Longitudinal Magnetization

T.O. 33B-1-1 3-35 3.3.11.4.3.1 When using a central bar conductor, alternating current is only to be used when inspecting for surface discontinuities on the inside circumference of the part. If only the inside surface is to be inspected, the diameter shall be the largest distance between two points, 180 degrees apart, on the inside circumference. Otherwise the diameter shall be determined as indicated in paragraph 3.3.11.4.2. The central bar conductor should have an outside diameter as close as practically possible to the inside diameter of the hole of the part that is being inspected. 3.3.12 Longitudinal Magnetization. 3.3.12.1 General. A part is said to have been longitudinally magnetized when the field in it is approximately parallel with a major axis. A part magnetized in a coil, for example, will be longitudinally magnetized in a direction approximately parallel with the coil axis. A characteristic of a part that is magnetized longitudinally will be the appearance of opposite magnetic poles, north and south, at the extreme ends of the part. The existence of the poles is a disadvantage when magnetizing and inspecting because much of the leakage flux from the pole-ends is not parallel with the part surface. This reduces the magnitude of flux that is parallel, thereby weakening the leakage fields at discontinuities in the end regions. The use of pole pieces as described in paragraph 3.3.12.6.3.7, overcomes this weakening effect in many cases. The poles are an advantage in demagnetizing since they make it easy to detect magnetized parts and to confirm removal of the residual fields after demagnetizing procedures. 3.3.12.1.1 Longitudinal magnetization is used for the detection of circumferential discontinuities that lie at approximately right angles to a part's axis. Circumferential discontinuities around a cylinder for example, are detected by magnetizing the cylinder longitudinally in a direction parallel with its axis. A portion of the longitudinal field will cross the discontinuities creating leakage fields that can capture and hold magnetic particles to form indications at the discontinuities. 3.3.12.2 Coil Shot Technique. The usual way to longitudinally magnetize a part is by placing the part in a rigid coil on a stationary magnetic particle inspection unit. The part may be laid on the bottom inside of the coil where the field is strongest, or the part may be supported in the coil by the contact heads of the unit. Special supports are provided on some inspection units for long heavy parts, permitting rotation of parts for inspection. Coils are usually mounted on rails permitting movement along a long part for multiple inspections (multiple coil shots). Because the effective field extends only 6 to 9 inches on either side of a coil, multiple inspections are needed on long parts. 3.3.12.3 Cable Wrap Technique. Cable wrapping a coil around large or heavy parts is another method of producing longitudinal magnetization. Flexible, insulated copper cable is used. A cable-wrapped coil is connected to a magnetic particle mobile or portable power pack or it can be connected to the contact heads of a stationary inspection unit. The type of power source to be used will depend upon the kind and level of current needed to accomplish the particular desired inspection, both magnetizing and demagnetizing. 3.3.12.3.1 Cable lengths used to connect cable-wrapped coils must be kept as short as practical to minimize cable resistance losses and obtain higher magnetizing currents. In the case of AC, and to some extent half-wave DC, in addition to cable resistance, there is the inductance of the coil circuit which further reduces current flow. Twisting or taping the coil cable leads together aids in reducing the inductance of the coil circuit. Coil inductance increases directly with the coil opening area and increases as the square of the turns in the coil. Keeping each of these factors as small as practical, particularly when using AC, assures the maximum current will be obtainable from the power supply. To help keep coil current losses low, cable coils should be wrapped directly on a part or on some insulating material only a little larger than the part. Multiple inspections along a long part, using a coil of only a few turns (3 to 5) is preferably to using a coil of many turns over the length of the part. The latter is occasionally done in some cases where performing multiple inspections is not possible or when a power pack having the required output voltage and current capacity is available.