Step-heating thermography NDT for new composite high-speed rail carbodies

The motivation of this paper is to explore the application of Step-Heating Thermography (SHT) as a technique capable of inspecting new composite rail carbodies using demanding requirements set by the rail manufacturing industry. A large composite sample, with Polytetrafluoroethylene (PTFE) artificial defects, replicating a side-wall section of a new rail carbody, was manufactured and inspected for surface and subsurface defects in this research. The sample, characterized by its large thickness, consists of a monolithic Carbon Fibre Reinforced Polymers (CFRP) component (20 mm thickness) and a CFRP-PET foam-CFRP sandwich (40 mm total thickness) component fused together. The main challenge of the inspection procedure was to apply reflection mode thermography and detect defects in the entire thickness of the sample that exhibits both low emissivity and thermal insulating properties, especially at the sandwich sections of the sample. The paper explored thermography procedures that would be able to detect large numbers of defects under one single acquisition and would be applied under an automated inspection process leading to the detection of defects only up to 5 mm in the CFRP sections of the sample while no defects were able to be detected at the back skin of the sample.