PSI - Issue 42

Valerii Matveenko et al. / Procedia Structural Integrity 42 (2022) 307–314 V.Matveenko, N.Kosheleva, G.Serovaev/ Structural Integrity Procedia 00 (2022) 000 – 000

310

4

Simultaneous sensitivity of FOSs to temperature changes and strain makes it necessary to provide temperature compensation of FOS readings when measuring strain. It is possible to compensate for the influence of temperature with the help of an additional FOS, free from external mechanical influence and located in the same temperature conditions with a sensor measuring strain (Matveenko et al. , 2018). In this case, the change in strain, taking into account temperature compensation, can be expressed:





0  −        , 0 T   T

1



T

(

)

(4)

 =  −  =

сK

K





where  is the central scanning wavelength, с is the speed of light,   is the difference between the resonant wavelength of the FBG in the loaded and unloaded ( 0  ) states,   - is the spectral shift,  is the mean optical frequency and superscript T denotes measurements by temperature sensor. 3. Experimental strain and temperature measurements in 3D printed samples For temperature measurements, it is proposed to use an optical fiber with a FBG in the case of point measurements and without a Bragg grating for distributed measurements enclosed in a hollow PolyTetraFluoroEthylene (PTFE) tube. In order to exclude the effect of mechanical strain on the temperature FOS, the bonding of the optical fiber and the PTFE tube should be carried out on one side of the tube using an adhesive joint (Fig. 1). A temperature sensor equipped in this way can also be embedded in the material during printing and used both for temperature measurements and for temperature compensation of fiber-optic strain sensors.

Fig. 1. Diagram of a temperature fiber-optic sensor

An experimental demonstration of strain and temperature measurements using embedded fiber-optic sensors was carried out on uniform strength beam manufactured by FDM technique from polylactide (PLA) material. Two samples with the same geometry and printing parameters were made from the same batch of material. In one of the samples, FOSs based on Bragg gratings were embedded, in the second – a similar optical fiber was used as a DFOS. In total, two optical fiber lines were embedded in the first sample (Fig. 2a): an optical fiber line with a polyimide coating with an outer diameter of 0.15 mm and two Bragg gratings each 5 mm long, located at a distance of 30 mm from each other, which were used to measure strain; an optical fiber line with one Bragg grating enclosed in a PTFE tube, with an inner diameter of 0.5 mm and an outer diameter of 1 mm, for temperature measurement and temperature compensation (Fig. 3). Due to one measuring channel on the OBR 4600, one optical fiber line is embedded in the second sample, divided into two sections (Fig. 2b): the first embedded optical fiber section is sensitive to changes in both strain and temperature, the second embedded optical fiber section is enclosed in a PTFE tube similar to the first sample to exclude mechanical strain while providing temperature measurements.