PSI - Issue 18

Jan Chvojan et al. / Procedia Structural Integrity 18 (2019) 443–448 Author name / Structural Integrity Procedia 00 (2019) 000–000

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But the maximum stress amplitudes are the same in both town parts. Detail of measured time histories of three strain gauges positioned on one insulator (position see Fig. 2c), two strain gauges on the bottom box beam T4, T5 and roof excitation are presented in Fig. 6. The maximum measured insulator strains during operational tests in Bologna were 122 μmꞏm -1 . According these results we estimated, that the insulator was exposed to low cycle damage during its operation. The level of sheer stresses was lower in comparison with bending stresses.

Fig. 6 An example of detail strain and acceleration time histories obtained during the test The main fundamental observation was made on the insulator stress during the trolleybus performance. The insulators were stressed in the transversal direction to the trolleybus axes. Strain gauge a, situated in the longitudinal direction, is very low stressed. The main stress was the bending of the insulator in the transversal axes. Both sides of the insulators (strain gauges b and c, see Fig. 2c) oscillate in opposite phases to each other. All three measured insulators oscillate in phase. Also strain gauge, positioned on the internal transversal beam in the brake resistor box, oscillates with the same frequency. The character of each response to the pavement irregularity is the same in form of resonance beat at the same frequency of about 13 Hz. The acceleration in transversal and vertical direction has the similar character. However the main frequency is about 11 Hz. The proximity of both frequencies makes the hypothesis, that the resonance frequency of the brake resistor box is well excited with the vibration mode of the trolleybus structure. When we compare the measured accelerations with those on another trolleybuses measured in the past, the extreme and effective values, obtained in Bologna are higher also in comparison with the standard EN 61 373. However, the peak at frequency characteristics of transversal acceleration component above 10 Hz is visible also for other trolleybuses. It is obvious that mentioned standard is not suitable for testing the devices for trolleybuses. 2.3. Laboratory vibration tests According performed operational tests; one part (Fig. 5) was selected and used for vibration table excitation based on measured acceleration time histories. This part was filtered and two times integrated to obtain the displacement, needed for vibration table excitation in three directions. Acceleration time histories used for vibration table excitation are visible in Fig. 7 (Ax .. transversal, Ay .. longitudinal, Az .. vertical directions). All insulators were instrumented with strain gauges.