Akiyoshi Shimada

Application of Fiber-Optic Distributed Sensors to Health Monitoring for Full-Scale Composite Structures

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 Americas 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.

Distributed Strain Sensing from Damaged Composite Materials Based on Shape Variation of the Brillouin Spectrum

Among fiber-optic distributed sensors, Brillouin optical time domain reflectometry (BOTDR) can effectively measure overall strain in a structure. However, since the spatial resolution of BOTDR is generally more than 1 m, it is difficult to detect inhomogeneous strain distributed within 1 m along a fiber. In this paper, we propose a new technique to detect strain changing sharply within the length of the spatial resolution. This technique is based on the fact that the profile of the Brillouin spectrum changes depending on strain distributions. We confirmed the dependency of the Brillouin spectrum on strain distributions theoretically and experimentally.

Dependence of the Brillouin gain spectrum on linear strain distribution for optical time-domain reflectometer-type strain sensors

We theoretically derive the shape of the Brillouin gain spectrum, that is, the Briilouin backscattered-light power spectrum produced in an optical fiber under conditions of a strain distribution that changes linearly with a constant slope. The modeled measurement system is an optical time-domain reflectometer-type strain sensor system. The linear strain distribution is one of the fundamental distributions and is produced in, for example, a beam to which a concentrated load is applied. By analyzing a function that expresses the shape of the derived Brillouin gain spectrum, we show that the strain calculated from the frequency at which the spectrum has a peek value coincide. with that at the center of the effective pulsed light. In addition, the peak value and the full width at half maximum of the Brillouin gain spectrum are both influenced by the strain difference between the two ends of the effective pulse. We investigate this influence in detail and obtain the relationship between strain difference and strain measurement error.

Linear strain distribution dependence of the Brillouin gain spectrum

We undertook a theoretical study of the Brillouin gain spectrum dependence on the strain distribution in an optical fiber, which changes linearly with a constant slope. We also investigated the strain measurement error in BOTDR induced by the linear strain distribution.

Damage Detection of Composites by Using Brillouin Spectroscopy.

Fiber-optic distributed sensors are candidates for sensing elements of structural health monitoring that improves reliability and safety of composite structures. Among fiber-optic sensors, Brillouin Optical Time Domain Reflectometer (BOTDR) can measure strain at an arbitrary position of a sensing fiber by detecting the variation in Brillouin frequency shift. In the case of the measurement based on the Brillouin frequency shift, however, it is difficult to detect the damage leading to nonuniform strain distributed within the length of the spatial resolution of BOTDR along a fiber. In this paper, we propose a new technique to detect the nonuniform strain changing sharply within the length of the spatial resolution. This technique is based on the fact that the profile of Brillouin gain spectrum changes depending on the strain distribution. We confirmed the fact theoretically and experimentally. The applicability of BOTDR to detecting the damage in composite structures was improved.