WHAT IS CIRCULAR MAGNETIZATION
Circular magnetization is used for the detection of radial discontinuities around edges of holes or openings in test objects. It is also used for the detection of longitudinal discontinuities that lie in the same direction as the current flows, either in a test object or in a test object that a central conductor passes through.
A circular magnetic field is generated in a test object whenever an electric current is passed through it or through an electrical conductor that passes through the test object. The circular field around the inside of the test object will be wholly contained within the test object in the case of a concentric cylinder. No magnetic
poles will be produced on the test object. Poles will be produced if the test object is not a concentric cylinder, is irregularly shaped, or if the path of the current flow is not located on the test object’sgeometric axis.
The magnetic poles, in these cases, are caused by a relatively small portion of the magnetic flux that passes out of the test object and into the air that surrounds the test object. The no-pole condition in a concentric cylinder occurs both while the magnetizing current is flowing and after current flow ceases. The test object is thus residually magnetized, but since no magnetic poles exist, the test object appears to be in an unmagnetized state. However, if the test object is cut, such as when a keyway is made, some of the field will pass out and over the cut producing opposite magnetic poles on each side of the cut. Such poles can hold chips or metal that can interfere with subsequent machining operations or damage bearing surfaces.
Care is needed in the case of circular magnetization which may not be detectable, and appropriate means to assure demagnetization must be taken. Two techniques are used to obtain circular magnetization in test objects: by passage of electric current through the test objects themselves, called the direct contact method; or by passage of the current through a conductor that in turn passes through the test
objects, called the central conductor method.
Direct contact is generally made by placing test objects between clamping heads. Lead face plates or copper braid pads must be used to prevent arcing, overheating and splatter. Wetting of the contact plates with the suspension media before current application helps to prevent overheating. On large test objects, current contact is sometimes made by clamping lug terminated cables to the test object
using nonmagnetic C clamps.
Regardless of how the contact is made, the test object should always make as good an electrical contact as practical. This will minimize any heating or arcing at the juncture. This means contact surfaces must be clean and free of paint
or similar coatings and have adequate pressure applied to achieve good contact over a sufficient area of the test surface. Any excessive heating at the contact points may bum the test object, affecting its temper and finish.
A test object can be circularly magnetized by passing electrical current through a conductor positioned coaxially in a hole or opening. A magnetizing field does exist outside a central conductor carrying current so the walls surrounding a central conductor become magnetized, making possible the detection of discontinuities that parallel the central conductor. Central conductors are any high conductive material, such as a copper bar or cable, placed in the center of the test object to be magnetized. The central conductor method should be used if
longitudinal discontinuities on the inside of tubular or cylindrically shaped test objects are to be detected.
Theoretically, the magnetic field is zero on the inside surface of such test objects unless a central conductor is used. The direct contact method may not produce reliable results in this case, particularly if the test object is a concentric tube or cylinder with good current contact at each end. Either the central conductor or the
direct contact method can be used to detect discontinuities on the outside surfaces of such test objects. Because the circular field around a central conductor is at right angle to the axis of the conductor, the central conductor method is useful for the detection of discontinuities that lie in a direction parallel with the conductor.
The central conductor method is also very useful for detecting discontinuities, usually cracks, which form radially out of holes in castings.
A test object having a hole or opening that is to be tested for inside and outside discontinuities is usually positioned with the conductor centered coaxially in the hole or opening. On very large test objects having large openings, the central conductor may be located close to the inside surface and several tests made around the inside periphery of the opening. Placing the conductor close to the
inside surface reduces the current requirement because the strength of the circular field decreases with distance away from the conductor.
A number of factors must be considered when determining what current amperage to use for circular magnetization.
1. The type of equipment and capacity available.
2. The type of discontinuity and its expected ease or difficulty of detection.
3. The test object’s size, shape and cross sectional area through which the current will flow.
4. The amount of heating that can be tolerated in the test object at the current contact areas.
Another method is the use of a small, metal adhesive indicator.
The indicator is a silicon iron material and measures about 1.3 cm (0.5 in.) long by 0.6 cm (0.25 in.) wide by 0.03 cm (0.01 in.) thick. A 0.008 cm (0.003 in.) square slot is cut across the 0.6 cm (0.25 in.) dimension. In practice, the indicator is placed in a critical area on the test object with the slotted side firmly against the test object surface to ensure intimate contact.
The testing process is carried out and if an indication forms at the slot, it is assumed that there is adequate field strength to reveal actual discontinuities in the
test object. The indicator has a length-to-diameter ratio that is different from the test object; there may be considerable difference between the permeability of the indicator and test object. The indicator could develop a greater field strength than the test object with the slot readily indicated but with insufficient field strength in
·the test object.
The use of an eddy current device in a prescribed manner can effectively indicate the direction and relative level of magnetic field intensity at the surface of a magnetized test object. To relate the meter readings of flux density necessitates the development of a separate calibration curve for each material with full consideration given to all the physical and metallurgical properties of the test
Since maximum material permeability, with its associated saturation level, is an optimum characteristic in selecting a magnetization level, it would appear from a practical aspect that an eddy current device would serve well to determine this
characteristic. Tests have shown that when an eddy current probe (insensitive to stray magnetic fields external to the test object) is placed against the test object and the test object is magnetized with increasing current, the indicating pointer will not significantly move off its zero position until near maximum material saturation has been achieved.
To test for discontinuities at a specific location on a test object without using formulas or rules of thumb, it is only necessary to place the probe on the test object in the area of suspected discontinuities and increase-the current until the indicating pointer deflects, signifying near maximum material saturation. At this time, further processing may be initiated with reasonable assurance that suspected discontinuities, if present, will be indicated. Unlike some laboratory instruments used to measure magnetic field intensity, eddy current devices are designed to withstand the comparative rough handling that may be found in a magnetic particle testing shop.