PSI - Issue 17

Kristýna Čápová et al. / Procedia Structural Integrity 17 (2019) 726 – 733 Kristýna Čápová et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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the cross-section corresponds to the location of FBG sensors. The FBG sensor and the extensometer placed in the middle of the span in the tensile zone of the cross-section were chosen for the data comparison, see Fig. 7 (b). It can be seen that the strain in that particular part increases every year at the same loading force.

a

b

Fig. 7. (a) GLT beam ready for the four point bending test, Velebil et al. (2018); (b) Cyclic loading test of GLT beam, year-on-year comparison.

4. Conclusions and recommendations

In particular, following points concerning the environmental testing deserve attention. From the first phase it is clear that the mounting of sensors for the accurate calibration must be done so as not to interfere with shifting on the pad or in the winding on a spool when expanding with temperature. In the tested range of 40 °C in the vicinity of the interrogator, the temperature inside the spectrometer is changed to less than 1 °C. The change of the temperature of the spectrometer to 1 °C causes the indicated change in order of hundredths of nanometer on the connected temperature and deformation sensors; in case of the athermal sensor in order of thousands of nanometers. The second phase brought an idea of compensation of the thermal effects by two parallelly working FBG sensors. This hypothesis will be subjected to testing. The third phase of testing proved that the sensors react well both to the temperature change and to the humidity change. However, in all cases considerable hysteresis probably due to the influence of air humidity was evident. Therefore, it is necessary to run more tests with much longer duration. Concerning the mechanical loading test, there is a year-to-year increase of measured strain at the same loading force. The increase corresponds well to the decrease of the modulus of elasticity of timber during the loading tests that occurs due to the gradual plastic deformation of the wood caused by mechanical stress. Acknowledgements This work has been supported by the Ministry of Education, Youth and Sports within National Sustainability Programme I, project No. LO1605.

References

Kersey, A.D., 1996. A Review of Recent Developments in Fiber Optic Sensor Technology. Optical Fiber Technology, 2/3. ISSN 1068-5200. Othonos, A., Kyriacos, K., 1999. Fiber Bragg Gratings: Fundamentals and Applications in Telecomunications and Sensing. ISBN 0-89006-344-3. Velebil, L., Čápová, K., Včelák, J., Kuklík, P., Demuth, J., Dvořák, M., 2018. Mechanical Stress Monitoring of Timber and Con crete Structures by Fibre Optic Sensors. WCTE 2018 Proceedings. ISBN 979-11-6019-235-3.