Radiography in Modern Industry - Kodak

a radiographic sensitivity level of 2-2T. The first symbol (2) indicates that the penetrameter shallbe 2 percent of the thickness of the specimen; the second (2T) indicates that the hole having adiameter twice the penetrameter thickness shall be visible on the finished radiograph. The qualitylevel 2-2T is probably the one most commonly specified for routine radiography. However, criticalcomponents may require more rigid standards, and a level of 1-2T or 1-1T may be required. Onthe other hand, the radiography of less critical specimens may be satisfactory if a quality level of2-4T or 4-4T is achieved. The more critical the radiographic examination--that is, the higher thelevel 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 PressureVessel Code require a penetrameter similar in general to the ASTM penetrameter. It containsthree 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 is1/6 inch. Penetrameters 0.010 inch, and less, in thickness also contain a slit 0.010-inch wide and1/4 inch long. Each is identified by a lead number designating the thickness in thousandths of aninch.Equivalent Penetrameter SensitivityIdeally, the penetrameter should be made of the same material as the specimen. However, this issometimes impossible because of practical or economic difficulties. In such cases, thepenetrameter may be made of a radiographically similar material--that is, a material having thesame radiographic absorption as the specimen, but one of which it is easier to makepenetrameters. Tables of radiographically equivalent materials have been published whereinmaterials having similar radiographic absorptions are arranged in groups. In addition, apenetrameter made of a particular material may be used in the radiography of materials havinggreater radiographic absorption. In such a case, there is a certain penalty on the radiographictesters, because they are setting for themselves more rigid radiographic quality standards thanare actually required. The penalty is often outweighed, however, by avoidance of the problems ofobtaining penetrameters of an unusual material or one of which it is difficult to makepenetrameters.In some cases, the materials involved do not appear in published tabulations. Under thesecircumstances the comparative radiographic absorption of two materials may be determinedexperimentally. A block of the material under test and a block of the material proposed forpenetrameters, equal in thickness to the part being examined, can be radiographed side by sideon the same film with the technique to be used in practice. If the density under the proposedpenetrameter materials is equal to or greater than the density under the specimen material, thatproposed material is suitable for fabrication of penetrameters.In practically all cases, the penetrameter is placed on the source side of the specimen--that is, inthe least advantageous geometric position. In some instances, however, this location for thepenetrameter is not feasible. An example would be the radiography of a circumferential weld in along tubular structure, using a source positioned within the tube and film on the outer surface. Insuch a case a "film-side" penetrameter must be used. Some codes specify the film-sidepenetrameter that is equivalent to the source-side penetrameter normally required. When such aspecification is not made, the required film-side penetrameter may be found experimentally. In theexample above, a short section of tube of the same dimensions and materials as the item undertest would be used to demonstrate the technique. The required penetrameter would be used onthe source side, and a range of penetrameters on the film side. If the penetrameter on the sourceside indicated that the required radiographic sensitivity was being achieved, the image of thesmallest visible penetrameter hole in the film-side penetrameters would be used to determine thepenetrameter 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 materialRadiography in Modern Industry 92