Developing procedures for RTR involves several of the same concerns as that for NDT. As has been stated before, certain industries are required to use RTR and others do so voluntarily. As a result, the procedures used differ: some are very formal and specific; others may not be formal. One example of a specific required procedure is discussed at the end of this section.
Discussions of these areas of procedure development follow next:
- Determining the inspection requirement and accept/reject criteria
- Defining part parameters
- Determining parameters for each area of interest
- Determining which quality control indicators, if any, to use
- Determining archival record needs
- Determining whether parameters meet specification requirements
- Defining personnel qualifications
- Verifying system resolution
- Determining fixtures, masking, shutters, etc.
- Determining defining equipment requirements
- Determining the inspection requirements and accept/reject criteria. What discontinuities must be detected? Cracks, voids, misalignment, or presence or absence of components? Welds may evidence discontinuities such as incomplete penetration, nonfusion, or slag inclusion. Rolled plates may evidence lamination; bars may evidence stringers or seams. Castings may evidence porosity, blowholes, or shrinkage. Forgings may evidence bursts, laps, cold shuts, etc. What inherent discontinuities exist? Does RTR afford the best probability of detection (POD) for this part of material? Determining inspection requirements may involve testing and experimentation to find critical areas if the product or material application is new. Previous industry and acceptance standards must be considered.
- Defining part parameters. What are the thickness and geometric considerations? What type of material is involved? Is 100% inspection required, or will a sampling suffice? Does the part lend itself to inspection on the fly? If image processing is to be done, then the part may have to be stopped. Here, also, testing will probably have to be done to determine parameters. In instances such as checking food products for contaminants, either the product can be penetrated and the system will show enough contrast between the contaminant and the product, or it cannot be done. Comparing RTR images with film images may be helpful.
- Determining parameters for each area of interest. Does energy level and the MA need to be varied for different ROIs? Which source-to-object and object-to-imaging detector distance needs to be used? Must it be varied during the inspection? A scan plan must be defined: How is the part to be moved? Tilted? Turned on its axis? What speed can be used which retaining and detection capability? Testing and experimentation is needed, and comparison to existing film-based imaging can be utilized if RTR has not previously been used on the part/material.
- Determining which indicators, if any, to use. Film-based penetrameters are options, as are, wire gauges, line pair gauges, and, in the case of food-product tests, a sphere of carbon and stainless steel down to .5 mm measured through the product. If wire penetrameters are used, decisions must be made as to what diameter of wire to use. A distinction must be made between gauges used to set up and to establish the image quality of the system and those used to check the radiographic image quality of production pieces/materials.
- Determining archival record needs. Several possibilities are available. The traditional film radiograph is considered to have a long archival life. In most instances, however, the archival life of RTR is even longer. There are instances when due to the subjectivity of the interpretations process, a need may arise to review an image. Analog recording involves the use of a videotape recorder. This method is fairly inexpensive and the recorded images have a long life. The videotape referred to in the application booklet was recorded this way. Hard-copy video recordings can be made on thermally sensitive paper with modest quality and a limited shelf life. Digital recording will produce an archival image of the same quality as the original image because it is stored as a numerical pixel array. Two methods of recording are magnetic and optical. Magnetic media include magnetic floppy and hard disc and magnetic tape. Optical media include the laser optical disc and the laser holograph. Attention must be paid to protecting recorded images that they are not written over. Digital image recording is suited best to single frames onto a few frames of motion because a great deal of space is required for each video frame.
- Determining whether parameters meet specification requirements. What specs are involved? As RTR gains wider acceptance and use, more and more specifications will emerge regarding the process. Customers needs must be considered. These can involve a variety of material from the traditional metals, to composites, to US Department of Agriculture standards for food inspection.
- Defining personnel qualifications. Because RTR is required at times, and optional at others, personnel qualifications also differ. Requirements may be those of the American Society for Nondestructive Testing (ASNT), or a written practice recognized by the ASME. Personnel may be required to have Level II or III qualifications in radiography, with minimal additional training in operating, imaging, and interpreting the system. Other sources of specifications may include the American Welding Society (AWS), American Petroleum Institute (API), National Aeronautics and Space Administration (NASA), the Department of Defense (DOD), and the Federal Aviation Administration (FAA).
- Verifying system resolution. Addressing issues of checking the system itself and checking to see if the resolution of an actual production piece with known defect(s) may be used, ASTM E1255 can be referred to for guidance. These resolution checks usually are conducted at least at the beginning and at the end of each shift. It must be determined whether the test is to be conducted in the static mode or on the fly.
- Determining fixtures, masking, shutters, etc. Such determinations depend on the specific part and material and can be determined best by means of testing and experimenting. Surfaces must meet application specifications. With RTR, as with film radiography, weld ripples may need to be conditioned so that they do not mask or become confused with the images of other discontinuities. The area or the section examined must be identified and be traceable back to the part at a later date, if need be.
- Determining defining equipment requirements. The necessary components to produce final image requirements is critical here. Application of RTR involves a wide variety of parts/materials. Equipment is relatively expensive and, to keep costs down, only the components necessary to produce the required image should be required. If magnification of images is not necessary, microfocus X-ray tubes probably are not either. If a new system is being considered, the decision needs careful consideration because a compromise between minimum system capability and cost must be achieved.