(See also "Film Graininess; Signal-to-Noise Ratio in Radiographs".) The image on an x-ray film is formed by countless minute silver grains, the individual particles being so small that they are visible only under a microscope. However, these small particles are grouped together in relatively large masses, which are visible to the naked eye or with a magnification of only a few diameters. These masses result in the visual impression called graininess.
All films exhibit graininess to a greater or lesser degree. In general, the slower films have lower graininess than the faster. Thus, Film Y (Figure 47) would have a lower graininess than Film X.
The graininess of all films increases as the penetration of the radiation increases, although the rate of increase may be different for different films. The graininess of the images produced at high kilovoltages makes the slow, inherently fine-grain films especially useful in the million- and multimillion-volt range. When sufficient exposure can be given, they are also useful with gamma rays.
The use of lead screens has no significant effect on film graininess. However, graininess is affected by processing conditions, being directly related to the degree of development. For instance, if development time is increased for the purpose of increasing film speed, the graininess of the resulting image is likewise increased. Conversely, a developer or developing technique that results in an appreciable decrease in graininess will also cause an appreciable loss in film speed. However, adjustments made in development technique to compensate for changes in temperature or activity of a developer will have little effect on graininess. Such adjustments are made to achieve the same degree of development as would be obtained in the fresh developer at a standard processing temperature, and therefore the graininess of the film will be essentially unaffected.
Another source of the irregular density in uniformly exposed areas is the screen mottle encountered in radiography with the fluorescent screens. The screen mottle increases markedly as hardness of the radiation increases. This is one of the factors that limits the use of fluorescent screens at high voltage and with gamma rays.
Penetrameters
A standard test piece is usually included in every radiograph as a check on the adequacy of the radiographic technique. The test piece is commonly referred to as a penetrameter in North America and an Image Quality Indicator (IQl) in Europe. The penetrameter (or lQI) is made of the same material, or a similar material, as the specimen being radiographed, and is of a simple geometric form. It contains some small structures (holes, wires, etc), the dimensions of which bear some numerical relation to the thickness of the part being tested. The image of the penetrameter on the radiograph is permanent evidence that the radiographic examination was conducted under proper conditions.
Codes or agreements between customer and vendor may specify the type of penetrameter, its dimensions, and how it is to be employed. Even if penetrameters are not specified, their use is advisable, because they provide an effective check of the overall quality of the radiographic inspection.
Hole Type Penetrameters
The common penetrameter consists of a small rectangular piece of metal, containing several (usually three) holes, the diameters of which are related to the thickness of the penetrameter (See Figure 65).
Figure 65: American Society for Testing and Materials (ASTM) penetrameter (ASTM E 142- 68).
The ASTM (American Society for Testing and Materials) penetrameter contains three holes of diameters T, 2T, and 4T, where T is the thickness of the penetrameter. Because of the practical difficulties in drilling minute holes in thin materials, the minimum diameters of these three holes are 0.010, 0.020, and 0.040 inches, respectively. These penetrameters may also have a slit similar to the ASME penetrameter described below. Thick penetrameters of the hole type would be very large, because of the diameter of the 4T hole. Therefore, penetrameters more than 0.180 inch thick are in the form of discs, the diameters of which are 4 times the thickness (4T) and which contain two holes of diameters T and 2T. Each penetrameter is identified by a lead number showing the thickness in thousandths of an inch.
The ASTM penetrameter permits the specification of a number of levels of radiographic sensitivity, depending on the requirements of the job. For example, the specifications may call for a radiographic sensitivity level of 2-2T. The first symbol (2) indicates that the penetrameter shall be 2 percent of the thickness of the specimen; the second (2T) indicates that the hole having a diameter twice the penetrameter thickness shall be visible on the finished radiograph. The quality level 2-2T is probably the one most commonly specified for routine radiography. However, critical components may require more rigid standards, and a level of 1-2T or 1-1T may be required. On the other hand, the radiography of less critical specimens may be satisfactory if a quality level of 2-4T or 4-4T is achieved. The more critical the radiographic examination--that is, the higher the level of radiographic sensitivity required--the lower the numerical designation for the quality level.
Some sections of the ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code require a penetrameter similar in general to the ASTM penetrameter. It contains three holes, one of which is 2T in diameter, where T is the penetrameter thickness. Customarily, the other two holes are 3T and 4T in diameter, but other sizes may be used. Minimum hole size is 1/6 inch. Penetrameters 0.010 inch, and less, in thickness also contain a slit 0.010-inch wide and 1/4 inch long. Each is identified by a lead number designating the thickness in thousandths of an inch.
Equivalent Penetrameter Sensitivity
Ideally, the penetrameter should be made of the same material as the specimen. However, this is sometimes impossible because of practical or economic difficulties. In such cases, the penetrameter may be made of a radiographically similar material--that is, a material having the same radiographic absorption as the specimen, but one of which it is easier to make penetrameters. Tables of radiographically equivalent materials have been published wherein materials having similar radiographic absorptions are arranged in groups. In addition, a penetrameter made of a particular material may be used in the radiography of materials having greater radiographic absorption. In such a case, there is a certain penalty on the radiographic testers, because they are setting for themselves more rigid radiographic quality standards than are actually required. The penalty is often outweighed, however, by avoidance of the problems of obtaining penetrameters of an unusual material or one of which it is difficult to make penetrameters.
In some cases, the materials involved do not appear in published tabulations. Under these circumstances the comparative radiographic absorption of two materials may be determined experimentally. A block of the material under test and a block of the material proposed for penetrameters, equal in thickness to the part being examined, can be radiographed side by side on the same film with the technique to be used in practice. If the density under the proposed penetrameter materials is equal to or greater than the density under the specimen material, that proposed material is suitable for fabrication of penetrameters.
In practically all cases, the penetrameter is placed on the source side of the specimen--that is, in the least advantageous geometric position. In some instances, however, this location for the penetrameter is not feasible. An example would be the radiography of a circumferential weld in a long tubular structure, using a source positioned within the tube and film on the outer surface. In such a case a "film-side" penetrameter must be used. Some codes specify the film-side penetrameter that is equivalent to the source-side penetrameter normally required. When such a specification is not made, the required film-side penetrameter may be found experimentally. In the example above, a short section of tube of the same dimensions and materials as the item under test would be used to demonstrate the technique. The required penetrameter would be used on the source side, and a range of penetrameters on the film side. If the penetrameter on the source side indicated that the required radiographic sensitivity was being achieved, the image of the smallest visible penetrameter hole in the film-side penetrameters would be used to determine the penetrameter and the hole size to be used on the production radiograph.
Sometimes the shape of the part being examined precludes placing the penetrameter on the part. When this occurs, the penetrameter may be placed on a block of radiographically similar material of the same thickness as the specimen. The block and the penetrameter should be placed as close as possible to the specimen.
Wire Penetrameters
A number of other penetrameter designs are also in use. The German DIN (Deutsche Industrie- Norm) penetrameter (See Figure 66) is one that is widely used. It consists of a number of wires, of various diameters, sealed in a plastic envelope that carries the necessary identification symbols. The thinnest wire visible on the radiograph indicates the image quality. The system is such that only three penetrameters, each containing seven wires, can cover a very wide range of specimen thicknesses. Sets of DIN penetrameters are available in aluminum, copper, and steel. Thus a total of nine penetrameters is sufficient for the radiography of a wide range of materials and thicknesses.
Figure 66: DIN (German) penetrameter (German Standard DIN 54109).
Comparison of Penetrameter Design
The hole type of penetrameter (ASTM, ASME) is, in a sense, a "go no-go" gauge; that is, it indicates whether or not a specified quality level has been attained but, in most cases, does not indicate whether the requirements have been exceeded, or by how much. The DIN penetrameter on the other hand is a series of seven penetrameters in a single unit. As such, it has the advantage that the radiographic quality level achieved can often be read directly from the processed radiograph.
On the other hand, the hole penetrameter can be made of any desired material but the wire penetrameter is made from only a few materials. Therefore, using the hole penetrameter, a quality level of 2-2T may be specified for the radiography of, for example, commercially pure aluminum and 2024 aluminum alloy, even though these have appreciably different compositions and radiation absorptions. The penetrameter would, in each case, be made of the appropriate material. The wire penetrameters, however, are available in aluminum but not in 2024 alloy. To achieve the same quality of radiographic inspection of equal thicknesses of these two materials, it would be necessary to specify different wire diameters--that for 2024 alloy would probably have to be determined by experiment.
Special Penetrameters
Special penetrameters have been designed for certain classes of radiographic inspection. An example is the radiography of small electronic components wherein some of the significant factors are the continuity of fine wires or the presence of tiny balls of solder. Special image quality indicators have been designed consisting of fine wires and small metallic spheres within a plastic block, the whole covered on top and the bottom with steel approximately as thick as the case of the electronic component.
Penetrameters and Visibility of Discontinuities
It should be remembered that even if a certain hole in a penetrameter is visible on the radiograph, a cavity of the same diameter and thickness may not be visible. The penetrameter holes, having sharp boundaries, result in an abrupt, though small, change in metal thickness whereas a natural cavity having more or less rounded sides causes a gradual change. Therefore, the image of the penetrameter hole is sharper and more easily seen in the radiograph than is the image of the cavity. Similarly, a fine crack may be of considerable extent, but if the x-rays or gamma rays pass from source to film along the thickness of the crack, its image on the film may not be visible because of the very gradual transition in photographic density. Thus, a penetrameter is used to indicate the quality of the radiographic technique and not to measure the size of cavity that can be shown.
In the case of a wire image quality indicator of the DIN type, the visibility of a wire of a certain diameter does not assure that a discontinuity of the same cross section will be visible. The human eye perceives much more readily a long boundary than it does a short one, even if the density difference and the sharpness of the image are the same.