strain gauge is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
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, Gauge Factor.
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 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.
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 .
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 as given below:
Material Gauge Factor
1.Metal foil strain gauge 2–5
2.Thin-film metal (e.g. constantan) 2
3.Single crystal silicon -125 to + 200
4.Polysilicon ±30
5.p-type Ge 102
6.Thick Film Resistors 100
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 and thus, making it difficult to measure the changes precise.
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’s any deformity 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 as significant measuring equipment for ensuring productivity and safety.