MEASUREMENT OF STRAIN

The problem of determining stresses acting on a body has been an important and interesting aspect of materials engineering. Prior to the invention of electrical resistance, strain gauge extensometers were extensively employed for the determination of strain. These were associated with a number of problems. One of the most important problems was their bulk, which was a major constraint in its use. Due to lack of precise knowledge of stress conditions of the material or structure, many engineering problems were solved on a theoretical basis, by employing trial and error methods and by assuming a large factor of safety.

In 1856, Lord Kelvin established the principle of electrical resistance strain gauges when he demonstrated that when stress is applied to a metal wire, apart from undergoing changes in both length and diameter, there will be some changes in the electrical resistance of the wire.

When external forces act on a body, the body is in a condition of stress and strain. Since direct measurement of stress is not possible, its effects, such as change of shape of the body or change in length, are measureable, thus providing a known relationship between stress and strain. If adequate information is available, stresses acting on a body can be computed.

When a force or load is applied to a body, it undergoes some deformation. This deformation per unit length is known as unit strain or simply strain, denoted by e, and is given by the

following equation:

e =δl/l

Here, dl is the change in the length of the body and l is the original length of the body.

Strain gauges are essentially employed for two different purposes

1. To determine the state of strain existing at a point on a loaded member for carrying out strain analysis

2. To act as a strain-sensitive transducer element in the measurement of quantities such as force, pressure, displacement, and acceleration

Normally, measurements are made over the shortest gauge lengths. The change in length measured over a finite length does not give the value of strain at a fixed point but rather gives the average strain over the entire length. A magnification system is essential since the change in length is over a small gauge length.

Two types of strain gauges are employed:

1. Mechanical strain gauges

2. Electrical strain gauges

Mechanical strain gauges

A mechanical strain gauge is a device used to measure the strain or deformation of an object or material. It typically consists of a thin, flexible strip of material, such as metal or plastic, that is bonded to the surface of the object being measured. As the object is deformed, the strain gauge also deforms, causing a change in its resistance. This change in resistance can be measured using an external circuit, and is proportional to the amount of strain experienced by the object.

Mechanical strain gauges are commonly used in engineering and materials science applications to measure the strain in structures such as bridges, buildings, and aircraft. They are also used in laboratory settings to measure the deformation of materials under controlled conditions.

One disadvantage of mechanical strain gauges is that they are prone to fatigue and can fail over time if subjected to repeated cycles of strain. They also require a physical connection to the object being measured, which can be difficult to achieve in some situations. As a result, electronic strain gauges, which use electrical resistance or capacitance to measure strain, have become more popular in recent years.

Electrical strain gauges

Electrical strain gauges are devices used to measure the strain or deformation of a material by converting mechanical deformation into an electrical signal. They consist of a thin metallic wire or foil that is bonded to the surface of a material that is undergoing deformation. When the material is subjected to a force, it stretches or compresses, causing the wire or foil to also stretch or compress. This results in a change in the electrical resistance of the wire or foil, which is proportional to the strain or deformation of the material.

The most common type of electrical strain gauge is the resistive strain gauge. It consists of a grid of thin metallic wire or foil that is arranged in a pattern on a flexible substrate, such as a polymer film. When the substrate is bonded to the surface of the material being tested, any deformation of the material causes the grid to stretch or compress, changing the resistance of the wire. This change in resistance is then measured using a Wheatstone bridge circuit, which provides an output signal proportional to the strain.

Other types of electrical strain gauges include capacitive strain gauges, which measure changes in capacitance due to deformation, and piezoelectric strain gauges, which generate an electrical signal in response to deformation through the piezoelectric effect. However, resistive strain gauges are the most commonly used type due to their accuracy, sensitivity, and ease of use. They are widely used in industries such as aerospace, automotive, and civil engineering for applications such as stress analysis, load testing, and structural health monitoring.