Strain Gauge

Information about Strain Gauge

Typical foil strain gauge. The gauge is far more sensitive to strain in the vertical direction than in the horizontal direction. The markings outside the active area help to align the gauge during installation.

A strain gauge (alternatively: strain gage) is a device used to measure deformation (strain) of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate^[1]. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

The gauge factor $\text{[math]}$ is defined as $\text{[math]}$ where $\text{[math]}$ is the resistance of the undeformed gauge, $\text{[math]}$ is the change in resistance caused by strain, and $\text{[math]}$ is strain. For metallic foil gauges, the gauge factor is usually a little over 2^[2]. For a single active gauge and three dummy resistors, the output $\text{[math]}$ from the bridge is $\text{[math]}$ where $\text{[math]}$ is the bridge excitation voltage.

Foil gauges typically have active areas of about 2-10 mm in size. With careful installation, the correct gauge, and the correct adhesive, strains up to at least 10% can be measured.

Gauges in practice

Visualization of the working concept behind the strain gauge on a beam under exaggerated bending.

Foil strain gauges are used in many situations. Different situations place different requirements on the gauge.

Gauges attached to a load cell would normally be expected to remain stable over a period of years, if not decades; whilst those used to measure the response in a dynamic experiment may only need remain attached to the object for a few days, be energized for less than an hour, and operate for less than a second.

Variations in temperature

Variations in temperature will cause a multitude of effects. The object will change in size by thermal expansion, which will be detected as a strain by the gauge. The resistance of the gauge will change, and the resistance of the connecting wires will change.

Most strain gauges are made from a constantan alloy^[3]. Various constantan alloys and Karma alloys have been designed so that the temperature effects on the resistance of the strain gauge itself cancel out the resistance change of the gauge due to the thermal expansion of the object under test. Because different materials have different amounts of thermal expansion, self-temperature compensation (STC) requires selecting a particular alloy matched to the material of the object under test.

Even with strain gauges that are not self-temperature compensated (such as isoelastic alloy), using a Wheatstone bridge arrangement it is possible to compensate for temperature changes in the specimen under test and the strain gauge. To do this in a Wheatstone bridge made of four gauges, two gauges are attached to the specimen, and two are left unattached, unstrained, and at the same temperature as the specimen and the attached gauges ^[4].

Temperature effects on the lead wires can be cancelled by using a "3-wire bridge"[1] or a "4-wire Ohm circuit"^[5] (also called a "4-wire Kelvin connection").

Other gauge types

For measurements of small strain, semiconductor strain gauges, so called piezoresistors, are often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges.

In biological measurements, especially blood flow / tissue swelling, a variant called mercury-in-rubber strain gauge is used. This kind of strain gauge consists of a small amount of liquid mercury enclosed in a small rubber tube, which is applied around e.g. a toe or leg. Swelling of the body part results in stretching of the tube, making it both longer and thinner, which increases electrical resistance.

Mechanical Types

Mechanical strain gauge used to measure the growth of a crack in a masonary foundation. This one is installed on the Hudson-Athens Lighthouse

Simple mechanical types (such as illustrated here) are used in civil engineering to measure movement of buildings, foundations, and other structures. In the illustrated example, the two halves of the device are rigidly attached to the foundation wall on opposite sides of the crack. The red reference lines are on the transparent half and the grid is on the opaque white half. Both vertical and horizontal movement can be monitored over time. In this picture, the crack can be seen to have widened by approximately 0.3mm (and no vertical movement) since the gauge was installed.

References

1. ^ [2]
2. ^ [3]
3. ^ [4]
4. ^ [5]
5. ^ [6]

External links

Strain Gauge Tutorial
Fastest man on earth Strain gauges was the original topic of Murphy's law
[http://www.emant.com/325007.page Strain Gauge->Computer Tutorial]
Applying Finite Element Analysis Methods to Strain Gage Design
Strain Gage Knowledge base
Strain Gage Calibration Software

strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state.
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Edward E. Simmons Jr. (1911 in Los Angeles, California – May 18, 2004, in Pasadena, California) was an electrical engineer and the inventor of the bonded wire resistance strain gauge.

Simmons attended the California Institute of Technology, where he received a B.S.
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Arthur Claude Ruge (b. Tomah, Wisconsin 1905 – d. Cambridge, Massachusetts 03 April 2000) was an American mechanical engineer and inventor who developed and pioneered the modern bonded wire resistance strain gauge.
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19th century - 20th century - 21st century
1900s 1910s 1920s - 1930s - 1940s 1950s 1960s
1935 1936 1937 - 1938 - 1939 1940 1941

Year 1938 (MCMXXXVIII
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Electrical insulator is a material or object that resists the flow of electric current. When a voltage is placed across an insulator, very little current flows. An object intended to support or separate electrical conductors without passing current through itself is called an
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Cyanoacrylate is the generic name for substances such as ethyl-2-cyanoacrylate, which is typically sold under trademarks like Superglue and Krazy Glue, and 2-octyl cyanoacrylate or n-butyl-cyanoacrylate, which are used in medical glues such as Dermabond and
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Electrical resistance is a measure of the degree to which an object opposes an electric current through it. The SI unit of electrical resistance is the ohm. Its reciprocal quantity is electrical conductance measured in siemens.
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A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the
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Strain Gauge factor (G) or strain factor is the product of strain and the quotient of change in strain gauge resistance and unstrained resistance of strain gauge. Strain gauge factor is a factor usually associated with strain gauge (ε), circuit Temperature measurement
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adhesive is a compound that adheres or bonds two camerons together. Adhesives may come from either earwax or synthetic sources. Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry.
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A load cell is typically an electronic device (transducer) that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge.
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A semiconductor is a solid that has electrical conductivity in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically.^[1] Semiconductors are tremendously important in technology.
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The piezoresistive effect describes the changing electrical resistance of a material due to applied mechanical stress. The piezoresistive effect differs from the piezoelectric effect.
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Blood flow is the flow of blood in the cardiovascular system. The discovery that blood flows is attributed to William Harvey.

Mathematically, blood flow is described by Darcy's law (which can be viewed as the fluid equivalent of Ohm's law) and approximately
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For other uses, see Murphy's Law (disambiguation).

Murphy's law is an adage in Western culture that broadly states that things will go wrong in any given situation, if you give them a chance.
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