Strain Gauge: Principle, Types, Features and Applications

Here’s everything that you need to know about Strain Gauges. We have covered its working principles, characteristics, features, as well as, application areas.

What is a Strain Gauge?

Strain Gauge or Strain Gage was invented in 1938 by Edward E. Simmons and Arthur C. Ruge. It is one of the significant sensors used in the geotechnical field to measure the amount of strain on any structure (Dams, Buildings, Nuclear Plants, Tunnels, etc.). The resistance of a strain gauge varies with applied force and, it converts parameters such as force, pressure, tension, weight, etc. into a change in resistance that can be measured later on.

Whenever an external force is applied to an object, it tends to change its shape and size thereby, altering its resistance. The stress is the internal resisting capacity of an object while a strain is the amount of deformation experienced by it.

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 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 the gauge Factor.

Read more: Types Of Strain Gauges: Construction & Specifications

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.

Suppose L1 is the original length of the wire and L2 is the new length after an external force is applied to it, the strain (ε) is given by the formula:

ε = (L2-L1)/L1

Now, 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. 

In the case of real-life monitoring, while constructing concrete structures or monuments, the load is applied at the load application point of a load cell that consists of a strain gauge underlying it. As soon as the force is exerted, the strain gauge is deformed and, this deformation causes a change in its electrical resistance which ultimately changes the output voltage.

The Gauge Factor 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

The Gauge Factor of different materials is given below:

What is the working principle of Strain Gauge?

A strain gauge works on the principle of electrical conductance and its dependence on the conductor’s geometry. Whenever a conductor is stretched within the limits of its elasticity, it doesn’t break but, gets narrower and longer. Similarly, when it is compressed, it gets shorter and broader, ultimately changing its resistance.

We know, resistance is directly dependent on the length and the cross-sectional area of the conductor given by:

R= L/A

Where,

R = Resistance

L = Length

A = Cross-Sectional Area

The change in the shape and size of the conductor also alters its length and the cross-sectional area which eventually affects its resistance.

Any typical strain gauge will have a long, thin conductive strip arranged in a zig-zag pattern of parallel lines. The reason behind aligning them in a zig-zag fashion is that they don’t increase the sensitivity since the percentage change in resistance for a given strain for the entire conductive strip is the same for any single trace. 

Also, a single trace is liable to overheating which would change its resistance thus, making it difficult to measure the changes precisely.

Read more: Vibrating Wire Piezometer – Types and Operating Principle

How do you measure strain with a strain gauge?

As mentioned earlier, strain gauges work on the principle of the conductor’s resistance which gives you the value of the Gauge Factor by the formula:

GF = [ΔR / (RG * ε)]

Now, in practice, the change in the strain of an object is a very small quantity that can only be measured using a Wheatstone Bridge. The Wheatstone Bridge circuit is given below.

Figure 1: Strain Gauge Circuit

A Wheatstone Bridge is a network of four resistors with an excitation voltage, Vex that is applied across the bridge. The Wheatstone Bridge is the electrical equivalent of two parallel voltage divider circuits with R1 and R2 as one of them and R3 and R4 as the other one. 

The output of the Wheatstone circuit is given by:

Vo = [(R3/ R3+ R4) – (R2/ R1+2)] * Vex

Whenever R1/ R2 = R4 / R3, the output voltage Vo is zero and the bridge is said to be balanced. Any change in the values of R1, R2, R3, and R4 will, therefore, change the output voltage. If you replace the R4 resistor with a strain gauge, even a minor change in its resistance will change the output voltage Vex which is a function of strain. The equivalent strain output and voltage output always have a relation of 2:1.

Characteristics of Strain Gauges 

The characteristics of strain gauges are as follows:

1. They are highly precise and don’t get influenced due to temperature changes. However, if they do get affected by temperature changes, a thermistor is available for temperature corrections.
2. They are ideal for long-distance communication as the output is an electrical signal.
3. Strain Gauges require easy maintenance and have a long operating life.
4. The production of strain gauges is easy because of the simple operating principle and the small number of components.
5. The strain gauges are suitable for long-term installation. However, they require certain precautions while installing.
6. All the strain gauges produced by Encardio Rite are hermetically sealed and made up of stainless steel thus, waterproof.
7. They are fully encapsulated for protection against handling and installation damage
8. The remote digital readout for strain gauges is also possible

Where are strain gauges used (Applications)

Strain gauges are extensively used in the field of geotechnical monitoring to keep a constant check on structures, dams, tunnels, and buildings so that mishaps can be avoided well on time. The applications of strain gauges include:

1. 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.

2. Cable Bridges

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

Well-instrumented bridges can alert responsible authorities about approaching failure 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 the full benefits of instrumentation. 

It becomes necessary to monitor the bridges regularly for any kind of deformation as it might lead to fatal accidents. Strain gauge technology is used in the real-time monitoring of huge bridges, making the inspections precise.

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

3. 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.

Why are strain gauges important?

Strain gauges are extensively used in the field of geotechnical monitoring and instrumentation to constantly monitor dams, inner linings of tunnels, structures, buildings, cable-stayed bridges, and nuclear power plants to avoid mishaps and accidents in case there are any deformities in them. 

Timely actions taken can avoid accidents and loss of life due to deformities. Hence, strain gauges are important sensors in the geotechnical field.

Strain gauges are installed on these structures and then, the complete data from them is remotely retrievable through data loggers and readout units. They are considered significant measuring equipment for ensuring productivity and safety.

Types of Strain Gauges

There are several strain gauge types based on the principle of their working viz. mechanical, optical, acoustical, pneumatic, or electrical. Considering the mounting, strain gauges can be either bonded or unbonded, and based on the construction, we can have foil, semiconductor, and photoelectric strain gauges.

Encardio Rite primarily deals with six different types of strain gauges:

1. Model EDS-11V/Hermetically Sealed Vibrating Wire Strain Gauge

The Model EDS-11V strainmeter is suitable for embedment in soil or concrete or for 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, sewer/metro/rail/road tunnels, mines, steel structures, and other areas of 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 eliminated.

Working Principle of Hermetically Sealed Vibrating Wire Strain Gauge

The Encardio Rite vibrating wire strainmeter 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 can 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 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 dams
4. Testing of wye sections  
5. Monitoring of stresses in pressure shafts

2. Model EDS-12V/Sister Bar Strain Meter

Encardio Rite Models EDS-12V Strain Gauge is specially designed for embedment in concrete structures. These are ideal for the 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

Reliable and precise measurements possible

1. Fully encapsulated for protection against handling and installation damage
2. 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. Model EDS-12V Strain measurement in tunnel linings, dams, and bridge abutments
5. Provides reliable and high-resolution readings

3. Model EDS-20V-Series/Vibrating Wire Strain Gauge

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

• EDS-20V-AW/Arc Wieldable Strain Gauge

Model EDS-20V-AW 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 gauge against ingress of water. Extra waterproofing is provided on the strain gauge 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 of the strain gauge. The strain gauge is better suited for locations subjected to dripping or running water or which may submerge in water.

To mount the strain gauge, two annular mounting blocks are accurately positioned and aligned with the help of a mounting jig and dummy gauge and welded to the structure. The dummy gauge is then finally replaced by the actual strain gauge 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 gauge to a concrete structure.

• EDS-20V-E/Low Range Embedment Strain Gauge

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

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

• EDS-20V-SW/Spot Weldable Strain Gauge

In Model EDS-20V-SW spot weldable strain gauge, a stainless steel foil tab fixed to each end block enables the gauge to be spot-welded or fixed in place by epoxy. The gauge 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 gauge is designed to be used only for flat surfaces. 

4. Model EDS-21V Series/High Range Strain Gauge

Encardio Rite’s series of High Range Vibrating Wire Strain Gauge is used to measure strains up to 5000 micro-strains. There are two different models in this series:

• EDS-21V-E/High Range Embedment Strain Gage

Model EDS-21V-E is a strain gauge pretty similar to the Model EDS 20V-E. The High Range Strain Gauge is employed to determine strain in concrete mass, underground cavities, tunnels, buildings, concrete, masonry dams, etc. The stainless steel flanges at either end of the instrument allow it to be embedded directly into any concrete.

The deformation in concrete mass is reflected in the tension change of the wire, leading to a modification in the frequency of vibrations. 

The High Range Embedment Strain Gauge can measure up to 3000 micro-strains.

• EDS-21V-AW/High-range Arc Weldable Strain Gauge

Encardio Rite Model EDS-21-AW finds its application in surface mounting by welding on steel structures such as bridges, piles, wye sections, pressure shafts, tunnel linings, and supports, etc. It is used to determine the strain in the steel or concrete surface of the composite structure. 

The Arc Weldable Strain Gage has a range of 5000 micro-strains with an active gage length of 150 mm. 

5. Model EDS-40D/Dynamic Strain Gauge

Encardio Rite Model EDS-40D is a precision strain gauge with a high-frequency response for dynamic strain or stress monitoring. The Dynamic Strain Gage is usually applied in structure health monitoring. It is installed on the structural steel membrane where the stress/strain has to be monitored. Model EDS-40D is useful in applications where stress changes swiftly and dynamic monitoring is necessary. 

Encardio Rite Dynamic Strain Gauge is suitable for long-term monitoring as it comes with a provision for waterproof installation and works well even in hostile environments. 

FAQs

Q1. What is gauge length?
Ans: 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.

Q2. What is gauge sensitivity?
Ans: 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.

Q3. What is the range in terms of strain gauge?
Ans: The next characteristic of the strain gauge is its range. The range represents the maximum strain that can be recorded without resetting or replacing the strain gauges. 
Also, the range and sensitivity are interrelated since highly-sensitive gauge responds to small strains with indicator deflections and the range is usually limited to the full-scale deflection of the indicator.

Q4. What is accuracy in terms of strain gauge?
Ans: In a mechanical strain gauge, inaccuracies may result from lost motion such as backlash gear train, friction, temperature changes, wear in the mechanism, slippage, or flexure or deflection of the components.

Q5. Are strain gauges reusable?
Ans: 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.

Q6. Why is a Wheatstone Bridge used in the measurement of strain?
Ans: 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.

Q7. What is stress?
Ans: 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 to 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)

Q8. What is a strain?
Ans: The amount of deformation that occurs in an object is known as strain. The strain 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. 
The stress formula is articulated as
σ = F/A
Where,
F = Force applied 
A = Area on which force is acting