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 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

and oriented with respect to penetrating rays to be reliably detected. Radiography is one of the most expensive and least sensitive methods for crack detection. It should only be used to detect flaws that are not accessible or favorabl y oriented for use by other test methods.

The extent of recorded information upon the following three prime factors:

1. The composition of the material. is dependent

2. The product of the density and the thickness of the material.

3. The energy of the X-rays, which is incident upon the material. Material discontinuities cause an apparent change in these characteristics, and thus make themselves detectable.

Figure 15-10 is a diagram of radiographic exposure showing the elements of the system. Radiation passes through the object and produces an invisible or latent image in the film. When processed, the film becomes a radiograph or shadow picture of the object. Since more radiation passes through the object where the section is thin or where there is a space or void, the corresponding area on the film is darker. The radiograph is read or interpreted by comparing it with the known nature of the object.