Railway structures: thermographic analysis on welded butt joints

In the railway sector, aluminium is becoming the preferred alloy for the car-body shell construction. Welding aluminium alloys for structural applications has to comply to specific Standards that allow to guarantee the proper design, the manufacturing and the in-service reliability. In the railway market, welding is often defined as Special Process. Indeed, even if the process parameters are controlled, the conformance of the resultant weld can be checked only with destructive tests. Although some destructive tests can be done in the welding validation phase, in the past years, the non-destructive evaluation of these structures has assumed a crucial role both at a manufacturing and at in-service levels. In this work, a preliminary examination of the most used Standards for aluminium welding in railway constructions has been done in terms of: arc welding process description, principal defects that can be generated during the arc welding process and the acceptance quality levels for the welded aluminium structures. The second part of this work focuses the attention on the most used non-destructive techniques applied to the welds. A general description of ultrasonic, radiographic, visual and penetrant testing is given with their reference Standards. The thermographic investigation is the emerging technique that can be used to assess the structure conformity of a welded component. Due to a lack of ISO standardization on thermography applied to the welds, the theory of both the active and the passive thermographic investigations is presented, with the main fields of application found in literature. Regarding the third part, an experimental procedure has been carried out with the goal of comparing the material response in a passive and active thermographic investigation. In this context some plates with a butt-joint in the middle have been prepared. In these samples the most occurrent defects that can be found in welding have been reproduced: inclusions, incomplete root penetration and porosities. A temperature-based post-processing technique has been elaborated to evaluate the temperature trend during the fatigue test with the passive thermographic investigation and the temperature trend before and after the fatigue test with active thermographic investigation.