Structural condition assessment based strain-stress behaviour for railway welded rail joint using rosette fibre Bragg grating optical sensor

The rail provides a medium for trains to operate along and a support system for the cargo trains to carry. Proper caution should be exercised when handling the thermite welded rail, as it can be vulnerable to damage and failure if not properly maintained. The thermite welded rail joint experiences the greatest amount of train impact and has the least material rigidity. However, measuring the deformation or strain that occurs on a rail is difficult as limitation of the strain gauge itself and challenges to perform the measurement on site due to surroundings. A rectangular rosette Fibre Bragg Grating (FBG) sensor setup was mounted on the web surface of the rail to obtain three distinct strain directions, in order to measure and monitor the direct strain on the thermite welded rail joint. The location for the FBG sensor setup is determined by the presence of visible defects and the production of load sounds when the railway passes through specific locations on the rail track. The FBG sensor's wavelength signal displays three distinct patterns for each strain direction and is used to compare the results between the thermite welded joint and the smooth base rail structure. The wavelength signal exhibits tensional strain, compressional strain and tensional and compressional strain in combination. The principal strain and principal stress are examined between four different locations. The strain value obtained from FBG sensors show instability and variations, which can be attributed to factors such as the dynamic wheel-rail interaction and the condition of the rail structure. Therefore, the FBG sensors are proven to be acceptable in rail track condition assessment based on direct strain measurement for running rail track applications.