Differential Absorption in Matter
A material discontinuity, such as a void or change in configuration, (see Figure 6-2) changes the effective thickness of a material, and thus changes the degree of radiation absorption. Since all radiation that is not absorbed or scattered within a material is transmitted, the amount of transmitted radiation varies with localized changes in effective material thickness.
It is the transmitted radiation intensity that is generally used to find a material defect. If the material discontinuity represented in Figure 6-3 were a foreign material inclusion, it also would cause a change in the apparent composition of the material and again result in a change in the transmitted radiation intensity. The degree of this change would be dependent on the relative effects of the base material and the included material on the incident radiation.
NOTE
Although radiography will reveal the interior of opaque objects, it cannot detect all types of irregularities or discontinuities. Small defects in thick objects such as fine cracks or indentations are difficult to detect. In applying radiography as an inspection method, the sensitivity of the method must be kept in mind. The limitations 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 of radiation. A most important example of this type of defect is the crack. A crack represents a tear or rupture within a homogeneous material. If a crack is open, that is, the opening is wide, (see Figure 6-3a) it appears to the radiation beam as a significant change in effective material thickness and is thus readily detected. However, if a crack is under compression and is very tight as illustrated in Figure 6-3b, then its detection may become very difficult, if not impossible, because the apparent change in material thickness is negligible. It is important to note that crack orientation also has a very significant effect on the detectability of the crack with radiation. In Figure 6-3b, if the crack were oriented parallel with the radiation beam, the effective change in material thickness would be enough to make the crack easily detectable. However, in most situations the probability of aligning a beam with a tight crack is low, so other NDI techniques must be relied upon as backups. The problems associated with crack detection will be dealt with at length in later paragraphs.
Exposure of Film. In their action on photographic film, X-rays differ from ordinary light. Examination of microscopic sections through the sensitive layer of exposed films has shown that X-rays, unlike light, produce an equal distribution of grains of reduced silver throughout the whole thickness of the layer whereas light produces an effect mainly on the surface of the emulsion. Consequently, a greater blackening of the emulsion can be produced by increasing the thickness of the emulsion and by coating both sides of the base of X-ray film. This darkening effect may then be used to obtain a photographic record, or radiograph, which is produced by the passage of X-rays or gamma rays through an object and onto a film. Thus a radiograph is a shadow picture of an object and its interior; dark regions on the film represent the more penetrable regions of the part and lighter areas on the film represent the more dense areas of the part. Film may be coupled with various screens to improve the image and reduce problems associated with scattered radiation.
Summary. Radiography satisfies the three primary requirements of any nondestructive inspection:
- There is an energy form that can be usefully produced in a controlled manner.
- This energy form is capable of interacting with material in a manner that causes a change in the energy form, but not in the material.
- After such interaction the energy form may be detected and may be interpreted to define what material condition produced the observed result.
