Strain Gauges are an 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. 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, 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 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 as given below ... [Continue Reading]
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