Most everyone is aware of changes affecting our everyday lives due to the evolution of digital technology. Primarily, the health care industry has taken the lead in implementing digital radiography, better known as digital radiology for medical use. The industrial community also has put digital radiography to use, but on a much smaller scale. Those that have invested and implemented digital radiography have driven research and development.
Many industrial manufacturing and nondestructive testing (NDT) companies-mostly aerospace-have prospered by implementing NDT technologies, such as digital radiography, and applying continuous improvement. Such implementations often result in improved inspection efficiency, productivity and profit margins.
A variety of product forms can benefit from digital radiography and related analog methods, including radiographic evaluation of metallic, composite and electronic components and structures. Sources of radiation involve X-ray (standard, fractional and micro-focus tubes), gamma and neutron sources.
Some inspection applications are not possible using radiographic film, such as real-time and 3-D computed tomography-making digital radiography an ideal choice. For example, one company converted to a non-film-based system, which required design specification and implementation of a real-time radiography micro-focus system using an image intensifier and an analog cassette tape recorder. The system was approved by Morton Thiokol and the U.S. Navy for evaluation of circumferential electron beam welding of standard missile cases in 1987.
Regardless of the application, when implementing a digital radiography system, the selection criteria include ergonomics such as safety, comfort, ease of use, productivity, performance and aesthetics.
When the right system and associated equipment is selected, benefits of implementing digital radiography include improved flaw detection and evaluation capabilities. Depending on the job, equipment consists of X-ray tubes; high-resolution panels or imaging plates if needed; robotics; and new operating system tools. The appropriate pre-qualification needs to be performed to ensure it is optimized and validated, and personnel need to be trained on the digital radiography equipment.
To evaluate digital radiography equipment, the inspector must pay attention to equipment reliability, improved efficiency, training and learning-curve objectives, OEM technical support and the rate of investment return.
In all cases, the key initiatives that drive conversion to digital or similar advanced radiography methods are: reduced cost, improved production and improved flaw detection.
Using digital radiography, whether the image is produced by a digital radiography panel or computed radiography imaging plate, the radioactive source power and exposure time is substantially reduced (usually by 30% to 50%) and, in the case of computed radiography, image transfer by means of electronic processing is at least five times faster than chemical film processing. Using digital radiography panels, the processing step is completely eliminated, and the exposure-to-read time is only a few seconds.
What Processes are Ideal?
With a few exceptions, most applications can be performed by, or converted to, digital radiography. Unlikely processes can be identified by a few simple questions about the current and proposed radiographic inspection plans.
Digital radiography companies typically are eager to provide onsite complimentary demonstrations and evaluations of the product to be inspected. Be prepared by having a sample replicating the item or structure with all representative manufacturing defects included. Keep in mind that simulated defects do not always validate actual detection capability.
The following queries can help determine if a product should be considered for digital radiography. This is not necessarily fail-safe but is a good starting point. In addition, it is highly recommended that the processes be evaluated by qualified and certified Level III personnel.
Is film radiography performed using flexible vinyl film holders and/or pre-packaged envelopes (one or more 14-inch by 17-inch may be overlapped) and are the parts to be inspected curved? The curvature must be matched by attaching the film to the structure using tape-use a quad film load to simulate the imaging plate in a flexible film holder.
If yes, then it is possible to use a digital radiography panel, likely with computed radiography imaging plate for this application.
Using a flexible film holder or pre-packaged film envelope only, is the film rolled into a tight circle for ID placement inside a small pipe or rounded structure?
If yes, then this application is not a likely candidate for digital radiography.
Does the inspection technique require difficult positioning or jamming of the film into restricted access areas, often resulting in film damage? Is custom cutting or folding of the film required, often resulting in marginally acceptable radiographic results?
If yes, then this application is not a likely candidate for digital radiography either.
Some computed radiography imaging plates can be hand-cut and still processed, depending on the OEM’s recommendations. A computed radiography system with a flat-bed scanner-type processor, similar to a standard photo or document scanner but of a much larger scale, is likely to work best with cut computed radiography imaging plates. The computed radiography feeding process typical of computed radiography imaging plate systems is then eliminated; therefore jam-ups from cut computed radiography imaging plates cannot occur.
Benefits and Considerations
Inspection professionals should consider the following benefits and elements of digital radiography inspection:
Image sharing. Engineering evaluation, component repair and customer support often require evaluation of the radiographic image. Electronic sharing capability via the Internet or a digital storage device eliminates the need to produce a duplicate radiographic film or risk handling or transporting damage of the original film. Time and materials are again preserved by digital radiography.
Chemical and film hazardous waste. The cost and burden of hazardous waste created by chemical processing and film with a traditional radiography system is eliminated by using a digital radiography process. Cost associated with film chemical processing and the radiographic film produced to evaluate the component or assembly creates a substantial amount of liquid and solid hazardous waste. Associated cost for silver recovery equipment and required proper waste disposal management must be considered. No chemicals are needed with digital radiography-whether the system uses direct and indirect signal processing, or indirect signal processing of the computed radiography panels by means of a scanner.
Film retention and archival. Design specifications often require radiographic film or other recording media to be archived for the design life of the component, which in some cases may well exceed a century. The burden of storing radiographic film in a controlled environment to maintain critical film characteristics for re-evaluation, if required, is most often the responsibility of the component manufacturer.
A tremendous amount of underlying cost is generated by radiographic film archival and associated management, making digital data storage a much more reasonable and manageable process.
Training and certification. Training is required for digital radiography inspections. Experience and time to get through the learning curve are needed to become proficient with operating the equipment and software programs. One benefit is that the certification level of the inspector for film radiography is transferable to filmless radiography. The inspector must become familiar with the process, however, to become proficient with the numerous new evaluation tools, high-resolution monitors that replace the high-intensity film viewer, processing, and file management and archiving.
Robotics and artificial intelligence. While robotics and artificial intelligence are not yet commonplace for industrial radiography, more companies are successfully developing and marketing this technology. As an example, a semi-automatic system was developed a few years ago that incorporates a six-axis robotic digital radiography system consisting of the robot with the 225 kilovolt X-ray tube and digital detector panel mounted on a C-arm fixture with precision motorized turntable. The system is capable of producing repeatable, better than 2% sensitivity images on aerospace investment castings up to 60 inches in diameter.
The system’s master control/ interpretation station allows for the coordinated control and communication between the X-ray system, digital panel, robot and turntable position, in addition to allowing the operator to perform image interpretation while additional images are being captured, which greatly increases inspection throughput. Due to easy coupling and the new advantages that equal and exceed film sensitivity and resolution using digital radiography, semiautomatic and automatic inspection technology is available or can often be custom designed for specific applications.
Digital Radiography and Computed Radiography Imaging Plates. Digital radiography panels are available up to 17 inches by 17 inches, but as the size increases there is a substantial increase in the panel cost. These panels can often be custom made for specific applications. Digital radiography panels and imaging plates can potentially provide thousands of exposures.
Accidental damage may render either irreparable. The cost for replacement can exceed $70,000 for a large high-end digital radiography panel. Computed radiography imaging plate manufacturers often make imaging plates like standard film sizes and roll film lengths up to 60 inches. Most offer only 14 inches by 17 inches or a small variety of different sizes to match their standard hard cassettes, also required for processing with most imaging plate systems. Imaging plate cost is the most expensive for larger sizes, from around $200 to $2,000.
Maintenance plans and service agreements. These are usually provided for a specific warranty period and will result in a yearly renewal fee at around $10,000 per year.
Upgrade plan. Software and PC upgrades are normally an additional charge and can result in a substantial cost to the user, usually similar to the cost of upgrading to a high-end PC. As with typical PC-based systems, upgrades are often required as the OEM’s system capability improves, and additional memory or operating systems are often needed to support these. Projected evolution can be expected to occur in three- to five-year intervals.
These benefits and equipment requirements are important considerations when evaluating whether to implement a digital radiography inspection system. It is important to remember that the customer has final approval for the use of a recording media other than film-and, therefore, it is important to obtain advanced approval to use digital radiography for any project. Q
Tech Tips
The key initiatives that drive conversion to digital or similar advanced radiography methods are: reduced cost, improved production and improved flaw detection.
Electronic sharing capability via the Internet or a digital storage device eliminates the need to produce a duplicate radiographic film or risk handling or transporting damage of the original film.
The cost and burden of hazardous waste created by chemical processing and film with a traditional radiography system is eliminated by using a digital radiography process.