Strain gauge factor derivation pdf

Report For System And Measurement Class

Abstract

Report about strain gauges as they are an important geotechnical tool 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.

The report gives a good knowledge about the strain gauges as it contains definition and how

they work with details about applications of them.

A strain gauge works on the principle of electrical conductance and its dependence on the

conductor’s geometry.

And this report considered as one of the measurement system applications and how its

applied in real life as it is useful for mechanical engineers as a reference about the strain gauges.

Contents

1. Introduction .................................................................................................................... 4

2. Literature Review: .......................................................................................................... 5

3.References ........................................................................................................................ 6

1. Introduction

Strain Gage 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.

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 (rite, 2020).

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, Gauge Factor.

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 wire and L2 is the new length after an external force is

applied on 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.

Figure 1:strain gauge

2. Literature Review:

The working principle of Strain Gauge is a principle of electrical conductance and its

dependence on the conductor’s geometry. Whenever a conductor is stretched within the limits

of its elasticity, it does not break but, gets narrower and longer. Similarly, when it is

compressed, it gets shorter and broader, ultimately changing its resistance (rite, 2020).

As mentioned earlier, strain gauges work on the principle of the conductor’s resistance which

gives you the value of Gauge Factor by the formula:

GF = [ΔR / (RG * ε)]

Now, in practice, the change in the strain of an object is an exceedingly small quantity which

can only be measured using a Wheatstone Bridge. The Wheatstone Bridge circuit is given below

(allaboutcircuits., 2020).

Strain gauges are extensively used in a wide range of fields, including:

• Geotechnical monitoring (to keep a constant check on structures, dams, tunnels, and

buildings) in order to avoid unpleasant conditions well on time.

• Real-time monitoring of huge bridges.

• Railway line stress/strain monitoring

• Ability to determine the torque, power measurements makes strain gauge useful in

rotating devices like wheel, propellor, fans, etc...

• To determine structural load bearing components to measure stresses along load paths

for wing deflection as well as in structural frames too (electrical4u, 2020).

Figure 2:wheatstone bridge connected with strain gauge

References

(2020, 11). Retrieved from electrical4u: https://www.electrical4u.com/strain-gauge/

allaboutcircuits. (2020, 11). Retrieved from https://www.allaboutcircuits.com/textbook/direct-

current/chpt-9/strain-gauges/

rite, E. (2020, 11). strain-gauge-principle-types-features-and-applications. Retrieved from

encardio.medium: https://encardio.medium.com/strain-gauge-principle-types-

features-and-applications-357f6fed86a5