Chuan Swee Tjin

Determination of load-strain characteristics of concrete slabs by using embedded fiber Bragg grating sensors

In this paper, we present our work on the fiber Bragg grating (FBG) sensors for structural health monitoring in 5m long concrete structures. Two sets of sensors were securely fastened onto the surfaces of the top and bottom reinforced bars respectively before concrete was poured in. Another set of the sensors was mounted onto the slab surface. These sensors were then monitored to observe the strain experienced at different locations within concrete slab. Loading and unloading cycle tests and failure test were performed on the completed structure. From the results obtained using the FBG sensors, we were able to correlate t he load-strain behavior of the slabs to the failure state as observed on the slab surface. These data are useful in determining the maximum allowable load before failure sets in. At the same time, we made comparisons of the data obtained using our FBG sensors with those obtained with electrical strain gauges. The two sets of data show a similar trend during the loading and unloading tests as well as during the failure tests.

Fiber optic pressure sensor using a fiber Bragg grating

A Fiber Optic Pressure Sensor utilizing a Fiber Bragg Grating (FBG) as a sensing element has been demonstrated. In the experiment, a broad band LED source with a wavelength range of 1520 nm to 1570 nm was launched into a single mode optical fiber within which the FBG was written. With no applied force, the fiber grating reflects light strongly at a wavelength of 1549.92nm. When the FBG was subjected to a range of forces (from 0.0N to 18.0N), applied perpendicularly to its axis, the reflected light from the FBG was found to be wavelength shifted in proportion to the applied force, but in a non- linear manner. The shift in wavelength is detected using fiber Fabry-Perot interferometric technique. With the FBG embedded in carbon fiber material, the test results showed an excellent linear relationship between the wavelength shift and the applied force (pressure). In addition, force sensing range of up to 60N was obtained with the pressure sensor still remaining in good condition.

Measurement of contact forces between human and vibrating floors using fiber Bragg grating foot sensors

We report on the use of fiber Bragg grating (FBG) foot sensors to study the reaction of human being subjected to floor vibration. The entire testing system comprises the multiplexed FBG foot sensors, the communication link between the sensors and the vibrating slab, the control and synchronization software, the graphical user interface, and the data acquisition and analysis system. The pressure distributions over different parts of the foot are obtained at different floor vibrating frequencies and with different distances from the vibrating source. These results show great potential in the application of the FBG foot sensors to study the biomechanics of balancing on a vibrating floor. It has also been demonstrated that the system is robust and able to work under harsh conditions. Further development towards practical implementation of the system is proposed.

Tabu search algorithm to optimize the design of fiber Bragg gratings

A Tabu Search (TS) algorithm, to our knowledge, for the first time, is introduced into the optimized design of Fiber Bragg gratings (FBGs). By combining the Transfer Matrix Method (TMM) for calculating the reflection spectrum and the TS algorithm, we obtained a new method for synthesis the FBGs with advanced characteristics. A new appodization profile is proposed as an example to demonstrate the effectiveness of the method that is general to be useful for inverse problems in FBGs application.

Simultaneous monitoring of strain and temperature in concrete structures with embedded fiber Bragg gratings

Fiber Bragg grating (FBG)-based strain and temperature sensor array were embedded into the concrete structure in order to provide real-time information on its strain and temperature distribution. The sensors were wavelength- multiplexed along a single fiber. The temperature and strain sensors were specially designed and optimized for their measurands. The calibration experiments of those FBG sensors, and parameter monitoring during the structural curing processes were also presented in this paper. These fiber optic strain and temperature sensor show many advantages over the traditional electrical strain gauges and thermocouples.

Measurement of temperature profile of concrete structures with embedded fiber Bragg grating sensors and thermocouples

It is known that certain design consideration must be given to the concrete structures to cater for possible ambient temperature variations. The thermal stress that results from these changes can cause damage in concrete structures ifthey are restrained or ignored in the design stage. The thermal effects in concrete structures are mainly due to the surrounding environmental conditions and some other special effects, such as the heat of hydration during construction and casting of the bitumen. In this paper, we present our preliminary results on monitoring the non-linear temperature profiles of simulated bridge decks due to various environmental contributing factors using fiber Bragg grating (FBG) sensors and thermocouples. We will also present the data on the variations of the temperature profiles in the concrete due to different thickness of the bitumen topping.

Simultaneous monitoring of the amplitude and location of loading with fiber Bragg grating sensor arrays

Development in sensing technologies and data acquisition systems is making it possible for engineers to acquire the actual state of strains and stresses in structures at construction stage and during their service life. The knowledge of actual internal forces in a structure is key to identification of parameters such as stiffness and support conditions enabling the structural performance to be accurately estimated. This paper reports on the theoretical and experimental work carried out using fibre Bragg gating (FBG) strain sensors, embedded into and attached onto the concrete beam, to obtain the strain distribution of the concrete beam, therefore deduce the information of the applied load.

Monitoring of concrete curing process with embedded fiber Bragg gratings

In concrete curing process, the temperature and the volume of the structure, and the inner pressure/stress exerted on the rebars will change in a fashion that depends on the composition of the concrete mixture. The temperature, strain and pressure changes at various points of a 3m long concrete structure with a mixture of cement, water and aggregate were monitored over a period of several days. Fiber Bragg grating based sensors were used to monitor these parameters in vivo. These sensors were designed and optimized to measure the temperature, strain and pressure respectively.

Development of electrically tunable optical filters based on fiber Bragg gratings

We demonstrate the development of tunable optical filters based on fiber bragg gratings (FBGs). A distributed on fiber resistive heater consisting of a thin metal film is deposited by means of an Electron Beam Evaporator onto the outer surface of an uniform FBG. The physics of heat flow and diffusion in these structures leads to resistive heating of the metal film that follow, to a remarkably good approximation, the local resistance of the coating. This generated heating induces changes in the fibers refraction index (thermo-optic effect), which together with the thermal expansion effect of fiber causes a shift in the Bragg wavelength thereby achieving the tunability. Experimental results show a dc tuning range of 4.284 nm (nickel as thin film) with a corresponding electrical power of 569 mW. A maximum efficiency of 8.133 nm/W (gold as thin film) was obtained for dc tuning.

Vibration measurement and mode analysis on concrete structures with embedded fiber Bragg grating sensors

This paper reports our work on the applications of fiber Bragg grating-based strain sensors for the vibration tests and mode analysis on concrete structures. The arrayed fiber grating strain sensors, which were wavelength-division-multiplexed along the fibers, were attached onto the reinforced bars (rebars) before concrete was poured in to form a 5.5m long, 0.3m wide, 0.15m deep reinforced concrete beam. The embedded sensors will provide quasi-distributed real-time dynamic strain information along the length of the beam. For verification with the FBG strain sensors, some electrical accelerometers were also placed on the top surface of the concrete beam. All the data from FBG sensors and electrical accelerometers were recorded and analyzed by a computer. In the experiments, a hammer and an electrical shaker were used to excite the structure. The experimental results obtained with the FBG sensors show good consistency with the theoretical analysis.