Embedded Fiber Bragg Grating Research Articles

Prediction of fatigue response of composite structures by monitoring the strain energy release rate with embedded fiber Bragg gratings

Composite materials are becoming increasingly more valuable due to their high specific strength and stiffness. Currently, most components are operated for a number of service cycles and then replaced regardless of their actual condition. Embedded fiber Bragg gratings are under investigation for monitoring these components in real time and estimating their remaining life. This article presents research conducted on a novel technique for prediction of the remaining life of composites under fatigue loading using embedded fiber Bragg grating sensors. A prediction is made of the remaining life at every cycle based on data collected from the sensors and the previous loading history.

Measuring mode I cohesive law of wood bonded joints based on digital image correlation and fibre Bragg grating sensors

This work addresses the experimental identification of mode I cohesive law of wood bonded joints. The approach combines the double cantilever beam (DCB) test with both digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. The spectrum geometric mean of the FBG reflected spectral response was determined, and the wavelength evolution was used to define the fracture process zone (FPZ) development phase. This evaluation allowed a consistent selection of experimental range of over which the identification procedure of mode I cohesive law is build up. Mode I crack length, Resistance-curve and cohesive law parameters are characterised and discussed. The strain energy release rate (GI) is determined from the P–δ curve by the compliance-based beam method (CBBM). The crack tip opening displacement (wI) is determined by post-processing displacements measured by DIC. The cohesive law in mode I (σI–wI) is then obtained by numerical differentiation of the GI–wI relationship.

Embedded Fiber Bragg Grating Based Strain Sensor as Smart Costume for Vital Signal Sensing

Embedded Fiber Bragg Grating Based Strain Sensor as Smart Costume for Vital Signal Sensing

Research on enhancement sensitivity of embedded fiber Bragg grating sensor

With its unique advantages, embedded fiber Bragg grating (FBG) sensor is more and more used in the field of engineering structural health monitoring. According to the actual needs of engineering monitoring, we combined with the internal structure to make sensitivity design of the sensor. It mainly through setting the elastic modulus and the radius of sensor's inner sheath intermediate section, to make the strain of middle section larger than average strain, In order to achieve the increasing sensitivity. Through the analysis, the enhancement degree is relevant to the length, radius and elastic modulus of the middle section, so it could achieved different requirement of sensitizing by adjusting these parameters. Through the analysis of the core strain in the sensitizing structure, it can obtain the strain transfer distribution in the scope of sensor. All these could provide the theory basis for the realization of embedded FBG sensor's sensitivity enhancement.

Detection of Membrane Puncture with Haptic Feedback using a Tip-Force Sensing Needle.

This paper presents calibration and user test results of a 3-D tip-force sensing needle with haptic feedback. The needle is a modified MRI-compatible biopsy needle with embedded fiber Bragg grating (FBG) sensors for strain detection. After calibration, the needle is interrogated at 2 kHz, and dynamic forces are displayed remotely with a voice coil actuator. The needle is tested in a single-axis master/slave system, with the voice coil haptic display at the master, and the needle at the slave end. Tissue phantoms with embedded membranes were used to determine the ability of the tip-force sensors to provide real-time haptic feedback as compared to external sensors at the needle base during needle insertion via the master/slave system. Subjects were able to determine the position of the embedded membranes with significantly better accuracy using FBG tip feedback than with base feedback using a commercial force/torque sensor (p = 0.045) or with no added haptic feedback (p = 0.0024).

Strain Sensing in Fiber-Reinforced Polymer Laminates Using Embedded Fiber Bragg Grating Sensor

This article demonstrates the use of an embedded fiber Bragg grating sensor for strain measurement at different directions to the loading axis of a fiber-reinforced polymer laminate. Four 16-layer samples have been fabricated with glass cloth and epoxy resin with the fiber Bragg gratings embedded at different angles. The fiber Bragg gratings were interrogated with the help of a paired long-period grating to convert the wavelength change into an optical intensity change. It has been seen that there is a ~30% decrease of strain at 30° and a ~90% decrease at 60° to the loading direction for an applied strain in the range of 0–9,000 μstrain.

Autonomous real-time interventional scan plane control with a 3-D shape-sensing needle.

This study demonstrates real-time scan plane control dependent on three-dimensional needle bending, as measured from magnetic resonance imaging (MRI)-compatible optical strain sensors. A biopsy needle with embedded fiber Bragg grating (FBG) sensors to measure surface strains is used to estimate its full 3-D shape and control the imaging plane of an MR scanner in real-time, based on the needle's estimated profile. The needle and scanner coordinate frames are registered to each other via miniature radio-frequency (RF) tracking coils, and the scan planes autonomously track the needle as it is deflected, keeping its tip in view. A 3-D needle annotation is superimposed over MR-images presented in a 3-D environment with the scanner's frame of reference. Scan planes calculated based on the FBG sensors successfully follow the tip of the needle. Experiments using the FBG sensors and RF coils to track the needle shape and location in real-time had an average root mean square error of 4.2 mm when comparing the estimated shape to the needle profile as seen in high resolution MR images. This positional variance is less than the image artifact caused by the needle in high resolution SPGR (spoiled gradient recalled) images. Optical fiber strain sensors can estimate a needle's profile in real-time and be used for MRI scan plane control to potentially enable faster and more accurate physician response.

Embedded optical probes for simultaneous pressure and temperature measurement of materials in extreme conditions

We present recent efforts at Los Alamos National Laboratory (LANL) to develop sensors for simultaneous, in situ pressure and temperature measurements under dynamic conditions by using an all-optical fiber-based approach. While similar tests have been done previously in deflagration-to-detonation tests (DDT), where pressure and temperature were measured to 82 kbar and 400°C simultaneously, here we demonstrate the use of embedded fiber grating sensors to obtain high temporal resolution, in situ pressure measurements in inert materials. We present two experimental demonstrations of pressure measurements: (1) under precise shock loading from a gas-gun driven plate impact and (2) under high explosive driven shock in a water filled vessel. The system capitalizes on existing telecom components and fast transient digitizing recording technology. It operates as a relatively inexpensive embedded probe (single-mode 1550 nm fiber-based Bragg grating) that provides a continuous fast pressure record during shock and/or detonation. By applying well-controlled shock wave pressure profiles to these inert materials, we study the dynamic pressure response of embedded fiber Bragg gratings to extract pressure amplitude of the shock wave and compare our results with particle velocity wave profiles measured simultaneously.

Evaluation of bending strain measurements in a composite sailboat bowsprit with embedded fibre Bragg gratings

Acrylate-coated Fibre Bragg Gratings (FBGs) were embedded in a sailing boat bowsprit tubular section manufactured using the vacuum bagging process in autoclave with the aim of measuring strains under realistic loading conditions. In order to establish an effective procedure for sensor integration, flat tensile specimens with embedded sensors were firstly produced under different processing conditions, using advanced composite material employed in the marine industry. Information obtained in this way was used to manufacture a sailboat bowsprit with an array of gratings embedded between the last inner plies. In order to assess the sensitivity and accuracy of the embedded sensors, the bowsprit was subjected to bending tests and the results were compared to analytical values and to data obtained from electrical strain gages. Results are very encouraging but reveal that great attention must be paid both to the integration process and to the in situ calibration of the embedded sensors.

Up-taper-based Mach-Zehnder interferometer for temperature and strain simultaneous measurement.

A novel all-fiber sensing configuration for simultaneous measurements of temperature and strain based on the up-taper Mach-Zehnder interferometer (MZI) with an in-line embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated. This configuration consists of two up-tapers fabricated by an excessive fusion splicing method and a short segment of inscribed FBG. Due to the different responses of the up-taper MZI and the FBG to the uniform variation of temperature and strain, the simultaneous measurement for these two variables could be achieved by real-time monitoring the transmission spectrum. For 0.01nm wavelength resolution, a resolution of 0.311°C in temperature can be achieved, and the average strain resolution is 10.07με.

Post-Impact Fatigue Damage Monitoring Using Fiber Bragg Grating Sensors

It has been shown that impact damage to composite materials can be revealed by embedded Fiber Bragg Gratings (FBG) as a broadening and splitting of the latter's characteristic narrow peak reflected spectrum. The current work further subjected the impact damaged composite to cyclic loading and found that the FBG spectrum gradually submerged into a rise of background intensity as internal damages progressed. By skipping the impact, directing the impact to positions away from the FBG and examining the extracted fibers, we concluded that the above change is not a result of deterioration/damage of the sensor. It is caused solely by the damages initiated in the composite by the impact and aggravated by fatigue loading. Evolution of the grating spectrum may therefore be used to monitor qualitatively the development of the incurred damages.

Steel Stress Redistribution and Fatigue Life Estimation of Partially Prestressed Concrete Beams under Fatigue Loading

The objective of this paper is to investigate the steel stress/strain variation and redistribution in partially prestressed concrete (PPC) post-tensioned T-beams with bonded tendons under various ranges of fatigue loading by using an embedded Fiber Bragg Grating (FBG) sensor. A total number of six beams were tested to failure under fatigue loading, and two control beams were tested for determing the monotonic bearing capacity. Test results indicated that the initial fatigue failure of all beams was caused by fatigue fracture of the non-prestressed reinforcement rather than failure of the prestressing strands. Based on the strain recorded, the development trends of non-prestressed steel strain and the ratio of strain range between prestressed and non-prestressed reinforcement during fatigue loading were described. Moreover, an analytical model for fatigue life prediction of partially prestressed concrete beams by using the sectional analysis and local strain-life method was developed. Good agreement between analytical and experimental results for fatigue life of PPC beams is observed.

Study of the nonuniform behavior of temperature sensitivity in bare and embedded fiber Bragg gratings: experimental results and analysis

This paper presents an experiment and analysis on the factors affecting nonlinear evolution of Bragg wavelength with change in temperature in typical bare and embedded fiber Bragg grating-based (FBG) temperature sensors. The purpose of the study was to find the constants in the function required to evaluate temperature from Bragg wavelength shift. The temperature sensitivity of bare FBGs was found to increase with temperature elevation, and is different for FBGs written in different fiber types. The average temperature sensitivity increased by about 20% when the bare FBG temperature was elevated from 25°C to 525°C. The average temperature sensitivity of the embedded FBG sensor, investigated in the temperature range of 30°C-90°C, was a factor of 2-3 times larger than for bare FBG, depending on its fastened length with the substrate. Analytically, it is shown that the nonuniform behavior of temperature sensitivity in bare FBGs is the result of both the thermal expansion effect of the fiber and the temperature derivatives of the effective refractive index. The strain transfer and temperature coefficients of thermal expansion of the substrate affect the nonuniform behavior of temperature sensitivity in embedded FBG sensors.

Monitoring of deployment process of shape memory polymers for morphing structures with embedded fibre Bragg grating sensors

This article demonstrates the use of embedded fibre Bragg gratings as vector bending sensor to monitor two-dimensional shape deformation of a shape memory polymer plate. The shape memory polymer plate was made by using thermal-responsive epoxy-based shape memory polymer materials, and the two fibre Bragg grating sensors were orthogonally embedded, one on the top and the other on the bottom layer of the plate, in order to measure the strain distribution in both longitudinal and transverse directions separately and also with temperature reference. When the shape memory polymer plate was bent at different angles, the Bragg wavelengths of the embedded fibre Bragg gratings showed a red-shift of 50 pm/° caused by the bent-induced tensile strain on the plate surface. The finite element method was used to analyse the stress distribution for the whole shape recovery process. The strain transfer rate between the shape memory polymer and optical fibre was also calculated from the finite element method and determined by experimental results, which was around 0.25. During the experiment, the embedded fibre Bragg gratings showed very high temperature sensitivity due to the high thermal expansion coefficient of the shape memory polymer, which was around 108.24 pm/°C below the glass transition temperature ( Tg) and 47.29 pm/°C above Tg. Therefore, the orthogonal arrangement of the two fibre Bragg grating sensors could provide a temperature compensation function, as one of the fibre Bragg gratings only measures the temperature while the other is subjected to the directional deformation.

New design for temperature–strain discrimination using fiber Bragg gratings embedded in laminated composites

A new smart structure based on fiber Bragg gratings (FBGs) embedded into composite laminates for temperature and strain simultaneous measurement has been designed and experimentally tested. Two holes have been drilled at preset locations in the composite plate to create different strain sensitivities at different locations. The proposed design has been compared to three reference sensing heads also based on embedding FBGs into composite materials. Experimental results agree remarkably well with mechanical simulations and validate all the tested designs for the temperature–strain discrimination. Based on the same principle, another sensing head with a long single FBG embedded has also been designed and experimentally tested, obtaining temperature independent strain measurement.

Fiber modal interferometer with embedded fiber Bragg grating for simultaneous measurements of refractive index and temperature

A highly sensitive and compact fiber-optic sensor for simultaneous measurement of refractive index (RI) and temperature has been proposed and experimentally demonstrated. The device is based on a fiber modal interferometer assisted by an embedded fiber Bragg grating, which helps to resolve the crosstalk issue. The fiber modal interferometer consists of an Up-Fusion-Bitaper (UFBT) pair to excite high-order cladding-modes and re-couple them to the fiber core. For high order cladding mode involved in the interference, the implemented sensor yields high spectral sensitivities of −197.03±7.35nm/RIU (refractive index unit) for a refractive index (RI) range of 1.365–1.415 and 63.7±1.84pm/°C for a temperature range from 20 to 80°C, respectively. Simultaneous measurement of ambient RI and temperature has been experimentally demonstrated. The maximum RI and temperature measurement errors are ±0.00054RIU and ±0.77°C, respectively. These results demonstrate the feasibility of this configuration for reliable RI-based chemical and biochemical monitoring.

Smart Structural Health Monitoring Based on Detecting Picometer-Scale Wavelength Shift of Fiber Bragg Grating

The real-time monitoring technologies of smart civil structure based on detecting picometer-scale wavelength shift of fiber Bragg grating (FBG), including the wavelength demodulation technology of FBG, are researched extensively at home and abroad. In the paper, using the technologies of wavelength division multiplex (WDM) and time division multiplex (TDM), fiber Bragg grating (FBG) sensor network was built for monitoring smart structure health condition. Based on SOPC (System on Programmable Chip) technology and fiber comb filter, a high-speed and high-precision wavelength demodulation scheme of FBG sensor network was proposed. The optical system and hardware circuit for demodulation system were designed specifically. To improve the accuracy of demodulation system of FBG, a constant temperature channel of the demodulation system connected with a fiber comb filter, which offered reference points to calibrate the Bragg grating center wavelength. Based on 32-bit soft-core processor NoisⅡ, the embedded system collected and processed the photoelectric signal voltage transformed to rectangular voltage pulse. The upper computer displayed dynamically the FBG wavelength demodulation process and calibrated the Bragg grating center wavelength. The experiments of FBG wavelength demodulation and health monitoring of smart structural embedded fiber Bragg gratings were done. Experimental results show that, the FBG wavelength demodulation method can be used to demodulate the FBG wavelength with high speed and high precision (± 2 pm), which can be used extensively in large-scale multipoint monitor engineering, and the strains of the smart structure can be measured accurately.

FBG Strain Sensors for Creak Measurement Due to Hydration Heat in Massive Concrete

A massive concrete structure has a lot of micro-cracks due to the hydration heat. In general, conventional electric sensors have a certain limit in measuring cracks because of the noise. The durability is also badly influenced by corrosion. Recently, optical fiber sensors have demonstrated their superiority over conventional sensors and are able to solve these problems. Thus, in this work, embedded Fiber Bragg Grating (FBG) sensors for measurement of cracks in massive concrete due to hydration heat are developed and investigated. The developed sensor has the potential to perform strain measurement under harsh conditions in the presence of electrical noise and electromagnetic interference. Particularly, these FBG strain sensors are easily installed and found to be durable for massive concrete structures such as dams, bridges, nuclear structures, etc.

Strain monitoring with embedded Fiber Bragg Gratings in advanced composite structures for nautical applications

Methods to effectively and reliably embed Fiber Bragg Grating sensors in the composite material have been investigated in this work, with particular regard to a sailing boat mast manufacturing process by means of bagging technique in autoclave.Small samples were produced in order to investigate the effect of the curing process parameters on the light transmission characteristics of the embedded optical fibers. Polyimide and acrylate coated optical fibers were tested measuring the relative coefficient of attenuation by Optical Time Domain Reflectometry. In particular, carbon fiber reinforced epoxy laminas with embedded FBGs were manufactured and specimens for tensile tests extracted from the laminas. Each specimen was instrumented with FBG and conventional electrical strain gage bonded on the surface. The comparison between the FBG and SG measurements recorded during static tensile tests permitted to assess the strain monitoring capability of the FBGs and their sensitivity and accuracy.

Development of a PZT phased array and FBG network for structural health monitoring based on guided Lamb waves

The development of a structural health monitoring (SHM) strategy based on a PZT phased array system is proposed. The objective is to increase the low signal-to-noise ratio (SNR) compared to PZT networks for Lamb wave-based SHM systems. This is achieved by constructive interference—beamforming—of the different waves generated by the different transducers in the array. By carefully selecting and changing the delays in between the actuation of consecutive transducers in the array, a generated wave front can be steered to different selected directions in the plane of the plate component being scanned. By increasing the amplitude of generated waves, through beamforming, potential damage reflected waves present also an increased amplitude and higher SNR, facilitating their assessment in sensor signals and consequently damage detection. The developed automatic system was designed based on the use of the fast propagating fundamental symmetric Lamb wave mode (S0). The accuracy of the method is strongly dependant on a precise multiple actuation system and particularly in the accuracy at which the diminutive time delays are introduced in between the actuation of the different array elements. This problem was addressed by developing a dedicated multiple actuation system. The developed system merits were explored particularly and more importantly considering the application of the fast propagating and dispersive fundamental Lamb wave mode, requiring alternative actuation strategies with respect to conventional NDI systems based on non-dispersive body waves. This is done also to considerably decrease the threshold for minimum detectable damage dimensions which can be verified by conventional NDI systems and by similar state-of-the-art SHM systems, in an attempt to benefit a subsequent damage repair and/or prognosis procedure. Tests using the developed system were performed with the successful and repeatable detection of the 1 mm damages applied cumulatively into both aluminum and composite plates, subjected to different boundary conditions. Damages were simulated by surface and through thickness holes and cuts with different orientations. Finally, a network and phased array were also applied to a more complex composite panel with embedded fiber Bragg grating (FBG) optical sensors. An FBG interrogation technique, based in a tunable laser and photo-detector, was developed. The laser is tuned just before test execution and it is not changed during scans. With no moving components, this technique does not impose intrinsically a maximum sampling frequency. At the same time, the influence of temperature and operational induced strains (both static and due to low frequency vibration) are eliminated. This technique proved to be robust and of great potential for a future aircraft implementation.