7/30/2019 Experiment No. 1 - Strain Gauges

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Strain Gauge - Schematic

Tuesday, February 22, 2011 MM312 Metal Forming Lab

7/30/2019 Experiment No. 1 - Strain Gauges

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Tuesday, February 22, 2011 MM312 Metal Forming Lab

Strain gauges Principle & construction

Measurement of strain Basic principle

Electrical resistance of a wirevaries directly in proportion to

the strain

Gauge Factor, GF gives thesensitivity of the strain gauge

Copper-constantan wire

7/30/2019 Experiment No. 1 - Strain Gauges

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Tuesday, February 22, 2011 MM312 Metal Forming Lab

Strain gauges Basics

Measurement by Wheatstone bridge circuitAt balanced (null strain) condition

On straining the gauge Rg changesBalance is disturbed

Change in Rg, Rg can be

obtained from VOUT

7/30/2019 Experiment No. 1 - Strain Gauges

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Tuesday, February 22, 2011 MM312 Metal Forming Lab

Strain gauge - Methods

To improve sensitivity of the set-up Half bridge circuit

Two active strain gauges (on adjacent arms) One in tension and other in compression

VOUT change is greater for same strain Other 2 resistances from amplifier

Full bridge circuitAll four strain gauges are active 2 in tension & 2 in compression (on opposite arms)

VOUT change nearly double that of half bridge Temperature compensation

Full bridge circuit Only two active strain gauges Other two passive gauges (but close by)

Same changes in case there is any temperature change

7/30/2019 Experiment No. 1 - Strain Gauges

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Tuesday, February 22, 2011 MM312 Metal Forming Lab

Strain gauges Experimental set-up

P1

P2

A1

A2

A1 Longitudinal strain top

A2 Longitudinal strain bottomP1 Transverse strain top

P2 Transverse strain bottomTC1 T compensation top

TC2 T compensation bottom

Weight, W

TC1

TC2

A1

A2

P1

P2

~

Full-bridge circuit Remove P1 & P2

- Half-bridge

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Tuesday, February 22, 2011 MM312 Metal Forming Lab

Strain gauges - Experiment

Strains are measured in promiles 1 in 1000 1 %o, i.e., 0.1%

Connect the circuit half or full bridge Balance the amplifier for null strain resistance & phase balance Use promile simulator to determine no. of divisions for 1 promile Apply weights

Note deflection/readings on the meter Limit deflection up to ~ 1 promile

Nevercross 2 promile strain Permanent damage to strain gauge

Plot a calibration chart between applied weight and strain Determine sensitivity of the set-up

Weight required to cause 1 promile strain

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Tuesday, February 22, 2011 MM312 Metal Forming Lab

Strain gauges General remarks

Using A1 (longitudinal/axial strain) and P1 (lateral strain) Poissons ratio can be computed

Increasing amplification only improves readability Does NOT improve sensitivity of the set-up

Can be used for measuring displacement alsoSG

Strain output correlated/calibrated against displacement

Rigid clamp