Evolution mechanism of optical fiber strain induced by multi-fracture growth during fracturing in horizontal wells

A forward model for optical fiber strain was established based on a planar 3D multi-fracture model. Then the forward method calculating distributed fiber strain induced by multi-fracture growth was proposed. Based on this method, fiber strain evolution during fracturing of the horizontal well was numerically simulated. Fiber strain evolution induced by fracture growth can be divided into three stages: strain increasing, shrinkage convergence, and straight-line convergence, whereas the evolution of fiber strain rate has four stages: strain rate increasing, shrinkage convergence, straight-line convergence, and strain rate reversal after pumping stops. Fiber strain does not flip after pumping stop, while the strain rate flips after pumping stop so that strain rate can reflect injection dynamics. The time when the fracture extends to the fiber and inter-well pressure channeling can be identified by the straight-line convergence band of distributed fiber strain or strain rate, and the non-uniform growth of multiple fractures can be evaluated by using the instants of fractures reaching the fiber monitoring well.When the horizontal section of the fiber monitoring well is within the height range of a hydraulic fracture, the instant of the fracture reaching the fiber can be identified;otherwise, the converging band is not apparent. In multi-stage fracturing, under the influence of stress shadow from previous fracturing stages, the tensile region of fiber strain may not appear, but the fiber strain rate can effectively show the fracture growth behavior in each stage. The evolution law of fiber strain rate in single-stage fracturing can be applied to multi-stage fracturing.

High sensitive fiber strain sensor using suspended-core fibers

A novel fiber strain sensor is proposed, based on the two-mode interference of a suspended-core fiber. A fullvectorial finite difference mode solver is employed as the numerical tool for characterizing the proposed strain sensor. The numerical results show that the proposed strain sensor has an estimated sensitivity of 0.05 rad/（m-με）, higher than that of the strain sensors using conventional multimode fibers, while the temperature sensitivity of the proposed sensor is relatively low.

DETERMINATION OF INTERNAL STRAIN IN 3-D BRAIDED COMPOSITES USING OPTIC FIBER STRAIN SENSORS

A reliable understanding of the properties of 3-D braided composites is of primary importance for proper utilization of these materials. A new method is introduced to study the mechanical performance of braided composite materials using embedded optic fiber sensors. Experimental research is performed to devise a method of incorporating optic fibers into a 3-D braided composite structure. The efficacy of this new testing method is evaluated on two counts. First, the optical performance of optic fibers is studied before and after incorporated into 3-D braided composites, as well as after completion of the manufacturing process for 3-D braided composites, to validate the ability of the optic fiber to survive the manufacturing process. On the other hand, the influence of incorporated optic fiber on the original braided composite is also researched by tension and compression experiments. Second, two kinds of optic fiber sensors are co-embedded into 3-D braided composites to evaluate their respective ability to measure the internal strain. Experimental results show that multiple optic fiber sensors can be co-braided into 3-D braided composites to determine their internal strain which is difficult to be fulfilled by other current existing methods.

Deformation and failure characteristics of sandstone under uniaxial compression using distributed fiber optic strain sensing

This paper investigates the deformation and fracture propagation of sandstone specimen under uniaxial compression using the distributed fiber optic strain sensing(DFOSS)technology.It shows that the DFOSS-based circumferential strains are in agreement with the data monitored with the traditional strain gage.The DFOSS successfully scans the full-field view of axial and circumferential strains on the specimen surface.The spatiotemporal strain measurement based on DFOSS manifests crack closure and elastoplastic deformation,detects initialization of microcrack nucleation,and identifies strain localization within the specimen.The DFOSS well observes the effects of rock heterogeneity on rock deformation.The advantage of DFOSS-based strain acquisition includes the high spatiotemporal resolution of signals and the ability of full-surface strain scanning.The introduction to the DFOSS technology yields a better understanding of the rock damage process under uniaxial compression.

Strain measurement of fiber optic sensor surface bonding on host material

Fiber optic sensor has been widely used as a structural health monitoring device by either embedding into or surface bonding onto the structures. The strain of optic fiber induced by the host material is strongly dependent on the bonding characteristics which include the protective coating, adhesive layer and the length of bonding. The strains between the fiber optics and host structure are not exact the same. The existence of the protective coating and adhesive layer would affect the strain measured by the surface bonding optic sensor. The analytical expression of the strain in the optic fiber induced by the host material was presented. The results were validated by the finite element method. The theoretical predictions reveal that the strain in the optical fiber is lower than the strain of host material. Parametric study shows that a long bonding length and high modulus of protective coating would increase the percentage of strain transferring into the optical fiber. Experiments were conducted by using Mach-Zehnder interferometer to measure the strain of the surface bonding optic fiber induced by the host structure. Good agreements were observed in comparison with the experimental results and theoretical predictions.

STRAIN REGULARITY IN REINFORCERS OF SHORT-FIBER/ WHISKER REINFORCED COMPOSITE AND ITS APPLICATION

Based on the study of strain distribution in short-fiber/whisker reinforced metal matrix composites, a deformation characteristic parameter λ is defined as the ratio of the root-mean-square strain of reinforcers to the macro-linear strain along the same direction. Quantitative relation between λ and microstructure parameters of the composite is obtained. As an example of applying and verifying λ, the stress-strain curve of ［AlBO］w/Al composite under tensile loading is predicted and favorably compared with experiments. By using λ, the stiffness modulus of the composite with arbitrary reinforcer orientation under any loading condition is predicted from the microstructure parameters of material.

Irradiation effect on strain sensitivity coefficient of strain sensing fiber Bragg gratings

The effect of irradiation on the strain sensitivity coefficient of strain sensing fiber Bragg gratings （FBGs） has been investigated through experiments. FBGs were fabricated in single mode fibers with 3 tool% Ge-concentration in the core and with a H2-1oading treatment. In experiments, the FBGs were subjected to y-radiation exposures using a Co6~ source at a dose-rate of 25 Gy/min up to a total dose of 10.5 kGy. The GeO defect in fiber absorbs photons to form a GeE＇ defect; the interaction with H2 is a probable reason for the y-radiation sensitivity of gratings written in hydrogen loaded fibres, The effect mechanism of radiation on the strain sensitivity coefficient is similar to that of radiation on the temperature sensitivity coefficient. Radiation affects the effective index neff, which results in the change of the thermo-optic coefficient and the strain-optic coefficient. Irradiation can change the strain sensitivity coefficient of FBGs by 1.48%-2.71%, as well as changing the Bragg wavelength shift （BWS） by 22 pm-25 pm under a total dose of 10.5 kGy. Our research demonstrates that the effect of irradiation on the strain sensitivity coefficient of FBG is small and that strain sensing FBGs can work well in the radiation environment.

Tensile Properties of Fiber Materials under Different Strain Rates

The quasi-static and dynamic tensile tests of aramid and high strength PVA fiber bundles are carried out under a wider range of strain rate by use of MTS (Materials Testing System) and bar-bar tensile impact apparatus. The influences of strain rate on mechanical properties of aramid and high strength polyvinyl alcohol fibers are studied. Micro failure mechanisms of fibers at different strain rates are examined by means of SEM.

Complete splitting process of steel fiber reinforced concrete at intermediate strain rate

The complete splitting process of steel fiber reinforced concrete(SFRC)at intermediate strain rate was studied by experiment.The basic information of a self-developed SFRC dynamic test system matching with Instron 1342 materials testing machine was given,and the experiment principle and the loading mode of cubic split specimen were introduced.During the experiment,30 cubes of 150 mm×150 mm×150 mm and 36 cubes of 100 mm×100 mm×100 mm,designed and prepared according to C20 class SFRC with different volume fractions of steel fiber(0,1%,2%,3%,4%)were tested and analyzed.At the same time,the size effect of SFRC at intermediate strain rate was investigated.The experimental study indicates that SFRC size effect is not influenced by the loading speed or strain rate.When the steel fiber content increases from 0 to 4%,the splitting strength of SFRC increases from 100%to 261%,i.e.increasing by 161%compared with that of the common concrete.The loading rate increases from 1.33 kN/s to 80.00 kN/s,and the splitting tensile strength increases by 43.55%.

A Dnal-Mode Bow Tie Optical Fiber Sensor for Measurement of Axial Strain

An optical fiber strain sensor utilizing the mode-mode interference of LP01x,andLR11x,modes in bow tie optical fibers is described.The heterodyne interference caused byfrequency modulation of laser diode（LD） with ramp injection current is used torealize the detection of linear phase.Experimental results show a resolution of about 2%of one fringe corresponding to a strain of 2.9με.m.

Complete Phase-strain Model for Interferometric Optical Fiber Sensor

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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 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.

Structure Damage Identification via Fiber Grating Strain Sensing Array Detecting and Wavelet Analysis

The measuring method of structure damage during vibrating has been developed by applying simple supported beam as object of study, fiber Bragg grating strain sensing array as the measuring method, and wavelet package analysis as signal extracting tools. The damage data of simple supported beam at vibrating state has been collected. The damage characteristic indexes have been extracted based on analyzing and handling the damage data with wavelet analysis. The experiment shows that fiber Bragg grating strain sensing array can sensitively measure the experimental data of simple supported beam at vibrating state. The fiber Bragg grating strain sensing array measuring is a new method in dynamic measurement.

Fiber Bragg Grating Strain Sensing Detecting Multi-Crack Damages under Vibrating Status

The multi-crack damages modal of simple supported beam has been build, at the vibrating status, the multi-damage detecting method of simple supported beam measured by fiber Bragg grating strain sensing array has been studied. From 0 hz to 200 hz, using exciter vibrating simple supported beam, with different damages, resonant frequency of simple supported beam has changed. So, when the damage appears in simple supported beam, the local rigidity will decrease, the resonant frequency of simple supported beam will be affected, the damage status of simple supported beam have been determined by this. The experimental result indicates that the resonant frequency of simple supported beam has changed when there is no damage, one damage, two damages, three damages on simple supported beam. According to this, the fiber Bragg grating strain sensing array can detect multi-crack damage of simple supported beam under vibrating status.

Charactersitics of Stress-strain Curve of High Strength Steel Fiber Reinforced Concrete under Uniaxial Tension

A whole of 110 specimens divided into 22 groups were tested with varying the volume fraction of steel fibers and the matrix strength of these specimens. The stress-strain behaviors of four types of steel fiber reinforced concrete （SFRC） under uniaxial tension were studied experimentally. When the matrix strength and the fiber content increase, the tensile stress and tensile strain vary differently according to the fiber type. The mechanisms of reinforcing effect for different types of fiber were analyzed and the stress-strain curves of the specimens were plotted. Some experimental factors for stress or strain of SFRC were given. A tensile toughness modulus Re0.5 was introduced to evaluate the toughness characters of SFRC under uniaxial tension. Moreover, the formula of the tensile stress-strain curve of SFRC was regressed. The theoretical curve and the experimental ones fit well, which can be used for references in construction.

基于纤维解离水曲柳压缩解离特征与能量耗散机制研究

目的以水曲柳为研究对象,研究高应变率压缩载荷作用下水曲柳试件的解离特征和能量耗散机制。方法利用压缩加载试验分析应变率、加载方向对受载水曲柳的形态特征影响和动力学特性,并利用弹塑性基本原理分析其受压解离的能量耗散机制。结果解离后径向加载试件主要呈火柴棍状,弦向加载试件主要呈片状,轴向加载试件主要呈不规则块状,试件的解离程度随应变率的增大而增大;当应变率在400~1000 s^(-1)时,水曲柳试件的应力-应变曲线由弹性阶段和屈服后弱线性强化阶段两部分组成;水曲柳试件的屈服强度随应变率的增大而增大,当应变率从400s^(-1)增加到1000s^(-1)时,径向、弦向和轴向加载试件的屈服强度分别增加了0.45倍、1.34倍和0.71倍;木材原料沿径、弦向解离时主要依靠木材细胞的压缩变形来耗散能量,沿轴向解离时主要依靠木材细胞纵向结构的弯曲来耗散能量。结论弦向最易解离,轴向最难解离;水曲柳是一种应变率敏感材料;木材原料径、弦向解离主要依靠压缩变形来耗散能量,轴向解离主要依靠弯曲变形来耗散能量,木材原料解离能够耗散能量的多少主要受加载方向、木材细胞的结构尺寸和力学性能的影响。

砂土压缩变形传感光缆耦合试验分析与预测模型研究

灾害监测、预测是地质与岩土工程领域一项极为重要的任务。分布式光纤传感技术凭借其连续、实时、抗干扰、稳定性好等优点在长距离、长周期、高隐蔽性、强突发性的灾害监测中发挥了显著的作用。然而,针对不同工程问题,评价与解决传感光缆与被测岩土体协调变形问题,是精细化分析岩土体变形分布与研究演化规律的关键所在。利用自主研制的三维主动变形可控围压光缆-砂土耦合试验装置,探究了0~4.0MPa围压条件下金属基索传感光缆与粗砂介质压缩变形的协调性问题。试验结果表明:低围压条件下金属基索传感光缆与砂土压缩变形协调性较差,变形表现为非线性变化特征;随着围压增大,传感光缆与砂土耦合效果提高,变形呈现线性变化特征,当围压达到1.6MPa后,两者协调变形一致性明显提高,其形变由非线性向线性转变。基于上述变形特性,提出了一种围压曲面投影数值模型,采用非扭结边界条件的样条插值算法对非试验数据实际位移进行预测,结果表明该模型具有较好的可靠性。试验研究认为金属基索光缆用于高围压条件下砂土变形测试具有更高的准确性,研究内容可以为分布式光纤传感测试技术用于深部地层变形监测提供科学的参考依据。

不同钢纤维类型混凝土动静态力学特性试验研究及应用

以云南某深部竖井钢纤维混凝土衬砌支护为背景,利用深部矿山现场使用的水泥、钢纤维及掘进工作面废矸石作为原材料,制作钢纤维混凝土试样,探究了不同钢纤维类型混凝土的动静态力学性能。通过单轴压缩、劈裂拉伸和动态压缩试验获得了钢纤维混凝土动静态力学性能及其破坏特征,结合高速摄像机和数字图像相关技术(DIC),探讨了钢纤维混凝土试样的裂纹演化规律。研究结果表明:双掺、三掺钢纤维混凝土的抗压强度均大于单掺钢纤维混凝土,三掺钢纤维在增强混凝土抗拉强度方面效果最佳,混凝土动态抗压强度随着掺杂钢纤维类型增加而增加;与素混凝土相比,钢纤维混凝土试样表面裂纹出现较晚;掺杂的钢纤维类型越多,试样峰值应变越大;素混凝土表现出脆性破坏特征,钢纤维混凝土则表现出延性破坏特征;将三掺钢纤维混凝土应用于矿山竖井衬砌支护,有效提高了竖井井壁的完整性和稳定性,为深部矿山工程安全高效开采提供了保障。