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PROCEDURE
DEVELOPMENT
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. Customer’s
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.
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