This paper presents the interface transferring mechanism and error modification of the Optical Fiber Bragg Grating
(OFBG) sensors based on the mono-scalar isotropic damage constitutive model. The OFBG sensor is made up of optical
fiber and encapsulated materials which include protective coating, adhesive layer et al. The accuracy of OFBG sensor is
highly dependent on the physical and mechanical properties of the optical fiber and encapsulated materials. The
encapsulated materials were regarded as scatheless continuum in the prevenient researches and the elastic modulus, for
example, the Young's modulus (E) or shear modulus (G), was keeping constant from the beginning to the end of
transformations. However, there is lots of damage, such as microcracks, inclusions and voids, in the encapsulated
materials. And these micro-defects can become cores, expand and joint up with together which induce the gradual bad in
the materials. Hence, the modulus (E and G) is no longer assumed as a constant but as a variable with the damage. So the
mono-scalar isotropic damage model (damage modulus D) is employed to describe the shear constitutive equation of the
encapsulated materials along the optical fiber axes. The general expression of multilayer interface strain transferring
mechanism of OFBG sensor is given based on the isotropic damage theory. And the error rate and error modification
coefficient of OFBG sensor are obtained under the defining of average strains. The results indicate that the damage of
encapsulated materials affects the interface strain transferring property of the OFBG sensor.
This paper presents the interface transferring mechanism and error modification of the Fiber Reinforced Polymer-Optical Fiber Bragg Grating (FRP-OFBG) sensing tendons, which including GFRP (Glass Fiber Reinforced Polymer) and CFRP (Carbon Fiber Reinforced Polymer), using standard linear viscoelastic model. The optical fiber is made up of glass, quartz or plastic, et al, which creep strain is very small at room temperature. So the tensile creep compliance of optical fiber is independent of time at room temperature. On the other hand, the FRP (GFRP or CFRP) is composed of a kind of polymeric matrix (epoxy resins or the others) with glass, carbon or aramid fibers, which shear creep strain is dependent of time at room temperature. Hence, the standard linear viscoelastic model is employed to describe the shear creep compliance of FRP along the fiber direction. The expression of interface strain transferring mechanism of FRP-OFBG sensors is derived based on the linear viscoelastic theory and the analytic solution of the error rate is given by the inverse Laplace transform. The effects of FRP viscoelasticity on the error rate of FRP-OFBG sensing tendons are included in the above expression. And the transient and steady-state error modified coefficient of FRP-OFBG sensors are obtained using initial value and final value theorems. Finally, a calculated example is given to explain the correct of theoretical prediction.
The application of adhered FBG strain sensor is affected by interface strain transfer and error modification. In his paper, firstly, based on the characterstics of forces and damage, the fundamental hupotheses are given, and the general expression of interface transferring mechanism is derived. After that, united form of the characteristic value-λ for the general equation is geven for the multi-layer coatings. Finally, according to the error-modified equation of adhered FBG sensor, the relationships the error rate η against the shear modulus and the thckness of the glue are given. With regard to the glue applied in engineering (thickness is from 4mm to 60mm, shear modulus is from 30MPa to 200MPa), the error rate η is about 5~10%, and the correction coefficient k is about 1.05~1.11. Hence, the error modification must be considered when adhered FBG strain sensors are used in civil engineering.
As strain-sensing elements in the structural health monitoring, the research and application of the Fiber-optic Bragg Grating sensor (FBG) has been widely accepted. Although there are some significant achievements on the interface transferring mechanism and error modification of FBG, the theoretical research on the creep behavior of FBG sensors is rarely taken into account. Because the optical fiber and adhesive is macromolecular polymer, when loaded in long term or high temperature, the creep characteristic of these material is emerging which influences the accuracy of FBG sensors. This paper presents the theoretical part on creep behavior of FBG sensors. Firstly, based on the linear viscoelastic constitutive relations, the general expression of multiplayer interface strain transferring mechanism is derived, and the error modified equation of FBG sensors is obtained. Secondly, the transient and steady-state responses of FBG sensors are presented. Finally, the elasticity-viscoelasticity corresponding principle is proposed, which is used for solving a class of interface strain transferring linear viscoelastic problem.
As the strain sensing element of structural health monitoring, the study and application of the fibre-optic Bragg grating (FBG) has been widely accepted. The accuracy of the FBG sensor is highly dependent on the physical and mechanical properties among the bare optical fibre, protective coating, adhesive layer and host material, namely, the strain interface transferring characteristics among layers. This paper takes the embedded FBG sensor as an object of study. Firstly, the general expression of multiplayer interface strain transferring mechanism is derived. Secondly, based on the defined average strain, the error modified equation of FBG sensor is obtained. Finally, in the light of embedded tube-packaged FBG and FRP-OFBG strain sensors, which developed by ourself, the corresponding strain transferring error, laws were studied, and the corresponding error modification coefficients were given too, which were validated by experiments. The achievements provide theories for the development and application of the embedded FBG sensors.
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