What Is a Strain Gauge? Working Principle & Applications

Table Of Contents

External force applied to an elastic material generates stress, which subsequently generates deformation in the material. At this time, the length of the material L extends to L+ΔL if the applied force is a tensile force. The ratio of Δ L to L, that is Δ L/L, is called strain. On the other hand, if a compressive force is applied, the length L is reduced to L−ΔL. Strain at this time is (−ΔL/L).

What is Strain gauge and working?

The electric resistance of a metal change proportionally to the mechanical deformation caused by an external force applied to the metal. By bonding a thin metal to a measurement object through a thin electrical insulator, the metal deforms depending on deformation of the measurement object and its electric resistance changes. The strain gauge (electric resistance strain gauge) is a sensor to measure the strain by means of measuring the resistance change. A strain gauge is constructed by forming a grid made of fine electric resistance wire or photographically etched metallic resistance foil on an electrical insulation base (backing), and attaching gauge leads.

Deformation and resistance: When a force is applied, it causes the object to stretch (tensile strain) or compress (compressive strain). This deformation also causes the strain gauge's grid to elongate or shorten.
Physical principle: According to the principles of electrical resistance, the resistance of a wire is proportional to its length and inversely proportional to its cross-sectional area.
When stretched, the wire becomes longer and thinner, increasing its resistance.
When compressed, the wire becomes shorter and thicker, decreasing its resistance.
Electrical measurement: The strain gauge is wired into a circuit, typically a Wheatstone bridge, which is designed to be balanced to produce zero voltage when there is no strain.
Signal conversion:  As the strain gauge's resistance changes, it unbalances the bridge and produces a small, proportional output voltage. This voltage is then amplified to get a meaningful reading.

Strain gauges are generally used for one of three reasons:

To ascertain the amount of deformation caused by strain
To ascertain the stress caused by strain and the degree of safety of a material or of a structural element that uses that material.
To indirectly ascertain various physical quantities by converting them to strain. There are a number of ways of measuring strain mechanically and electrically, but the vast majority of stress measurement is carried out

using strain gauges due to their superior measurement characteristics.

When strain is generated in a measurement object, the strain is transferred to the resistance wire or foil of the strain gauge via the gauge base (backing). As a result, the wire or foil experiences a resistance change.

Application of strain gauges

Strain gauges have a wide range of applications, including the measurement of force, pressure, and torque in industries like automotive, aerospace, and biomedical engineering. They are also used in structural health monitoring for bridges and buildings, material testing to analyse stress and strain, and in various industrial and consumer products like weighing scales.

Material testing:

Widely used in tensile, compression, vibration, and fatigue testing to study material properties under different conditions.

Engineering and structural monitoring:

Structural Health Monitoring (SHM): Used to monitor the integrity of structures like bridges and buildings by detecting stress and strain, which helps in preventing failure.

Geotechnical engineering:

Monitors the deformation of soil, rock, and underground structures like tunnels and retaining walls to assess stability.

Industrial applications

Weight and force measurement: Found in weighing scales (for process, crane, and platform scales), tank weighing, and other applications where force needs to be measured. Torque and power measurement: Measures torque in rotating equipment like motors, turbines, and engines in power plants, ships, and industrial machinery. Industrial automation: Used for process monitoring, quality control, and automation in various manufacturing settings.

Automotive and aerospace

Vehicle testing: Used in crash simulations, brake testing, and vehicle emission testing. Aerospace: Applied in aircraft and missile production to monitor strain on components. Rotating equipment: Measures torque in applications like generators and propeller shafts.

Biomedical and consumer products

Biomedical equipment: Integrated into surgical tables, dental chairs, and MRI equipment for patient monitoring. Prosthetics: Used in prosthetic legs to sense usage mode and adjust settings. Consumer electronics: Found in products like smart phones and bathroom scales.