Strain Gauge: Principle, Types, Features and Applications

Strain Gauge

Strain Gauges are important geotechnical tools that measure strain in underground cavities, tunnels, buildings, concrete, masonry dams, bridges, embedment in soil/concrete. etc. The main purpose of a strain gauge is to indirectly determine stress and its variation with time, quantitatively. Change in stress is determined by multiplying the measured strain by the modulus of elasticity.

Well, if you are confused regarding strain gauges, here’s everything that you need to know about it. We have covered its working principles, characteristics, features, as well as, application areas.

What Is A Strain Gauge?

Before we jump into the more technical part of the instrument, let’s first clearly define Strain Gauge.

Strain GaugeStrain Gage measures the strain (amount of contraction as well as expansion caused by external forces) on any object. The strain gauge was invented in 1938 by Edward E. Simmons and Arthur C. Ruge.

Any basic strain gauge consists of an insulating flexible backing that supports a metallic foil pattern. The gauge is attached to the object under stress using an adhesive. The deformation in the object causes the foil to get distorted as well which ultimately changes the electrical resistivity of the foil.

This change in resistivity is measured by a Wheatstone bridge which is related to strain by a quantity called Gauge Factor.

Strain Gauge Types – Encardio-rite Models

If you’re wondering what are the types of Strain Gauges, we have the answer for you. Encardio-rite primarily deals with five different types of strain gauges. Let’s take an in-depth look at them.

Model EDS-11V/Hermetically Sealed Vibrating Wire Strain Gage

The model EDS-11V strain meter is suitable for embedment in soil or concrete or surface mounting by welding on steel structures. It provides significant quantitative data on the magnitude and distribution of compressive and tensile strain and its variations with time.

The Encardio-rite strain meter incorporates the latest vibrating wire technology to provide the remote digital readout of compressive and tensile strain in dams, bridges, underground cavities, tunnels, mines, steel structures and other areas of strain gauge application where strain measurement is required.

Long term stability is achieved by thermal and load cycling, the unique method of wire clamping by generating a vacuum of 1/1000 Torr inside the sensor by electron beam welding. This results in the effect of oxidation, moisture, environmental conditions and any ingress of water being completely eliminated.

Strain Gauge Working Principle (Hermetically Sealed Vibrating Wire)

The Encardio-rite vibrating wire strain meter basically consists of a magnetic, high tensile strength stretched wire, one end of which is anchored and the other end is displaced proportionally to the variation in strain.

Any change in the strain directly affects the tension of the wire, resulting in a corresponding change in the frequency of vibration of the wire. The resonant frequency, with which the wire vibrates, is read by the readout unit. The strain is proportional to the square of the frequency and the readout unit is able to display this directly in strains.

Characteristics of Hermetically Sealed Vibrating Wire Strain Gauge

  1. The strain gauge is accurate, robust and low cost
  2. It ensures long term stability with high reliability
  3. It’s hermetically sealed under a vacuum of 0.001 Torr
  4. The strain gauge is made up of stainless steel construction
  5. It doesn’t require any special installation and maintenance
  6. A wide range of accessories are available along with this strain gauge
  7. A thermistor is available for temperature correction
  8. Remote digital readout for strain measurement
  9. Ease in data logging

Applications of Hermetically Sealed Vibrating Wire Strain Gage

There is the general query of what are strain gauges used for, and here is the answer for you.

These are the main application area of Strain Gauge:

  1. Measurement and monitoring of strain in concrete, rock and steel constructions
  2. Study of stress distribution in the supporting ribs of underground cavities and tunnels
  3. Determination and monitoring of stress distribution in concrete & masonry damsTesting of wye sections
  4. Monitoring of stresses in pressure shafts

Model EDS-12V/Vibrating Wire Sister Bar Strain Meter

Encardio-rite models EDS-12V/EDS-12V-EX vibrating wire sister bar strain meters are specially designed for embedment in concrete structures. These are ideal for measurement of strain in concrete structures such as piles, diaphragm/slurry walls, bridge abutments, tunnel lining, dams, foundations etc.

Characteristics of Vibrating Wire Sister Bar Strain Meter

  1. Suitable for long-term installation
  2. Easy to install and waterproof
  3. Reliable and precise measurement possible
  4. Fully encapsulated for protection against handling and installation damage
  5. Rugged construction

Applications of Vibrating Wire Sister Bar Strain Meter

  1. Suitable for measurement of strain in reinforced concrete structures
  2. Strain measurement in concrete piles and cast in place concrete piles.
  3. Strain measurement in diaphragm slurry walls
  4. The model EDS-12V Strain sensor is used in tunnel linings, dams and bridge abutments
  5. Provides reliable and high-resolution readings

Model EDS-20V-Series/Vibrating Wire Strain Gage

Vibrating Wire Strain Gage has three different models in its series:

EDS-20V-AW/Arc Wieldable Strain Gauge

Model EDS-20V-AW basically consists of two end pieces joined by a tube that encloses a length of magnetic, high tensile strength stretched wire. The wire is sealed in the tube by a set of double “O” rings fixed on each end piece. The double ‘O’ ring seals suitably protect the strain gage against ingress of water. An extra waterproofing is provided on the strain gage by a heat-shrinkable tube to prevent the ingress of water.

The tube is flattened in the middle to accommodate a sensor assembly in the constriction. The sensor forms an integral part with the strain gage. The strain gage is better suited for locations subjected to dripping or running water or which may submerge in water.

To mount the strain gage, two annular mounting blocks are accurately positioned and aligned with the help of a mounting jig and dummy gage and welded to the structure. The dummy gauge is then finally replaced by the actual strain gage and clamped in position by a pair of set screws on each block.

Groutable reinforced bar annular mounting blocks are available for surface mounting the strain gage to a concrete structure.

EDS-20V-E/Embedment Strain Gauge

Model EDS-20V-E vibrating wire strain gage is designed to measure strain in underground cavities, tunnels, buildings, concrete and masonry dams etc. The strain gage is suitable for embedding in soil or concrete.

The embedment strain gage is similar to the arc weldable strain gage except for the fact that mounting blocks are replaced with stainless steel flanges.

EDS-20V-SW/Spot Wieldable Strain Gauge

So, how does a strain gauge work?

A stainless steel foil tab fixed to each end block enables the gage to be spot-welded or fixed in place by epoxy. The gage is pre-tensioned by a small compression spring. Initial tension can be set during installation, allowing for maximum range in tension or compression as required. The garage is designed to be used only for flat surfaces.

How does a Strain Gauge Work?

A strain gauge depends on the electrical resistivity of any conductor. The resistance in any conducting device is dependent on its length as well as the cross-section area.

Whenever an external force changes the physical parameters of an object, its electrical resistivity also changes. A strain gauge measures this deformity by using the Gauge Factor formula.

That’s how strain gauge works!

What is the working Principle of Strain Gauge?

Whenever the external compressive or expansion force is increased or decreased on any object, its electrical resistivity also increases or decreases proportionally.

The Gauge Factor of a Strain Gauge is the sensitivity coefficient of strain gauges and, is given by the formula:

GF = [ΔR / (RG * ε)]

where,

ΔR = Change in the resistance caused due to strain

RG = resistance of the undeformed gauge

ε = Strain

The gauge factor for common metallic foil is usually a little over 2. The output voltage of the Wheatstone Bridge, SV is given by the formula:

SV = {EV x [(GF x ε)/4]}

where,

EV is the bridge excitation voltage.

Characteristics of Strain Gauge

Here are all the characteristics of Strain Gauge that are needed for the accurate functioning of the instrument.

Gauge Length

It’s impossible to measure nonlinear strain fields without some degree of error being introduced because strains cannot be measured at a point with any type of gauge.

In such cases, the error depends on the gauge length and the gauge width. The gauge size for a mechanical strain gauge is calculated by the distance between the two knife-edges in contact with the specimen and by the width of the moveable knife edge.

Gauge Sensitivity

Sensitivity can be defined as the smallest value of strain which is readable on the strain gauge scale.

The choice of a gauge is highly dependent upon the degree of sensitivity required, and quite often the selection of a gauge with a very high sensitivity increases the complexity of the measuring method.

Range

The next characteristic of the strain gauge is its range. The range represents the maximum strain which can be recorded without resetting or replacing the strain gauges.

Also, the range and sensitivity are interrelated since a highly-sensitive gauge responds to small strains with indicator deflections and the range is usually limited to the full-scale deflection of the indicator.

Accuracy

In a mechanical strain gage, inaccuracies may result from lost motion such as backlash gear train, friction, temperature changes, and wear in the mechanism, slippage, or flexure or deflection of the components.

Real-Life Applications of Strain Gauge

The scope of strain gauges in real life is unlimited. It can be used to test deformity in multiple structures such as high-rise buildings, underground metro corridors, bridges, dams, deep vacations, railway lines, etc.

Here are some practical applications of strain gauges:

Aerospace

Let’s talk about the application of strain gauge in aerospace.

Strain gauges are fixed to the structural load-bearing components to measure stresses along load paths for wing deflection or deformation in an aeroplane.

The strain gauges are wired into the Wheatstone Bridge circuits and its application areas include onboard signal conditioning units, excitation power supplies, and the telemetry necessary to read in site measurements.

Cable Bridges

Instrumentation of bridges is done to verify design parameters, evaluate the performance of new technologies used in the construction of bridges, to verify and control the construction process and for subsequent performance monitoring.

Well-instrumented bridges can alert responsible authorities about approaching failure so as to initiate preventive measures. Choosing proper sensor types, technology, a measurement range and their location on the bridge is very important to optimize costs and to extract full benefits of instrumentation.

It becomes necessary to monitor the bridges regularly for any kind of deformation as it might lead to fatal accidents. This calls for application of strain gauge in the engineering field.

Strain gauge technology is used in the real-time monitoring of huge bridges, making the inspections precise.

For example, Yamuna Bridge in Allahabad-Naini is a 630 m cable-stayed bridge across river Yamuna. The bridge is installed with many measurement channels that sense wind speed and strain on its cables.

Rail Monitoring

Strain Gauges have a long history in the safety of rails. It is used to measure stress and strain on rails. Strain gauges measure axial tension or compression with no impact on the rails. In case of an emergency, the strain gauges can generate a warning so maintenance can be done early to minimize the impact on rail traffic.

Torque and Power Measurements in Rotating Equipment

Strain gauges can measure the torque applied by a motor, turbine, or engine to fans, generators, wheels, or propellers. You will find such types of equipment in power plants, ships, refineries, automobiles and industries.

Limitations of Strain Gauge

Strain Gauges are pretty reliable and widely used sensing elements applied in the field of Geotechnical Instrumentation. However, there are very minor limitations that you can mention when it comes to drawbacks.

Each Model of Strain Gauge offers limitations in terms of fatigue, temperature, amount of strain, and the measurement environment. However, with the complete range from Encardio-rite, you can easily examine the strain gauge features and specification and use it as per your preference.

Frequently Asked Questions

Are Strain Gauges Reusable?

The main advantages of mechanical strain gauges are the ease with which they can be used, their relatively low cost and the fact that they are reusable. Electrical resistance strain gauges overcome most of the disadvantages of mechanical gauges.

Why is a Wheatstone Bridge Used in the Measurement of Strain?

It’s not always necessary to use the Wheatstone Bridge circuit to measure electrical resistance. The main advantage of a four-wire ohm circuit is that the lead wires do not affect the values because the voltage is detected directly across the strain gauge element.

What are the types of Strain Gauges?

Encardio-rite offers many make and models of Strain Gauges which includes:

Model EDS-11V Hermetically Sealed Vibrating Wire Strain Gauge

Model EDS-12V Vibrating Wire Sister Bar Strain Meter

Model EDS-20V-AW Vibrating Wire Arc Weldable Strain Gauge

Model EDS-20V-E Vibrating Wire Embedment Strain Gauge

Model EDS-20V SW Vibrating Wire Spot Weldable Strain Gauge

Model EDS-21V Concrete Embedment Strain Gauge

What is the unit in which the Strain measured?

The unit for strain in the SI (Système International) is “one” i.e. 1 ε= 1 = 1 m/m. In practice, the “unit” for strain is called “strain” and the symbol e is used.

What is Stress?

It is a measure of internal pressure on an object whenever an external force acts on it. The larger the force or the smaller the area over which it acts, the more likely the material is going to deform.

The strain is a unitless quantity since the values in the numerator and denominator always have the same units.

S = Δx/X

Where,

S = strain (unitless)

Δx = change in dimension (m for longitudinal or shearing strain, m3 for volumetric strain)

X = original dimension (m for longitudinal or shearing strain, m3 for volumetric strain)

What is a Strain?

The amount of deformation that occurs in an object is known as strain. There can be different types of strain.

It is defined as the change in length the force produces divided by the material’s original length.

Stress is denoted by σ. It is represented as N/m2.

Stress formula is articulated as

σ = F/A

Where,

F = Force applied

A = Area on which force is acting

This was all about strain gauges and, if you have any further queries, do let us know in the comments below.

 

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