In this paper, we review our researches on the topics of the structural health monitoring (SHM) with the fiber-optic distributed strain sensor. Highly-dense information on strains in a structure can be useful to identify some kind of existing damages or applied loads in implementation of SHM. The fiber-optic distributed sensors developed by the authors have been applied to the damage detection of a single-lap joint and load identification of a beam simply supported. We confirmed that the applicability of the distributed sensor to SHM could be improved as making the spatial resolution higher. In addition, we showed that the simulation technique considering both structural and optical effects seamlessly in strain measurement could be powerful tools to evaluate the performance of a sensing system and design it for SHM. Finally, the technique for simultaneous distributed strain and temperature measurement using the PANDA-fiber Bragg grating (FBG) is shown in this paper, because problems caused by the cross-sensitivity toward strain and temperature would be always inevitable in strain measurement for SHM.

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.

We have developed a fiber-optic distributed sensor which can measure strain distributions along fiber Bragg grating (FBG) with the high spatial resolution. This sensing system is based on optical frequency domain reflectometry and a long-length FBG whose length is about 100 mm can be used. We can identify the longitudinal strain at an arbitrary position along the FBG using signal processing technology. In this study, long-length FBGs were embedded into the adhesive layers of the two single-lap joints and we could successfully measure the strain distributions inside the adhesives. In one single-lap joint, the adherends were carbon fiber reinforced plastics and in another one, they were aluminum. Theadhesive was epoxy in both cases. The measured results were compared with the calculated ones by nonlinear finite element (FE) analysis in which the large displacement and the elasto–plastic response of the adherend or adhesive material were account for. We found that in most of the applied loads, the agreement between the measured results and the calculated ones obtained from an intact FE model is excellent. While the measured strain distributions inside the adhesive layer of the aluminum single-lap joint were varied at the end of the overlap in the higher applied loads and they were much different from those of the intact model, an FE model with debonding was made and it could represent such variations. We could also monitor the strain distributions inside the adhesive during the manufacturing process and we observed the perturbation in residual strain distributions after curing. Consequently, we can say that the fiber-optic distributed sensor with the high spatial resolution is very useful not only to assess the structural integrity of adhesive joints but also to improve numerical analysis techniques and manufacturing processes for them.