Hirotaka Igawa

Strain monitoring of a single-lap joint with embedded fiber-optic distributed sensors

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.

Measurements of strain distributions with a long gauge FBG sensor using optical frequency domain reflectometry

We developed a strain measurement system based on optical frequency domain reflectometry and applied it to measuring strain distributions of a specimen in tensile tests. In the sensing region on the specimen, five FBG sensors of 6 mm gauge length or one of 100 mm gauge length was bonded and strain measurements were implemented with both configurations. By using the former configuration, we could successfully carry out accurate quasi-distributed strain measurements. The later allowed fully-distributed measurements for 100 mm at the high spatial resolution. Such performance of high resolution sensing can be applied to health monitoring of a structure which may have stress concentration. In this paper, we describe the principle of the measurement system and the results in the tensile tests.

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.

Lateral Load Measurements Based on a Distributed Sensing System of Optical Frequency-Domain Reflectometry Using Long-Length Fiber Bragg Gratings

In this paper, we demonstrate the measurements of lateral loads in a distributed manner using long-length fiber Bragg gratings (FBGs) based on optical frequency-domain reflectometry (OFDR). Birefringence is induced at specific areas of the long-length FBGs by lateral compression. The distributions of the Bragg peaks were monitored by the OFDR sensing system, from which we calculated the applied lateral loads. Theoretical calculation results confirmed the validity of the experimental results. In addition, position-wise beat signals of the power of the obtained spectra were observed with smaller lateral loads that were not sufficient to provide two distinguished Bragg peaks. This unique phenomenon for the frequency-domain reflectometry scheme is explained theoretically. We proposed a method for utilizing the beat cycle to calculate smaller lateral loads.

A hydrogen curing effect on surface plasmon resonance fiber optic hydrogen sensors using an annealed Au/Ta 2 O 5 /Pd multi-layers film

In this paper, a response time of the surface plasmon resonance fiber optic hydrogen sensor has successfully improved with keeping sensor sensitivity high by means of hydrogen curing (immersing) process of annealed Au/Ta2O5/ Pd multi-layers film. The hydrogen curing effect on the response time and sensitivity has been experimentally revealed by changing the annealing temperatures of 400, 600, 800°C and through observing the optical loss change in the H2 curing process. When the 25-nm Au/60-nm Ta2O5/10-nm Pd multi-layers film annealed at 600°C is cured with 4% H2/N2 mixture, it is found that a lot of nano-sized cracks were produced on the Pd surface. After H2 curing process, the response time is improved to be 8 s, which is two times faster than previous reported one in the case of the 25-nm Au/60-nm Ta2O5/3-nm Pd multi-layers film with keeping the sensor sensitivity of 0.27 dB for 4% hydrogen adding. Discussions most likely responsible for this effect are given by introducing the α-β transition Pd structure in the H2 curing process.

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.

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.

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.

Structural health monitoring of a full-scale composite structure with fiber-optic sensors

Structural health monitoring systems capable of assessing structural integrity during manufacture and service would allow us to keep them up-to-date and to increase their lifetime and safety in use. We installed fiber-optic sensors into a full-scale composite structure of a Japanese experimental re-entry vehicle and monitored temperature and strain distributions of the fuselage during the manufacturing process. The results obtained us with important information about the process control and the structural quality. The sensing system used in the manufacture could measure strain also in structural tests with static load. Although the strain measured by the fiber-optic sensor was averaged data depending on the spatial resolution, the overall deformation of the structure could be found, because strain was acquired extensively and continuously along the sensing fiber. The achievement of this study shows applicability of fiber-optic sensors to structural health monitoring for composite structures.

A Challenge of Modeling Thermo-Mechanical Response of Silica-Phenolic Composites under High Heating Rates

We have been building up a brand new ablation analysis code which is intended to predict simultaneously thermo-chemical and thermo-mechanical response of the SilicaPhenolic (SiFRP) ablator. In this paper, the model is applied to the RTG tests, FTE tests and the laser heating tests for model validation. The comparison to the SiFRP’s actual thermomechanical behavior shows that the present model gives better match by introducing the dependence of the elastic coefficient on the pore pressure in the high temperature region.