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THE DISTRIBUTOR’S LINK
ROB LaPOINTE MAGNETIC PARTICLE TESTING VS. PENETRANT TESTING – WHICH TEST IS BEST? from page 8
FIGURE 3 - A CUTAWAY SECTION OF A FULLY THREADED STUD
SHOWING A VOID OF MATERIAL KNOW AS A BURST
ASTM F788, Standard Specification for Surface
Discontinuities of Bolts, Screws, Studs, and Rivets, Inch
and Metric Series, provides us with a list of and examples
of surface discontinuities common to fasteners. F788
also provides tolerances for allowable discontinuities.
Discontinuities that are not open to the surface such
as internal voids, internal bursts, or inclusions are not
able to be detected by PT and many are not detectable
by MT. Magnetic particle testing does offer the ability
to detect subsurface discontinuities provided they are
close to the surface and the technician is skilled at
spotting them. Figure 3 shows a subsurface discontinuity
that would not be detectable by penetrant or magnetic
particle testing. The burst in Figure 2 is not connected
to the surface and is too deep below the surface for the
possibility of detection by MT. For discontinuities like
these, other NDT methods like ultrasonic or radiograph
testing would be useful. These methods can image the
inside of something without cutting it open. They are
essentially the same as the medical techniques used to
image unborn babies (ultrasound) or a broken leg (x-ray).
It would also be worth our time to understand a
little more about nondestructive testing. Nondestructive
testing is just that – nondestructive. This testing does
not destroy, mark, or otherwise injure the part to collect
data. Specifically, nondestructive testing accesses the
quality of the material and workmanship of the product to
ensure that there are no hidden flaws that would render
the part unfit for service. Also, since it’s nondestructive,
test parts can be sold and used along with the production
lot after testing. For highly controlled products, it is
common for 100 % of the production lot to be tested for
flaws using a nondestructive method.
Nondestructive testing includes many detection
methods such as Magnetic Particle Testing (MT),
Penetrant Testing (PT), Visual Testing (VT), Ultrasonic
Testing (UT), Radiographic Testing (RT) and many others.
Personnel training, qualification and certification is
typically an employer-based system through guidelines
provided by the American Society for Nondestructive
Testing (ASNT) using the SNT-TC-1A standard and the
National Aerospace Standard NAS 410. It is a highly
technical and regulated branch of materials testing.
FIGURE 4 - THE RIGHT-HAND RULE FOR LOCATING THE MAGNETIC
FIELD RELATING TO AN ELECTRICAL CURRENT
Magnetic particle testing makes use of magnetic
fields created by the flow of charges to detect breaks or
discontinuities in conductive and magnetizable materials.
To understand how this works, we need to be acquainted
with some physics of electromagnetism. Don’t worry, we
won’t be deriving Maxwell’s equations here, just some
basic spatial relationships between electrical current and
magnetic fields.
When electrical current (electrons) flows through a
metal, a magnetic field is created which incircles the
flow. Physics students around the world use what’s
called the “Right-hand Rule” for understanding this
relationship. Figure 4 shows the right-hand rule. Notice
that if your right thumb points in the direction of the
current, then your curled fingers point in the direction of
the magnetic field created by the flow.
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