Strain Gauge

What is a Strain Gauge?

A strain gauge is a sensor that translates force, pressure, tension, weight, etc. into a change in electrical resistance that can subsequently be measured. The resistance of a strain gauge varies with applied force. Stress and strain happen when outside forces are applied to a stationary item.

A strain gauge is one of the most crucial sensors used in the electrical measurement method used to assess mechanical quantities.

Download Formulas for GATE Civil Engineering – Fluid Mechanics

Strain Gauge Working Principle

Strain gauge 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 will not break, but it may be narrower and longer. Similarly, when it is compressed, it gets shorter and broader, and ultimately it changes resistance.

Types of Strain Gauge

Based on the position and arrangement of the strain gauge, it is classified into the following types.

• Linear strain gauge
• Rosette strain gauge
• Diaphragm strain gauge
• Shear strain gauge
• Double parallel strain gauge

Based on the type of resistance material, it is classified into the following four types.

• Fine wire strain gauge
• Metal foil strain gauge
• Semiconductor strain gauge
• Photo-electric strain gauge

Based on the construction method, it is classified into the following two types.

• Unbonded type
• Bonded type

Download Formulas for GATE Civil Engineering – Environmental Engineering

Characteristics of Strain Gauge

The gauge dimensions, resistance, gauge factor, resistivity, temperature coefficient, and thermal stability mainly define the characteristics of a strain gauge. Gauge dimensions and shape are essential in choosing the right strain gauge type for a given application.

• A wide range of accessories are available along with the strain gauge
• It is highly precise and doesn’t get influenced due to temperature changes. However, a thermistor is used for temperature corrections if they get affected by temperature changes.
• A strain gauge is ideal for long-distance communication as the output is an electrical signal.
• Strain gauges require low maintenance and have a long operating life.
• The production of strain gauges is easy because of their simple operating principle and a small number of components.
• Strain gauge suitable for long-term installation. However, Strain gauges require certain precautions during installation.
• All the strain gauges produced by Encardio-Rite are hermetically sealed and made up of stainless steel thus, it is waterproof.
• Strain gauges are fully encapsulated for protection against handling and installation damage.
• The remote digital readout resistivity is also possible in strain gauges.

Design Consideration of Strain Gauge

The strain gauges are made from metals. It can be further divided into wire-wounded and metal foil types. the earliest form of the device is Wire wounded type. Now-a-day, metal-foil strain gauges are the most common type. Metal foil strain gauge manufactured through photochemical etching or circuit printing. Some raw materials used for producing metal strain gauges are copper-nickel, nickel-chromium, and platinum-alloys.

Strain Gauge Applications

Strain gauges are extensively used in geotechnical monitoring to constantly check structures, dams, tunnels, and buildings so that accidents can be avoided well on time. The applications of strain gauges include:

• Airplane: Strain gauges are fixed to the structural load-bearing components to measure stresses along load paths for wing deformation in an airplane.
• Cable bridge: Strain gauge becomes important to monitor the bridges regularly for any kind of deformation as it might lead to fatal accidents. Strain gauge technology is used in the real-time monitoring of huge bridges, making the inspections precise.
• Rail Monitoring: Strain Gauges have a vast history in the safety of rails. The strain gauge is used to measure strain corresponding to stress on rails. Strain gauges measure axial tension or compression without impact on the rails. While in an emergency, the strain gauges can generate a warning. Hence, maintenance can be done early to minimize the impact on rail traffic.

Strain Gauge Rosette

We use the most common type of strain gauge to find the strain in a different direction.

• In order to measure strain, forces, and deformation in multiple axes, strain gauge rosettes are used. A strain gauge rosette with two strain gauges can be used if the direction of principal stresses is known. A combination of 3 strain gauges was placed to find the strain in any direction.
• Strains that occur in most engineering structures and machines are very small. For example, In a metallic tie rod, the maximum allowable axial strain will be less than the offset strain of 0.2%.
• A widely used method of strain gauge measurement is based on the electric resistance strain gauge. These strain gauges measure normal or longitudinal strain whether extension or contraction at a point on the surface of the deforming solid.
• When shear strain can not be measured directly by one single strain gauge. An arrangement of three gauges mounted at the same point is used to define the state of strain of a point; such arrangement is known as a strain gauge rosette.

Strain Gauge Rosettes are further classified into three types:

• Delta strain gauge rosettes (the 2nd and 3rd strain gauges 60⁰and 120⁰away from the 1st strain gauge)
• Rectangular strain gauge rosettes (the 2nd and 3rd strain gauges45⁰and 90⁰away from the 1st strain gauge)
• Y-type or Star strain gauge rosettes (the 2nd and 3rd strain gauge 120⁰and 240⁰away from 1st strain gauge)

Strain Gauge Limitations

Each strain gauge has its limitations regarding temperature, fatigue, amount of strain, and the measurement environment. These limitations must be examined before using of strain gauge.

• Strain gauges measure strain in only one direction, a single strain gauge is often referred to as an axial gauge which is short for uniaxial gauge and reflects its sensitivity of the strain in one direction only.
• If the temperature is more, resistance will be more and vice versa. This is a common property for all the conductors. It can be resolved by using strain gauges that are self-temperature compensated (or) by a dummy strain gauge technique.

Important Topics for Gate Exam
Non-Newtonian Fluids	Open Loop Control System
Pattern Allowances	Poissons Ratio
Pressure Measurement	Prestressed Concrete
Prestressing Systems	Principle of Conservation of Energy
Properties of Aggregate	Properties of Concrete