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Relative Humidity (RH) sensors based on Fiber Bragg Grating (FBG) technology have been fabricated by polymer functionalization, using hygroscopic material, namely agar. The sensor is specifically intended to monitor moisture in stones of cultural heritage buildings and monuments, in particular to be positioned under hydrophobic coatings used for protecting stones from absorption of water and dissolved chemicals, thus providing early warning of coating deterioration by RH monitoring. The performance of the prototype was first tested exposing the sensors in controlled environment and then assessed for the application simulating “in field” operating conditions, with sensors installed on marble and travertine tiles. Results show that the proposed sensor is able to monitor the presence of moisture inside the stones. © 2018 Elsevier Ltd

This paper details a study which was carried out on the application of FBG sensors for investigation of delamination crack status in glass fibre reinforced composite materials. A woven glass (0/90) epoxy composite sample containing a purposely created delamination and an embedded FBG sensor was investigated to study the behaviour of delamination crack under applied axial quasi-static tensile load. The reflected spectra from FBG sensor and the thermo-elastic response using an infrared camera were recorded to detect the propagation of delamination crack tip. In addition a finite element analysis (FEA) was performed using cohesive elements to simulate delamination crack tip. It has been seen that the propagation of delamination crack tip monitored from thermal stress analysis (TSA) was consistent with the prediction of the FBG sensor response. Further it has been observed that the experimentally observed delamination damage propagation was in a good agreement with FEA simulation results.

In this paper, the polyimide-coated fiber Bragg grating (PI-FBG) humidity sensor suitable for use in unsaturated soils was developed. Through a series of laboratory tests, the pore gas relative humidity in different sand-kaolin mixtures were measured and analyzed to verify the feasibility of the sensor in unsaturated soils. The results shows that the pore gas relative humidity in unsaturated soils first increases rapidly and then tends to be stable with the increase of water content. The relationship between them can be fitted by quadratic function when the pore gas relative humidity is less than 100%. In addition, the critical water content ( w 100 % R H ) of mixed soils increases linearly with the increase of kaolin content; when the water contents of mixed soils are same, higher kaolin content induces higher pore gas relative humidity.

Sensing characteristics of FBG sensor embedded in carbon fiber reinforced polymer (CFRP) laminate were experimentally investigated in this work. In order to evaluate the monitoring accuracy, two upper/lower surface bonded FBG sensors were also included as the measurement references. Results reveal that the wavelength of embedded FBG changes sensitively corresponding to the temperature or strain variation during the experiments: responses to temperature and static loads follow highly linear trend, showing a good agreement with that of surface bonded FBGs, while the frequency responses to low velocity impact give the same resonant frequency but lower amplitudes compared with the ones of surface bonded FBGs. This work confirms that embedded FBG sensor can effectively and reliably monitor the inner conditions of composite materials. Due to the advantages of precise measurement, small size, easy integration and so on, the embedded FBG sensors have broad application prospects in the field of intelligent composites.

The hydraulic characteristics of pavement, such as pore water pressure, hydrodynamic pressure, and permeability, have a great influence on the functional performance and durability of pavement. The main objective of this study is to design a device for measuring the hydrodynamic pressure of pavement surface and analyze its characteristics with different vehicle speeds. In order to investigate the influence of vehicle speeds on the hydrodynamic pressure of pavement surface, a device equipped with five fiber Bragg grating (FBG) sensors was designed and utilized to measure the surface hydraulic characteristics at four vehicle speeds in the field. The calibration of FBG sensor was conducted by the interrogator (SmartScan Aero) under given pressures from 0 MPa to 1 MPa with an interval of 0.1 MPa. Then, the device was embedded in the pavement and compacted by an auto wheel, which traveled over the device at different speeds. Furthermore, the experimental data was collected by the interrogator during the process of a moving tire touching the surface of the device. The experimental results showed that the correlation coefficient between the given pressure and the center wavelength change of the FBG sensor was 0.99, which means the FBG sensor was accurate and reliable. The hydrodynamic pressure of the pavement surface increased with the increase in vehicle speed from 40 km/h to 100 km/h. Moreover, the directional anisotropy of the hydraulic pressure was found to be dependent on vehicle speed.

In this paper, an ORMOCER® (OR)-coated fiber Bragg grating (FBG) sensor encapsulated with stainless steel was proposed for relative humidity (RH) sensing. Various sensing characteristics of the sensor, including RH and temperature sensitivity, hysteresis, repeatability, stability, and water resistance, were investigated systematically and compared with a polyimide (PI)-coated FBG humidity sensor. The results show that the OR-coated FBG humidity sensor can measure simultaneously the humidity and temperature, with sensitivity coefficients of 2.4 pm/%RH and 12.9 pm/°C, respectively. The OR-coated sensor has obvious advantages over the PI-coated sensor in terms of sensitivity, response speed, accuracy, and mechanical strength. Furthermore, two practical encapsulation methods were proposed for the sensor for applications in complex and harsh environments. Combined, the developed OR-coated FBG sensor provides an innovative means for RH monitoring.

Flexible barriers, as an effective protective measure to mitigate landslide hazards, have attracted a lot of interest from the geotechnical community. The dynamic response of flexible barriers subjected to impact loads is of great concern. However, the forces developed on ring-net barriers during impacts have not been effectively measured and the dynamic behavior of barriers has not been fully understood. In this study, new mini tension link transducers based on fiber Bragg grating (FBG) sensing technology are uniquely developed to both link adjacent rings of a flexible barrier and measure the forces between rings. Besides, a novel monitoring system based on the new FBG mini tension link transducers is established for detecting dynamic response of a flexible barrier under impact loads. Calibration results of the FBG mini tension link transducers demonstrate that the wavelength shift of the FBG sensor has a linear relationship with the applied force with high accuracy. A single boulder impact test and a debris flow impact test were conducted to investigate the performance of the FBG-based system. The results reveal that the FBG mini tension link transducers are capable of capturing the evolution of the forces between rings of the flexible barrier, and the FBG-based system can be used for monitoring the dynamic response of the flexible barrier under impact loads.

An integration of Structural Health Monitoring (SHM) into composite structures contribute towards the development of smart composites structure. Smart-structure offers an ability have a continuous and real-time information of its circumstances under critical loading applications. One of the main objectives in this research study was the development of the Fiber Bragg grating (FBG) sensor system for SHM of thin composite laminates. Not only that, this work has been utilizing the FBG sensors in the Near Infra-red (NIR) range; ∼830 nm, as an alternative to the applications of the conventional 1550 nm FBG sensors. The capability of this sensor system was validated with the impact location determination test on a thin composite plate. It showed a very promising result whereby the relative error falls below 10%.

An approach is proposed for obtaining simultaneous measurements of the level and specific gravity of a liquid using a dual-pressure-sensor system comprising a fiber Bragg grating (FBG) pressure sensor and a Fabry–Perot (FP) pressure sensor. In the FBG sensor, the pressure is derived from the FBG wavelength shift induced when the sensor is immersed in the liquid. Meanwhile, in the FP sensor, the pressure is calculated from the change in cavity length which takes place when the sensor is immersed. The advantageous concept of the dual-pressure-sensor system is atmospheric pressure compensation. The experimental results show that the FBG and FP pressure sensors have sensitivities of 0.1495 nm/kPa and 0.1569 μm/kPa, respectively. Analytical formulae are derived for the level and specific gravity of the liquid in terms of the FBG wavelength shift, the change in cavity length, and the vertical separation distance between the two sensors.

A novel electromagnetic force sensor has been proposed. The force created by the input current results in the distortion of a cantilever structure. A couple of fiber Bragg gratings (FBGs) are mounted at either side of the cantilever. By monitoring the wavelength shift difference of the two FBGs, the electromagnetic force can be obtained and the cross-sensitive problem of the FBG sensor can be solved by compensating the temperature effect. Measurement theory has been described. Simulation and preliminary experiments have been carried out, and the results verify the feasibility of the proposed sensor.