Strain Gauge Principle, Types, Features and Applications

1. STRAIN GAUGE PRINCIPLE, TYPES, FEATURES AND APPLICATIONS

2. 1 6 INTRODUCTION WHERE ARE STRAIN GAUGES USED? 7 2 TYPES OF STRAIN GAUGES WHAT IS A STRAIN GAUGE? TABLE OF CONTENT 8 3 WHAT IS THE WORKING PRINCIPLE OF STRAIN GAUGE? ADVANTAGES OF STRAIN GAUGES 9 4 HOW DO YOU MEASURE STRAIN WITH A STRAIN GAUGE? LIMITATIONS OF STRAIN GAUGES 5 CHARACTERISTICS OF STRAIN GAUGES

3. 1 INTRODUCTION Strain Gauges are important geotechnical sensors 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. The change in stress is determined by multiplying the measured strain by the modulus of elasticity.

4. 2 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 like 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.

5. 2 3 WHAT IS MEANT BY STRUCTURAL HEALTH MONITORING? WHAT IS THE WORKING PRINCIPLE OF STRAIN GAUGE? 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.

6. 2 3 WHAT IS MEANT BY STRUCTURAL HEALTH MONITORING? WHAT IS THE WORKING PRINCIPLE OF STRAIN GAUGE? 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 = [∆R / (RG * ε)] Where, ∆R = Change in the resistance caused due to strain RG = resistance of the undeformed gauge ε = Strain

7. 2 3 WHAT IS MEANT BY STRUCTURAL HEALTH MONITORING? WHAT IS THE WORKING PRINCIPLE OF STRAIN GAUGE? 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

8. 4 WHY IS GEOTECHNICAL MONITORING IMPORTANT? HOW DO YOU MEASURE STRAIN WITH A STRAIN GAUGE? Strain gauges work on the principle of the conductor’s resistance which gives you the value of Gauge Factor by the formula: GF = [∆R / (RG * ε)] In practice, the change in the strain of an object is a very small quantity which can only be measured using a Wheatstone Bridge. A Wheatstone Bridge is a network of four resistors with an excitation voltage, Vex that is applied across the bridge.

9. 4 WHY IS GEOTECHNICAL MONITORING IMPORTANT? HOW DO YOU MEASURE STRAIN WITH A STRAIN GAUGE? The Wheatstone Bridge is the electrical equivalent of two parallel voltage divider circuits with R1 and R2 as one of them and R3and R4as 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.

10. 5 WHY IS GEOTECHNICAL MONITORING IMPORTANT? CHARACTERISTICS OF STRAIN GAUGES • • • • • • • • 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. They are ideal for long distance communication as the output is an electrical signal. Strain Gauges require easy maintenance and have a long operating life. The production of strain gauges is easy because of the simple operating principle and a small number of components. The strain gauges are suitable for long-term installation. However, they require certain precautions while installing. All the strain gauges produced by Encardio-Rite are hermetically sealed and made up of stainless steel thus, waterproof. They are fully encapsulated for protection against handling and installation damage. The remote digital readout for strain gauges is also possible. https://www.encardio.com/geotechnical-instruments/

11. 6 WHY IS GEOTECHNICAL MONITORING IMPORTANT? WHERE ARE STRAIN GAUGES USED? 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.

12. 6 WHY IS GEOTECHNICAL MONITORING IMPORTANT? WHERE ARE STRAIN GAUGES USED? Cable Bridges Strain gauge technology is used in the real-time monitoring of huge bridges, making the inspections precise. For example, Yamuna Bridge in Allahabad-Naini is a 630 m cable-stayed bridge across river Yamuna. The bridge is installed with many measurement channels that sense wind speed and strain on its cables.

13. 6 WHY IS GEOTECHNICAL MONITORING IMPORTANT? WHERE ARE STRAIN GAUGES USED? 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.

14. 6 WHY IS GEOTECHNICAL MONITORING IMPORTANT? WHERE ARE STRAIN GAUGES USED? 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.

15. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES There are several types of strain gauges 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. Model EDS-11V/Hermetically Sealed Vibrating Wire Strain Gage It 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.

16. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES The Encardio-rite strain meter incorporates the latest vibrating wire technology to provide the remote digital readout of compressive and tensile strain in dams, bridg- es, underground cavities, tunnels, mines, steel structures and other areas of applica- tion where strain measurement is required.

17. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES Model EDS-12V/Vibrating Wire Sister Bar Strain Meter Encardio-rite models EDS-12V/EDS-12V-EX vibrating wire sister bar strain meters are specially designed for embedment in concrete structures. These are ideal for mea- surement of strain in concrete structures such as piles, diaphragm/slurry walls, bridge abutments, tunnel lining, dams, foundations etc.

18. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES Model EDS-20V-Series/Vibrating Wire Strain Gauge Vibrating Wire Strain Gage has three different models in its series: EDS-20V-AW/Arc Wieldable Strain Gauge Model EDS-20V-AW basically consists of two end pieces joined by a tube that en- closes 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.

19. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES The double ‘O’ ring seals suitably protect the strain gage against ingress of water. The strain gage is better suited for locations subjected to dripping or running water or which may submerge in water.

20. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES EDS-20V-E/Embedment Strain Gauge Model EDS-20V-E vibrating wire strain gage is designed to measure strain in under- ground cavities, tunnels, buildings, concrete and masonry dams etc. The strain gage is suitable for embedment in soil or concrete. The embedment strain gage is similar to the arc wieldable strain gage except for the fact that mounting blocks are replaced with stainless steel flanges.

21. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES DS-20V-SW/Spot Wieldable Strain Gauge A stainless steel foil tab fixed to each end block enables the gage 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 re- quired. The gage is designed to be used only for flat surfaces.

22. 7 WHY IS GEOTECHNICAL MONITORING IMPORTANT? TYPES OF STRAIN GAUGES Model EDS-21V Concrete Embedment Strain Gauge Model EDS-21V vibrating wire strain gauge is suitable for direct embedment in con- crete. It is extensively used in applications where strain measurement of up to 5000 macrostrains is required. The sensor is of stainless steel construction, hermetically sealed, and has water- proofing to prevent any ingress of water.

23. 8 ADVANTAGES OF STRAIN GAUGES • • • • • • • • • Strain Gauges are accurate, robust and low cost. They provide long term stability with high reliability. Vibrating wire strain gauges are hermetically sealed under a vacuum of 0.001 Torr The stainless steel construction makes it versatile for all kinds of environments. Strain gauges are easy to install and require low maintenance. A wide range of accessories available that can be paired up with strain gauges for extensive usage. Most of the strain gauges have thermistor available for temperature correction. You can get a remote digital readout for strain measurement. Strain gauges are fully encapsulated for protection against handling and installation damage.

24. 9 LIMITATIONS OF STRAIN GAUGES • • Strain Gauges are non-linear and they require regular calibration to avoid errors in the output readouts. A few of the strain gauges might require special assistance.

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