Radiographic Examination – Physical Principles

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X-rays and gamma rays penetrate steel, but the intensity of the incident radiation will be attenuated as it passes through the material. The degree of attenuation depends on the thickness and density of the material.

Flaws can have the effect of reducing the thickness of material through which the radiation must pass by interposing cavities or impurities of lower density in the workpiece. Therefore, there is less attenuation of radiation passing through the flaw than through the surrounding material, as illustrated in Figure 700-22.

Attenuation of Radiation by Workpiece Containing a Flaw

Photographic film placed opposite the source of radiation will be exposed by the radiation that has passed through the workpiece (transmitted radiation) and, consequently, the flaw will appear as a dark image on the developed negative (referred to as a radiograph). It is important to recognize that the radiation passing through the workpiece does not directly interact with the flaw. The flaw is detectable only because it alters the thickness of material through which the radiation passes. The image of the flaw on the radiograph is actually the silhouette of the three-dimensional flaw projected onto the two-dimensional surface of the film, as illustrated in Figure 700-23.

Projection of Three-Dimensional Flaw in Workpiece onto Two-Dimensional Surface of Film

Flaws will not always significantly reduce the thickness of material through which the radiation must pass. The flaw in Figure 700-22b is identical to the flaw in Figure 700-22a, except that it is rotated 90 degrees. With this orientation, the flaw will not significantly reduce the thickness of material. Therefore, the attenuation of radiation passing through this flaw will be essentially the same as that for the radiation passing through the surrounding material, and there will be no indication of the flaw.

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