MAGNETIC FIELD THEORY
Magnetic Domains
Some materials that can be magnetized possess atoms that are classified as submicroscopic regions, called magnetic domains. These domains have a positive and negative polarity at opposite ends because of internal atomic alignment. If the material is not considered to be magnetized, the domains are randomly aligned, usually parallel to the crystalline axes of the material.
When the material is subjected to a magnetic field, the domains align themselves parallel to the external magnetic field. The material then acts as a magnet. Figure illustrates the domain alignment in nonmagnetized and magnetized material.
Alignment of magnetic domains: (a) in an unmagnetized material; (b) in a
magnetized material.
Magnetic Poles
A magnet has the property of attracting ferromagnetic materials. The ability to attract or repel is not uniform over the surface of a magnet, but is concentrated at localized areas called poles. In every magnet, there are two or more poles with opposite polarities. These poles are attracted to the Earth’s magnetic poles and therefore are called north and south poles.
Can be duplicated by placing a sheet of paper over a bar magnet and sprinkling iron filings on the paper. It shows the magnetic field leaving and entering the ends or poles of the magnet. This characteristic pattern illustrates the term lines of force used to describe a magnetic flux field. There are a number of important properties associated with lines of force.
1. They form continuous loops that are never broken.
2. They do not cross one another.
3. They are considered to have direction: leaving from the north pole, traveling to the south pole.
4. Their density decreases with increasing distance from the poles.
5. They seek the path of least magnetic resistance or reluctance in completing their loop.
When a bar magnet is broken into two or more pieces, new magnetic poles are formed. The opposing poles attract one another, as shown in Figure.
If the center piece in Figure is reversed so that similar poles are adjacent, the lines of force repel one magnet from the other. If one of the bars is small enough, the lines of force can cause it to rotate so that unlike poles are again adjacent. This illustrates the most basic rule of magnetism: unlike poles attract and like poles repel.
Broken bar magnet illustrating the location of newly formed magnetic poles.
Magnetic Fields
The magnetic particle testing method uses magnetic fields to reveal material discontinuities in ferromagnetic materials. The common horseshoe magnet attracts ferritic materials to its ends or poles. Magnetic lines of flux flow from the south pole through the magnet to the north pole.
Magnets only attract materials where the lines of flux leave or enter the magnet. When magnetic material is placed across the poles of a horseshoe magnet, the lines of flux flow from the north pole of the magnet through the material to the south pole. Magnetic lines of flux flow preferentially through magnetic material rather than nonmagnetic material or air.
Magnetized Ring
If a horseshoe magnet is bent so that its poles are close together, the poles still attract magnetic materials. Iron filings or other magnetic materials cling to the poles and bridge the gap between them. In the absence of a slot, the magnetic flux lines are enclosed within the ring. No external poles exist, and magnetic particles dusted over the ring are not attracted to the ring even though there
are magnetic flux lines flowing through it. Magnetized materials attract externally only when poles exist. A ring magnetized in this manner is said to contain a circular magnetic field that is wholly within the object.
Small changes in the cross section of the ring or in the permeability of its material may cause external flux and the attraction of magnetic particles.
Effect of Cracks in a Magnetized Ring
A radial crack in a circularly magnetized object creates north and south magnetic poles at the edges of the crack. This forces some of the magnetic lines of force out of the metal path. These disrupted lines of force are called magnetic flux leakage. Magnetic particles are attracted to the poles created by such a crack, forming an indication of the discontinuity in the metal test object.
Bar Magnet
When a horseshoe magnet is straightened, it becomes a bar magnet with poles at each end, as shown in Figure a. Magnetic flux lines flow through the bar from the south pole to the north pole, but the flux density is not uniform along the bar. Magnetic particles are attracted to any location where flux emerges and particularly to the ends of the magnet where the concentration of external flux lines is greatest. Since the magnetic flux within a bar magnet may run the
length of the bar, it is said to be longitudinally magnetized or to contain a longitudinal field.
Effect of Cracks in a Magnetized Bar
A crack in a bar magnet, shown in Figure, distorts the magnetic lines of force and creates poles on either side of the crack.
These poles attract magnetic particles to form an indication of the crack. The strengths of poles formed at a crack depend on the number of magnetic flux lines interrupted. A crack at a right angle to the magnetic lines of force interrupts more flux lines and creates stronger poles than a crack that is parallel to the flux lines. Test indications of maximum size are formed when discontinuities are at right angles to the magnetic lines of flux.
Bar magnet illustrating longitudinal magnetization:
(a) horseshoe magnet I straightened into a bar magnet with north and south poles; and (b) bar magnet I containing a machined slot and corresponding flux leakage field.
