Seiji Kojima

Evaluation of debonding progress in composite bonded structures using ultrasonic waves received in fiber Bragg grating sensors

We have developed a damage detection system that generates ultrasonic waves with a piezo-ceramic actuator and receives them in a fiber Bragg grating (FBG) sensor. In this research, this system was applied to evaluate the debonding progress in carbon fiber reinforced plastic (CFRP) bonded structures. First, small-diameter FBG sensors were embedded in adhesive layers of a double-lap-type coupon specimen consisting of CFRP quasi-isotropic laminates bonded with epoxy adhesive films. Then, an ultrasonic wave at 300 kHz was propagated through the debonded region, and the wavelet transform was applied to the received waveform. The obtained results showed clear differences depending on the debonding length. Hence, a new damage index was proposed using the difference in the distribution of the wavelet transform coefficient. The damage index increased with an increase in the debonded area. Furthermore, this system was applied to a skin/stringer structural element of airplanes made of CFRP laminates. In this case, a correlation coefficient was also calculated from the results of the wavelet transform. As a result, the damage index increased and the correlation coefficient decreased with an increase in the debonded area. Hence the length of the debonding between the skin and the stringer could be easily evaluated.

High-speed optical wavelength interrogator using a PLC-type optical filter for fiber Bragg grating sensors

We study a new sensing system for health monitoring of aircraft structures made of composite materials. This sensing system is composed of fiber Bragg grating (FBG) sensors and a piezoelectric transducer (PZT). The FBG sensors receive elastic wave generated by the PZT. For high-frequency vibration monitoring, or acoustic emission (AE) detection, we have developed high-speed optical wavelength interrogator for FBG sensors. In this FBG interrogator, we used an optical filter which converts wavelength shift of the reflected light from the FBG into the output optical power change. This system is suitable for high-speed wavelength detection because there is no mechanical moving part. We studied two types of optical filter. One is a Mach-Zender interferometer and the other is an arrayed waveguide grating (AWG). Both of them were fabricated using silica-based planar lightwave circuit (PLC) technology. The optical filters based on the PLC technology have the advantage in integration of optical components. By combining the FBG interrogator using the AWG optical filter with the PZT actuator, we succeeded to detect elastic wave propagating in the CFRP laminated plate. As a result, we found that this FBG/PZT hybrid sensing system is very promising for detection of internal defects in composite materials.

Development of high-speed optical wavelength interrogation system for damage detection in composite materials

We have been studying optical sensing technologies that use fiber Bragg gratings (FBGs) for health monitoring of aircraft structures made of carbon fiber reinforced plastic (CFRP) composite materials. The sensing system is composed of a piezoelectric transducer (PZT) actuator, which generates an elastic wave of several hundred kHz, and FBG sensors that receive the elastic wave. When some damage occurs in the composite materials, the elastic wave that propagates through those materials changes. Therefore the damage can be detected by analyzing the elastic waveform to be received by FBG sensors. For detecting this wave, we developed a high-speed optical wavelength interrogator for FBG sensors, and FBG sensor modules that can be embedded in the composite materials. In this interrogator, we employed an arrayed waveguide grating (AWG) as an optical filter that can convert the wavelength shift of the FBG sensors into optical power change. Using this interrogator and FBG sensor modules, we detected elastic waves of 300 kHz in frequency. We determined the required characteristics of FBG sensor both through simulation and experiments for improving the sensitivity of this health monitoring system.

Small-diameter optical fiber and high-speed wavelength interrogator for FBG/PZT hybrid sensing system

We have been developing a sensing system for checking the health of aircraft structures made of composite materials. In this system, lead zirconium titanate (PZT) actuators generate elastic waves that travel through the composite material and are received by embedded fiber Bragg grating (FBG) sensors. By analyzing the change in received waveforms, we can detect various kinds of damage. The frequency of the elastic waves is several hundred kHz, which is too high for a conventional optical spectrum analyzer to detect the wavelength change. Moreover, a conventional single-mode optical fiber cannot be used for an embedded FBG sensor because it is so thick that it induces defects in the composite material structure when it is embedded. We are thus developing a wavelength interrogator with an arrayed waveguide grating (AWG) that can detect the high-speed wavelength change and a small-diameter optical fiber (cladding diameter of 40µm) that does not induce defects. We use an AWG to convert the wavelength change into an output power change by using the wavelength dependency of the AWG transmittance. For this conversion, we previously used two adjacent output ports that cover the reflection spectrum of an FBG sensor. However, this requires controlling the temperature of the AWG because the ratio of the optical power change to the wavelength change is very sensitive to the relationship of the center wavelengths between an FBG sensor and the output ports of the AWG. We have now investigated the use of a denser AWG and six adjacent output ports, which covers the reflection spectrum of an FBG sensor, for detecting the elastic waves. Experimental results showed that this method can suppress the sensitivity of the power change ratio to the relationship of the center wavelengths between an FBG sensor and the output ports. Although our improved small-diameter optical fiber does not induce structural defects in the composite material when it is embedded, there is some micro or macro bending of the fiber, which causes propagation loss. To suppress this embedment loss, we adjusted the refractive index difference of the fiber to have larger value. Experimental result showed that this reduced the embedment loss by about 0.3 dB/cm. These enhancements make our sensing system more practical and should promote the use of composite materials in a wider range of applications.

Lamb wave sensing using fiber Bragg grating sensors for delamination detection in composite laminates

The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor attached on the laminate using a newly developed high-speed optical wavelength interrogation system. At first, the optimal gauge length of the FBG to detect ultrasonic waves was investigated through theoretical simulations and experiments. Then, the directional sensitivity of the FBG to ultrasonic waves was evaluated experimentally. On the basis of the above results, the 1mm FBG sensors were applied to the detection of Lamb waves propagated in carbon fiber reinforced plastic (CFRP) cross-ply laminates. The piezo-actuator was put on the laminate about 50mm away from the FBG sensor glued on the laminate, and three-cycle sine waves of 300kHz were excited repeatedly. The waveforms obtained by the FBG showed that S0 and A0 modes could be detected appropriately. Then, artificial delamination was made in the laminate by removing of a Teflon sheet embedded in the 0/90 interface after the manufacturing. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element method. Furthermore, since the amplitude and the velocity of the new mode increased with an increase in the delamination length, this system has a potential to evaluate the interlaminar delamination length quantitatively.

Evaluation of debonding progress in composite bonded structures by ultrasonic wave sensing with fiber Bragg grating sensors

The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates ultrasonic waves in a carbon fiber reinforced plastic (CFRP) laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor using a newly developed high-speed optical wavelength interrogation system. In this research, this system was applied to the evaluation of debonding progress in CFRP bonded structures. At first, small-diameter FBG sensors, whose cladding diameter is about 1/3 of common optical fibers, were embedded in an adhesive layer of a double-lap type coupon specimen consisting of CFRP quasi-isotropic laminates, and the ultrasonic wave was propagated through the debonded region. After that, the wavelet transform was applied to the received waveforms and the results showed clear difference depending on the debonding length. Hence, a new damage index was proposed, which could be obtained from the difference in the distribution of the wavelet transform coefficient. As a result, the damage index increased with an increase in the debonded area. Furthermore this system was applied to the skin/stringer structural element of airplanes made of CFRP laminates. Both of the waves received by a bonded FBG and by an embedded FBG changed sensitively to the debonding progress. Also, the damage index could evaluate the length of the debonding between the skin and the stringer.

Development of multipoint strain measurement systems using small-diameter fiber Bragg gratings

We have been studying optical sensing technologies using fiber Bragg gratings (FBGs) for health-monitoring systems in the fields of constructions, civil engineering, aerospace and so on. In these fields, various kinds of sensing techniques such as strain, temperature, vibration and crack detection are required. To meet these needs, we have fabricated the FBGs by precisely controlling photo-induced refractive index modulation or the grating period along a fiber. Also, as embedded sensors, we developed small-diameter FBGs which are embedded in fiber reinforced plastics (FRP) composite materials without inducing any mechanical deterioration. In this paper, we present the progress of our small-diameter FBGs and propose a new wavelength detection technique for FBG sensors using a wavelength division multiplexing (WDM) coupler. The reflected light is divided at the ratio depending wavelength through the WDM coupler. Especially, we adopted a PLC-type WDM coupler which has the advantage in low polarization sensitivity and integration compared with a fused-optical fiber-type WDM coupler. This technique is useful in high-speed detection for vibration or impact damage. Another application is a multipoint measurement system by using together with optical time domain reflectometry (OTDR) method. In this system, pulses of light incident into an optical fiber return from the FBGs to a detector with different arrival time. As a result, we can use FBGs with the same Bragg wavelength, because measuring delay time of the pulses enables to distinguish each FBG location. In addition, using together with wavelength division multiplexing within wavelength region for optical communications can increase the number of FBG sensors. The results of the basic performance in this system showed that it is very promising for the multipoint measuring system. These wavelength detection techniques will expand sensing applications using small-diameter FBGs.

Development of small-diameter optical fiber sensors and high-speed optical wavelength interrogator for damage detection in composite materials

We have been developing a sensing system for monitoring the structural health of aircraft structures made of composite materials. The sensing system is composed of fiber Bragg grating (FBG) sensors, a wavelength interrogator and piezoelectric actuators. The FBG sensors receive 100 kHz to 1 MHz elastic waves generated by the PZT actuators. For the FBG sensors, we previously developed a polyimide-coated optical fiber with a cladding diameter of 40 μm and core-cladding relative refractive index difference Δ of 0.65 %, that can be embedded in composite materials without inducing any mechanical defects. Since the cladding of that fiber is so thin, however, under embedded conditions, the transmission loss of the fiber is larger than that of a normal single-mode optical fiber. We therefore developed a new small-diameter optical fiber with an Δ of 1.8 %, in order to suppress the loss increase caused by micro-bending or transversely applied strain under the embedded condition. On the other hand, the small-diameter optical fiber needs to be connected to a normal optical fiber whose claddingding diameter is 125 μm, because it is fragile and difficult to handle. For practical use, we developed a small-diameter optical fiber module that has a special connector on both ends of the small-diameter optical fiber. The special connector can connect the small-diameter optical fiber to a normal optical fiber that has a standard MU connector. We also developed a high-speed optical wavelength interrogator that can detect the high-frequency vibration of the FBG sensors. It uses an arrayed waveguide grating (AWG) as an optical filter that converts the wavelength shift of the light reflected from the FBG into the output optical power changes. This wavelength interogator is suiatable for high-speed wavelength detection because it has no mechanical moving parts. The development of these components will help put this system to practical use and thus extend the use of composite materials to a wider range of applications.

Embedded small-diameter fiber Bragg grating sensors and high speed wavelenth detection

We have been studying optical sensing technology using embedded fiber Bragg grating (FBG) sensors. The FBG is inscribed in a small-diameter optical fiber with the cladding diameter of 40 μm. This technology is very promising for health monitoring in aviation components, because the diameter of the sensor optical fiber is so small that embedding of the sensor does not deteriorate mechanical properties of the composite materials. For practical use, we have also studied high reliable fused-splicing method between the small-diameter optical fiber and an ordinary optical fiber in order to improve handling. The embedded FBG sensors are useful for vibration or impact detection as well as static strain detection. For the purpose of detecting dynamic phenomena, we have developed a high speed wavelength detection unit for the FBG sensors which uses wavelength division multiplexing (WDM) coupler based on planar lightwave circuit (PLC) technique. WDM coupler converts wavelength of the light reflected from the FBG sensor into output powers. Since there is no mechanical moving part, this type of wavelength detection technique is suitable for high speed detection.

Optical water-level sensing systems using fiber Bragg grating

We have developed the all optical high-precision water level sensors based on fiber Bragg grating (FBG) technique, which are applied for actual fields such as rivers, lakes, sewage systems and so on. The sensor head consists of a diaphragm, a customized Bourdon tube and two FBGs, one for tensile measurement and other for temperature compensation. The FBG attached to the Bourdon tube is strained as the water level increases, and causes center wavelength shift of the reflected light from the FBG, which is detected by the wavelength interrogation equipment composed of a tunable Fabry-Perot filer. We have achieved the sensor accuracy of +/- 0.1% F.S., i.e. +/- 1 cm in case of full measurement range of 10 m. Several sensor heads can be connected in series through one optical fiber and each water level at different places can be measured simultaneously by one wavelength interrogation equipment.