Figure 15-9.—Coil creates a longitudinal field to show crack in a part. the part and induce a longitudinal magnetic field. Inspection of a cylindrical part with longitudinal magnetism is shown in figure 15-9. If there is a transverse discontinuity in the part, such as that in the illustration, small magnetic poles are formed on either side of the crack. These poles will attract magnetic particles, forming an indication of the discontinuity. Compare figure 15-9 with figure 15-5, and note that in both cases a magnetic field has been induced in the part that is at right angles to the defect. This is the most desirable condition for a reliable inspection. ALTERNATING CURRENT.—The use of alter- nating current (ac) in magnetic particle inspection is recommended only for the detection of surface discontinuities, which comprise the majority of service-induced defects. Fatigue and stress corrosion cracks are examples of cracks usually open to the surface. Alternating current, which must be single phase when used directly for magnetizing purposes, is taken from commercial power lines or portable power sources, and is usually 50 or 60 hertz. DIRECT CURRENT.—Direct current (dc) mag- netizes the entire cross section more or less uniformly in the case of longitudinal magnetization. Magnetic fields produced by direct current penetrate deeper into apart than fields produced by alternating current, which makes it possible to detect subsurface discontinuities. Generally, direct current is used with wet magnetic particle methods. In the presence of dc fields, dry powder particles behave as though they were immobile, tending to remain wherever they happen to land on the surface of a part. This is in contrast to what happens with dry powder particles in the presence of ac fields. In these fields, the particles have mobility on a surface due to the pulsating character of the fields. Particle mobility aids considerably the formation of particle accumulations (indications) at discontinuities. PARTICLES AND METHODS OF APPLI- CATION.—The particles used in magnetic particle testing are made of magnetic materials, usually combinations of iron and iron oxides, that have a high permeability and low retentivity. Particles that have high permeability are easily magnetized by and attracted to the low-level leakage fields at discontinuities. Low retentivity is required to prevent the particles from being permanently magnetized. Strongly retentive particles tend to cling together and to any magnetic surface, resulting in reduced particle mobility and increased background accumulation. Particles are very small and are various sizes. Each magnetic particle formulation always contains a range of sizes and shapes to produce optimum results for the intended use. The smallest particles are more easily attracted to and held by the low-level leakage fields at very fine discontinuitics; larger particles can more easily bridge across coarse discontinuities, where the leakage fields are usually stronger. Elongated particles are included, particularly in the case of dry powders, because these rod-shaped particles easily align themselves with leakage fields not sharply defined, such as those that occur over subsurface discontinuities. Global-shaped particles are included to aid in the mobility and uniform dispersion of particles on a surface. Magnetic particles may be applied as a dry powder, or wet, by using either water or a high flash point petroleum distillate as a liquid vehicle carrier. Dry powder is available in various colors, so the user can select the color that contrasts best with the color of the surface upon which it is used. Colors for use with ordinary visible light are red, grey, black, or yellow. Red- and black-colored particles are available for use in wet baths with ordinary light, and yellow-green fluorescent particles for use with a black light. Fluorescent particles are widely used in wet baths, since the bright fluorescent indications produced at discontinuities are readily seen against the dark backgrounds that exist in black light inspection areas. Radiographic Inspection Radiographic is a nondestructive inspection method that uses a source of X-rays to detect discontinuities in materials and assembly components. Radiation is projected through the item to be tested, and the results are captured on film. Radiography may be used on metallic, nonmetallic, and combination metallic/ nonmetallic materials and assemblies without access to the interior. However, defects must be correctly aligned 15-8