PSI - Issue 42

Nikola Schmidová et al. / Procedia Structural Integrity 42 (2022) 1306–1313 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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increase of the measured electrical resistance from the beginning of loading. For all specimens, we observed step change during fracture of the specimens, see Fig. 4. In Table 1 and Fig. 5 are given detailed information about the step change of measured electrical resistance. In order to identify the step change we introduce three values: electrical resistance before fracture ( RBF ), peak electrical resistance during fracture ( RP ) and electrical resistance after fracture in loaded state ( RAF ). It can be seen, that the step change in measured electrical resistance change

Fig. 6 Measured electrical resistance change and relative resistance moment of cyclic and static torsional tests.

was bigger than the signal fluctuation, which was observed for specimen SN-11. Based on this observation, we can suppose, that the fracture detection could be possible under continual electrical resistance measurement of such construction.

3.2. Torsional loading The initial electrical resistance of the specimens was measured for each specimen. For the specimen SN-20 the value of 3.96 Ω was measured, the value of 0.74 Ω was measured for the specimen SN-21. The relative torsional moment Mt rel was calculated in the same way as Mo rel see equation 1 and 2. In Fig. 6 a clearly dependence between applied torsional loading and measured electrical resistance change can be observed. If we compare the flexural loading, where part of the cross section of the specimen is loaded by tension and part by the tension, and the torsional loading, where all the cross section is loaded in the same manner, the torsional loading is more convenient for the monitoring of operational loading. According to our measurement, fracture detection using continual electrical resistance measurement is also possible for specimen loaded by the torsional loading, see Fig. 6. We also observed step change in measured electrical resistance, when the fracture of the specimen occurred.

3.3. Electrical contacts and influence to the structure

There were two basic requirements to the electrical contacts. The electrical contacts had to be prepared on the inner surface of the specimens and they must not influence the strength of the specimens. In order to prove that the strength of the specimens was not affected, we compared the values of maximal strength for the specimens with integrated