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Measurement of Strain

Introduction:

If a metal conductor is stretched or compresssed, its resistance changes on accountof the fact that both the length and the diameter of conductor change. Also, there isa change in the resistivity of the conductor when it is strained and this is calledpiezoresistive effect. Therefore resistive strain guages are also called Piezoresistiveguages. The strain gauge has been in use for many years and is thefundamentalsensing element for many types of sensors, including pressuresensors,load cells, torque sensors, position sensors, etc.

Operating Principle:On calculation of the guage factor, and having the knowlegdge about thepiezoresistive effect, one can calculate the Poission's ratio (measure of strain). The

guage factor is defined as:Guage factor=Resistance change due to change of length+ resistance change due tochange inarea+ resistance change due to piezoresistive effect

Types of strain guages:Unbonded metal strain guages,Bonded metal foil dtrain guages ,Bondedmetal foil strain guages,Vacuum deposited thin metal strain guages,Sputterdeposited thin metal strain guages,Bonded semionductor strainguages,Diffuesed metal strain guages.

The guage:

Measuring Circuit: In order to measure strain with a bonded resistance strain gage, it must beconnected to an electric circuit that is capable of measuring the minute changes inresistance corresponding to strain. Strain gage transducers usually employ fourstrain gage elements electrically connected to form a Wheatstone bridge circuit(Figure 1). A Wheatstone bridge is a divided bridge circuit used for themeasurement of static or dynamic electrical resistance. The output voltage of the

Wheatstone bridge is expressed in millivolts output per volt input. The Wheatstonecircuit is also well suited for temperature compensation. The number of activestrain gages that should be connected to the bridge depends on the application.

THe figure below shows the wheatstone bridge circuit applied in this operation.

Structure of rosette:Generally a combination of strain guages are available. They are called rosettes.

The figures below illustrate the same:

Materials Used: The basic materials used for strain guages are Nichrome, Constantan, Isoelastic,Nickel, Platinum. Several types of base or carrier materials are used to support the

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