Home - Teaching Resources - NDT Tips

-

Radiographic Setup - Source/Part Orientation

The orientation of the material or component being radiographed with respect to the radiation source can have a significant effect on the interpretation of the image. This experiment illustrates that with one exposure, it is sometimes difficult to accurately identify or size a defect. Student will make two exposures of the same components at slightly different orientations and evaluate the differences in the radiographic images.

Procedure:

  1. Load a film cassette with a 10 by 12 inch piece of film (type 1 film is recommended).
  2. Cover one-half of the film cassette with a lead sheet so that only half of the film will be exposed during each of the two exposures.
  3. Place two coins about one-half inch apart on the side of the film cassette not covered by the lead sheet..
  4. Place the film/lead/coins assembly so that it will be in the center of the radiation source and perpendicular to the beam.
  5. Expose the film to produce a density of two or three.
  6. For the second exposure, move the lead sheet so that it covers the now exposed half of the film and reposition the coins on the unexposed side of the film. Be sure to keep the distance between the coins at about one-half inch.
  7. Reposition the film/lead/coin assembly back to the center for the radiation beam. This time, tilt the assembly so it is about 20 degree from being normal to the beam and so that the two coins are aligned with this angle. In other words, one coin should be closer to the source than the other.
  8. Expose the film using the same conditions as the first exposure.
  9. Develop the film and compare and contrast the two images.

The results should show that the change in relationship between the source and two nickels can have a significant effect on the image produced. Since radiography produces a shadow graph image, it is possible for two features to appear as one in the image. This is very important when attempting to interpret a radiograph. Two small defects can appear as one bigger defect, which would affect defect sizing results. Also, one defect could go completely undetected if it were hidden by the shadow of another defect or feature of the component being radiographed.


NDT
Teaching Tips

Ultrasonic
Reflectors
Signal Interpretation
DAC Curve

Eddy Current
Depth of Penetration
Lenz Law

X-ray
Backscatter Radiation
Film Handling Artifacts
Focal Spot Size
Lead Screens
Source/Object Orientation