Operating Manual

12
Defect orientation, image distortion
and useful film length
107
12.1 Defect detectability and image distortion
On a radiograph, a three-dimensional object is presented in a two-dimensional plane
(the film). The appearance of both the object and its defects depends on the orientation
of radiation relative to the object. As shown in figure 1-12, the image of a gas cavity in a
casting may be circular or elongated depending on beam orientation.
In general, the beam of radiation should be at right angles to the film and a specimen
should whenever possible be laid flat on the film cassette. Special angle shots are, howe-
ver, sometimes useful to detect defects which are unfavourable oriented with regard to
the X-ray direction. This influence of X-ray beam angles relative to the orientation of a
defect is also described and illustrated in section 17.4.
Figure 2-12 (A) shows a situation whereby detection of lack-of-side wall fusion in a
V-weld is not performed optimally. Angled radiation (B) is more likely to show up this
type of weld defect.
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Fig. 1-12. Distortion of the image of a gas cavity due to beam orientation
source source
circular image
elongated image
source source
lack of side
wall fusion
lack of side
wall fusion
blurred image of the defect
sharp image
Fig. 2-12. Lack of sidewall fusion in a V-shaped weld joint.
Shot A is unlikely to detect the defect; shot B will.
AB