A Contactless Approach to Monitor Rail Vibrations

Abstract

Continuous welded rails (CWR) are subjected to thermal effects that may lead to buckling or fracture during warm or cold seasons, respectively. The modal characteristics (frequency and mode shapes) of CWR may reveal important information about the thermal stress that can be used to prevent rail failures. The primary objective of this study is to prove a contactless method to monitor the vibration and to extract the modal characteristics of rails using a high-speed camera and advanced image processing. This study is the first step towards a general noninvasive monitoring paradigm aimed at measuring axial stress in CWR. To prove the principles of the proposed paradigm, a finite element model of an unrestrained rail segment under varying length, boundary conditions, and axial stresses was formulated. The results of the model were then used to interpret the experimental results relative to a 2.4 m-long rail subjected to compressive loading–unloading cycles. During the experiment, the rail was subjected to the impact of an instrumented hammer and the triggered vibration was recorded with a high-speed camera. The videos were then processed using the phase-based displacement extraction, motion magnification, as well as dynamic mode decomposition techniques to extract the modal characteristics of the specimen. The results show that the frequencies extracted from the images matched well those obtained with two conventional accelerometers bonded to the rail while the mode shapes extracted from the videos matched those predicted numerically. Additionally, the numerical analysis enabled the interpretation of some unexpected experimental results. The results presented here proved that the proposed method to infer axial stress in CWR requires proper modeling in order to link the modal characteristics of the rails to the axial stress. In the future, the finite element formulation presented here will be expanded to model CWR under given cross-ties and fasteners conditions in order to link the modal characteristics of the rail of interest to its axial stress.