Electrical Electronic

Understanding strain gauge

A passive transducer, such as a strain gauge, transforms mechanical displacement into a change in resistance. It’s a thin, wafer-like device that measures applied strain and may be connected to a variety of materials with an appropriate adhesive. The ensuing strain deforms the strain gauge attached to the structure as it is stressed. It increases the gauge’s resistivity, resulting in an electrical signal proportionate to the deformation. When subjected to a displacement, the strain gauge displacement sensor comprises a structure attached to the strain gauge that deforms elastically.

In this article, you’ll learn the definition, application, requirements, parts, diagram, types, working, advantages, and disadvantages of a strain gauge.

strain gauge

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What is a strain gauge?

A strain gauge (also known as a strain gauge) is a device that measures the amount of strain on an item. The most common type of strain gauge, invented by Edward E. Simmons and Arthur C. Ruge in 1938, consists of an insulating flexible backing that supports a metallic foil pattern. An appropriate adhesive, such as cyanoacrylate, is used to connect the gauge to the item. As the object deforms, the foil deforms as well, changing the electrical resistance. The gauge factor is a quantity that is connected to the resistance change, which is commonly measured using a Wheatstone bridge.

Engineers frequently use strain gauges to measure the influence of external forces on an object. They directly measure strain, which can be used to infer stress, torque, pressure, deflection, and a variety of other metrics.

Strain is a non-dimensional measurement, that is a ratio of an object’s changed length to its initial length. As a result, a positive strain occurs when a material is stretched, while a negative strain occurs when a material is compressed. Stress is defined as the force applied divided by the object’s initial cross-sectional area or the object’s internal resisting ability.

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Application

Strain gauges are commonly used in civil engineering and geotechnical monitoring to identify breakdowns in structures such as bridges, buildings, and other structures. Because any considerable deformation could result in injury or death, these structures must be constantly monitored. These gauges are widely used because of their high precision, ability to perform at vast distances from the test object, and ease of setup and maintenance over long periods of time.

Testing in the outdoors is frequently not the same as testing in a lab under ideal conditions. One of the reasons strain gauges are so valuable is that they can be used in severe conditions and produce reliable, high-precision data. A specialized gadget like a strain gauge is frequently required when an engineer tests products with irregular shapes in harsh locations with difficult-to-access setups. For example, millions of strain gauges are used in aerospace applications to check the findings of CAD (Computer-Aided Design) and FEA (Finite Element Analysis) simulations. These tests are frequently carried out under dynamic conditions in order to provide a realistic portrayal of how various forces affect airplanes.

Static testing is also routinely done with strain gauges. Wireless telemetry is used by some bridges, which sends the test results through Ethernet. Other bridges, on the other hand, are subjected to visual inspection or penetrant testing to detect surface flaws. While these technologies are cost-effective, they do not allow for continuous inspection, which can lead to catastrophic collapses like the I-35 Minneapolis steel truss bridge. The federal government designated the bridge as “structurally weak” in 1990, which meant it had to undergo yearly inspections. The bridge eventually collapsed in 2007, killing 13 people, due to a lack of continual strain gauge monitoring, significant repairs, or replacement. The following are the common applications of a strain gauge:

  • Measurement of strain
  • Measuring residual stress
  • Vibration analysis
  • Measurement of torque
  • Measurement of bending and deflection
  • Measurement of compression and tension

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Requirement of strain gauge material

A good strain gauge must be able to perform the followings:

  • The strain gauge should be modest in size and light in weight.
  • It should be extremely strain-sensitive.
  • The gauge factor of a strain gauge should be high.
  • It must be simple to attach to the specimen.
  • It should have a fast response time and little time lag.
  • It should be able to tell the difference between static, transient, and dynamic strain.
  • It should be able to indicate and record information from a distance.
  • Temperature, humidity, vibration, and other environmental factors should not affect it.
  • It should be affordable, dependable, and readily available in a variety of sizes.

Diagram of a strain gauge:

strain-gauge-diagram

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Types of strain gauge

There are several types of strain gauges available on the market. The followings are the various types of strain gauges:

  • Linear strain gauges
  • Membrane Rosette strain gauges
  • Double linear strain gauges
  • Full bridge strain gauges
  • Shear strain gauges
  • Half-bridge strain gauges
  • Column strain gauges
  • 45°-Rosette (3 measuring directions)
  • 90°-Rosette (2 measuring directions).

Below are some other types of stain gauges you might come across:

Semiconductor strain gauges, also known as piezoresistive, are generally favored over foil gauges for measuring small strains. The gauge factor of a semiconductor gauge is usually higher than that of a foil gauge. Semiconductor gauges are often more expensive, more temperature-sensitive, and more delicate than foil gauges.

Strain gauges made of nanoparticles appear to be a promising new technique. Due to their high impedance, these resistive sensors with an active area comprised of an assembly of conductive nanoparticles, such as gold or carbon, have a high gauge factor, a large deformation range, and low electrical consumption.

A mercury-in-rubber strain gauge is employed in biological measurements, particularly blood flow and tissue swelling. This strain gauge is made up of a little amount of liquid mercury encased in a small rubber tube that is wrapped around a toe or leg, for example. When a bodily portion swells, it causes the tube to expand, making it both longer and thinner, increasing electrical resistance.

To measure strain along with an optical fiber, fiber optic sensing can be used. Measurements might be done at random locations along with the fiber or at specified points. Embedded sensors of this type are used on the 2010 America’s Cup boats Alinghi 5 and USA-17.

Strains can also be measured using other optical techniques such as electronic speckle pattern interferometry or digital picture correlation.

Microscale strain gauges are frequently employed in MEMS to monitor stresses caused by force, acceleration, pressure, or sound. MEMS accelerometers, for example, are frequently used to activate airbags in automobiles. Integrated optical ring resonators can be used to monitor strain in microoptoelectromechanical systems as an alternative to piezo-resistant strain gauges (MOEMS).

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A variable capacitor is used in capacitive strain gauges to detect the level of mechanical deformation.

Geotechnical and civil engineering applications use vibrating wire strain gauges. A vibrating, tensioned wire makes up the gauge. The strain is calculated by determining the wire’s resonance frequency (an increase in tension increases the resonant frequency).

Geotechnical applications also use quartz crystal strain gauges. DART’s crucial sensor is a pressure sensor with a resonant quartz crystal strain gauge and a bourdon tube force collector.  DART detects tsunami waves at the ocean’s deepest depths. When monitoring pressure at a depth of many kilometers, it has a pressure resolution of about 1mm of water.

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Working Principle

The working of a strain gauge is quite interesting, it is less complex and easy to understand. watch the video below to have a visual view of how a strain gauge work.

 

Advantages and disadvantages of a strain gauge

Advantages:

The followings are the benefits of a strain gauge in its various applications:

  • Because there is no moving part, there is no wear.
  • Strain gauges are extremely accurate.
  • It’s compact and low-cost.
  • It has a wide frequency range.

Disadvantages:

Despite the advantages of a strain gauge, some limitations still occur. Below are the disadvantages of strain gauges in their various applications.

  • It’s a non-linear system.
  • It is extremely temperature-sensitive.
  • It must be calibrated on a regular basis.
  • Strain gauges must be manually applied. It takes time and money to put them in their proper places. It is one of their most serious flaws.

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Conclusion

A passive transducer, such as a strain gauge, transforms mechanical displacement into a change in resistance. It’s a thin, wafer-like device that measures applied strain and may be connected to a variety of materials with an appropriate adhesive. The ensuing strain deforms the strain gauge attached to the structure as it is stressed. That is all for this article, where the definition, application, requirements, parts, diagram, types, working, advantages, and disadvantages of a strain gauge are been discussed.

I hope you learn a lot from the reading, if so, kindly share with other students. Thanks for reading, see you around!