What is Strain Gauge : Working and Its Applications

A strain gauge was invented by Edward E. Simmons and Arthur C. Ruge, in 1938. This invention led to the measurement of a significant amount of strain on various structures. A strain gauge is a type of sensor used in a wide variety of applications to measure the strain of an object. It is an essential geotechnical tool that determines strain in a range of structures such as tunnels, underground cavities, buildings, bridges, concretes, masonry dams, embedment in soil/concrete, etc. Here’s everything that a reader can know about the Strain gauge including working principle, characteristics, and applications.

What is a Strain Gauge?

Definition: A strain gauge is one of the imperative devices used in the field of geotechnical engineering to measure strain on diverse structures. By applying an external force, there would be a change in resistance of a strain gauge.

The basic construction of a gauge has an insulating flexible backing to support a metallic foil structure. This metallic coil is glued to a thin backing called a carrier, and the entire setup is fixed to an object using a suitable adhesive. As the object is deformed due to force, pressure, weight, tension, etc., the electrical resistance of foil changes. A Wheatstone bridge measures the change in resistivity, which is related to strain through a quantity known as Gauge Factor.

The small changes in resistance of a gauge are measured using the concept of Wheatstone bridge. The figure below illustrates the general Wheatstone bridge, which has four resistive arms and an excitation voltage, V_EX.

The Wheatstone bridge has two parallel voltage divider circuits. R1 and R2 form a one voltage divider circuit, R3 and R4 form second voltage divider circuit. The output voltage VO is given by:

Vo=[R3/(R3+R4 )-R2/(R1+2)]* V_EX

If R1/R2 = R4/R3, then the output voltage is zero and the bridge is said to be a balanced bridge.

A small change in resistance leads to a nonzero output voltage. If ‘R4’ is replaced with a strain gauge and any changes in the resistance of strain gauge will unbalance the bridge and produce nonzero voltage.

Gauge Factor of Strain Gauge

The gauge factor GF is given as

GF= (∆R⁄RG )/∈

Where,

‘ΔR’ is the change in resistance due to strain

‘RG’ is the resistance of the undeformed gauge

‘ε’ is the strain

The gauge factor of common metallic foils is around 2. The output sensor voltage SV of a Wheatstone bridge is given by,

SV=EV (GF.∈)/4

Where EV is the bridge excitation voltage

Strain Gauge Working

The functioning of a strain gauge entirely depends on the electrical resistivity of an object/conductor. When an object gets stretched within its limits of elasticity and does not break or buckle permanently, it becomes thinner and longer, resulting in high electrical resistance. If an object is compressed and does not deform, but, broadens and shortens, results in decreased electrical resistance. The values obtained after measuring the electrical resistance of a gauge helps to understand the amount of stress-induced.

The excitation voltage is applied at the input terminals of a gauge network, while the output is read at the output terminals. Normally, these are connected to a load and are likely to remain stable for longer periods, sometimes decades. The glue used for gauges depends on the duration of a measurement system – cyanoacrylate glue is suitable for short term measurements and epoxy glue for long term measurements.

Strain Gauge Working Principle

As we know that the resistance is directly dependent on the length and cross-sectional area of a conductor, which is given by R = L/A

Where,

‘R’= Resistance

‘L’ = Length

‘A’ = cross-sectional area

Clearly, the length of a conductor is altered with the change in size and shape of a conductor, eventually, altering the cross-sectional area and resistance.

Any normal gauge has a conductive strip that is long and thin, placed in a zig-zag fashion of parallel lines. The purpose of this zig-zag alignment is to elaborate on the small amount of stress that occurs between the parallel lines with much accuracy. Stress is defined as the resisting force of an object.

Strain Gauges Rosettes

Two or more gauges positioned closely in a rosette-like structure to measure the number of components to evaluate precise strain on the surface are known as strain gauge rosettes. The illustration is shown in the figure below.

Strain Gauge Load Cells

These load cells are most commonly found in industrial applications. It is highly precise and economical. Basically, a load cell consists of a metal body on which strain gauges are attached. For the metal body to be sturdy and less elastic, alloy steel, aluminum, or stainless steel are employed to design.

When an external force is applied on a load cell, the load cell is slightly deformed, and if not overloaded, it returns to its original shape.

If the load cell deforms, the gauge changes in shape, causing the change in electrical resistance of gauge, which in turn measures voltage.

There are common types of strain gauge load cells, which include bending beam, pancake, single point shear beam load cell, double-ended shear beam, wire rope clamps, and so on.

Characteristics of Strain Gauges

The important characteristics of strain gauges are:

• These are appropriate for longer periods with certain precautions
• They provide precise values with a change in temperature and other factors
• These are easy to manufacture because of simple components
• They are easy to maintain and has a long operating life
• This is totally encapsulated to protect from damages like handling and installation

Applications of Strain Gauge

The exceptional features enable these gauges to be used in the field of geotechnical engineering to monitor structures like dams, tunnels, etc. constantly and to avoid accidents well in advance. Some of the applications of strain gauges include –

• Rail monitoring
• Cable bridges
• Aerospace
• Nuclear power plants

FAQs

1). What is the sensitivity of strain gauge?

The flow stress differs with respect to the strain rate. Also, the strain rate relies on the grain size of an object or a work material. It is defined as the ratio of the change in the flow stress to the change in the strain.

2). What is the unit of strain?

The strain is a dimensionless quantity. However, the strain rate is the reciprocal of time and the SI unit is reciprocal of seconds (s-1).

3). How do I choose a strain gauge?

This is selected based on the type of applications and other related elements. Such as –

• Based on gauge length and resistance
• Based on labor-saving cost
• Based on material and measurement environment

4). Why Wheatstone bridge is used for strain gauge?

Wheatstone bridge is capable of measuring output voltages in terms of millivolts. For a bonded strain gauge, a change in resistance can be measured when it is connected to an electrical circuit (Wheatstone bridge) that measures the minute change in resistance. As the output voltage on the Wheatstone bridge becomes non-zero, the circuit loses its balance and helps to determine the strain on the object.

5). How do you install strain gauges?

Here are the steps to install strain gauge

Thus, an extensive description of the strain gauge, the principle of working, gauge factor, characteristics, and applications are provided in this article. Besides this, digital image correlation (DIC) is a technique employed nowadays to measure strain. It is used in many industries because of accuracy and as a replacement of conventional types of sensors like accelerometers, string pots, LVDT, and many more. Here is a question for you, what is the main function of strain gauge?