Experimental study on uplift mechanism of pipeline buried in sand using high-resolution fiber optic strain sensing nerves

Reliable assessment of uplift capacity of buried pipelines against upheaval buckling requires a valid failure mechanism and a reliable real-time monitoring technique.This paper presents a sensing solution for evaluating uplift capacity of pipelines buried in sand using fiber optic strain sensing(FOSS)nerves.Upward pipe-soil interaction(PSI)was investigated through a series of scaled tests,in which the FOSS and image analysis techniques were used to capture the failure patterns.The published prediction models were evaluated and modified according to observations in the present study as well as a database of 41 pipe loading tests assembled from the literature.Axial strain measurements of FOSS nerves horizontally installed above the pipeline were correlated with the failure behavior of the overlying soil.The test results indicate that the previous analytical models could be further improved regarding their estimations in the failure geometry and mobilization distance at the peak uplift resistance.For typical slip plane failure forms,inclined shear bands star from the pipe shoulder,instead of the springline,and have not yet reached the ground surface at the peak resistance.The vertical inclination of curved shear bands decreases with increasing uplift displacements at the post-peak periods.At large displacements,the upward movement is confined to the deeper ground,and the slip plane failure progressively changes to the flow-around.The feasibility of FOSS in pipe uplift resistance prediction was validated through the comparison with image analyses.In addition,the shear band locations can be identified using fiber optic strain measurements.Finally,the advantages and limits of the FOSS system are discussed in terms of different levels in upward PSI assessment,including failure identification,location,and quantification.

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.

In-Fiber Mach-Zehnder Interferometer Based on Waist-Enlarged Taper and Core-Mismatching for Strain Sensing

An in-fiber Mach-Zehnder interferometer for strain measurement is proposed and experimentally demonstrated. The sensor consists of a taper followed by a short section of a multi-mode fiber （MMF） and a dispersion com- pensating fiber （DCF）, which is sandwiched between two single mode fibers （SMFs）. The taper is used as a fiber coupler to excite cladding modes in the SMF, and these cladding modes transmit within the MMF and the DCF. The core mode and the cladding modes interfere in the DCF SMF fusion point to form intermodal interference. A well-defined interference spectrum is obtained in the experiment. Selected interference dips are used to measure the strain changes. The experimental results show that this device is sensitive to strain with the wavelength-referenced sensitivity of 2.6 pm/με and the power-referenced sensitivity of 0. 0027 dB/με, respectively.

Integrated wellbore-reservoir-geomechanics modeling for enhanced interpretation of distributed fiber-optic strain sensing data in hydraulicfracture analysis

Fiber-optic distributed strain sensing(FO-DSS)has been successful in monitoring strain changes along horizontal wellbores in hydraulically fractured reservoirs.However,the mechanism driving the various FO-DSS responses associated with near-wellbore hydraulic fracture properties is still unclear.To address this knowledge gap,we use coupled wellbore-reservoir-geomechanics simulations to study measured strain-change behavior and infer hydraulic fracture characteristics.The crossflow among fractures is captured through explicit modeling of the transient wellbore flow.In addition,local grid refinement is applied to accurately capture strain changes along the fiber.A Base Case model was designed with four fractures of varying properties,simulating strain change signals when the production well is shut-in for 10 d after 240 d of production and reopened for 2 d.Strain-pressure plots for different fracture clusters were used to gain insights into inferring fracture properties using DSS data.When comparing the model with and without the wellbore,distinct strain change signals were observed,emphasizing the importance of incorporating the wellbore in FO-DSS modeling.The effects of fracture spacing and matrix permeability on strain change signals were thoroughly investigated.The results of our numerical study can improve the understanding of the relation between DSS signals and fracture hydraulic properties,thus maximizing the value of the dataset for fracture diagnostics and characterization.

Packaging and Temperature Compensation of Fiber Bragg Grating for Strain Sensing： A Survey

During last decades, sensor elements based on the fiber Bragg grating （FBG） have been widely studied and developed due to the advantages of immunity to electromagnetic interference, compact size, high precision, and so on. The FBG itself is sensitive to axial strain and temperature variation directly and can indirectly measure these complex physical parameters, such as pressure, displacement, and vibration, by using some specially designed elastic structures to convert them into the axial strain of the FBG. Whether the FBG is fixed on the measured object to measure the strain directly or fixed on an elastic structure body to measure other physical quantities, these types of FBGs could be collectively called as strain sensing FBGs. The packaging of the FBG has important influence on FBG characteristics that directly affect the measurement accuracy, such as strain transfer, temperature characteristic, and spectral shape. This paper summarizes the packaging methods and corresponding temperature compensation methods of the currently reported strain sensing FBGs, focusing especially on fully pasted FBG, pre-stretched FBG with double-end fixed, and metallic packaging. Furthermore, the advantages and drawbacks of different packaging methods have been analyzed, which can provide a reference for future researches.

Self-healing carrageenan-driven polyacrylamide hydrogels for strain sensing

Conductive hydrogels have attached considerable attention due to their good stretchability,excellent conductivity when they are applied in soft electronics. However,to fabricate a flexible hydrogel sensor with excellent toughness and good self-healing properties remains a challenge. In this work,we assembled a dual physical-crosslinking(DPC) ionic conductive polyacrylamide/carrageenan double-network(DN) hydrogel. This hydrogel has excellent fracture tensile stress and toughness,and demonstrates rapid self-recovery and self-healing ability due to the unique dual physical-crosslinking structures. Besides,the hydrogel is highly conductive by adding some conductive ions. As a result,the hydrogel-based sensor can stably detect human motions and physiological signals. The work provides novel ideas for the development of flexible sensing devices.

Breathable Kirigami‑Shaped Ionotronic e‑Textile with Touch/Strain Sensing for Friendly Epidermal Electronics

Flexible ionotronic devices have great potential to revolutionize epidermal electronics.However,the lack of breathability in most ionotronic devices is a significance barrier to practical application.Herein,a breathable kirigami-shaped ionotronic e-textile with two functions of sensing(touch and strain)is designed,by integrating silk fabric and kirigami-shaped ionic hydrogel.The kirigami-shaped ionic hydrogel,combined with fluffy silk fabric,allows the ionotronic e-textile to achieve excellent breathability and comfortability.Furthermore,the fabricated ionotronic e-textile can precisely perform the function of touch sensing and strain perception.For touch-sensing,the ionotronic e-textile can detect the position of finger touching point with a fast response time(3 ms)based on the interruption of the ion field.For strain sensing,large workable strain range(>100%),inconspicuous drift(<0.78%)and long-term stability(>10,000 cycles)is demonstrated.On the proof of concept,a fabric keyboard and game controlling sleeve have been designed to display touch and strain sensing functions.The ionotronic e-textile break through the bottlenecks of traditional wearable ionotronic devices,suggesting a great promising application in future wearable epidermal electronics.

An in situ monitoring campaign of a hard rock pillar at great depth within a Canadian mine

A recent research campaign at a Canadian nickel-copper mine involved instrumenting a hard rock sill drift pillar with an array of multi-point rod extensometers,distributed optical fibre strain sensors,and borehole pressure cells(BHPCs).The instrumentation spanned across a 15.24 m lengthwise segment of the relatively massive granitic pillar situated at a depth of 2.44 km within the mine.Between May 2016 and March 2017,the pillar’s displacement and pressure response were measured and correlated with mining activities on the same level as the pillar,including:(1)mine-by of the pillar,(2)footwall drift development,and(3)ore body stoping operations.Regarding displacements of the pillar,the extensometers provided high temporal resolution(logged hourly)and the optical fibre strain sensors provide high spatial resolution(measured every 0.65 mm along the length of each sensor).The combination of sensing techniques allowed centimetre-scale rock mass bulking near the pillar sidewalls to be distinguished from microstrain-scale fracturing towards the core of the pillar.Additionally,the influence and extent of a mine-scale schistose shear zone transecting the pillar was identified.By converting measured rock mass displacement to velocity,a process was demonstrated which allowed mining activities inducing displacements to be categorised by time-duration and cumulative displacement.In over half of the analysed mining activities,displacements were determined to prolong for over an hour,predominately resulting in submillimetre cumulative displacements,but in some cases multi-centimetre cumulative displacements were observed.This time-dependent behaviour was more pronounced within the vicinity of the plumb shear zone.Displacement measurements were also used to assess selected support member load and elongation mobilisation per mining activity.It was found that a combined static load and elongation capacity of reinforcing members was essential to maintaining excavation stability,while permitting gradual shedding of stress through controlled pillar sidewall displacements.

Field monitoring of differential frost heave in widened highway subgrade

In cold regions,the widened subgrade could produce uneven frost heave that is detrimental to the pavement.This study investigates the differential frost heave characteristics in a widened subgrade.The field monitoring system mainly consists of temperature,moisture,and displacement sensors and distributed optical fiber cables for strain measurement.The monitoring results show that the cooling period in the subgrade is longer than the warming period.Water content in the subgrade changes significantly within 0−2 m below the subgrade surface but stabilizes within 2−5 m.The maximum frost heave occurs from February to March.In comparison,the existing subgrade has a longer freezing period and larger heave value,caused by the higher density and water content inside.Water in the existing subgrade migrates into the new one after widening,leading to frost heave reduction in the existing subgrade.Simultaneously,the traffic loads result in the consolidation of the new subgrade,thus reducing the heave value in the second year.In the third year,the water supply from the existing subgrade facilitates the frost heave in the new subgrade.The tensile strain distributions obtained by the distributed optical fiber cables show that the maximum differential frost heave occurs at the joint between the existing and new subgrades.The differential frost heave gradually stabilizes after three years.Finally,an improved frost heave prediction model is developed based on the segregation potential concept and monitoring results.

Use of carbon nanotubes for strain and damage sensing of epoxy-based composites

The interest in structural health monitoring of carbon fiber-reinforced polymers using electrical methods to detect damage in structures is growing because once the material is fabricated the evaluation of strain and damage is simple and feasible.In order to obtain the conductivity,the polymer matrix must be conductive and the use of nanoreinforcement seems to be the most feasible method.In this work,the behavior of nanoreinforced polymer with carbon nanotubes(CNTs)and composites with glass and carbon fibers with nanoreinforced matrices was investigated.These composites were evaluated in tensile tests by simultaneously measuring stress,strain and resistivity.During elastic deformation,a linear increase in resistance was observed and during fracture of the composite fibers,stronger and discontinuous changes in the resistivity were observed.Among other factors,the percentage of nanotubes incorporated in the matrix turned out to be an important factor in the sensitivity of the method.

Tensile and Electro-Mechanical Properties of Carbon Nanotube Film Twisted Yarn with Adjustable Diameter

Carbon nanotube(CNT)yarns with adjustable diameters were manufactured by twisting CNT films with varied twists.Different from traditional CNT fibers,CNT yarns exhibited a larger diameter(423μm)and a higher tensile force(1988 cN).The results showed that CNT yarns with the twist angle of 35°exhibited the highest conductivity(886 S/cm)and the highest tensile strain(35%).

Design of an All-Fiber Fabry-Perot Sensor for Strain Measurement in Radiative Environment

A length-matched micro Fabry-Perot(FP)interferometer is proposed for strain measurement under irradiation environment.Theoretical simulation shows that a well length-matched FP sensor can achieve a very low drift of the cavity length and strain sensitivity in irradiation environment.In experiment,such an FP cavity is realized by laser micromachining.It shows a low cavity length drift of−0.037μm and a strain sensitivity deviation of 0.52%,respectively,under gamma irradiation.Meanwhile,the intensity of interference fringes is also stable.As a result,such a length-matched FP cavity is a very promising candidate for strain sensing in radiative environments.

Facile and Large-scale Fabrication of Self-crimping Elastic Fibers for Large Strain Sensors

Stretchable conductive fibers offer unparalleled advantages in the development of wearable strain sensors for smart textiles due to their excellent flexibility and weaveability.However,the practical applications of these fibers in wearable devices are hindered by either contradictory properties of conductive fibers(high stretchability versus high sensing stability),or lack of manufacturing scalability.Herein,we present a facile approach for highly stretchable self-crimping fiber strain sensors based on a polyether-ester(TPEE)elastomer matrix using a side-by-side bicomponent melt-spinning process involving two parallel but attached components with different shrinkage properties.The TPEE component serves as a highly elastic mechanical support layer within the bicomponent fibers,while the conductive component(E-TPEE)of carbon black(CB),multiwalled carbon nanotubes(MWCNTs)and TPEE works as a strain-sensitive layer.In addition to the intrinsic elasticity of the matrix,theTPEE/E-TPEE bicomponent fibers present an excellent form of elasticity due to self-crimping.The self-crimping elongation of the fibers can provide a large deformation,and after the crimp disappears,the intrinsic elastic deformation is responsible for monitoring the strain sensing.The reliable strain sensing range of theTPEE/E-TPEE composite fibers was 160%-270%and could be regulated by adjusting the crimp structure.More importantly,the TPEE/E-TPEE fibers had a diameter of 30-40 pm and tenacity of 40-50 MPa,showing the necessary practicality.This work introduces new possibilities for fiber strain sensors produced in standard industrial spinning machines.

Fiber optic sensing technologies potentially applicable for hypersonic wind tunnel harsh environments

Advanced sensing techniques are in big demand for applications in hypersonic wind tunnel harsh environments,such as aero(thermo)dynamics measurements,thermal protection of aircraft structures,air-breathing propulsion,light-weighted and highstrength materials,etc.In comparison with traditional electromechanical or electronic sensors,the fiber optic sensors have relatively high potential to work in hypersonic wind tunnel,due to the capability of responding to a wide variety of parameters,high resolution,miniature size,high resistant to electromagnetic and radio frequency interferences,and multiplexing,and so on.This article has classified and summarized the research status and the representative achievement on the fiber optic sensing technologies,giving special attention to the summary of research status on the popular Fabry-Perot interferometric,fiber Bragg gratings and(quasi)distributed fiber optic sensors working in hypersonic wind tunnel environment,and discussed the current problems in special optical fiber sensing technologies.This article would be regarded as reference for the researchers in hypersonic wind tunnel experiment field.

Research Progress in the Key Device and Technology for Fiber Optic Sensor Network

The recent research progress in the key device and technology of the fiber optic sensor network （FOSN） is introduced in this paper. An architecture of the sensor optical passive network （SPON）, by employing hybrid wavelength division multiplexing/time division multiplexing （WDM/TDM） techniques similar to the fiber communication passive optical network （PON）, is proposed. The network topology scheme of a hybrid TDM/WDM/FDM （frequency division multiplexing） three-dimension fiber optic sensing system for achieving ultra-large capacity, long distance, and high resolution sensing performance is performed and analyzed. As the most important device of the FOSN, several kinds of light source are developed, including the wideband multi-wavelength fiber laser operating at C band, switchable and tunable 2 μm multi-wavelength fiber lasers, ultra-fast mode-locked fiber laser, as well as the optical wideband chaos source, which have very good application prospects in the FOSN. Meanwhile, intelligent management techniques for the FOSN including wideband spectrum demodulation of the sensing signals and real-time fault monitoring of fiber links are presented. Moreover, several typical applications of the FOSN are also discussed, such as the fiber optic gas sensing network, fiber optic acoustic sensing network, and strain/dynamic strain sensing network.

Soft curvature sensors for measuring the rotational angles of mechanical fingers

The design, fabrication, and testing of soft sensors that measure elastomer curvature and mechanical finger bending are described in this study. The base of the soft sensors is polydimethylsiloxane (PDMS), which is a translucent elastomer. The main body of the soft sensors consists of three layers of silicone rubber plate, and the sensing element is a microchannel filled with gallium-indium-tin (Ga-In-Sn) alloy, which is embedded in the elastomer. First, the working principle of soft sensors is investigated, and their structure is designed. Second, the relationship between curvature and resistance is determined. Third, several sensors with different specifications are built in accordance with the structural design. Experiments show that the sensors exhibit high accuracy when the curvature changes within a certain range. Lastly, the soft sensors are applied to the measurement of mechanical finger bending. Experiments show that soft curvature sensors can effectively reflect mechanical finger bending and can be used to measure the bending of mechanical fingers with high sensitivity within a certain working range.

Ultrahigh-speed distributed Brillouin reflectometry

Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements.However,real-time distributed strain measurement has been achieved only for two-end-access systems;such systems reduce the degree of freedom in embedding the sensors into structures,and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre.Here,we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique.In this method,the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator,and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform;the phase delay is subsequently converted into a voltage,which can be directly measured.When a single-point measurement is performed at an arbitrary position,a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz.When distributed measurements are performed at 100 points with 10 times averaging,a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre.Some drawbacks of this ultrahigh-speed configuration,including the reduced measurement accuracy,lowered spatial resolution and limited strain dynamic range,are also discussed.