Daichi Wada

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.

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.

Dependence of measurement accuracy on the birefringence of PANDA fiber Bragg gratings in distributed simultaneous strain and temperature sensing

By both simulation and experiment, we studied the relationship of the measurement accuracy and the birefringence of the distributed simultaneous strain and temperature sensor using polarization-maintaining fiber Bragg gratings (PANDA-FBGs). The PANDA-FBGs were applied to an optical frequency domain reflectometry (OFDR) which is capable of distributed measurement at high spatial resolution and sampling rate. The simulated results had agreement with the experimental results that the measurement accuracy of both strain and temperature were improved by increasing the birefringence. Additionally, the efficiency of the accuracy improvements decreased when accuracy increased.

An optical fiber sensing technique for temperature distribution measurements in microwave heating

We introduce an optical fiber sensing technique that can measure the temperature distributions along a fiber during microwave heating. We used a long-length fiber Bragg grating (FBG) as an electromagnetic-immune sensor and interrogated temperature distributions along the FBG by an optical frequency domain reflectometry. Water in a glass tube with a length of 820 mm was heated in a microwave oven, and its temperature distribution along the glass tube was measured using the sensing system. The temperature distribution was obtained in 5 mm intervals. Infrared radiometry was also used to compare the temperature measurement results. Time and spatial variations of the temperature distribution profiles were monitored for several microwave input powers. The results clearly depict inhomogeneous temperature profiles. The applicability and effectiveness of the optical fiber distributed measurement technique in microwave heating are demonstrated.

Measurement of longitudinal strain and estimation of peel stress in adhesive-bonded single-lap joint of CFRP adherend using embedded FBG sensor

In this research, longitudinal strain and peel stress in adhesive-bonded single-lap joint of carbon fiber reinforced plastics (CFRP) were measured and estimated by embedded fiber Bragg grating (FBG) sensor. Two unidirectional CFRP substrates were bonded by epoxy to form a single-lap configuration. The distributed strain measurement system is used. It is based on optical frequency domain reflectometry (OFDR), which can provide measurement at an arbitrary position along FBG sensors with the high spatial resolution. The longitudinal strain was measured based on Bragg grating effect and the peel stress was estimated based on birefringence effect. Special manufacturing procedure was developed to ensure the embedded location of FBG sensor. A portion of the FBG sensor was embedded into one of CFRP adherends along fiber direction and another portion was kept free for temperature compensation. Photomicrograph of cross-section of specimen was taken to verify the sensor was embedded into proper location after adherend curing. The residual strain was monitored during specimen curing and adhesive joint bonding process. Tensile tests were carried out and longitudinal strain and peel stress of the bondline are measured and estimated by the embedded FBG sensor. A two-dimensional geometrically nonlinear finite element analysis was performed by ANSYS to evaluate the measurement precision.

Signal processing method based on group delay calculation for distributed Bragg wavelength shift in optical frequency domain reflectometry

A signal processing method based on group delay calculations is introduced for distributed measurements of long-length fiber Bragg gratings (FBGs) based on optical frequency domain reflectometry (OFDR). Bragg wavelength shifts in interfered signals of OFDR are regarded as group delay. By calculating group delay, the distribution of Bragg wavelength shifts is obtained with high computational efficiency. We introduce weighted averaging process for noise reduction. This method required only 3.5% of signal processing time which was necessary for conventional equivalent signal processing based on short-time Fourier transform. The method also showed high sensitivity to experimental signals where non-uniform strain distributions existed in a long-length FBG.

Distributed monitoring of fiber Bragg gratings under local lateral loads using optical frequency domain reflectometry

We demonstrate distributed monitoring of long-length fiber Bragg gratings (FBGs) under lateral loads using optical frequency domain reflectometry (OFDR). Local lateral loads are applied to a certain area of the long-length FBGs. We observe splits of Bragg peaks due to the lateral loads in a distributed manner. We calculated the lateral loads based on the amount of Bragg peak splits, which showed the good agreement with the applied loads.

Self-Evaluation of PANDA-FBG Based Sensing System for Dynamic Distributed Strain and Temperature Measurement

A novel method is introduced in this work for effectively evaluating the performance of the PANDA type polarization-maintaining fiber Bragg grating (PANDA-FBG) distributed dynamic strain and temperature sensing system. Conventionally, the errors during the measurement are unknown or evaluated by using other sensors such as strain gauge and thermocouples. This will make the sensing system complicated and decrease the efficiency since more than one kind of sensor is applied for the same measurand. In this study, we used the approximately constant ratio of primary errors in strain and temperature measurement and realized the self-evaluation of the sensing system, which can significantly enhance the applicability, as well as the reliability in strategy making.

Detection of local birefringence in embedded fiber Bragg grating caused by concentrated transverse load using optical frequency domain reflectometry

We investigate the capability of local birefringence detection in an embedded fiber Bragg grating (FBG) using optical frequency domain reflectometry. We embed an FBG into carbon fiber reinforced plastic specimen, and conduct 3-point bending test. The cross-sectional stresses are applied to the FBG at the loading location in addition to the non-uniform longitudinal strain distribution over the length of the FBG. The local birefringence due to the cross-sectional stresses was successfully detected while the non-uniform longitudinal strain distribution was accurately measured.