Thus, a strain gage is a device which uses change in electrical resistance to measure strain. E25192, astm bsi bs6888 methods for calibration of bonded electric resistance strain gauges draft for development 6. Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge. A strain gauge takes advantage of the physical property of electrical conductance and its dependence on the conductors geometry. Various means like mechanical, optical, acoustical, pneumatic or electrical can be used to measure deformation strain of an object.
The strain gauges are based on the resistive properties of the metal. The strain gage was invented by ed simmons at caltech in 1936. The resistance in any conducting device is dependent on its length as well as the crosssection area. This resistance may change only a fraction of a percent for the full force range of the gauge, given the limitations imposed by the elastic limits of the gauge material and of the test specimen. A strain gauge figure 1 is a sensor used tomeasure strain. A strain gauge is an electrical transducer which is used for measuring mechanical surface strain.
A strain gage s electrical resistance varies in proportion to the amount of strain in the device. Normally, this requires the use of heavy machinery, but the application of special electrical strain gauges provides a simplified solution for measuring the axial load on bolt elements. A strain gauge works on the principle of electrical conductance and its dependence on the conductors geometry. There are some materials whose resistance changes when strain is applied to them or when they are stretched and this change in resistance can be measured easily. Endloops and solder tabs are considered insensitive to strain because of their relativly large cs area and low electrical resistance. Flight strains are to be measured by gauges, strategically. It operates on the principle, discovered by the british physicist william thompson later lord kelvin in 1856, that the electrical resistance of a copper or iron wire changes when the wire. Gauge length defnition gauge length of a strain gauge is the active or strain sensitive length of the grid.
Strain gauge working principle your electrical guide. The gauge is attached to the object by a suitable adhesive. This strain gauge works on the principle that when a. This type of gauge is frequently employed as a sensor in transducers to measure load, torque, pressure and acceleration. Electrical strain gauge when an electrical wire is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end toend. Measuring strain with strain gages national instruments. In this circuit diagram, a strain gauge is connected into a wheatstone bridge.
The electrical resistance strain gage is by far the most popular device for measuring strain today. Automatic strain measurement is usually done with an electrical resistance strain gauge or gage. The following diagram shows the fatigue life of electrical strain gauges for load cycles between and 10,000,000 for yand mseries gauges. Jul 04, 2019 a strain gauge depends on the electrical resistivity of any conductor. A strain gauge is basically used to measure the strain in a work piece. It can be attached to a pressure sensing diaphragm as shown in fig 2 when diaphragm flexes due to the process pressure applied on it, the strain gauge stretches or compresses due to this resistance changes. The strain gage is bonded to the surface with an adhesive. It is very difficult to measure the dimensions like force, stress and strain that cannot be really sensed directly by any instrument.
Strain measurement strain can be measured using many techniques, but the resistance strain gauge is generally the most direct and convenient tool. January 27, 2016 comparing strain gage measurements to force calculations in a simple cantilever beam 4 p a g e mqp, hazel figure 1 stressstrain graph showing elastic and plastic region davis stress when a body is loaded with a force, the material must compensate for this load by altering the shape of. However, since strain is an invisible infinitesimal phenomenon, the resistance change caused by strain is extremely small. For the determination of the absolute value of a resistance by comparison with a known resistance, for the determination of relative changes in resistance. Effect of pressure on electrical resistance strain gages article pdf available in experimental mechanics 373. An illustration of arthur ruges original electrical resistance strain gauge from a us patent he filed in september 1939. The electrical resistance strain gage in the basic form known today was first used in 1936. Sep 21, 2017 measuring the electrical resistance was clearly advantageous to these mechanical methods, but wasnt used. Due to the correct fit to the material being examined, the strain gauge is. Figure 12 is a picture of a bonded resistance strain gage with a constantan foil grid and polyimide carrier material. It operates on the principle, discovered by the british physicist william thompson later lord kelvin in 1856, that the electrical resistance of a copper or iron wire changes when the wire is either stretched or compressed.
The electrical resistance r of a wire of length l and crosssectional area a is given by l r a, where is the resistivity of the wire material. Hi friends, in this article, i am going to describe you strain gauge working principle, gauge factor and strain gauge load cell working. Electrical resistance strain gauges pdf editor salam pajak. The resistance strain gage may be employed in shock or vibration instrumentation in either of two ways. Resistancetype strain gauges like this were invented in 1938 by mit professor arthur ruge 19052000 to help with earthquake detection. They can measure the surface strains in any direction.
A strain gage consists of a small diameter wire actually an etched metal foil that is attached to a backing material usually. Deformation of the surface element forces the strain gage to change its length. Strain may be either tensile positive or compressive negative. For example, the piezoresistive strain gauge is a semiconductor device whose resistance varies nonlinearly with strain. The electrical resistance of the wire changes with strain.
Measuring the electrical resistance was clearly advantageous to these mechanical methods, but wasnt used. The two ratio arms of the bridge r 1 and r 3 are set equal to each other. Therefore, resistance strain gauges are also known as piezoresistive gauges. A strain gauge is a passive transducer, that converts mechanical displacement into the change of resistance. In this application note, we will be more specific and define the term strain to mean deformation per unit length or fractional change in length and give it the symbol. For the special conductor gage materials, the variation in length of the parallel segments of the wiretype conductor will be directly proportional to the variation in electrical resistance of the conductor.
The bonded resistance strain gage is an ana log electrical sensor ideally suited to the task of measuring surface stains on solid materials. When external forces are applied to a stationary object, stress and strain are the result. Strain gauge working, types, diagram, applications. The electrical resistance strain gauge is a resistance element which changes resistance when subject to strain. The wheatstone bridge can be used in various ways to measure electrical resistance. The entire device is typically 10 mm long, with 16 or more parallel. However, it will also change resistance when subject to a temperature change. Electrical strain gauge unbonded strain gauge bonded strain gauge foil strain gauge weldable strain gauge general working principle in electrical resistance strain gauge the displacement or strain is measured as a function of resistance change produced by the displacement in the gauging circuit. The most widely used strain gage is the bonded metallic strain gage.
Thus, a strain gage using this alloy for the sensing element enables conversion of mechanical strain to a corresponding electrical resistance change. Electrical resistance strain gages can be used with this resolution and with an accuracy varying from numbers as low as 1% to as high as 0. Metal strain gauge an overview sciencedirect topics. For example, lets calculate the resistance change on a strain gage. A strain gauge depends on the electrical resistivity of any conductor. The electrical resistance strain gage discussed in this chapter is basically a piece of very thin foil or fine wire which exhibits a change in resistance propor. Similarly, when it is compressed, it gets shorter and broader, ultimately changing its resistance. Strain gauge seminar report, ppt, pdf for mechanical. Do not confuse with density, for which the same symbol is used.
The strain gage applies this principle to strain measurement through. Standard resistance are 120 and 350 ohms, whereas 500, and 3000 ohms are for special purpose gauges. For such cases special devices called strain gauges are very useful. January 27, 2016 comparing strain gage measurements to force calculations in a simple cantilever beam 5 p a g e mqp, hazel eq 3 where the stress vector is now equal to the normal and sheer stress components. Figure 1 resistive strain gage with electrical leads on the right side in the laboratory we will be using a strain gage that is glued to an aluminum band. It is a very fine metal grid which is cemented in paper base material on the surface of the any structural component to measure the surface normal strains in any desired direction. This change in resistance is a measure of its mechanical motion. When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, which increases its electrical resistance endtoend. The selection criteria for strain gages used on transducer spring elements, while similar in many respects to the considerations presented here, may vary significantly from application to application and should be treated accordingly. Change in the length divided by the original length is called normal strain. Stress is defined as the objects internal resisting forces, and. Electrical resistance strain gauges are the most frequently used devices in experimental stress analysis today. In application, the strain gauge will be attached to a structural member with the help of special cement.
Comparing strain gage measurements to force calculations. A strain gages electrical resistance varies in proportion to the amount of strain in the device. A strain gage sometimes refered to as a strain gauge is a sensor whose resistance varies with applied force. Strain gauge is a passive transducer that converts a mechanical elongation or displacement produced due to a force into its corresponding change in resistance r, inductance l, or capacitance c. Strain gauges a re available with the most common resistance values of 120, 350.
Bonded electric resistance strain gauges with metallic measurement grids characteristics and testing conditions. The strain gauge while there are several methods of measuring strain, the most common is with a strain gauge, a device whose electrical resistance varies in proportion to the amount of strain in the device. Electrical resistance strain gauge an overview sciencedirect. When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end. The resistance in any conducting device is dependent on its length as well as the. The inverse quantity is electrical conductance, and is the ease with which an electric current passes. Pdf strain gauge principle, types, features and applications. Thus, in order to use it to determine strain, compensation has to be made for temperature effects.
The strain gage selection criteria considered here relate primarily to stress analysis applications. The discovery of the principle upon which electrical resistance. Upon stretching, the metal material loses or increases its electrical resistance. A strain gauge with a gauge factor gf 2 will exhibit a change in electrical resistance of only 2500. Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The maximum achievable values depend on different factors such as the installation quality.
The metallic strain gage consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. Elastic stressstrain uniaxial mechanical behavior relations. Pdf effect of pressure on electrical resistance strain gages. Both the wire and foil type have been developed extensively. Whenever a conductor is stretched within the limits of its elasticity, it doesnt break but, gets narrower and longer. The resistance strain gauge is a valuable tool in the field of experimental stress analysis. Strain gauge theory types, formulas and applications. If the area of cs is large, resistance is small and vice versa. Aug 05, 2011 electrical resistance type strain gauge. Basic principle of strain gauge when stress is applied to the metal conductor, its resistance changes owing to change in length and crosssectional area of the conductor. Once bonded to the surface of the material, any physical strain in the material is transmitted to the strain.
The strain gage may be the active element in a commer. Specialized semiconductor strain gauges based on silicon have gauge factors of. Pdf application of strain gauges in measurements of strain. Wiring of a bolt strain gauge it is difficult to bond a strain gauge to a bolt while measuring the tightening stress. Strain can be inferred by measuring change in resistance. The electrical resistance of an object is a measure of its opposition to the flow of electric current. The correlation between load cycles and maximum load amplitude is not linear at all logarithmic scaling. Strain gaugetransducer,sensor,wheatstone bridge,electrical. A thin plastic base supports thin ribbons of metal, joined in a zigzag to form one long electrically conductive strip. A strain gauge sensor is a thin waferlike device that can be attached to a variety of materials to measure applied strain.
Change in angle or shape of to the original angle or shape of the body is called shear strain. If a metal piece is subjected to a tensile stress, the metal length will increase and thus will increase the. Normally, this requires the use of heavy machinery, but the application. A strain gauge is a sensor whose resistance varies with applied force. Gauge factor needs to be found experimentally in strain gauge. It took almost 80 years for the technology to become cost effective.
Strain gauge sheet metal electrical resistance and. Metal strain gauges have a gauge factor of about 2. Stress analysis by using strain gages theory page 9 15, 1 1. The discovery of the principle upon which the electrical resistance strain gauge is based was made in 1856 by lord kelvin, who loaded copper and iron wires in tension and noted that their resistance increased with the strain applied to the wire. The most commonly used strain gauge is an electrical resistance strain gauge. As this change in resistance is very small in magnitude so its effect can be only sensed by a wheatstone bridge. In an electrical resistance strain gauge, the device consists of a thin wire placed on a flexible paper tissue and is attached to a variety of materials to measure the strain of the material. A strain gauge sometimes refered to as a strain gage is a sensor whose resistance varies with applied force. This change in resistance is proportional to the applied strain. Photoelectric strain gauge was also introduced which uses a light beam to produce electric current corresponding to deformation. This resistance change, usually measured using a wheatstone bridge, is related to the strain by the.
Comparing strain gage measurements to force calculations in a. Tml strain gauge testing and inspection standards astm e25174 standard test methods for performance characteristics of metallic bonded resistance strain gauges designation. Pdf the pressure effect is defined as the indicated strain response minus the expected response of a strain gage under hydrostatic pressure. Strain gages are invaluable for evaluating not only airplane frames, but also automobiles, bridges, and buildings. Mar 15, 2020 the change in resistance normally has very small value, and to sense that small change, strain gauge has a long thin metallic strip arrange in a zigzag pattern on a nonconducting material called the carrier, as shown below, so that it can enlarge the small amount of stress in the group of parallel lines and could be measured with high accuracy. Stress, strain, and strain gages, page 2 strain gage the principle discussed above, namely that a wires resistance increases with strain, is key to understanding how a strain gage works. The strain gauge foil is glued on the material, which will be tested using a specially adapted glue for strain gauges. A strain gauge also strain gage is a device used to measure the strain of an object. For example, lets calculate the resistance change on a strain gage caused by x106 strain. This is the strain that we typically measure with a bonded resistance strain gage.
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