Development of a new laser doppler vibrometer-based non-contact damage detection system for cracks in rail head

Rail defects are one of the dominant causes of train derailments and an essential factor affecting transportation safety. Among the rail defects, transverse defects (TDs), which are cracks located transversely in rail heads, are one of the main causes of derailments. When TDs are left undetected, their size expands, leading to rail breaks. Therefore, the railway transportation community is interested in the detection of such defects at speeds that do not obstruct the routine railroad operation. The goal of this research is to develop a novel LDV-based noncontact damage detection system for TDs. The tasks performed herein to achieve this goal (i.e., the objective of the study) were: (i) extensive literature review and in-situ testing to understand the vibrations resulting from the propagating waves in rail, (ii) numerical modeling of the damage detection system, (iii) rigorous laboratory and in-situ testing to understand the noise in LDV measurements as well as to evaluate the performance of the damage detection system, and (iv) analytical work to develop filters to minimize the noise in the LDV measurements. Accordingly, the configuration of the developed damage detection consists of two LDVs attached vertically in front of a rail car to measure guided waves in the rail head, which are induced by rail-wheel interaction. This system uses the LDV measurements to detect a change in the relative amplitudes of the recorded waves caused by a defect in the frequency range between 30 kHz to 100 kHz. The lower cut-off frequency was selected conservatively since it was shown in the literature that guided waves start to localize in the rail head after approximately 15 kHz. The higher cut-off frequency was selected since (i) the guided waves below 100 kHz can be used for transverse defect detection (as the frequency exceeds 100 kHz, waves are susceptible to surface defects), and (ii) the measurements collected from rail during the passage of operating trains showed that the power of the excitations induced by wheel-rail interactions is dominant up to approximately 100 kHz. The main challenge during the development of the system was speckle noise, which is inevitable due to the inherent nature of the measurements performed by LDVs placed on a moving platform. Consequently, the damage detection framework associated with the system operates as follows: 1) in the pre-processing stage, time-varying mean and impulsive noise in the recorded LDV signals are filtered and then the changes in the LDV signals in the frequency range of interest are quantified and monitored using moving standard deviation, 2) in the post-processing stage, two damage features, which are based on the relative change in the moving standard deviations and transfer functions between two measurement points are combined using multivariate statistical analysis to create a damage index that shows the location of rail segments which are affected by a defect. The goal of impulsive noise filtering and transfer functions in the framework is to minimize the speckle noise. The field tests demonstrated that rail segments consisting of a defect can be identified by the developed system.Civil, Architectural, and Environmental Engineerin

Numerical Simulations to Examine the Interaction of Train-Induced Guided Waves With Transverse Cracks

This paper presents the results of a numerical study aimed at investigating the propagation of guided waves generated by train wheels on rails. The particular application of interest is to exploit such waves for the identification of transverse cracks located in the rail head. The research presented is part of an ongoing project aimed at developing a non-contact damage detection system based on laser Doppler vibrometer measurements. In this study, numerical simulations were carried out using three rail models. The first two models consisted of a transverse crack whose size is 20% and 10% of the rail head cross-section (RHC), respectively. The third model consisted of the same crack size as the first model, but it was designed to be longer before and shorter after the location of the crack compared with the first model. The goal of this model was to examine the location where the effect of cracks on the waves first appeared. To acquire train-induced guided waves to use in the simulations, an accelerometer was placed under a rail head, and propagating waves were recorded during the passage of an operating train. Afterward, on each rail model, the location of excitation and two measurement points were shifted forward over the rail to replicate the movement of the defect detection system. Two damage functions were used to examine the change in wave propagation caused by cracks, while a multi-dimensional damage index consisting of the damage functions was used to identify the location of the crack (20% of RHC). </jats:p

Experimental Investigation of the Modal Response of a Rail Span during and after Wheel Passage

This paper examines the vibrations of a rail span (rail section between two consecutive sleepers) during and after the passage of a rail car’s wheel as well as under impact hammer excitation. In literature, the dynamic response of railway tracks under moving loads has been studied extensively. Many of these studies focus on the responses in relation to displacement/force-time histories and wave propagation parameters. These responses are investigated for the time instants when rail car wheels transverse over the rail spans of interest. In this context, an investigation of responses in relation to modal parameters during and after moving loads might provide additional information. Such information can be used to examine how the loading and additional masses induced by the moving wheels affect the dynamic responses. To this end, field tests were carried out at Transportation Technology Center Inc. (TTCI) facility in Colorado, U.S. First, to find the flexural modes of a rail span under no loading, data was collected from three accelerometers placed on the span under vertical impact hammer excitation. Next, the accelerometers were placed underneath the rail span, and data was collected while a rail car traveled over the span. The signal segments corresponding to during and after a wheel passage were analyzed for the identification of modal parameters. The comparison of the results demonstrated that the frequencies of the rail span increased as the loading induced by the wheel increased. </jats:p

System Identification Of Bogazici Suspension Bridge During Hanger Replacement

Bogazici suspension bridge is one of the long-span bridges crossing over the Bogazici strait. It was opened to traffic in 1973 and from that day on it has been one of the most important links in the traffic network of Istanbul. It has a main span of 1074m and the original configuration of the suspension hangers was inverted - V shape. In 2015, the suspension cables were replaced and the orientation of the cables was changed to vertical. During this campaign, vibration measurements were recorded on the tower, deck and the suspension cables of the bridge. Measurements were taken at different stages of this campaign to have an understanding of the effects of cable replacement on the overall dynamic behaviour. Additionally, measurements were recorded for at least several days to several weeks to observe operational variations on the modal frequencies of vibration. Results show that frequencies of the deck motion dominant modes decreased by 6 to 12% and there were almost no change in the tower motion dominant modal frequencies due to replacement of the hangers