Small-diameter Fiber Bragg Grating Research Articles

Loading Localization by Small-Diameter Optical Fiber Sensors

Structural health monitoring (SHM) in service has attracted increasing attention for years. Load localization on a structure is studied hereby. Two algorithms, i.e., support vector machine (SVM) method and back propagation neural network (BPNN) algorithm, are proposed to identify the loading positions individually. The feasibility of the suggested methods is evaluated through an experimental program on a carbon fiber reinforced plastic laminate. The experimental tests involve in application of four optical fiber-based sensors for strain measurement at discrete points. The sensors are specially designed fiber Bragg grating (FBG) in small diameter. The small-diameter FBG sensors are arrayed in 2-D on the laminate surface. The testing results indicate that the loading position could be detected by the proposed method. Using SVM method, the 2-D FBG sensors can approximate the loading location with maximum error less than 14 mm. However, the maximum localization error could be limited to about 1 mm by applying the BPNN algorithm. It is mainly because the convergence conditions (mean square error) can be set in advance, while SVM cannot.

Natural frequency detection of smart composite structure by small diameter fiber Bragg grating

Structural health monitoring (SHM) in-service is very important and definitely demanded for the safe working of high performance composite structures in situ. Fiber Bragg grating-based sensors were usually embedded inside a composite to compose a smart structure for a real time monitor. However, the mechanical performance of the materials would be unfavorably influenced. To lower the influence, a smaller diameter optical fiber is preferred. In this paper, a new non-destructive evaluation sensor system based on small diameter fiber Bragg grating (SDFBG) was proposed. The impact results show that the SDFBG sensor could be used in dynamic strain measurement and detect the natural frequency of the structure. Furthermore, the sensor would be applied in SHM.

Smart Honeycomb Sandwich Panels With Damage Detection and Shape Recovery Functions

In this research, optical fiber sensors and shape memory alloys (SMA) were incorporated into sandwich panels for development of a smart honeycomb sandwich structure with damage detection and shape recovery functions. First, small-diameter fiber Bragg grating (FBG) sensors were embedded in the adhesive layer between a CFRP face-sheet and an aluminum honeycomb core. From the change in the reflection spectrum of the FBG sensors, the debonding between the face-sheet and the core and the deformation of the face-sheet due to impact loading could be well detected. Then, the authors developed the SMA honeycomb core and bonded CFRP face-sheets to the core. When an impact load was applied to the panel, the cell walls of the core were buckled and the face-sheet was bent. However, after the panel was heated over the reverse transformation finish temperature of the SMA, the core buckling disappeared and the deflection of the face-sheet was relieved. Hence the bending stiffness of the panel could be recovered.

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

We have developed a damage detection system that generates ultrasonic waves with apiezo-ceramic actuator and receives them in a fiber Bragg grating (FBG) sensor. In thisresearch, this system was applied to evaluate the debonding progress in carbon fiberreinforced plastic (CFRP) bonded structures. First, small-diameter FBG sensors wereembedded in adhesive layers of a double-lap-type coupon specimen consisting of CFRPquasi-isotropic laminates bonded with epoxy adhesive films. Then, an ultrasonic wave at300 kHz was propagated through the debonded region, and the wavelet transform wasapplied to the received waveform. The obtained results showed clear differences dependingon the debonding length. Hence, a new damage index was proposed using thedifference in the distribution of the wavelet transform coefficient. The damage indexincreased with an increase in the debonded area. Furthermore, this system wasapplied 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 thewavelet transform. As a result, the damage index increased and the correlationcoefficient decreased with an increase in the debonded area. Hence the lengthof the debonding between the skin and the stringer could be easily evaluated.

Debonding monitoring of composite repair patches using embedded small-diameter FBGsensors

Small-diameter fiber Bragg grating (FBG) sensors were applied to the debondingmonitoring of composite repair patches. The target of the monitoring consists of carbonfiber reinforced plastic (CFRP) patches and an aluminum substrate. The specimens wereprepared through simple means: the tabular patches adhered to both sides of the aluminumsubstrate using epoxy adhesive films. Under cyclic loading conditions, the debondingprogressed between the aluminum substrate and the adhesive layers. Before theembedding of the FBG sensors, the embedding location was determined usingthe calculation of the sensor signals, also called the reflection spectra. When thecyclic loading was stopped at a pre-determined numbers of cycles, the reflectionspectra were measured at stress conditions of both 0 and 200 MPa. The newlyappearing peak in the reflection spectrum indicated the arrival of debonding atthe FBG sensors. As the debonding length increased, the intensity of the peakwas found to increase. The relationship between the spectral change and thedebonding length was verified by the simulated spectra and the strain distributionscalculated by finite element (FE) analysis. As a result, the debonding lengthcould be evaluated quantitatively by the spectral changes in the FBG sensors.

Real-time Detection of Debonding between Honeycomb Core and Facesheet using a Small-diameter FBG Sensor Embedded in Adhesive Layer

The authors have developed a novel technique to detect debonding in honeycomb sandwich structures using small-diameter fiber Bragg grating (FBG) sensors. A small-diameter FBG sensor is embedded in the adhesive layer between the core and the facesheet during the curing process of the adhesive layer. After the curing process, the reflection spectrum from the small-diameter FBG sensor is distorted because the formation of fillets induces a non-uniform strain distribution in the adhesive layer. However, after debonding, the core with the fillets peels off the facesheet, and, as a result, the reflection spectrum recovers its original shape due to the release of the non-uniform strain. Debonding can thus be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. In addition, a more reliable debonding detection technique using a small-diameter chirped FBG sensor is proposed, in which two indicators in the reflection spectrum have been successfully used for a more reliable judgment of debonding.

3216 衝撃損傷検知および形状回復能力を有する知的サンドイッチパネル(J06-4 損傷検知・診断,J06 知的材料・構造システム)

In this research, the authors developed a smart sandwich structure with damage detection and suppression functions using a small-diameter fiber Bragg grating (FBG) sensor and shape memory alloy (SMA) honeycomb core. Impact damage was detected using the change in the reflection spectrum of the FBG sensor due to strain change along a bend of a facesheet caused by core buckling. After the damage detection, in order to heat the SMA honeycomb, a voltage was applied to a nichrome wire embedded in the facesheet. The core buckling disappeared and the bend of the facesheet was relieved. These abilities could also be confirmed by theoretical and numerical simulations.

Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber and its fiber Bragg grating (FBG) sensor for embedment inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. First, a brief summary is presented for applications of small-diameter FBG sensors to damage monitoring in composite structures. Then, we propose a new damage detection system for quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. 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 an FBG sensor attached on or embedded in the laminate using a newly developed high-speed optical wavelength interrogation system. This system was applied to detect interlaminar delamination in CFRP cross-ply laminates. 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 analysis. From the changes in the amplitude ratio and the arrival time of the new mode depending on the delamination length, it was found that this system could evaluate the delamination length quantitatively. Furthermore, small-diameter FBG sensors were embedded in a double-lap type coupon specimen, and the debonding progress could be evaluated using the wavelet transform.

Delamination monitoring of laminated composites subjected to low-velocity impact using small-diameter FBG sensors

Abstract Small-diameter fiber Bragg grating (FBG) sensors were applied for the monitoring of delamination induced by low-velocity impact. The FBG sensors were embedded into carbon fiber reinforced plastic (CFRP) laminates [04/904/04]. Using a drop-weight impact tester, an impact loading was applied to the laminates at four impact energy levels. After the impact tests, the internal damages including delaminations were observed by ultrasonic C-scan, and the reflection spectra from the embedded FBG sensors were measured. The form of the spectrum changed sensitively depending on the delamination size. Furthermore, the spectra were calculated theoretically for confirmation of the measured spectra. Since the change in the measured spectrum was consistent with that in the calculated spectrum, the relationship between the delamination size and the form of the spectrum could be clarified. From the results, the present method using small-diameter FBG sensors was found to be effective for the monitoring of the delamination.

Evaluation of Crack Suppression Effect of TiNi SMA Foil Embedded in CFRP Cross-Ply Laminates with Embedded Small-Diameter FBG Sensor

A Ti-Ni shape memory alloy (SMA) foil and a small-diameter fiber Bragg grating (FBG) sensor were embedded simultaneously into a CFRP cross-ply laminate. When the specimen was heated, the recovery compressive force was generated from the embedded SMA foil, which homogenized the non-uniform strain distribution caused by cracks in the 90° ply. Then, the tensile stress in the 90° ply was relaxed and the occurrence of new transverse cracks was suppressed. This effect was evaluated with the embedded FBG sensor. When the specimen was heated, the deformed reflection spectrum of the FBG returned to its original shape, which suggested that Ti-Ni SMA foil was effective to suppress the damage. However, relaxation of thermal residual tensile stress in the 90° ply was also effective. The result of the 3D FEA suggested that the suppression of damage occurrence and growth was mainly caused by the relaxation of thermal residual tensile stress.

Quantitative evaluation of transverse cracks in carbon fiber reinforced plasticquasi-isotropic laminates with embedded small-diameter fiber Bragg grating sensors

The authors have applied newly developed small-diameter fiber Bragg grating (FBG) sensors, whose claddingis 40 µm in diameter, for the detection of transverse cracks in carbon fiber reinforced plastic (CFRP)laminates. In previous research, the small-diameter FBG sensors were embedded in CFRPcross-ply laminates. When transverse cracks occurred, reflection spectra from the FBGsensors broadened with an increase in the crack density. Thus, the authors showed thatsmall-diameter FBG sensors had the potential to detect the occurrence of cracks. In thepresent research, this technique is applied to the detection of the transverse crack evolutionin CFRP quasi-isotropic laminates, whose laminate configuration is more suitable forpractical use. Through the experiment and the theoretical calculation, it was found thatthe small-diameter FBG sensor could also detect transverse cracks in quasi-isotropiclaminates quantitatively.

Detection of edge delamination in CFRP laminates under cyclic loading using small-diameter FBG sensors

Small-diameter FBG sensors were applied for the detection of edge delamination in carbon fiber reinforced plastic (CFRP) quasi-isotropic laminates. Reflection spectra from the embedded fiber Bragg grating (FBG) sensor were measured at various lengths of delamination initiated from the edge of the specimen under cyclic loading. The form of the spectrum changed sensitively as the edge delamination grew. For confirmation of the measured results, the strain distribution in the FBG sensor was calculated by FEM analysis and the spectrum was simulated from the strain distribution theoretically. The change in the form of the measured spectrum was consistent with that of the calculated spectrum. From these results, the spectrum was found to depend on the size and location of the edge delamination. Moreover, the intensity ratio of the two peaks in the spectrum was proposed as an effective indicator for the quantitative evaluation of the edge delamination size.

426 細径 FBG センサを利用した補修用複合材料パッチのスマート化に関する研究

Since airworthiness of aging aircrafts is required to be extended, bonded composite patches are used for repairing fatigue cracks. However, the repair deteriorates when the patch is debonded. Thus, in this research, the smart patch that has a sensor function for the debonding is developed. A small-diameter fiber Bragg grating (FBG) sensor was embedded in this smart patch for detection of the delamination between substrate and the patch. It was theoretically revealed that the delamination could be evaluated quantitatively by the monitoring of the change in the reflection spectrum from the sensor.

752 細径 FBG センサを利用した複合材料パッチの剥離挙動の検出

A small-diameter fiber Bragg Grating (FBG) sensor is easily embedded in adhesive layer between a repair patch and a structure, since the diameter of this sensor is very small. Thus, the FBG sensor was embedded into the adhesive layer to detect the debonding between the patch and the substrate. First, it was confirmed that debonding progress of the patch was not affected by embedment of the FBG sensor. Secondly, it was experimentally and theoretically revealed that the delamination length could be evaluated quantitatively by the comparison of intensity ratio of the two peaks in the reflection spectrum from the sensor.

108 細径 FBG センサによる CFRP クロスプライ積層板の衝撃損傷の定量的検出

Small-diameter fiber Bragg grating (FBG) sensors were embedded into the 90°ply on the border of the lower 0°ply of carbon fiber reinforced plastic (CFRP) cross-ply [0_4/(90)_4/0_4] laminates. When impact loading was applied to the specimens with a drop-weight impact tester, reflection spectrum from the sensors changed sensitively. In order to clarify these results, the spectrum was simulated theoretically. Furthermore, the intensity ratio of two peaks in the spectrum was found to be a good indicator for the quantitative evaluation of impact damage in real time.

744 細径光ファイバセンサによる CFRP 積層板の動的挙動の検出

In order to monitor damages in Carbon Fiber Reinforced Plastic (CFRP) laminates, it is attempted to detect transmitted waves in laminates using embedded small-diameter fiber Bragg Grating (FBG) sensors. In this research, velocities of elastic waves in CFRP unidirectional laminates and quasi-isotopic laminates were calculated theoretically. Then the relationship between the grating length and the wavelength of the elastic wave was simulated. From these results, the optimum grating length to detect elastic waves was determined.

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates. Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers. The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test. The form of the spectrum changed sensitively, as the delamination length increased. Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum. The calculated result reproduced the change in the measured spectrum very well. Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length. This indicator has a potential to be applied to other laminate configurations.

Detection of microscopic damages in composite laminates

Small-diameter fiber Bragg grating (FBG) sensors, of outside diameter 52 μm, have been developed by the authors and Hitachi Cable Ltd. for embedding inside a laminate without deterioration of the mechanical properties of the composite laminate. In this research, the small-diameter FBG sensor was embedded in the 0° ply of a CFRP cross-ply laminate for the detection of transverse cracks in the 90° ply. The reflection spectra from the FBG sensor were obtained at various tensile stresses. As a result of damage, the spectrum became broad and had some peaks with increase in the transverse crack density. Furthermore, theoretical calculation reproduced the change in the spectrum very well. These results show that small-diameter FBG sensors have the potential to detect the occurrence of transverse cracks through the change in the form of the spectrum, and to evaluate the transverse crack density quantitatively by the spectrum width.