Optical fiber sensors are promised candidates as sensor elements in structural health monitoring (SHM). Especially
fiber-optic distributed strain sensors that return a strain value as a function of linear position along an optical fiber have
been attractive for people in the field of SHM. We have developed a distributed strain sensing system using long-length
fiber Bragg gratings (FBGs), based on optical frequency domain reflectometry (OFDR). We employ long-length FBGs
whose length is about 100 mm and the sensing region, in other words the gauge length, can be expanded up to more than
1 m by serially-cascaded long-length FBGs. This sensing system has the high spatial resolution of less than 1 mm. In this
paper the distributed sensing system with OFDR and its application to SHM are described.
A new fiber-optic vibration sensor has been developed and applied to structural health monitoring. The sensor is based on new finding that frequency of light wave transmitted through an bended optical fiber is shifted by vibration at the bended region. The principle can be explained that Doppler's effect of the light wave which changes its direction in the optical fiber. Several configurations of the sensor have been designed, and very high sensitivity has been achieved in the extremely wide frequency band by applying laser Doppler velocimetory. Practically numberless sensing points can be arranged on a single optical fiber, and regional monitoring which covers large area of the structures can be achieved. The performance of the sensor is examined experimentally by applying to detection of AE signals and elastic wave propagation in the composite material and adhesive joint test specimens. The experimental results show that the sensitivity is almost equivalent to PZT sensor and that failure of composite materials and debonding in the adhesive joint can be detected successfully. The durable, low-cost and high-sensitive sensor can show a new scope to structural health monitoring in the very variety of applications, for examples, composite aerospace structures, energy plants, piping system, infrastructures such as bridges and tunnels, surface and underground facilities. University and industry collaboration initiated a new business of new NDE for health monitoring and diagnostics.