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What is Strain Gauge -Types Of Strain Gauges

What is Strain Gauge ?

The most widely used pressure and force sensitive transducer is the strain gauge. The principle of the strain gauge is based on the resistive properties of electrical conductors.

Electrical conductor possesses resistance based on the relationship

R = ρ ( L / A)

where R is the resistance, ρ is the resistivity, L is the length and A is the area of cross-section.

When a metal conductor is stretched or compressed, its resistance changes because of the fact that both length and diameter of the conductor change. These effects, called piezoresistive effect, can be used for measurement of several variables like strain and associated stress in experimental stress analysis, and small dimensional changes.

At the top of the figure, the conductor is unstressed. At the bottom of the figure, the conductor is in tension, increasing its length and reducing its area. The resistance of the strain gauge changes in proportion to its change in dimensions.

The gauge factor, G, of a strain gauge is the ratio of relative change in resistance to the relative change in length.

There are two primary constructions used in making strain gauges : bonded and unbonded. These are shown in Figure . In the unbonded strain gauge, the wire resistance element is stretched between two flexible supports. The wire stretches in accordance with the force applied to the diaphragm. The resistance of the wire changes due to these forces.

types of strain gauges -bonded and unboned strain gauges

In a bonded strain gauge, a wire metal foil is placed in a thin metal diaphragm. When the diaphragm is flexed, the element deforms and change in resistance occurs. Generally, bonded strain gauge is more durable than unbonded.

There are three types of strain gauges :

(a) Metallic resistance strain gauge made of metallic wires such as constantan (Cu-Ni alloy) Nichrome V or Platinum alloy.

(b) Foil strain gauge consists of a thin, 8-to 15 µm nitro-cellulose impregnated paper on which photo etched metal alloy filaments are attached as resistance material. For higher temperature, an epoxy backing is used instead of paper.
The active length of the gauge is along the transverse axis. The gauge should be mounted with its transverse axis in the same direction as the direction of application of force or strain. Thus, the elongation of the gauge reduces the length and consequently the resistance.

(c) The third type is the semiconductor gauge. It depends on the piezoresistive properties of silicon and germanium. They have high sensitivities with gauge factor from 50 to 200. Their chief defects are fluctuations due to temperature and non-linear output. The p-type gauges increase resistance with applied tensile strain while n-type gauge resistance decreases. The gauge is generally bonded to the structure by epoxy adhesive or ceramic cement.