MULTIDIRECTIONAL MAGNETIZATION
Multidirectional magnetization uses several contact points and relay switches to produce constantly varying magnetic fields. Typical uses of multi directional magnetization are large castings, high volume small test objects with automatic detectors and critical test objects such as aircraft landing gear.
Retentivity and Coercive Force
The retentivity of a particular magnetic material is its property to retain, to a greater or lesser degree, a certain amount of magnetism after the magnetic force is removed. Coercive force is defined as the reverse magnetizing force necessary to remove the residual magnetism to demagnetize a test object.
LONGITUDINAL MAGNETIZATION
Longitudinal Fields
The field in and around a coil is shown in Figure. Longitudinal fields are typically induced in the test object by current flow in a coil. A coil is a long, nonmagnetic conductor wrapped around the test object. The magnetic field is greatest at the surface of each line of the coil. Thus, the test object should be placed near the inner diameter of the wrapped coil.
Longitudinal magnetic field in and around a coil.
Field Direction
In a direct current coil, the magnetic field direction is from the electron emitter or negative source to the positive or electron collector, as shown in Figure.
Field produced by using a central conductor.
Field Strength
The force to successfully magnetize a test object in a coil may require several wraps of the coil to build up enough magnetic flux density for proper testing.
A yoke may be used to magnetize a test object longitudinally.
A yoke is a temporary horseshoe magnet made of soft, low retentivity iron that is magnetized by a small coil wound around its horizontal bar. When the energized yoke is placed on a test object, the flux flowing from the yoke’s north pole through the test object to the south pole induces a local longitudinal field in the test object.
However, the magnetic field produced by the yoke does not lie entirely within the test object. An external field is present, which is a deterrent to locating subsurface discontinuities. If magnetic particles are applied sparingly at the area between the poles, indications of surface discontinuities are produced.
Discontinuities Discovered by Longitudinal Fields
Longitudinal magnetism typically finds transverse discontinuities. Both alternating and direct current techniques may be used, depending on test object shape.