Isamu Ohsawa

Smart marine structures: an approach to the monitoring of ship structures with fiber-optic sensors

Prevention of serious damage of marine structures might be achieved by monitoring the loading conditions and by inspecting the structural integrity. The concept of smart structures with a fiber-optic sensor network can be applied to marine applications. There are plenty of marine structures in huge dimensions, for example, the overall length of a very large crude carrier exceeds 200 m. A fiber-optic laser-Doppler velocimeter was developed as a displacement sensor with long gage length. The optical fiber sensor has potential advantages as a structure monitoring sensor: (i) unlimited gage length, (ii) applicability to dynamic measurement and (iii) less affected by temperature. Free vibration behavior of the coupon specimen could be monitored with excellent dynamic resolution. Flexural deformation of the specimen under cyclic bending load was also monitored, and linearity and sensitivity of the developed sensor system was confirmed. An optical time-domain reflectometer (OTDR) was applied to the damage detection of composite materials in which optical fibers were embedded. Onset and location of fiber breaks could be detected by using the OTDR technique, which was applied to tensile tests of resin, and tensile and flexure tests of cross-plied GFRP. The failure strain of the optical fiber had rather wide scatter when it was embedded in the composite, and the average values also depended on the condition of embedment. Location of the failure points could be measured with sufficient accuracy.

Doppler effect in flexible and expandable light waveguide and development of new fiber-optic vibration/acoustic sensor

New principle and a geometrical arrangement of an optical fiber for a vibration/acoustic measurement are proposed in the present paper. The sensor is based on a new finding that a frequency of light wave transmitted through a bent optical fiber is shifted by vibration at the bent region. The phenomenon can be explained as Dopplers effect in flexible and expandable light waveguide. Several configurations of the sensor have been designed, and very high sensitivity is achieved in the extremely wide frequency range. Principle, sensor configuration and theoretical sensitivity, measurement system, and some experimental consideration are described in the present paper. The sensor sensitivity was examined experimentally in the low and middle frequency range, and the detectability was confirmed experimentally in the frequency range of acoustic emission signals.

Structural health monitoring using fiber optic distributed sensors for vacuum-assisted resin transfer molding

In this study we implemented manufacturing process and strain monitoring of a composite structure by optical fiber sensors for vacuum-assisted resin transfer molding (VaRTM). Optical fibers with fiber Bragg gratings were embedded into a glass fiber reinforced plastic specimen made by VaRTM and the applicability of structural health monitoring with fiber Bragg grating (FBG) sensors based on optical frequency domain reflectometry (OFDR) was investigated. In this study, long-gage FBGs which are 10 times longer than ordinary FBGs (which are about 10 mm long) were employed for distributed sensing. We can easily map the strain or temperature profile along gratings by OFDR and the spatial resolution of this sensing technique is about 1 mm. The resin flow process in VaRTM could be monitored by measuring the difference in temperature between the resin and preform. Then, the shrinkage of resin could be also monitored during the curing process. The specimen was then subjected to a bending load in a three-point bending test and the strain distributions along the FBGs were measured. From these results we could show the applicability of distributed sensors to quality assurance of a composite structure made by VaRTM and assessment of the structural integrity of in-service composite structures.

Dynamic strain distribution measurement and crack detection of an adhesive-bonded single-lap joint under cyclic loading using embedded FBG

In this study, the dynamic strain distribution measurement of an adhesive-bonded single-lap joint was carried out in a cyclic load test using a fiber Bragg grating (FBG) sensor embedded into the adhesive/adherend interface along the overlap length direction. Unidirectional carbon fiber reinforced plastic (CFRP) substrates were bonded by epoxy resin to form the joint, and the FBG sensor was embedded into the surface of one substrate during its curing. The measurement was carried out with a sampling rate of 5 Hz by the sensing system, based on the optical frequency domain reflectometry (OFDR) throughout the test. A finite element analysis (FEA) was performed for the measurement evaluation using a three-dimensional model, which included the embedded FBG sensor. The crack detection method, based on the longitudinal strain distribution measurement, was introduced and performed to estimate the cracks that occurred at the adhesive/adherend interface in the test.

Multiple damage assessment in composite laminates using a Doppler-effect-based fiber-optic sensor

In this paper, carbon fiber-reinforced plastic (CFRP) laminates are addressed for the purpose of multiple damage assessment. Doppler-effect-based fiber-optic (FOD) sensors were used to capture guided waves propagating in the CFRP laminates. Characteristics of the fundamental symmetric (S0) and anti-symmetric (A0) Lamb waves in captured guided-wave signals were extracted by taking advantage of linear-phase finite impulse response filter and Hilbert transform, so as to systematically investigate the influence of delaminations on guided-wave propagation. Both dispersive characteristics of multi-mode Lamb waves and features of the Lamb wave-excited fundamental shear horizontal (SH0) guided wave were applied for damage evaluation and multiple damage identification. Results demonstrate that the FOD sensor is effective in multiple damage identification for composite laminates because it is omnidirectional in ultrasonic detection.

Measurement of distributed strain and load identification using 1500 mm gauge length FBG and optical frequency domain reflectometry

High spatial resolution and sensitivity are required in distributed strain measurements for structural health monitoring. We have developed a distributed strain sensing technique with long gauge FBG sensors, which enables to measure strain at an arbitrary position along the FBG sensors with the high spatial resolution less than 1 mm based on optical frequency domain reflectometry (OFDR). In this paper this technique with a 1500 mm gauge length FBG was applied to monitoring strain distributions of a simply supported beam subjected to bending loads. The agreement between the measured strain and the theoretical one is excellent. Also we succeeded to identify the applied load by the inverse analysis from the measured strain distribution data, and confirmed the validity of these methods.

A Fiber Optic Doppler Sensor and Its Application in Debonding Detection for Composite Structures

Debonding is one of the most important damage forms in fiber-reinforced composite structures. This work was devoted to the debonding damage detection of lap splice joints in carbon fiber reinforced plastic (CFRP) structures, which is based on guided ultrasonic wave signals captured by using fiber optic Doppler (FOD) sensor with spiral shape. Interferometers based on two types of laser sources, namely the He-Ne laser and the infrared semiconductor laser, are proposed and compared in this study for the purpose of measuring Doppler frequency shift of the FOD sensor. Locations of the FOD sensors are optimized based on mechanical characteristics of lap splice joint. The FOD sensors are subsequently used to detect the guided ultrasonic waves propagating in the CFRP structures. By taking advantage of signal processing approaches, features of the guided wave signals can be revealed. The results demonstrate that debonding in the lap splice joint results in arrival time delay of the first package in the guided wave signals, which can be the characteristic for debonding damage inspection and damage extent estimation.

Process/health monitoring for wind turbine blade by using FBG sensors with multiplexing techniques

In this study, we applied fiber Bragg grating sensors to conduct process/health monitoring of wind turbine blade manufactured by VaRTM. In this study, we used a long gauge FBG (about 100mm) based optical frequency domain reflectometory (OFDR) and 8 FBGs on a single fiber based wavelength division multiplexing (WDM). Resin flow front and resin cure were detected during VaRTM. After manufacturing, structural health monitoring was conducted with the blades. These sensors with multiplexing techniques were able to monitor VaRTM process and wind turbine blade successfully.

Sensor for measuring velocity of vibration using light waveguide

This invention provides a device or method for measuring vibration with high sensitivity and over a wide bandwidth. A curved section is formed in an optical fiber. The curved section is disposed on a place to be measured. Light is input in the optical fiber, and then variation of frequency between the input light and the output light is detected. Infinitesimal vibration applied to the curved section can be measured as variation of frequency between the input light and the output light. Further, it is possible to measure vibration over a wide bandwidth.

Debonding detection using a self-calibration sensor network

Adhesively bonded joints between two plates are gradually receiving increasing acceptance in safety-critical structures, and therefore methods for nondestructive evaluation (NDE) of adhesive joints have drawn much attention. In this paper, specimens with lap splice joints are prepared for the purpose of guided-wave-based debonding damage detection. Fibre optic Doppler (FOD) sensors are used to acquire guided ultrasonic waves propagating in two kinds of bonded specimens, namely aluminium and carbon fibre reinforced plastic (CFRP) assemblies. To overcome the drawbacks of sensor networks in conventional structural health monitoring systems, a self-calibration sensor network is constructed based on elastic analyses of the bonded structures. By using the proposed sensor network, both the baseline measurement and the defect-related guided wave signals can be acquired in one excitation experiment. Moreover, complicated signal interpretation is also avoided by using the simplest feature, arrival time, of the ultrasonic wave signals.