Strain-Insensitive Fiber Bragg Grating Composite Structure for Wide-Range Temperature Sensing

This paper reports on the design,fabrication,and temperature strain sensing performance of a fiber Bragg grating composite structure for surface mounted temperature measurements over a wide temperature range,with highly reduced strain cross-sensitivity.The fiber Bragg grating sensor is encapsulated in a polyimide tube filled with epoxy resin,forming an arc-shaped cavity.This assembly is then placed between two layers of glass fiber prepreg with a flexible pad in between and cured into shape.Experimental results,supported by finite element simulations,demonstrate an enhanced temperature sensitivity is 26.3 pm/°C over a wide temperature range of–30°C to 70°C,and high strain transfer isolation of about 99.65%.

Measurement of Temperature and Residual Strain during Fatigue of a CFRP Composite Using Fiber Bragg Grating Sensors

Fatigue behaviour has important implications for engineering composite structures in sectors ranging from automotive to aerospace. Optical sensing technology displays excellent performance in these fields for monitoring. In this paper, temperature and residual strain during fatigue of a carbon fiber reinforced polymer（CFRP） are investigated. Four autoclaved CFRP beam specimens, with fiber Bragg grating（FBG） sensors and thermocouples embedded at selected locations, are subjected to three-point bending cyclic loading on the BOSE testing machine for fatigue testing. Thennocouples are used to measure the temperature while FBGs can sense the temperature and strain as well. Seven tests in total are conducted at different frequencies, and each test lasts for several days. From the experimental results, transient steep peaks of temperature increases （up to 2.3℃） are discovered at the beginning of the load. The following constant temperature increments are around 1.0℃, which is not relevant to frequencies from 0.1 Hz to 20 Hz and suspected due to fatigue. Residual strains of 1×10^-5-2×10^-5 during fatigue, fading away rapidly when unloading, are also reported. Embedded FBGs here are validated to sense temperature and strains in composite structures, which demonstrates promising potentials in structure monitoring fields. CFRP are verified to have an excellent performance during fatigue with low temperature increase and residual strain.

Fiber Bragg grating monitors for thermal and stress of the composite insulators in transmission lines

Running composite insulators are prone to failure due to their harsh surrounding work environment, which directly affects the safe operation of transmission lines. This paper puts forward the method of using fiber Bragg grating(FBG) as the monitors to parameters correlated with thermal and stress of the composite insulators in transmission lines at working status. Firstly, monitoring points are found out by the mechanical test on composite insulator samples. Secondly, based on the monitoring theory, this paper introduces the feasibility design frame of the composite insulator with FBG implanted in the rod and the online monitor system. At last, it describes applications of this monitor system in the field of transmission lines.

Time Division Multiplexing Fiber Bragg Grating Sensor System Using Composite Codes

A novel multi-point sensing scheme to improve the signal-to-noise ratio(SNR),temperature measurement range of the fiber Bragg grating(FBG)sensors is proposed.The new sensing scheme adopted a composite coding technique based on Simplex codes and Golay codes and time division multiplexing technique to enhance the performance of the sensing system.Simulation results,in good agreement with the theory,confirmed that the maximum SNR enhancement of the sensor system employing composite coding based on 15-bit Simplex codes and 16-bit Golay codes was approximately 6.01 dB in comparison to the conventional single pulse case.

Modification of Interfacial Performance of Fiber Bragg Grating Embedded in the Composite Materials

The interfacial performance of the Fiber Bragg grating（FGB） embedded in the composite was studied and the influence of interface modification on the final profile of the spectra of the FBG sensor was examined. A type of polyamine（Pentaethylenehexamine, PEHA） was proposed to modify the coating of PI on FBG, and the interfacial performance was evaluated by a pull-out test. Sharp improvements of the interfacial shear strength（77%） were obtained by 40 min treatment of PEHA. Compared with untreated specimen, FGB spectra of treated specimen in the tensile tests show improved linearity within the test regime, which proves that the enhanced interface is beneficial for the sensing performance.

Non-destructive Evaluation of Composite Pressure Vessel by Using FBG Sensors

In recent years, advanced composite structures are used extensively in many industries such as aerospace, aircraft, automobile, pipeline and civil engineering. Reliability and safety are crucial requirements posed by them to the advanced composite structures be- cause of their harsh working conditions. Therefore, as a very important measure, structural health monitoring （SHM） in-service is deft- nitely demanded for ensuring their safe working in-situ. In this paper, fiber Bragg grating （FBG） sensors are surface-mounted on the hoop and in the axial directions of a FRP pressure vessel to monitor the strain status during its pressurization. The experimental results show that the FBG sensors could be used to monitor the strain development and determine the ultimate failure strain of the composite pressure vessel.

Fiber Grating-Based Strain Sensor Array for Health Monitoring of Pipelines

Pipelines are one of the most important modern energy transportation methods,used especially for the transportation of certain dangerous energy media materials such as crude oil,natural gas,and chemical raw materials.New requirements have been put forward for the health monitoring and early security warning of pipelines because of the large-scale and complicated development trend of the pipe network system.To achieve an accurate assessment of the health conditions of pipeline infrastructure,obtaining as many precise operating parameters as possible,particularly at some critical parts of the pipeline,is necessary.Therefore,a novel type of fiber grating strain sensor array is proposed herein to monitor the pipeline hoop strain.The sensor utilizes fiber grating characteristics such as light weight,corrosion resistance,remote transmission,and strong environmental adaptability.The fiber containing the grating measurement points is implanted into the composite material to complete the sensitization encapsulation and protection of the bare fiber grating.The design of the sensor array fulfills the requirements for monitoring pipeline mass data,making it easy to form a pipeline health monitoring sensor network.The sensor sensitivity is researched by using a combination of theoretical and experimental analysis.A sensitivity test,as well as linearity and stability tests,are performed on the sensor.The experimental results show that the average sensitivity of the sensor is 14.86 pm/με,and the error from the theoretical calculation analysis value is 8.75%.Due to its high reliability,good linear response and long-term stability,and the ability to reflect the exact strain change of the outer wall of the pipeline,the designed sensor can support longterm online pipeline monitoring.The fiber grating sensor array network has successfully realized the monitoring of the pipeline’s internal operation by using external strain changes.In addition to the performance benefits,there are other merits associated with the applicability of the sensor namely simple structure,compact size,manufacturing ease,and exterior installation ease.

Fiber Bragg Grating Strain Rosette

In this paper a Fiber Bragg Grating (FBG) Strain Rosette is designed, developed and tested. Traditional FBGs measure strain in only one direction. However, in-plane strain at a point consists of two normal strains and one shear strain. Hence a FBG strain rosette needs to be designed. The sensing principle of FBGs as a strain and temperature sensor and fundamental principles of strain transformation and strain gage rosettes are discussed.FBG strain rosettes are fabricated and embedded in two materials namely, Silicon Gel RTV 146-9 and Glass Fiber Composite Laminates. Experiments were conducted on the FBG strain rosette structures that were embedded in Silicon Gel (RTV 146-9). Initial findings from the experiments as well as preferred embedding material are presented.

Review on Composite Structural Health Monitoring Based on Fiber Bragg Grating Sensing Principle

Fiber Bragg grating (FBG) based sensors offer important advantages over traditional instrumentation with regards to real-time structural health monitoring (SHM) of composite materials and structures in recent years. FBG sensors, integrated into existing structures or embedded into new ones, have played a major role in assessing the safety and integrity of engineering structures. In this paper, a review on the latest research of the FBG-based SHM technique for composite field is presented. Firstly, the FBG sensing principle is briefly discussed and FBG and several other optical fiber sensors (OFSs) for SHM are performance-compared. Then, several examples of the use of FBG sensors in composite SHM are illustrated, including those from the field of cure monitoring, civil engineering, aviation, aerospace, marine and offshore platform. Finally, some existing problems are pointed out and some proposals for further researches are provided.

Large Strain Detection of SRM Composite Shell Based on Fiber Bragg Grating Sensor

There may be more than 2% strain of carbon fiber composite material on solid rocket motor （SRM） in some extreme cases. A surface-bonded silica fiber Bragg grating （FBG） strain sensor coated by polymer is designed to detect the large strain of composite material. The strain transfer relation of the FBG large strain sensor is deduced, and the strain transfer mechanism is verified by finite element simulation. To calibrate the sensors, the tensile test is done by using the carbon fiber composite plate specimen attached to the designed strain sensor. The results show that the designed sensor can detect the strain more than 3%, the strain sensitivity is 0.0762pm/με, the resolution is 13.13με, and the fitting degree of the wavelength-strain curve fitting function is 0.9988. The accuracy and linearity of the sensor can meet the engineering requirements.

Use of fiber Bragg grating sensors for monitoring delamination damage propagation in glass-fiber reinforced composite structures

Embedded fiber Bragg grating（FBG）sensors have been widely used for damage monkoring of fiber composite structures for a few decades.However,many remaining engineering challenges have delayed FBG based in situ structural health monitoring（SHM）systems.One of the major problem associated with FBG based SHM system is the unavailability of reliable data processing algorithms.The present work details a study which has been undertaken for identification of delamination crack propagation in composite plate under fiber reinforced polymer（FRP）uniaxial loading.The strain measured by embedded FBG sensors closer to the crack tip was used to qualitatively and quantitatively analyze delamination damage propagation using recently proposed elasto-plastic model.Strain energy release rate was calculated and compared with the model prediction.The study has concluded that the delamination crack propagation in a FRP composite can be monitored successfully using an integral approach of FBG sensors measurements and the predictions of proposed elasto-plastic model.

A NEW METHOD TO DETERMINE THE R-CURVE OF CERAMICS-FRACTURE GRATING METHOD

The etching technique of the single-lined zero-thickness specimen grating is applied to the surface of the SiC fiber toughening Si3N4 ceramic composite specimen. The position of the crack and the crack length during the process of crack extension when the load is applied and gradually increased can be determined by recording the output voltage value of the Wheatstone bridge in which the ceramic specimen with the fracture grating on is located. The crack-growth-resistance(R-curve) of this material is thus obtained.

Ultrasensitive liquid level sensor based on slice-shaped composite long period fiber grating

In this paper,a novel liquid level sensor with ultra-high sensitivity is proposed.The proposed sensor is configured by a sliceshaped composite long period fiber grating(SSC-LPFG).The SSC-LPFG is prepared by polishing two opposite sides of a composite multimode-single-mode-multimode fiber structure using a CO;laser.The method improves the sensitivity of the sensor to external environment.Based on the simulation calculation,a liquid level sensor with a length of 3 mm is designed.The experimental transmission spectrum agrees well with the simulation result.The experimental results show that the sensitivity reaches 7080 pm/mm in the liquid level range of 0-1400 μm in water.The temperature sensitivity is24.52 pm/℃ in the range of 20℃-90℃.Due to the ultra-high sensitivity,good linearity,and compact structure,the SSC-LPFG has potential application in the field of high-Drecision liquid level measurement.

利用FBG传感器测试高温合金和陶瓷基复合材料的应变

利用光纤布拉格光栅(FBG)应变传感器测量了高温合金和陶瓷基复合材料的应变特性,并与电阻应变片的测量结果进行了对比分析.将FBG应变传感器与电阻应变片分别粘贴在高温合金和陶瓷基复合材料试件的正、反面上,利用力学试验机对2种试件施加载荷,分别利用FBG解调仪和应变测试仪对FBG应变传感器和电阻应变片的信号进行实时解调.研究得出:FBG应变传感器与电阻应变片在一定的应变范围内,测量结果符合较好.因此,FBG应变传感器可代替电阻应变片,实现对材料的应变测量.

变温域纤维增强复合材料的损伤定位研究

纤维增强复合材料因其轻质高强、易成型和可设计性强等优异性能,成为众多工程结构用材首选,然而其结构易产生多种形式损伤,降低了材料的可靠性和安全性,故对复合材料进行实时健康监测及损伤定位研究具有重要意义。声发射(Acoustic emission,AE)技术可以有效地检测复合材料中的微小损伤,基于光纤光栅声发射监测系统不受电磁干扰,可在多点同时测量多个物理量,但实现高精度损伤定位仍需进一步研究。为此,构建了一种四通道温度自适应光纤布拉格光栅(Fiber Bragg gratings,FBG)声发射监测系统,可以快速追踪、及时反馈、实时监测FBG的中心波长变化,提高定位精度,在变温域内对碳纤维增强复合材料的损伤定位进行监测,研究结果表明,该系统平均损伤定位误差为12%。

复合材料层板光纤冲击判位与载荷历程重构方法

针对典型航空航天器复合材料层板冲击载荷识别需求,提出了一种基于光纤Bragg光栅传感器的复合材料层板冲击响应特征辨识方法。基于光纤Bragg光栅传感器离散应变感知信息,发展了基于距离倒数乘方原理的复合材料层板冲击应变响应反演方法。基于冲击响应突变特征并结合极值比较法,辨识出冲击载荷对应的应变响应峰值所在位置。在辨识出冲击位置的基础上,提出采用贝叶斯正则化法对复合材料层板冲击载荷进行重构,有效解决了常规载荷历程重构中脉冲响应矩阵病态性造成的不适定性问题。实验结果显示,冲击点与辨识点坐标之间平均误差约为13.00 mm,重构所得冲击载荷有效作用历程内平均相对误差约为4.21%,载荷峰值辨识相对误差为3.85%。

基于光纤光栅监测固化工艺参数对层合板内部残余应变的影响

复合材料以其比强度、比模量高,具有较好设计性等优点,成为轻量化装备的重要材料。复合材料生产过程中产生的残余应变是制约其快速发展和应用的重要因素之一,因此研究成形过程中材料内部应变变化十分必要。本文采用内埋光纤布拉格光栅(Fiber Bragg grating,FBG)的方法,对T700/C11预浸料在不同工艺参数下成形时内部应变及温度进行实时监测,探究了工艺条件对其内部应变的影响,建立了预测层合板残余应变数值分析模型,从而优化复合材料固化工艺参数。结果表明,合理地降低升温速率、使用较低热膨胀系数和导热系数的模具材料以及避免铺放误差可以有效降低残余应变。

复合绝缘子表面异常发热点识别方法

【目的】为了避免复合绝缘子热故障造成的高压输电线路运行风险,文章研究了复合绝缘子表面异常发热点识别的方法。【方法】文章分析了电流发热型、电压发热型以及综合发热型3种复合绝缘子表面异常发热形式,基于分析结果,利用光纤光栅集中测温技术,将光纤光栅传感器布设于复合绝缘子附近,采用光纤光栅集中测温单元计算相应波长偏移量及温度变化值,将变化值经数字信号处理单元解析后,生成复合绝缘子热图像。之后,文章使用最大类间方差法分割复合绝缘子热图像,计算复合绝缘子热图像的平均灰度,获取最优阈值划分复合绝缘子热图像中的像素。根据复合绝缘子热图像分割结果,利用像素统计方法,统计复合绝缘子分割图像横向的行像素点,划分复合绝缘子结构区域。依据带电设备热图像诊断应用规范,结合光纤光栅集中测温原理,将光纤光栅传感器采集的复合绝缘子热图像以伪彩色呈现复合绝缘子表面温度分布情况。最后,通过目标图像各结构区域间的温度关系,实现复合绝缘子表面异常发热点的识别。【结果】实验结果表明,该方法能够准确测量复合绝缘子表面任意点温度,最大均方根误差仅为1.39%;能有效划分复合绝缘子分割图像的结构区域,各结构之间分界明显,不存在重合现象。【结论】使用文章所提方法识别高压输电线路上不同区域的复合绝缘子表面异常发热点的性能均较为理想,复合绝缘子结构区域划分效果较为优良。

基于小波分析和深度学习的钢-混组合梁损伤识别

为提高钢-混组合梁损伤识别的准确率,提出一种基于小波分析和深度学习的损伤识别方法。设计并制作6种不同损伤的工字钢-混凝土组合梁,通过钢球自由落体冲击组合梁表面,采用光纤光栅应变传感器获取不同损伤组合梁的应变信号;采用haar、sym2、sym4、db2和db4五种小波基函数对采集到的应变信号进行降噪处理;搭建ResNet-18、ResNet-50、ResNet-101、InceptionV3、InceptionResNetV2、MobileNetV2六个深度学习模型对降噪前、后的应变信号进行训练和预测,分析得出预测准确率最高的模型,以实现对组合梁损伤的分类和定位。结果表明:haar小波基函数降噪效果优于其它函数;ResNet-50模型预测准确率均高于其它模型,降噪前、后的预测准确率平均值分别为96.73%、97.91%,小波降噪使ResNet-50模型预测准确率提高了1.18%;远离损伤位置ResNet-50模型预测准确率平均值为96.82%,仍有较高的准确率。该方法可为钢-混组合梁的运营维护以及损伤评估提供参考。

智能型碳纤维复合材料筋材的制备及应变传感特性的研究

现有裸光纤光栅的应用方式主要为粘贴在受力筋、结构表面或者埋入混凝土,易受到混凝土振捣、泌水、离析、干缩等影响,导致其数据失常。为了对裸光纤光栅实施有效的保护,基于碳纤维复合材料筋材独特的拉挤成型工艺,在筋材的制备过程中预埋入裸光纤光栅,制备出智能型碳纤维复合材料筋材,并对其应变传感特性进行测试,探索将其应用于桥梁监测的可能性。试验结果表明:初次加载时,智能复合筋的应变传感灵敏度分别为1.05 pm/με、0.96 pm/με;重复加载后,应变传感灵敏度分别为1.12 pm/με、1.07 pm/με。初次加载与重复加载应变传感灵敏度仍然保持在1.0 pm/με左右,基本没有改变其应变传感特性。表明在碳纤维复合材料筋材中预埋入裸光纤光栅制备智能型复合筋实施监测是可行的,可将其应用于桥梁结构的监测中。