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Carbon fiber reinforced plastic (CFRP) structures are vulnerable to low-speed impacts, which will lead to almost invisible impact damage. Therefore, the timely localization of impact is of great si...

Two types of fiber optics, namely macro etched silica single mode fiber (SMF) and Fiber Bragg Grating (FBG) (Bragg wavelength of 1550 nm and 1554 nm) had been used to detect various types of honey samples, such as Apis Dorsata Honey, Trigona Honey and Capilano Australian Honey. To study the effect of exposure period in open environment at room temperature, all honey samples were exposed in open environment at room temperature from 2 to 10 days. In comparison with macro etched SMF and FBG (λB=1554 nm), the FBG (λB=1550 nm) portrays an excellent sensing properties with sensitivity and selectivity of 33.56 dB/RIU and 24.07 dB/RIU respectively. The output of this work concludes that the quality of honey based on optical output power reduces up to 0.35% as period of exposure to the open environment increased.

Multi-parameter sensing technology plays an important role in structural health monitoring. In this paper, a novel FBG sensor with a pendulum structure for simultaneous measurement of tilt and acceleration is proposed. The design, principle and performance of the FBG sensor is introduced and tested in detail. The results show that the tilt sensitivity and acceleration sensitivity of the proposed FBG sensor can reach 408.3 pm/° and 2.43 nm/(m/s2), respectively, and that it works below 32 Hz in weak vibration detection. The theory analysis and the test result are in good agreement with each other. A wavelet transform method is utilized to separate the tilt and acceleration from the original signal received by the proposed FBG sensor. Cross sensitization of tilt and acceleration can be avoided effectively using this method. Finally, a field test is conducted in a railway slope. The recorded data is analyzed, which confirms further that the proposed FBG sensor can detect tilt and acceleration simultaneously. This novel FBG sensor provides a new method for multi-parameter monitoring.

We propose a conductive carbon coated fiber Bragg grating (FBG) temperature sensor for enhancing temperature sensitivity. The temperature sensitivity of the proposed sensor is effectively enhanced by approximately 4 times higher than those of the bare-type FBG sensors due to thermal expansion and physical stress possessed by the conductive carbon coating, with the operating temperature ranging from 40 °C to 260 °C. The proposed sensor is much more convenient to fabricate compared to other similarly coated FBG sensors, giving a leap advantage of easy reformation of the coating to fit various housings. This proposed sensor is significantly ideal for various applications such as optoelectronic circuit and waveguide temperature measurement.

We investigate the performance of a cantilever-type force sensor based on PDMS-coated fiber Bragg gratings for robot-assisted surgical applications. The cross section area of PDMS matrix and the location of the FBG sensor inside it are optimized through simulations. The sensitivity of the proposed sensor is obtained to be around 0.5 mN.

The deformation of Carbon Fibre Reinforced Polymer (hereafter referred to as CFRP) will affect the reliability and safety of products when it is applied in the fields of aeronautics & astronautics ...

Optical receiver modules play an important role in receiving signals from the Fiber Bragg Grating (FBG) sensors used for monitoring in civil structures, power facilities, and other sensing applications. When multiple sensors are used, more photo diodes receive optical signals; thin-film beam splitters with polarization characteristics are therefore used to reduce the size of the optical receiver module. Using a reflective semiconductor optical amplifier (R-SOA) with polarization characteristics in a sensing system with an FBG sensor, a structure that is less sensitive to polarization was developed by packaging a quad optical receiver module. A stabilization of ±4.67% was achieved using the dual optical receiver module in polarization and of ±1.20% with the quad optical receiver module in polarization.

We are working on the development of a sensor that can calculate various vital signs from the signals measured by installing a Fiber Bragg Grating (FBG) sensor at the pulsation point of a living body. In this paper, the influence of blood flow parameters and sensor installation conditions on the FBG sensor signal was verified using two blood flow simulation models. From the results of our study, the following observations were made. The fluid pressure increases, the wavelength shift of the FBG sensor beca me larger. By contrast, an increase in the viscosity of the fluid was seen to cause a decrease in the displacement length of the Bragg wavelength. Fluid pressure and viscosity were observed to opposite effects on the FBG sensor signal. The Bragg wavelength shift of the FBG sensor was observed to be larger when the installation direction of the FBG sensor was perpendicular to the blood flow direction than in the parallel direction. However, FBG sensor signals with higher signal-to-noise (S/N) ratio were measured at an angle of 75 degrees than at an angle of 90 degrees, which is perpendicular to the blood flow direction. From the above results, the installation condition of the FBG sensor on the measurement signal and the influence of each parameter of blood flow were established.

In this paper, a simple uniform fiber Bragg grating (FBG) sensor is designed, and maximum reflection of 7.1 dB at 1.55 μm is obtained. As this FBG shows a better reflection, it deserves well for the purpose of sensing and communication predominantly as they are resistant to electromagnetic radiations, light weight, low cost, firm size, and ease of handling. The proposed FBG is used for explosive sensing applications to detect explosives like trinitrotoluene (TNT), nitroglycerin, and royal demolition explosive (RDX). The different strain and temperature wavelength shifts of the explosives also analyzed and plotted.

Compared with the traditional electrical resistance strain gauge, the fibre Bragg grating (FBG) is a new material that can be used to produce the strain sensor, which has many advantages such as convenient installation, low temperature drifting, less signal interference and higher transmitted quality. In this study, FBG sensor system configuration, design principle, numerical simulation and loading tests for the FBG strain gauge are expatiated extensively. At the last, one-axial and three-axial strain gauges are produced and some conclusions have been put forward. So the FBG strain gauge can meet the precision of the structure monitoring and replace the traditional sensors.

Structural health monitoring (SHM) of engineering structures plays a crucial role in ensuring their safety and integrity. Acoustic emissions (AE) can be one of the several ways to monitor the structural health. Fiber Bragg grating (FBG)-based AE measurement technique is emerging as a solution to assess cracks in structures remotely and has advantages of being less bulky and having low noise, high bandwidth and ease of implementation. This paper presents a study on the impact of AE signals on FBGs attached to a substrate, focusing on the directional sensitivity using different configurations. For ease of installation, the FBG sensors are metallically packaged similar to lead zirconium titanate (PZT) sensors in which they can withstand temperatures of up to 250 °C. The directional AE sensitivity of the packaged FBG sensor device is studied and compared with that of a surface attached FBG. Experimentally, the AE signals are generated by a metal ball drop impact on aluminum plate and are measured using an FBG AE interrogation system. A numerical simulation of the design was carried out using ANSYS explicit dynamics and the AE wave propagation in structure was analyzed and experimentally verified.

Ventricular assist devices are used to support patients with end-stage heart failure. Many patients on long-term VAD support are treated out-of-hospital where monitoring technology for patients is severely limited; leading to sub-optimal patient management. This paper describes a novel fibre Bragg grating (FBG) pressure sensor which was developed for improved monitoring of patients with ventricular assist devices. The FBG sensor was fabricated in-house and characterised in a circulating heat bath. The FBG pressure sensor demonstrated a sensitivity of −0.0046 nm/mmHg with an accuracy of ± 0.35 mmHg, a resolution of 0.0019 mmHg and a response time of 89.7 mmHg/s. The high sensitivity and resolution combined with the adequate accuracy exhibited by the FBG sensor demonstrated its potential value of the FBG pressure sensor for monitoring patients with ventricular assist devices. Further investigation into device drift, improved manufacturing, and gas-impermeable biocompatible materials are required to prepare the sensor for pre-clinical trials.

In this paper, the effects of three types of commonly used adhesives were studied via experiment and simulation. The results show that the strain sensitivity of fiber Bragg grating (FBG) sensor is about 1.201 pm/μe and the effects of three adhesives of epoxy resin, AB glue and 502 glue with the same thickness and length on sensitivity coefficients of FBG sensors are 99.20%, 93.80% and 96.50%, respectively. Finally, the sensitivity coefficient of the simulated FBG sensor is the same as the test result by ANSYS modeling.

Glass has been widely used as an important component in structures such as reflection glass curtainwalls, high speed trains, and landscape glass bridges with advantages of transparent and easy to clean, which are exposed to extreme weather conditions and external loads. Over time, these factors can lead to a damage of glass. So the health status of glass structure is critical, which should be routinely monitored to improve safety and provide reliable maintenance strategy. In this paper, fiber Bragg grating (FBG) sensors are used to monitor glass damage based on the fact that the main components of both the optical fiber and the glass are silica, which hints that both optical fiber and glass have the similar mechanical properties. Furthermore, the diameter of FBG installed on the glass structure is small, which has little effect on the beauty of glass. In order to validate the feasibility of the damage monitoring method, one common glass panel model with two-side fixations is loaded impact and static loads respectively, on the upper and lower surfaces of which four FBG sensors and two resistance strain gages are installed. A comparison study among the measured strains from the FBG sensors, those from the resistance strain gages, and those calculated from finite element model (FEM) analysis is conducted and the result obtained with experiments agrees with the element result. Test results show that the FBG sensors can effectively measure the glass deformation or damage under the impact and static load.

L-shaped composites are fundamental parts of complex-shaped structures, and it is well-known that shape distortion (i.e., spring-in deformation) arises after curing due to the orthotropic nature of...

Carbon fibre-reinforced plastic (CFRP) shafts feature lightweight, high intensity, good dimensional stability and high drive efficiency. However, built-in stress formed during curing process may ca...

According to the reflection behavior of the fiber Bragg grating, the monitoring method of reinforced concrete was presented in this paper. A postprocessing method to extract crack openings from a three-dimension finite element computation was proposed. The three-axial stresses of the FBG were extracted also from the same finite element computation, and then, were converted as a wavelength deformation of the FBG reflected spectrum. The three-dimension computation result of crack width versus the FBG spectrum deformation was investigated using laboratory experiments. The results showed that the numerical values approximated the experimental ones.

In practical engineering applications, the Carbon Fiber Reinforced Polymer(CFRP)encapsulated Fiber Bragg Grating (FBG) sensor was pasted on the substrate surface by the adhesive layer to monitor the strain of the substrate. The strain of the substrate was transferred to the packaging layer through the adhesive layer, and the packaging layer transferred the strain to the FBG sensor. Performance parameters of the FBG sensor in the sensor system stability depends on the measure precision of the sensor system, different kinds of thickness of adhesive, paste, paste the length affects the performance parameters of the FBG sensor, based on the long-term cyclic loading fatigue test to study the adhesive layer to the influence of the performance parameters of the FBG sensor. By comparing the effect of 0.14mm and 0.28mm paste thickness on the performance parameters of the FBG sensor, the performance parameters of the sensor with 0.14mm paste thickness have less change. In engineering application, the small paste thickness should be selected as far as possible to ensure no degumming.

The paper made an experimental investigation on reliability of fiber Bragg grating (FBG) sensors packaged with carbon fiber reinforced polymer (CFRP) which would be used in fatigue test of wind turbine blades (WTBs). More and more FBG sensors are applied to the full-scale fatigue test of WTBs. For the size of FBG sensor is very small and easy to break, it must be packaged before used. Compared with other packaging materials, fiber reinforced polymer (FRP) is more suitable for packaging FBG sensors used in WTBs because of its good compatibility with the surface material of the blade. The performance of FRP packaged FBG sensor has been studied in the available literatures, while influences of different FRP layers and sizes on the reliability of FRP packaged FBG sensors are unknown. Thus, in this paper, CFRP is chosen as the packaging material of FBG sensors, and the influences of packaging layers and sizes on the reliability of CFRP packaged FBG sensors are studied by a series of experimental tests. The results show that the orientation angles and lengths have negligible influences on the reliability of CFRP packaged FBG sensors, the layers and sizes of CFRP must be considered when packaging FBG sensors.

A fiber Bragg grating (FBG) sensor integrated with an epoxy diaphragm was developed for the measurement of pressure and water level. The bending strain of a circular diaphragm induced by uniform pressure was transferred to the FBG sensor. The response of the FBG sensor to the pressure was observed in terms of the Bragg wavelength shift which is linearly proportional to the strain. The effect of epoxy diaphragm thickness on the sensitivity and accuracy was investigated. The experimental results show that the sensitivity of FBG/epoxy diaphragm pressure sensor is decreasing with the increase of the diaphragm thickness. The sensitivities of the FBG pressure sensors with diaphragm thicknesses of 0.5 mm, 0.7 mm, and 1.0 mm were 175.5 pm/kPa, 89.5 pm/kPa, and 43.7 pm/kPa, respectively. The pressure measured by the proposed FBG sensor was compared with theoretical prediction and a close agreement was observed.

A hydrogen and humidity simultaneous measurement system based on an integrated FBG sensor is proposed. Experimentally results indicate the sensor shows a high degree of hydrogen discrimination ranging from 100ppm-8000ppm and a high degree of humidity discrimination ranging from 20% - 80%, along with the high performance of anti-mutual interference of hydrogen and humidity.

An interrogation technique for fiber Bragg grating (FBG) sensor is devised with customized in-line edge filtering single-multi-single mode fiber (SMS) component and optical time-domain reflectometer (OTDR). The performance of the proposed SMS-OTDR interrogation is established by temperature and strain sensing analysis of a standard FBG. The temperature and strain sensitivity values which are verified by theoretical analysis are estimated as 1.01 × 10−2 dB/℃ and 3.9 × 10−4 dB/µɛ respectively. Furthermore, we have optimized and achieved controlled etching on FBG and tested it for hiking the sensitivity. Using the etched FBG, an enhanced temperature and strain sensitivity of 6.7 × 10−2 dB/℃ and 3.2 × 10−3 dB/µɛ covering the range of 20–200 °C and 100–2015 µɛ respectively are also recorded and analysed by this integration method. Eventually, the efficiency and cost-effectiveness of the proposed method are compared with various reported techniques and presented here.

We evaluate the capabilities of optical fiber sensors to measure traffic flow, speed, weight estimation and pavement degradation. The proper operation of the system has been proved 3.5 years after the installation in a metropolitan road. This work was supported by the Spanish AEI project TEC2016-76021-C2, FEDER Funds, and the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 838143 and the institute of Smart Cities (UPNA) talent grant.

Smartphone-based interrogation of FBG sensor is reported for the first time. Setup consists of a smartphone and low cost off-the shelf available components: a section of DVD disk, razor blade slit and a 3D-printed holder. Smartphone flashlight LED was used as a light source, and FBG spectra were projected on the smartphone camera by a DVD disk.

We demonstrate a self-referencing FBG sensor for damage detection. The sensor compensates for changing environmental conditions by measuring the reference and distorted signal simultaneously and uses traveling acoustic waves in optical fibers.

A small-diameter FBG sensor was embedded in 0° ply of a CFRP cross-ply laminate, and reflection spectra from the FBG sensor were measured at various tensile stresses. As a result, it was found that the small-diameter FBG sensors have a potential to detect the occurrence of transverse cracks in 90° ply through the change in the form of the spectrum, and to evaluate the crack density quantitatively by the spectrum width. On the other hand, pre-strained SMA films were embedded between laminas in CFRP laminates and heated. The effects of recovery stresses were demonstrated in the experiments and well predicted using the shear-lag analysis and the nonlinear constitutive equation of SMA films.

A fast FBG sensor interrogation method based on silicon on insulator (SOI) microring resonators is reported. The simple and effective FBG sensor interrogation scheme is based on the detection of the signal intensity at the drop port of a microring resonator, and its interrogation speed depends only on the photodetector and the front end electronics. The performance of the FBG interrogation method is evaluated in terms of minimum detectable wavelength shift and dynamic strain resolution and compared with a commercial FBG interrogator.

Recently, it has become necessary to develop a monitoring technology that combines an FBG (fiber Bragg grating) sensor as a means for continuously monitoring whether the reinforcing effect of an FRP (fiber-reinforced polymer) is maintained on FRP-reinforced structural members. However, most existing research focuses on the insertion of FBG sensors into bar-shaped FRPs, and there is a lack of studies that analyze the details for an FRP strip combined with FBG sensors. In this regard, this paper seeks to develop a reinforcement for an NSM (near-surface-mounted) retrofit in which an optical fiber with an FBG sensor is combined with FRP strips. For this, a series of experiments were performed to find the adhesive strength of optical fiber-epoxy-FRP interfaces, the tensile strength of the FBG sensor part of optical fiber with a reflection lattice, and the sensing performance depending on the adhesion length between the optical fiber and the FRP strips. As a result of the study, the adhesion length not less than 20 mm in one direction from the center, with a total adhesion length of 40 mm, needs to be secured when the optical fiber with an FBG is attached and fixed between the two FRP strips with epoxy. In addition, it is expected that the proposed model can be used to properly predict the strain transfer of an FRP strip with a fiber optic sensor and can also be utilized when determining optimum dimensions.

We demonstrated in this work a filterless, multi-point and temperature-independent FBG (fiber Bragg grating) dynamical demodulator using pulse-width-modulation (PWM). In this approach, the FBG interrogation system is composed of a tunable laser and a demodulator that is designed to detect the wavelength shift of the FBG sensor without any optical filter making it very suitable to be used in harsh environments. In this work, we applied the proposed method that uses the PWM technique for FBG sensors placed in high pressure and high-temperature environments. The proposed method was characterized in the laboratory using an FBG sensor modulated in a frequency of 6 Hz, with a 1 kHz sweeping frequency in the wavelength range from 1527 to 1534 nm. Also, the method was evaluated in a field test in an engine of a thermoelectric power plant.

The crack width of concrete structure was investigated and discussed in this paper. The paper used a post-processing method to calculate the crack width with a 3D FEM of loaded concrete slab. The FBG stresses of three directions were extracted from the 3D FEM, and then were used to calculate the spectrum distortion of FBG sensor. Experiments were carried out with a FBG sensor installed inside a concrete slab, and the experimental results of crack width of concrete slab were given also. The results demonstrated that the spectrum deformation could be applied as an effective indicator to evaluate the crack width of reinforced concrete.

An important factor for research in understanding the hand arm system is quantifying the hand grip strength (HGS). HGS is the prognosticator of various biochemical conditions like muscle strength, upper-limb strength, neuromuscular function, bone mass density etc., this paper highlights the demonstration of Fiber Bragg Grating Hand Grip Device (FBGHGD) and Electromyogram (EMG) for measuring the hand grip strength. The FBGHGD is useful in evaluating the muscle activity. It is calibrated using compression testing machine, which shows the relationship between the forces exerted on FBG based HGD and shift in Bragg wavelength. FBGHGD shows 184.067 pm shift in wavelength for a maximum applied weight. The demonstration shows the FBGHGD is more accurate and provides a real time data which is useful in accessing several vital parameters like grip force, muscle fatigue, stamina index etc., which in turn serves as a reference in understanding the functional capabilities of skeletal muscle.

Fiber Bragg grating (FBG) sensors are obviously attractive for many prominent advantages, such as strong antielectromagnetic interference ability, long transmission distance, and fast transmission speed. Due to these advantages, FBG sensors can be used for the health monitoring of high-voltage transmission lines. The FBG sensor can not only ensure the security of the transmission lines, but can realize real-time monitoring, which can effectively prevent serious accidents caused by rust breakage of overhead steel strand. In this paper, a new type of corrosion sensor based on FBG was developed to monitor and evaluate the degree of corrosion damage of the steel strand. To verify the performance of the FBG corrosion sensors, an electrochemical corrosion accelerated experiment was conducted on an actual steel strand. The experimental results show that the FBG corrosion sensors have a large measurement range of steel strand corrosion rate, and can monitor steel strand corrosion expansion damage in early stage effectively. The FBG corrosion sensor has practical application value in the field of electricity transmission engineering.

In this paper, a new damage feature, spectral area, was extracted to effectively detect crack location by studying the deformation mechanism of fiber Bragg grating (FBG) reflection spectra. In order to verify the robustness and reliability of spectral area to detect crack location, the following work was carried out: Firstly, the strain information was extracted by extended finite element method (XFEM) with fatigue crack propagation. The transmission matrix method (TMM) was used to simulate FBG reflection spectra using numerical results. Secondly, the fatigue crack growth monitoring experiment based on FBG sensors was carried out, and the digital image correlation (DIC) method was used to measure the strain values at the placement of FBG sensors with crack propagation. The temperature characteristic test of FBG was carried out to investigate the influence of temperature variation on the spectral area. The results presented that the spectral area was insensitive to temperature variation and experimental noise, and was greatly sensitive to the complex non-uniform strain field cause by crack damage. Moreover, compared with the 5 mm FBG sensor, the 10 mm FBG sensor showed a larger critical detection range for crack damage. Therefore, the spectral area can be used as a reliable damage feature to detect the crack location quantitatively based on the simulated and experimental results.

In this paper, a cladding etched fiber Bragg grating (FBG) sensor is proposed to monitor chemicals (Ca(OH)2 and Mg(OH)2) which produces in civil structures. Further, to enhance the sensitivity, an indium tin oxide is coated on cladding etched FBG sensor. It is observed that the indium tin oxide coated sensor is highly sensitive in terms of chemical detection and inducing wavelength shift of 12 nm and 10 nm with Ca(OH)2 and Mg(OH)2 respectively. On the other hand, cladding etched FBG sensor produces a wavelength shift of 8 nm and 6 nm for Ca(OH)2 and Mg(OH)2 respectively.

Due to the appearance of temperature gradient and non-uniform gelation , fully understanding of residual stresses is even more complex in thick laminates. In this paper, basic mechanisms of cure process were presented firstly. Then, experimental investigation of the non-uniform gelation was carried out together with characterization of necessary material properties. Tailed FBG sets were applied to monitor the cure of thick laminates with temperature gradient. The non-uniform gelation and chemical shrinkage gradient were captured successfully. Lastly, FEM analysis integrating thermal-chemical sub-model and stress-displacement sub-model was carried out to reveal the development of residual stresses. It is demonstrated that the high temperature layer gelled earlier and showed more chemical shrinkage. The chemical strain gradient would introduce compression internal stresses in prior gelled layer and introduces tensile internal stresses in later gelled layer. For the case studied here, transverse residual stress generated by non-uniform chemical shrinkage is small. The strategy used here is promising to estimate the residual stresses for more complex cases further.

The deployment of the spinal needle of an appropriate gauge is highly essential in order to significantly reduce the side effects of lumbar puncture procedures performed for spinal anaesthesia and spinal surgeries. The gauge of the spinal needle used for the procedure plays a significant role in determining the force required for its insertion into the tissue and is chosen by the medical practitioner performing the procedure based on the expertise. The selection of the optimum gauge of the needle is subjective to the level of discretion exercised by the surgeon. This paper shows a comparative analysis of the force required for lumbar puncture with various tissue layers traversal by four different gauges of the spinal needle, making use of a real time dynamic force monitoring device which employs a fiber Bragg grating sensor. The fiber Bragg grating (FBG) force device (FBGFD) is used to measure the force variations on different gauges of spinal needle during the lumbar puncture procedure performed first on a simulator model and subsequently on a human cadaver specimen. The FBGFD facilitates dynamic measurement of force variation on the spinal needle during the penetration by the sensing bar mechanism which is acquired by the FBG sensor bonded over it with a resolution of 0.021 N. This comparative study of force may serve as a guideline for selection of proper gauge spinal needle during tissue puncture procedures minimizing post puncture effects on patients.

Relative Humidity (RH) sensors based on Fiber Bragg Grating (FBG) technology have been fabricated by polymer functionalization, using hygroscopic material, namely agar. The sensor is specifically intended to monitor moisture in stones of cultural heritage buildings and monuments, in particular to be positioned under hydrophobic coatings used for protecting stones from absorption of water and dissolved chemicals, thus providing early warning of coating deterioration by RH monitoring. The performance of the prototype was first tested exposing the sensors in controlled environment and then assessed for the application simulating “in field” operating conditions, with sensors installed on marble and travertine tiles. Results show that the proposed sensor is able to monitor the presence of moisture inside the stones. © 2018 Elsevier Ltd

Fiber Bragg gratings (FBGs) are ideally suited for monitoring strain in civil structures, aircrafts, ocean engineering due to their low cost, reliable and high accuracy. A new encapsulation technology for FBG using polyurethane is proposed to help the sensor withstand large displacement in hash environment. This paper aims to provide experimental packaging procedures, calibration and field tests of developed FBG sensors for strain measurement. The principles, basic chemical reaction processes and prepare procedures of polyurethane are demonstrated. Suitable protective encapsulation to the bare fiber by polyurethane ensures that there is no relative slip at the interface. The encapsulated FBG sensor has good linearity from the result of calibration test conducted on a universal testing machine. Further the sensor is embedded into the freezing soil to evaluate the performance in severe environment.

This paper details a study which was carried out on the application of FBG sensors for investigation of delamination crack status in glass fibre reinforced composite materials. A woven glass (0/90) epoxy composite sample containing a purposely created delamination and an embedded FBG sensor was investigated to study the behaviour of delamination crack under applied axial quasi-static tensile load. The reflected spectra from FBG sensor and the thermo-elastic response using an infrared camera were recorded to detect the propagation of delamination crack tip. In addition a finite element analysis (FEA) was performed using cohesive elements to simulate delamination crack tip. It has been seen that the propagation of delamination crack tip monitored from thermal stress analysis (TSA) was consistent with the prediction of the FBG sensor response. Further it has been observed that the experimentally observed delamination damage propagation was in a good agreement with FEA simulation results.

One of the important application forms of Fiber Bragg grating (FBG) is to measure strain by pasting on the surface of structure. In order to improve the strength of surface-bonding FBG, packaging substrate in metal slice is designed and applied. On the basis of previous studies, an analytic model of the substrate-bonded FBG sensor is developed to predict the strain transfer rate. Moreover, the effects of substrate material and sizes on the strain transfer rate of substrate-bonded FBG are analyzed and compared. Finally, the theoretical analysis results are verified by experimental results. There is a good correspondence between the calculated values and experimental results.

In this paper, the polyimide-coated fiber Bragg grating (PI-FBG) humidity sensor suitable for use in unsaturated soils was developed. Through a series of laboratory tests, the pore gas relative humidity in different sand-kaolin mixtures were measured and analyzed to verify the feasibility of the sensor in unsaturated soils. The results shows that the pore gas relative humidity in unsaturated soils first increases rapidly and then tends to be stable with the increase of water content. The relationship between them can be fitted by quadratic function when the pore gas relative humidity is less than 100%. In addition, the critical water content ( w 100 % R H ) of mixed soils increases linearly with the increase of kaolin content; when the water contents of mixed soils are same, higher kaolin content induces higher pore gas relative humidity.

FBG based pressure sensors are becoming more important due to their vast application in various fields. This paper presents a new approach to improve the sensitivity of Fiber Bragg Grating based pressure sensor. The material chosen is a cyclic olefin copolymer named TOPAS with refractive index of the core is 1.53 and the cladding is 1.525.The sensitivity of both silica based FBG and TOPAS based FBG has been compared. Strain or temperature shifts the Bragg wavelength through compressing the grating and changing the effective index and it is observed that TOPAS FBG responds more efficiently in sensing the applied strain compared to silica FBG indicating improved sensitivity. The simulations are done using Opti Grating Tool.

To apply Fiber Bragg Grating (FBG) sensors in the hole structural health monitoring of aircraft, this study investigates a temperature compensation algorithm based on Fourier curve-fitting, combined with an analysis of FBG layout. The results identify that the wavelengths have a linear relationship with stress and temperature and the linear correlation coefficients are all greater than 0.9. In the experiment, the temperature range was 17 °C-100 °C (step is 10 °C) and the tensile force changed from 0 N to 10 KN (step is 1 KN). With the Fourier curve-fitting of the wavelength variation, the temperature compensation can be actualized, and the average error of temperature is 0.17 °C. By integrating the algorithm in the self-developed FBG interrogation system, the real-time monitoring of stress is achieved with the average error of stress is 3.9 με.

Aerospace composites are susceptible to barely visible impact damage (BVID) produced by low-velocity-impact (LVI) events. Fibre Bragg grating (FBG) sensors can detect BVID, but often FBG sensors are embedded in the mid-plan, where residual strains produced by impact damage are lower, leading to an undervaluation of the damage severity. This study compares the residual strains produced by LVI events measured by FBG embedded at the mid-plan and other through-thickness locations of carbon fibre reinforced polymer (CFRP) composites. The instrumented laminates were subjected to multiple low-velocity impacts while the FBG signals were acquired. The FBG sensor measurements allowed not only for the residual strain after damage to be measured, but also for a strain peak at the time of impact to be detected, which is an important feature to identify the nature and presence of BVID in real-life applications. The results allowed an adequate optical fibre (OF) embedding location to be selected for BVID detection. The effect of small- and large-diameter OF on the impact resistance of the CFRP was compared., This research was funded by the European Regional Development Fund, grant number NORTE-01-0145-FEDER-000015, and the APC was funded by the European Space Agency’s Network/Partnering Initiative Program, with contract number 4000123315.

A sensing module composed of a carbon fiber reinforced polymer (CFRP) packaging and an embedded fiber Bragg grating (FBG) sensor is proposed for strain monitoring of wind turbine blades. The sensing unit is designed and manufactured with the aim of maximizing the strain transfer from a CFRP wind blade to the FBG sensor. The performance of the packaged FBG sensors is experimentally investigated through exhaustive aging and fatigue tests. Then, a series of CFRP-packaged FBG sensors is used to monitor a full-scale 56.85 m-long wind blade during flapwise and edgewise fatigue tests over a few million dynamic strain cycles. Results demonstrate that the independent sensing modules provide efficient strain transfer, verifying a longer lifetime and more reliable measurements compared to conventional electrical strain gauges.