Radiography in Modern Industry - Kodak

each separate radiograph may seem laborious, but it prevents invalidation of the measurementby an unnoticed or unavoidable variation in radiographic technique.The kilovoltage used should be as low as considerations of exposure time permit, for the sake ofincreased subject contrast, resulting in a higher accuracy of the measurements. By the sametoken, radiographs should be exposed to the highest density the available densitometer can readreliably. Because the contrast of industrial x-ray films increases with density (see "TheCharacteristic Curve"), the accuracy of the thickness measurement likewise increases withdensity.Special precautions should be observed if the specimen is narrow, or if measurements are to bemade near the edges of a large specimen. In these cases, the specimen should be surroundedwith a lead mask (see "Masks and Diaphragms)", to prevent the unattenuated primary radiationfrom striking the film near the boundaries of the specimen. Otherwise, large errors can arise inthe thickness determinations.Determination of the composition of parts is a special application of this technique. For example,it might be possible for parts identical in appearance to be made of different alloys. In general,though not invariably, different alloys have different absorptions. Thus, if a part of knowncomposition is radiographed along with a part, or group of parts, being checked, a difference indensity between corresponding areas of the control and specimens indicates a difference incomposition. Note that identical densities merely make it highly probable, though not certain, thatthe compositions are the same. This test, of course, provides no information on heat treatment,crystal size, or the like.It is much more difficult to estimate the size of a void or inclusion in the direction of the radiation.The density of the radiographic indication of a void depends not only on its dimensions along thedirection of the beam, but also on its location within the thickness of the material and on itsshape. Thus, it is necessary to prepare, by a separate experiment, calibration curves of void sizeversus density for each of several locations within the depth of the specimen, and perhaps foreach of several shapes of void. In addition, the location of the void must be determined by one ofthe methods described in "Depth Localization Of Defects", in order to know which of the abovementionedcalibration curves should be used.Such estimates of the dimensions of inclusions can be further complicated if the chemicalcomposition and density, and hence the radiation absorption, of the inclusion is unknown.This procedure for measuring the dimensions of voids has been applied successfully but,because of the extreme care and large amount of preliminary work required, has been limited tospecimens of high value and to circumstances where all parts of the radiographic process couldbe kept under the most rigorous control.High Speed RadiographyExposure times of one-millionth of a second, or even less, can be achieved by the use ofspecially designed high-voltage generating equipment and x-ray tubes. Such exposure times aresufficiently short to "stop" the motion of projectiles, high-speed machinery, and the like (SeeFigure 95).Radiography in Modern Industry 154