Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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Strain sensors for strain measurement:A Review

1Shivendra , 2Garima Saini1ME scholar, 2Assistant Professor

1,2 Department of Electronics, NITTTR, Chandigarh, IndiaShivendra.986@gmail.com

Abstract— Structural health monitoring (SHM) is atechnology used for the safety assurance of mechanical,aerospace, building structure and human life. By periodicalinspection using embedded sensors like piezorestive sensor,the optical fiber sensor a SHM system can provide advancedwarnings that prevent structural failures or damage. Strain isone of the most important mechanical parameters acquiredby SHM systems. The factors that could potentially causestructural failures or excessive loading, vibration, foundationdamages, crack development and environmental aging, etc.By monitoring strain changes in load-bearing structures thefailure can be avoided. Various strains sensors have beendeveloped. A detailed review of different strain sensingmechanisms can be found in this paper. The most commonlyused strain sensor is the piezoresistive thin-film strain gauge,made using semiconductor. On the other hand, Optical fiber-based strain sensors are an attractive choice of sensor forSHM systems. OFS has characteristics like small size, light inweight, remote monitoring, ability to multiplex and immuneto electromagnetic interferences.

Index Terms—SHM, OFS, Piezoresistive, Multiplex,Electromagnetic interference.

INTRODUCTION.SHM can be understood as the system that have the

capability of sensing, intelligence and possibly alsoactuation devices to allow the loading and damage-causingconditions of a structure to be stored, calculate, identified,and predicted in such a way that autonomous testingbecomes an essential part of the structure. According to thefunction and degree of complexity, SHM systems can beclassified in different levels.these levels are

To detect the damage and fatigue periodically.To estimate the effect of external load on the structure.

To estimate the remaining life.To make the structure more reliable, cost effective, long

life time.The higher the level, the higher will be the complexity andfunctionalities [1]. SHM is associated with self-working,maintenance, optimized technical structures in addition tothe minimization of the potential social, economicimpacts. With SHM systems, unusual structural behaviorlike vibration, fatigue can be detected at an early stage,decreasing the risks of sudden damage and conservingnature, goods and even human lives. In addition, thesesystems enable in-time refurbishment intervention, theextension of their life-time guaranteeing fewer directeconomic losses (repair, maintenance, and reconstruction)and also helping to avoid losses for users due to structuralfailures. Using SHM systems, hidden structural issues canbe detected early, enabling better exploitation of thematerials and components of the current structures. A keyissue in the SHM systems is the measurement of

Chemicals (pH, oxidation, corrosion, penetration, andtimber decay); Mechanical (strain, deformation,displacement, crack opening, stress, and load); andPhysical (temperature, humidity, pore pressure, etc.) [1].Several types of sensors, embedded or attached to astructure, can be used for this task, but only those based onfiber technology offer the capability to perform integrated,quasi-distributed, and distributed measurements on or evenwithin the structure, in addition to other advantages. Asmain Challenges for SHM systems, two fundamentaltechnical Challenges are identified: the development ofreliable and Sensitive techniques to detect early structuralmalfunction or unusual structural behavior and thedevelopment of data selection, storage and processingmodels, and robust algorithms to detect structuralmalfunctions. In addition, user-friendly and simpleinterfaces with the infrastructure are needed [2].A system of sensors allows the detection and

characterization of damages that could have a significanteffect on the operational capability of the structure. Ideally,this will provide warnings, prevent complete failure, andfacilitate countermeasures. SHM sensors detect variousparameters such as temperature, humidity, or strain. Thecharacterization of strain gives information on cracks,deformations, or vibrations in the structure [2]. It istherefore an essential parameter for the conclusion onOperability. However, in the course of their lifetimes,structures are subject to adverse changes in their structuralhealth conditions due to potential damage or deteriorationinduced by environment, wear, falts in design andmanufacturing, overloads and some unexpected events likeearthquakes or impacts or, simply, through their normalworking life [1].Structural degradation can be induced by a wide set offactors.1) Unsatisfactory inspection and monitoring of existinginfrastructure mean problems become apparent only whenStructures are in dire need of repair and then, repair costscan be comparable to replacement costs.2) Corrosion of conventional steel reinforcement withinConcrete can provoke expansion of steel, which leads tocracking, fragmentation or further deterioration. It leads toa reduction in strength and serviceability resulting in theneed for repair and/or replacement.3) Increased loads or design requirements over time likeheavier trucks, overload on ships, planes, etc. inducesdeterioration due to overloads or to structural inadequaciesresulting from design. Then the structures are deemedunsafe or unserviceable and strengthening or replacementis required.4) The overall deterioration and aging can inducedetrimental effects on structural performance, safety and

Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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serviceability, and then repair, rehabilitation, strengtheningor replacement may be needed.

. II. PIEZORESISTIVE STRAIN SENSORStrain is the relative change in shape or size of an object

due to externally-applied force. When external forces areapplied to objects made of elastic materials, they producechanges in shape and size of the object. In other wordstrain is defined as extension per unit length. Strain isdimensionless and has no units.

Strain = extension / original length

ɛ = …………(1)Where,ɛ = strain experienced by the structure,L = original length of the structure,ΔL = change in length of structure.

The basic function of the strain gauge is based ontransforming the strain in certain directions as to change itselectrical resistance. It allows measuring plenty ofnonelectrical quantities such as deformation, bending, force,acceleration, etc. Piezoresistivity of a material is thedependence of electrical resistivity on strain and normally isquantified by the gauge factor [3]. Strain gauges are fairlystraight forward devices that output a voltage signal basedon a change in resistance when the object to which they areattached to undergoes tension or compression. Forexample, the piezoresistive strain gauge is a semiconductordevice whose characteristic is resistance varies non linearlywith strain. The most widely used gauge, however, is thebonded metallic strain gauge [3]. Gauge factor is definedas the ratio of the fractional change in electrical resistanceto the fractional change in length (strain)GF = ∆ ⁄ɛ ……………….

(2)Where,R – Change in resistance caused by strain,Rg – Resistance of the unformed gauge,ɛ - Applied strain.Piezoresistive Sensors operate on a sensor principlewhereby an electrical resistor will change its resistancewhen it is subjected to a strain or deformation.Semiconductor strain gauge called the piezoresistor, basedon the piezoresistive effect is the commonly used straingauge [4]. Several piezoresisitive strain sensors have beendesigned to be used for various strain measurements withan excellent gauge factor. Semiconductor’s conductivitydepends on temperature change. Piezoresistive sensors arefabricated with semiconductor, so it is very sensitive totemperature changes. It is proved that by increasing thedoping level, piezoresistive sensors which are lesssensitive to temperature variation can be obtained, butgauge factor of piezoresistive strain sensor get reduced andstrain sensor required high gauge factor [4].

III. OPTICAL FIBER SENSORSAn OFS can be understood as the device in which the

measurand, introduces modifications or modulatescharacteristics of light in some part of an optical fibersystem, reproducing it faithfully in the electric domain [5].

In general terms, an OFS is usually made up of atransducer device, a communication channel and anoptoelectronic unit.A fiber optic sensor is a sensor that uses optical fiber eitheras the sensing element, or as a relaying signals from aremote sensor to the electronics that process the signals. Inan OFS is usually made up of a transducer device, acommunication channel and an optoelectronic unit. It isfound that OFS technology is attractive in those caseswhere it offers superior performance compared to the moreproven conventional sensors. Its other benefits are [5]1) Improved quality in the measurements.2) Better reliability.3) Manual readings, and automatic measurements.4) Easier installation and maintenance.

An optical fiber sensor with a linear relation betweentransmitted light intensity and applied stress. Optical fibershave the advantage of high bandwidth and free fromelectromagnetic interference, but the high cost ofimplementation has made it prohibitive for usage alongwith the robustness of the sensor system, transduction, datainterpretation, stability and reliability. In present time OFStechnology is attractive in those cases where it offerssuperior performance compared to the more provenconventional sensors offering, in addition to improvedquality in the measurements, better reliability, and thepossibility of replacing manual readings and operatorjudgment with automatic measurements, easier installationand maintenance or a lower lifetime cost [6].A fiber Bragg grating (FBG) is a sensor of distributedBragg reflector constructed in a short segment of opticalfiber that reflects particular wavelengths of light andtransmits to all remaining wavelengths. This is achieved bycreating a periodic variation in the refractive index of thefiber core, which generates a wavelength-specific dielectricmirror [7]. A fiber Bragg grating can therefore be used asan inline optical filter to block certain wavelengths, or as awavelength-specific reflector.Although fiber-optic sensors are apparently expensive for

widespread use in health monitoring however, they arebetter approaches for applications where reliability inchallenging environments is essential. When reliability is akey problem in certain critical health monitoringapplications, price is often no longer an issue [8]. Theapplication potential for OFSs in structural monitoring isvast, including civil or industrial structure monitoring likeconcrete beam tests, bridge girders, ore mines, nuclearcontainers, tunnels, and hydroelectric dams, compositematerials spacecraft, aircraft tail spars, helicopter andwindmill rotor blades, ship and submarine hulls, compositecure monitoring, and composite girders for bridges,acoustic sensing in-plant or distribution of electric powerutilities, gas pipelines, and industrial control, monitoringand processes [8].

IV. CONCLUSIONSHM sensors detect various parameters such astemperature, humidity, or strain. The characterization ofstrain gives information on cracks, deformations, orvibrations in the structure. It is therefore an essentialparameter for the conclusion on operability. Piezoresistivestrain sensors are Very sensitive to temperature changes,difficult to implement them in a commercial environmentconsidering the complexity of the network involved.

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Optical fibers based strain sensors: have the advantage ofhigh bandwidth and freedom from electromagneticinterference. For strain measurement of a buildingstructure, need to drill the building at many points, whichis called weak point, the drilling affects the quality ofbuilding.

FUTURE WORKSIn order for the installation of a permanently installedsensing system in buildings to be economically viable, thesensor modules must be wireless to reduce installationcosts, must operate with a low power consumption toreduce servicing costs of replacing batteries, and use lowcost sensors that can be a microstrip patch antenna.

ACKNOWLEDGEMENTThe authors would like to thank Dr, Umesh TiwariScientist, CSIO,Chandigarh, India for their continuous andvaluable guidelines during work.

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