Long-gauge fiber Bragg grating (FBG) strain response is characterized by sensitivity to local damage and obtaining global behavior (e.g., deflection, natural frequency) of structures, and can be measured with high-accuracy and high sampling frequency. Recent research about performance assessment of engineering structures using long-gauge FBG sensors is reviewed in this paper. Firstly, description of long-gauge FBG sensing technique is presented. Secondly, assessment methods of structural local behavior using long-gauge strain response are classified into two types (time domain-based and frequency domain-based method). Time domain-based methods are those methods which directly use long-gauge strain to identify local damage of structures. Frequency domain-based methods are those methods which extract frequency features (e.g., modal macro-strain) of structures for damage identification. Thirdly, assessment methods of structural global behavior using long-gauge strain response are summarized. These methods include calculating deflection of structures from long-gauge strain response, extraction of natural frequency from dynamic strain time-history, and calculating displacement mode shape from modal macro-strain. Finally, suggestions on the selection of methods for performance assessment of engineering structures are proposed and some challenges are discussed.
Recent reports show that modal macro-strain vector (MMSV) obtained by using distributed long-gage FBG sensors is an
effective indicator for damage detection. However, in previous researches, MMSV was always obtained under impulsive
load such as hammer impact. In structural health monitoring of real large-scale structures, however, it is often very
difficult to apply such impulsive load. This paper therefore introduces a new method to abstract MMSV under ambient
excitation. Theoretical deduction reveals that MMSV can be uniquely determined by auto-spectrum of dynamic
macro-strain responses under ambient excitation. Both numerical simulation and experiment were conducted to verify
the proposed methods. Simulation results showed that that the identified frequencies and MMSV vectors under random
excitation are in good agreement with those obtained from theoretical analysis, while experimental results showed the
identified frequencies and MMSV agreed well with those obtained using point impulsive excitation.
In general, macro-strain is an effective index for health monitoring of civil infrastructures, which can reveal the
unforeseen damage accumulation. However, it is difficult to acquire precise strain distribution with existing
fully-distributed optical fiber sensing techniques. Based on the distributed optical fiber strain sensing technique of
pulse-prepump Brillouin Optical Time Domain Analysis (PPP-BOTDA), a new optical fiber sensor with improved strain
sensitivity (OFSISS) is proposed to enhance the precision of macro-strain measurements. The most advantage of the
OFSISS sensor is that it can markedly reduce the measurement error of strain data with a proper designed magnified
coefficient. The OFSISS has also good designability and durability according to detailed sensing requirements. Results
of uniaxial tensile experiment show not only the high accuracy and precision of the OFSISS but also an important fact
that the measured magnified coefficients of the manufactured OFSISSs with a recoating process agree well with the
designed values. The bending experiment of using a steel beam illustrates that the linearity and reliability of macro-strain
measurement from the OFSISS are good enough for the application in actual macro-strain monitoring and structural
deformation monitoring.
In this paper, a new type of self-sensing basalt fiber reinforced polymer (BFRP) bars is developed with using the
Brillouin scattering-based distributed optic fiber sensing technique. During the fabrication, optic fiber without buffer and
sheath as a core is firstly reinforced through braiding around mechanically dry continuous basalt fiber sheath in order to
survive the pulling-shoving process of manufacturing the BFRP bars. The optic fiber with dry basalt fiber sheath as a
core embedded further in the BFRP bars will be impregnated well with epoxy resin during the pulling-shoving process.
The bond between the optic fiber and the basalt fiber sheath as well as between the basalt fiber sheath and the FRP bar
can be controlled and ensured. Therefore, the measuring error due to the slippage between the optic fiber core and the
coating can be improved. Moreover, epoxy resin of the segments, where the connection of optic fibers will be performed,
is uncured by isolating heat from these parts of the bar during the manufacture. Consequently, the optic fiber in these
segments of the bar can be easily taken out, and the connection between optic fibers can be smoothly carried out. Finally,
a series of experiments are performed to study the sensing and mechanical properties of the propose BFRP bars. The
experimental results show that the self-sensing BFRP bar is characterized by not only excellent accuracy, repeatability
and linearity for strain measuring but also good mechanical property.
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