Application of distributed sensing with longlength FBG to structural health monitoring

In the field of fiber-optic sensing technology, distributed sensors that return a value of the measurand as a function of linear position along an optical fiber are regarded as a promising sensor which can be applied to structural health monitoring (SHM). We have developed a distributed strain sensing technique using long-length fiber Bragg grating (FBG) based on optical frequency domain reflectometry (OFDR). FBGs functioning as mirrors with wavelength selective reflectivity have been used as strain or temperature sensors. OFDR is a technique designed to measure back reflections from optical fiber networks and components. In our system, we use a longer FBG whose length is ordinarily about 100 mm and we can measure strain at an arbitrary position along the FBGs. Therefore, we can obtain continuous strain data along the FBGs. Furthermore, since the spatial resolution in strain measurements is less than 1 mm, it enables us to measure the strain distribution of stress concentrated area, such as welded and bonded joints, precisely. In this paper, we describe the principle of the distributed sensing technique with long-length FBGs and the applications to strain monitoring of a bonded joint and a wing structure. We also describe the technique to identify the load distribution applied to a structure based on strain information measured by fiber-optic strain sensors.