The development of high-speed trains brings further challenges in terms of safety. New materials that are used on railways, such as high-strength structural steel, must be checked , among other things, for internal damage (defects, cracks). NCBJ researchers in cooperation with the Railway Institute are developing new solutions for the use of radiography for non-destructive testing of rails and rail joints.

The railway is a very efficient means of transporting people and goods, which is why the rail network is constantly expanding. At the same time, the technologies used in the industry continue to improve, resulting in high-speed railways that can reach speeds of over 250 km/h. Adapting the infrastructure to such operating conditions requires innovative solutions and modern materials. Specially designed alloys, such as manganese steel, are used for railway turnouts. However, due to the difficulty of combining this type of steel with the structural steel used in the rails, it is necessary to use an additional material, austenitic steel. We are then dealing with rail fragments with a non-homogeneous structure, which adds to the difficulty of examining such structures with standard methods.

Non-destructive methods such as ultrasonography (UT), acoustic emission (AE) or electromagnetic methods such as eddy current testing (ECT), magnetic flux leakage inspection (MFL), alternating field current measurement (AFCM), Barkhausen noise testing (MBN) are typically used to detect surface or internal defects in railway tracks. Radiography, i.e. the examination of components using ionising radiation, is not a standard method for rails, but it allows the most accurate examination of internal structure and defects, especially in welded joints. For this reason, scientists from the NCBJ's Nuclear Techniques and Equipment Department, in collaboration with the Railway Institute, decided to investigate the possibility of constructing a system that would allow rails over 70 mm thick to be scanned, while remaining mobile, simple for a trained operator to use and, most importantly when dealing with radiation, safe. A paper on the research was published in the Journal of Nondestructive Evaluation.

Although a radioactive isotope such as Ir-192 is the typical solution used in the research area outside the laboratory, due to the safety and administrative procedures of using a radiation source, the X-ray tube was chosen as the target solution. "A very important aspect of the research was the selection of optimal parameters for the entire system, such as a sufficiently high voltage of the X-ray tube to enable the thickest parts of the welds in the rails to be scanned," - says Professor Slawomir Wronka of NCBJ, co-author of the publication. "Equally important were parameters typical of digital radiography, such as the distances between the object under examination, the radiation source and the detector. This is because the quality of the images obtained depends on them."

The team of researchers conducted a series of experiments involving the examination of a real section of a 70mm thick rail, according to the radiographic testing standard. The tests included investigating the effect of the distance between the object under examination and the detector (ODD) and between the radiation source and the generated image (SID) on the quality of the images obtained from the detector. "It turned out that it is difficult to optimally select the parameters of the system using digital radiography based only on intuition and experience," explains Jacek Rzadkiewicz, Ph.D., professor of NCBJ and co-author of the paper. "A compromise has to be reached between an optimal system geometry, promoting larger distances, and maintaining a certain minimum dose on the digital imaging detector. Studies have shown that it is possible to use a relatively lightweight tube of only 300 kV to obtain images of the desired quality. It provides sufficient conditions to recognise defects as small as 0.5 mm in size in the image, while keeping the size and weight of the whole device small." The results obtained are superior to those of a comparative study carried out for a radiation source in the form of the isotope iridium-192.

The researchers' work has shown that it is possible to use digital radiography with a commercially available X-ray tube to create a portable device capable of screening welded joints in normal (1435 mm wide) and high-speed railway tracks. The research was conducted as part of the project “Mobile system for radiographic (radiological) inspection of R60E1 or E2 profile rails on PKP PLK railway lines”, funded by the National Centre for Research and Development. Also part of the project is the construction and implementation of a prototype mobile device together with a power and transport platform, which will enable the system to quickly reach the assumed location, efficiently perform the radiographic examination in a safe manner and obtain X-ray images for further analysis in a short time. "With the growth of the railway industry in Poland in recent years, through the modernisation of PKP PLK's railway lines and the introduction of modern fleet, there is a need to maintain supervision of the railway infrastructure in terms of its technical condition," comments Dariusz Kowalczyk, Ph.D., of the Railway Institute, the project consortium leader. "The introduction of this solution will increase the level of safety on railway lines through the timely detection of internal defects, defects and cracks in the rails."

Original publication: Chmieliński, Z., et al. Feasibility Study of Radiographic Rail Inspections Using a Compact X-ray Tube. J Nondestruct Eval 44, 57 (2025). https: //doi. org/10.1007/s10921-025-01193-x