A Guide to Using Digital Radiography in the Aerospace Industry

Customer approvals may create some obstacles as aerospace suppliers enter into the world of digital radiography. It will be easy to receive customer approvals for production using digital X-ray if this recommended path is followed.

The transition from film-based radiography to a digital radiography system is similar to the chicken and the egg story. Which one came first? Suppliers have questioned the investment in capital equipment from the mid 2000s. How can the supplier invest in capital equipment unless and approval is guaranteed?

According to the contractor, before granting approval, the supplier has to invest in training, equipment, and demonstrate the use and proficiency of the equipment.

This endeavor is critical to the aerospace industry. Millions of dollars are spent each year on consumables such as; processing equipment, film, water, disposal fees, and processing chemicals. One targeted area for cost reduction is reducing cost at X-ray. Labor is another big variable for cost reduction.

A company entering in this new endeavor is normally not sure of the labor savings based on finding the correct equipment to increase output and production requirements. For castings, a digital system with CNC motion is vital for cost-savings. A mid-sized casting of around 20" cubed went from two hours on film, exposed to reaching the film interpreter, to 20 minutes with digital.

Another casting, three to four times larger, took about six hours from exposure to interpreter compared to one hour and 15 minutes on digital.

This article covers some basic concerns that suppliers need to be aware of in order to receive customer approval; equipment decisions, training and certification of personnel, part manipulation and magnification, image review, documentation, and data storage.

Equipment Decisions, Tube and Detector

Based on customer’s needs for digital radiography (suppliers must have such specifications with them), the relevant questions are: what are the goals at radiographic inspection? Is there a certain detectable size requirement relative to defects or is a required (IQI) Penetrameter T-hole a concern for image quality?

Based on the goal, a specific size anomaly, the correct tube and detector pair or multiples (dual tube/dual detector) can be matched. The correct tube/detector pair can ensure that proper total image unsharpness is satisfied. The material and the material thickness range are also analyzed so that adequate penetration, contrast-to-noise (CNR), and signal-to-noise (SNR) are satisfied for producing quality images.

If the material is very dense with thin and thick regions, it could be an application for dual tubes. Pairing a micro focus tube to detect smaller anomalies, and a larger 450 Kv, for example, for penetrating the thick regions, all packaged in a single system.

When selecting a detector, resolution is a major factor in the panel choice. Other vital options in a panel other than physical size would be: pixel or resolution pitch, frame rate, dose rating, type of scintillator, X-ray tube combination, and initial cost.

In certain applications, having a smaller panel with a faster frame rate that allows quicker acquisitions with extra frame averaging can provide better image quality and increased throughput.

Part Manipulation

The use of geometric magnification of the object is vital for digital systems, which is the ability to use magnification to change resolution based on the requirements in order to achieve image quality. To ensure the most versatile system for various components, a fully programmable CNC machine enables the use of various distances and magnifications.

This also shortens production times where close tube-to-detector (TDD) distances provide brighter values and as a result, frame rate can be increased and things can be done quickly.

Training and Certification of Personnel

Having qualified and certified operators is a vital aspect for digital system approval. With the availability of aerospace documents like NAS410-revision 4, and prime contractors having specifications, a baseline has been set for digital radiography.

The revised NAS410 reflects the needs for digital radiography for the classroom training as well as on-the-job training hours along with a requirement for Level III to have 40 hours of classroom training. Certification of personnel is not very different to film radiography.

Previous Level I technicians set up parts, exposed and processed films, 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 decides to define this position differently. The acquisition portion has evolved with increased accuracy and consistency and less error due to motion programming.

The Level II technician will have the benefit of revised procedures for the new equipment and general practice. Theory will remain similar in some aspects to film X-ray regarding: Kv, milli-amperage, latitude, and contrast, but many new items will also pose problems during the transition. Terms like, SNR, spatial resolution, CNR, beam hardening, and gray values are just a few.

The company’s Level III technicians will need to have a minimum of 40 hours of classroom training, and in some cases a mentor who can be used for Q&A while the transition is made. New specifications and standards will be used for digital radiography; taking time for research, consulting with other Level III technicians, and attending training/seminars will increase their understanding and knowledge of this new technology.

Documentation

A commonly overlooked item during digital conversion is the documentation portion. The procedures need to be complete and accurate, and must be audited regularly. The written practice portion covers personnel qualification and certification as well as the working "how to" practice for hardware and process checks. A few items in this area are: work instructions, procedures, techniques, logs, forms, and qualification and certification records.

Technique Strategy

Some things are similar but others are different with digital radiography. Some techniques used for film radiography can also be used for digital radiography. For getting approvals easily, the following procedure is recommended:

Parts must be grouped as per material and thickness range, while also keeping physical size in mind. Creation of a technique to image the material (example: aluminum) and a range of thickness can be explored, by imaging step wedges and proving image quality using SNR, CNR, and T-holes, if required.

Once a material and thickness range is established, that is the technique, for instance, Aluminum 0.12" – 1.5" at a specific tube to detector distance and magnification factor. Now, the decision can be made as to which part numbers work well for this technique. The number of techniques required for customer approval can be limited, but each unique part number will have a CNC motion program for part coverage.

Image Review

Interpretation of images will be not be different as a person will still make, reject or accept decisions, but many features or tools are new. Using software and having an electronic image can intimidate some, but others may enjoy this.

All of these new features such as reference image comparison, image annotation, contrast, measurements, and normalization, can turn into second nature with use. Retrieving and filing images can become much easier in the electronic mode.

New tools such as visualization filters exist today to improve and make the images faster and easier to view. Production software also exists when many parts are viewed and time savings are needed for image management and rework.

Data Storage

Electronic data can be helpful, because there is no need to visit a warehouse, or shipping and crating film now. The server must be of optimal size based on image data size, archival time period, and production needs. Currently, the common image formats are TIFF or DICONDE with many shifting to the DICONDE platform. With electronic data, the use of storage device should be considered as it might be required to generate a second copy for data backup.

Conclusion

This article has addressed the basic concerns faced during the conversion from film radiography to digital radiography, providing a starting point and the ability to ask the appropriate questions.

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This information has been sourced, reviewed and adapted from materials provided by North Star Imaging, Inc.

For more information on this source, please visit North Star Imaging, Inc.

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