Basically, radiography can be described as an examination that uses a beam of penetrating electromagnetic radiation to create the shadow image of a specimen's internal and external structure. Any examination that does not show apparent discontinuities is meaningless, misleading, and can create a false sense of security in a less than qualified product.

One of a radiographer's most important tools to ensure that this doesn't happen is his "radiographers eye." To examine an item properly, the radiographer should have an understanding of the anomalies that could be present, and a grasp of the best way to display them. The radiographer must be able to visualize the item's projected image (shadow form) on a film or other recording medium.

Shadow formation is easy to understand. If a group of people were to stand side by side in front of a wall and be illuminated by an auto's headlight, the projected image (shadow) of the individual most central to the beam would be true to his shape while the image of the other individuals would become more and more distorted as their distance from the center increased. If the same group of people were to stand in front of a curved wall whose radius matched the distance of the auto headlight to the wall, this would cause them all to "face" the beam in the same manner: all their projected images would then be true to shape.

This same condition of projection exists in radiographic examination and can be controlled and manipulated by the radiographer (see Figure 1). The beam of radiant energy that will cast the X-ray shadow image of the item(s) being examined comes from a very small focal spot and diverges as it gets farther from its source, much like the auto headlight's beam.

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Some specifications place specific limits on the extent of beam angle deviation from the normal plane.

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Visualizing the proper beam to component orientation can mean the difference between imaging relevant indications or hiding them in the projected shadow of other component details.

• Volumetric (100 percent) inspection of castings is meaningless if the projected shadow of a large detail masks indications in thinner areas.
• Inspection of a fusion weld is meaningless if the part to beam orientation is such that lack of complete penetration or fusion cannot be imaged.
• Shrinkage in a casting radiographed from the wrong direction can appear to be a crack or hot tear. The definition of fine line indications such as cracks, hot tears, and incomplete penetration or fusion can be greatly affected if the beam is not orientated correctly to the fissure interface.

Figure 1 - (a) Good geometry, (b) poor geometry.

 

Some specifications place specific limits on the extent of beam angle deviation from the normal plane. This is especially true for the examination of electron beam welds where little or no filler material is used and preweld fit-up is so tight that an exposure taken only a few degrees off parallel to the weld joint interface probably would not show lack of fusion even in an unwelded joint!

If a radiographer were to position his head at the radiation source, he would "see" the outcome of his radiographic setup. By visualizing the projection of the image of the item(s) being examined onto the recording medium, the radiographer can arrange the items to ensure the best possible resultant image.

A proper setup of parts should not look like an army of soldiers, all in straight lines and all facing the same way This apparent neatness may look good when the setup is made but the resultant image of each of the parts will be different because of the projection caused by the small size of the radiation beam source and its divergence. Often all the items being examined must be aligned to "face" the beam in the same manner. The most important product should be the X-ray image and not the neat appearance or convenience of the straight line setup.

A simple method for proper imaging of small objects when using film radiography is to curve the long dimension of the film to match the arc of the source to film distance (SFD) and then position the parts on the film. This small change (raising each end of the film 12 to 25 mm [0.5 to 1 in.] for a typical 1.2 to 1.5 m [48 to 60 in.] SFD) allows the beam to impinge upon all objects positioned on the central axis of the film at the same angle. The image of objects positioned away from the central axis of the film will still be affected by distortion Ñ but not as much as if the film were laying flat.

Some parts, though small, might best be examined using only single row of parts or even a single piece placed in the center of the beam and film

Radiographic examination is more than a science, it is an art form that requires forethought and an ability to "see" the image of a specimen even before the exposure is taken. In a world where everything from cigars to storage tanks to space station components is being examined, convenience is important, economics is important, but using the proper set up to image all significant indications must always take precedence.

* 190 Bluebird Drive, Naugatuck, CT 06770.

 

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