# Understanding strain gauge

It converts force, pressure, tension, weight, etc. into a change in electrical resistance that is then measured. A strain gauge is a sensor whose resistance varies with applied force. Stress and strain happen when outside forces are applied to a stationary item. Stress and strain are terms used to describe opposing internal forces within an object and the resulting displacement and deformation, etc.

One of the most important tools in the electrical measurement approach used to measure mechanical quantities is the strain gauge. They are used to measure strain, as their name suggests. A positive or negative sign differentiates tensile and compressive strain, which are referred to as “strain” in a technical sense. As a result, strain gauges may detect both expansion and contraction. A body’s stress is always brought on by an internal or external factor.

Forces, pressures, moments, heat, changes to the material, and other factors may result in strain. The amount or value of the influencing quantity can be determined based on the measured strain value if certain requirements are met. This function is frequently used in experimental stress analysis. Experimental stress analysis states the stress in the material and also indicates its safety and durability using the strain values obtained on the surface of a specimen, or structural part. For the measurement of forces or other derived quantities, such as moments, pressures, accelerations, displacements, vibrations, and others, special transducers can be created. The pressure sensitive diaphragm in the transducer typically has strain gauges bonded to it.

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## What is a strain gauge?

When talking about strain gauges, it is important to first comprehend what they measure. A sensor known as a strain gauge measures electrical resistance that changes in relation to differences in strain. Strain is the material’s displacement or deformation as a result of applied stress. are made to concentrate stress through beam elements where strain gauges are positioned.

Stress is defined as the force applied to a material divided by the cross-sectional area of the material. Strain gauges turn the applied force, pressure, torque, etc. into an electrical signal that can be measured. Strain is caused by force, and the strain gauge measures it by detecting a change in electrical resistance. The voltage measurement is then collected using.

Read more: Understanding Bourdon Pressure Gauge

## 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 penetrable 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

## 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:

Read more: Understanding Diaphragm Pressure Gauge

## 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).

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.

## 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.

## How is Strain Gauge Measured?

The next stage in employing strain gauges is to gather relevant data now that the method of strain measurements has been developed.The strain gauge needs to be coupled to an electrical circuit that can precisely react to the tiny resistance variations brought on by strain. In a divided bridge circuit, several strain gauges can be utilized to track tiny changes in electrical resistance. A Wheatstone bridge is what you have here. An excitation voltage is delivered across the circuit in a Wheatstone bridge configuration, and the output voltage is measured across two points in the middle of the bridge.

The Wheatstone bridge is balanced and has zero output voltage when there is no load on the load cell. As the strain gauge deforms together with the material under it, even the smallest change causes a change in resistance. The output voltage changes as a result of the bridge being knocked out of balance. As was previously said, the resistance change is minute, hence signal amplification is often needed to accurately identify changes. The strain signal changes are boosted by the amplification process, but extra undesired noise is also picked up in the signal as a side effect. Signal conditioning removes the extra noise to produce accurate and understandable data.

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.

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.