PSI - Issue 17

João Morais et al. / Procedia Structural Integrity 17 (2019) 448–455

453

João Morais et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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the video camera was equipped with an optical bandpass filter that cuts radiation with a wavelength of less than about 750 nm, thus eliminating most of the visible radiation spectrum. As an alternative to traditional passive targets, these tests used active LED targets, developed in LNEC, which emit radiation in the near infrared spectrum (emission peak at 875 nm). These active targets were mounted on the side of the rails of the railway track, facing the video camera. The six uniaxial accelerometers used, type Force Balance (reference FBA ES-U2), produced by EpiSensor, were also mounted on the side of the rails. Signal conditioning and data acquisition were performed using a monitoring system developed in LNEC. For the vision system, data was collected either with several static photos or through films acquired at a rate of 10 frames per second. The on-site accelerometers data was also acquired at 1000 Hz (Fig. 5).

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Fig. 5. Instrumentation installed on the railway track: a) accelerometers on the rails; b) industrial-type video camera, c) active LED targets; d) image taken with the video camera, using an optical bandpass filter.

The calibration tests were performed in two phases: off-set determination for all the on-board instrumentation (on the concrete slab of the maintenance yard) and measurements of the structural response of the selected railway track using the on-board and on-site instrumentation, due to the passage of the railway vehicle. The first phase consisted on establishing the zeros of the displacement transducers, i.e., registering data from these transducers acquired under conditions corresponding to infinite stiffness of the underlying platform. To this end, it was considered that the concrete slab of the maintenance yard met these conditions, when compared to the typical stiffness ranges from railway tracks. Initial off-set determinations were also carried out for all accelerometers. Thus, with the vehicle stationary in this area, the necessary measurements were made. The number of samples collected allowed us to make a statistical analysis and to determine the so-called "zeros of the system". The second phase tests were performed at low vehicle speeds (around 1 m/s). The tests consisted on several passages of the instrumented vehicle over the selected railway track. Due to the low velocities that were possible to employ safely in such a small track length, all the data corresponding to accelerometers was not usable (low energy levels that could not produce relevant dynamic results). Post-processing of the obtained data and the subsequent analysis of the results allowed us to conclude that the vision system was successful in monitoring the deformed state of the rail, resulting from the passage of the vehicle. The peak deformation values obtained with this method were comprised between 2.0 mm and 2.7 mm for the several vehicle passages. The embedded displacement transducers, after applying the previously described processing method to the data, were also capable of measuring railway deformations between 2.0 mm and 2.6 mm. However, an unforeseen problem was detected on the data provided by the two central displacement transducers. The zones selected to install these transducers appear to be highly susceptible to the engine induced vibration frequency, thus entering controlled resonance vibration conditions under specific circumstances. Still, it is considered that the results obtained with the two systems are compatible with each other. Fig. 6 shows the obtained results with the vision system on one of the vehicle passages. Apart from the induced vibration on the railway track, due to the presence of the vehicle, this figure clearly shows the passage of both axles over the instrumented area. Also, other important features, such as the slight negative deformation of the railway track when the vehicle’s middle section passes over the targets, are possible to acquire with this vision based system.