SECTION VI DEMAGNITIZATION

Figure 3-34. Drawing Fine Magnetic Particles from Vehicle with Horseshoe Magnet

Figure 3-35. Hysteresis Loops Produced During Demagnetization

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 3-76 the lumen. The units of measurements for visible light intensity are foot-candles where one foot-candle equals one lumen per square foot. Another term often used is lux, which equals one lumen per square meter. The conversion between the two terms is 1 foot-candle equals approximately 10 lux. 3.5.7.9 Dark Adaptation. The human eye becomes much more sensitive to light under dark conditions. This increased sensitivity gradually occurs when the light conditions change from light to dark. When entering a darkened area from a lighted area, little or nothing can be seen at first. The pupil of the eye must widen to admit more light. The time required for the eye to adjust to darken condition depends upon the overall health and age of the individual. Full sensitivity or dark adaptation requires about 20 minutes. A dark adaptation time of 5 minutes is usually sufficient for magnetic particle inspection with black light. An inspector entering a darkened area SHALL allow at least 5 minutes for dark adaptation before examining parts. Once dark-adapted, the pupil of the eye responds very rapidly to bright light. A very short bright light exposure cancels the slowly acquired dark adaptation. Time for dark adaptation must be allowed whenever an inspector enters the darkened booth or is exposed to a bright light. A timer capable of measuring this time period should be available within the darkened area. 3.5.7.10 Cleanliness. The inspection area as well as the hands and clothing of the inspector should be clean and free of extraneous fluorescent materials. Nonrelevant indications may be formed when parts contact extraneous fluorescent materials. In addition, the fluorescence from this material will raise the ambient light level, thus increasing the amount of blacklight necessary to produce a visible indication of a small defect. SECTION VI DEMAGNITIZATION 3.6 DEMAGNETIZATION. 3.6.1 General. a. Any ferromagnetic material subjected to magnetic particle inspection requires demagnetization. When performing magnetic particle inspection of aircraft parts, it is essential to demagnetize them. The inspector should understand the reasons for this step, as well as the problems involved and the available means for solving them. b. The earth's magnetic field can contribute to the difficulty of demagnetizing parts. A long part to be demagnetized should be placed so that its principal axis is in an east-west direction. A long part lying in a north and south direction can never be demagnetized below the level of the earth's field. Rotating the part or structure on its east-west axis while demagnetizing often helps reduce the field in transverse members that are not lying east and west. Vibration of the structure during the demagnetization process is also helpful under these circumstances. Complete removal of all magnetic fields is virtually impossible. 3.6.2 Purpose. Ferromagnetic materials retain a certain amount of residual magnetism (or remnant field) after application of a magnetizing force. This does not affect the mechanical properties of the part. However, a residual field can impede the operation of some parts as well as affect the operation of adjacent equipment that are sensitive to low level stray magnetic fields. 3.6.3 Principles Of operation. Demagnetization may be accomplished in a number of different ways. The method used depends upon the electrical power and equipment available, the degree of demagnetization required and the skill of the inspector.

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