Quantifying radiographic image quality improvement (POD) by reducing grain diffraction patterns
- DuPont NDT Systems, Chadds Ford, PA (United States)
Grain diffraction patterns in radiographs of castings have been observed for at least several decades. The 1959 NDT Handbook says this, Mottling (diffraction patterns) due to x-ray diffraction is occasionally encountered. (...in thin sections with large grain structure). Today, with increased radiography of thin sections and more importantly larger grain structure alloys, diffraction (mottling) is seen more frequently and is complicating radiographic interpretation. Therefore, mottling has increased proportionally in importance. Since radiographic NDT uses polychromatic x-ray, it is this ``very complicated`` pattern the authors are trying to reduce in this paper. To reduce the pattern, the equation give {lambda} (wavelength) and d (distance between planes) as the only control variables. Since d is given, {lambda} must be used to avoid (reduce) diffraction patterns. ``Mottling due to diffraction can be reduced and in some cases eliminated by raising kV (shortening {lambda})``. ...filtering the beam may be necessary to reduce wavelength enough. By reducing wavelength, the radiographic contrast is reduced. Required radiographic sensitivity may be lost. They propose as a solution using ultrafine grain film with very high contrast at density 3+ (if needed). System contrast is a function of the exposing x-ray wavelength and to instantaneous gradient of the film at the density of radiograph. A good approximation is given by: Contrast (System) = Contrast (Part) {times} Contrast (Film). Additionally, as shown in a previous talk, a sharper image (hi-res film) allows lower contrast for equal EPS perception. This lead to the choice, NDT 30. While very slow, they are using high kV. High kV increases tube output (mr/sec.) as well as the well known penetrability. The exposure using NDT 30 is thus actually shorter.
- OSTI ID:
- 64641
- Report Number(s):
- CONF-931193--; ISBN 0-931403-23-5
- Country of Publication:
- United States
- Language:
- English
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