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AC 43.13-1B 9/8/98 Curing time for silicone rubbers varies from about 30 minutes and up depending upon the particular silicone rubber, the catalyst, and the amount of catalyst used to produce the curing reaction. b. Curing. While curing is taking place, the insides of the hole must be maintained in the required magnetized state. This can be ac- complished using a permanent magnet, a DC yoke, an electromagnet, or some other suitable means. Whatever method of magnetization is used, the leakage fields at any discontinuities inside the holes must be maintained long enough to attract and hold in position the mag- netic particles until a partial cure takes place. A two-step magnetizing procedure has been developed. (1) The first magnetization is accom- plished for a short time in one direction fol- lowed by a second at 90 degrees to the first for the same length of time. This procedure must be repeated for whatever period of time is needed until the cure prevents particle mobil- ity. Magnetization in two directions 90 degrees apart ensures formation of indica- tions at discontinuities in all directions inside the holes. (2) After curing, the rubber plugs, which are exact replicas of the holes, are re- moved and visibly examined for indications which will appear as colored lines against the lighter colored background of the silicone rub- ber. Location of any discontinuities or other surface imperfections in the holes can be de- termined from the location of the indications on the plugs. The magnetic rubber inspection method is covered in detail in Air Force Tech- nical Order 33B-1-1, section XI. c. Critical Examination for Sharp Radii Parts. A critical examination is required for cracks in sharp radii; such as threaded parts, splines, gear teeth roots, and abrupt changes in sections, that cause obscuring and nonrelevant indications during normal examination prac- tices. The procedure provided herein is the most sensitive method for detecting the early beginnings of in-service fatigue cracks in the sharp, internal radii of ferro-magnetic parts. Magnetic particle examination equipment may be used; however, alternating fields are not re- liable to provide the necessary high level of re- sidual magnetism. Optical aids are necessary to realize the maximum sensitivity provided by this magnetic particle procedure. Low-power (10x-30x) binocular microscopes are recom- mended. As a minimum, pocket magnifiers of 7 to 10 power may be used with the following procedure. (1) Thoroughly clean the part at the sharp radii and fillets where soils, greases, and other contaminants tend to accumulate and at other places where they might be overlooked during a casual or hasty examination. (2) The residual method should be used as an aid in particular problem areas, even though it is not considered the best practice in most of the instances. The conventional wet continuous methods should be used initially for overall examination and the residual tech- nique should be applied only for supplemental, local examination of the sharp radii. It should not be applied except in those cases where nonrelevant indications have proven to be a problem in the initial examination. (3) Methods of magnetization should be done according to standard procedures; how- ever, alternating fields should not be used, and the level of magnetizing force imposed should usually be increased above the normal levels to ensure a higher residual field within the part. (4) Following magnetization, apply particles in liquid suspension. The application should be liberal and in a manner to cause maximum particle buildup. Immersion of Page 5-26 Par 5-50PDF Image | AFS-640
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