Abstract
Optical fiber strain sensors with light weight, small dimensions and immunity to electromagnetic interference are widely used in structural health monitoring devices. As a sensor, it is expected that the strains between the optical fiber and host structure are the same. However, due to the shear deformation of the protective coating, the optical fiber strain is different from that of host structure. To improve the measurement accuracy, the strain measured by the optical fiber needs to be modified to reflect the influence of the coating. In this investigation, a theoretical model of the strain transferred from the host material to the optical fiber is developed to evaluate the interaction between the host material and coating. The theoretical predictions are validated with a numerical analysis using the finite element method. Experimental tests are performed to reveal the differential strains between the optical fiber strain sensor and test specimen. The Mach-Zehnder interferometric type fiber-optic sensor is adopted to measure the strain. Experimental results show that the strain measured at the optical fiber is lower than the true strain in the test specimen. The percentage of strain in the test specimen actually transferred to the optical fiber is dependent on the bonded length of the optical fiber and the protective coating. The general trend of the strain transformation obtained from both experimental tests and theoretical predictions shows that the longer the bonded length and the stiffer the coating the more strain is transferred to the optical fiber.
Highlights
The use of optical fiber sensors has grown considerably over the past decade in applications ranging from the aerospace industry to civil engineering [1,2]
Several fiber optic technologies have been proposed for strain/temperature measurements, such as fiber Bragg grating (FBG) sensors [6,7,8], high-polarimetric fiber optic sensors [9,10] and white-light interferometric fiber sensors [11]
An electric resistance strain gauge with gauge length of 5 mm was adhered to the specimen to measure the strain 0 of the host material and compared to the results from the optical fiber strain sensor
Summary
The use of optical fiber sensors has grown considerably over the past decade in applications ranging from the aerospace industry to civil engineering [1,2]. Ansari and Libo [18] proposed a theoretical model to study the strain transferred from the host material to the surface bonded optical fiber. They assumed that the optical fiber (with coating) was perfectly bonded with the host material. An analytical formula is proposed to evaluate the real strain in the host structure using the surface bonded optical fiber This formula provides accurate prediction of the strain transformation rate from the host material through the adhesive layer and protective coating to the optical fiber strain sensor. A numerical study is performed to evaluate the influence of the coating and bonded length on the strain transferred from the host structure to the optical fiber. Mach-Zehnder interferometery to reveal the differential strains between the fiber-optic sensor and host structure
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