy is limited but longer than that
of low grade tapes often used for simple
Such coatings are typically applied by tape transfers. The removed test
spray and may be water based or organic indications may be mounted on paper
materials. The coating must dry before it backing, wrapped in foil and archived in a
can perform its protective function. In the cool, moist environment.

Recording of Magnetic Particle Indications 241

PART 4. Alginate Impressions1

Alginate impression compounds are based compound paste. If the area of interest is
on mixtures of potassium alginate, large or has a complex geometry, the
calcium sulfate, sequestering agents such recording compound can be applied to a
as sodium phosphate, and fillers such as thin polyethylene sheet and the sheet is
silica, diatomaceous earth or calcium then smoothed over the full test area.
carbonate. When the compound is mixed Movement of the sheet or excessively
with a prescribed amount of water, a soft working the compound will smear the test
paste is formed. In 3 or 4 min, the paste indications.
becomes a rubbery solid with the
characteristic of accurately conforming to After the paste sets into a solid, the
and replicating, in negative, the surface to magnetic particles are gently separated
which it is applied. This physical record in from the object by slowly peeling the
one-to-one scale has proved valuable in polyethylene sheet away from the surface
many areas, as in dentistry. or gently dislodging the solid from its
cavity. In a successful impression
The impression solid also absorbs and recording, the discontinuity indication is
lifts particulate matter from the object transferred to the compound material in
surface and it is this absorption property its original size and shape. Test results
that makes alginate compounds useful for may then be examined and interpreted at
recording magnetic particle indications. a convenient location remote from the
The primary benefit of these compounds test object surface.
is their ability to lift particle indications
from test object locations that cannot be Archival Quality of
directly viewed by the inspector. Alginate Impression
Records
Application of Alginate
Impression Techniques Because of their size, alginate impressions
are difficult to retain for archival
To make alginate impressions, the test purposes. One procedure is to use the
object is first processed through a typical alginate impression to retrieve a
magnetic particle test procedure. discontinuity indication from an
Discontinuity indications are allowed to inaccessible location and to then
form normally. photograph the indication for filing
purposes. Supplementary documentation
After the test, the alginate impression details the magnetic particle test
compound is mixed according to the technique, the impression recording
instructions of the manufacturer. The procedure and the photographic data
mixed compound is immediately applied required for complete archival record
to the inspected part. The test object does keeping.
not have to be dried before application of
the recording media.

If the area of interest is a cavity, it
should immediately be filled with the

242 Magnetic Testing

PART 5. Digital Photography and Image
Archiving2

Digital photography is a valuable sort test photographs later. A ruler or
inspection tool. It can be used to record tape measure in the photograph will
the test object in any method but is provide scale.
especially valuable to preserve test results 3. Photograph the reference standard at
in qualitative surface methods — liquid the time of inspection and include its
penetrant, magnetic particle and visual image with the documentation.
testing. Test results can be saved as an 4. Specify or describe photography in the
archive of the inspection, as is done with written procedures.
radiographs and ultrasonic displays. The visible characteristics of
photographs are affected by the
In the twentieth century, articles about photographer’s choice of settings. Some
photography of surface tests featured options are hardware specific and can be
advice specific to photographic film.1,3,4 selected at the moment a test is
Digital techniques have obviated much of documented — zoom, for example.
this information. Camera models change every year: for
desired settings, inspectors should consult
Also, in liquid penetrant and magnetic the owner’s manual of the camera used. A
particle testing, surface indications have good rule of thumb is that the resulting
been documented by using transfers, image should provide visible information
strips of cellophane or transparent like that in the reference standards used
adhesive tape, to lift indications and for discontinuity evaluation.
attach them to reports.1,5 These transfers Some conditions are controlled at the
had drawbacks: photocopy machines time of photography — illumination, for
picked up glare from the tape, and lifting example. Some photographic settings can
was impractical for wet techniques. be modified later — a color image may be
Fluorescence itself would not be visible changed to gray scale for printing in a
later and could not be photocopied. Film manual, for example.
photography also kept the inspector from
judging the adequacy of an image until Photographing of
the film was developed. Digital Indications
photography provides an immediate
image so that a photograph can be Photography is art. Digital technology has
retaken immediately if an image is made it faster, easier and less expensive
unsatisfactory, without having to return than it was in the twentieth century, but
to the site or repeat a test. doing it well requires training and
practice. Basic principles still apply, with
Documentation changes due to the digital medium. Books
are available on the subject.6,7
Inspectors need to save images of test
objects for various reasons — comparison, Compact Camera versus Reflex
process control, training, documentation Cameras
and quality assurance. Photographs
demonstrate that the job was performed. There are two general types of digital
The following measures can make cameras in use today. They are the
photography a useful part of the single-lens reflex and the compact
inspection. camera. The single lens reflex is more
sophisticated and more versatile. Its
1. Few cameras let the operator name special feature is that the scene is viewed
files, so keep a written log of through the lens by a system of mirrors
photograph file names, who and prisms. This gives a very clear view of
performed the tests, when, test 95 percent or more of the picture area.
parameters and any circumstances of These cameras have a liquid crystal
interest. display on the back where images already
in the camera and operational menus can
2. Write information such as the test be viewed. These cameras use
number, location and time — interchangeable lenses allowing a large
information like that used to identify
radiographs — on a small card or piece
of paper and place it by the test object
within the camera’s field of view. This
step makes it easier to identify and

Recording of Magnetic Particle Indications 243

selection of focal lengths. Most (20 in.). Closer work needs a macro lens,
professionals use this type of camera. close-up screw-on lenses or extension
tube. Most compact cameras will focus
The other type of camera is the down to 75 mm (3 in.) so special
compact camera. Here, the scene is viewed attachments are not necessary.
on the liquid crystal display on the back
of the camera. Some of these cameras also Very close-up photography usually
allow viewing with another liquid crystal means very limited depth of field. If a test
display in an eyepiece or by an optical exposure shows some desired areas are out
viewer above the lens. This optical of focus, depth of field can be increased
eyepiece shows only about two thirds of by using a smaller lens aperture (higher
the picture area and does not exactly align f-stop). A smaller aperture may require a
with the lens, so this feature should not slower shutter speed and blurring from
be used for scene selection in indication camera movement.
photography. Compact cameras are the
normal consumer’s choice because they Digital cameras focus automatically,
are smaller, simpler and less expensive but the object of interest must be centered
than the digital single lens reflex. They in the camera’s field of view. Also, the
can often do an adequate job of range finder used for focusing in many
indication photography. cameras is acoustic and will not work
through transparent obstacles such as
Focal Length and Field of View window glass.

To capture maximum detail, the inspector Illumination
wants the area of interest to be as large as
possible within the camera’s field of view. Direct lighting from incandescent lamps
Many cameras offer optical zoom, letting can cause artifacts — light reflecting from
the user move the lens telescopically to the test object, for example, or refracted
“zoom in on” a subject and capture more light creating glare in the camera lens.
detail in a closeup. The same advantage Indirect lighting and overhead
can be achieved simply by moving closer illumination from fluorescent lamps help
to the subject. Some digital cameras minimize this problem.
achieve zoom inexpensively by something
called digital zoom, which amounts to Anybody taking photographs outdoors
cropping the image; this expedient is aware of light and avoids dark shadows.
sacrifices details because not all the pixels A portable flood lamp or camera flash
are retained. may remedy dim lighting for a test.

Camera focal length determines the Digital cameras adjust automatically to
angle of view and the depth of field. Short dim illumination, and subsequent image
focal lengths give wide angles of view; processing may salvage a dark
long focal lengths give narrow angles of photograph. However, a test should be
view. Short focal lengths give large depth performed only if reference standard
of field; long focal lengths give small indications are visible on-site and at the
depth of field. The closer the subject is to moment of inspection.
the camera, the smaller the depth of field
will be. Digital cameras are so sensitive that
flash is not often necessary. Flash
Indication photography is basically illumination with digital cameras should
close-up or macro photography, macro be avoided at close range, less than 0.6 m
meaning that the image is viewed in a (2 ft). Because most indication
scale larger than the original subject. For photographs will be made at this distance
indication photography at close range, or closer, such photographs will be
depth of field is a problem. Small depth of overexposed. Furthermore, in most cases
field may cause parts of the subject to be the flash is very close to the camera lens
out of focus, making the photographic and cannot be removed. Flash from such
record unacceptable. Therefore, camera equipment is sometimes reflected directly
equipment and settings should be used back into the camera, producing glare.
that keep the indication in focus. Many Therefore flash photographs of
cameras have a setting for macro indications will normally be made only if
photography that changes the lens to all else fails.
improve focus at close range. The macro
setting does not alter the number of Exposure Duration and Sensitivity
pixels.
Because a macro image is enlarged,
In general, focal lengths should be camera motion during exposure will be
short for indication photography. Short magnified and proper procedures are
focal lengths maximize depth of field and needed to avoid blurring. Briefer
permit closer working distances. On a exposures and image stabilization help
digital lens single-reflex camera with but do not eliminate the problem. Camera
typical midrange lens, the minimum motion can be eliminated by using a
focusing distance is around 500 mm tripod or by bracing the camera against a
solid object. If this is not possible, one

244 Magnetic Testing

must use the highest practical shutter Therefore, if one must err in exposure
speed. Even with image stabilization, with digital photography, underexposure
shutter speed should not exceed 0.03 s. is better than overexposure.
With a tripod, it is wise to use a cable
release or self timer to avoid moving the White Balance
camera while pressing the shutter release.
The self timer function lets the camera Photographs can have unnatural tints.
release the shutter without motion from When the picture is taken, this effect can
hand pressure. be mitigated through a control called
white balance on digital cameras.
Digital cameras let the sensitivity of the Unrealistic tints can be removed so that
camera be adjusted over a very wide objects that appear white on site are white
range. With film, sensitivity or exposure in the image. If skin appears too pink or
index could only be changed by changing too green, for instance, then the problem
the film, and even the most sensitive may be corrected through white balance.
films were not as sensitive as the digital The white balance control lets the
cameras. In more sensitive films, increased photographer adjust the tint of an image
grain size causes spots in the picture. by scrolling through a spectrum of tint
Unfortunately, digital cameras have the options in a single dial and selecting the
same problem. Photographs made with most realistic one.
high exposure sensitivities also show
graininess, called noise. This noise gets Most digital cameras also have a way
worse as sensitivity increases. True, one for the user to tell the camera under what
can make digital photographs in the dark, type of illumination a photograph is
but the photographs may be so grainy as being taken. White balance takes into
to be unacceptable. Thus, the account the colors of the light source and
photographer must compromise all the adjusts the relative amounts of red, green
above factors to get acceptable indication and blue primary colors so that neutral
photographs. colors are correct before the image is
recorded.
Effect of Exposure Duration on
Contrast Color can be balanced again later, in
image processing. The selection of the
The purpose of an indication is to make a RAW file format, discussed below, makes
discontinuity visible. This means making this easier.
the indication contrast strongly with the
background. Usually there is enough When photographing indications,
contrast, but for emphasis it is often color is often unimportant. Therefore,
desirable to increase this contrast. Most white balance is usually not critical as it is
digital cameras can be set to give more in most photography. In fact, gray scale
contrast. However, it is often easier and (black and white) indication photographs
more practical to adjust contrast with an may be preferable. Digital cameras
image editing program in the computer. normally record in color, but in some
cases they can be set to produce gray
Similarly, the inspector should try to scale. And it is easy for software to
expose the photograph properly. In convert images to gray scale.
indication photography, a bad exposure
may be redone unless the inspector or the Photography is a process which has
test object moves on before the problem many variables. It is necessary to do some
is noticed. A great aid to getting proper experimentation to get good results. As
exposure is the histogram display, usually equipment becomes better, the process
available via the menu. The histogram should become still easier and more
appears as a mountain and valley profile accurate.
in the display. The right side of the
display indicates the amount of bright Fluorescence
tones; the left side, dark tones. Ideally, the
display should look like a mountain more In many cases, the indications to be
or less in the middle. If the mountain is photographed will be fluorescent. The
off to either side of the display, the magnetic particle inspector does not want
photograph is either overexposed or to photograph ultraviolet radiation as
underexposed. Should this be the case, such but instead wants to photograph the
reset the camera and retake the shot if nonultraviolet radiation known as
possible. fluorescence. Ultraviolet radiation is
invisible to the human eye; to be seen, its
Badly underexposed shots can often be energy needs to be converted into visible
brightened to a useable degree with image light. A fluorescent magnetic particle
editing software, but excessive graininess indication is not ultraviolet, or else no
may be present. Software can often make one could see it. Similarly, photographs
slightly overexposed shots useable with called ultraviolet are usually visible
some loss of detail. Badly overexposed translations of ultraviolet originals.
areas cannot be corrected and are lost.
As in night photography, the ambient
area will be partially dark and bright

Recording of Magnetic Particle Indications 245

indications will stand out in the display of 800 × 600 pixels is called a super
photograph. The following measures may video graphics array. It is superseded in
help the inspector photograph fluorescent most computers but remains popular in
indications. mobile devices.

1. Try night camera settings if the image Each pixel is part of a larger image, and
is taken in a dark area. more samples provide a more accurate
representation of the original. Pixels are
2. Notice if wavelengths or colors are rectangular and on computer monitors are
being filtered or enhanced, either with usually square. The square shape is an
hardware (by a screen at the lens, for artifact of the display; the original
instance) or digitally. A test object electronic datum is dimensionless. Pixels
viewed with protective goggles may may be translated into dots for printing.
look different in a photograph. Dot and pixel are interchangeable terms:
for example, dots per inch (DPI) and
3. Consult reference standards with pixels per inch (PPI) are equivalent
known discontinuities to select camera measures of resolution, discussed below.
settings and filters.
There are two classes of graphic file
4. Keep notes about circumstances that formats. Line drawings use mathematical
might affect discontinuity evaluation. calculations called vectors to plot lines and
are not considered here. Raster images, or
Dark Background bit mapped images, assign hue and
intensity values to each pixel and are used
To prevent a haze on dark background, for photography. Table 1 lists common file
the ultraviolet radiation must be filtered. formats for digital images. Some cameras
A standard photographic ultraviolet
radiation filter is effective for this TABLE 1. File formats of computer images.
purpose. If the camera has filter threads
on the front of the lens, a filter of the Designation Format
proper size can easily be attached.
Unfortunately, many compact cameras do Lossless Raster Formats
not have such threads. In such cases, it is
necessary to improvise. An ultraviolet BMP bit mapped picture
filter can be held over the lens during
exposure. It is also possible to tape a filter DIB device independent bitmap
over the lens. However, most compact
cameras have a battery saving feature PIC three-letter file extension for PICT
which shuts off the camera after a short
period, and often retracts the lens into the PCT three-letter file extension for PICT
body at the same time. Tape placed
around the lens body may then prevent PICT picture metafile
the lens from retracting and cause a
mechanical failure. If a filter is attached RAW extension for undeveloped image from digital camera
by tape, the tape should be on the camera (virtually lossless)
body, not the lens. Also, the shutdown
time might be adjustable via a menu to TIF three-letter file extension for TIFF
minimize the problem at the cost of
decreased battery life. TIFF tagged image file format

When photographing fluorescent Lossy Raster Formats
indications, the background should be
dark but still visible. One should be able GIF graphics interchange format, native to CompuServe®
to see the test object as well as the internet service provider
indications. Usually, there is enough
visible light leaking through the IFF Interchange file format
ultraviolet filters both on the lamp and
the camera to make the test object visible. JFF JPEG file format
This illumination can often be adjusted
with the image adjusting software. It is JFIF JPEG file interchange format
also possible to use a brief visible light
exposure by turning on a white light in JIF JPEG image format
the area.
JPEG Joint Photographic Experts Group, a standards
Pixels organization

Digital photographs are designed for a JPG three-letter file extension for JPEG
computer display. A display consists of a
grid of squares called picture elements, or PBM portable bit map
pixels. Displays range from 320 × 240
pixels to over 3840 × 2400. A standard PGM portable gray map

PNG portable network graphics

RAW extension for undeveloped image from digital camera
(virtually lossless)

XBM X bit map

Vector Formats (for line drawings)

CGM computer graphics metafile

EMF enhanced metafile, version of WMF

EPS encapsulated postscript, native to Adobe® software

PDF portable document format, native to Adobe®
software

SVG scalable vector graphics

SWF Shockwave Flash®

246 Magnetic Testing

use proprietary formats similar to these The purpose of the camera RAW format
common file formats. is to record all of the exact data from the
camera’s sensor and any other metadata,
A bit map defines an image and color such as time and camera settings. Camera
for all pixels in the image. TIFF and JPG RAW files are much larger than JPG files.
files are bitmapped. A bit map does not Some RAW formats do not use
need to contain color coded information compression; others implement lossy data
for each pixel in every row. It needs to compression to reduce file size without
contain information indicating a new affecting image quality. Camera RAW
color as the display scans along the row. formats avoid the compression artifacts
For this reason, an image with much solid inherent in JPG files. RAW files have
color requires a small bit map. many advantages over JPG files, including
the following:
The term pixel can also mean an
element in a sensor array. An array is a 1. Image quality is higher.
group of adjacent receptors each of which 2. RAW conversion software gives the
records a single point of radiation and
which together provide a composite user fine control of more parameters
picture. Such arrays are in cameras and such as lightness, color balance, hue
other radiation detectors (X-ray and saturation.
fluoroscopes and infrared cameras) at 3. Edits are performed nondestructively,
various wavelengths. without degrading image quality.
4. The image is not compressed; if
File Formats lossless, the maximum amount of
image detail is always kept within the
File formatting has been evolving rapidly file.
with advances in digital technology. Camera manufacturers use different
Several standards may be consulted or versions of RAW format. As of 2008, there
referenced for digital image archives.8-11 is no widely accepted standard for RAW
format, so more specialized software may
The file formats for digital photographs be required to open RAW files than to
fall into two classes: lossy and lossless open standardized formats like JPG or
(Table 1). Lossy formats compress files to TIFF. Cameras that support RAW files
save hard disk space. A format is said to typically come with proprietary software
be lossy when digital information and for conversion of their RAW format to
hence image detail are lost. JPG files are TIFF or JPG. To retrieve an image from a
lossy and are used in many digital RAW file, the data must first be converted
cameras and web pages because their into a red, green and blue (RGB) image.
compression makes small files and Because of proprietary formatting,
because they can be viewed on personal some formats listed in Table 1 are in fact
computers. Lossless TIFF and PICT and families of related formats. There are
nearly lossless RAW formats provide several forms of TIFF and of PNG, for
similar files but use more disk memory. example, and a given version may not
work with a given program. Before using
Inspectors who archive digital new equipment, the inspector will want
photographs need to be aware that to check that test images can be
converting files from a lossless to a lossy transferred to and viewed with hardware
format (for example, from a TIFF to a JPG) at the archiving location.
can degrade images so that some details,
such as crack tips, are harder to see. There Image Characteristics
are several occasions when this sort of
image degradation can take place: Many computer programs are widely
(1) when a file is changed from a lossless available for processing digital images.
to a lossy format, such as TIFF to JPG; They let the user adjust settings that, in
(2) when a lossy image is converted to a the twentieth century, used to rely on the
file with lower size or resolution; (3) when skill of a photographer at the moment of
an image is imported into a document image capture or film development.
created by a word processing or layout
program. This reduction may be Color
inadvertent, caused by program settings.
Of course, an image conversion may have Hue. A color is described by its hue,
no effect on the usefulness of the saturation and value (HSV). Hue is what is
resulting image: though degraded, it may normally thought of as color. A digital
still show all features of interest. color in a computer display is a blend of
three primary hues: red, green and blue.
Procedures need to be clear so that Saturation is the intensity or dominance
they can be followed. The procedures of a hue: a hue can be increased or
describing how photographs are archived diminished in an image just as black
should include definitions of lossy and
lossless and of file formats so that the
results can be interpreted later.

Recording of Magnetic Particle Indications 247

versus white can be adjusted in an image lines, for instance, and not as one thicker
through all shades of gray. Value is the line. Resolution depends on the number
lightness or darkness of a hue and is of pixels in the image and hence on its
usually called brightness when the three file size. In Fig. 2, two versions of the
hues are combined in white light. Image same image show the difference between
processing software permits the intensity high and low resolution. Details in an
and value of these hues to be adjusted image cannot be restored simply by
independently within each image. converting it to a higher resolution or
Conversion to Gray. If the image is from a lossy to a lossless format. Once an
converted to gray scale, as in a report image’s resolution is reduced, details are
printout, care must be taken that features lost. If a display’s resolution exceeds an
of interest remain visible: a fluorescent image’s resolution, the image will look
nondestructive test indication in brilliant blurry.
green may disappear entirely if it changes FIGURE 1. Example of scaling of digital
to a shade of gray like that in an adjacent image.
area.
Color Balance. The camera setting of 228 pixels gives
white balance, discussed above, can resolution of
improve the realism of color photographs 58 pixels·cm–1
by preventing unnatural tints. Image (144 DPI)
processing programs also enable
improvement of color balance. Program 50 mm (2 in.)
menus may refer to it by other terms, Scaling
such as color intensity or hue saturation. 228 pixels gives resolution of
29 pixels·cm–1 (72 DPI)
If a photograph is taken in a digital
camera’s RAW file format, image colors 100 mm (4 in.)
can be fixed without information loss.
Most cameras take RAW photos in FIGURE 2. Visible light photograph showing
twelve-bit color (4096 shades per color) magnetic particle indication under
instead of eight-bit color (256 shades per ultraviolet lamp: (a) high resolution, at
color), enabling powerful balance 40 pixels·cm–1 (100 PPI); (b) low resolution,
adjustments without visible loss in at 10 pixels·cm–1 (25 PPI). Lower resolution
quality. blurs details. Lowering resolution sometimes
enhances contrast or increases hue intensity
Size — side effects better achieved by image
processes that do not sacrifice detail.
The more pixels in an image, the more (a)
information it carries. An image’s quantity
of pixels is its true size in the (b)
dimensionless electronic space of the
computer.

To be viewed, the image must be
translated to a medium such as a
computer display or printed page, where
details become visible to human eyes in
order to be interpreted. In a process
known as scaling, the pixels can be
compressed or spread out to fit the desired
viewing surface (Fig. 1). Scaling affects
resolution, discussed below.

Image size and resolution are usually
expressed in terms of image width, in the
horizontal dimension. The size and
resolution of the vertical scale can be
manipulated independently but to
prevent distortion usually undergo the
same processes as the horizontal.

The number of pixels in an image can
be decreased or increased in a step called
resampling or conversion. Resampling
cannot add more data to a photograph
once it has been shot.

Resolution

The resolution of an image is its ability to
distinguish small adjacent objects. Two
thin adjacent lines appear as two thin

248 Magnetic Testing

Resolution is measured in dots per inch adjusted to compensate for poor visibility
(DPI) for printers or square pixels per inch on overcast days. Too little brightness can
(PPI) for computer displays, measured make a picture dark as night, and too
along the horizontal axis. Alternatives much brightness can make it washed out
using SI metric units are to specify pixels so no features stand out.
per centimeter (pixels·cm–1) or the pixel
width in micrometers (µm). Saturation. In a color image, saturation is
the intensity of a hue. A hue of zero
Because the number of pixels in a saturation is black or gray. If all three hues
scalable image is fixed, however, an have zero saturation, the image is
expression of resolution is meaningless colorless, black.
unless the viewed image size is also Contrast. The setting of contrast controls
specified. If a given digital image is the degree of difference between light
reduced to half its width, for instance, the intensities at different points in the
number of pixels per unit of width and image. Like brightness, contrast is
the resolution are doubled (Fig. 1). The adjusted to affect all pixels in an image at
data in the image file may remain the once. As contrast is increased, for
same, but smaller size makes details example, a dark indication becomes easier
harder to see. Someone processing digital to see on a bright background and a
images must make decisions balancing bright indication becomes more visible on
image size versus resolution. a dark background.

Brightness Contrast and brightness interact
closely, and the inspector must sometimes
Brightness is the common term for the adjust them to find the correct settings for
luminance of light emitted by each pixel. each photograph. Figure 3 shows the
Usually, one control setting adjusts effect of brightness and contrast on an
brightness to affect all pixels in an image image.
simultaneously. This parameter can be

FIGURE 3. Photograph of cracked rod: (a) high contrast, high brightness; (b) high contrast, low brightness; (c) low contrast,
high brightness; (d) low contrast, low brightness.

(a) (b) (c) (d)

Recording of Magnetic Particle Indications 249

Image Integrity Closing

Inspectors may be tempted to enhance In practice, the viewing quality of a
images to justify accept/reject decisions. digital photograph depends on several
This temptation should be treated with factors.
caution. The inspector may consider
several rules of thumb. 1. The vision acuity of the viewing
human can be impaired by medical
1. Local features including indications conditions such as myopia and color
must not be exaggerated or obscured blindness.
using image processing tools having
descriptors such as pencil, eraser, flood, 2. The visibility of a shot is affected by
spray or smudge. These alterations of things such as light and camera
an image could be considered position.
falsification of test results.
3. The useful detail in an image is
2. Image settings such as zoom, affected by its size and by its quality in
brightness and contrast may be freely terms of features such as contrast and
adjusted, as photographers have done resolution.
for generations. Technology has given the inspector

3. A photograph of the reference options beyond the scope of the present
standard should be included in the discussion: (1) light measurement and
archive and used to evaluate illumination, (2) optical gaging and range
modifications made to test images. finding, (3) photography sensitive to
How do the same modifications affect other wavelengths, including ultraviolet
discontinuity visibility in the reference and infrared, and (4) video
standard? documentation of inspection, more for
For some applications, a written procedures than for indications. These
and other options depend on hardware.
statement on image processing may be a The present discussion also does not
desirable part of an inspection procedure. consider marine environments or
Accept/reject decisions are usually made extremes of altitude or temperature.
at the moment of inspection and in the
presence of the test object. If some Most inspectors already use a computer
decisions are made later, that protocol can for communication, for research and for
be documented in the procedure. writing procedures. With the investment
of a few hundred dollars for a camera and
In some archives, a file’s metadata, software, an inspector can add digital
including its creation date, are part of its photography to the inspection options.
documentation. These metadata can be With practice and planning, digital
lost if an image is saved in another photography can be a valuable tool for
format. inspection and quality control.

250 Magnetic Testing

PART 6. Magnetic Rubber1

Magnetic rubber provides a Its principal applications are in certain
nondestructive test technique for problem areas having (1) limited visual or
detecting cracks or other discontinuities mechanical accessibility, (2) coated
on or near the surface of ferromagnetic surfaces, (3) complex shapes or poor
materials. Magnetic rubber is a mixture of surface conditions and (4) discontinuities
silicones and petroleum hydrocarbon that require magnification for detection
distillate. Its controlled viscosity rubber and interpretation.
base holds a dispersion of magnetic
particles and cures at room temperature. FIGURE 5. Magnetic leakage at material
The test object is magnetized with the discontinuity causes particles in magnetic
uncured rubber covering the area of rubber to migrate and concentrate over
interest. The magnetic particles then discontinuity: (a) yoke positioning; (b) cross
migrate to the leakage field caused by a section of inspected hole.
discontinuity. (a) Electromagnet

As the rubber cures, discontinuity
indications remain in place on the surface
of the rubber touching the test object.
After curing, the rubber is removed from
the test object and the test results are
interpreted in a convenient location.

The technique is described in SAE AMS I
83387.12 Figures 4 to 6 depict the
magnetic rubber inspection process.
Figures 7 and 8 show the preparation for
magnetic rubber tests of horizontal and
vertical fastener holes.

FIGURE 4. Typical magnetic test using (b)
magnetic rubber: (a) rubber application;
(b) cross section of inspected hole. Electromagnet
(a) Syringe

Dam Crack Discontinuity
Duct seal Tape
putty
(b) FIGURE 6. After curing, magnetic rubber replica is removed
Duct seal putty from test surface, revealing crack as dark line. Other surface
Dam discontinuities and conditions are also shown.

Crack

Rubber replica

Discontinuity Discontinuity

Crack

Tape

Recording of Magnetic Particle Indications 251

Magnetic Rubber Supplies There are three formulations of
magnetic rubber (Tables 3 to 5). The more
Magnetic rubber techniques require viscous and slower curing provides
special supplies and equipment (Table 2) medium sensitivity. The one with lowest
Necessary cleaning materials include viscosity is most sensitive. A slightly more
organic solvents. The solvents must be viscous and very slightly less sensitive
able to dissolve oil and grease and must formula has a yellow coloring agent.
be stored with appropriate precautions. Yellow makes the indications more
Water, air pressure and strong detergents noticeable to the inspector reading the
may also be used for cleaning. replica, thereby improving the probability
of detection of very small cracks.
FIGURE 7. Horizontal hole preparation for magnetic rubber
inspection. Magnetizing Equipment

Syringe Magnetic rubber test techniques require
the same kinds of magnetizing systems
Typical used for dry technique magnetic particle
identification tag tests. The system may include yoke,
prods, clamps, coils or central conductors
Air vent Slots for paper with alternating or direct current. The
Magnetic identification tag direct current yoke is preferred for
magnetization. Permanent magnets may
rubber Aluminum be used in certain applications.
Tape cup dam
Duct seal putty TABLE 2. Materials and accessories for magnetic rubber
inspection.
FIGURE 8. Vertical hole preparation for magnetic rubber
inspection: (a) rubber application; (b) site identification. Basic Materials
Magnetic rubber compound
(a) Catalysts (dibutyl tin dilaurate and stannous octoate)
Cure stabilizer (water with acetone)
Syringe Duct seal putty
Masking or duct tape
Aluminum or plastic Magnetic Slots for paper Aluminum foil
cylinder dam rubber identification tag Release agent (not silicone based) to help remove replicas

Duct seal putty Material Preparation Accessories
Small disposable cups (paper or plastic)
Balance scale (0.1 to 1000 g)
Disposable syringes, 20 to 50 mL (20 to 50 cm3)
Disposable stirrers

Optional Special Equipment
Central conductor and dam (assorted sizes)
Electromagnetic yoke with fixed or articulated legs
Electronic magnetic field indicator (tesla meter)
Geometric pole pieces that fit test object
Mechanical shaker (such as paint shaker)
Microscope (7× to 10×) for tiny indications
Permanent bar magnets
Vacuum chamber

(b) Tape TABLE 3. Magnetic rubber formulations.
Typical identification tag
Slots for paper
identification tag Viscosity Curing Sensitivity
Duct seal putty
50 Pa·s (500 cP) slow medium
10.5 Pa·s (105 cP) fast high, colored
10 Pa·s (100 cP) fast high

Tape

252 Magnetic Testing

Test Object Preparation putty, a reservoir is formed for the
magnetic rubber. Virtually any test object
Using the chosen cleaning solvent, oil, configuration can be prepared for
grease and soils are removed from the test magnetic rubber inspection using a dam
surface. Also remove metal burrs, smears assembly. Upside down surfaces can be
and sharp edges. Be sure the surface is dry tested by building a reservoir beneath the
and free of loose debris before proceeding. test area and pressure filling it with
magnetic rubber. A vent hole must be
If needed, a central conductor and dam provided in the reservoir to prevent
assembly (Fig. 9) are then installed and entrapment of air.
connected. Using tape, foil and duct seal

TABLE 4. Recommended magnetic field intensities and durations for magnetic rubber. Variations are
required for specific applications.

Tested Viscosity Field Intensity Magnetization Duration
Area Pa·s (cP) mT (G) in Each Direction (s)

Holes, uncoated 50 (500) 5 to 10 (50 to 100) 30
Holes, coated 10.5 (105) 3 to 5 (30 to 50) 30
10 (100) 3 to 5 (30 to 50) 30
same as uncoated same as uncoated longer than uncoated; depending
on coating thickness
Surfaces, uncoated 50 (500) 15 (150) 60
Holes, coated 50 (500) 10 (100) 180
50 (500) 600
10 (100) 5 (50) 30
10.5 (105) 10 (100) 60
10.5 (105) 120
same as uncoated 5 (50) longer than uncoated; depending
3 (30) on coating thickness
same as uncoated

TABLE 5. Formula for magnetic rubber inspection material FIGURE 9. Typical central conductor setup for magnetic
starting with 10 mL (10 cm3) of rubber base. rubber inspection of horizontal or vertical fastener holes:
(a) cutaway view of fastener; (b) cross section through the
T-12 T-9 Cure Pot Cure fastener hole.
Catalysta Catalystb Stabilizerc Life Time
(drops) (drops) (drops) (min) (min) (a) Syringe

Formula for High Viscosity, 50 Pa·s (500 cP) Fastener hole
Magnetic through structure
rubber
10d 0d 2 8 60
120
5 0 2 15 240 Vent

2 0 2 30 15 to 20 Dam Wing nut
10 to 15 Spacer
Formula for Medium Viscosity, 10.5 Pa·s (105 cP)
5 to 10
220 5 75e

330 3 15 to 20 Central conductor
10 and dam assembly
440 2 75e

15e 0 2 10e Dam Central
conductor
Formula for Low Viscosity, 10 Pa·s (100 cP)

110 5

220 3

15e 0 2 10e To power supply

a. Dibutyl tin dilaurate. (b)
b. Stannous octoate.
c. Water with acetone.
d. Do not use more catalyst than indicated for 1 h cure. Use only this catalyst

system.
e. Very sensitive formula for long pot life.

Recording of Magnetic Particle Indications 253

Magnetization and Current Good magnetic contact is critical.
Parameters Magnetic field intensity is greatly reduced
with poor contact between the test object
Magnetization may be applied with and magnetizing yoke. To increase
(1) portable electromagnets or yokes, contact, auxiliary pole pieces may be
(2) central conductors, (3) conventional machined from soft iron and attached to
magnetic particle units with clamps, prods the poles of electromagnets. Pole pieces
or coils or (4) permanent magnets. Direct should be designed to have the smallest
current yokes are preferred. In areas of possible reduction of cross section
limited access, steel extensions or pole consistent with space requirements.
pieces may be used to extend the Figures 10 to 13 demonstrate good and
magnetic circuit to include the test object. bad magnetic contact arrangements and
Permanent magnets may be useful in the use of pole pieces.
certain specialized situations where test
object shape makes magnetization with Magnetic field requirements include
yokes difficult. The magnetic fields both field intensity, duration and
produced by permanent magnets are often direction of the field. Field intensity and
quite low and sometimes unreliable. duration are detailed in Table 4.
Permanent magnets should be used only
if specific procedures have been developed Determine the direction of the field.
using reference standards with known Cracks and other discontinuities are
discontinuities. displayed more strongly when they lie
perpendicular to the magnetic lines of
Central conductors are best suited for force. When the discontinuity direction is
fastener and attachment holes, uncertain, the magnetism should be
particularly when there are layers of applied from two directions to increase
material requiring replication. reliability: after magnetizing in one
direction, the magnetization source is
FIGURE 10. Good and bad magnetic contact rotated 90 degrees and applied again.
for magnetic rubber inspection: (a) very When the direction of a discontinuity is
little magnetic flux getting through known, only one magnetizing direction is
aluminum to steel; (b) better magnetic field required.
from leaving steel bolts in place; (c) best
magnetic field from magnetizing from steel FIGURE 11. Improved contact and improved
side. magnetic field using pole pieces: (a) poor
(a) contact (field partially lost); (b) field
improved with addition of pole pieces;
(c) magnetic field and contact improved
with correct pole piece configuration.
(a)

Aluminum
Steel

(b)
(b)

Aluminum Auxiliary pole piece
Steel

(c)
(c)

Aluminum

Steel

Auxiliary pole piece

254 Magnetic Testing

Measure the magnetic field intensity material than can be used within the
using a tesla meter by placing the probe allowable magnetization period.
in the hole or on the surface to be
inspected. The transverse probe can Deaerate the material if air bubbles are
measure the field parallel to the part a problem. This is done by placing the
surface and will be used most often. mixed material in a vacuum chamber and
reducing the pressure to between 85 and
Preparing and Applying of 100 kPa (25 to 30 in. Hg) for 60 to 120 s.
Magnetic Rubber
Catalyst and Stabilizer
Magnetic rubber is prepared according to
manufacturer specifications and is mixed The catalyst, stabilizer or both are added
only a few minutes or seconds before as in Table 5. Higher humidity or higher
application. temperature will increase the cure rate.
When temperature or humidity change,
First, to resuspend the magnetic or when material from a different batch is
particles, the base rubber is thoroughly used first, mixing a small test batch to
mixed with a wooden tongue depressor or determine optimum ratios of catalyst to
spatula. Mixing should continue until the base material is recommended. If the cure
material contains no streaks or color is too fast and the rubber starts to gel
variations. Materials that have settled before magnetization is complete, the
should be agitated by a mechanical process will need to be repeated. Using a
shaker, such as a paint shaker. tongue depressor or equivalent,
thoroughly stir the mixture. Avoid
Weigh or measure the specified amount whipping air into the mixture.
of magnetic rubber into a paper cup or
other disposable container. About 1 g of Applying of Rubber to Test
base material is equivalent to 1 mL (1 cm3) Object
of material. Do not measure out more
When testing holes with scored surfaces,
FIGURE 12. Improved contact and improved small diameters or unusual
magnetic field using pole pieces: (a) very configurations, the test area may be
poor contact (field partially lost); (b) radius coated with a thin layer of petrolatum, or
magnetism much improved with pole piece. release agent to aid removal of the replica.
(a) The release agent must not be of the
silicone variety. Rubber may be applied by

FIGURE 13. Correct positioning of pole
pieces: (a) pole pieces reduce cross section
unnecessarily; (b) greater cross section
reduces loss of field.
(a)

(b)

Auxiliary pole piece

(b)

Recording of Magnetic Particle Indications 255

pouring from the disposable container if cleaned with solvent or vapor degreased if
sufficient access is available. Inaccessible necessary.
areas, or overhead applications, may be
filled with a disposable syringe. Such areas Archival Quality of
must be equipped with vent holes to let Magnetic Rubber Records
air escape and let rubber completely fill
the cavity. These vent holes must often be The magnetic rubber technique is not a
closed with tape or putty after filling to means of recording indications from a
prevent escape of the rubber compound typical magnetic particle test. It is a
different test procedure, requiring its own
Magnetization commences and must test parameters and its own specialized
be completed in the allotted time. Allow equipment.
the magnetic rubber to cure for the time
specified by the chosen formula. Avoid Advantages of the technique are as
movement of the test object and follows.
contamination of the magnetic rubber
while it cures. To determine that the 1. Inspections may be performed in areas
magnetic rubber is completely cured, of limited visual or mechanical access.
lightly touch the replica or the mixture
remaining in the mixing container to 2. Inspections may be performed
check that it is no longer sticky. through paint, coatings and on parts
of any shape.
To remove the replica, first remove
tape, aluminum dam, duct seal putty and 3. Inspections may be performed using
central conductor dam assembly, then low levels of magnetization by varying
carefully extract the replica from the test cure times of the rubber.
area. The replica must be identified by a
tag or label or by placing it in a labeled 4. Inspections may be performed in very
container. small or threaded surfaces.

The replica is then visually examined 5. Magnetic rubber provides a permanent
for its condition and discontinuity replica of the test object surface as well
indications. A low powered microscope as the particle discontinuity
(at least 10×) will aid in locating minute indications. This record may be
discontinuities in the inspection site. retained intact for as long as space is
After inspection, test objects may be available. Magnetic rubber replicas
demagnetized per instructions and may also be photographed for archival
purposes.

256 Magnetic Testing

References

1. Schmidt, J.T. Section 11, “Recording of 7. Kelby, S. The Digital Photography Book.
Magnetic Particle Test Indications.” Berkeley, CA: Peachpit (2006).
Nondestructive Testing Handbook,
second edition: Vol. 6, Magnetic Particle 8. ANSI/I3A IT10, Photography — Digital
Testing. Columbus, OH: American Still Cameras. Washington, DC:
Society for Nondestructive Testing American National Standards
(1989): p 271-290. Institute (2004).

2. Moore, P.O. “Digital Photography and 9. ISO DIS 12232, Photography — Digital
Image Archiving.” Materials Evaluation. Still Cameras — Determination of
Vol. 66, No. 7. Columbus, OH: Exposure Index, ISO Speed Ratings,
American Society for Nondestructive Standard Output Sensitivity, and
Testing (July 2008): p 717-722. Recommended Exposure Index. Geneva,
Switzerland: International
3. Schmidt, J.T. “Color Photography of Organization for Standardization
Magnetic Particle and Penetrant (2004).
Indications.” Materials Evaluation.
Vol. 31, No. 3. Columbus, OH: 10. ISO/IEC 10918, Information Technology
American Society for Nondestructive — Digital Compression and Coding of
Testing (March 1973): p 39-42. Continuous-Tone Still Images. Geneva,
Switzerland: International
4. Mosher, T.A. “Ultraviolet Photography: Organization for Standardization
A System for the Nonprofessional (2005).
Photographer.” Materials Evaluation.
Vol. 45, No. 7. Columbus, OH: 11. ISO TS 22028, Photography and Graphic
American Society for Nondestructive Technology — Extended Colour Encodings
Testing (July 1987): p 778-779, 781. for Digital Image Storage, Manipulation
and Interchange. Geneva, Switzerland:
5. Stephens, P. “Liquid Penetrant Panel International Organization for
Calibration.” Materials Evaluation. Standardization (2006).
Vol. 65, No. 10. Columbus, OH:
American Society for Nondestructive 12. SAE AMS I 83387, Inspection Process,
Testing (October 2007): p 1016-1018. Magnetic Rubber. Warrendale, PA: SAE
International (1999).
6. Ang, T. Advanced Digital Photography:
Techniques and Tips for Creating
Professional-Quality Images, revised
edition. Garden City, NY: Amphoto
Books (2007).

Recording of Magnetic Particle Indications 257

10

CHAPTER

Reference Standards for
Magnetic Particle Testing1

Gina R. Caudill, Naval Air Warfare Center, Patuxent
River, Maryland
Thomas S. Jones, Alcoa Howmet, Whitehall, Michigan
Roderic K. Stanley, NDE Information Consultants,
Houston, Texas

PART 1. Fundamentals of Reference Standards for
Magnetic Particle Testing

Nondestructive tests are typically designed enormous complexity of magnetic fields
to reveal the presence of discontinuities or and their interactions with ferromagnetic
to measure specific properties in a components. Unfortunately, rules of
structure or material. The discontinuity thumb have sometimes been used
may be an anomaly in a homogeneous exclusively for determining the adequacy
material or a change in one of the of certain test setups. As with most
material’s properties, such as thickness, empirical data, the rules developed for
hardness or density. magnetic particle testing should be used
with caution and with an understanding
Before testing, some form of artificial of their limits. Caution dictates that
discontinuity or reference standard is regular system monitoring be used to
commonly used to verify the operation of verify acceptable test sensitivity — most
a magnetic particle testing system. This existing formulas cause overmagnetization
verification is performed (1) to provide a of some test objects.
sensitivity check of the testing procedure
and (2) to establish a known correlation It is easy to demonstrate the
between the response of the test system, connection between misused rules of
the magnitude of the material property or magnetization and inaccurate testing.
the severity of a discontinuity. Figure 1 shows where a null field is
produced at the fork in a simple test
Magnetic particle testing uses magnetic object configuration. Failure to use some
fields to inspect ferromagnetic materials. form of field intensity indicator to verify
Discontinuities in the material cause the presence of a valid magnetic flux
disturbances in the magnetic field which could lead to inadequate testing of this
in turn produce a leakage flux. It is this critical area. Measurements with the
leakage flux that permits the formation of means described here have shown a
particle indications. Both the direction variation from 0.3 to 8.5 mT (3 to 85 G)
and intensity of the magnetic field are within the same test object, because of its
critical in determining the sensitivity of geometric differences.
the test procedure. Both of these factors
are affected by the nature of the material, Reliability Studies
the test object geometry and the way in
which the magnetic field is induced. The absence of magnetic field verification
has led to widely varying test
All of these parameters are interrelated effectiveness. Two often cited reliability
to determine the direction and intensity studies are excellent examples of the
of the magnetic field in a particular problems associated with the lack of a
location in a test object. The mathematics sensitivity check. The first of the studies2
of these interrelationships does not lend was the result of a round robin among
itself to straightforward, closed form four major aerospace contractors, two jet
solutions even for relatively simple
geometries. A generalized solution for FIGURE 1. Magnetic particle testing problem area in simple
more complex test objects has not been part geometry.
obtained and would probably not
completely solve the problem of Magnetic flux lines
establishing magnetic particle testing
procedures.

Empirical Rules for Magnetizing Area of zero field
Reference Standards current

Perhaps more than any other
nondestructive technique, magnetic
particle testing has based its procedures
on empirical data (rules of thumb)
developed by trial and error in the early
days of the technique. These rules have
persisted through the years in various
standards and specifications. The reliance
on empirical data occurred because of the

260 Magnetic Testing

engine manufacturers, three landing gear and the generation of an appropriate
manufacturers, one major forging supplier magnetic field.
and a commercial test lab. Each company
processed and tested 24 pedigreed System Standardization
specimens with known discontinuities. All
of the discontinuities were considered When multiple variables can affect the
detectable at that time in the technique’s outcome of a test, a means should be used
development. to normalize or standardize the test. This
ensures that consistent, repeatable results
The companies varied from less than are achieved, independent of the
20 percent detection to over 90 percent. machine, the operator or the time of the
Only one of the eleven companies scored test.
better than 60 percent of the
discontinuities. At the time of its The most direct way to achieve
completion, the study drew few consistent results is to regularly use a
conclusions about the cause for this reference standard to compare system
variability. Another study3 concluded that sensitivity to preestablished tolerances. If
the maximum probability of detection the desired sensitivity is not achieved,
was 55 percent on jet turbine blades made testing should be stopped to allow
of magnetic alloys. The reasons for the required system adjustments.
poor results were not individually
assessed. Parametric Evaluations

Needs for Known Indicator It is at times useful to examine a system’s
sensitivity to changes in one or more
The goal of system monitoring is to verify variables. For example, to evaluate the
that the system is performing in the effectiveness of magnetic particle testing
desired way and at the desired sensitivity. on chromium plated components, it
The most direct way to achieve this goal is would be appropriate to investigate (1) the
to verify the system’s ability to detect one effect of various plating thicknesses,
or more known discontinuities. Ideally (2) the sensitivity of the test to changes in
this would be done with a discontinuity current levels or field intensity and (3) the
of the smallest critical size in an exact effect of changes in the particle type or
duplicate of the test piece. This ideal is bath concentration.
rarely if ever practical.
Reference standards are used to study
More often, some form of artificial these changing parameters. Indications of
discontinuity indicator is used. This the known discontinuities help determine
reference standard is designed to help the effect of the individual parameters on
evaluate several aspects of a magnetic test sensitivity. The results of such studies
particle system’s performance, including are used to generate or modify testing
(1) testing the magnetizing equipment, procedures for the material and geometry
(2) checking the sensitivity of the of interest.
magnetic particle compound and
(3) verifying the adequacy of a test Technique Development
procedure for detecting discontinuities of
a predetermined magnitude. In the past, it was common for some
operators to rely solely on empirical rules
System Evaluation for establishing magnetic particle testing
procedures. This practice frequently lead
Unlike equipment for other to overmagnetization, poor coverage,
nondestructive testing methods, magnetic inappropriate selection of test geometries
particle testing systems give little evidence or some combination of all three
of malfunction. The absence of a test disadvantages.
indication could mean (1) that tests were
properly performed on samples without The selection of an appropriate test
discontinuities or (2) that the testing technique may be the single most
system was not working and therefore not important factor in the success of a
locating existing discontinuities. magnetic particle test. Reference standards
and artificial indications can significantly
As a result, some form of reference improve system performance and may
standard is needed to determine proper also reduce the cost of testing by
system performance and adequate eliminating unnecessary configurations or
sensitivity. Such a system evaluation tool scrap caused by excessive current.
should check for contamination of the Reference standards used during
magnetic particle bath, material visibility technique development can quickly verify
(loss of fluorescence on fluorescent the completeness of coverage, the
oxides), particle concentration (for wet direction of magnetizing fields and the
techniques), adequate particle mobility level of field intensities.

In many cases, it can be demonstrated
that common rules of thumb produce
field intensities far in excess of those

Reference Standards for Magnetic Particle Testing 261

necessary for detecting particular standards are often used to regulate field
discontinuities. Excessive field intensity intensity to avoid excess flux while
might appear to provide a margin of achieving accurate indications.
safety for unknown effects of test object
material and geometry. However, in many Two kinds of artificial discontinuities
cases, this excess produces a significant are used for magnetic particle test
field component normal to the test object systems: (1) those designed to indicate the
surface. This in turn reduces particle adequacy of the field in an unknown test
mobility, increases particle background object and (2) those designed to measure
and actually reduces rather than enhances the effectiveness of the testing system
the sensitivity of the test. Reference independent of the test object.

262 Magnetic Testing

PART 2. Reference Standards for System
Evaluation

Reference standards may be used to The ring standard is used by passing a
evaluate the functionality or performance specified direct current through a
of a magnetic particle testing system. On conductor which in turn passes through
a periodic basis, reference standards are the ring’s center. The magnetic particle
used as test objects to monitor the system testing procedure (or system) is evaluated
for changes in magnetic field production, based on the number of holes detected at
particle concentration, visibility or various current levels. The number of
contamination. It is helpful to have holes that should be detected at a
graduated discontinuities so that a
numerical indicator of the system TABLE 1. Comparative dimensions for tool
performance can be recorded and steel ring having diameter of 125 mm
monitored. (5 in.) (Fig. 2). Hole diameters are
1.78 mm (0.07 in.) each.
Tool Steel Ring Standard
Hole Distance from Edge
The tool steel ring is a commonly used Number _____t_o__C_e_n__te_r__o_f_H__o_l_e_____
and universally recognized reference
standard for magnetic particle testing mm (in.)
systems (Fig. 2 and Table 1) but essentially
indicates only particle efficacy. Since its 1 1.8 (0.07)
introduction in the 1940s, it appears in 2 3.6 (0.14)
virtually all United States codes and 3 5.3 (0.21)
specifications as the means for checking 4 7.1 (0.28)
magnetic particle performance.4 5 9.0 (0.35)
6 10.7 (0.42)
The ring is widely used for measuring 7 12.5 (0.49)
system performance for both wet and dry 8 14.2 (0.56)
techniques. It is important, however, to 9 16.0 (0.63)
recognize the ring’s limits. For example, a 10 17.8 (0.70)
current density level less than 20 percent 11 19.6 (0.77)
of that usually applied (Table 2) is all that 12 21.4 (0.84)
is needed to indicate a surface
discontinuity 0.25 mm (0.01 in.) in
length.

FIGURE 2. Tool steel ring standard (see TABLE 2. Amperage and indications for test
Table 1). rings per SAE AS 5282.7

Full-Wave or Minimum
Suspension Half-Wave Quantity
Type Direct Current (A) of Holes
D

12 Wet fluorescent 500 3
11 Wet visible 1000 5
Dry powder 1500 6
10 1 2500 7
3500 9
92 3
500 4
3 1000 5
84 1500 6
2500 8
7 19 mm 3500 4
65 (0.75 in.) 6
500 7
32 mm 1000 8
(1.25 in.) 1500 9
2500
125 mm (5.0 in.) 3500

22 mm
(0.87 in.)

Reference Standards for Magnetic Particle Testing 263

particular current level is provided by (3) m = µ1 − µ0 H0 a2
written specifications (Tables 2 and 3).5-7 µ1 + µ0

Standard test objects such as the ring The field from the cylindrical
have proven to be a valuable aid in discontinuity is exactly that which would
controlling magnetic particle test system be obtained from a magnetic dipole m of
parameters.8 However, in addition to intensity per unit length centered at the
magnetizing current level, other factors origin and pointing in the negative X
influence test results, including the direction (because µ1 > µ0), or that which
properties of the particles, operator skill, would be obtained from a current dipole
magnetization level, direction of the with a current separation product of 2πm
magnetic fields produced and particle pointing in the negative Y direction.
concentration. An evaluation of all the
contributing factors requires the Boundary Influence
development of mathematical models9 to
describe their effect on the formation of The field from a cylindrical discontinuity
test indications.10 next to a plane boundary can be obtained
by using the technique of images. The
Ring Standard Magnetic Fields field above the plane surface is the sum of
the fields from a dipole at the center of a
All magnetic leakage fields are a discontinuity (Fig. 4) with intensity as
superposition of dipolar fields. This dipole
character is usually evident when the field FIGURE 3. Coordinate system for cylindrical
from the discontinuity is measured.11,12 discontinuities.
The field arising from a long cylindrical
discontinuity in a linear isotropic medium Y
can be exactly calculated and has a pure
dipolar character. The coordinate system H0 µ1 Hθ Hr
is defined in Fig. 3. The applied field has a
value of H0 and is in the X direction. r

This is a classic problem in θ
magnetostatics13 whose solution in the µ0 X
region of permeability m1 consists of the
vector sum of the applied field H0 with a
dipole field centered at the origin. The
dipole field can be written in polar
coordinates as:

(1) cos θ Cylindrical discontinuity
r2
Hr = − m Legend

H0 = applied magnetic field intensity
Hr = axial magnetic field intensity
sin Hθr = radial magnetic field intensity
(2) Hθ = − m r2 θ = radial distance of field of interest

X,Y = directional axes
µ0 = original permeability
Figure 3 shows these variables. Dipole µ1 = permeability of test material
moment m of length along the cylinder
axis is given by:

TABLE 3. Amperage and hole indication FIGURE 4. Dipole images for field from
requirements for tool steel rings with cylindrical discontinuity next to plane
hardness of 187 to 221 brinnell. surface.

Full-Wave or Minimum Y
Suspension Half-Wave Quantity
Type Surface
µ0

Direct Current (A) of Holes

Wet fluorescent 1400 3 h µ1 2—ah
Wet visible 2500 5
Dry powder 3400 6 µ0 X
1400 3
2500 5 a
3400 6
1400 4 Legend
2500 6 a = radius of cylindrical discontinuity
3400 7 h = depth of discontinuity

X,Y = directional axes
µ0 = original permeability
µ1 = permeability of test material

264 Magnetic Testing

given by Eq. 3 and an infinite series of The numerical values are based on the
images. To a good approximation, when h hole depth (h = 3.56 mm or 0.14 in.) and
> 2a, all image dipoles are assumed to be hole radius (a = 0.89 mm or 0.03 in.) of
at the center of the cylinder.14 The field hole 2 in the ring, with a central
above the surface can then be described conductor current of 1000 A. This
by Eqs. 1 and 2 (rather than by Eq. 3), corresponds to an H0 value of 2.5 kA·m–1
with m given by: (31.5 Oe). This calculation applies to the
ideal case of a linear isotropic magnetic
(4) m = 2µ1 medium with a high permeability, a
two-dimensional geometry (a cylindrical
µ1 + µ0 discontinuity of infinite length) and a
plane parallel boundary. Although the test
× ⎡ + ⎛ µ1 − µ0 ⎞2 ⎛ a ⎞2 ⎤ ring of Fig. 2 deviates considerably from
⎢⎢1 ⎜⎜⎝ µ1 + µ0 ⎠⎟⎟ ⎝⎜ 2h ⎠⎟ ⎥ this ideal condition, comparison of
⎣ ⎥ measured leakage fields with the form
⎦ shown in Fig. 5 reveals that the leakage
fields from the ring can be closely
× H0 a2 approximated by Eqs. 1 and 2.

Magnetic particle test materials generally Consistency of Ring Standard
have µ1 >> µ0. The major effect of the
surface is an approximate doubling of the A discussion of magnetic particle bath
effective dipolar field from the first term quality says that specification SAE AS 5282,
on the right of Eq. 4 and a small increase Tool Steel Ring for Magnetic Particle
from the second term (about 7 percent Inspection, “has identified and corrected
when h = 2a). inconsistencies that have clouded the
usefulness of the ring in the past.
Sample Leakage Field Calculation Previously, baths were used to verify the
rings which, in turn, were used to verify
The calculated field from a subsurface the baths. The use of a mechanism using
cylindrical discontinuity (as obtained by magnetic probes to indicate the magnetic
Eqs. 1, 2 and 4) has the familiar form properties of a ring have eliminated this
illustrated in Fig. 5. The illustration shows seeming conundrum and resulted in a
the components tangential and much more uniform industry standard.
perpendicular to the surface (the most
commonly measured components).

FIGURE 5. Calculated components of magnetic leakage field at surface of subsurface
cylindrical discontinuity in linear isotropic magnetic medium with high permeability and with
applied field in positive X direction.

Distance (in.)

6 –0.3 –0.2 –0.1 0 0.1 0.2 0.3

H 400
4

Leakage field (Oe) H⊥
Leakage field (A·m–1)2
—hm2 200

h

0 1——3——0———m 0
h2
0.58 h

–2
H0 –200

–4 –5 0 5 10
–10 Distance (mm)

Legend

h = depth of discontinuity
H0 = applied magnetic field intensity
HH⊥ = perpendicular magnetic field intensity
= parallel magnetic field intensity
m = dipole moment per unit length

Reference Standards for Magnetic Particle Testing 265

Still, the procedure for magnetizing the magnet, so that longer discontinuity
ring and applying the bath needs to be indications reveal higher test sensitivity.
uniformly administered for consistently
meaningful observations.”7,15 This same task can be fulfilled by
another block standard using a slightly
Reference Standard Test different test principle: a residually
Blocks magnetized block is manufactured to
contain a network of many different crack
Split Prism Test Block widths on its surface (Fig. 7). A typical
standard of this type is 50 mm (2 in.) in
The prism block shown in Fig. 6 is diameter and 10 mm (0.4 in.) thick. Very
another reference standard containing an fine cracks are situated between grosser
artificial discontinuity.16 Truncated discontinuities across the block standard’s
half-prisms are built with one face at an face. As an example of this standard’s use,
angle. When two such components are the loss of indications for the fine cracks
bolted together, an artificial crack is (or their appearance as points rather than
formed. The sloped surface of the block lines) indicates that the magnetic particle
provides variable distances from the bath is no longer effective.
conductor.
Other Tools for
When current is passed through the Performance Checks
conductor, the leakage field from the
crack gradually weakens along the prism For performance tests, in addition to tool
face. A current of specified amperage is steel rings and test blocks, ASTM E 7096
applied through the conductor and the lists several tools available to the
length of the magnetic particle indication inspector: the hall effect device, pie gage,
is used to measure the test sensitivity. shim and test plate. They can help to
quickly confirm system settings. Some are
Magnetized Test Blocks also used during discontinuity evaluation
to determine parameters such as field
Another version of the block standard direction and are discussed in that
consists of two ground steel blocks context elsewhere in this chapter.
forming an artificial crack at their contact
surfaces, similar to the discontinuity Test Plate
formation in the split prism test block. On
one of the face ends, a small permanent For performance tests, in addition to the
magnet is fixed below a brass cover, tool steel ring, ASTM E 7096 describes a
causing magnetic flux leakage from the test plate that should be of a composition
artificial discontinuity. This leakage field and thickness like that of the test object.
decreases with greater distance from the The test plate measures at least 250 ×
300 mm (10 × 12 in.) and bears electro
FIGURE 6. Prism sensitivity indicator: discharge machined notches like those in
(a) separate blocks; (b) artificial crack. shims and pie gages.
(a)
FIGURE 7. Residually magnetized block
standard.

(b)

Current

266 Magnetic Testing

PART 3. Magnetic Discontinuity Standards

Artificial discontinuity standards and material so they won’t get clogged with
magnetic field indicators of the shared particles or grime. The sections are
flux type all have some common furnace brazed and copper plated. The
characteristics. The basic shared flux gage gage is placed on the test piece copper
was developed by B. Berthold and was side up, and the test piece is magnetized.
first used in Germany in the late 1930s to After particles are applied and the excess
indicate magnetic field direction. removed, the indications provide the
Refinements have resulted in the pie gage, inspector the orientation of the magnetic
widely used in weld testing applications field. The pie gage as illustrated in Fig. 8
(specified by T9074-AS-GIB-010/271).17 or the flexible laminated strips as shown
This device responds to applied magnetic in Fig. 9 may only be used as a tool to
fields independently of the test object. demonstrate the direction of the external
magnetic field.
On flat ferromagnetic surfaces, the
shared flux devices respond well and The principal application is on flat
verify magnetic field intensities that can surfaces such as weldments or steel
detect surface discontinuities about 25 µm castings where dry powder is used with a
(0.001 in.) long and 10 µm (0.0004 in.) yoke or prods. The pie gage is not
deep. On complex test objects with recommended for precision parts with
convex surfaces, the value of shared flux complex shapes, for wet techniques or for
devices is limited and invariably results in proving field magnitude. The gage should
overmagnetization when so applied. be demagnetized between readings.

Shared flux discontinuity standards The main advantages of the pie gage
should only be used with the continuous are that it is easy to use and it can be used
technique of particle application. When indefinitely without deterioration. The pie
the continuous technique is not used, gage’s disadvantages are that it retains
commercial pie gages may indicate a some residual magnetism so indications
magnetic field, but this is evidently will prevail after removal of the source of
residual magnetism in some gages or magnetization, that it can only be used in
possibly even physical entrapment of FIGURE 8. Pie gage magnetic field indicator: (a) diagram;
particles. (b) photograph.
(a) Nonferrous trunions
All artificial discontinuity standards
can produce results comparable to a hall 19.1 to 25.4 mm
effect probe and tesla meter. A shim with (0.75 to 1.00 in.)
30 percent discontinuity depth and pie
gages can indicate discontinuities 0.25 ×
0.05 mm (0.01 × 0.002 in.) at 0.9 to
1.5 mT (9 to 15 G), depending on the
permeability of the test object material.

Pie Gages and Flexible Nonferrous handles 0.79 mm
Laminated Strips of convenient length (0.03 in.)

Pie Gages and shape
Eight low carbon steel pie sections furnace
The pie gage is a disk of highly permeable
material divided into four, six or eight brazed together and copper plated
sections by nonferromagnetic material.
The divisions serve as artificial (b)
discontinuities that radiate out in
different directions from the center. The
diameter of the gage is 18 to 25 mm (0.75
to 1.0 in.). The divisions between the low
carbon steel pie sections are to be no
greater than 0.8 mm (0.03 in.); the
dividing interstices between sections are
usually filled with epoxy or another inert

Reference Standards for Magnetic Particle Testing 267

relatively flat areas and that it cannot be (0.0020 in.) thick. The bottom brass layer
reliably used for determination of acts as a liftoff of 0.05 mm (0.0020 in.)
balanced fields in multidirectional from the examination surface. The brass is
magnetization. nonmagnetic and functions only to
provide liftoff and to protect the steel
Flexible Laminated Strips layer. The entire strip may have a
polymeric coating for further protection.
Flexible laminated strips, also known as The longitudinal dimension of the strips
slotted strips, are pieces of highly is 50 mm (1.95 in.) and the width of the
permeable ferromagnetic material with strip is 12 mm (0.47 in.).
slots of different widths. These strips are
typically used to ensure proper field Both types of strips (general and
direction during magnetic particle aerospace) contain three longitudinal slots
examination. They are placed on areas of in the center steel layer. The widths of the
interest on the test object as it is slots in the general strip are 191 µm
inspected. The longitudinal axis of the (0.0075 in.), 229 µm (0.009 in.) and
strip should be placed perpendicular to 254 µm (0.010 in.). The widths of the
the direction of the magnetic field of slots in the aerospace strip are 76 µm
interest to generate the strongest particle (0.003 in.), 102 µm (0.004 in.) and
indications on the strip. A strip whose 127 µm (0.005 in.). Figure 9 shows the
longitudinal axis is parallel to the applied aerospace strip in visible light and under
field will not provide any particle ultraviolet radiation.
indications. The indications produced on
the strips assure the inspector that the Advantages of these strips are that they
proper field direction is obtained in the are relatively easily applied to the
area of interest. component, that they can be used
successfully with either the wet or dry
The strips are available in two types, technique when using continuous
for general and aerospace use. Both types magnetization, that they are repeatable as
of strip contain a steel layer sandwiched long as orientation to the magnetic field
between two brass plates 0.05 µm is maintained and that they can be used
repetitively. Two disadvantages are that
FIGURE 9. Slots are cut into center steel layer they cannot be bent to complex
of flexible laminated strips: (a) diagram; configuration and that they indicate
(b) visible light photograph of aerospace discontinuities in only one direction,
strip; (c) photograph under ultraviolet making them unsuitable for
irradiation. multidirectional fields.
(a)
Both pie gages and flexible laminated
(b) strips are used to determine the
approximate orientation and, to a limited
extent, indicate the adequacy of field
intensity. However, they do not measure
the internal field intensity of the test
object. They merely detect external fields
in the vicinity of the 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 most distinct indications. Slots
parallel to the flux lines produce little or
nothing.

Notched Shim Standards

Notched shim standards are widely used
to establish proper field direction and
ensure adequate field intensity during
technique development in magnetic
particle testing. The shims in Figs. 10 and
(c) 11 may be used to ensure the
establishment and balance of fields in the
multidirectional magnetization technique.

The shims are available in two
thicknesses, 0.05 mm (0.002 in.) and
0.102 mm (0.004 in.). Thinner shims are
used when the thicker shims cannot
conform to the part surface in the area of
interest. The shims are available in two

268 Magnetic Testing

sizes, 19 mm (0.75 in.) square for Figs. 11 test object (slotted side in close contact
and 12 and 20 mm (0.79 in.) square for with the part), in areas where the
Fig. 13. The shims of Fig. 13 are cut, by intensity and direction of the magnetic
the user, into four 10 mm (0.395 in.) field are in question. The slots share
square shims for use in restricted areas. magnetic flux with the test object and
simulate discontinuities slightly below the
Shims are made of AMS 506218 or surface.
equivalent low carbon steel and must be
used as specified in AS 5371.19 Shims are An American version of the Japanese
placed in the areas of interest with shim is manufactured in 0.05 mm
notches toward the test surface. Use (0.002 in.) and 0.1 mm (0.004 in.)
enough shims or place the shim in thicknesses, with discontinuity depths of
multiple areas to ensure that proper field 15, 30 and 60 percent of the shim
directions and intensities are obtained. thickness. The controlled discontinuities
are available as either linear grooves or
These shim standards are often used circular grooves. The circular
with a tesla meter and a hall effect probe
to establish the test procedure during FIGURE 11. Shims for magnetic particle
technique development for a particular testing: (a) type CX-230, notch 30 percent
component. They are used with the wet of 50 µm (0.002 in.); (b) type CX4-430,
technique only, and like other flux notch 30 percent of thickness 0.10 mm
sharing devices, can only be used with (0.004 in.).
continuous magnetization.
(a)
Some of the advantages of the shim
standards are that they can be quantified About 19 mm (0.8 in.)
and related to other parameters, that they
can accommodate virtually any 6.35 mm
configuration with suitable selection and (0.25 in.)
that they can be reused with careful
application and removal practices. Some Outside
disadvantages are that the application diameter
process is somewhat slow, that the parts 12.88 mm
must be clean and dry, that shims cannot (0.507 in.)
be used as a residual magnetism indicator
as they are a flux sharing device, that they 230 About
can be easily damaged with improper 0.18 mm
handling and that they will corrode if not (0.007 in.)
cleaned and properly stored. width

Figures 14 and 15 are photographs of 50 µm Notch depth
some of these shims. Figure 14 shows (0.002 in.) 30 percent of
fluorescent tests. shim thickness

Another type of shim (sometimes (b)
called a block) has been used in Japan
since the 1960s. These indicators help About 19 mm (0.8 in.)
check the magnetic particle apparatus and 6.35 mm
suspension and the intensity and (0.25 in.)
direction of effective magnetic field on
the test surface.20 The blocks are available Outside
in a variety of thicknesses and slot depths. diameter
Linear and circular slots are available. 12.88 mm
Circular slots are particularly effective (0.507 in.)
when the direction of the magnetic flux is
not known. The shims are taped to the 430 About
FIGURE 10. Shim indicators for magnetic 0.18 mm
field verification: (a) simple shim; (b) slotted (0.007 in.)
strip. width
(a)
0.10 mm Notch depth
Nonmagnetic (0.004 in.) 30 percent of
shim thickness
(b) material

Reference Standards for Magnetic Particle Testing 269

discontinuities have the added advantage Application of Shims
of indicating the direction of maximum
field intensity and the angular tolerance Shims are sometimes called paste-on
of sensitivity. Such shim standards can be discontinuities because they must be
used when setting up multidirectional taped to the test object. Shims are used
magnetization to ensure that balanced most often during the development of
fields exist in all directions at any given test procedures, where they help indicate
point in the test object during the the relative intensity and direction of a
magnetization cycle. magnetic field for a particular test
configuration.

FIGURE 12. Shims for magnetic particle testing: (a) type FIGURE 13. Shims for magnetic particle testing: (a) type
3C4-234, thickness 0.102 mm (0.004 in.); (b) type 3C2-234, CX4-230, thickness 0.05 mm (0.002 in.); (b) type CX4-430,
thickness 0.05 mm (0.002 in.). Notch depth 20 percent for thickness 102 µm (0.004 in.). Notch depth 30 percent of
outer circle, 30 percent for center circle, 40 percent for inner shim thickness.
circle.

(a) (a)

About 20 mm (0.8 in.)

About 19 mm (0.8 in.) About 5.97 mm
(0.235 in.)
About Outside diameter
0.18 mm Outside diameter 6.48 mm
(0.007 in.) 12.88 mm (0.507 in.) (2.55 in.)

Outside diameter Width about
9.73 mm (0.383 in.) 152 µm
(0.006 in.)
Outside diameter About About 5 mm
6.55 mm (0.258 in.) 10 mm (0.2 in.)
(0.4 in.)

4-234

0.10 mm Notch depth 20 percent 0.05 mm 230
(0.004 in.) of plate thickness for (0.002 in.)
outer circle Notch depth
(b) 30 percent of
Notch depth 30 percent shim thickness
About 19 mm (0.8 in.) of plate thickness for
center circle
Notch depth 40 percent
of plate thickness for
inner circle

About Outside diameter (b)
0.18 mm 12.88 mm (0.507 in.)
(0.007 in.) About 20 mm (0.8 in.)

About 5.97 mm Outside diameter
(0.235 in.) 6.48 mm
(2.55 in.)
Outside diameter
9.73 mm (0.383 in.)

Outside diameter Width about
6.55 mm (0.258 in.) 152 µm
(0.006 in.)
2-234 About
10 mm About 5 mm
Notch depth 20 percent (0.395 in.) (0.2 in.)
of plate thickness for
0.05 mm outer circle
(0.002 in.) Notch depth 30 percent
of plate thickness for
center circle 102 µm 430
Notch depth 40 percent (0.004 in.)
of plate thickness for Notch depth
inner circle 30 percent of
shim thickness

270 Magnetic Testing

Because shims are often made of high These devices can be used with either
permeability foils, they are generally small dry technique or wet technique materials.
and flexible enough to fit into fairly They can be used (1) to compare different
complex test object geometries to help materials to each other, (2) to check the
determine the adequacy of field intensity visibility of materials under varying
in these critical areas. Once the field illumination conditions, (3) to track the
distribution is found adequate, the testing performance of a material over time and
procedure is recorded and the (4) to make other relevant comparisons.
components are tested with the With suitable care, the magnetization in
parameters established by the shims. these devices should not change over
time. These devices should not be
Magnetic Media magnetized or demagnetized. Contact the

Encoded magnetic media — permanently FIGURE 15. Visible light photographs of
magnetized devices or devices containing shims: (a) CX-430; (b) 3C2-234;
permanent magnets — are used to (c) CX4-230.
evaluate the performance of particle (a)
materials in a manner independent of
system factors including magnetization.

FIGURE 14. Photographs of shims seen under
ultraviolet irradiation: (a) CX4-430;
(b) 3C2-234; (c) CX4-230.
(a)

(b)
(b)

(c)
(c)

Reference Standards for Magnetic Particle Testing 271

device manufacturer in regard to any evaluate quantitatively the performance
magnetization or performance issues. of magnetic particle materials by
displaying the number of indications that
Magnetic Stripe Cards can be observed after the powder or
solution has been applied to the stripe.
A magnetic stripe card is an encoded Particle sensitivity can be graded on the
magnetic medium for instantly evaluating basis of the weakest visible indication.
the performance of magnetic particle test
materials. ASTM standards recognize The card can be exposed to fluid
magnetic stripe cards as tools for suspensions by pouring or spraying the
evaluation of magnetic particle test bath over the card or by dipping the card
materials.5,6 Figure 16 shows samples of in the bath and allowing excess fluid to
the encoded strips on a magnetic stripe drain off. Observations can be made
card using dry particles and wet, immediately. The fluid may be allowed to
fluorescent, magnetic particle materials. evaporate, allowing permanent recording
as described below. For dry powders, blow
The magnetic stripe contains two or sprinkle the powder over the card.
encoded tracks. The upper track is labeled Remove the excess powder by gently
from 0 to 10X when read from left to blowing over the stripe.
right. The lower track is a mirror image of
the upper track and is labeled from 10X to Using the scales printed next to the
0 when read from left to right. Each track stripe, observe the number of visible
is magnetically encoded with a series of indications. A reading may be recorded as
gradients that decrease in intensity in the the number of observable indications on
direction of 0 to 10X. A more sensitive either or both of the magnetic tracks.
magnetic particle material will display a Both tracks should display close to the
greater number of observable gradients or same number of indications, as long as
indications than a less sensitive material. the particle medium is evenly applied.
The magnetic stripe card can be used to
FIGURE 16. Particle indications of decaying Permanent documentation is easy.
encoded pattern (top track) and reverse Firmly press transparent tape over the
decaying pattern (bottom track) on dried indications and transfer the tape to
magnetic strip of magnetic strip card: the report form; or photograph the card.
(a) dry, visible light particles; (b) wet, Records can be used to document bath
fluorescent particles. conditions, make onsite bath standards
(a) and for other purposes.

(b) Particle sensitivity can be determined
immediately when particles are applied.
Then the surface of the card must be
cleaned of fluid and particles.
Precautions. The surface of the stripe card
must be clean of any fluid or foreign
matter before application of particles. The
encoded strip shall not be remagnetized
in any manner before use or
demagnetized in any manner after use.
When not in use the magnetic stripe card
should be kept away from excessive heat
and cold, and strong magnetic fields. For
optimum performance, it should be used
at room temperature. The stripe material
has a coercivity of 2200 A·m–1 (2750 Oe).
If indications are not visible, the particles
must be verified with another card or test
material before use. Under no
circumstances will this strip replace the
steel ring or the particle concentration
tests for magnetic particle materials.

Permanent Magnets

Ferromagnetic objects can be permanently
magnetized and devices containing
permanent magnets can be fabricated to
provide and serve as qualitative indicators
of magnetic particle sensitivity.

272 Magnetic Testing

PART 4. Electromagnetic Reference Devices

Hall Effect Meters less than 240 A·m–1 (3 Oe) usually does
not attract conventional magnetic
Hall effect meters are commonly used to particles.
measure the intensity of the magnetizing
force tangential to the surface of a test The voltage generated in the setup of
object (Fig. 17). The device measures not Fig. 17a can be related to the external
the magnetic flux B within the magnetic field:
component but the corresponding
magnetic field intensity H next to it. The (5) Vh = I BRh
hall effect meter is a relatively effective b
indicator and is in widespread use for
establishing magnetic particle testing where B is the magnetic flux leakage
procedures. It effectively measures residual density field component (tesla) at a right
fields and indicates the direction of the angle to the direct current in the hall
remanence. element, b is the thickness (meter) of the
hall element, I is the applied direct
Various specifications call for different current (ampere), Rh is the hall coefficient
tesla (gauss) or ampere per meter (oersted) of the hall element and Vh is the potential
values in particular applications. Required generated (volt).
values commonly range from 1.6 kA·m–1
(20 Oe) to 4.8 kA·m–1 (60 Oe) when the Probes are available with either
residual technique is used. A residual field tangential (transverse) or axial sensing
elements (Fig. 18). Probes can be
FIGURE 17. Digital magnetic field indicator purchased in a wide variety of sizes and
(tesla gage): (a) diagram; (b) photograph. configurations and with different
(a) measurement ranges. The probe is placed
in the magnetic field such that the
B Vh I magnetic lines of force intersect the major
Fm dimensions of the sensing element at a
right angle.
Fm
The advantages of hall effect devices
(b) are that they provide a quantitative
measure of the intensity of magnetizing
force tangential to the surface of a test
object, that they can be used for
measurement of residual magnetic fields
and that they can be used repetitively.
Their main disadvantages are that they
must be periodically calibrated and that
they cannot be used to establish the
balance of fields in multidirectional
applications.

FIGURE 18. Hall effect probes: (a) tangential;
(b) axial.
(a) Magnetic flux

Legend Hall effect
B = magnetic flux density (tesla)
Fm = magnetic force on electrons (newton) (b) sensor
I = primary current (ampere)
Vh = high potential (volt) Magnetic
flux

Reference Standards for Magnetic Particle Testing 273

Eddy Current Devices permeability materials containing
controlled graduations of artificial
The ability of a material to hold discontinuities. Reference standards are
electromagnetic energy in the form of also available for evaluating the
eddy currents is a function of both the effectiveness of a magnetic particle test for
conductivity and permeability of the a particular test object. These devices
material. Because the permeability of a include a variety of shim configurations
ferromagnetic material changes as the containing known discontinuities.
material is magnetized (from a relatively
low initial permeability through a higher Electronic reference standards such as
maximum value), the eddy current coil hall effect meters and eddy current
impedance also changes. devices can also be used to evaluate the
adequacy of a test procedure.
Several eddy current procedures have
been developed to detect this change in Widespread use of reference standards
permeability and thereby indicate the and test discontinuities is needed to
degree of magnetization. Because of poor improve the consistency of magnetic
repeatability, few of these procedures are particle tests and to increase the detection
widely used. The repeatability problem reliability of the technique.
stems from the large number of variables
that can affect eddy current response in a FIGURE 19. Analog magnetic field indicator
ferromagnetic material. (tesla gage): (a) radiographic view from
side; (b) photograph showing face plate.
A magnetization level indicator has (a)
been developed to detect imbalance in the
permeability along the lines of flux
compared with the permeability
transverse to the lines of flux as the
material approaches magnetic saturation.

Magnetic Field Indicators (b)

Field indicators are small mechanical
devices, each with the shape of a hockey
puck, that use a soft iron vane deflected
by a magnetic field. The X-ray image in
Fig. 19 shows the inside working of a field
meter seen from the side. The vane is
attached to a needle that rotates and
moves the pointer along the scale. Field
indicators can be adjusted and calibrated
so that quantitative information can be
obtained. However, the measurement
range of field indicators is usually small
due to the mechanics of the device. The
one shown in Fig. 19 has a range from
2 mT (20 G) to –2 mT (–20 G). This
limited range makes it best suited for
measuring the residual magnetic field
after demagnetization.

Closing

Magnetic particle testing historically
relied on empirical guidelines for the
development of test procedures. This
practice led to widely varying
discontinuity detection capabilities.
Various codes and specifications have
perpetuated the problem by citing rules of
thumb for establishing procedures.

Several forms of reference standards are
available for verifying procedures and for
evaluating the performance of a magnetic
particle testing system (as a whole or by
components). Reference standards for this
purpose are usually made of high

274 Magnetic Testing

References

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Society for Nondestructive Testing Northampton, United Kingdom:
(1989): p 337-348. British Institute of Non-Destructive
Inspection (1975): p 304.
2. Lewis, W.H., W.H. Sproat, B.D. Dodd
and J.M. Hamilton. Report 13. Stratton, J.A. Electromagnetic Theory,
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Nondestructive Inspections — Final for IEEE Press (2007): p 258.
Report. Kelly Air Force Base, TX: San
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“Calculating the Effect of the
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by Nondestructive Testing Methods. New York, NY: Plenum/Consultants
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15. Chedister, W.C. “MT Bath Condition:
4. Betz, C.E. Principles of Magnetic Particle Time for a Change?” Materials
Testing. Chicago, IL: Magnaflux Evaluation. Vol. 57, No. 6. Columbus,
Corporation (1967). OH: American Society for
Nondestructive Testing (June 1999):
5. ASTM E 1444, Standard Practice for p 571-575.
Magnetic Particle Testing.
West Conshohocken, PA: 16. Shelikov, G.S. and A.G. Aleksandrov.
ASTM International (2005). “Coagulation of the Particles in a
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6. ASTM E 709, Standard Guide for on the Sensitivity of the Magnetic
Magnetic Particle Examination. Powder Method of Testing.” Soviet
West Conshohocken, PA: Journal of Nondestructive Testing.
ASTM International (2001). Vol. 13. New York, NY:
Plenum/Consultants Bureau (1977):
7. SAE AS 5282, Tool Steel Ring for Magnetic p 26.
Particle Inspection. Warrendale, PA:
SAE International (1998, 2007). 17. T9074-AS-GIB-010/271, Requirements
for Nondestructive Testing Methods.
8. Gregory, C.A., V.L. Holmes and Supersedes NAVSEA 271. Washington,
R.J. Roehrs. “Approaches to DC: Naval Sea Systems Command
Verification and Solution of Magnetic (1996).
Particle Inspection Problems.”
Materials Evaluation. Vol. 30, No. 10. 18. SAE AMS 5062, Steel, Low Carbon Bars,
Columbus, OH: American Society for Forgings, Tubing, Sheet, Strip, and Plate
Nondestructive Testing (October 1972): 0.25 Carbon, Maximum. Warrendale,
p 219-228. PA: SAE International (2004).

9. Beissner, R.E. Report AFML-TR-76-236, 19. SAE AS 5371, Reference Standards
An Investigation of Flux Density Notched Shims for Magnetic Particle
Determinations. San Antonio, TX: Inspection. Warrendale, PA:
Southwest Research Institute (1976). SAE International (2007).

10. Swartzendruber, L. “Magnetic Leakage 20. JIS Z 2320-1, Non-Destructive Testing —
and Force Fields for Artificial Defects Magnetic Particle Testing — Part 1,
in Magnetic Particle Test Rings.” General Principles. Supersedes
Proceedings of the Twelfth Symposium on JIS G 0656. Akasaka, Minato-ku, Japan:
Nondestructive Evaluation. San Antonio, Japanese Standards Association (2007).
TX: Southwest Research Institute
(1979).

11. Lord, W., J.M. Bridges, W. Yen and
R. Palanisamy. “Residual and Active
Leakage Fields around Defects in
Ferromagnetic Materials.” Materials
Evaluation. Vol. 36, No. 7. Columbus,
OH: American Society for
Nondestructive Testing (July 1978):
p 47.

Reference Standards for Magnetic Particle Testing 275

11

CHAPTER

Demagnetization1

Roderic K. Stanley, NDE Information Consultants,
Houston, Texas

PART 1. Theory of Demagnetization1,2

Demagnetization is the process of potentially defective welds. An important
removing magnetism from a reason for demagnetizing is to clean the
ferromagnetic material. Ferrous materials test object before it moves to any other
usually retain some residual magnetism surface related production step.
after the magnetizing current is removed.
The amount of residual magnetism Residual fields can also interfere with
depends on (1) the retentivity of the the function of the test object after it is
material, (2) the coercive force of the placed in service. Magnetized milling
material and (3) the intensity and cutters, for instance, hold ferromagnetic
direction of the magnetic field. A chips and this not only interferes with the
longitudinal residual field is often simple cutter’s function but damages the surfaces
to detect with a tag wire or a field meter. of cut objects. Residual fields can interfere
A circular magnetic field is completely with certain magnetic instruments
contained in the test object and may not (compasses, for example) or with any
exhibit the leakage fields and poles number of sensitive electronic
needed for typical detection means. components. A residual magnetic field is
particularly destructive to closely fitted
Complete demagnetization is virtually moving components such as crankshafts,
impossible to obtain. Consequently, the connecting rods and bearing surfaces.
demagnetization process is actually a
reduction of the residual field to a level When Demagnetization Is Not
that is appropriate for the test object’s Needed
subsequent service or manufacture.
Demagnetization is accomplished on the It is not always necessary to demagnetize
domain level by establishing or after magnetic particle testing. Below are
reestablishing random orientations in the some factors that determine whether
material’s magnetic domains. demagnetization is needed to reduce the
residual magnetic field. These factors
The demagnetization process exposes focus on the test object’s subsequent
the test object to a reversing magnetizing manufacture and its service life.
field that gradually diminishes in
intensity. This causes a corresponding When test objects have low magnetic
reversal and reduction in the magnetic retentivity (an automotive block, for
field intensity in the object by scrambling example, or low carbon steel used for
the domains. welded tanks), the residual field dissipates
as soon as the magnetizing current is
Need for removed. Sometimes, this is also true in
Demagnetization2 large structures and no demagnetization is
required after magnetic particle testing.
Demagnetizing is often required for
magnetic particle testing but may also be Demagnetization occurs as a side effect
required for other reasons. of heat treatment if the test object
temperature is taken to the curie point or
It is sometimes necessary to above (about 750 °C or 1400 °F for steel).
demagnetize an object before magnetic At the curie temperature, magnetic
particle testing, particularly if the test domains return to their random
object has a strong residual field from orientations and the material is
previous operations such as magnetic demagnetized when it cools, making
crane handling or from contact with a further demagnetization unnecessary.
magnetic chuck.
Finally, when an object is remagnetized
When an object is magnetized for in a different direction to a level equal to
magnetic particle testing, the residual or below its previous magnetization, then
field might adversely affect later stages in demagnetization is not required. Some
its production. A residual field interferes early specifications required
with machining operations by causing demagnetization between circular and
chips to adhere to the test object and this longitudinal magnetization steps of
is detrimental to both tool life and the magnetic particle tests. This was
object’s finish. A magnetic field can also unnecessary for the accuracy of the test.
interfere with welding operations by
actually shifting the arc and producing The written procedure document
should specify if demagnetization is
unneccessary.

278 Magnetic Testing

Ferromagnetic Materials Plating quality also can be affected by
residual magnetism. In an operation such
Ferromagnetic materials are characterized as chrome plating on a steel surface, the
by a relative ease of magnetization when presence of a strong residual magnetic
exposed to a magnetizing force. This can field can divert the plating current away
be attributed to the relatively high from its intended location. Such mishaps
magnetic permeability µ exhibited by incur the expenses of the first faulty
these materials. Once magnetized, plating, of possible repair and of a second
ferromagnetic material retains a level of plating operation.
magnetic flux density B after the
magnetizing field intensity H has been Inservice operation of an object may be
removed. This remanent flux density is impaired if it is the source of excessive
variously referred to as residual flux, leakage fields, attracting metallic chips or
residual magnetism or the residual field. The tramp particles. This condition can cause
magnitude of residual flux density is a malfunctions in rotating assemblies such
function of the following factors: (1) the as bearings or excessive wear on bearing
magnetic characteristics of the material, surfaces. Strong residual flux densities also
(2) the immediate history of the material’s may cause a substantial magnetic
magnetization, (3) the intensity of the attraction between parts moving next to
applied magnetizing field, (4) the each other. This can produce increased
direction of magnetization (longitudinal friction or may in other ways interfere
or circular) and (5) the test object’s with the intended function of the
geometry. component.

See the introduction to this volume for Cleaning operations for the removal of
information about measurement units. metallic chips and particles can be
hampered by the presence of residual
Depending on the direction of the flux fields. This can be of special concern
with respect to the surface of the test when cleaning objects that have internal
object, these lines may leave the object at openings such as oil passages.
one point and continue their path
through air before reentering at another Residual leakage fields can have a
point. The points where magnetic lines of direct adverse effect on certain types of
flux leave and reenter a ferromagnetic instrumentation. The magnetic compass
material are called surface magnetic poles. aboard aircraft is an example, as are many
The north pole is near where flux lines electronic components.
leave the material. Flux lines reenter the
material near to the south pole. Such Rotating shafts containing residual
fields emanating from a test object are magnetism may act as electric generators
called leakage fields and are the with the Earth’s magnetic field or other
phenomena responsible for attracting stray fields. The generated electricity can
nearby magnetic particles. heat the shafts, causing operating losses
and other service problems.
Requirement for
Demagnetization Finally, objects sometimes need to be
demagnetized before using established
Test objects that retain a relatively strong magnetizing current levels for magnetic
residual flux density can be a source of particle testing. Strong residual induction
problems during subsequent can lead to erroneous results and faulty
manufacturing processes or during service. test results because magnetic particles may
Some typical problems are detailed below. not move freely on the test surface.

When subsequent machining is Demagnetization is not required when
performed on a test object, the presence the test material exhibits very low
of a strong residual flux density can magnetic remanence. Low carbon or
attract and hold chips or particles. This nodular steels are included in this
can adversely affect surface finish or tool category. If the next manufacturing
life. process calls for the object to be heated
above the curie point, the material
In an arc welding operation, the becomes nonmagnetic and
presence of strong leakage fields can demagnetization is accomplished by
deflect the arc away from its intended heating. At the curie point, steel
location. This phenomenon is sometimes temporarily transforms from
called arc blow. Electron beam welding is ferromagnetic to paramagnetic and then
also adversely affected by residual cools with zero net induction.
magnetism. Even leakage fields of
moderate intensity can deflect the If the part does not require additional
electron beam away from its intended machining and its intended function is
target. not compromised by the presence of a
residual field, then demagnetization is
unnecessary.

Demagnetization 279

Common Sources of Effect of Magnetic Field
Residual Magnetic Fields Origin on
Demagnetization
Ferromagnetic material can become
magnetized in a number of ways, The type of magnetizing source becomes
intentionally and unintentionally. significant when considering the nature
of residual induction and its subsequent
Parts may be purposely magnetized to demagnetization.
perform magnetic particle tests or to
facilitate a magnetic flux leakage test. Alternating currents tend to flow near
Magnetization may also come from the surface of a conductor. This skin effect
biasing magnetic fields such as those used produces residual fields that are surface
to negate the effects of permeability oriented. Such fields respond well to
changes in eddy current tests. demagnetization with alternating current.

Magnetic chucks are a common source There is practically no skin effect
of residual fields. Pronounced fields can associated with a direct current
be left in an object if a chuck’s built-in magnetizing source and consequently the
demagnetizing cycle is faulty or entire cross section of an object can be
inadequate. Prematurely removing test residually magnetized. Deep seated
objects before completion of the residual fields in larger objects may not be
demagnetizing cycle can also be a source affected by alternating current
of residual magnetism demagnetizing techniques because the
skin depth is only about 1 mm (0.04 in.)
Lift magnets are convenient material for steel of relative permeability 100 at
handling devices, but because they 60 Hz.
operate with direct current, they can leave
strong residual fields in fabricated objects Types of Residual
or in raw material. Physical contact with Magnetic Fields
any permanent magnet or highly
magnetized object (a machine table or Longitudinal Magnetic Fields
fixture) can also establish a residual field.
Materials magnetized by a coil or solenoid
Low frequency induction heating that sometimes can be left with a longitudinal
is abruptly terminated can induce very residual induction. The field is oriented
strong residual flux density. The lower the longitudinally, lengthwise, in the test
frequency, the deeper the magnetizing object, and there is a high concentration
field intensity penetrates into the of emergent fields at each end. These
material. fields constitute poles, lines of magnetic
flux entering or leaving the material.
With certain orientations of high
amperage cables, an electric arc welding Longitudinal magnetization is easily
operation can magnetize ferromagnetic detected by field measuring devices such
material. as the tesla meter or by the attraction of
other ferromagnetic materials. Although
Residual magnetization can occur with this type of field can adversely affect
improper operation of alternating current subsequent machining, it is usually very
through a coil demagnetizer. This occurs responsive to demagnetizing techniques.
when the coil current is terminated and
the test object is still within the coil or In some situations, such as magnetic
within its influence. Penetration depth is particle testing of oil field tubes, leaving a
determined by the frequency of the circular induction in a pipe may be more
alternating current. advantageous than leaving a longitudinal
induction.
Finally, under certain conditions the
Earth’s magnetic field can impart a Circular Magnetic Fields
longitudinal residual field. This occurs
only with low coercivity steels, usually Unlike longitudinal residual induction,
when the object is shocked or vibrated circular residual induction can exhibit
while its long axis is parallel to the Earth’s little or no external evidence of its
magnetic field. Such a residual field may presence. The flux may be entirely
become quite significant for long parts confined within the material, depending
subjected to severe vibrations in service or to some extent on part geometry and the
transport. The intensity (horizontal magnetizing procedure.
component) of the Earth’s field in the
United States is about 16 A·m–1 (0.2 Oe). For example, if magnetizing current is
This is equivalent to the magnetic field passed through a homogeneous length of
intensity at the center of a 5 ampere-turn ferromagnetic bar stock having a circular
coil 300 mm (12 in.) in diameter. cross section, the resulting circular

280 Magnetic Testing

residual flux density is for all practical magnetized object can be very difficult.
purposes undetectable without altering Confirmation of an adequate
the bar in some manner. Virtually no demagnetization level is an additional
leakage fields emanate from its surface problem. Leakage field measuring devices
because the magnetic flux path is closed are ineffective because there may not be
within the object. Because of this, the an external leakage field to monitor. In
internal residual flux density B may be this case, reorientation of the circular field
much stronger than the magnetization of into a longitudinal field before
the test object longitudinally in a coil or demagnetization may be advantageous in
solenoid having a comparable magnetic some instances when such a procedure is
field intensity H. compatible with part geometry and size.

The circular residual flux density Multiple Magnetic Poles
becomes an apparent problem when the
geometry of the object is altered by Multiple magnetic poles can be induced
subsequent machining. For example, if a in and retained by ferromagnetic material
keyway is cut in a piece of shafting that is that has been exposed to direct current
circularly magnetized, the circular field magnetization, as in a magnetic chuck or
becomes quite evident. Strong leakage lift magnet. These fields can be
fields occur on either side of the keyway. pronounced and can take the form of
All holes or slots cutting through a alternate north and south poles at
circular field produce magnetic poles that relatively close intervals. Demagnetization
can attract materials such as chips or dust of multiple poles may be difficult using
from subsequent machining. conventional alternating current through-
coil procedures.
Without special equipment,
demagnetization of a circularly

Demagnetization 281

PART 2. Principles of Demagnetization

Ferromagnetic material differs from other materials. Therefore, harder materials
material in that it contains magnetic usually offer more resistance to
domains, localized regions in which the demagnetization and require a higher
atomic or molecular magnetic moments demagnetizing field than softer materials.
are aligned in parallel. When a material is
not magnetized, the domains are No definition has been made for the
randomly oriented and their respective dividing line between hard and soft
magnetic inductions sum to zero. When materials. However, if Hc >> 8000 A·m–1
the material is exposed to a magnetizing (100 Oe), then the material is typically
field intensity H, the domains tend to considered hard. If Hc << 400 A·m–1
align with the applied magnetic field and (5 Oe), then the material is considered
add to the applied field. soft. Coercive forces as large as
8 × 105 A·m–1 (104 Oe) and as small as
When the magnetizing source H is 0.08 A·m–1 (10–3 Oe) have been observed.
removed, some of the magnetic domains
remain in their new orientation rather FIGURE 1. Hysteresis curve: (a) showing
than returning to the original random reference points; (b) downcycling.
orientation and the material retains a (a) Magnetic flux density
certain level of residual magnetic field.
(level of magnetization)
Magnetic Hysteresis +B

Magnetic hysteresis is a lag in the change Magnetic field +Bm Virgin
of magnetization values in response to a (magnetizing source) magnetic
change in the magnetizing force. There is
a relationship (Fig. 1a) between the +Br curve
magnetic field intensity H (magnetizing
source) and the magnetic flux density B –H –Hm –Hc +H
(level of magnetization). When an +Hc +Hm
unmagnetized material is magnetized by
gradually increasing H from zero to +Hm, –Br
the level of magnetization B increases –Bm
along the virgin magnetization curve to a
maximum value corresponding to +Bm. –B

As H is reduced to zero, the level of B (b) +B
falls to +Br rather than zero. This level of
retained magnetization (+Br) is the –H +H
remanent or residual field remaining in
the material. As negative values of H
(opposite polarity) are applied, the curve
passes through –Hc and on to Hm and
reaches a magnetization level of –Bm.
Reversing the applied field to a value of
+Hm completes the magnetic hysteresis
loop.

Retentivity and Coercive –B
Force Legend

The value Hc is the coercive force and is B = magnetic flux density
an indicator of the difficulty involved in Bm = maximum magnetic flux density
demagnetizing a material. The value of Br Br = residual magnetic flux density where H = 0
is the material’s retentivity and indicates H = magnetic field intensity
the residual induction within a section of Hc = magnetic field intensity where B = 0
the material. As a rule, high coercive Hm = maximum magnetic field intensity
forces are associated with harder materials
and low coercive forces with softer

282 Magnetic Testing

The fact that an object retains a strong to zero. The diminishing hysteresis curve
residual field Br does not necessarily in Fig. 1b illustrates this principle.
indicate a high coercive force Hc. Some
materials retain an appreciable residual The value of the total coercive force Hc
induction yet are easily demagnetized is often unknown and varies near the
(transformer steels are an example). On ends of the test object. However, it is
the other hand, some materials that retain always less than the maximum
relatively weak residual fields can be magnetizing force Hm value. This may
extremely difficult to demagnetize serve as a guide in cases where the
because of a high coercive force. magnetizing field intensity Hm is known,
as is sometimes the case in magnetic
Downcycling particle testing. Successful
demagnetization procedures often start
Practically all demagnetizing techniques with an initial demagnetizing field that
are based on a common procedure called equals or exceeds that of the magnetizing
downcycling — cyclical application of force Hm.
opposed magnetic fields of successively
decreasing intensity.3 A magnetizing field An object may also be demagnetized by
intensity H high enough to overcome the raising its temperature above the curie
initial coercive force Hc is alternately point. Although this technique provides
reversed in polarity and gradually reduced thorough demagnetization, it is often
impractical.

The magnetic field of the Earth can
affect ferromagnetic objects but is weak,
about 0.03 mT (0.3 G).

Demagnetization 283

PART 3. Demagnetization Procedures

Alternating Current The magnitude of the field can also be
Demagnetization reduced by withdrawing a coil such as a
cable coil away from a stationary test
Through-Coil Demagnetization1,2 object. This technique is advantageous for
high production rates because a properly
Alternating current demagnetization with designed coil can be continuously
a specially built demagnetizing coil is the energized while a steady stream of test
most convenient and widely used objects is conveyed through the coil
demagnetization method. Single-phase opening. Typical alternating current
alternating current is used to power a through-coil demagnetizers are shown in
multiturn coil. Various voltages (120, 240 Figs. 2 and 3.
or 480 V) are used, depending on FIGURE 2. Manually operated alternating
availability and the application. current coil demagnetizer with roller
carriage and automatic timer.
The simplicity of the alternating
current through-coil technique makes it FIGURE 3. Production alternating current coil
one of the most prevalent demagnetizer in magnetic particle testing
demagnetization means. The technique conveyor unit.
uses a coil powered from a current source
alternating at line frequency (usually
60 Hz in the United States). As a test
object is conveyed through the coil, it is
subjected to an intense magnetic field
within the coil. Operating at a fixed
amplitude, the coil produces a
continuously reversing magnetic field
because of the cyclic, alternating current.

The typical demagnetization procedure
comprises three main steps: (1) the test
object is positioned within the coil, (2) the
coil is energized to a predetermined level
and (3) the test object is removed from
the coil and placed outside the coil’s
magnetic field.

As the single-phase alternating current
reverses in the coil, the magnetic field in
the test object also reverses. As the object
is moved out of the coil’s magnetic field,
the object’s field is weakened as it is
reversed. Moving the test object
completely away from the coil (usually
1 m away) effectively demagnetizes the
part. The current through the coil is
maintained until the test object is
completely out of the coil’s field. If the
current is stopped before this, or if the
object’s movement is stopped within the
coil’s field, residual magnetism could
remain in the test material.

To facilitate handling, a small rolling
carriage and track are often furnished
with coil demagnetizing systems. The test
object is positioned on the carriage next
to the coil and pushed through the coil
and beyond it to a neutral area.

Demagnetization of test shims may be
performed by removing them slowly from
one pole of an alternating current yoke.

284 Magnetic Testing

Circular Field Demagnetization Direct Current
Demagnetization1,2
Alternating current demagnetization may
also be achieved with a circular magnetic Direct current demagnetization requires
field. The test object is placed between the equipment with (1) a means of reversing
contact points (headstock and tailstock) of the direction of current flow and (2) a
a magnetizing system. An applied means of gradually reducing the current
alternating current is gradually reduced to level. A motorized 30-point tap switch
zero and this demagnetizes the object. connected to a tapped transformer is the
The procedure is widely used with most common current reduction means.
alternating and direct current systems The reducing voltage is fed into rectifiers
designed for testing heavy objects. with each tap setting.

Reducing alternating current was Reversing Direct Current Contact
formerly done with a step down switch Coil Technique
connected to a tapped power transformer.
The step down was either a motorized Reversing direct current contact coil
switch or the manual current control tap demagnetization is usually associated with
switch of the magnetizing unit. Similar relatively large test objects that have been
circuitry and procedures were used for magnetized using a direct current
mobile equipment, where contact with magnetic field. It is also applicable in
the object was made with cables and certain instances where alternating
clamps. current demagnetization proves
ineffective.
A faster system of demagnetizing heavy
objects with alternating current is the The technique requires high amperage
solid state decaying technique. The direct current or full-wave rectified
amperage flowing through the object is alternating current that can be directed to
rapidly and smoothly reduced by the a coil or contact plate. There must also be
demagnetizing system’s electronics. provisions for reversing the polarity of the
current and suitable means for gradually
Cable Wrap Technique reducing its amplitude to zero.

There are applications (such as large or The coil of a magnetic particle testing
immobile test objects) where a cable wrap, unit can be used for demagnetization, but
with an appropriate high amperage a more effective technique is to put the
alternating current power source, can demagnetizing current directly through
provide a convenient means of the test object with what is called a head
demagnetization. The high amperage shot. Head shots are not permitted in
power source must also incorporate a some circumstances, such as for oil field
suitable current control to facilitate the tubular test objects.
gradual reduction of current from
maximum to zero. The direct current is alternately reversed
in polarity (direction) and reduced in
Portable or mobile alternating current amplitude to zero. Although fewer steps
power packs are available for such may provide satisfactory results, greater
applications. Equipment of this type reliability is achieved by using about 30
usually features stepless solid state current reversals and current reductions to
control. Some units incorporate special approach zero asymptotically.
current decay circuitry that automatically
reduces the current from maximum to The cycle is usually controlled
zero in a matter of 3 or 4 s. automatically and requires about 30 s to
complete. When using a coil, the test
It is difficult to demagnetize the ends object should remain stationary within
of line pipe in the field by using welding the coil until the demagnetizing cycle has
cables because the local BH curve at the been completed. When the contact
end of a pipe is variable. The technician technique is used, contact should be
performing the demagnetization must maintained until the cycle is completed.
have a good understanding of the process. The direct current provides deep
penetration and is usually very successful
Through-Current Technique on objects otherwise difficult to
demagnetize.
Through-current demagnetization is a
contact technique. High amperage For economy, the reversing direct
alternating current is caused to flow current demagnetizing feature is usually
directly through a test object starting at incorporated into the design of direct
contact electrodes. The current control current horizontal wet magnetic particle
system must provide gradual reduction of testing units or relatively large stationary
current as required for demagnetization. A power packs.
portable alternating current power pack
used for cable wrap demagnetization may
also be used for the through-current
technique.

Demagnetization 285

Reversing Cable Wrap Technique Specialized
Demagnetization
This technique is used for demagnetizing
objects too large or heavy to process on a There is a wide variety of miscellaneous
horizontal wet testing unit. The object to demagnetizing techniques that have
be demagnetized is wrapped with multiple specific applications designed around a
turns of high amperage flexible cable, specific test object.
such as American Wire Gage 4\0 (12 mm
[0.45 in.] in diameter), connected to a Demagnetization with Yokes
stationary direct current power pack.
A yoke configuration is sometimes used to
The current is alternately reversed in demagnetize small objects having high
direction and reduced in amplitude coercive force. Yokes are also used for
through multiple steps until the current demagnetization of objects with small
reaches zero. This current is reduced length-to-diameter ratios, including
usually by using built-in automatic spheres with nearly perfect
circuitry similar to that designed into length-to-diameter ratios of 1. Yokes are
some wet testing units. Power packs that usually designed for a specific type or
lack an automatic demagnetizing cycle range of test objects.
can sometimes be used to accomplish
demagnetization by manually Some alternating current yokes are
interchanging cable connections (current similar in operation to the alternating
reversal) and manipulating manual current coils: the test object is passed
current controls (current reduction). between or across the surfaces of the pole
However, this is time consuming and faces at maximum field intensity and are
requires an appropriately fine current then withdrawn. Direct current yokes are
control to be successful — for example, sometimes used with reversing direct
demagnetizing the ends of a pipe in the current techniques on specific objects
field. requiring deep penetration. However, this
process is inherently slow because of the
Pulsating Reversing Technique associated high inductance of the circuit.

A high amperage direct current coil Some coil or yoke arrangements use
demagnetizer has been designed to damped oscillation to obtain the required
produce alternate pulses of positive and reversing and diminishing field. The
negative current. The pulses are generated oscillation is derived from a special circuit
at a fixed amplitude and a repetition rate design based on specific values of
of five to ten cycles per second. This capacitance, inductance and resistance.
permits relatively small objects to be
demagnetized by the through-coil Demagnetization of Oil Field
technique. Tubes

The object is subjected to a constantly Various testing processes involving direct
reversing magnetic field as it passes current magnetization leave the test
through the coil and the magnitude of object with a significant longitudinal
the effective field is reduced to zero as the residual field. Demagnetization is usually
object is gradually withdrawn from the required to eliminate any possibility of
coil. This mode of operation is identical to the field having an adverse effect on
the alternating current through-coil subsequent machining or welding. Some
technique described above, except for the tubular products have lengths of 9 to
reduced repetition rate (five to ten cycles 15 m (30 to 50 ft) — a formidable
per second rather than sixty cycles per challenge to standard demagnetization
second). procedures. Some unusual techniques
have been used when facilities for full
The lower repetition rate substantially length demagnetization are not available.
reduces the skin effect with a
corresponding increase in magnetic field One such procedure applies a bucking
penetration. Therefore, the technique (opposite polarity) field at the ends of the
provides additional demagnetizing object using a direct current coil
capabilities on some relatively small test positioned near the ends. By trial and
objects with thick cross sections that are error, magnetizing current to the coil is
difficult to handle with line frequency increased until external field readings are
coil demagnetizers. Another standard reduced to an acceptable level. The result
mode of operation allows the object to is a redistribution of the field at each end
remain stationary within the coil while of the tube. Unfortunately, mechanical
the current gradually decays to zero. agitation from handling or transport after
this procedure can result in the
redistribution of the longitudinal field
and can reestablish strong leakage fields at
the ends of the test object.

286 Magnetic Testing

Another specialized demagnetization Selection of
procedure uses a full length internal Demagnetization
conductor and a unidirectional current Technique
pulse to reorient the longitudinal field
and form a circular field. The magnitude Selecting a suitable demagnetization
of the circularly magnetizing current is procedure is a matter of matching
increased until acceptable external field capabilities with the specific application.
readings are achieved. While this is not Each of the procedures discussed above
demagnetization, it does reduce leakage has advantages and limitations based on
fields to a level suitable for butt welding test object size, hardness, production rate
operations. It also creates the undesirable and source of magnetization (Table 1).
conditions associated with circular
magnetization and hinders subsequent Limitations of Alternating Current
operations, including weld preparation or Techniques
threading.
The alternating current through-coil
Overall demagnetization is the best technique is probably the most widely
solution for long tubes and this is usually used demagnetization technique because
carried out using some form of reversing of its simplicity and its adaptability to
direct current. For static conditions, an high production rates. However, the skin
internal conductor is used with a effect associated with alternating current
conventional reversing direct current magnetic fields results in limited
cycle. A cable, wrapped to give several penetration. Consequently, alternating
internal conductors, could be used in current demagnetization may be
place of the central conductor. However, ineffective for removing deep residual
when the object’s length is taken into fields such as those from direct current
consideration, the central conductor magnetization.
approach is much more time efficient.
This factor becomes significant on large
The advent of pulsating direct current test objects. As a rule, alternating current
equipment has led to dynamic techniques can effectively demagnetize
demagnetizing systems for overall most objects with a cross section less than
demagnetization of long test objects. In 50 mm (2 in.), regardless of the original
such systems, the object is conveyed magnetizing source. Larger test objects
through a special coil arrangement at with deep residual fields usually require
speeds up to 1.5 m·s–1 (300 ft·min–1). The some form of reversing direct current
coil current consists of constant demagnetization. If alternating current is
amplitude pulses of direct current the only source of magnetization,
alternating relative to polarity (positive alternating current demagnetization is
and negative) at a rate of about five cycles effective regardless of test object size.
per second.

TABLE 1. Demagnetizing method selection guide based on test object size, hardness and production rate for residual
fields from direct current magnetizing source.

______T_e_s_t_O__b_j_e_c_t_S_i_z_e______ ____M__a_t_e_r_i_a_l _H_a_r_d__n_e_s_s____ _____P__ro__d_u_c_t_io__n_R__a_te______
Smalla Mediumb Largec Soft Medium Hard Low Medium High

Alternating Current (50 to 60 Hz)

Through-coil yes maybe no yes yes no yes yes yes
yes yes no
Cable wrap no maybe maybe yes yes no yes no no
yes yes no
Through-current (30-point step down) no yes no yes yes maybe yes maybe no

Through-current (current decay) no yes no yes yes yes yes maybe no
yes yes yes
Yoke yes no no yes yes yes yes no no
yes yes yes
Reversing Direct Current

Through-coil (pulsating) yes yes yes yes yes yes

Coil (30-point step down) yes yes maybe yes yes no

Cable wrap (30-point step down) no maybe yes yes no no

Through-current (30-point step down) no yes yes yes maybe no

a. Hand held.
b. Diameter < 50 mm (2 in.).
c. Diameter > 50 mm (2 in.).

Demagnetization 287

Maximum Effective Field Intensity available for continuous duty. Various coil
openings are commercially available.
The maximum effective field intensity Hm
of a particular demagnetization procedure Portable alternating current half-wave
is an important indicator of the units are lightweight magnetization and
procedure’s capability. Basically, the Hm demagnetization units used to power high
value must be sufficient to overcome the amperage cables. Solid state stepless
coercive force Hc associated with the current control permits demagnetization
material to be demagnetized. Generally, by coil, cable wrap or contact techniques.
Hc increases with hardness. Limited alternating current output usually
restricts application of these systems to
Test object size and geometry relatively small or medium sized objects.
(including the length-to-diameter ratio)
are also factors that influence field Mobile alternating current half-wave
intensity requirements. The units are heavy duty, dual output systems
demagnetization potential of a procedure used with high amperage cables. Newer
inherently increases with field intensity designs have solid state current control and
capabilities. When related to various built-in current decay circuitry. Relatively
alternating current coil sizes with equal large test objects can be demagnetized by
ampere turn ratings, the field intensity the coil, cable wrap or contact techniques.
increases as coil size decreases.
Horizontal wet alternating current
Direct Current Demagnetization units with solid state current control
of Large Objects1,2 feature current decay circuitry that can be
used for demagnetizing either by the coil
Alternating current does not penetrate technique or the contact technique.
ferromagnetic materials very deeply. For
this reason, large test objects and those Reversing Direct Current
magnetized with direct current cannot be Demagnetization Equipment
demagnetized with alternating current.
Reversing direct current procedures are Equipment for reversing direct current
usually required when alternating current demagnetization is often expensive. The
techniques prove inadequate. reversing feature is commonly offered as
an option on direct current magnetization
For heavy objects, putting the equipment but separate units are also
demagnetizing current directly through available. Some typical units are listed
the test object in a head shot is efficient below.
because it breaks down longitudinal and
circular fields. Head shot demagnetization Coil demagnetizers are designed for
is usually done with large power packs. demagnetization only. A constant
amplitude, high amperage direct current is
On production equipment, directed through a fixed coil. The polarity
demagnetization circuitry can be a part of of the current is continuously reversed at a
the test system design. For example, the fixed rate. Relatively small test objects may
unit’s conveyor can pass through a be demagnetized dynamically with the
demagnetizing coil beyond the test station through-coil technique or statically with
or a circular magnetization shot can be modified current decay techniques. Coil
incorporated into the conveyor operation. sizes may be limited.

For those difficult applications, Horizontal wet magnetization and
reversing direct current has unexcelled demagnetization units permit objects to
demagnetization capabilities. Although be magnetized, tested and demagnetized
production rate capabilities are limited, in place on the system. Demagnetization
this is usually not a significant hindrance. is based on automatic polarity reversal
Because larger objects are often involved, and a step down cycle using coil or
the associated production rates are contact techniques. These systems are
relatively low. Almost any object can be suitable for relatively large test objects.3
demagnetized to an acceptable level with
reversing direct current. Stationary magnetization and
demagnetization power packs produce
Demagnetizing Equipment high amperage directed to the test object
through flexible high amperage cables.
Alternating Current Either the contact or cable wrap
Demagnetization Equipment technique can be used to demagnetize
large objects. The reversing direct current
Alternating current coil demagnetizers demagnetization process is automatically
function only as demagnetizers. They are controlled through a fixed cycle.
designed for operation at line voltage and
line frequency. Most of these units are Specially designed systems are available
rated for intermittent duty but some are for applications that require high rates of
productivity. Special power pack and coil
configurations are available to
demagnetize 9 to 15 m (30 to 50 ft)
lengths of tubing on a continuous basis at
rates up to 1.5 m·s–1 (300 ft·min–1).

288 Magnetic Testing

PART 4. Typical Demagnetization Problems

Demagnetizing Field Poor Length-to-Diameter
Intensity Too Weak Ratio

Demagnetizing capability is directly The ratio of an object’s length to its
related to the technique’s effective diameter is as important to coil
magnetic field intensity induced in the demagnetization as it is to coil
test object. magnetization. Values below 3:1 can be a
problem, but the situation may be
Conventional alternating current coils corrected by effectively increasing the
are usually rated in terms of ampere turns. length-to-diameter ratio.
However, for the same applied ampere
turns, the intensity of the field increases This correction is done by adding
as coil diameter decreases. It should be ferromagnetic pole pieces at both ends of
noted that the field intensity within a coil the object. Pole pieces should be about
is greater near the inside wall than at the 150 mm (6 in.) long and nearly the same
center. diameter as the object. With pole pieces
in place, the assembly can be passed
Alternating Current through the demagnetizing coil in the
Demagnetization after usual way. For large lots, it is sometimes
Direct Current convenient to pass the objects through
Magnetization the demagnetizing coil end to end in an
unbroken chain.
Objects magnetized by a direct current
source can be difficult if not impossible to Magnetic Shielding
adequately demagnetize with an
alternating current process. This is more Whenever possible, it is desirable to
prevalent in test objects with diameters demagnetize an object before its assembly
greater than 50 mm (2 in.) because of the with other components. Once assembled,
pronounced skin effect associated with the test object may be adjacent to or
alternating current fields. surrounded by other ferromagnetic
material. In such cases, the demagnetizing
Deep interior residual magnetism field may be shunted through the
remains unaffected by surface oriented adjacent material rather than through the
alternating current fields. On such object itself and demagnetization will be
objects, some form of reversing direct ineffective.
current demagnetization must be used.
Small objects should not be passed
Test Object Orientation through a demagnetizing coil in bundles
or layered in handling baskets. Test
Improper test object orientation relative objects in the center of a stack are
to an alternating current demagnetizing shielded from the demagnetizing field by
coil can adversely affect demagnetization. the outer layer of material. For the same
For the best results, a test object should be reason, ferromagnetic baskets or trays are
passed through the coil lengthwise or not used.
with its longest axis perpendicular to the
coil — that is, parallel to the coil’s central Calibration of Field
axis. Indicators

Ring shaped test objects such as Field indicators are convenient for
bearing races can be rolled through an monitoring of demagnetization, but
alternating current coil to obtain the obscure changes in calibration can cause
desired results. In a similar manner, problems. Inadvertently subjecting the
objects with a complex configuration may device to a demagnetizing field can
require some form of rotation as they are partially demagnetize the field indicator’s
passed through a demagnetizing coil. internal permanent magnet and
significantly increase the device’s

Demagnetization 289

sensitivity. Subsequent measurements process and its verification. For example,
would then falsely indicate that an objects exhibiting a field indicator reading
established demagnetization process is around 0.3 mT (3 G) usually will not have
inadequate. In such cases, the results of an adverse effect on subsequent
demagnetization should be verified with machining or welding, instrumentation or
another device known to be accurate. end use.

Demagnetization Strictly specified limitations should be
Specifications placed only on a limited number of
demagnetization applications (particular
Demagnetization specifications can cause test objects manufactured for specific
serious production problems. service) as required by experience or
Specifications are often written without laboratory data. To eliminate ambiguity, a
accommodation to the practical specification should define how a reading
limitations of the demagnetization is made — on the surface with a hall
effect meter or with a field indicator of a
specific design.

290 Magnetic Testing

PART 5. Demagnetization of Elongated Test
Objects

Much finished or semifinished tubular value below a specified minimum after
product is tested for surface breaking tight magnetic flux leakage testing.4 The field is
discontinuities by magnetic flux leakage. generally reduced so that such materials
In the case of ferromagnetic oil field can more easily be welded.
tubes, for example, the ends of the tube
might first be tested for transversely Typically a specification might require
oriented discontinuities, with the that not more than 800 A·m–1 (10 Oe) be
magnetic flux induced by placing the end measured with a tesla meter at the ends of
of the tube in a coil. The tube is then the material. After magnetic flux leakage
tested for longitudinal discontinuities testing for transversely oriented
using circumferential induction from a discontinuities, the magnetic field
current pulse along an internally placed intensities emerging from the ends of
conductor connected to a capacitor tubes might easily be 40 kA·m–1 (500 Oe):
discharge system. Both of these demagnetization is essential.
techniques are shown in Fig. 4.
Circumferential
In some testing specifications, it is also Remagnetization
required that the external longitudinal
magnetic fields close to the ends of the In some cases, after longitudinal
tubular product (especially oil field magnetization has been established and
transmission line pipe) be reduced to a transverse discontinuity testing
completed, it is necessary to remagnetize
FIGURE 4. Magnetization of elongated only in the circumferential direction. The
products as commonly practiced for oil field resulting magnetization then causes little
tubular tests: (a) circumferential or no external field. Tubes are easy to
magnetization of tube by capacitor remagnetize by using the internal
discharge internal conductor; conductor technique5 shown in Fig. 4a.
(b) longitudinal magnetization of end region The remaining longitudinal field intensity
using encircling coil. can be measured easily with a tesla meter
(a) Capacitor discharge unit as shown in Fig. 5.

Ic In this test, a calibrated tesla meter is
C SCR used to measure the maximum value of
the magnetic field intensity at the end of
IE the material. If it exceeds a specified
IE value, additional shots from the capacitor
discharge internal conductor are applied
(b) to reduce this emergent field. For this
application, a tesla meter capable of
H reading up to 100 mT (1000 G) is
preferred over other types of field
I indicators.

FIGURE 5. Hall element tesla meter
measuring longitudinal magnetic field in air
close to end of tube.

Tube

Legend Hall element
C = capacitor Tesla meter
H = lines of magnetic flux
Ic = comparator output current Read out
IE = induced current

SCR = silicon controlled rectifier

Demagnetization 291

Angled Nature of Resulting threads are particularly prone to
Field cracking.7 A commonly practiced
magnetic flux leakage test is that of
The presence of small axial field intensity longitudinally magnetizing the ends of
at the end of a tube (or other elongated drill pipe (Fig. 4b) and looking for thread
part) does not indicate that the tube is root cracks with wet fluorescent magnetic
not circumferentially magnetized.6 It particles. Following such a test, the
merely indicates that the circumferential rotation of the flux into the
and axial flux densities involved form circumferential direction might require
spiral flux lines, obeying a vector several pulses from a capacitor discharge
relationship such as that shown in Fig. 6. internal conductor system. Under such
circumstances, longer pulses (25 ms) are
In this particular example, the material more effective than shorter ones (3 ms).
just inside the end of the tube is saturated
with magnetic flux that is virtually In the resulting circumferential
circumferential at a value very close to the magnetization, the drill stem can be
remanence Br for the steel. Suppose that tested for longitudinally oriented
an external flux density of 1 mT (10 G) discontinuities and the tool joints can be
occurs for a material with a remanence of tested for heat check cracks with magnetic
1400 mT (14 kG). This flux density particle tests. The threads are also
measurement was made with a tesla meter relatively easy to clean of particles.
after attempted circumferential
magnetization by the technique outlined Upset Area Problems
above. The external flux density indicates
that the direction of the flux inside the It is generally necessary to apply only 3.2
material is only tan–1 (1400–1) or to 4 kA·m–1 (40 to 50 Oe)
0.04 degrees from being perfectly circumferentially to magnetize oil field
circumferential. tubular materials to a point near
saturation. This is especially true of the
Drill Pipe End Area Tests region between the ends of the material
(Q-R in Fig. 7). Some materials may
When the threaded end of a pipe becomes require higher field intensity values to
strained longitudinally, the roots of the cause saturation — the BH curve for the
material should be consulted.
FIGURE 6. Possible magnetic flux density
configuration at end of tube: (a) emergent However, before threading the ends of
flux density is measurable with tesla meter such tubes, it is sometimes necessary to
but circumferential component is not; bell or upset the ends (P-Q and R-S) of the
(b) resulting field within tube is almost material. This process may lock in existing
circumferential here. magnetic fluxes from earlier tests, so the
(a) 2 flux density at the ends can require a

1 FIGURE 7. Different circumferential magnetic
field BH curves: (a) at the belled or upset
(b) 2 ends of tubes; (b) at unworked center
section of tube. Curve (a) shows lower flux
density Br and higher field intensity Hc than
curve (b).

(a)

P
Q

1 Resulting (b)
field

Legend R
1. Circumferential field, 1.4 T (14 kG). S
2. Longitudinal field, 1 mT (10 G).

292 Magnetic Testing