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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of the box producer was that the damage is caused through the box twisting due to wrong mounting the box on the roof with insufficient torsion stiffness. However mentioned laboratory vibration tests are assigned for rail vehicles but they may not be sufficient for road vehicles.

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Fig. 1. (a) Tested brake discharging resistor block; (b) position of the box on the front part of trolleybus roof

2. Experimental work The goal of presented work was to find out the really reason of the damage of the insulators. The assumption was that the failure was caused due to extremely rugged relief of the pavement in the Bologna town center which is hard comparable with standard roadways. The test schedule was divided to following steps. The insulators were provided with strain gauges and static tests were performed to determine the strength of the insulators and its relation to the real surface strain. Using the same instrumented insulators assembled to the discharging box and added accelerometers, the operational tests were performed along Bologna trolleybus lines. According measured quantities the tests on vibration table of the discharging box were performed in the laboratory looking for lowering the stress of the insulators. 2.1. Static tests Tensile and sheer tests of the empty and with strain gauges provided insulators were performed in the laboratory on hydraulic testing machine Schenck. For the tensile tests, the axial oriented three rectangular strain gauge pairs were used. For sheer test, the same strain gauge pairs were used, but they were turned about 45°to insulator axis to be able to measure doubleꞏε 45 strain and thus the sheer γ (Fig. 2).

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Fig. 2 (a) Insulator with turned strain gauge pairs, (b) broken insulator after tensile test, (c) lay-out of strain gauges