Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Some Materials and Structures Monitoring Cases. <s> The purpose of structural health monitoring (SHM) is to lead a structure to be safer at lower cost. SHM systems capable of assessing structural integrity during manufacture and in-service operation would allow timely maintenance actions to increase safety and lifetime of structures. In such systems, it is important to evaluate the actual state of a structure. Recently, fiber-optic sensors have been actively developed, and one can measure many kinds of the physical measurands by them. Since they also have excellent characteristic, such as immunity of electromagnetic interference, durability and capability to realize distributed sensing, they are supposed to be suitable sensors for SHM systems. We installed fiber-optic sensors into full-scale composite structures to monitor strain or temperature during manufacture or to monitor in-service structural performance, i.e., stiffness. The structures applied with the sensors are International America's Cup Class (IACC) yachts and a Japanese experimental reentry vehicle, namely, HOPE-X, that are made of carbon fiber reinforced plastic. The fiber-optic sensors used in this study are two kinds of distributed sensors using Brillouin scattering and Raman scattering, respectively. The former can measure strain or temperature and the latter can measure temperature at an arbitrary region along an optical fiber. We could successfully measure strain or temperature of the full-scale composite structures in field and access the structural state. The results of this study demonstrate the great potential of fiber-optic distributed sensors for practical applications to large composite structures. <s> BIB001 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Some Materials and Structures Monitoring Cases. <s> Water accumulation in honeycomb sandwich structures is a perceived problem for aircraft operators since it significantly degrades the structural integrity. The authors developed a fiber-optic-based technique to detect water accumulation in large-scale aircraft honeycomb sandwich structures. An optical-fiber network was formed in the adhesive layer and a Brillouin-based sensing system with high spatial resolution (specifically, pre-pump pulse Brillouin optical time domain analysis (PPP-BOTDA)) was utilized to detect the non-uniform internal temperature distribution during cooling in aircraft ascent. First, the temperature change during the ascent was investigated to evaluate the feasibility of the proposed technique. A verification test was then conducted using a Nomex honeycomb sandwich panel. The non-uniform temperature induced by water accumulation was detected from the peak-frequency distributions and the width of the Brillouin gain spectrum, which is the output of the PPP-BOTDA. The spectrum width, wh... <s> BIB002 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Some Materials and Structures Monitoring Cases. <s> Abstract This study demonstrated fiber-optic-based life cycle monitoring of a representative carbon fiber reinforced plastic (CFRP) stiffened panel manufactured by vacuum assisted resin transfer molding (VARTM). A single optical fiber was embedded between the stiffeners and the skin during the laminate lay-up process and the formed fiber-optic network was then utilized to monitor the manufacturing process and subsequent impact tests. A Brillouin-based system with a spatial resolution of 10 cm was utilized for distributed strain measurement. Strain changes induced during the manufacturing process and the impact tests were comprehensively presented and discussed. The internal state of the panel was successfully monitored throughout its life, confirming the effectiveness of life cycle monitoring by fiber-optic-based distributed sensing for developing highly-reliable composite structures. <s> BIB003 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Some Materials and Structures Monitoring Cases. <s> We report for the first time, to the best of our knowledge, online monitoring of the distributed lateral displacement in large AC power generators using high spatial resolution differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). To perform the measurement of distributed lateral displacements with periods of only a few cm in large AC power generators, a 2?cm spatial resolution strain measurement is realized using the differential pulse pair of 8/8.2 ns in DPP-BOTDA, and then the lateral displacements are reconstructed according to the strain?displacement relation with the assumption of a sine shape function. Using different fiberglass ripple springs, two types of lateral displacement with periods of 3 and 3.25?cm are demonstrated, obtaining a maximum displacement of 0.43?mm with a measurement accuracy of ~ 40??m. This provides the information on the stator coil tightness through online monitoring of the distributed lateral displacement caused by the fiberglass ripple springs, and ensures safe operating conditions for large AC power generators. In addition, the large number of sensing points associated with distributed optical fiber sensors make it economically and technically practical to monitor large numbers of key components in a generator without any interference from the large magnetic and electrical fields. <s> BIB004 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Some Materials and Structures Monitoring Cases. <s> In this paper, a novel kind of method to monitor corrosion expansion of steel rebars in steel reinforced concrete structures named fiber optic coil winding method is proposed, discussed and tested. It is based on the fiber optical Brillouin sensing technique. Firstly, a strain calibration experiment is designed and conducted to obtain the strain coefficient of single mode fiber optics. Results have shown that there is a good linear relationship between Brillouin frequency and applied strain. Then, three kinds of novel fiber optical Brillouin corrosion expansion sensors with different fiber optic coil winding packaging schemes are designed. Sensors were embedded into concrete specimens to monitor expansion strain caused by steel rebar corrosion, and their performance was studied in a designed electrochemical corrosion acceleration experiment. Experimental results have shown that expansion strain along the fiber optic coil winding area can be detected and measured by the three kinds of sensors with different measurement range during development the corrosion. With the assumption of uniform corrosion, diameters of corrosion steel rebars were obtained using calculated average strains. A maximum expansion strain of 6,738 μe was monitored. Furthermore, the uniform corrosion analysis model was established and the evaluation formula to evaluate mass loss rate of steel rebar under a given corrosion rust expansion rate was derived. The research has shown that three kinds of Brillouin sensors can be used to monitor the steel rebar corrosion expansion of reinforced concrete structures with good sensitivity, accuracy and monitoring range, and can be applied to monitor different levels of corrosion. By means of this kind of monitoring technique, quantitative corrosion expansion monitoring can be carried out, with the virtues of long durability, real-time monitoring and quasi-distribution monitoring. <s> BIB005
Applications of the Brillouin distributed sensing include a large variety of structures, such as competition yachts or experimental vehicles. Monocoque structures such as those made of carbon fiber reinforced plastic (CFRP) were instrumented with this fiber distributed technology to measure and monitor strain and temperature during manufacturing and in-service structural performance like its stiffness BIB001 . The sensing technique was embedded in a couple of International America's Cup Class (IACC) yachts (Asura and Idaten). The yachts were equipped with a fiber-optic distributed sensor using Brillouin scattering to monitor the longitudinal and transverse strains of the yachts to assess the stiffness of the structures in America's Cup 2000 (held at Auckland in New Zealand). Another monocoque structure is a fullscale model built as a prototype to demonstrate feasibility of a Japanese experimental reentry vehicle, namely, HOPE-X (H-II Orbiting Plane-Experimental). The same type of sensor that was applied to the IACC yachts and a sensor using Raman scattering were used for strain and temperature measurements during a manufacturing process and for strain measurements in structural tests. Another Brillouin-based system application with a spatial resolution of 10 cm was utilized for distributed strain measurement in a representative carbon fiber reinforced plastic (CFRP) stiffened panel manufactured by vacuum assisted resin transfer molding (VARTM). Strain changes induced during the manufacturing process and the impact tests were comprehensively presented and discussed. Carbon fiber reinforced plastic (CFRP) is being used on almost all modern commercial aircraft as a primary structural material, but it is still difficult to precisely manufacture cocured large-scale CFRP structures and ensure their structural integrity during operation. Hence there is an urgent need to develop innovative techniques to monitor the internal states of composite structures and utilize the obtained data to improve structural design, processing technologies, and maintenance methods. By combining all the information obtained by the fiber-optic network, the structural health can be accurately evaluated BIB003 . Honeycomb sandwich structures are integral constructions consisting of two thin facesheets and a lightweight honeycomb core. They are widely utilized in aircraft structures on account of their excellent mechanical properties. However, the lightweight structure absorbs a large amount of water while in service. Over time, cracks form in the facesheets and the adhesive layer as a result of mechanical and thermal loading, creating leak paths from the surface to the honeycomb core. Then, the water enters the core through the paths and accumulates in the cells of the honeycomb structure. Mechanical deterioration of the honeycomb core such as delamination of the face-sheet from the core can be provoked by the accumulated water. This trouble can be detected measuring the nonuniform temperature distribution induced by the water in the cell of the honeycomb core. Using this principle and a Brillouin distributed fiber optic system the water accumulation monitoring of large-scaled structures was effectively detected BIB002 . In large AC power generators, fatigues on electrical insulation and mechanical components could cause deterioration of the stator coil tightness and provoke a failure. Additionally, during the installation of an AC power generator, the fiberglass ripple spring, which is located just below the stator wedge, must be sufficiently compressed to be flat or nearly flat to ensure the maximum tightness, to compensate for the stator coil ground-wall insulation creep that is the primary factor in stator coil looseness resulting in "slot pounding". Using a DPP-BOTDA this deformation is measured BIB004 . The sensing fiber is glued to a fiberglass flat plate, which is subjected to a periodic side force from a fiberglass ripple spring and is consequently characterized by a periodic deformation. The measurement of the longitudinal strain of the flat plate caused by the compression of the ripple spring is performed with the Brillouin technique. Then the shape of the flat plate is reconstructed according to the strain-displacement relation. Measurements of distributed lateral displacement of the flat plate with different periods of 3 and 3.25 cm are reported by the authors with a maximum displacement of 0.43 mm and a minimum measurable displacement of ∼40 μm BIB004 . Steel corrosion has become a major problem worldwide, especially for structures exposed to aggressive environments. When corrosion happens, the volume of the steel rebar will increasingly expand due to the rust product accumulation on the surface of the steel rebar. Using this principle, a fiber BOTDA sensing technique was implemented to monitor corrosion expansion of steel rebars in steel reinforced concrete structures BIB005 . This kind of fiber optic coil winding Brillouin corrosion expansion sensors with different fiber optic coil winding packaging schemes was embedded into concrete specimens to monitor expansion strain caused by steel rebar corrosion. It was demonstrated by the authors that the expansion strain along the fiber optic coil winding area can be detected and measured using this kind of sensors during development of the corrosion. Three different types of these transducers were constructed by winding the authors around a polished steel rebar into a pretensioned fiber optic coil ( Figure 17 ). The fiber optic will be stretched when the volume of the steel rebar expands due to corrosion and hence the tension strain change of the fiber optic coil in corrosion area can be monitored using a BOTDA analyzer. The first kind of sensor offered a rather high sensitivity being able to detect signals of steel corrosion just at the beginning of the process. The second kind of sensor offered a relative low sensitivity compared with the first one; however, the larger monitoring range with a maximum expansion of 5690 με compensates for this to make it suitable for application in the resinous corrosion situations. Finally, the third kind of sensor showed the lowest sensitivity but the largest monitoring range (6738 με) of the three ones. It can perform steel corrosion monitoring where there may exist an extremely corrosive environment.