Electric currents are used to create or induce magnetic fields in
electrically conducting materials. Since it is possible to alter the
directions of magnetic fields by controlling the direction of the
electrical magnetizing current, the arrangement of current paths is
used to induce magnetic flux lines at right angles to expected
discontinuities in the ferromagnetic test object.


Electric current passing through a straight conductor (a wire or
bar, for example) creates a circumferential magnetic field around
that conductor, as shown in Figure IO.la. The magnetic lines of
force are always at right angles to the direction of the current that
induces the magnetic field.
To determine the direction taken by magnetic lines of force
around a conductor, imagine that the conductor is grasped with the
right hand so that the thumb points in the direction of the electric
current. The fingers then point in the direction taken by the magnetic
field lines surrounding the conductor. This is called the right hand
rule that you can check in our website tutorials.
The passage of current induces a magnetic field strength in the
conductor as well as in surrounding space. An object magnetized in
this manner is said to have a circular field or to be circularly
magnetized, as shown in Figure.

Fields in circular magnetization: (a) circumferential magnetic field
surrounding a straight conductor carrying an electric current; (b) circular magnetization of
a test object through which a magnetizing electric current passes.


Circular Magnetization of Solid Test Objects

Circular magnetization can be induced into a test object by prod,
direct or indirect contact methods. It is also possible to generate a
circular field in localized areas of the test object using prods to pass
current through the area being tested.

Circular Magnetization with Prods

The prod electrodes (generally solid copper or braided copper
tips) are first pressed firmly against the test object. The magnetizing
current is passed through the prods and into the area of the object in
contact with the prods. This establishes a circular magnetic field in
the test object around and between each prod electrode.
The use of alternating current limits the prod technique to the
detection of surface discontinuities. Half wave rectified direct
current is more desirable here because its greater particle mobility
helps detect surface and near surface discontinuities with greater
particle mobility.
The prod technique is generally used with dry magnetic particle
materials because of increased particle mobility on rough surfaces
and better penetration. In the United States, wet magnetic particles
are not normally used with the prod technique because of electrical
and fire hazards. In Europe, wet particles are regularly used with
prods to achieve higher sensitivity. Care should be taken to maintain
clean prod tips, to minimize heating at the point of contact and to
prevent prod arc strikes and local heating of the test surface.
Aluminum or copper braided tip prods or pads (rather than solid
copper tips) are recommended because of the possibility of copper
penetration if arcing occurs. A remote control switch should be built
into the prod handles to permit control of the current after
positioning and before removing to minimize arcing.

Circular Magnetization with Direct Contact

To induce a circular magnetic field in a solid test object, current
may be passed through the object. This creates poles on both sides
of discontinuities that are parallel to the length of the test object.
These poles attract fine magnetic particles and form an indication of
the discontinuity. Figure shows the direct contact method for
producing circular fields in a ring to indicate circumferential cracks.
To achieve a reliable test of the entire cylindrical surface, two
magnetizations are required.
This is done because the points of contact (where the current
enters and leaves the ring) are not adequately magnetized for
discontinuity indication. The ring must therefore be turned
90 degrees and then retested.

Direct contact method of
1 magnetizing ring shaped objects to locate
• circumferential discontinuities.

Circular Magnetization with Induced Current

Figure  shows a current flowing circumferentially around the
ring, which can be induced by making the ring a single turn, short
circuited secondary transformer. To accomplish this effect, a
standard magnetizing coil can be used.
The ring is placed inside the coil with its axis parallel to that of
the coil. When the coil is energized with alternating current, the
arrangement constitutes an air core transformer; the magnetizing coil
is the primary and the ring is the single turn secondary. The total
current induced in the ring is greatly increased by inserting a
laminated core of ferromagnetic material through the ring.
For materials with high magnetic retentivity, direct current can be
applied in the technique called quick break, and the objects may then
be tested by the residual method. Quick break is when a direct
current field is caused to collapse suddenly because of an abruptly
interrupted magnetizing current. The circular field generated by the
induced current leaves the test object with a strong residual
induction. A bearing race is a good example of the type of object
that can be tested advantageously by this method.
For test objects made of soft material with low retentivity, the
continuous method must be used and the collapsing direct current
field method is not applicable. By using alternating current (or half
wave direct current) in the magnetizing coil, the current may be left
on and an alternating current ( or half wave direct current) of the
same frequency as the magnetizing current is induced in the ring.
This current should be allowed to flow long enough to produce
indications by the continuous method.

Induced current method of
magnetizing ring shaped objects to locate
circumferential discontinuities.


Circular Magnetization of Hollow Test Objects

With hollow objects or tubes, the inside surfaces may be as
important for testing as the outside surfaces. Since a magnetic field
surrounds a current carrying conductor, it is possible to induce a
satisfactory magnetic field by sliding the test object onto an internal
conducting bar, as shown in Figure a. Passing current through
the bar induces a circular magnetic field throughout the volume of
the test object.
When a conducting bar is not available, an electrical cable may
be passed through the test object and connected to receptacles in the
magnetic particle unit, as shown in Figure b. For large diameter
cylinders, the cable can be brought back on the outside of the test
object, then threaded through again; each pass through increases the
effective field by a factor of two. For long finished tubes,
uninsulated conductors are not permitted because of arc bums.

Circular magnetization of cylindrical test objects using an internal current
carrying conductor: (a) internal bar conductor; and (b) internal cable conductor.



You may also like...

Leave a Reply

%d bloggers like this: