strain how to guide

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Strain Measurements with Strain Gages:How-To Guide

OverviewThis document is part of theHow-To Guide for Most Common Measurementscentralized resource portal.

Strain and Strain Gage OverviewStrain is the amount of deformation of a body due to an applied force.More specifically, strain (e) is defined as the fractional change in length,as shown in Figure 1 below.

While there are several methods of measuring strain, the most common iswith a strain gage, a device whose electrical resistance varies inproportion to the amount of strain in the device. The most widely usedgage is the bonded metallic strain gage.

View a 60-second video on

how to take a Strain

Gage Measurement

Figure 1. Definition of Strain

The metallic strain gage consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The

grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction (Figure 2). The gridis bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strainexperienced by the test specimen is transferred directly to the strain gage, which responds with a linear change inelectrical resistance. Strain gages are available commercially with nominal resistance values from 30 to 3000 #, with120, 350, and 1000 # being the most common values.

National Instruments. All rights reserved. LabVIEW, National Instruments, NI, ni.com, the National Instruments corporate logo, and the Eagle logo are trademarks of National Instruments. Seeni.com/trademarksfor other NI trademarks. Other product and company names are trademarks or trade names of their respective companies. For patents covering National Instruments products, referto the appropriate location: Help>>patents in your software, the patents.txt file on your CD, or ni.com/patents.

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Figure 2. Bonded Metallic Strain Gauge

In practice, the strain measurements rarely involve quantities larger than a few millistrain (e x 10 -3). Therefore, tomeasure the strain requires accurate measurement of very small changes in resistance. To measure such small changesin resistance, strain gages are almost always used in a bridge configuration with a voltage excitation source. The generalWheatstone bridge, illustrated below, consists of four resistive arms with an excitation voltage, VEX, that is appliedacross the bridge.

Figure 3. Full-Bridge Circuit

The output voltage of the bridge, VO, will be equal to:

From this equation, it is apparent that when R1/R2= R4/R3, the voltage output VOwill be zero. Under these conditions,the bridge is said to be balanced. Any change in resistance in any arm of the bridge will result in a nonzero outputvoltage.

Therefore, if we replace R4in Figure 3 with an active strain gage, any changes in the strain gage resistance will

unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of the strain gage is designated asRG, then the strain-induced change in resistance, DR, can be expressed as DR = RG*GF*e. Assuming that R1= R2and

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3= RG, the bridge equation above can be rewritten to express VO/VEXas a function of strain (see Figure 4). Note the

presence of the 1/(1+GF*e/2) term that indicates the nonlinearity of the quarter-bridge output with respect to strain.

Figure 4. Quarter-Bridge Circuit

Ideally, we would like the resistance of the strain gage to change only in response to applied strain. However, straingage material, as well as the specimen material to which the gage is applied, will also respond to changes intemperature. Strain gage manufacturers attempt to minimize sensitivity to temperature by processing the gage materialto compensate for the thermal expansion of the specimen material for which the gage is intended. While compensatedgages reduce the thermal sensitivity, they do not totally remove it.

By using two strain gages in the bridge, the effect of temperature can be further minimized. For example, Figure 5illustrates a strain gage configuration where one gage is active ( RG+ DR), and a second gage is placed transverse to theapplied strain. Therefore, the strain has little effect on the second gage, called the dummy gage. However, any changesin temperature will affect both gages in the same way. Because the temperature changes are identical in the two gages,the ratio of their resistance does not change, the voltage VOdoes not change, and the effects of the temperature changeare minimized.

Figure 5. Use of a Dummy Gage to Eliminate Temperature Effects

The sensitivity of the bridge to strain can be doubled by making both gages active in a half-bridge configuration. Forexample, Figure 6 illustrates a bending beam application with one bridge mounted in tension ( R G+ DR) and the othermounted in compression ( RG+ DR). This half-bridge configuration, whose circuit diagram is also illustrated in Figure6, yields an output voltage that is linear and approximately doubles the output of the quarter-bridge circuit.

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Figure 6. Half-Bridge Circuit

Finally, you can further increase the sensitivity of the circuit by making all four of the arms of the bridge active straingages in a full-bridge configuration. The full-bridge circuit is shown in Figure 7.

Figure 7. Full-Bridge Circuit

Thus, a single arm is an active strain gage in a quarter-bridge circuit; two arms are active strain gages in a half-bridgecircuit while all four arms are active strain gages in a full-bridge circuit.

Strain gages do not have polarity, although depending upon which one of the above three categories a strain gage, therewill be different number of connections that you will have to make to the measurement hardware as explained in thesection below.

How to Make a Strain Gage Measurement

Most strain gage measuring solutions will provide an option to measure quarter-, half- and full-bridge configurations.

Lets take an example of a NI CompactDAQ system with a NI 9237 4-channel simultaneous bridge module (Figure 8).

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Figure 8: NI CompactDAQ and NI 9237 bridge module

Figure 9 below shows connection diagram for wiring a strain gage in quarter-bridge configuration to this module.Connect one end of a quarter bridge gage to CH+ terminal on the module and other end to the QTR terminal. Noticethat the EX- terminal on the module is left unwired because for quarter-bridge configuration, R3 is internal to themeasurement hardware (Figure 9)

Figure 9: Wiring in quarter-bridge configuration

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For measuring a half-bridge configuration, connect two wires from the two active elements to EX+ and EX- terminalson the module. Lastly, connect a wire between the common point of the two active elements to the QTR terminal on themeasurement module.

Figure 10: Wiring in half-bridge configuration

For measuring in a full-bridge configuration, connect the common point between R1 and R4 to EX+ and common pointbetween R2 and R3 to EX-. Also, connect the common point between R3 and R4 to CH+ and common point betweenR1 and R2 to CH-.

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Figure 11: Wiring in full-bridge configuration

Getting to see your measurement:

Now that you have your sensor connected to the measurement device, you can bring that data into computer andvisualize using NI LabVIEW graphical programming software.

Figure 12 shows an example of displaying measured strain data on a chart indicator inside the LabVIEW programming

environment.

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Figure 12: Strain Data Measurements with LabVIEW

Recommended Hardware and Software

Example Strain Measurement System

NI CompactDAQ: Three-minute out of the box video

Take a Virtual Tour of NI CompactDAQ

Learn about and test-drive LabVIEW software for free

Strain Webcasts, Tutorials, and Other How-to Resources

Measuring Strain with Strain Gages

NI Strain Measurement Solutions

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