Aerospace Suppliers Entering the World of Digital Radiography
Computed Tomography for Aerospace
As aerospace suppliers enter into the world of digital radiography, there may be some obstacles with customer approvals. Receiving customer approvals for production using digital X-ray can be easier when following this recommended path.
The scenario of transitioning from film based radiography to a digital radiography system has been recently seen as the chicken or the egg. Which one came first? Since the mid 2000’s suppliers have questioned investing in capital equipment. How can the supplier invest in capital equipment unless an approval is guaranteed? From the contractor’s perspective, the supplier would need to invest in equipment, training, and demonstrate proficiency and use of the equipment before granting approval.
When we look at the rationale behind why this endeavor is so critical to the aerospace industry, it goes back to the good ol’ dollar. When millions of dollars are spent each year on consumables such as; film, processing equipment, processing chemicals, water, and disposal fees. Reducing cost at X-ray has been a key targeted area for cost reduction. Another large variable for cost reduction is associated with labor. A company moving into this new endeavor is normally unsure of the labor savings based on finding the correct equipment to increase throughput and production needs. From my personal experience with castings, a digital system with CNC motion was key for cost savings. A midsized casting of around 20” cubed went from 2 hours on film, exposed to reaching the film interpreter, to 20 minutes with digital. A different casting, 3 to 4 times larger, took about 6 hours from exposure to interpreter compared to 1 hour and 15 minutes on digital.
In order to receive customer approval, I’ll cover some basic concerns suppliers need to be aware of; equipment decisions, part manipulation and magnification, training and certification of personnel, documentation, image review and data storage.
Equipment Decisions, Tube and Detector
Based on customer requirements for digital radiography (specification which you must have), what are the goals at radiographic inspection? Is there a certain detectable size requirement relative to defects or are you looking for a required (IQI) Penetrameter T-hole for image quality? Based on your goal, a specific size anomaly, we can now match the correct tube and detector pair or multiples (dual tube/dual detector). The correct tube/detector combination will ensure proper total image unsharpness is met. The material and the material thickness range is also reviewed so adequate penetration, contrast to noise, and signal to noise are met in order to produce quality images. If your material is very dense with thick and thin regions, this could be an application for dual tubes. Pairing a micro focus tube for detecting smaller anomalies, and a larger 450Kv (for example) for penetrating the thick regions, all packaged in one system.
When choosing a detector, resolution is a huge factor in panel choice. Other important options in a panel besides physical size would be; resolution or pixel pitch, frame rate, type of scintillator, dose rating, initial cost, and X-ray tube combination. In some applications a smaller panel with a faster frame rate, allowing quicker acquisitions with more frame averaging can produce increased throughput and better image quality.
A vital piece of your digital systems puzzle is use of geometric magnification of the object. The ability to use magnification to change resolution based on your requirements to achieve image quality. To ensure the most versatile system for differing components, a fully programmable CNC machine allows you to use different distances and magnifications. This also reduces production times where close TDD (tube to detector) distances give brighter values allowing you to increase frame rate and do things quicker.
Training and Certification of Personnel
One of the key components to your digital system approval will be having qualified and certified operators. With the evolution of aerospace documents like NAS410 reaching revision 4, and Prime Contractors having specifications, a baseline has been set for digital radiography. The revised NAS410 now reflects the requirements for digital radiography for both classroom and On the Job training hours along with a requirement for Level III to have 40 hours classroom training. Certification of personnel is still similar to film with a few differences.
Previous Level I’s set up parts, exposed and process film, and performed basic process checks. Now that position can be turned into a Load and Go operator if the company has a complete CNC motion package and chooses to define this position differently. The acquisition portion has evolved with less error and increased accuracy and repeatability due to motion programming.
The Level II technician will see revised procedures for the new equipment and general practice. Theory will remain similar in some aspects to film X-ray regarding; Kv, latitude, milli-amperage and contrast, but many new items will also create challenges during the transition. Terms like; spatial resolution, signal to noise, beam hardening, contrast to noise and grey values are just a few.
The company’s Level III’s will be required to have a minimum of 40 hours classroom, and in some cases a mentor who they can use for Q&A while the transition is made. New specifications and standards will be used for digital radiography; taking the time to research, consult with other Level III’s, and attending training/seminars will increase their knowledge and understanding of this new technology.
One of the most overlooked items during digital conversion is the documentation portion. The dreaded procedures, to be complete and accurate, and expected to be audited repeatedly. The written practice portion covering personnel qualification and certification and also the working “how to” practice for hardware and process checks. Some of the items in this area could include; work instructions, techniques, procedures, forms, logs, and qualification and certification records.
Things are similar but different with digital. If you choose to make techniques like you did with film, you are free to do so. If you would like to make life easier for you and your approving agency, look at this scenario.
Start with grouping parts by material and thickness range, while also keeping physical size in the back of your mind. Look at creating a technique to image the material (example: Aluminum) and a range of thickness. By imaging step wedges and proving image quality using CNR, SNR and T-holes if required. Once a material and thickness range is established, that is your technique. For example, Aluminum .12” – 1.5” at a specific tube to detector distance and magnification factor. Now you can decide what part numbers work well for this technique. You will limit the number of techniques required for customer approval, but each unique part number will have a CNC motion program for part coverage.
Interpretation of images will be similar because a person will still make, accept or reject decisions, but many tools or features are new. Using software and having an electronic image may seem intimidating to some while others will enjoy this. All of these news features such as, image annotation, reference image comparison, contrast normalization, and measurements, will become second nature with use. Retrieving and filing images will become much easier in the electronic world. New tools like visualization filters are available to enhance and make the images easier and faster to view. Production software is also available when many parts are viewed and time savings is required for image management and rework.
Electronic data will become your friend, because you are not traveling to a warehouse, or shipping and crating film anymore. Properly size your server based on image data size, production needs, and archival time period. Most image formats today are TIFF or DICONDE with most moving towards the DICONDE platform. Keep in mind, with electronic data, requirements will exist for the storage device and you may be required to create a second copy (backup data).
I have addressed the basic concerns you will face in the conversion from film to digital radiography. This will give you a starting point and ability to ask the right questions.