T.O. 33B-1-16-46.1.2.1.2NOTEAlthough radiography will reveal the interior of opaque objects, it cannot detect alltypes of irregularities or discontinuities. Small defects in thick objects such as finecracks or indentations are difficult to detect. In applying radiography as aninspection method, the sensitivity of the method must be kept in mind. Thelimitations of radiography will become more apparent in subsequent discussions.Some voids are difficult to detect, because they present a very slight change in material thickness to a beam ofradiation. A most important example of this type of defect is the crack. A crack represents a tear or rupture within ahomogeneous material. If a crack is open, that is, the opening is wide, (see Figure 6-3a) it appears to the radiationbeam as a significant change in effective material thickness and is thus readily detected. However, if a crack is undercompression and is very tight as illustrated in Figure 6-3b, then its detection may become very difficult, if notimpossible, because the apparent change in material thickness is negligible. It is important to note that crackorientation also has a very significant effect on the detectability of the crack with radiation. In Figure 6-3b, if the crackwere oriented parallel with the radiation beam, the effective change in material thickness would be enough to make thecrack easily detectable. However, in most situations the probability of aligning a beam with a tight crack is low, soother NDI techniques must be relied upon as backups. The problems associated with crack detection will be dealt withat length in later paragraphs.Figure 6-3. Effect of Change in Thickness of Cracks.
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