Scanning inductive and line laser thermography for surface crack detection and characterization in railway rails

Head checks and transversal cracks are very common defects on rails due to rolling contact fatigue (RFC) phenomena. Despite not reaching the nominal mechanical strength threshold during operation, these cracks tend to propagate and expand, potentially culminating in abrupt component failure during service. This underscores the crucial role of assessing the remaining service life of these components. Therefore, the development of non-destructive testing methodologies is mandatory. Nowadays, railway rails are usually inspected through ultrasound, visual inspection, and eddy current testing, with some limits due to the contact with the component and the number and type of detectable defects. In the last few years induction and laser thermography have been proposed as innovative non-destructive techniques for rail inspection, with interesting results, considering low speeds and laboratory setups. In this work, we show some results from inspecting rail pieces after induction and line laser thermographic tests, considering the inspected component moving at speeds up to about 12 km/h. With an image reconstruction algorithm and some filter operations unique images of rail piece have been obtained, showing the information related to surface defects and their dimensions and position. Additionally, different coil geometries have been developed and proposed to optimize induction inspection. To align with on-site applications, the experimental tests have been performed considering a setup configuration with mirrors. Following an investigation into the effects of test parameters on the detectability of the relevant cracks, such as scan speed, frame rate and type of excitation, a preliminary procedure for detecting real cracks is introduced, tailored for on-site applications, and validated through experimental results.