A Brillouin optical time domain reflectometer (BOTDR) is a fiber optic strain distributed sensing system which can measure strain along arbitrary regions of an optical fiber. We have used this system for structural health monitoring of composite structures, such as racing yachts. Although the BOTDR is useful to measure strain distributions of a large area efficiently, it is difficult to accurately measure strain changing sharply within the length of the spatial resolution. The resolution depends on the width of the pulse light emitted from the BOTDR and it is 2 m for the pulse width of 20 ns. In this paper, we propose a new technique to improve the spatial resolution of a commercially available BOTDR in strain measurements. This technique is based on analysis of the Brillouin gain spectrum along a sensing fiber. We confirmed experimentally that the BOTDR using 20 ns pulse could measure strain which has a rectangular distribution of 0.3 m in the sensing fiber.
KEYWORDS: Control systems, Sensors, Actuators, Ferroelectric materials, Fiber Bragg gratings, Digital signal processing, Finite element methods, Ferroelectric polymers, Composites, Vibration control
A new vibration control system, named 'block-by-block' distributed cluster control system, is presented with a CFRP board with stiffeners. Distributed cluster control system which had been applied for flat simple board is a control system includes 'cluster sensing' which classified numberless vibration modes into some limited number of clusters by using a group of sensors, and 'cluster actuation' which can actuate only specific cluster. It means, this system controls only target clusters. When the system is applied to complex structure such as this CFRP board with stiffeners, it is not possible to applied directly since the forms of vibration modes are not as simple as the one of flat board but there exist three blocks; 2 side blocks and one center block between two stiffeners. In this paper, after a rough explanation of distributed cluster control system, the idea of 'block-by-block' control is explained and verified experimentally with FEM analysis using some kinds of sensors and actuator.
We propose a damage detection method based on frequency responses measured by FBG sensors during vibration tests. When random noise vibrates a laminated composite panel and a stiffener fastened with bolts, the peak gain of the resonance frequencies can be obtained. We calculated the correlation coefficient of the normalized gain for the frequency responses, and then predicted the location of a missing bolt. This method will make possible to predict the location of damage with a limited number of FBG sensors while a structure is vibrating even if the excitation point changes.
In this study, we developed a health monitoring system using a fiber optic distributed strain sensor for International America's Cup Class (IACC) yachts. Most structural components of an IACC yacht consist of an aluminum honeycomb core sandwiched between carbon fiber reinforced plastic (CFRP) laminates. In such structures, delamination, skin/core debonding and debonding between adhered members will be result in serious fracture of the structure. We equipped two IACC yachts with fiber optic strain sensors designed to measured the distributed strain using a Brillouin optical time domain reflectometer (BOTDR) and to detect any deterioration or damage to the yacht's structures caused by such failures. And based on laboratory test results, we proposed a structural health monitoring technique for IACC yachts that involves analyzing their strain distribution. Some important information about structural conditions of the IACC yachts could be obtained from this system through the periodical strain measurements in the field.
We constructed a new health monitoring system to detect damage using a fiber optic distributed sensor, namely a Brillouin optical time domain reflectometer (BOTDR), and installed it in International America's Cup Class (IACC) yachts, the Japanese entry in America's Cup 2000. IACC yachts are designed to be as fast as possible, so it is essential that they are lightweight and encounter minimum water resistance. Advanced composite sandwich structures, made with carbon fiber reinforced plastic (CFRP) skins and a honeycomb core, are used to achieve the lightweight structure. Yacht structure designs push the strength of the materials to their limit and so it is important to detect highly stressed or damaged regions that might cause a catastrophic fracture. The BOTDR measures changes in the Brillouin frequency shift caused by distributed strain along one optical fiber. We undertook two experiments: a pulling test and a four point bending test on a composite beam. The former showed that no slippage occurred between the optical fiber glass and its coating. The latter confirmed that a debonding between the skin and the core of 300 mm length could be found with the BOTDR. Next we examined the effectiveness with which this system can assess the structural integrity of IACC yachts. The results show that our system has the potential for use as a damage detection system for smart structures.
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