Strain sensors for strain measurement: A Review

Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

Strain sensors for strain measurement: A Review 1

Shivendra , 2Garima Saini ME scholar, 2Assistant Professor

1 1,2

Department of Electronics, NITTTR, Chandigarh, India Shivendra.986@gmail.com

Abstract— Structural health monitoring (SHM) is a technology used for the safety assurance of mechanical, aerospace, building structure and human life. By periodical inspection using embedded sensors like piezorestive sensor, the optical fiber sensor a SHM system can provide advanced warnings that prevent structural failures or damage. Strain is one of the most important mechanical parameters acquired

by SHM systems. The factors that could potentially cause structural failures or excessive loading, vibration, foundation damages, crack development and environmental aging, etc.

By monitoring strain changes in load-bearing structures the failure can be avoided. Various strains sensors have been developed. A detailed review of different strain sensing mechanisms can be found in this paper. The most commonly used strain sensor is the piezoresistive thin-film strain gauge, made using semiconductor. On the other hand, Optical fiberbased strain sensors are an attractive choice of sensor for SHM systems. OFS has characteristics like small size, light in weight, remote monitoring, ability to multiplex and immune to electromagnetic interferences.

Index Terms—SHM, OFS, Electromagnetic interference.

Piezoresistive,

Multiplex,

INTRODUCTION. SHM can be understood as the system that have the capability of sensing, intelligence and possibly also actuation devices to allow the loading and damage-causing conditions of a structure to be stored, calculate, identified, and predicted in such a way that autonomous testing becomes an essential part of the structure. According to the function and degree of complexity, SHM systems can be classified 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 and functionalities [1]. SHM is associated with self-working, maintenance, optimized technical structures in addition to the minimization of the potential social, economic impacts. With SHM systems, unusual structural behavior like vibration, fatigue can be detected at an early stage, decreasing the risks of sudden damage and conserving nature, goods and even human lives. In addition, these systems enable in-time refurbishment intervention, the extension of their life-time guaranteeing fewer direct economic losses (repair, maintenance, and reconstruction) and also helping to avoid losses for users due to structural failures. Using SHM systems, hidden structural issues can be detected early, enabling better exploitation of the materials and components of the current structures. A key issue in the SHM systems is the measurement of NITTTR, Chandigarh

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Chemicals (pH, oxidation, corrosion, penetration, and timber decay); Mechanical (strain, deformation, displacement, crack opening, stress, and load); and Physical (temperature, humidity, pore pressure, etc.) [1].

Several types of sensors, embedded or attached to a structure, can be used for this task, but only those based on fiber technology offer the capability to perform integrated, quasi-distributed, and distributed measurements on or even within the structure, in addition to other advantages. As main Challenges for SHM systems, two fundamental technical Challenges are identified: the development of reliable and Sensitive techniques to detect early structural malfunction or unusual structural behavior and the development of data selection, storage and processing models, and robust algorithms to detect structural malfunctions. In addition, user-friendly and simple interfaces with the infrastructure are needed [2].

A system of sensors allows the detection and characterization of damages that could have a significant effect on the operational capability of the structure. Ideally, this will provide warnings, prevent complete failure, and facilitate countermeasures. SHM sensors detect various parameters such as temperature, humidity, or strain. The characterization of strain gives information on cracks, deformations, or vibrations in the structure [2]. It is therefore an essential parameter for the conclusion on Operability. However, in the course of their lifetimes, structures are subject to adverse changes in their structural health conditions due to potential damage or deterioration induced by environment, wear, falts in design and manufacturing, overloads and some unexpected events like earthquakes or impacts or, simply, through their normal working life [1].

Structural degradation can be induced by a wide set of factors.

1) Unsatisfactory inspection and monitoring of existing infrastructure mean problems become apparent only when Structures are in dire need of repair and then, repair costs can be comparable to replacement costs.

2) Corrosion of conventional steel reinforcement within Concrete can provoke expansion of steel, which leads to cracking, fragmentation or further deterioration. It leads to a reduction in strength and serviceability resulting in the need for repair and/or replacement.

3) Increased loads or design requirements over time like heavier trucks, overload on ships, planes, etc. induces deterioration due to overloads or to structural inadequacies resulting from design. Then the structures are deemed unsafe or unserviceable and strengthening or replacement is required.

4) The overall deterioration and aging can induce detrimental effects on structural performance, safety and 144

Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

serviceability, and then repair, rehabilitation, strengthening or replacement may be needed. .

II. PIEZORESISTIVE STRAIN SENSOR Strain is the relative change in shape or size of an object due to externally-applied force. When external forces are applied to objects made of elastic materials, they produce changes in shape and size of the object. In other word strain is defined as extension per unit length. Strain is dimensionless 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 on transforming the strain in certain directions as to change its electrical resistance. It allows measuring plenty of nonelectrical quantities such as deformation, bending, force, acceleration, etc. Piezoresistivity of a material is the dependence of electrical resistivity on strain and normally is quantified by the gauge factor [3]. Strain gauges are fairly

straight forward devices that output a voltage signal based on a change in resistance when the object to which they are attached to undergoes tension or compression. For example, the piezoresistive strain gauge is a semiconductor device whose characteristic is resistance varies non linearly with strain. The most widely used gauge, however, is the bonded metallic strain gauge [3]. Gauge factor is defined as the ratio of the fractional change in electrical resistance to 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 principle whereby an electrical resistor will change its resistance when it is subjected to a strain or deformation.

Semiconductor strain gauge called the piezoresistor, based on the piezoresistive effect is the commonly used strain gauge [4]. Several piezoresisitive strain sensors have been designed to be used for various strain measurements with an excellent gauge factor. Semiconductor’s conductivity depends on temperature change. Piezoresistive sensors are fabricated with semiconductor, so it is very sensitive to temperature changes. It is proved that by increasing the doping level, piezoresistive sensors which are less sensitive to temperature variation can be obtained, but gauge factor of piezoresistive strain sensor get reduced and strain sensor required high gauge factor [4].

III. OPTICAL FIBER SENSORS An OFS can be understood as the device in which the measurand, introduces modifications or modulates characteristics of light in some part of an optical fiber system, reproducing it faithfully in the electric domain [5].

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e-ISSN: 1694-2310 | p-ISSN: 1694-2426

In general terms, an OFS is usually made up of a transducer device, a communication channel and an optoelectronic unit.

A fiber optic sensor is a sensor that uses optical fiber either as the sensing element, or as a relaying signals from a remote sensor to the electronics that process the signals. In an OFS is usually made up of a transducer device, a communication channel and an optoelectronic unit. It is found that OFS technology is attractive in those cases where it offers superior performance compared to the more proven 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 between transmitted light intensity and applied stress. Optical fibers have the advantage of high bandwidth and free from electromagnetic interference, but the high cost of implementation has made it prohibitive for usage along with the robustness of the sensor system, transduction, data interpretation, stability and reliability. In present time OFS technology is attractive in those cases where it offers superior performance compared to the more proven conventional sensors offering, in addition to improved quality in the measurements, better reliability, and the possibility of replacing manual readings and operator judgment with automatic measurements, easier installation and maintenance or a lower lifetime cost [6].

A fiber Bragg grating (FBG) is a sensor of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits to all remaining wavelengths. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror [7]. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

Although fiber-optic sensors are apparently expensive for widespread use in health monitoring however, they are better approaches for applications where reliability in challenging environments is essential. When reliability is a key problem in certain critical health monitoring applications, price is often no longer an issue [8]. The application potential for OFSs in structural monitoring is vast, including civil or industrial structure monitoring like concrete beam tests, bridge girders, ore mines, nuclear containers, tunnels, and hydroelectric dams, composite materials spacecraft, aircraft tail spars, helicopter and windmill rotor blades, ship and submarine hulls, composite cure monitoring, and composite girders for bridges, acoustic sensing in-plant or distribution of electric power utilities, gas pipelines, and industrial control, monitoring and processes [8].

IV. CONCLUSION SHM sensors detect various parameters such as temperature, humidity, or strain. The characterization of strain gives information on cracks, deformations, or vibrations in the structure. It is therefore an essential parameter for the conclusion on operability. Piezoresistive strain sensors are Very sensitive to temperature changes, difficult to implement them in a commercial environment considering the complexity of the network involved.

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Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

Optical fibers based strain sensors: have the advantage of high bandwidth and freedom from electromagnetic interference. For strain measurement of a building structure, need to drill the building at many points, which is called weak point, the drilling affects the quality of building.

FUTURE WORKS In order for the installation of a permanently installed sensing system in buildings to be economically viable, the sensor modules must be wireless to reduce installation costs, must operate with a low power consumption to reduce servicing costs of replacing batteries, and use low cost sensors that can be a microstrip patch antenna.

ACKNOWLEDGEMENT

The authors would like to thank Dr, Umesh Tiwari Scientist, CSIO,Chandigarh, India for their continuous and valuable guidelines during work.

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[3]. Kulha, Pavel, Adam Boura, Miroslav Husak, Alexander Kromka, and Oleg Babchenko. IEEE International Symposium In Industrial Electronics, on "Design and characterization of NCD piezoresistive strain sensor.", pp. 121-126, 2009.

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[7]. Antunes, Paulo FC, M. Fátima F. Domingues, Nélia J. Alberto, and P. S. Andre.", IEEE Photonics Technology Letters, 26, "Optical fiber microcavity strain sensors produced by the catastrophic fuse effect”, vol. no. 1, pp. 78-81, 2014.

[8]. Abe, Tetsuji, Yutaka Mitsunaga, and Hiroaki Koga. Journal of Lightwave Technology, 7 "A strain sensor using twisted optical fibers.",vol. no. 3, pp. 525-529, 1989.

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