Bonded and Unbonded Strain Gauges

Working Principle: 

The strain gauge working principle is based on the fact that the electrical resistance of materials varies with deformation. A strain gauge is an example of a passive transducer that converts the mechanical displacement into electrical quantity.

We know, the resistance of the conductor depends on the length and cross-sectional area.

 The resistance

R = ρ L/A

 L = length of the conductor or semiconductor element.

A = Cross sectional area.

 ρ = Resistivity.

Electrical strain gauges:

A strain gauge is a sensor whose resistance varies with an applied force. The strain gauge converts the force, pressure, tension, and weight into electrical quantity which can be measured.

The electrical strain gauges measure the changes that occur in resistance, capacitance, or inductance due to the strain transferred from the workpiece to the basic gauge element.

Based on mounting, strain gauges are classified as

Bonded strain gauges.

Unbonded strain gauges.

Bonded strain gauge :

Bonded strain gauges are so-called because they are attached to the elastic element surface. The most commonly used are bonded resistance type strain gauges. They are primarily used for strain analysis.  

In bonded resistance wire strain gauges resistance element is cemented to the base which may be a thin sheet of paper or a thin sheet of bakelite or Teflon. The bonded strain gauge is connected to the Wheatstone bridge circuit.

Wire strain gauge:

The resistance element is in the form of wire foil or film of the material.

In a metal bonded strain gauge a fine wire element, about 0.025 mm or less in diameter is looped back and forth on the base carrier or mounting plate. The base is cemented to the member subjected to the stress.

The grid fine wire is cemented on a carrier which may be a thin sheet of paper bakelite or Teflon.

Metal Foil :

The strain is detected by using a metal foil. The metal foil is pasted on one side of the plastic The leads are soldered to the metal foil for connecting the Wheatstone bridge.

The metal and alloys used for the foil or wire are Constantan(Cu-Ni), Nichrome V(Ni-Cr), Isoelastic(Fe-Ni-Cr-Mn-Mo), Platinum-Tungsten(Pt-W).  

Metal Foil strain gauges exhibit a higher gauge factor than wire foil strain gauges.  

Rosette Strain gauges: 

Strains that are parallel to the strain axis are measured by a single strain gauge.

Whereas if there is a requirement of measurement of the strain in more than one direction, placing more than one strain gauge in a number of directions at the proper locations.

Types of Adhesives: 

  • Epoxy adhesive
  • Ceramic cement
  • Cellulose nitrate cement
  • Cyanoacrylate cement
  • Polyimide adhesive
  • Phenolic adhesive

Gauge factor G:

The sensitivity of the strain gauge is expressed in the terms of a characteristic called the Gauge factor.

Strain wire Gauge factor G is the ratio between relative change in resistance due to the change in relative length.   

Unbonded strain gauge:

It is exclusively used in transducer applications. The unbonded resistance strain gauge uses strain-sensitive wire with one end fixed or the other end attached to a movable element as shown in figure 2.

When the strain gauge is connected to the Wheatstone bridge circuit, at initial preload, the strain and resistance of the four arms are approximately equal. The resultant output voltage of the bridge is equal to zero. Application of pressure causes a small displacement causes a decrease in resistance of the strain gauge. This results in an unbalance in the output voltage of the Wheatstone bridge which is proportional to the small displacement hence the pressure.

Un bonded strain gauges measure the very small motion of the order of 50 micrometers and very small forces.

The device is less robust than the bonded gauges. Unbonded wire strain gauges become less common with the advancement of the improved bonding cement.

Semiconductor strain gauges:

The basic principle of operation of semiconductor strain gauge is the piezo-resistive effect. Change in value of resistance due to the change in resistivity of the semiconductor because of the strain applied.

Semiconductor materials used are germanium and silicon.

Semiconductor filament used has a thickness of 0.05 mm bonded on a suitable insulative material such as Teflon. Gold leads are used for making contacts.

Advantages of Semiconductor strain gauges :

  1. Semiconductor strain gauges have a higher gauge factor.
  2. Hysteresis characteristics of semiconductor strain gauges are excellent (as very low).
  3. Semiconductor strain gauges can be very small in size.

Disadvantages:

  1. They are very sensitive to changes in temperature.
  2. The linearity of semiconductor strain gauges is poor.
  3. They are more expensive.

Strain gauge connection methods to bridge circuit:

  • Quarter bride
  • Half-bridge
  • Full bridge

Quarter Bridge connection:

Only one arm is connected with the strain gauges. Half-bridge and full-bridge are more sensitive than the quarter bridge circuit.

Half Bridge connection:

Two arms R2 and R4 are connected with the strain gauges. Two strain gauges are mounted on opposite side surfaces of the test specimen with one as stressed strain and the other as compressed strain. R1 and R3 are dummy resistances not subjected to stress. Therefore the temperature effects will be canceled and no effect on the circuit.

Full Bridge connection:

All four arms are connected with the strain gauges. It is used where the complementary pairs of strain gauges bonded to test the specimen. A full-bridge circuit is more linear than a half-bridge and quarter bridge circuit.

Desirable characteristics of strain gauges:

  • Strain gages should have a high value of gauge factor.
  • Strain gauges should have high resistance.
  • Strain gauges high resistance temperature coefficient.
  • Should have linear characteristics (resistance versus strain).
  • Strain gauge lead wires should have low and stable resistivity also a low-temperature coefficient.
  • If strain gauges are used frequently for dynamic measurements, the frequency response over the entire range should be linear.

Uses:

  • Strain gauges are used in weight measurement.
  • Strain measurement in concrete and metal structures.

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