Fig. Strain Gauges (source: omega.com)

Strain gauge
Strain gauge is an electrical transducer which is used for measuring mechanical surface strain.

Basic Principle of Strain Gauge

When stress is applied on the metal conductor, its resistance changes owing to change in length and cross-sectional area of the conductor. The resistance of the conductor under stress is also changed due to change in resistivity of the conductor.
When there is a change in the value of resistivity of the conductor when it is strained, this property is called as piezoresistive effect. So strain gauges are also called as piezoresistive gauges.

What is Strain?

Strain (ε) is defined as the fractional change in length. Strain is the amount of deformation of a body due to an applied force as shown in Figure below.

Strain can be positive (tensile) or negative (compressive). When the conductor is stretched, it is under tensile strain. In this case length of the conductor increases. Strain is positive in this case. Other the other hand if the conductor is compressed, it is under compressive strain. In this case length of the conductor decreases. Strain is negative in this case.

Poisson’s Ratio (ν)

The Poisson’s Ratio (ν) of a material is defined as the negative ratio of the strain in the transverse direction (perpendicular to the force) to the strain in the axial (longitudinal) direction (parallel to the force).

OR

Poisson’s Ratio (ν) of a material is a negative ratio of lateral strain to the longitudinal (axial) strain. It is a material specific property and is dimensionless.

Gauge factor (GF)

It is defined as the ratio of per unit change in resistance to per unit change in length.

OR

where, ν = Poisson’s Ratio
ε = Strain
Δ⍴/⍴ = per unit change in resistivity

If we look the individual terms in Gauge factor formula then

1 corresponds to resistance change due to change in length of the conductor.
2ν corresponds to resistance change due to change in area of the conductor.
(Δ⍴/⍴)/ε  corresponds to resistance change due to piezoresistive effect.

In many problem, the piezoresistive effect is neglected. Now the formula becomes

GF = 1 + 2ν

Note: (a). Gauge factor is a unit less number.
(b). Gauge factor varies with temperature and the type of material.

Types of Strain Gauges

i). Unbonded metal strain gauge
ii). Bonded metal wire strain gauge
iii). Bonded metal foil strain gauge
iv). Vacuum deposited thin metal film strain gauge
v). Sputter deposited thin metal strain gauge
vi). Bonded semiconductor strain gauge
vii). Diffused Semiconductor strain gauge
viii). Diffused metal strain gauge

Q. What are Rosettes?
Ans. The cluster of strain gauges is called the Rosettes. Practically, stress can be applied in any direction and it is not possible to orient the strain gauges along the direction of principle stress. So with the help of combination of strain gauges, it is possible to find the strain and stress values without actually knowing their directions. It is used for specific stress analysis or transducer applications. A three element rosette 60° planar (foil) is shown in figure below.

(source: veda.com.ua)
Q. Why are strain gauges called the piezoresistive strain gauges?
Ans. The resistance of conductor under strain is also changed due to change in resistivity of the conductor. This property is known as piezo-resistive effect and hence strain gauges are also called as piezoresistive strain gauges.
Q. Write are the different applications of Strain Gauges?
Ans. 1. Strain Gauges are used for analysing the dynamic strains in complex structures like roads, bridges, buildings, etc.
2. It is used for measuring tension, force, torque and stresses in structures.
3. It is used for the measurement of force by the strain produced in load rings.
Q. What is the selection criterion of strain gauge?
Ans. Operating temperature, Stability requirements and  Nature of the strain to be detected are the three primary considerations in strain gauge selection. Also, choosing the right carrier material, adhesive, grid alloy, and protective coating plays an important role in particular application.
Q. Write a Short note on Semiconductor Strain Gauges. Mention advantages and disadvantages.
Q. Write a Short note on Thin-film Strain Gauges. Mention advantages and disadvantages.
Q. Write a Short note on Diffused Semiconductor Strain Gauges. Mention advantages and disadvantages.
Q. Write a Short note on Bonded Resistance Strain Gauges. Mention advantages and disadvantages.

Q. Explain the construction of wire wound strain gauges.

Q. Derive the expression for the gauge factor of strain gauge.

Q. Explain with diagrams, the bonded and unbonded type of strain gauges.

Q. What are the materials used for base, adhesive and lead in case of bonded strain gauges?

Strain Gauge Working Principle & Applications
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