RADIOGRAPHIC INTERPRETATION

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Figure 15-10.—Diagram of radiographic exposure. 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. RADIOGRAPHIC INTERPRETATION.—The usefulness of the information obtained from the radiographic process depends upon the intelligent interpretation of the derived image. To successfully interpret the radiograph, the radiographic interpreter must have a working knowledge of the component or material and be able to relate the images to the conditions likely to occur. Specifications are used to spell out the discontinuities that maybe considered detrimental to the function of the part and the acceptable magnitudes of the discontinuities. It is the duty of the film interpreter to recognize the various discontinuities, their magnitudes, and be capable of relating them to the particular specification required. The responsibility and capability of the radiographic interpreter cannot be overemphasized. Often, many human lives and investments of millions of dollars are depending on the judgement of the radiographic interpreter. RADIATION HAZARD.—Radiation from X-ray units is destructive to living tissue. It is universally recognized that in the use of such equipment, adequate protection must be provided to personnel. Personnel must keep outside the primary X-ray beam at all times. Radiation produces changes in all matter that it passes through. This is also true of living tissue. When the radiation strikes the molecules of the body, the effect may be no more than to dislodge a few electrons; but an excess of these changes could cause irreparable harm. When a complex organism is exposed to radiation, the degree of damage, if any, depends on which of its body cells have been changed. The more vital parts are in the center of the body; therefore, the more penetrating radiation is likely to be the more harmful in these areas. The skin usually absorbs most of the radiation; therefore, it reacts earliest to radiation. If the whole body is exposed to a very large dose of radiation, it could result in death. In general, the type and severity of the pathological effects of radiation depend on the amount of radiation received at one time and the percentage of the total body exposed. The smaller doses of radiation may cause blood and intestinal disorders in a short period of time. The more delayed effects are leukemia and cancer. Skin damage and loss of hair are also possible results of exposure to radiation. 15-9

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