Stress-induced birefringence can lead to distortion in the reflection spectra of fiber Bragg grating (FBG) sensors, thereby resulting in the loss of accuracy and stability of strain measurements. The bonding layer is a direct factor in producing stress birefringence within FBGs. To assess the impacts quantitatively, a theoretical model that links the bonding layer and the reflection spectrum was established. At the same time, the finite element method, based on the theoretical model, was used to study the relationships between characteristics of the bonding layer and reflection spectrum in detail. The analytical results indicate that high elastic modulus and mismatched Poisson's ratio of bonding layer decrease the available strain measuring range of FBGs remarkably, and that unreasonable geometric parameters of the bonding layer should be avoided. In addition, a validation experiment was conducted and experimental results proved the prediction of the theoretical analysis. It can be concluded from the results that the bonding layer is the major limiting factor for the application of surface-bonded FBG sensors in large strain measurements. The bonding materials and bonding processes used in producing FBG sensors deserve serious consideration.
Steel cable plays an important role in modern infrastructure due to its special characteristics. Because most of structure load is transformed to the cable tension in cable stayed structures, it is very important to monitor cable tension. Being a slender element, Fiber Grating Strain Sensor is sensitive to axial strain and is regarded as a most prospective way to monitor the cable tension. The paper reviews a series of problems of FBG when embedded into the cable, and introduced five different embedded FBG strain sensors. Principle, characteristics, and application states of these five sensor has been discussed in details. The prospective of embedded FBG Strain Sensor for cable tension has been forecast.
Bonding layers, serving as the strain transmission mediums, may bring undesirable effects to the sensing properties of fiber Bragg grating (FBG) strain sensors during their fatigue process. To analyze the strain sensitivity and the reflected spectrum of FBG strain sensors in different fatigue stages of bonding layers, their strain sensitivities were derived according to strain transfer models. Resorting to T-matrix formalism, the affected reflected spectra were stimulated. Theoretical analysis results show that there is a gradual decline in the strain sensitivity during the stage of fatigue crack initiation, and that significant distortions emerge in the reflected spectrum during the stage of fatigue crack propagation. In addition, a cyclic loading fatigue test was conducted and the phenomenon observed in the test showed a good agreement with the theoretical prediction. Zhang et al.: Degradation of sensing properties of fiber Bragg grating strain sensors. . . Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/14/2015 Terms of Use: http://spiedl.org/terms
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