Strain gauges are resistance elements that change resistance when subjected to strain. They consist of a grid wire or foil cemented between thin paper and bonded to a specimen. Strain gauges can be used to measure stress, force, torque, bending moment, and deflection proportionally to strain within the elastic limit. They are classified as metallic or nonmetallic and metallic gauges can be wire or foil types. Strain gauges are often mounted on elastic members called strain gauge transducers to provide a larger output that can be recorded or indicated remotely.

1. Strain gauges
Experimental Stress Analysis
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2. Introduction
• Strain gauge are resistance element when subjected to strain its resistance changes
• For tensile strain resistance increases and compressive strain it will decrease
• The gauge consist of a grid wire or foil cemented between 2 pieces of thin paper
• This gauge is bonded to the specimen being analyzed with thin layer of adhesive
• Adhesive serves to transmit the surface strain of specimen to the gauge wire
• Thin paper protects the gauge and serve as insulation between wire and specimen
• Stress, force, torque, bending moment, deflection etc. are proportional to the strain
within elastic limit, strain gauges can be used to measure these quantities
• Classified into metallic and nonmetallic (semiconductor)
• Metallic classified into wire (0.025mm dia) and foil strain gauges(thin foil by
photoetching process)
• Foil strain gauge got larger area and better heat dissipation characteristics
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3. Strain gauge transducers
• Strain gauges are mounted on some form elastic members called
strain gauge transducer
• The gauges are arranged to give large output
• Output from transducer is fed to the indicator and calibrated to read
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6. Construction
• Strain gauge transducer is generally smaller in size
• The output can be recorded or indicated at a remote place
• Meter deflection is calibrated in terms of quantity to be measured by
applying known inputs
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7. Strain indicators
• Strain gauge bridge amplifier
meter/recorder indicator static strains
• Oscilloscope recorder Dynamic strains
• Height of the pulse gives the magnitude of the strain
• For static strain measurement meter of deflection type
bridge directly indicates microstrains while for null type
bridge difference of null position before and after load
application give microstrains.
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8. Strain indicator control unit
• On/off switch
• Arrangement for two arm and four arm bridge
• Amplifier balance control
• Bridge balance control
• Calibration switch
• Switch for gauge factor selection
• Connection for multichannel
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9. Operating procedure
• Put on the supply and wait for 10/15 minutes
• Balance the amplifier by balance knob
• Balance the bridge with bridge balance control
• Check calibration
• Put the gauge factor switch to appropriate gauge factor position, to
calibrate the meter in microstrains
• Test component is then subjected to loads
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10. Measurement of Force or Load
• This type of transducer is called load cell
• Suitable for measurement of tensile and compressive loads
• Strain gauges are bonded on short pillar of high tensile steel
• The bridge is temperature compensated and the bending effects due to
column action are eliminated automatically
• Sensitivity can be improved by placing strain gauges at different locations
• Proving type strain gauges are provided for increasing sensitivity
• Used for measuring the cutting force on the lathe tool
• Separate bridge arrangement are provided for sensing bending strains in
cantilever due to the radial force and feed force
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