Experimental Analysis of Fatigue Influence on Diaphragm-Embedded Fiber Bragg Grating Sensors

This article presents the fatigue analysis on a diaphragm-embedded optical fiber sensor after more than 40000 stress-strain cycles. The optical fiber sensor is based on a fiber Bragg grating (FBG) embedded in a rubber diaphragm through a vulcanization method. To evaluate the influence of the temperature variation in the sensor system, the diaphragm-embedded FBG is characterized as a function of the temperature, where a linear response (determination coefficient higher than 0.99) is obtained. To compensate the temperature influence on the sensor response, a bare FBG is added. Furthermore, the diaphragm-embedded FBG is subjected to the transverse displacement cycles and the wavelength shift is analyzed as a function of time (and number of cycles). Results show that the sensor performance is affected by the number of cycles, related to the fatigue on both diaphragm and optical fiber materials. A 10% reduction on the sensor sensitivity is obtained, where such reduction follows an exponential trend with sharper reduction in the middle of the test, followed by a saturation trend with higher number of cycles. Considering the sensitivity, the experimental analysis indicated a necessity to perform additional calibrations on the sensor system after 5000 cycles, after the middle region of the test. In addition, as the rubber is a viscoelastic material, the number of cycles also resulted in variations in the dynamic responses of the sensor. The decay of the sensor responses and the delay between strain and stress responses resulted in variations in 22.2% and 3.9%, respectively.