Strain Gauge Principles: How They Work, Applications & Types

Do you know that the strain gauge could have prevented the Minneapolis Bridge collapse in 2007?

It’s true, with its ability to change its resistance to compression and expansion, it could measure the deformation of bridges caused by the load. Read ahead to know how a strain gauge can be used to prevent bridge collapse. But before that, you have to understand what is a strain gauge, how strain gauge works, its types and its use cases.

 

What Is a Strain Gauge?

As the name says, a strain gauge is a device that could measure even a small amount of strain caused in an object. The resistance of this device changes with the strain caused in it. Edward E. Simons and Arthur C. Ruge invented the strain gauge in 1938. 

With a strain gauge, you can calculate the strain of an object or body. You can use this high-precise electrical instrument to measure force, pressure or any factor that causes stress in an object. In a strain gauge, you can find metal strips arranged in a zig-zag manner on non-conductive material. The thin metal strips are called measuring grids and are of 3-5 micrometres in thickness. It is equivalent to the thickness of one strand of spider web, which is about 3-8 micrometres. 

        

 

Working Principle of Strain Gauge

The strain gauge’s working principle is the electrical resistivity and conductivity of an object. We all know that electrical resistivity and conductivity are any object's innate property. However, it varies from material to material and also depends on factors like length, temperature and cross-section area. The relation between resistance, length and cross-sectional area is mentioned below.

 

R ∝ L/A

R→Resistance

L→Length

A→Cross-sectional area

From this, you can say when a strain gauge is stretched, length increases and cross-sectional area shrinks; thereby, its resistance would increase. When it is compressed, length decreases and cross-sectional area increases; thereby, its resistance is decreased. This is how a strain gauge works.

You must couple the strain gauge with a wheat stone bridge for practical usage. It is because the change in resistance is very small, and only with the help of a wheat stone bridge you can determine it. 

 

 

 

Strain Gauge for Bridge Applications

Now it's time for you to understand how a strain gauge prevents bridge collapse. When a bridge undergoes deformation, the strain can be easily measured using a strain gauge. Strain is nothing but the ratio of change in dimension to the original dimension. Understand the relationship between change in resistance and strain from the below equation,

ΔR/R = G𝜀

ΔR→Change in Resistance

𝜀→Strain

G→Gauge factor of strain gauge

The Gauge factor of a strain gauge is the ratio of change in electrical resistance to mechanical strain. This is a direct indicator of the sensitivity of a strain gauge. With this, engineers could determine the strain caused in a bridge and prevent it from collapsing.

 

Other Use Cases of a Strain Gauge

Stain gauge is used in many areas, especially in geotechnical fields. They are used to monitor dams, tunnels, powerplants, etc. constantly. You could find their applications in rails, vehicle testing, non-destructive testing of beams and trusses, tunnels, etc. In all these areas strain gauge remains an essential component in preventing failures and gives an alert when maintenance is needed. A strain gauge could also be used to measure the force required to rotate an object like wheels, motors and propellors. In aeroplanes, they are attached to load-bearing components along load paths to detect deflection. You could also find their applications in rails, vehicle testing, non-destructive testing of beams and trusses, load cells and so on. These are a few examples of strain gauge applications.              

Types of Strain Gauge

We have different types of strain gauges used for various applications. Knowledge of this will help you choose the right one for the right application.

• Linear Strain Gauges
• Shear Strain Gauge
• Rosette Strain Gauge
• Chain Strain Gauge
• Full-bridge Strain Gauge
• Half-bridge Strain Gauge 

 

 

Advantages of Strain Gauge

• They are very small in size, and it is very easy to install. You can glue it to an object using adhesive, for which you need to conduct an experiment.
• They are highly precise. 
• Also, they have no moving parts. It means there is no chance for wear and tear, and it can be long lasting.

 

Disadvantages of Strain Gauge

You have to be careful while choosing strain gauges as they have their own limitations.

• Strain gauges are non-linear. It means they can only function within their elastic limit. If the strain exceeds its elastic limit, strain gauge material may fracture. Elastic limit is the point to which an object could be deformed and can come back to its original shape.
• It is sensitive to temperature. The temperature it could bear depends on the material used in the strain gauge.
• Also, they have to be regularly calibrated.

 

The strain gauge principle may be simple, but its use cases are many. After reading, you will have a basic understanding of what is strain gauge, the gauge factor of strain gauge and strain gauge applications. With this knowledge, you will be able to determine whether a strain gauge is necessary for your project or not. Also, if you want to upskill yourself by working on industry projects, you can pursue a course at Skill-Lync, an e-learning platform for engineers.